ui
CHEMICAL EMBRYOLOGY
IN THREE VOLUMES
VOL. I
New Tork
The Macmillan Co.
London
The
Cambridge University Press
Bombay, Calcutta and
Madras
Macmillan and Co., Ltd.
Toronto
The Macmillan Co. of
Canada, Ltd.
All rights reserved
i"
f) ^
4'
Vir PraecLarissinius
GULIELMUS HARVEY.
Mysterii Generationis Indaqator Diliqens.
Q) r^^Lji(iJ<^i~^4^(tJ^^^i~^^L3(*J<^^>^L3(*^^ 0
CHEMICAL
EMBRYOLOGY
BY
JOSEPH NEEDHAM
M. A., Ph.D.
Fellow ofGonville & Cains College, Cambridge, and
University Demonstrator in
Biochemistry
VOLUME ONE
NEW YORK: THE MACMILLAN COMPANY
CAMBRIDGE, ENGLAND: AT THE UNIVERSITY PRESS
1931
3 'j>^t>r^<?>^t>r^)(r>^t>f7)(j>^t>r^cp^<jt>^ ^
/
PRINTED IN GREAT BRITAIN
JOSEPHO NEEDHAM
in Univ. Aberdon. Anat. olim Professori
FREDERIGO GULIELMO SANDERSON
Schol. Undel. olim Praeposito
HUGONI KERR ANDERSON
Coll. Gonv. et Caii olim Custodi
FREDERIGO GOWLAND HOPKINS
Artis chemicae ad animantia spectantis in
Univ. Cantabrig. Professori
et
DOROTHEAE MOYLE
hanc suam disquisitionem
auctor
sacram voluit
For more, and abler operations are required to the Fabrick
and erection of Living creatures, than to their dissolution,
and plucking of them down : For those things that easily
and nimbly perish, are slow and difficult in their rise and
complement.
William Harvey, Anatomical Exercitations concerning
the generation of living creatures, London, 1653,
Ex. XLi, p. 206.
That discouraging Maxime, Nil dictum quod non
dictum prius, hath little room in my estimation, nor can
I tye up my belief to the Letter of Solomon ; I do not think,
that all Science is Tautology; these last Ages have shown
us, what Antiquity never saw; no, riot in a dream.
Joseph Glanville, Scepsis Scientifica, an essay of the
vanity of dogmatizing, and confident opinion, London,
1 66 1, Chap. XXII.
CONTENTS
VOLUW
Prolegomena page 2
PART I
The Theory of Chemical Embryology
Philosophy, Embryology, and Chemistry 7
The Historical Perspective 10
Obstacles to Chemical Embryology 1 3
The Stumbling-block of Hormism 1 4
Finalism as a Rock of Offence 1 6
Organicism as an Occasion of Falling 25
Organicism and Emergence 3^
Neo-Mechanism as a Theory for Chemical Embryology 32
PART II
The Origins of Chemical Embryology
Preliminary Note 41
Section i . Embryology in Antiquity 44
I • I . Non-Hellenic Antiquity 44
1-2. Hellenic Antiquity; the Pre-Socratics 50
1-3. Hippocrates; the Beginning of Observation 53
1-4. Aristotle ^g
1-5. The Hellenistic Age 77
1-6. Galen 85
Section 2. Embryology from Galen to the Renaissance gi
2-1. Patristic, Talmudic, and Arabian Writers gi
2-2. St Hildegard; the Lowest Depth g^
23. Albertus Magnus gy
2-4. The Scholastic Period 103
2-5. Leonardo da Vinci 1 07
2-6. The Sixteenth Century; the Macro-iconographers I lO
NE h
'> i
/ i\ '\ ^ ,
VUl
CONTENTS
Section 3. Embryology in the Seventeenth and Eighteenth Centuries page 125
3-1. The Opening Years of the Seventeenth Century 1 25
32. Kenelm Digby and Nathaniel Highmore 1 29
3-3. Thomas Browne and the Beginning of Chemical Embryology 135
3-4. William Harvey 138
35, Gassendi and Descartes; Atomistic Embryology 1 56
36. Walter Needham and Robert Boyle 1 60
3-7. Marcello Malpighi; Micro-iconography and Preformationism 166
3-8. Robert Boyle and John Mayow 169
3-9. The Theories of Foetal Nutrition 176
3-10. Boerhaave, Hamberger, Mazin 1 82
3-11. Albrecht v. Haller and his Contemporaries 1 88
3-12. Ovism and Animalculism 1 99
3- 1 3. Preformation and Epigenesis 205
3-14. The Close of the Eighteenth Century 215
3-15. The Beginning of the Nineteenth Century 220
PART III
General Chemical Embryology
Preliminary Note
Section i. The UnfertiUsed Egg as a Physico-chemical System
1 . Introduction
2. General Characteristics of the Avian Egg
3. The Proportion of Parts in the Avian Egg
4. Chemical Constitution of the Avian Egg as a Whole
5. The Shell of the Avian Egg
6. The Avian Egg-white
7. The Avian Yolk
8. The Avian Yolk-proteins
9. The Fat and Carbohydrate of the Avian Yolk
10. The Ash of the Avian Egg
1 1 . General Characteristics of non-Avian Eggs
12. Egg-shells and Egg-membranes
13. Proteins and other Nitrogenous Compounds
14. Fats, Lipoids, and Sterols
15. Carbohydrates
16. Ash
231
232
232
232
236
242
255
265
280
287
294
302
306
321
331
346
355
357
CONTENTS
IX
Section 2. On Increase in Size and Weight
page 368
368
2-1. Introduction
2-2. The Existing Data 369
2-3. The General Nature of Embryonic Growth 383
2-4. The Empirical Formulae 389
2-5. Percentage Growth-rate and the Mitotic Index 399
2-6. Yolk-absorption Rate 405
2 '7. The Autocatakinetic Formulae 408
2-8. Instantaneous Percentage Growth-rate 420
2-9. Growth Constants 434
2-10. The Growth of Parts 440
2-1 1. Variability and Correlation 455
2-12. Explantation and the Growth-promoting Factor 460
2-13. Incubation Time and Gestation Time 470
2-14. The Effect of Heat on Embryonic Growth 498
2-15. Temperature Coefficients 503
2 • 1 6. Temperature Characteristics 515
2-17. The Effect of Light on Embryonic Growth 533
2-i8. The Effect of X-rays and Electricity on Embryonic Growth 536
2-19. The Effect of Hormones on Embryonic Growth 538
Section 3.
3-1
3-2
33
3-4
3-5
3-6
37
3-8
3-9
On Increase in Complexity and Organisation
The Independence of Growth and Differentiation
Differentiation-rate
Chemical Processes and Organic Form
The Types of Morphogenetic Action
Pluripotence and Totipotence
Self-differentiation and Organiser Phenomena
Functional Differentiation
Axial Gradients
Organised and Unorganised Growth
3-10. Chemical Embryology and Genetics
541
541
544
552
559
567
570
580
582
606
608
VOLUM]
Section 4. The Respiration and Heat-production of the Embryo 615
4- 1 . Early Work on Embryonic Respiration 6 1 5
4-2. Respiration of Echinoderm Embryos in General 623
4-3. Rhythms in Respiratory Exchange 64 1
6-2
X CONTENTS
Section 4-4. Heat Production and Calorific Quotients of Echinoderm page 649
Embryos
4-5. Respiration of Annelid, Nematode, Rotifer, and Mollusc 659
Embryos
46. Respiration of Fish Embryos 665
47. , Respiration of Amphibian Embryos 67 1
4-8. Heat-production of Amphibian Embryos 682
49. Respiration of Insect Embryos 687
4- 1 o. Respiration of Reptile Embryos 692
4-11. Respiration of Avian Embryos in General 693
4-12. Heat-production of Avian Embryos 7^4
4-13. Later Work on the Chick's Respiratory Exchange 7^8
4- 1 4. The Air-space and the Shell 7 1 9
4-15. Respiration of Mammalian Embryos 726
4" 1 6. Heat-production of Mammalian Embryos 732
4-17. Anaerobiosis in Embryonic Life 742
^, 4" 1 8. Metabolic Rate in Embryonic Life 746
4-19. Respiratory Intensity of Embryonic Cells fn wVro 755
4-20. Embryonic Tissue-respiration and Glycolysis 758
4-21. The Genesis of Heat Regulation 772
4-22. Light-production in Embryonic Life 776
Section 5. Biophysical Phenomena in Ontogenesis 777
5-1. The Osmotic Pressure of Amphibian Eggs 777
5-2. The Genesis of Volume Regulation , 786
53. The Osmotic Pressure of Aquatic Arthropod Eggs 790
54. The Osmotic Pressure of Fish Eggs 793
5-5. Osmotic Pressure and Electrical Conductivity in Worm and 799
Echinoderm Eggs
5-6. The Osmotic Pressure of Terrestrial Eggs 8l2
5-7. Specific Gravity 820
5-8. Potential Differences, Electrical Resistance, Blaze Currents 825
and Cataphoresis
5-9. Refractive Index, Surface Tension and Viscosity 833
Section 6. General Metabolism of the Embryo 839
6-1. The j&H of Aquatic Eggs 839
6-2. The j&H of Terrestrial Eggs 855
63. rH in Embryonic Life 865
6-4. Water-metabolism of the Avian Egg 870
CONTENTS xi
Section 6-5. Water-content and Growth-rate page 883
6-6. Water-absorption and the Evolution of the Terrestrial Egg 889
6-7. Water-metabolism in Aquatic Eggs 906
6-8. The Chemical Constitution of the Embryonic Body in Birds 911
and Mammals
69. Absorption-mechanisms and Absorption-intensity 917
6- 10. Storage and Combustion; the Plastic Efficiency Coefficient 934
6-1 1. Metabolism of the Avian Spare Yolk 939
6-12. Maternal Diet and Embryonic Constitution 943
Section 7. The Energetics and Energy-sources of Embryonic Development 946
7-1. The Energy Lost from the Egg during Development 946
7-2. Energy of Growth and Energy of Differentiation 956
7-3. The Relation between Energy Lost and Energy Stored 962
7-4. Real Energetic Efficiency • 969
7-5. Apparent Energetic Efficiency 972
7*6. Synthetic Energetic Efficiency 98 1
7-7. The Sources of the Energy Lost from the Egg 986
Sections. Carbohydrate Metabolism 1000
8-1. General Observations on the Avian Egg lOOO
8-2. Total Carbohydrate, Free Glucose, and Glycogen lOOl
8-3. Ovomucoid and Combined Glucose 1 007
8-4. Carbohydrate and Fat 1014
8-5. The Metabolism of Glycogen and the Transitory Liver 1018
8-6. Free Glucose, Glycogen, and Insulin in the Embryonic Body IO29
8-7. General Scheme of Carbohydrate Metabolism in the Avian Egg 1035
8-8. Embryonic Tissue Glycogen 1 036
8'9. Embryonic Blood Sugar 1039
8- 10. Carbohydrate Metabolism in Amphibian Development 1043
8*i I. Carbohydrate Metabolism of Invertebrate Eggs I047
8-12. Pentoses 1 05 1
8-13. Lactic Acid 1 051
8-14. Fructose 1054
Section 9. Protein Metabolism 1055
9* I. The Structure of the Avian Egg-proteins before and after 1055
Development
9*2. Metabolism of the Individual Amino-Acids I059
9-3. The Relations between Protein and non-Protein Nitrogen 1065
9-4. The Accumulation of Nitrogenous Waste Products 10 76
xii CONTENTS
Section 9-5. Protein Catabolism page
9-6. Nitrogen-excretion; Mesonephros, Allantois, and Amnios
9-7. The Origin of Protective Syntheses
9*8. Protein Metabolism of Reptilian Eggs
9-9. Protein Metabolism of Amphibian Eggs
9' 10. Protein Metabolism in Teleostean Ontogeny
9-11. Protein Metabolism in Selachian Ontogeny
9* 1 2. Protein Metabolism of Insect, Worm, and Echinoderm Eggs
9-13. Protein Utilisation in Mammalian Embryonic Life
9-14. Protein Utilisation of Explanted Embryonic Cells
9-15. Uricotelic Metabolism and the Evolution of the Terrestrial Egg
Section 10. The Metabolism of Nucleins and Nitrogenous Extractives
[O-i. Nuclein Metabolism of the Chick Embryo
IO-2. The Nucleoplasmatic Ratio
[0-3. Nuclein Synthesis in Developing Eggs
[0-4. Creatinine, Creatine, and Guanidine
Section 11. Fat Metabolism
I • I . Fat Metabolism of Avian Eggs
1-2. Fat Metabolism of Reptilian Eggs
I •2- Fat Metabolism of Amphibian Eggs
I -4. Fat Metabolism of Selachian Eggs
1-5. Fat Metabolism of Teleostean Eggs
1-6. Fat Metabolism of Mollusc, Worm, and Echinoderm Eggs
1-7. Fat Metabolism of Insect Eggs
1-8. Combustion and Synthesis of Fatty Acids in Relation to
Metabolic Water
1 1 -9. Fat Metabolism of Mammalian Embryos
Section 12. The Metabolism ofLipoids, Sterols, Cycloses, Phosphorus
and Sulphur
1 2- 1. Phosphorus Metabolism of the Avian Egg
12-2. Tissue Phosphorus Coefficients
I2'3. Choline in Avian Development
12-4, The Metabolism of Sterols during Avian Development
1 2*5. The Relation between Lipoids and Sterols; the Lipocytic
Coefficient
12-6. Cycloses and Alcohols in Avian Development
12-7. Sulphur Metabolism of the Avian Egg
12-8. Phosphorus, Sulphur, Choline, and Cholesterol in Reptile Eggs
CONTENTS
Section 12-9. Lipoids and Sterols in Amphibian Eggs page 1237
12-10. Lipoids, Sterols, and Cycloses in Fish Eggs 1239
i2'ii. Phosphorus, Lipoids and Sterols in Arthropod Eggs 1241
12-12. Phosphorus, Lipoids, and Sterols in Worm and Echinoderm 1243
Eggs
12-13. Lipoids and Sterols in Mammalian Development 1252
Section 13. Inorganic Metabolism 1255
[ 3- 1 . Changes in the Distribution of Ash during Avian Development 1 255
32. Calcium Metabolism of the Avian Egg 1260
33. Inorganic Metabolism of other Eggs 1 268
3-4. The Absorption of Ash from Sea-water by Marine Eggs 1 27 1
3-5. The Ani on/Cation Ratio 1 2 74
3-6. Inorganic Metabolism of Mammalian Embryos 1277
3-7. Calcium Metabolism of Mammalian Embryos 1285
Section 14. Enzymes in Ontogenesis 1289
4-1. Introduction 1 289
4-2. Enzymes in Arthropod Eggs 1290
4-3. Enzymes in Mollusc, Worm, and Echinoderm Eggs 1293
4-4. Enzymes in Fish Eggs 1 295
4-5. Enzymes in Amphibian Eggs 1300
4-6. Enzymes in Sauropsid Eggs 1303
4-7. Changes in Enzymic Activity during Development 1307
4-8. Enzymes of the Embryonic Body 1 3^0
4-9. Enzymes in Mammalian Embryos 13 12
4-10. The Genesis of Nucleases 1326
4-11. Foetal Autolysis 1 329
Section 15. Hormones in Ontogenesis 1335
5-1. Introduction 1335
5-2. Adrenalin ^337
53. Insulin 1342
5-4. The Parathyroid Hormone 134^
5-5. The Hormones of the Pituitary 134^
5-6. Secretin 134^
5-7. Thyroxin 134^
5-8. Oestrin and other Sex Hormones • 1353
VOLUM
xiv CONTENTS
Section i6. Vitamins in Ontogenesis page 1359
[6-1. Vitamin A 1359
[6-2. Vitamin B 1360
[6-3. Vitamin C 1 360
[6-4. Vitamin D 1 360
[6-5. Vitamins in Mammalian Development 1 363
[6-6. Vitamin E 1 365
Section 17. Pigments in Ontogenesis 1368
[7-1. The Formation of Blood Pigments 1 368
[7-2. The Formation of Bile Pigments 137^
[7-3. The Formation of Tissue Pigments 1 375
[7-4. The Pigments of the Avian Egg-shell 137^
[7-5. The Pigments of the Avian Yolk 1378
[7-6. Egg-pigments of Aquatic Animals 1380/
[7-7. Melanins in Ontogenesis 13^^
Section 18. Resistance and Susceptibility in Embryonic Life 1383
•I. Introduction 1 3^3
•2. Standard Mortality Curves 1 3^3
[8-3. Resistance to Mechanical Injury ^3^5
$-4. Resistance to Thermal Injury 1 388
5-5. Resistance to Electrical Injury ^392
[8-6. Resistance to Injury caused by Abnormal j&H 1 397
5-7. Resistance to Injury caused by Abnormal Gas Concentrations 1 399
(non-Avian Embryos)
!-8. Critical Points in Development 1 409
!-g. Resistance to Injury caused by Abnormal Gas Concentrations 1 4 1 4
(Avian Embryos)
>-io. Resistance to Injury caused by Toxic Substances 1420
••I I. Resistance to Injury caused by X-rays, Radium Emanation, 1 43 1
and Ultra-violet Light
Section 19. Serology and Immunology in Embryonic Life 1444
ig-i. Antigenic Properties of Eggs and Embryos ^444
19-2. The Formation of Natural Antibodies 1446
19-3. The Natural Immunity of Egg-white ^447
19-4. Inheritance of Immunity in Oviparous Animals HS^
19-5. Serology and Pregnancy 1452
19-6. Resistance of the Avian Embryo to Foreign Neoplasms 1 454
CONTENTS
XV
Section 20. Biochemistry of the Placenta page 14.^6
20-1. Introduction 1 45"
20-2. General Metabolism of the Placenta 145^
20-3. Placental Respiration 1 46 1
20-4. Nitrogen Metabolism of the Placenta 1 462
20-5. Carbohydrate Metabolism of the Placenta 14^9
20-6. Fat and Lipoid Metabolism of the Placenta 1472
20-7. Placental Enzymes 1481
Section 21. Biochemistry of the Placental Barrier 1485
2 1 • I . The Autonomy of the Foetal Blood 1 4^5
21-2. Evolution of the Placenta -4^7
21-3. Histotrophe and Haemotrophe 149^
21-4. Mesonephros and Placenta 1 493
21-5. Colostrum and Placenta ^497
21-6. Placental Transmission and Molecular Size 1497
21-7. QuaHtative Experiments on Placental Permeability 1 505
21-8. The Passage of Hormones 15^^
2 1 -9. Factors Governing Placental Transmission 15^2
2I-IO. Quantitative Experiments on the Passage of Nitrogenous 15 14
Substances
2 1 -I I. Quantitative Experiments on the Passage of Phosphorus, Fats, 1520
and Sterols
2i'i2. Quantitative Experiments on the Passage of Carbohydrates 1525
2i*i3. Quantitative Experiments on the Passage of Ash 1 52 7
21-14. The Passage of Enzymes 15^9
2i*i5. The Unequal Balance of Blood Constituents 1530
Section 22. Biochemistry of the Amniotic and Allantoic Liquids 1534
22-1. Introduction 1 534
22-2. Evolution of the Liquids ^535
22-3. Avian Amniotic and Allantoic Liquids 1 537
22-4. Amount and Composition of Mammalian Amniotic and Allan- 1539
toic Liquids
22-5. Maternal Transudation and Foetal Secretion 154^
22-6. Interchange between Amniotic and Allantoic Liquids 15^2
22-7. Vernix Caseosa 1 5^4
Section 23. Blood and Tissue Chemistry of the Embryo 1565
23-1. Blood 1565
23-2. Lung 1 57 1
23-3. Muscle 1574
xvi
CONTENTS
Section 23-4.
Heart
23'5-
Nervous Tissue
23-6.
Connective Tissue
237.
Lymph
23-8.
Sense Organs
23-9-
Intestinal Tract
Section 24.
Hatching and Birth
24-1.
Introduction
24-2.
Hatching Enzymes
24-3.
Osmotic Hatching
24-4.
Egg-breakers
24-5-
Hatching of the Avian Egg
24-6.
MammaUan Birth
page 1577
1583
1592
1593
1594
1594
1595
1595
1595
1600
1602
1602
1605
Epilegomena
The Two Problems of Embryology 1 6 1 3
The Cleidoic Egg and its Evolution 16 1 3
Chemical Synthesis as an Aspect of Ontogeny 1 623
Biochemistry and Morphogenesis 1 624
Transitory Functions in Embryonic Life 1627
The Theory of Recapitulation 1629
Recapitulation and Substitution ' 1632
Chemical Recapitulation 1638
Provisional Generalisations for Chemical Embryology 1 647
The Organisation of Development and the Development of Organisation 1659
The Future of Embryology 1 664
PART IV
Appendices
i. Normal Tables of Magnitudes in Embryonic Growth 1669
ii. A Chemical Account of the Maturation of the Egg-cell 1679
iii. The Chemical Changes during the Metamorphosis of Insects (by 1685
Dorothy Needham)
iv. The Development of the Plant Embryo from a Physico-chemical View- 1 7 1 1
point (by Muriel Robinson)
PART V
Bibliography and Author-Index
Subject-Index
Index Animalium
1725
1971
2013
PLATES
VOLUME I
William Harvey frontispiece
I. Primitive methods of incubation : (A) Egyptian, (B) Chinese facing page 46
II. The oldest known drawing of the Uterus (gth century) . „ „ 82
III. Illustration from the Liber Scivias of St Hildegard (ca.
1150A.D.) jj J3 96
IV. A page from Leonardo da Vinci's Anatomical Notebooks
(ca. 1490 A.D.) ........„„ 108
V. Illustration from the De Formatione Ovi et Pulli of Fabricius
(1604) „ „ 116
VI. Illustration {rom Highmore's History of Generation {16^1) . „ „ 134
VII. Illustrations from Malpighi: i)e Or;o in^M^a/o (1672) . ,, „ 168
VIII. Reaumur's Illustration of his Incubators (1749) . . „ „ 198
IX. Microphotograph of the yolk of the hen's egg at the time
of laying, to show the vitelline globules . . . . ,, ,, 236
X. Microphotograph of the yolk of the hen's egg, not yet
liberated from the ovary, to show the stratification . . „ ,, 288
VOLUME II
The frontispiece of William Harvey's Generation of Ani-
mals ( 1 65 1 ) ; Zeus liberating living beings from an egg . frontispiece
XI. Microphotograph of the yolk of the hen's egg at the
eleventh day of incubation, showing its heterogeneous
state .......... facing page 836
XII. Microphotograph of the yolk of the hen's egg at the second
day of incubation, showing the cholesterol esters . . „ ,,1218
VOLUME III
An embryological investigation in the eighteenth century frontispiece
TABLES
27. Ash of the avian egg ....
34. Distribution of amino-acids in egg-proteins
47. Ash content of egg .
195. Enzymes in the hen's egg
199. Enzymes in the human embryo
201. Enzymes in the pig embryo
220. Placental enzymes .
227, Passage of substances through the placenta
Appendix 1, Table 3. Embryonic growth of the hen
facing page
302
»5 55
330
.J 55
356
55 55
1304
55 55
I3I4
55 55
I316
55 55
1469
55 5'
1506
55 5?
1670
ACKNOWLEDGEMENTS
OF
INDEBTEDNESS
THOSE who have assisted me in the preparation of this work are so
numerous that it is impossible to mention them all by name. Its
original impetus was derived from a discussion with Professor Sir
F. G. Hopkins in 1923 on the observation of Klein that inositol, though
absent from the undeveloped hen's egg, was present in considerable
quantity at hatching; and throughout the period of preparation his
encouragement, help, and advice were never-failing. I have derived great
benefit from the discussion of various points with Miss Marjory Stephenson,
M. Louis Rapkine, Dr R. A. Fisher, and my wife. Professor J. T. Wilson
has been repeatedly helpful to me on anatomical points, and in the
Zoological Laboratory I was always sure of obtaining expert advice from
Mr James Gray, Mr J. T. Saunders, Mr C. F. A. Pantin and Dr Eastham.
I have relied much upon the kindness and wide biological knowledge of
Dr D. Keilin and Dr F. H. A. Marshall. As regards the historical chapters,
I am most grateful to Dr Charles Singer, who annotated them with
valuable comments, and to Professor R. C. Punnett who placed un-
reservedly at my disposal his knowledge of the history of generation, and
his library of old and rare biological books. To Dr Arthur Peck I am
indebted for the correction of my Greek, and it was Professor A. B. Cook
who guided me to the embryology of the ancients. Without the assiduous
backing of Mr Powell, the Librarian of the Royal Society of Medicine, and
his assistants, I should have dealt much more inadequately than I have
with the papers which cannot be consulted in Cambridge. I have also to
thank the administrators of the Thruston Fund of Gonville and Caius
College for a grant which was devoted to incidental expenses. For the
indexes I wish to thank Miss Helen Moyle, and for other services which
have made the book possible, Mrs V. Townsend. My thanks are also
due to the Editors of the following journals: Biochemical Journal, Journal
XX ACKNOWLEDGEMENTS
of Experimental Biology, Biological Reviews, Science Progress, and the Monist,
for permission to reprint passages from papers. I must record my gratitude
to the following friends, who very kindly read through and criticised the
proofs of the various sections:
Part I
Professor A. E. Boycott
Dr J. H. Woodger
Part II
Professor R. C. Punnett
Dr Charles Singer
Dr Reuben Levy
Dr Arthur Peck
Sir William Dampier
Professor A. B. Cook
The Rev. W. Elmslie
Professor F. M. Cornford
Part III
Section
1 Professor R. H. A. Plimmer
Mr J. B. S. Haldane
2 Dr Samuel Brody
Mr James Gray
Dr E. N. Willmer
3 Mr G. R. de Beer
Mr C. H. Waddington
Mr J. B. S. Haldane
4 Dr D. Keilin
Professor Munro Fox
5 Mr T. R. Parsons
Dr Malcolm Dixon
6 M. Louis Rapkine
Mr C. Forster Cooper
7 Miss Marjory Stephenson
M. Louis Rapkine
Dr D. Keilin
8 Dr Eric Holmes
Dr Bruce Anderson & Mrs Margaret
Whetham Anderson
OF INDEBTEDNESS
Section
9
Dr Dorothy Jordan Lloyd
Professor J. Murray Luck
Mr C. Forster Cooper
lO
Mile Eliane LeBreton
II
Professor J. B. Leathes
12
Dr Irvine Page
13
Dr Elsie Watchorn
14
Dr Barnet Woolf
MrJ. B. S. Haldane
15
Dr Howard Florey
16
Dr Leslie J. Harris
Dr A. L. Bacharach
18
Dr Howard Whittle
19
Mr C. F. A. Pantin
Professor A. R. Moore & Mrs Moore
20
Dr John Hammond
21
Dr St G. Huggett
22
Dr Arthur Walton
23
Dr Barbara Holmes
24
Dr F. H. A. Marshall
Epilegomena
Professor L. G. M. B. Becking
Dr D. Keilin
Dr G. S. Carter
Professor Lancelot Hogben
Mr G. R. de Beer
Professor A. R. Moore & Mrs Moore
Appendix III
Dr L. E. S. Eastham
I am indebted to the Master of Gonville and Caius College for permission
to reproduce the portrait of William Harvey (attributed to Rembrandt)
in the Senior Combination Room. Finally, I am glad to record here my
gratitude to the StafTof the Cambridge University Press for the unremitting
care which they gave to my book during the course of its preparation.
J. N.
Note: The use of the shortened
and (&) indicates collaboration
between two or more authors.
PROLEGOMENA
The Sciences, unlike the Graces or the Eumenides, are not limited
in number. Once born, they are immortal, but, as knowledge in-
creases, they are ever multiplying, and so great is now the dominion
of the scientific mind that every few years sees a new one brought
into the world. Some spring, fully armed, from the brains of one or
two men of genius, but most of them, perhaps, come only gradually
to their full development through the labours of very many obscure
and accurate observers.
If the analogy may be permitted, physico-chemical embryology has
so far been living an intra-uterine existence. Its facts have been
buried in a wide range of scientific journals, and its theories have
lain dormant or in potentia in reviews of modest scope. Physico-
chemical embryology has, indeed, arrived at the stage immediately
priox to birth, and all it needs is a skilful obstetrician, for, when once
it has reached the light of day and has passed for ever out of the
foetal stage, it will be well able to take care of itself. This obstetrical
task is that which I have chosen and obviously enough it divides
into three principal heads: first, to collect together out of all the
original papers on the subject the facts which are known about the
physico-chemical basis of embryonic development; second, to relate
these facts to each other and to the facts derived from the labours
of investigators in morphological embryology and " Entwicklungs-
mechanik," and, third, to ascertain whether, from what is at present
known, any generally valid principles emerge.
I may as well say at the outset that in order to do this certain
arbitrary boundary-lines are inevitable. The following arrangement
has been adopted. Chronologically speaking, the prelude to all em-
bryonic development is the maturation of the egg-cell, but this is not
strictly embryology, and so has been relegated to an appendix. The
egg-cell as a physico-chemical system is dealt with at the opening
of Part III, and thereafter the physico-chemical aspects of develop-
ment follow in order. No mention will be made of fertilisation, for
this has been treated exhaustively by other writers (Lillie, Dalcq)
and, after all, embryology presupposes fertiUsation whether natural
or artificial. Nor in later chapters will any complete treatment be
2 PROLEGOMENA
given of the events going on in the maternal organism during preg-
nancy : for the present purpose the discussion will go as far into the
mother as the placenta but no farther. Again, hatching or birth will
put an end to the discourse as to the foetal state itself, save that, in
the cases of animals which hatch before the yolk-sac is absorbed,
their embryonic life is assumed to end when they first take food for
themselves. Appendices are added dealing with the plant embryo
and the insect pupa, which, in the later stages of metamorphosis,
have points both of resemblance to and of difference from the growth
of the embryo. It is natural to hope that the outcome of all this labour
may be an increase of interest among biologists in this section of their
domain, and a great accession to the number of those investigators
who devote their energies to actual experiments in this new field.
For it must be confessed that it is a new field. It has been opened
up in very gradual stages: fitful and sporadic experiments on the
constitution of embryonic tissues in the seventeenth century, a gradual
growth of knowledge about the chemical composition of eggs in the
eighteenth, a big increase of activity in the early nineteenth; d'lTxiug
which appear the first observations on the physico-chemical changes
taking place in the embryo during its development, and then in our
own time a mass of very widely scattered work bringing the subject
up to the "obstetrical" stage. Such a work as this, in my opinion,
should not be compared with laboratory experiments in a derogatory
sense, for, while it is true that facts are the ultimate court of appeal
in any scientific discussion, yet at the same time the number of in-
vestigators has grown to such extraordinary proportions in this century
that some danger exists lest we should be so busily engaged in accu-
mulating new facts as to be left with no time at all to devote any
thought to those we have already. Classification, indexing, and
maturer consideration about the facts we actually possess are at least
as great a need at the present moment as the invention of new facts.
"Everyone must realise", says Eugenio Rignano, "how much this
theoretical elaboration, performed by means of analyses and com-
parisons, of generalisations and hypotheses controlled and verified by
the correspondence of facts with the results of the reasoning, is useful
and necessary if one wishes to reach a progressive systematisation and
an ever more synthetic vision of the confused mass of facts which
experimentalists pour daily in a continuous stream into the scientific
market."
PROLEGOMENA 3
My predecessors in this work have been few in number. The volumes
of Haller's, Buffon's, and Milne-Edwards' great treatises, in which
they deal with the phenomena of generation, contain as much in-
formation as was available up to 1863, but this is purely of historical
interest to us. In 1885, W. Preyer, Professor of Physiology at Jena,
published his Spezielle Physiologie des Embryo, which still remains a most
valuable review, and indeed, even to-day, is the only existing book
specially devoted to embryonic physiology. The present century
has produced only three books which even touch upon my subject,
namely, T. B. Robertson's Chemical Basis of Growth and Senescence,
F. H. A. Marshall's Physiology of Reproduction and E. Faure-Fremiet's
La Cinetique du Developpement. The first of these was admittedly written
to support a particular theory, and in any case says comparatively
little about physico-chemical embryology. The second and the third
deal with it only as a constituent part of a much wider field. In
Marshall's case, the whole array of facts relating to oestrus and
breeding, fertilisation and fertility, lactation and sex determination,
have to be dealt with, and only three chapters out of sixteen are
devoted to the subject of this book. The first of these is contributed
by W. Cramer, and covers the biochemistry of the sexual organs, in-
cluding the unfertiUsed egg ; the second, which deals with foetal
nutrition and the placenta, is by J. Lochhead ; and the third, by these
two investigators together, is concerned with changes in the maternal
organism during pregnancy. Admirable as these chapters are, they
are now rather out of date. Moreover, though one or two corners
of the field I have before me were covered in Marshall's book, it was
from a quite different standpoint.
Faure-Fremiet's work is exactly analogous; it deals with physico-
chemical embryology only, as it were, in passing. The relevant dis-
cussion takes up only two chapters out of seven ; the rest are occupied
with tissue culture, growth of protozoal populations, and general
cytology. His book covers, it might be said, the third and fourth
corners : all the main expanse of the field remains.
Thus neither of these books deals with physico-chemical embryo-
logy in an exhaustive and comprehensive fashion, treating it as, in
my view, it ought to be treated, with the thoroughness which is
deserved by a new branch of natural knowledge. Inseparable, how-
ever, from thoroughness of treatment is the submergence of the parts
of more general interest under a mass of detail, and it may be well.
4 PROLEGOMENA
therefore, to mention now what sections of the book could be said
to be most valuable to any student of general biology. Part i comes
in this class, and of Part iii, the middle portion of Section i, all of
Sections 2, 3, and 5, thelatter half of Section 7, Sections 8, 9 (especially
the end), 11, possibly 18, and finally the Epilegomena.
For my models in the preparation of this book, if it is permissible
to name them, I have taken, Growth and Form by d'Arcy Thompson,
surely the most scholarly work produced by a biologist in our time,
and The Physiology of Reproduction by F. H. A. Marshall, already
mentioned, which showed to all successors, in my opinion, how a
colossal array of facts can be welded together into an absorbing and
readable book, I am conscious that I shall not attain the level of
these classics of modern biology, but then
.... Pauci, quos aequus amavit
Jupiter, aut ardens evexit ad aethera virtus.
The progress of any branch of natural knowledge can be best
described as a continual pilgrimage towards the quantitative. QuaUties
can never be altogether left out of account and this is what makes
it impossible for science to achieve its end with absolute finality. Yet
an association with the probably unattainable is common to all the
great types of man's activity. But "Fuyez toujours les a peu pres",
as O. W. Holmes used to put it, is a proper maxim for the scientific
mind, and whatever this book can do towards making embryology
an exact science will be its final justification.
PART I
THE THEORY
OF
CHEMICAL EMBRYOLOGY
. . . .to measure all things that can be measured, and to
make measurable what cannot yet be measured.
Galileo.
THE THEORY OF ^^^-^S^aj^,
CHEMICAL EMBRYOLOGY ^^' 0 ^'"
Philosophy, Embryology, and Chemistry
The penetration of physico-chemical concepts into embryology
has not been entirely peaceful. "In experimental embryology", it
has been said, "concepts borrowed from the physical sciences do not
admit of calculations being made, and until they do they are not
really playing the same role as they do in the sciences from which
they have been borrowed and for which they were devised." "Nothing
is more clear", says another writer, "in chemistry and physics than
that identical results follow upon identical causes. Introduce a dis-
turbing element, even a small one, into your experiment, and the
experiment will fail. Such is not the case with the developing egg."
W. McDougall, too, endows the egg with good intentions. "The
embryo", he says, "seems to be resolved to acquire a certain form
and structure, and to be capable of overcoming very great obstacles
placed in its path. The development of the forms of organisms seems
to be utterly refractory to explanation by mechanical or physico-
chemical principles." Finally, J. A. Thomson goes farther than them
all, and does not hesitate to say, "It is a mere impious opinion that
development will one day be described in terms of mechanics".
Chapter iv of his Gifford Lectures illustrates the antagonistic attitude
to physico-chemical embryology in its most acute form.
It can hardly be a coincidence that so many among the great
embryologists of the past were men of strongly philosophic minds.
It would be absurd to support this opinion by citing Aristotle, but
it holds less obviously true of William Harvey, whose book on genera-
tion is full of thoughts about causation, and in the cases of Ernst
von Baer, Ernst Haeckel, Wilhelm Roux, Hans Driesch, dArcy
Thompson and J. W. Jenkinson, there is no doubt about it. It is not
really surprising, for of all the strange things in biology surely the
most striking of all is the transmutation inside the developing egg,
when in three weeks the white and the yolk give place to che animal
with its tissues and organs, its batteries of enzymes and its dehcately
regulated endocrine system. This coming-to-be can hardly have failed
8 THE THEORY OF [pt.
to lead, in the minds of those most intimately acquainted with it, to
thoughts of a metaphysical character. Nor, it seemed, did those who
worked on it do much to diminish its wonder. "Neither the schools
of physicians", as Harvey said, "nor Aristotle's discerning brain,
have disclosed the manner how the Cock and its seed, doth mint and
coine, the chicken out of the Ggg,'^ Or, in the words of Erycius
Puteanus, "I will neglect gold, and will praise what is more precious
than any metal, I will despise feasts, and will set forth praises of
something better than any food or drink. If you would know of what
it is that I intend to speak, it is the egg; men marvel at the sun, at
meteors flung from heaven, at stars swimming therein, but this is
the greatest of all wonders". Here, however, there is one significant
thing. It is that the very chapter of Harvey's book in which the
preceding remark is found has as its heading "The Efficient Cause
of the Chicken, is hard to be found out". It certainly was, but the
right clue was in the heading to that exercitation.
This close association of embryology with philosophy, then, made
it necessary to discuss at the outset of this book certain points in the
more theoretical regions of biology, and, as it were, to defend from
a theoretical angle the extension of the domain of physics and
chemistry over embryology. I might have entitled this part of the
book "The philosophy of embryology", but, in deference to those
metaphysicians who rightly insist that the word philosophy should
only be used of a definite system of experience which looks at the
universe as a corporate whole, I adopted the present heading. Under
it I propose to discuss the exact status of the chemical aspect of
embryology. For many biologists, having perhaps insufficiently con-
sidered the nature of the scientific method, think it likely that
the discoveries of modern times may allow of some other basis for
biology than mathematical physics and that the scientific niethod
may rightly be different in biology from what it is in chemistry. It
is this factor in our present intellectual climate which makes it neces-
sary to preface by a philosophical discussion a book in which the
concepts of physics and chemistry are extended to a field of biology
where they have never before received more than a conventional and
formal reverence.
The aim of all studies in physico-chemical embryology must be
that expressed by T. H. Huxley when he said, " Zoological Physiology
is the doctrine of the functions or actions of animals. It regards
I] CHEMICAL EMBRYOLOGY 9
animal bodies as machines impelled by certain forces and perform-
ing an amount of work which can be measured and expressed
in terms of the ordinary forces of nature. The final object of
physiology is to deduce the facts of morphology on the one hand
and those of oecology on the other hand from the laws of the mole-
cular forces of matter". It may be regarded as very noteworthy
that Huxley here puts morphology as secondary to physiology and
as it were derivable from it; he does not place morphology and
physiology on two high places, "neither afore or after other", as
has so often been done, but he plainly states his view that the
anatomical aspect of animals, their external and internal forms, could
be deduced from the interplay of physico-chemical forces within
them, if we only knew enough about those forces. This is the idea
of the primacy of function. It seems always to have two meanings,
firstly, the Epicurean-Lucretian one which Huxley adopts here and
Roux so brilliantly developed, in which shape is regarded as the
outward and visible sign of the properties of matter itself, and,
secondly, the Aristotelian one emphasised by J. B. de Lamarck's
writings in the eighteenth century, and in our time by E. S. Russell's
great work Form and Function, in which psychical factors are intro-
duced as the essential elements in the ultimate analysis of shape. In
both these interpretations, function has the priority over form, but
the meaning of function is the point of difference. Some biologists,
however, seem to think that physiology and morphology are cate-
gorical, and the latter is emphatically not reducible to or derivable
from the former. The two spheres of study represent, for them,
correlative and immiscible disciplines, morphology aiming ultimately
at solid geometry, physiology at causation, and "rerum cognoscere
causas" is not the basic desire of the scientific mind. They object
to the view which regards "the ovum as a kind of chemical device
wound up and ready to go off on receipt of a stimulus, the task of
the causal morphologist being to disentangle the complex of events
which constitute the unwinding process" (Woodger), complaining
that in this view no account is taken of the past history of the
race, which is left to genetics, again a causal discipHne. To some
extent these opinions spring from a conviction that the analytical
method is inapplicable to a living being because it is an organism, and
of that there is more to be said. But they also arise from a profound
unwillingness to subsume biology under physics and a desire to uphold
10 THE THEORY OF [pt.
"the autonomy of biology". This precludes the promise of an ever-
increasing homogeneity in the structure of science, and hence an
ever-increasing simplicity.
The Historical Perspective
That the older embryologists awaited the extension of physico-
chemical conceptions to embryology is no mere matter of conjecture.
Until the mechanical theory of the universe had been consolidated
by the " corpuscularian philosophy" of the seventeenth century it
would be useless to look for illustration of this, but by 1674 John
Mayow was tracing the part played by the " nitro-aerial particles"
in the development of the embryo, and in 1732 Hermann Boerhaave
was discussing chemical problems with explicit reference to embryonic
development. Many other examples of this point of view in the
eighteenth century will be given later. Then, when the second decade
of the nineteenth century had nearly gone, von Baer, perhaps the
greatest of all embryologists, was careful to preface his Entwicklungs-
geschichte by a careful account of all that was known about the
chemical constitution of the Qgg, and that, although his philosophical
inclinations were deeply vitalistic, and even his practical interests
morphological. In Roux, of course, this future reference came out
explicitly, and the extension of biochemistry into embryology was
allowed for and foreseen. An early instance was the association be-
tween Wilhelm His and Hans Miescher. Miescher, writing to Hoppe-
Seyler in 1872 said, "I am now collecting material from fishes,
birds, and amphibia to lead to a chemical statics of development.
With this end in view I shall do analyses of ash, nuclein, and lecithin".
Embryology before Harvey, however, was rigidly Aristotelian, a
statement the meaning of which George Santayana has lucidly ex-
plained. "Aristotle", said he, "distinguished four principles in the
understanding of Nature. The ignorant think that these are all,
equally, forces producing change, and the cooperative sources of all
natural things. Thus, if a chicken is hatched, they say that the Efficient
Cause is the warmth of the brooding hen, yet this heat would not
have hatched a chicken out of a stone, so that a second condition,
which they call the Material Cause, must be invoked as well, namely,
the nature of an egg; the essence of eggness being precisely a capacity
to be hatched when warmed gently — because, as they wisely observe,
boiling would drive away all potentiality of hatching. Yet, as they
I] CHEMICAL EMBRYOLOGY ii
further remark, gentle heat-in-general joined with the essence-of-
eggness would produce only hatching-as-such and not the hatching
of a chicken, so that a third influence, which they call the Final
Cause, or the End-in-view, must operate as well, and this guiding
influence is the divine idea of a perfect cock or a perfect hen presiding
over the incubation and causing the mere eggness in the egg to
assume the likeness of the animals from which it came. Nor, finally,
do they find that these three influences are sufficient to produce here
and now this particular chicken, but are compelled to add a fourth,
a Formal Cause, namely, a particular yolk, a particular shell, and
a particular farmyard, on which and in which the other three causes
may work, and laboriously hatch an individual chicken, probably
lame and ridiculous despite so many sponsors." The Aristotelian
account of causation could not be better expressed. Santayana puts
this description of it into the mouth of Avicenna in his imaginary
dialogue, and makes him go on to say, "Thus these learned babblers
would put nature together out of words, and would regard the four
principles of interpretation as forces mutually supplementary com-
bining to produce material things ; as if perfection could be one of
the sources of imperfection or as if the form which things happen
to have could be one of the causes of their having it. Far differently
do these four principles clarify the world when discretion conceives
them as four rays shed by the light of an observing spirit". In this
last observation we may perhaps trace the germ of the Copernican
revolution in philosophy effected by Kant, if we may take it to enclose
the idea of the activity of the experient subject in all perception.
In science generally, however, the x\ristotelian conceptions went
without serious contradiction, and thus formed the framework for all
the embryological work that was done, as, for instance, by Albertus
Magnus. Owing to its association with the idea of the plan of a
divine being, the final cause tended in the Middle Ages to eclipse
the others. In the seventeenth century this feeling is well shown in
a remarkable passage, which occurs in the Religio Medici of Sir Thomas
Browne: "There is but one first cause, and four second causes of all
things; some are without Efficient, as God; others without Matter, as
Angels; some without Form, as the first matter; but every Essence
created or uncreated, hath its Final cause, and some positive End both
of its Essence and Operation ; this is the cause I grope after in the
works of Nature ; on this hangs the providence of God ; to raise so
12 THE THEORY OF [pt.
beauteous a structure as the World and the Creatures thereof, was
but his Art; but their sundry and divided operations, with their
predestinated ends, are from the Treasure of his Wisdom. In the
causes, nature, and affections of the EcHpses of the Sun and Moon
there is most excellent speculation, but to profound farther, and to
contemplate a reason why his providence hath so disposed and
ordered their motions in that vast circle as to conjoyn and obscure
each other, is a sweeter piece of Reason and a diviner point of
Philosophy; therefore sometimes, and in some things, there appears
to me as much Divinity in Galen his books De Usu Partium, as in
Suarez' Metaphysicks: Had Aristotle been as curious in the enquiry
of this cause as he was of the other, he had not left behind him an
imperfect piece of Philosophy but an absolute tract of Divinity".
This was written in Harvey's time, and in Harvey's thought the four
causes were still supreme ; his De Generatione Animalium is deeply con-
cerned with the unravelling of the causes which must collaborate in
producing the finished embryo. But the end of their domination was
at hand, and the exsuccous Lord Chancellor, whose writings Harvey
thought so little of, was making an attack on one of Aristotle's causes
which was destined to be peculiarly successful. There is no need to
quote his immortal passages about the "impertinence", or ir-
relevance, of final causes in science, for they cannot but be familiar
to all scientific men. Bacon demonstrated that from a scientific point
of view the final cause was a useless conception; recourse to it as an
explanation of any phenomenon might be of value in metaphysics,
but was pernicious in science, since it closed the way at once for
further experiments. To say that embryonic development took the
course it did because the process was drawn on by a pulling force,
by the idea of the perfect adult animal, might be an explanation of
interest to the metaphysician, but as it could lead to no fresh experi-
ments, it was nothing but a nuisance to the man of science. Later
on, it became clear also that the final cause was irrelevant in science
owing to its inexpressibility in terms of measurable entities. From
these blows the final cause never recovered. In England the seven-
teenth century was the time of transition in these aflfairs, and in such
books as Josfeph Glanville's Plus Ultra and Scepsis Scientifica, for in-
stance, and Thomas Sprat's Defence of the Royal Society, the stormy
conflict between the "new or experimental philosophy" and the
Aristotelian "school-philosophy" can be easily followed. Francis
I] CHEMICAL EMBRYOLOGY 13
Gotch has given a delightful account of the evening of AristoteUanism,
but it involved a stormy sunset, and the older ideas did not give
way without a struggle. Harvey's work is perfectly representative of
the period of transition, for, in his preface under the heading "Of the
Method to be observed in the knowledge of Generation", he says,
"Every inquisition is to be derived from its Causes, and chiefly from
the Material and Efficient". As for the formal cause. Bacon expressly
excluded it from Physic, and it quietly disappeared as men saw that
scientific laws depended on the repeatableness of phenomena, and
that anything unique or individual stood outside the scope of science.
Thus in the case of the developing egg, the formal (the particular
farmyard, etc.) and the final causes are scientifically meaningless,
and if it were desired to express modern scientific explanation in
Aristotelian terminology, the material and efficient causes would
alone be spoken of, essence-of-eggness being a "chymical matter"
as well as the heat of the brooding hen.
Obstacles to Chemical Embryology
The complexity of living systems, however, is such that many
minds find it difficult to accept this physico-chemical account as the
most truly scientific way of looking at it. This is doubtless due in part
to an erroneous notion, which is yet very tenacious of existence, that
the mechanical theory of the universe must, if accepted at all, be
accepted as an ultimate ontological doctrine, and so involve its sup-
porter in one of the classical varieties of metaphysical materialism.
It cannot be too strongly asserted that this is not the case. To imagine
that it is, is to take no account of the great space that separates us
from the last century. "When the first mathematical, logical, and
natural uniformities", said WilHam James, "the first Laws, were
discovered, men were so carried away by the clearness, beauty, and
simplification that resulted that they believed themselves to have
deciphered authentically the eternal thoughts of the Almighty. His
mind also thundered and reverberated in syllogisms. He also thought
in conic sections, squares, and roots and ratios, and geometrised like
Euclid. He made Kepler's laws for the planets to follow, he made
velocity increase proportionately to the time in falhng bodies; he
made the laws of the sines for light to obey when refracted; he
established the classes, orders, families, and genera of plants and
animals, and fixed the distances between them."
14 THE THEORY OF [pt.
Far different is the account of itself which science has since learned
to give. But this change of attitude is not a revolt against thought
as such, or against reason as such ; it is only a loss of belief in the
literal inspiration of the formulae proper to science. It would be just
as extravagant to claim that the scientific investigator of the twentieth
century sets down absolute truths in his laboratory notebook, and,
armed with an infallible method, explores the real structure of an
objective world, as it would be fantastic to claim that Jehovah
dictated an absolute code of the good to Moses on Mount Sinai. To
say that the development of a living being can best be described in a
metrical or mechanical way is not to say that it is metrical or me-
chanical and nothing else. The physico-chemical embryologist is not
committed to any opinion on what his material really is, but he is
committed to the opinion that the scientific method is one way of
describing it, and that it is best to apply that method in its full rigour
if it is to be applied at all. In other words, following the train of
thought of William James, he does not assert that the courts of Heaven
as well as those of our laboratories resound with expressions such as
"organisers of the second grade," and "so many milHgrams per cent."
The mechanical theory of the world, which is, as many beHeve,
bound up indissolubly with one of the ultimate types of human
experience, can no longer be considered as necessarily involving the
exclusion of other theories of the world. Or, put in another way, it
is a theory of the world, and not a pocket edition of the world itself
But before bringing forward any arguments in support of this
attitude and in defence of physico-chemical embryology, it will be
well to consider briefly those theoretical tendencies in modern biology
which go together under the inexact adjective "neo-vitalistic", for
their influence in scientific thought has been far-reaching. To deal
critically with them is not a waste of time, for, were we to adopt
any one of them, we should find that the notion of embryology as
complicated biophysics and biochemistry would have to be abandoned,
and quite other means of approach (never, indeed, very well defined)
would have to be used.
The Stumbling-block of Hormism
Hormism, or "Psychobiology," may be dealt with in a few
words. Chiefly supported by A. Wagner in Germany, and by
E. S. Russell and L. T. Hobhouse in this country, it holds that — to
I] CHEMICAL EMBRYOLOGY 15
use Lloyd Morgan's terminology — a physiological tale cannot be told
separately from a psychological tale. Instead of expressing living
processes in terms of physical causes and effects, the hormists wish
to regard unconscious striving as the essential urge in life, and such
conceptions as food, rest, fatigue, etc., as irreducible biological cate-
gories. These thinkers do not often acknowledge their debt to Galen
of Pergamos, who put forward, as early as a.d. 170, an essentially
similar conception as the basis of his biology. In the treatise On the
Natural Faculties he says, "The cause of an activity I term a faculty....
Thus we say that there exists in the veins a blood-making faculty,
as also a digestive faculty in the stomach, a pulsatile faculty in the
heart, and in each of the other parts a special faculty corresponding
to the function or activity of that part". He also said, "We call it
a faculty so long as we are ignorant of the cause which is operating",
but he never actually suggested any such underlying cause, and
seems to have thought it impossible to ascertain. So do the hormists.
According to them the actions of protozoa are to be described in
terms of avoiding responses, seeking responses and the like, language
which, as they claim, is much simpler than the complex terminology
of surface tension and molecular orientation. Everything, of course,
depends on what is meant by simple. To say that a protozoon seeks
the light is evidently more naive than to say that a dimolecular
photochemical reaction takes place in its protoplasm leading to an
increase of lactic acid or what not on the stimulated side, but since
the latter explanation fits into the body of scientific fact known
already it is open to the biochemist to say that, for his part, he. con-
siders the latter explanation the simpler. It is, in fact, simpler in
the long run. Psychobiology or hormism differs from the other
forms of neo-vitalism because it insists on retaining " commonsense "
explanations in biology as categories of biological thought beneath
which it is impossible to go. It dismisses the entelechy of dynamic
Teleology, on the ground that it acts, as it were, in addition to the
mechanistic schema, accepting the latter fully but interfering in it.
It resembles much more finaUsm and organicism, but lays stress
rather on the unconscious striving force which seems to animate
colloidal solutions of carbohydrates, fats, and proteins. It resembles
the Behaviourism of J. B. Watson superficially by emphasising animal
behaviour, but it fundamentally differs, for it asks the question — Does
an animal see the green light and the red light in this experiment
i6 THE THEORY OF [pt.
as we do, or does it see them as two shades of grey as colour-blind
people do? while the behaviourist asks — Does it respond according
to difference of light-intensity or difference of wave-lengths ? Hormism,
in fact, recurs continually to psychical factors. Samuel Butler, for
instance, one of its principal exponents, wrote, "I want to connect
the actual manufacture of the things a chicken makes inside an egg
with the desire and memory of the chicken so as to show that one
and the same set of vibrations at once change the universal sub-
stratum into the particular phase of it required" (cf. ^ rov hwdixei,
6vTo<i ivreXex^ta fj tolovtov) "and awaken a consciousness of and a
memory of and a desire towards this particular phase on the part of
the molecules which are being vibrated into it". "The Hormist
contends", says Lloyd Morgan, "that something which is very
difficult to distinguish from a ' plan-in-mind ' on the part of the
embryo chick or rabbit does freely determine the course of events
in specific growth from egg to adult. This, I urge, is a metaphysical
hypothesis which goes beyond biology or psychology as branches of
science."
Finalism as a Rock of Offence
Finalism and dynamic Teleology are closely connected, for both
of them embody an attempt to go back to the Aristotelian inclusion of
the final cause as an integral essential of scientific explanation, and
to regard the Baconian attitude to teleology as a mistake. They solve
Kant's antinomy of the teleological judgment simply by deleting the
proposition, and leaving the counter-proposition. They are weakest
on their practical side, for their supporters do not suggest any altera-
tions which might be made in scientific method, although their
fundamental assumptions plainly require it. The principal repre-
sentative of finalism is Eugenio Rignano, and dynamic teleology
has been for the most part upheld by Hans Driesch.
Rignano, in his Qii' est-ce-que la Vie? and his Biological Memory has
contended that, though the mechanical concept of the universe may
be perfectly satisfactory as a description of the world of physics, yet
animals and plants show so much purposiveness that mechanical
categories are absolutely inadequate for them. Biology, therefore,
cannot be complicated physics and chemistry, but must be something
sui generis and with its own methods and laws. "The long debate
between vitalists and mechanists," he says, "in attempting to give
I] CHEMICAL EMBRYOLOGY 17
an explanation of life, cannot lead to any conclusion unless that
fundamental characteristic common to all vital phenomena of pre-
senting a purposive, teleological, or finaUstic aspect in their most
typical manifestations is first thoroughly examined." The most suc-
cinct account of his views and of the exact biologist's answer to them will
be found in his Man Not a Machine, and in the volume Man a Machine
in the same series. The way in which they affect embryology is
significant, and may be found in his chapter called "Finalism of the
generative and regenerative phenomena". "Even if", he says, "the
organism could be explained as a physico-chemical machine, there
would still remain to be explained the most fundamental thing — how
the machine constructed itself The purposiveness of the ontogenetic
development is too evident to be denied. It results from the con-
vergence of manifold morphogenetic activities to one sole end, that
is, to the formation of a marvellous functional unity, every part of
which serves to maintain the life and guarantee the well-being of the
whole. The embryo in its development manifests at every stage a
' harmony of composition ' as Driesch calls it, which has a touch of
the marvellous ; parts and elements of an organ develop independently,
but when they have finished their development they are found to
fit together perfectly like the parts of a machine and the one so
answers to the other that they unitedly form one complex organ.
Thus the mouth and intestine of the sea-urchin begin their develop-
ment at two points distant from each other and develop indepen-
dently, but as they grow the one moves towards the other, so that
when development is ended they fit together perfectly and form a
single canal." This passage illustrates the line of argument found
throughout Rignano, and I will not remark on it further than to
draw attention to the mention of the marvellous in it, another hint,
if any were needed, of that strain of misplaced "numinous instinct"
which seems to be present in all biological vitaHsts. Omnia exeunt
in mysterium would seem to be "a discouraging maxim" for the
scientific worker. "The direction", Rignano goes on to say, "of
ontogenetic development toward a predetermined end is also in-
fluenced by the fact that the embryo overcomes early disturbances
which might deflect it from its course. Ontogenesis thus seems to be
marshalled by some occult intelligence or entelechy in the same way
that the construction of a machine and the direction of its work is
presided over by the mind of the engineer."
i8 THE THEORY OF [pt.
Rignano's arguments are open to grave objection on two main
grounds, first, that he regards biology as suffering more than physics
from the teleology of things, and, second, that he wishes to bring the
concept of purposiveness back into natural science.
The first of these is inadmissible both on philosophic and scientific
grounds. Bernard Bosanquet best expresses the former attitude. He
was led to his conclusions by the conviction that James Ward and
other opponents of scientific naturalism had gone too far in their
polemics against the mechanical theory of the universe, and had
rested the case for teleology only "on the capacity of the finite
consciousness for guidance and selection". This he considered a
mistake. "Things are not teleological", he said, "because they are
de facto purposed but necessary to be purposed because they are
teleological.,.. The foundations of teleology in the universe are far
too deeply laid to be accounted for by, still less restricted to, the
intervention of finite consciousness. Everything goes to show that
such consciousness should not be regarded as the source of teleology
but as itself a manifestation falling within wider manifestations of
the immanent individuality of the real." Bosanquet proceeds, fol-
lowing out the thought of his teacher, Lotze, "The contrast, then,
of mechanism with teleology, is not to be treated as if elucidated
at one blow by the antithesis of purposive consciousness and the
reactions of part on part. It is rooted in the very nature of totality,
which is regarded from two complementary points of view, as an
individual whole, and as constituted of interacting members". But
Rignano's arguments are unsatisfactory also from a scientific angle,
and here the objection comes from Lawrence J. Henderson, whose
book The Fitness of the Environment, probably the most important
contribution to biological thought in this century, is never referred to
by Rignano. It cannot now be necessary to recount how Henderson
examined the question of the finality of our present scientific know-
ledge, and, judging that it was considerable, went on to enquire into
the properties of the elements and compounds principally associated
with life. His conclusion was that living animals and plants exist in
an environment just as fitted for them as they are for it, that the
Darwinian concept of fitness works, indeed, both ways, and that
there is a reciprocity between organism and environment so that
every teleological action done by an individual organism bears upon
it the image and superscription of universal teleology. Thus the
I] CHEMICAL EMBRYOLOGY 19
conclusion of the thought of Bosanquet and Henderson was that,
though teleology was a conception which it was impossible to
do without, yet any limitation of it to, or special association of it
with, living organisms, was inadmissible. The question remained,
What has teleology to do with science?
This point has been approached best by J. W. Jenkinson with his
usual clarity. "Those who uphold teleological doctrine", he said,
"seem to have fallen into a confusion between two different things,
the formal and the final cause. The material, efficient, and formal
causes, if we mean by the last the idea of the effect in the mind of a
sentient being, all precede in time the occurrence of the effect; and
this kind of teleology is not, as it is asserted to be, a doctrine of final,
but one of formal causes. The final cause stands for the use to which
an object is to be put, the effect it will produce, the function it will
perform, which obviously succeed in time the existence of the object
itself The final cause, then, cannot be taken as ever determining in
time the existence of the object itself, and is therefore a conception
which belongs not to science but to metaphysics. The only necessary
conditions of a phenomenon ascertainable by science are those
material and efficient causes which precede it." Or, as Streeter puts
it, "If there is purpose in nature, we ought not to expect science to
reveal it. Purpose is activity, the direction of which is determined
by an end, that is, by an apprehension of quality or value. But
quaUty cannot be measured, and therefore from its essential nature
it — and along with it purpose — lies outside the sphere of science".
Or, finally, to go straight to the fountain-head, "If I say", says
Kant, "that I must judge according to merely mechanical laws of
the possibility of all events in material nature and consequently of all
forms regarded as its products, I do not therefore say: they are
possible in this way alone. All that is impUed is: I must always
reflect on them according to the principle of the mere mechanism
of nature and consequently investigate this as far as I can ; because
unless this Ues at the basis of investigation there can be no proper
knowledge of nature at all". Purposiveness, in fact, is not a concep-
tion which interlocks with quantitative treatment; that mathematical
expression of relationships which is the ideal type of all science
has here nothing upon which to impinge, and the pulling force,
perpetually going on before, eludes and must always elude, if this
analysis is correct, the advancing web of mechanical explanation.
20 THE THEORY OF [pt.
If, then, Bacon and his successors were right in banishing the
concept of teleology from scientific thought, the physico-chemical
embryologist need not be alarmed by the finalism with which,
according to Rignano, the whole of ontogenetic development is suf-
fused. In fact, it is not the phenomenon, but only one way of looking
at it, that is finalistic, and it is this aspect of it that must be neglected
in scientific work if the gravest confusion is to be avoided. Is there
need for the biologist to be any more afraid of the Drieschian en-
telechy (Aristotle's eVreXe^em Actuality) making what might be into
what is and directing from within the development of the embryo in
the egg or the uterus? The word "entelechy" as used by Aristotle
meant that which exists in the highest sense of the word, whether
actually or potentially, e.g. the sword in the mind of the swordmaker
before a single one of the necessary operations of manufacture had
been begun. The entelechy therefore operated on the process in
question by means of the final cause, and did not reside in the
changing entity if it was dead like the sword, though it did if it was
alive like the embryo. Driesch frequently says that he uses the word
in a quite different sense from Aristotle, but the majority of his
readers find it impossible to discover any essential point of divergence.
He does at any rate make it much more precise than Aristotle, for he
defines it as a non-spatial element in the living being, which at one
time suspends possible action and at another time relaxes such
suspension, acting in this way as the bearer of "individualising
causality" and bringing the animal from potentiality into actuality.
It seems that this inherent immanent formative power has been
translated by biologists of every period since Aristotle into the lan-
guage of their time. Just as Driesch now tries to acclimatise it to the
unfavourable environment of a post-Cartesian world, so St Gregory of
Nyssa, who lived about a.d. 370, clothed it in patristic terminology,
and produced a theological variety of neo-vitalism. His most im-
portant biological works, the irepl KaraaKevrjq dvdpcoirov, On the making
of Man, and Trepl '^v^V'^i On the Soul, contained such passages as these,
"The thing so implanted by the male in the female is fashioned
into the different varieties of limbs and interior organs, not by the
importation of any other power from without, but by the power
which resides in it transforming it". And elsewhere, "For just as a
man when perfectly developed has a soul of a specific nature, so at
the fount and origin of his life, he shows in himself that conformation
i] CHEMICAL EMBRYOLOGY 21
of soul which is suitable for his need in its preparing for itself its
peculiarly fit dwelling-place by means of the matter implanted in
the maternal body, for we do not suppose it possible that the soul is
adapted to a strange building, just as it is not possible that a certain
seal should agree with a different impression made in wax".
Thus the soul makes its body as if it were a gem making a stamp
upon some soft substance, and acting during embryogeny from
within — a conception essentially like that of Driesch. We shall see
later how many Renaissance authors adopted similar views, e.g.
Fienus. "No unsouled thing", says Gregory, "has the power to
move and to grow. Yet there is no doubt that the embryo moves
and waxes big as it is fed in the body of the mother." There is nothing
new about dynamic teleology; it is by no means the outcome of
newly ascertained facts : it recurs from time to time in the history of
biological thought because it is the natural result of an unscientific
attitude.
I do not propose to discuss here the facts which originally led
Driesch to the views expressed in his Science and Philosophy of the
Organism, for they are very well known, and have been shown by
J. W. Jenkinson, H. S. Jennings, H. C. Warren and A. E. Boveri,
among others, to be interpretable on quite other lines. Nor shall I
demonstrate by a comparison of passages from Driesch and Paracelsus
how closely the conception of immanent formative force or entelechy
approaches the master-archaeus of Paracelsus and the later iatro-
chemists such as Stahl, for Driesch has done it himself in his History
and Theory of Vitalism. The inference from it is that the Drieschian
entelechy has been and will be of no more use as a practical working
hypothesis for the laboratory than the archaeus was in the past.
Driesch's dynamic teleology is open to more serious and funda-
mental objections. These were not obvious at the first appearance
of his Gifford Lectures, but were clearly brought to light through
the controversy which Jacques Loeb had with H. S. Jennings and
which resulted in the publication of their respective books, Forced
Movements, Tropisms, and Animal Conduct and The Behaviour of the Lower
Organisms. Loeb's theory of tropisms entirely dispensed with any
psychological factors, but Jennings upheld the view that they might
be legitimately brought under scientific discussion, provided they
were regarded as being determined as well as determining. This led
him to make a new enquiry into scientific methodology, and he
22 THE THEORY OF [pt.
published his results in a valuable series of papers from 1911 to 191 8.
He concluded that the pursuance of laboratory work demands as its
minimum of system what he called "Radical Experimental Deter-
minism", and that there was difference of opinion as to whether
this might regard conscious or unconscious mental processes as links
in the chain of determinate causation. On this point Jennings and
Loeb were antagonists, but both were united against Driesch, from
whose writings it now appeared that psychical events might or might
not affect physical events according to circumstances, and that the
entelechy was subject to no general laws. Neal had maintained that
the experimentally discoverable perceptual determiners in living
things were insufficient to account for the effects produced in them.
Jennings pointed out that, if this meant that the non-perceptual
(mental) determiners acted supplementarily to the others and not
instead of them, it was compatible with radical experimental deter-
minism. But, if it was said that some of the determiners were non-
perceptual and could not be known at all, then it was incompatible.
Now it was just this that Driesch had been saying. "A complete
knowledge", he wrote, "of all physico-chemical things and relations
(including possible relations) of a given system at a time t would
not give a complete characterisation of that system if it is a living
system. . . . Practically we may say that complete knowledge of the
physico-chemical constitution of a given egg in a given state and of
the behaviour following this constitution in one case, implies the
same knowledge for other cases (in the same species) with great
probability. But this is a probability in principle and can never be
more. It would not even be a probability if we did not know the
origin of a given egg in a given state, i.e. that the egg was the tgg,
say, of an ascidian. But to know this history or origin, is of course,
already more than simply to know its physico-chemical constitution
and its consequences in one case, which suffices in the realm of the
inorganic. It may be that the eggs of echinoidea, fishes, and birds,
are the same in all the essentials of physico-chemical constitution.
Something very different happens in each case on account of the
different entelechies. In spite of this we know with great probability
what will happen from one case if we know that this egg comes
from a bird and that from an echinoid. Therefore, practically, ex-
perimental indeterminism is not a great danger for science."
But the matter was taken up by various writers, and Lovejoy,
I] CHEMICAL EMBRYOLOGY 23
especially, defended Driesch from the charge of interfering with the
fundamental necessities of scientific thought. Jennings, however,
was able to publish in reply letters from Driesch in which these
implications of his position were fully admitted. "Two systems abso-
lutely identical in every physico-chemical respect may behave differ-
ently under absolutely identical conditions if the systems are living
systems. For the specificity of a certain entelechy is among the
complete characteristics of a living organism and about this entelechy
knowledge of physico-chemical things and relations teaches abso-
lutely nothing." Such a basis for experimental work was generally
felt to carry with it its own condemnation.
It is interesting to recall, in this connection, the vivid account
given by Claude Bernard of the polemic he had with Gerdy at
the Philomathic Society in Paris, for the Driesch-Lovejoy-Jennings
controversy simply repeated on a larger scale the arguments
of the two Parisian biologists sixty years before. "In 1859,"
says Claude Bernard, "I made a report to the Philomathic
Society in which I discussed the experiments of Brodie and
Magendie on ligature of the bile-duct, and I showed that the divergent
results which the two experimentalists reached depended on the fact
that one operated only on dogs and tied only the bile-duct, while
the other operated only on cats, and, without suspecting it, included
in his ligature both the bile-duct and a pancreatic duct. Thus I
explained the difference in the results they reached and concluded
that in physiology as everywhere else experiments are rigorous and
give identical results wherever we operate in exactly similar Conditions.
A propos of this a member of the Society took the floor to attack my
conclusions ; it was Gerdy, a surgeon at the Charite, professor in the
faculty of medicine and known through various works in surgery
and physiology. 'Your anatomical explanation of these experiments',
said he, 'is correct, but I cannot accept your general conclusions.
You say, in fact, that the results of experiments in physiology are
identical; I deny it. Your conclusion would be correct for inert
nature but cannot be true for living nature. Whenever life enters
into phenomena', he went on, 'conditions may be as similar as we
please, the results may still be different.' To support his opinion Gerdy
cited cases of individuals with the same disease, to whom he had
given the same drugs with different results. He also recalled cases
of like operations for the same disease, but followed by cure in one
24 THE THEORY OF [pt.
case and death in another. These differences, according to him, all
depended on life itself altering the results, though the experimental
conditions were the same, but this could not happen, he thought,
in the phenomena of inert bodies, where life does not enter. Opposi-
tion to these ideas was prompt and general in the Philomathic
Society. Everyone pointed out to Gerdy that his opinions were
nothing less than a denial of biological science, but he would not
give up his ideas and entrenched himself behind the word 'vitality'.
He could not be made to understand that it was only a word, devoid
of meaning and corresponding to nothing, and that saying that some-
thing was due to vitality amounted to calling it unknown." The
only difference between Driesch and Gerdy seems to be that Driesch's
arguments rested on a more solid basis of fact, and were put forward
with greater eloquence.
It is worth while to study the thought of Claude Bernard more
closely. Bernard was so subtle a thinker that it has always been
difficult to classify him with any of the main currents of biological
thought, but the following passage seems to me to sum up as well as
any other the main shape of his ideas. "When a chicken develops in
an egg", said he, "the formation of the animal body as a grouping of
chemical elements is not what essentially distinguishes the vital force.
This grouping takes place only according to laws which govern the
physico-chemical properties of matter ; but the guiding idea of the
vital evolution is essentially of the domain of life and belongs neither
to chemistry nor to physics nor to anything else. In every living germ is
a creative idea which develops and exhibits itself through organisation.
As long as a living being persists it remains under the influence of
this same creative vital force, and death comes when it can no longer
express itself; here, as everywhere, everything is derived from the idea
which alone creates and guides ; physico-chemical means of expres-
sion are common to all natural phenomena and remain mingled
pell-mell, like the letters of the alphabet in a box, till a force goes to
fetch them to express the most varied thoughts and mechanisms. This
same vital idea preserves beings by reconstructing the vital parts
disorganised by exercise or destroyed by accident or disease. To the
physico-chemical conditions of this primal development, then, we
must always refer our explanation of life, whether in the normal or
pathological state." Here Bernard seems to recognise the significance
of universal teleology, for he says, "here, as everywhere, everything
I] CHEMICAL EMBRYOLOGY 25
is derived, etc.", and at the same time he lays stress on the identifica-
tion of the physico-chemical aspect with the scientific aspect, going
on, indeed, to say that "physiologists can only act indirectly through
animal physico-chemistry, i.e. physics and chemistry worked out in
the field of life, where the necessary conditions of all living organisms
develop, create, and support each other according to a definite idea
and obedient to rigorous determinism". It is true that elsewhere he
identifies this "force that goes to arrange the letters of the alphabet"
with the "mediating nature" of Hippocrates and the archaeus
faber of van Helmont. Had he read more in Lucretius than in
Aristotle, he might rather have spoken of it as a necessary outcome
of the constitution of nature, a suggestion more profoundly in
harmony, perhaps, with the natural bent of the scientific conscious-
ness (cf Bacon's remarks on Democritus in De Augmentis Scientiarum) .
But, even so, he evidently regards it as the subject-matter of meta-
physics and not of science, for he says in the next paragraph, "The
term 'vital properties' is only provisional because we call properties
' vital ' which we have not yet been able to reduce to physico-chemical
terms, though doubtless we shall succeed in that some day".
Organicism as an Occasion of Falling
By making use of the thought of Bernard, we pass by an imper-
ceptible transition from finalism and dynamic teleology to or-
ganicism, another of the principal forms which the opposition to
mechanistic biology has taken. This, like some of the other doctrines
I have mentioned, has a long history behind it. The notion, "We
murder to dissect", finds clear expression as early as a.d. 200, when
Q^. Septimius Tertullianus, of Carthage, one of the Western Fathers,
spoke thus of Herophilus, the Alexandrian anatomist, "Herophilus,
the physician, or rather butcher, dissected 600 persons that he might
scrutinise nature; he hated man that he might gain knowledge.
I know not whether he explored clearly all the internal parts of man
for death itself changes them from their state when alive, and death
in his hands was not simply death, but led to error from the very process
of cutting up". No more excellent statement of the organicistic view-
point could be devised. Sir Kenelm Digby in 1644 gave a still
clearer summary of this point of view, and even in the rationalistic
eighteenth century there were scientific men who objected to the
use of the term machine-like as applied to animals, and insisted that
26 THE THEORY OF [pt.
the living being was an organism. Cuvier took a very definite stand
on this question when he said, "All the parts of a body are inter-
related, they can act only in so far as they all act together; trying
to separate one from the whole means transferring it to the realm
of dead substance and entirely changing its essence". But the name
most familiarly associated with biological organicism in this country
is that of J. S. Haldane, who has frequently set forth his views upon
this subject. His attitude is so well known that it need not be de-
scribed here at any length, but, in brief, he points out that the living
animal is an entity with a far higher degree of internal relatedness
than any non-living system, and holds that the organic cannot
be understood by a study of its parts though the inorganic very
possibly can. In other words, an organism is an entity whose parts
lose all their characteristic properties when they are studied away
from the organism itself; they fall, as it were, into meaninglessness
as soon as they are abstracted from the whole of which they are
parts. Consequently that kind of physiology, and a fortiori biophysics
and biochemistry, which analyses living organisms, is insufficient
as an apparatus for understanding living things and should give
place to studies in which organisms are regarded intact. Moreover,
it is only in the untouched organism that those wonderfully well-
balanced actions are seen by which the animal or plant holds to its
own niche in the economy of nature, resisting every attempt to dis-
lodge it, provided the attempt be not so successful as to disorganise the
living thing. This power of maintaining a constancy in its external and
internal environment is what Haldane regards as the deus ex machina, the
property of living things essentially inexplicable by physico-chemical
hypotheses and requiring special biological language for its formula-
tion. (For a discussion of the "inconceivability argument" in bio-
logy, see Mackenzie and Needham.) "All attempts", he says, "to
trace the ultimate mechanism of life must be given up as meaningless.
The aim of biology becomes a very different one — to trace in
increasing detail, and with increasing clearness, the organic deter-
mination which the organic conception formulates." It is to be
noted, however, that Haldane vigorously criticised Drieschian neo-
vitalism, adducing against it the argument of impossibility of inter-
ference with the second law of thermo-dynamics, and the dubiousness
of the theory of guidance without work done, a discussion of which
on much better foundations was subsequently given by Lotka.
i] CHEMICAL EMBRYOLOGY 27
Haldane was not convincing in his criticism of Driesch, and there
can be little doubt that Driesch's position is a perfectly tenable one,
provided its supporter closes his eyes to the nature of the scientific
method on the one hand, and the actual history of recent scientific
progress on the other. Another side of Haldane's teaching was the
view that the living animal was in some way less abstract than the
world of physics ; physics and biology, he thought, might some day
coalesce, but it would then be found that physics would not have
swallowed up biology; rather the contrary would occur and biology
would swallow up physics. "The idea of life", he said, "is nearer
to reality than the ideas of matter and energy, and therefore the
presupposition of ideal biology is that inorganic can ultimately be
resolved into organic phenomena, and that the physical world is thus
only the appearance of a deeper reality which is as yet hidden from
our distinct vision and can only be seen dimly with the eye of scien-
tific faith."
There had been precursory voices of all this in the nineteenth
century, as when, in spite of the discoveries of Cagniard de Latour
and others that the yeast-cell played an essential part in fermenta-
tion, Justus von Liebig refused to credit them, fearing that their
suggestions were a return to explanations by vital force. "Chemical
actions may very well explain physiological actions, but certainly
not vice versa ", said Moritz Traube. Claude Bernard, moreover, dis-
cussed the matter with his usual subtlety. "Physiologists", he said,
"must not forget that a living being is an organism with its own
individuality. Since physicists and chemists cannot take their stand
outside the universe they study bodies and phenomena in themselves
and separately without necessarily having to connect them with
nature as a whole. Physiologists, on the contrary, find themselves
outside the animal organism which they see as a whole, even when
trying to get inside so as to understand the mechanism of every part.
The result is that physicists and chemists reject all idea of final causes
for the facts which they observe while physiologists are inclined to
acknowledge an harmonious and pre-established unity in an organised
body, all of whose actions are independent and mutually generative.
If we break up an organism for the sake of studying its parts it is only
for the sake of ease in experimental analysis, and by no means in
order to conceive them separately. Indeed, when we wish to ascribe
to a physiological quality its value and true significance we must
28 THE THEORY OF [pt.
always refer it to this whole and draw our final conclusions only in
relation to its effect on the whole. It is doubtless because he felt this
necessary interdependence among the parts of an organism that
Cuvier said that experimentation was not applicable to living beings
since it separated organised parts that should remain united. For
the same reason vitalists proscribe experiments in medicine. These
views, which have their correct side, are nevertheless false in their
general outcome and have greatly hampered the progress of science."
Bernard did not commit himself to an absolutely unambiguous state-
ment as to the correct and incorrect sides of organicism, and seems to
have regarded it as true only in the sense that imaginative synthesis
must follow radical experimental analysis. He was therefore quite
opposed to that true and keen-edged organicism represented by
Cuvier and other biologists, which denied the bare utility and
legitimacy of the experimental analysis, and which was not un-
justly satirised by Woolf in 1927:
You cannot demonstrate the soul
Except upon the animal as a whole;
Spiritual autolytic changes begin
As soon as you push a needle through the skin.
Haldane's writings and those of his school, such as J. A. Thomson
and G. G. Douglas, had extremely little effect on the direction taken
by biological science in the first years of this century. As A. D.
Ritchie pointed out, it is extraordinarily difficult to find out anything
about living systems, unless their parts are treated in isolation, even
if that be recognised as but the preliminary for imaginative synthesis,
and, as many observers said, Haldane's own researches in the physio-
logy of breathing afforded an excellent example of the usual scientific
method. Biological research proceeded steadily on the usual lines,
for Haldane's practical counsels could only be followed by those who
were willing to abandon causal explanations in biology or to give
up the hope of biology becoming an exact science.
An influence was at hand, however, which was to lessen very much,
if not to destroy altogether, the attraction of Haldane's opinions for
biologists. A. N. Whitehead had in his earlier works, The Concept
of Mature and The Principles of Natural Knowledge, elaborated his theory
of extensive abstraction, but it was not until the publication of his
Science and the Modern World that it began to exercise any wide-
i] CHEMICAL EMBRYOLOGY 29
spread effect upon scientific men other than mathematical physicists.
Whitehead boldly extended the concept of the organism to cover all
objects, i.e. all events, non-living as well as living. The word "in-
organic" would thus cease to apply to non-living nature and all
physical systems would be regarded as in a sense incomprehensible,
except when regarded as wholes composed of parts owing their very
existence to their share and arrangement in the whole in question.
Quoting Tennyson's, '"The stars', she whispers, 'blindly run'", he
says, "An electron within a living body is different from an electron
outside it, by reason of the plan of the body. The electron blindly runs
within or without the body, but it runs within the body in accordance
with the general plan of the body and this plan includes the mental
state. But this principle of modification is perfectly general throughout
nature, and represents no property peculiar to living bodies".
Lloyd Morgan recognised in Whitehead's organisms his "systems
of relations going together in substantial unity", which he had con-
ceived of as stretching in degrees of ever vaster complexity from the
smallest physical event to the universe itself. It was Lloyd Morgan,
indeed, who pointed out first the significance of Whitehead's argu-
ments for biological thought. He showed that the extension of
organicism to cover the entire world of physics had no serious con-
sequences for biological mechanists (who would continue to employ
physico-chemical methods as before), provided that they had not
adopted some form of scientific naturalism. At the same time, it
could have little help for those who had insisted that the principal
characteristic of living things was their organismic character, and
had been led by this to propose far-reaching alterations in scientific
logic or to give up the hope of causal explanation in biology. If,
as it would seem, there are organisms everywhere, then the position
that there are organisms nowhere turns out to be better placed than
the position that living things are organisms and not other things ;
for, in the former case, peace can be at once secured by attention
to definitions, while in the latter case the irreducible characteristic
of life is not organicism, whatever else it may be. The difference
between the living and the non-living becomes a quantitative one,
expressible in degrees of organisation. As has been pointed out,
Haldane's prophetic observations concerning the eventual meeting-
place of physics and biology have perhaps at last been justified, but,
if so, with a barren benefit to neo- vitalism.
30 THE THEORY OF [pt.
Organicism and Emergence
We may now pass by another small transition, from organicism
to theories of emergence. Neo- vitalism in this form practically ceases
to have any claim to the name, and approaches extremely closely
to neo-mechanism. The principle of emergence in its simplest form
is the statement that there are levels of existence in the universe, at
each of which some more complicated form of being comes into
existence, containing some essence absolutely new, and which could
not have been predicted, even if all the properties of the constituents
of the lower order had been known. This is evidently a conception
very close to that of the organism, for just as the living or non-living
system, looked at from one point of view, ceases to be itself as soon
as it is dismembered, so the new level of complexity, looked at from
one point of view, consists of lower levels of complexity joined together
in a way that could not have been foreseen, because its properties
and peculiarities are not the sum of the properties and peculiarities
of its constituents. But it is important to note that there are here two
parentheses, namely, "looked at from one point of view", implying
that there is another point of view, and though the discussion is
complicated here by the doubt as to whether Whitehead intends his
organisms to be taken as a metaphysical theory, or as a scientific
theory, yet in Lloyd Morgan the statement is clear that the emergent
point of view has always a complementary one, the resultant point
of view, "the emergent web and the resultant woof" as he calls it.
This is really a new and more accurate way of putting the ancient
antithesis of mechanism and teleology, for the scientific method in-
volves the concept of resultance, since it continually seeks for the
predetermining causes which must be in some way uniform with
their effects, while the advent of something absolutely new at each
level, i.e. atom to molecule, colloidal aggregate to living cell, etc.,
is a speculation hardly germane to science and resembling the final
cause.
We find ourselves back again, then, at the distinction between
metaphysics and science, which was first seriously studied by Kant.
Most of the confusion has arisen in the past through an insufficiently
clear decision as to the nature of biology. Biology cannot be philo-
sophical and scientific, emergent and resultant, indeterminate and
determinate, teleological and mechanical at one and the same time.
i] CHEMICAL EMBRYOLOGY 31
No doubt the most powerful solvent of vitalism will turn out to
be the set of changes now taking place in physics. It is as yet too
early to describe very definitely the effects of these, and several
modern writers on the subject are rather free in their use of the word
"mysticism", but it is at any rate clear that physicists are coming to
see more clearly than before the impHcations and the limitations of
the study of the metrical aspects of the world, which is what science is.
Behind these sets of numbers and quantities the background of the
world is enigmatic, and Heisenberg's Principle of Indeterminacy
indicates that extreme accuracy can only be obtained at a cost.
Again, the abandonment of the model in physics is a highly important
step, and physics seems to have reached a point at which there is no
analogy in our everyday experience for the phenomena with which
it is dealing, so that we cannot even picture in ordinary words what
is happening. Eddington, in his great work The Nature of the Physical
World, also shows that physics no longer speaks of the motion of
every individual particle in the universe being rigidly determined but
rather of the relative probabilities of its motions, i.e. the odds on
them. If this funeral of Laplace's Calculator should turn out to
be enduring, a good deal of difficulty may vanish from the biological
sphere. But I should prefer to leave the working out of these possi-
bilities to those better qualified than myself, and to suggest simply
that if the mechanical theory of the world is being reconstructed by
modern physics, there may be a widespread sapping of the force of
neo-vitaHstic contentions in the near future. I have often thought
that neo-vitalists were thinkers whose religious sense had got into the
wrong place; unable to set up commonsense watertight compart-
ments on the one hand, or to work out a philosophy of the forms of
experience on the other, they brought the numinous into biology and
abused biophysics. The fundamental contention of the mechanists
always was that science and the scientific method were one and that
biology was complicated physics — this remains unaltered. But if
physics is more and more obviously throwing off the links which
bound it in the past to metaphysical materiahsm, and admitting
itself to be the study of the metrical aspects of the world, there will
be perhaps less excuse for the misplacement of the numinous, as it
might be called; and the affirmation that biology must be based
upon physics will cause less antagonism than in the past. In a word,
the vitaHsts wished to introduce mysticism at the wrong level; the right
32 THE THEORY OF [pt.
level would seem to be, as it were, underneath the metrical abstrac-
tions of physics, i.e. outside science altogether, and if this is realised,
the vitalist and the mechanist will fuse into one and the same
person — a happy consummation. It is curious that at the present
time, when physics is so markedly freeing itself from scientific
naturalism, biologists are sometimes found to support that untenable
world-view, while still farther away from the inorganic world,
literary criticism tries to make itself modern by using a psychological
jargon, and applying to its problems all the rigour of a materialist
metaphysic.
Neo-mechanism as a Theory for Chemical Embryology
The principal philosophical obstacles to physico-chemical embryo-
logy have now been assessed and nothing remains but to outline its
own theoretical basis. Anyone who was dissatisfied twenty years ago
with the various forms of neo-vitalism which have already been
discussed would have had no alternative but to accept the simple,
though rather incredible, scientific naturalism of the preceding
century, unless, indeed, he was acquainted with German philosophy,
and understood the momentous consequences which flowed from the
apparently technical question, "How are a priori synthetic judgments
possible?" The varieties of neo-vitalism may perhaps be thought of,
in so far as they are not modern forms of difficulties which have
for many centuries perplexed philosophers, as a series of reactions
against mechanistic biology insufficiently distinguished from scientific
naturalism. This confusion is well seen in the earlier phases of the
American discussion which led up to the symposium of 191 8. It is
often difficult to tell, as in the papers of Nichols; Ritter; More and
Fraser Harris, whether the writer is attacking the mechanical theory
of the universe regarded as an ultimate metaphysical faith or the
mechanical methodology of science.
Before the eighteenth century, of course, there had always been
thinkers who felt the necessity of including both teleology and its
antithesis in their systems of thought. This pull in two directions
accounts for those very interesting mediaeval theologians, such as
Siger of Brabant and John of Jandun, who wished to acknowledge
two kinds of truth, theological and philosophical (see Maywald and
Gilson). A right balance had to be struck in some way between
Democritus and Plotinus, and in the seventeenth century, for in-
I] CHEMICAL EMBRYOLOGY 33
stance, Sir Thomas Browne wrote to his son advising him to study-
Lucretius yet not to read too much in him, "there being divers
impieties in him". Perhaps the greatest figure in this line of descent
is the Cardinal Nicholas of Cusa, who, in his book De Docta Ignorantia
of 1440, contended that the principle of contradiction was only valid
for our reason, and so foreshadowed Kant and Hegel (cf. Vansteen-
berghe) . But the starry heavens with their infinite spaces that terrified
Pascal and their mechanical order and metrical uniformity revealed
by Kepler and Copernicus, on the one hand, and the moral law
with its tremendous purposiveness and its theological implications,
on the other hand, were never really faced at one and the same
moment and taken seriously together until the time of Kant, who
first subjectivated what had before struggled in the external world,
and suggested that the contradictions of our thought might spring
from the constitution of our own intelligence. "It has always been
assumed", he said, "that all our knowledge must conform to objects.
The time has now come to ask, whether better progress may not be
made by supposing that objects must conform to our knowledge."
Omnis enim longe nostris ab sensibus infra
primorum natura jacet. . .
Lucretius had said, but Kant went farther, and suggested that our
intelHgence can help us no more than our senses in the attempt
to see things as they really are.
This was the great service that Kant performed for philosophy, and
in the light of it the scientific mind was relieved of the burden of
having to believe finally in its own account of the world. But in the
scientific controversies of the last century, Kant was forgotten, and
the continual successes of the scientific method led to a naturaUstic
outlook, which was wholly unsatisfactory. It had been supported by
T. H. Huxley, Herbert Spencer, W. K. CUfford, Tyndall, Ray
Lankester, and many others: it apparently still is by Chalmers
Mitchell. But, as a widely accepted attitude, it did not live long into
the present century, and from such blows as James Ward's Naturalism
and Agnosticism and Antonio Aliotta's The Idealistic Reaction against
Science, it never recovered. Physico-chemical biologists were thus left,
as it were, without visible means of support, and existed for some time
on a purely pragmatic basis, devoid of any epistemological comfort.
Gradually, however, a more satisfactory attitude came into being.
34 THE THEORY OF [pt.
Karl Pearson pro\-ided an intimation of it when he remarked,
"Those who say that mechanism cannot explain life are perfectly
correct, but then mechanism does not explain anything. Those, on
the other hand, who say that mechanism cannot describe hfe are
going far beyond what is justifiable in the present state of our know-
ledge". A clear enunciation of it, informed with the charm of all
his writings, was given by d'Arcy Thompson in the .\ristoteUan
Society^ S\Tnposium of 191 8. "In the concepts of matter and energy",
he said, " the Whole is not enshrined, mechanism is but one aspect
of the world. These are the proper categories of objective science,
but they are no more: the physicist is, ipso facto, a mechanist, but he
is not by impHcation a materialist; nor is the biologist of necessity
a materiahst, even though he may study nothing but mechanism in
the material fabric and bodily acti\ities of the organism." R. S.
Lillie's paper of 1927 might be taken as one of the best expositions
of this point of \-iew. "Every biologist is aware", he says, "in his
non-professional moments that the possibiUties of hfe are larger than
the mechanistic \iew impHes. This is only another way of acknow-
ledging that the whole mechanistic conception is an incomplete,
derivative, or abstract one. To regard it as philosophically final is a
grave mistake." Thus Lillie remains cominced of the adequacy of
physico-chemical biologv', but expressly repudiates the elevation of
mechanism into a metaphysic.
LilUe goes on to discuss the abstract, distorted and incomplete
character of the world which is presented to us by the employment
of the scientific method. "To say", he proceeds, "that Hfe is 'nothing
but' a combination of chemical reactions in a colloidal substratum
is unscientific. Life may be and apparently is that in part but to
regard any such scientific formulation as a complete and adequate
representation of its total reaUt}- is simply to misconcei\-e the structure
of science." This is well said; Ufe is indeed a "dynamic equihbrium
in a polyphasic system", but also "Life is a manifest of emergent
creativity", "Life is a pure flame and we five by an imisible sun
within us", "Life is a sad composition, we five with death and die
not in a moment", and — if you will,
Life, like a dome of many-coloured glass,
Stains the white radiance of eternity
Until death tramples it to fi-agments.
I] CHEMICAL EMBRYOLOGY 35
"The dilemma of vitalism is irresolvable", says Lillie, "so long as
we regard the units, concepts, and formulae found vaHd in physical
science not as abstractions but as primary and self-existent reaUties,
by a combination of which all the properties of living beings as of
other natural phenomena can be derived."
In a former discussion I contrasted what might be called the
Democritus-Holbach-Huxley attitude in biology with the Driesch-
Haldane-Russell-Rignano attitude, and concluded after examining
them that both involved insuperable difficulties. Lotze was the
philosopher to whom I went for help in the elaboration of a better
standpoint. "The true source of the life of science", said he, "is to
be found, not indeed in admitting now a fragment of one view and
now a fragment of the other, but in showing how absolutely universal
is the extent and at the same time how completely subordinate is
the significance of the mission which mechanism has to fulfil in the
structure of the world." And in another place, "We granted v^aUdity
to the mechanical view in so far as concerns the examination of the
relations between finite and finite and the origin and accomplish-
ment of any reciprocal action whatever; we as decidedly denied its
authority when it claimed acceptance, not as a formal instrument of
investigation, but as a final theorv' of things". "Nowhere is me-
chanism", says Lotze, "the essence of the matter, but nowhere does
being assume another form of finite existence except through it."
I went on to argue that, although Lotze made everv^ effort to demon-
strate how mechanism and teleolog)' could fit together in the universe,
he failed to do so convincingly, and that it was more satisfactory to
make them necessary results of the a priority of our ways of thought.
"We may regard", I said, "the mechanistic view of the world as a
legitimate methodological distortion, capable of appHcation to any
phenomenon whatever, and possessing no value at all as a meta-
physical doctrine." Mechanism, whichever of its forms ^defined by
C. D. Broad, M. R. Cohen and Y. H. Krikorian turns out to be
the minimum requirement of science, is, in fact, not metaphysical
materiahsm. It is necessary to maintain it on methodological grounds,
but it is pernicious to allot it any v\ider value. It stands, in fact,
as one of the kinds of way in which the human spirit reacts to the
universe in which it finds itself, and it springs, as R. G. CoUingwood
puts it, directly from the ultimate root of all science, the assertion
of the abstract concept. "He who generaUses is an idiot", said
3-2
36 THE THEORY OF [pt.
William Blake, but that was a revelation of the poetic or the religious
mind. The scientific spirit is as profound as these, but it sets out
upon a different path from the very beginning and reaches in the
end a country different in every way. It directs its interest from the
first to the correlation of differences between phenomena rather than
to individual phenomena themselves, and the impulse to classify
leads inevitably to the supremacy of the mechanical cause and the
mathematical formula. It stands "at diameter and sword's point"
with such aphorisms as "Everything is itself and not something else"
or "Nothing is ever merely anything".
R. G. Collingwood has expressed this in a memorable passage:
"Mathematics, mechanics, and materialism are the three marks of
all science, a triad of which none can be separated from the others,
since in fact they all follow from the original act by which the scientific
consciousness comes into being, namely, the assertion of the abstract
concept. They are all, it may be said, products of the classificatory
frame of mind, corollaries from the fact that in this frame of mind
the universal and the particular are arbitrarily separated and the
universal asserted in its barren an'd rigid self-identity. It is this
barrenness and this rigidity which confer their character upon the
doctrines of scientific materialism. Hence it is idle to imagine that
materialism is justified in some sciences and not in others. It is idle
to protest that science ought to surrender its materialistic prejudices
when it finds itself face to face with a non-material object such as
the soul. No object is material, in the metaphysical sense of the word,
except in so far as scientific thinking conceives it so; for materiality
means abstractness, subjection to the formulae of mechanical deter-
mination and mathematical calculation, and these formulae are
never imposed upon any object whatever except by an arbitrary
act which falsifies the object's nature. This only appears paradoxical
when we fail to see the gulf which separates the common-sense
materiaHty of a table, its sensible qualities, from its metaphysical
materiality, the abstract conceptual substrate of those qualities. It is
this substrate whose transcendent or abstract existence is asserted by
materialism. Hence we cannot distinguish objects like tables which
are really 'material' from objects in whose presence science must
unlearn its materialistic habits of thought. MateriaHsm is no more
the truth in physics than in psychology, and no less. It is the truth
about any object, just in so far as this object is by abstraction re-
I] CHEMICAL EMBRYOLOGY 37
ducible to terms of pure mathematics; and no object is so reducible
except by consciously or unconsciously shutting our eyes to every-
thing which differentiates it from anything else. This conscious or
unconscious act of abstraction is the very being of the scientific con-
sciousness; and it is therefore no matter for pained surprise when
science shows a bias towards determinism, behaviourism, and
materiaHsm generally".
By this time the general outlines of this theoretical excursus should
have become clear. Embryology, to put it plainly, has been for so
many years the happy hunting-ground of vitaHstic and neo-vitalistic
theory that the first treatise on the physico-chemical aspect of it
could hardly go without some kind of theoretical introduction.
"VitaHstic conceptions", said Claude Bernard in 1875, with the
voice of authentic prophecy, "can no longer hover over physiology
as a whole. The developmental force of the egg and the embryonic
cells is the last rampart of vitalism, but in taking refuge there, it
transforms itself into a metaphysical concept and snaps the last link
connecting it with the physical world, and the science of physiology."
It is to be hoped that what has now been said will place in a right
light the aims of physico-chemical embryology, and provide it, as
it were, with its decretals. That they are not false will be the hope
of every exact biologist.
Chemical embryology is now indeed at a critical point in its
history. On the one hand, it links up with the morphological work
of the classical embryology and the experimental work of the
Entwicklungsmechanik school, while on the other hand it has
affinities with genetics, a science which is every day becoming more
physiological and which will more and more seek for the effects
of its factors in the biochemistry of development. Goldschmidt's
book, and the work of Dunn, who found a lethal gene in an inbred
strain of fowl which caused a regular chick mortaUty at the seven-
teenth day of incubation, are important examples of this. Another
relationship of chemical embryology is to obstetric medicine, for such
problems as the toxaemias of pregnancy will not be solved by Hippo-
cratic observation unassisted by a knowledge of the chemistry of
the embryo and the placenta. The attention devoted by the Medical
Research Council to such problems is an acknowledgment of this
fact. Nor is veterinary physiology in a position to do without the
aid of chemico-embryological researches, as is shown by the incident
38 THE THEORY OF CHEMICAL EMBRYOLOGY [pt.i
of the myxoedematous pig foetuses of western America in the work
of Smith.
But all practical applications, how valuable soever, must give
place to the increase of knowledge itself, and therefore the physico-
chemical history of embryonic development, from the egg-cell to the
loosing of the individual into the activity of post-natal life, is to be
the theme of this present book. "The history of a man for the nine
months preceding his birth", said S. T. Coleridge, "would probably
be far more interesting, and contain events of far greater moment
than all the three-score and ten years that follow it."
PART II
THE ORIGINS
OF
CHEMICAL EMBRYOLOGY
DII LABORIBUS OMNIA VENDUNT
Nobilissimo juveni Medico Philippo
de Glarges, amicitiae ergo libenter
Gulielmus Harveus scripsit, Anglus,
Med. Reg. et Anat. Prof. Londin.
Mai. 8 1641
From the commonplace book of
Philip de Glarges.
Not to prayse or disprayse : all did well.
William Harvey's MS. notes,
Canones Anatomiae Generalis, 6.
PRELIMINARY NOTE
It is open to anyone to say with some appearance of truth that
physico-chemical embryology has no history, since the attempt to
unravel the causes of embryological phenomena by physico-chemical
means has only recently begun. But such a statement would betray
a superficial and jejune mentality. Physico-chemical embryology has
its roots in the history of embryology as a whole, and it is those roots
which I shall try to uncover in what follows. It must be remembered
that morphological must theoretically precede biophysical analysis,
as it has actually preceded it chronologically, and to that extent
physico-chemical embryology cannot be properly understood without
reference to its descriptive husks, and their historical growth. More-
over, even in antiquity there are hints that the chemistry of the
embryo was dimly envisaged (as in Aristotle, see p. 70). Again, that
philosophical problem which we have already considered, plays a great
part in the history of embryology, and as we watch the pendulum swing-
ing from Democritus to Aristotle, back again over to Herophilus,
and back once more to Galen, we almost feel as if we were spectators
looking on, like Hardy's spirits, at a great drama, with the movement of
which we are powerless to interfere, but knowing that the existence of
exact biology depends upon which side wins. Lastly, such unmorpho-
logical questions as the respiration and nutrition of the embryo were
discussed from the most ancient times, and it is surely no unduly wide
interpretation of the word which leads us to include an account of
these opinions under the heading of chemico-embryological history.
Nor could the present moment be more appropriate for such an
historical survey. Embryology is entering a new phase: and on the
threshold it is very fitting that some retrospective attention should
be paid to the phases which it has already passed through. The
events of the past, moreover, throw Hght on those of the future, and
this is true not necessarily in Spengler's sense but also because the
historical approach to problems actually unsolved protects them,
by a kind of gentle scepticism, from too severe a subjection to doc-
trinaire presentations. "Die Geschichte einer Wissenschaft ist der
Hort ihrer Freiheit; sie duldet ihr keine einseitige Beherrschung",
said Louis Choulant in 1842. Theoretical bhnd alleys, such as the
final cause, practical blind alleys, such as preformationism and
phlogiston, are always able to remind us that we may be mistaken.
42 PRELIMINARY NOTE [pt.
No exhaustive treatise on the history of embryology at present
exists. The nearest approach to it is the very valuable memoir of
Bruno Bloch with its epitome but this only covers the era of the
Renaissance with thoroughness. Hertwig's account, which he printed
at the beginning of his great Handbuch der Entwicklungslehre, does not
deal very fully with any aspect of the subject before 1800, nor do
the much shorter ones of Henneguy and Minot. The latter paper is
interesting in that it ends with an emphasis on the need for a physico-
chemical attitude in the future. The introduction to Keibel's book
is much slighter, but contains some useful information. There are
various monographs and papers on special points, such as Pouchet's
rather untrustworthy treatment of the embryology of Aristotle, and
Lones' discussion of it, which is worse. Camus' notes are still the best
commentary on the Historia Animalium. Again, useful information on
some cultural points is to be had from the treatise of Ploss & Bartels.
The introductions to certain books also contain valuable information,
and in this class comes Dareste's remarkable book on teratology.
The bibliographies contained in v. Haller's eighth volume and in
Heffter's book, are, naturally, of the greatest assistance.
These reservations made, the principal reviews of the subject are
chiefly to be found in histories of science in general, such as Sarton's;
histories of biological theory, such as Radl's; histories of obstetrics,
such as V. Siebold's, Spencer's and Fasbender's; histories of gynaeco-
logy, such as McKay's; and histories of anatomy, such as Singer's
and V. Toply's. Histories of medicine as a whole are numerous
and helpful: I have found those of Garrison and Neuburger-
Pagel most useful. Those which deal with special periods are also
of assistance, such as Schrutz and Browne on Arabian, Bloch on
Byzantine, and Harnack on Patristic medicine. Histories of chemistry
provide no help, for ancient chemistry was so oriented towards
"practical" results, such as the lapis philosophorum and elixir vitae,
that the egg was only considered as a raw material for various
preparations. The investigation of its change of properties during
the development of the embryo did not occur to the alchemists.
Detailed studies of particular subjects, such as those contained in
Singer's two excellent volumes The History and Method of Science,
may also be of some assistance. Again, there are books which give
a wonderful orientation and an articulate survey of vast tracts : of
these Clifford Allbutt's Greek Medicine in Rome, with its mass of
references, is among the most valuable. And Miall's Early Naturalists
II] PRELIMINARY NOTE 43
must not be omitted, for, apart from the peculiar charm of style which
marks it, it contains some singularly helpful bibliographical data. But
the study of the original sources, so far as that is possible, is a duty
which cannot be avoided, and in what follows I have been careful
to copy down no statement from a previous review when it was
possible to read the actual words of the writer himself.
This practice of going to the originals is made peculiarly necessary
in a case such as the present one, when the history of a subject is
to be regarded from an entirely new angle. My intention is to give
here the sketch of a history of embryology consistently from the
physico-chemical angle, and to show, at one and the same time, how
our knowledge of the development of the embryo has come into being,
and how throughout the process what we now call the physico-
chemical foundations of embryonic growth have from time to time
received attention, even though it was largely speculative. Since few
have previously examined the history of the subject, and none from
this angle, I have in many cases come upon interesting facts which
have remained unknown owing to the very attitude of mind pre-
viously adopted.
Finally, I would defend the arrangement of my Sections only on
the ground that it is suitable in the present state of historical know-
ledge. I say little about embryology in ancient China and ancient
India, because on the basis of what we know there is little to say,
not because it was intrinsically less interesting than the embryology
of Mediterranean antiquity and the later West, though this may turn
out to be the case. I do not propose a framework for historical facts
in what follows ; I only attempt to bring them together, and to reveal
some of the relationships between them. If the traditional frame-
work turns out to be badly constructed — and there are many signs
that it may — the facts can be rearranged.
The history of single forms of scientific knowledge is in a way
happier because containing more of continuity than that of civilisa-
tion as a whole. The assiduity with which men of different periods
in the rise and decHne of a culture pursue the different forms of
human experience may, as Spengler has shown, vary much, but those
forms remain fundamentally the same, even if their manifestations
are profoundly changed. That science, at any rate, does maintain
some sort of continuity whatever gaps there may be between the
phases of its progress, is a belief agreeable with all the available facts,
and one which no criticism will easily shake.
SECTION I
EMBRYOLOGY IN ANTIQUITY
I -I. Non-Hellenic Antiquity
Since biological science as a whole was little cultivated in ancient
Egypt and the ancient civilisations of Babylonia, Assyria and India,
the study of embryology, we may assume, was equally little pursued.
Doubtless the undeveloped embryo,
whether in egg or uterus, carried
with it, for these persons of remote
antiquity, some flavour of the ob-
scene in the literal sense of the word.
But embryology stands in a peculiar
relation to the history of humanity,
in that even at the most remote times
children were being born, and,
though the practitioners of ancient
folk-medicine might confine their
ideas for the most part to simple
obstetrics, they yet could hardly
avoid some slight speculation on the
growth and formation of the embryo.
Fig. I illustrates this level of culture.
It is a painted and carved door from
a house in Dutch New Guinea, taken
from de Clercq's book; the original
was of yellowish brown wood. The
male embryo is clearly shown, but
the artist evidently had a hazy con-
ception of the umbilical cord. The
line passing from the uterus to the head may or may not be merely
ornamental. The movement of the foetus in utero played and still
plays a large part in the folklore of primitive peoples, as may be read in
the exhaustive treatise of Ploss & Bartels. For information concerning
god-embryos in primitive religion see Briffault.
Ancient Indian embryology achieved a relatively high level.
Structures such as the amniotic membrane are referred to in the
Fig,
I . Painted and carved door from
New Guinea (de Clercq).
SECT. I] EMBRYOLOGY IN ANTIQUITY 45
Bhagavad-Gitd. Susriita believed that the embryo was formed of a
mixture of semen and blood, both of which originated from chyle.
In the third month commences the differentiation into the various
parts of the body, legs, arms and head, in the fourth follows the
distinct development of the thorax, abdomen and heart, in the sixth
are developed hair, nails, bones, sinews and veins, and in the seventh
month the embryo is furnished with any other things that may be
necessary for it. In the eighth there is a drawing of the vital force
to and from mother and embryo (is this comparable with the
Hippocratic eXKeiv? see Peck) which explains why the foetus is not
yet viable. The hard parts of the body are derived from the father,
the soft from the mother. Nourishment is carried on through vessels
which lead chyle from mother to foetus. (For further details see
Vullers.) Ancient Chinese embryology was very similar, if we
may judge from Hureau de Villeneuve; Maxwell & Liu and von
Martuis.
Egyptian medicine did not venture on embryological speculation,
or so it would seem from the writings which have come down to
us — the Ebers medical papyrus does not once mention the embryo
(Brugsch) . But there are points of interest as regards Egypt in this
connection. The Egyptians were responsible for one of the greatest
helps in systematic embryological study, namely, the discovery of the
artificial incubation of the eggs of birds. The success of this process was
to have so obvious an effect on embryology and the abortive attempts
to bring it to completion were so frequent in the West right up to
the nineteenth century a.d. that it is remarkable to find artificial
incubation practised "probably", in Cadman's words, "as far back
as the dawn of the Old Kingdom, about 3000 B.C." It is doubtful
whether the very remote date could be supported by Egyptological
evidence, for, according to Hall and Lowe, hens were not introduced
into Egypt from Mesopotamia or India until the time of the eighteenth
dynasty {ca. 1400 b.g.) when there was much intercourse with the
East (cf Queen Tiy and the Tell-el-Amarna correspondence) : before
then the Egyptians had only goose or duck's eggs. Artificial incubation
is certainly as old as Diodorus Siculus and Pliny, for both of them
refer to the practice, the latter in connection with a curious piece of
ancient sympathetic magic. " Livia Augusta, theEmpresse," says Pliny,
"wife sometime of Nero, when she was conceived by him and went
with that child (who afterwards proved to be Tiberius Caesar) being
46 EMBRYOLOGY IN ANTIQ^UITY [pt. ii
very desirous (like a yong fine lady as she was) to have a jolly boy,
practised this girlish experiment to foreknow what she should have
in the end; she tooke an egge, and ever carried it about her in her
warme bosome; and if at any time she had occasion to lay it away,
she would convey it closely out of her own warme lap into her
nurses for feare it should chill. And verily this presage proved true,
the egge became a cocke chicken, and she was delivered of a sonne.
And hereof it may well be came the device of late, to lay egges in
some warme place and to make a soft fire underneath of small straw
or light chaffe to give a kinde of moderate heate ; but evermore the
egges must be turned with a mans or womans hand, both night and
day, and so at the set time they looked for chickens and had them"
(Philemon Holland's translation).
Pliny also says, "Over and besides there be some egs that will
come to be birds without sitting of the henne, even by the worke
of Nature onely, as a man may see the experience in the dunghills
of Egypt. There goeth a prettyjeast of a notable drunkard of Syracusa,
whose manner was when hee went into the Taverne to drinke to lay
certaine egges in the earth, and cover them with moulde, and he
would not rise nor give over bibbing untill they were hatched. To
conclude, a man or a woman may hatch egges with the very heate
onely of their body". This story occurs also in Aristotle.
The Emperor Hadrian — curiositatum omnium explorator as Tertullian
calls him — writing in a.d. 130 to his brother-in-law, L. Julius
Servianus, from Egypt, says, "I wish them no worse than that
they should feed on their own chickens, and how foully they hatch
them, I am ashamed to tell you". In the Description de VEgypte,
written by the members of the scientific staff of Napoleon's Egyptian
expedition, and published at Paris in 1809, Roziere and Rouyer
wrote on the artificial incubation of the Egyptians. They conjecture
very probably that the Emperor was shocked owing to a misunder-
standing shared by Aristotle, PUny, de Pauw and Reaumur, namely,
that the "gelleh" or dung was used to heat the eggs by its fermenta-
tion, and not, as is and was actually the case, by being slowly burnt
in the incubation ovens. Bay gave an account of the ovens in modern
times, but the best one is that of Lane. "The Egyptians", said Lane
in 1836, "have long been famous for hatching fowls' eggs by artificial
heat. This practice, though obscurely described by ancient authors,
appears to have become common in ancient Egypt from an early
PLATE 1
(A) EGYPTIAN PEASANT INCUBATOR
(from Cadman)
(B) CHINESE PEASANT INCUBATOR
(from King)
SECT, i] EMBRYOLOGY IN ANTIQUITY 47
time. In Upper Egypt there are over fifty establishments, and in
Lower Egypt more than a hundred. The furnace is constructed of
sun-dried bricks and consists of two parallel rows of small ovens and
cells for fire divided by a narrow vaulted passage, each oven being
about 9 or 10 feet long, 8 feet wide and 5 or 6 feet high, and having
above it a vaulted fire-cell of the same size but rather less in height.
The eggs are placed upon mats or straw, one tier above another
usually to the number of three tiers and the burning fuel is placed
upon the floors of the fire-cells above. The entrance of the furnace is
well closed. Each furnace consists of from twelve to 24 ovens and
receives about 150,000 eggs during the annual period of its con-
tinuing open, one quarter or one third of which generally fail. The
peasants supply the eggs and the attendants examine them and after-
wards generally give one chicken for every two eggs that they have
received. The general heat maintained during the process is from
100 to 103 of Fahrenheit's thermometer. The manager, having been
accustomed to the art from his youth, knows from long experience
the exact temperature that is required for the success of the operation
without having any instrument like our thermometer to guide him.
The eggs hatch after exactly the same period as in the case of natural
incubation. I have not found that the fowls produced in this manner
are inferior in point of flavour or in other respects to those produced
from the egg in the ordinary way." The accompanying picture
(Plate I a), taken from Cadman, shows the interior of a modern
peasant's incubator. There is reason to beheve that its construction
and operation vary very little, if at all, from that of the ovens used in
dynastic Egypt.
When Bay visited the native incubators in 191 2 he took with him
a flask of lime water and a thermometer. The former showed a large
precipitation of calcium carbonate and the latter stood at 40° C. He
was led to speculate on the value of a high CO2 tension in the at-
mosphere, and concluded that it must have a beneficial effect, since
the loss in the native incubator was not more than 4 per cent., while
that in the oil-heated agricultural incubators of his time was as
much as 40 per cent. Cadman, writing in 1921, suggests that the
well-known non-sitting instinct of Egyptian poultry is an effect of
the ancient practice of artificial incubation. But enough has been
said of the Egyptian "Ma'mal al katakeet", or chicken factory.
In spite of the remarkable opportunity thus afforded for acquiring
48 EMBRYOLOGY IN ANTIQUITY [pt. ii
facts in experimental embryology, no use was apparently ever made
of it, though there seems to be a certain amount of traditional
information current among the peasant operators, as, for example,
that the "ruh" or life enters into the egg at the eleventh day. It
w^ould be interesting to investigate this aspect of the subject further.
In ancient China also it would appear that artificial incubation
was successfully carried on in remote antiquity, if we may judge by
the account given by King. Native incubation in China is carried
on in wicker baskets, heated with charcoal pans (Plate Ib). The
attendants sleep in the incubator itself, and use the same thermometer
as the Egyptians, namely, their eyelids, to which they apply the blunt
end of the egg. The Egyptian success was known generally in the
West in later times though it could not be imitated. ' ' The Aegyptians ' ' ,
said Sir Thomas Browne, "observed a better way to hatch their
Eggs in Ovens, than the Babylonians to roast them at the bottom of
a sling, by swinging them round about, till heat from motion had
concocted them; for that confuseth all parts without any such effect."
Browne's slightly rueful tone suggests that he tried it himself. It is
interesting that this quaint experiment was the cause of a controversy
between Sarsi, who asserted on the authority of Suidas that it was
possible, and GaHleo, who thought the idea ridiculous. Modern work
on the instability of albumen solutions, such as that of Harris, lends
some colour to the legend. (See p. 275.)
Ancient Egypt supplies the starting-point for another and pro-
founder train of thought which recurs constantly throughout the
history of embryology, and to which I shall have to refer again more
than once. This was concerned with the problem of deciding at what
point the immortal constituent universally regarded as existing in
living beings took up its residence in the embryo. Some fragments
of ancient Indian philosophy assure us that the Vedic writers occupied
themselves with this question, and according to Crawley the Avesta
theorises upon it. But as early as 1400 B.C., i.e. during the eighteenth
dynasty in Egypt, something was said regarding this, for we have
extant at the present day a very beautiful hymn to the sun-god,
Aton, written by no less a person than Akhnaton (Nefer-kheperu-Ra
Ua-en-Ra, Amen-hetep Neter heq Uast), generally known as
Amenophis IV or the "heretic" king, who abandoned the traditional
worship of the Theban god Amen-Ra and established an Aton-cult,
as has been described by Baikie and others. One of his hymns, which
SECT, i] EMBRYOLOGY IN ANTIQUITY 49
bears considerable resemblance to the one hundred and fourth
psalm, runs as follows (in Breasted's translation) :
{the sun- god is addressed)
Creator of the germ in woman,
Maker of seed in man,
Giving life to the son in the body of his mother,
Soothing him that he may not weep,
Nurse (even) in the womb.
Giver of breath to animate every one that he maketh
When he cometh forth from the womb on the day of his birth.
Thou openest his mouth in speech.
Thou suppliest his necessity.
When the fledgling in the egg chirps in the shell
Thou givest him breath therein to preserve him alive.
When thou hast brought him together
To the point of bursting out of the egg,
He cometh forth from the egg
To chirp with all his might.
He goeth about upon his two feet
When he hath come forth therefrom.
The important point here is that life = soul. At this early period
there is no trace of the notions which appear later, such as the idea
that embryos are not aUve until the time of birth or hatching, or
the idea that the soul is breathed into the embryo at some particular
point in development. But in later times these considerations carried
great weight, and with the rise of theology a definite stand had to
be taken about them, for otherwise no ethical status could be assigned
to abortion. Speculation on these matters has continued without
cessation since the time of Akhnaton, reaching a climax perhaps in
Christian times with Cangiamilla's Embryologia Sacra, and living on
embedded in Roman Catholic theology up to our own era. In the
last century the subject seems to have had a special fascination for
Ernst Haeckel, who frequently mentioned it. But the future holds no
place for the discussion of such themes, and what has been called
"theological embryology" is already dead, though we may perhaps
descry its successor, psychological embryology, in such researches as
those of Teuscher, Cesana, y Gonzalez, Swenson and Coghill.
50 EMBRYOLOGY IN ANTIQUITY [pt. ii
Ancient Greek thought shows many evidences of appreciation of
the mystery of embryonic growth, as for example
in the Orphic cosmogonies, which had their origin
about the seventh or eighth century b.g. In these
rehgious and legendary descriptions of the world,
which have been exhaustively discussed by A. B.
Cook and F. Lukas, the cosmic egg plays a large
part, and has been shown to occur also in the . ^ , , .
. • f T^ T 1- -r. • 1 Fig- 2. Eros hatching
ancient cosmogonies ot Lgypt, India, Persia and from the cosmic egg.
Phoenicia. A familiar reference to this cosmic (A Hellenistic gem de-
egg, out of which all things were produced at ^^^^ ^ y . . oo .j
the beginning of the world, is in Aristophanes' comedy. The Birds,
where the owl, as leader of the Chorus, says in the Parabasis
(J. T. Sheppard's translation) :
Chaos was first, and Night, and the darkness of Emptiness, gloom
tartarean, vast ;
Earth was not, nor Heaven, nor Air, but only the bosom of Darkness ;
and there with a stirring at last
Of wings, though the wings were of darkness too, black Night was inspired
a wind-egg to lay.
And from that, with the turn of the seasons, there sprang to the light
the Desired,
Love, and his wings were of gold, and his spirit as swift as the wind when
it blows every way.
Love moved in the Emptiness vast, Love mingled with Chaos, in spite of
the darkness of Night,
Engendering us, and he brought us at last to the light.
And perhaps another reference to the place of the egg in ancient
cosmogony occurs in The Arabian Nights, where Aladdin's request
for a roc's egg is treated as a blasphemy by the genie. Still more
fantastic is the speculation of C. H. Rice (in Psyche, 1929!) that the
world is an egg; living matter being the embryo and inorganic
matter the yolk. But none of the facts which have so far been
mentioned bears more than obliquely upon the main centre of
interest, the study of embryology. For its direct ancestry, Greece, as
might be expected, is responsible.
1-2. Hellenic Antiquity: the Pre-Socratics
The pre-Socratic philosophers nearly all seem to have had opinions
upon embryological phenomena, many of which are worth referring
SECT. I] EMBRYOLOGY IN ANTIQUITY 51
to. These investigators of nature who Hved in Greece from the eighth
century onwards are only known to us through the writings of
others, or in some cases in the form of fragments, for all their
complete books have perished. Diels' collection of the Fragmente der
Vorsokratiker is the most convenient source for what is left, but the
assembling of their opinions has not been left to modern times, for
a collection of them occurs in the writings of Plutarch of Chaeronea^
(3rd century a.d.). It is necessary to make use of some caution
in describing their views, for Aristotle, as an instance, frequently
gives the most unfair versions of the views of his predecessors. The
account which follows is based upon Plutarch, in Philemon Holland's
translation, and Diels. Empedocles of Akragas, who lived about
444 B.C., believed that "the embryo derives its composition out of
vessels that are four in number, two veins and two arteries, through
which blood is brought to the embryo". He also held that the sinews
are formed from a mixture of equal parts of earth and air, that
the nails are water congealed, and that the bones are formed from
a mixture of equal parts of water and earth. Sweat and tears, on
the other hand, are made up of four parts of fire to one of water.
Empedocles also had opinions about the origin of monsters and twins,
and asserted that the influence of the maternal imagination upon the
embryo was great so that its formation could be guided and interfered
with. "Empedocles", says Plutarch, "saith that men begin to take
forme after the thirtie-first day and are finished and knit in their
parts within 50 dales wanting one. Asclepiades saith that the members
of males because they are more hot are joynted and receive shape in
the space of 26 dales, and many of them sooner, but are finished and
complet in all limbes within 50 dales but females require two moneths
ere they be fashioned, and fower before they come to their perfection,
for that they want naturall heat. As for the parts of unreasonable
creatures they come to their accomplishment sooner or later, ac-
cording to the temperature of their elements." Empedocles did not
consider that an embryo was fully alive. "Empedocles ", says Plutarch,
"denieth it to be a creature animall, howbeit that it hath life and
breath within the bellie, mary the first time that it hath respiration
is at the birth, namely, when the superfluous humiditie which is in
such unborne fruits is retired and gone, so that the aire from without
entreth into the void vessels lying open."
^ It is now certain that this collection is not by Plutarch himself but by an earlier
compiler, Aetius (see Burnet).
4-2
52 EMBRYOLOGY IN ANTIQUITY [pt. ii
Anaxagoras of Clazomenae (500-428 B.C.) may have said that the
milk of mammals corresponded to the white of the fowl's egg, but
that observation is also attributed to Alcmaeon of Croton. It is more
certain that he spoke of a fire inside the embryo which set the parts
in order as it developed, and that the head was the part to be formed
first in development. This thesis was supported by Alcmaeon, and
by Hippon of Samos, a Pythagorean, in the fifth century B.C., but
Diogenes of Apollonia maintained about the same time that a mass
of flesh was formed first, and afterwards the bones and nerves were
differentiated. Plutarch remarks about this: "Alcmaeon affirmeth
that the head is first made as being the seat of reason. Physicians
will have the heart to be the first, wherein the veines and arteries are.
Some thinke the great toe is framed first, others the navill".
The other contributions of Diogenes to this primitive embryology,
were the view that the placenta is the organ of foetal nutrition, and
the view that the male embryo was formed in four months but the
female embryo not till five months had elapsed — a notion also found
in Asclepiades and Empedocles, as we have seen. He also associated
heat with the generation of little animals out of slime, and compared
this with the heat of the uterus. He agreed with Empedocles that
the embryo was not alive. "Diogenes saith that infants are bred within
the matrice inanimate, howbeit in heat, whereupon it commeth that
naturall heat, so soon as ever the infant is turned out of the mother's
wombe, is drawen into the lungs." But the principal pre-Socratic
embryologist was, as Zeller points out, Alcmaeon of Croton, who
lived in the sixth century B.C., a disciple of Pythagoras, though ap-
parently an independent one. He is said to have been the first man
to make dissections. The fragments of Alcmaeon (who is not to be
confused with Alcman, the Lacedaemonian poet) have been col-
lected together by Wachtler; the most important are xviii and xix.
Athenaeus in the Deipnosophists says, in his usual chatty way, "Now
with respect to eggs Anaxagoras in his book on natural philosophy
says that what is called the milk of the bird is the white which is in
the eggs". This may be a wrong ascription; it may refer to Alcmaeon,
for Aristotle says in his book on the generation of animals, "Nature
not only places the material of the creature in the egg but also the
nourishment sufficient for its growth, for since the mother-bird cannot
protect the young within herself she produces the nourishment in the
egg along with it. Whereas the nourishment which is called milk
SECT, i] EMBRYOLOGY IN ANTIQUITY 53
is produced for the young of vivipara in another part, in the breasts,
Nature does this for birds in the egg. The opposite, however, is the
case to what people think and what is asserted by Alcmaeoii of
Croton. For it is not the white that is the milk, but the yolk, for it
is this that is the nourishment of the chick, whereas they think it is
the white because of the similarity of the colour". Whether Aristotle
was led to this conclusion because of his erroneous ideas about the
part played in foetal nutrition by yolk and white respectively or
whether he recognised a similarity between yolk and milk on account
of their fatty nature, we cannot tell. In any case, his correction of
Alcmaeon was in the right direction, and it is interesting to compare
the amino-acid distribution in the casein of milk and the vitellin of
yolk, as has been done by Abderhalden & Hunter (see p. 261).
Parmenides asserted a connection between male embryos and the
right side of the body and between female embryos and the left side
of the body — an idea which, considering its total lack of foundation,
has had a very long lease of life in the world of thought. There was
much controversy on the question of how foetal nutrition went on ;
the atomists, Democritus (born about 460 b.c.) and Epicurus (born
about 342 B.C.), said that the embryo ate and drank/>^r 0^. "Democritus
and Epicurus hold", says Plutarch, "that this unperfect fruit of the
wombe receiveth nourishment at the mouth; and thereupon it com-
meth that so soon as ever it is borne it seeketh and nuzzeleth with the
mouth for the brest head or nipple of the pappe : for that within the
matrice there be certain teats; yea, and mouths too, whereby they
may be nourished. But Alcmaeon affirmeth that the infant within
the mother's wombe, feedeth by the whole body throughout for that
it sucketh to it and draweth in maner of a spunge, of all the food,
that which is good for nourishment." It would appear also that
Democritus believed the external form of the embryo to be developed
before the internal organs were formed.
1-3. Hippocrates: the Beginning of Observation
But the foregoing fragments of speculation do not really amount
to much. The first detailed and clear-cut body of embryological
knowledge is associated with the name of Hippocrates, of whom
nothing certain is known save that he was born probably in the
forty-fifth Olympiad, about 460 b.c, that he lived on the island of
Cos in the Aegean Sea, and that he acquired greater fame as a
54 EMBRYOLOGY IN ANTIQUITY [pt. ii
physician than any of his predecessors, if we may except the legendary
names of Aesculapius, Machaon and Podalirius. It has not been
believed for many centuries past that all the writings in the collection
of Hippocratic books were actually set down by him, and much
discussion has taken place about the authenticity of individual
documents.
Most of the embryological information is contained in a section
which in other respects (style, etc.) shows homogeneity. We are there-
fore rather interested in that unknown biological thinker who wrote
the books in this class, for he could with considerable justice be
referred to as the first embryologist. Littre discusses his identity,
but there is no good evidence for any of the theories about it, though
perhaps the most likely one is that he was Polybus, the son-in-law of
Hippocrates. That the writings on generation are only slightly later
than the time of Hippocrates is more or less clear from the fact that
Bacchius knew of them, and actually mentions them.
For the most part the embryological knowledge of Hippocrates is
concerned with obstetrical and gynaecological problems. Thus in the
Aphorisms, d(f)opicr/iioi, the books on epidemics, eirchrifxiai, the treatise
on the nature of women, irepl rywaLKelr)'? (f)V(rio'?, the discussions of pre-
mature birth, Trepl eirraixrjvov, the books on the diseases of women,
irepl 'yvvaixeiaiv, and the pamphlet on superfoetation, there are many
facts recorded about the embryo, but all with obstetrical reference.
There are some curious notions to be found there, such as the asso-
ciation of right and left breasts with twin embryos and a prognostic
dependent on this.
But the three books which are most important in the history of
embryology are the treatise on Regimen, irepl StaLTr}<;, the work on
generation, irepl jovr]<;, and the book about the nature of the infant,
Trepl ^vaio<; TraiZiov. The two latter really form one continuous
discussion, and it is not at all clear how they came to be split up
into separate books. In the Regimen the writer expounds his funda-
mental physiological ideas, involving the two main constituents of all
natural bodies, fire and water. Each of these is made up of three
primary natures, only separable in thought and never found isolated,
heat, dryness and moisture, and each of them has the power of
attracting, eXKeiv, their like, an important feature of the system. Life
consists in moisture being dried up by fire and fire being wetted by
moisture alternately, rpo^-i'i, the nourishment (moisture) coming into
SECT, i] EMBRYOLOGY IN ANTIQUITY 55
the body, is consumed by the fire so that fresh rpocfir] is in its turn
required.
It is important to note that the Hippocratic school was far more
akin in its general attitude to living things to modern physiology
than the Aristotelian and Galenic physiology. For no considerations
of final causes complicate the causal explanations of the Hippocratic
school, and the author of the irepl SmtV?/? indeed devotes seven chapters
to a detailed comparison of the processes of the body {a) with the
processes of the inorganic world both celestial and terrestrial, and
(b) with the processes used by men in the arts and crafts, such as
iron-workers, cobblers, carpenters and confectioners. These dis-
cussions present distinct mechanistic features.
He then in Section 9 sets forth his theory of the formation of the
embryo. "Whatever may be the sex", he says, "which chance gives
to the embryo, it is set in motion, being humid, by fire, and thus it
extracts its nourishment from the food and breath introduced into
the mother. First of all this attraction is the same throughout because
the body is porous but by the motion and the fire it dries up and
solidifies — vtto Be r?)? Kivijcno^; Koi tov irvpoii ^rfpaiveTai koI arepeovraL
— as it solidifies, a dense outer crust is formed, and then the fire inside
cannot any more draw in sufficient nourishment and does not expel the
air because of the density of the surrounding surface. It therefore con-
sumes the interior humidity. In this way parts naturally solid being up
to a point hard and dry are not consumed to feed the fire but fortify
and condense themselves the more the humidity disappears — these
are called bones and nerves. The fire burns up the mixed humidity
and forwards development towards the natural disposition of the
body in this manner ; through the solid and dry parts it cannot make
permanent channels but it can do so through the soft wet parts, for
these are all nourishment to it. There is also in these parts a certain
dryness which the fire does not consume, and they become compacted
one to another. Therefore the most interior fire, being closed round
on all sides, becomes the most abundant and makes the most canals
for itself (for that was the wettest part) and this is called the belly.
Issuing out from thence, and finding no nourishment outside, it
makes the air pipes and those for conducting and distributing food.
As for the enclosed fire, it makes three circulations in the body and
what were the most humid parts become the venae cavae. In the
intermediate part the remainder of the water contracts and hardens
56 EMBRYOLOGY IN ANTIQUITY [pt. ii
forming the flesh." In this account of the formation of the embryo,
which seems at first sight a Httle fantastic, there are several interesting
things to be remarked. Firstly, there is to be noted throughout it a
remarkable attempt at causal explanations and not simply morpho-
logical description. The Hippocratic writer is out to explain the
development of the embryo from the very beginning on machine-like
principles, no doubt unduly simplified, but related directly to the
observed properties of fire and water. In this way he is the spiritual
ancestor of Gassendi and Descartes. The second point of interest is
that he speaks of the embryo drying up during its development, a
piece of observation which anyone could make by comparing a
fourth-day chick with a fourteenth-day one, and which we express
to-day in graphical form (see Fig. 220). Thirdly, the ascription of the
main driving force in development to fire has doubtless no direct
relation to John Mayow's discovery, two thousand years later, that
there is a similarity between a burning candle and a living mouse
each in its bell-jar, and may mean as much or as little as Sir Thomas
Browne's remark, "Life is a pure flame, and we live by an invisible
sun within us". Yet the essential chemical aspect of living matter
is oxidation, and the development of the embryo no less than the
life of the adult is subject to this rule, so that what may have been
a mere guess on the part of the Hippocratic writer, may also have
been a flash of insight due to the simple observation which, after all,
it was always possible to make, namely, that both fires and li\dng
things could be easily stifled.
Preformationism is perhaps foreshadowed in Section 26 of the
same treatise. "Everything in the embryo is formed simultaneously.
All the limbs separate themselves at the same time and so grow, none
comes before or after other, but those which are naturally bigger
appear before the smaller, without being formed earlier. Not all
embryos form themselves in an equal time but some earlier and some
later according to whether they meet with fire and food, some have
everything visible in 40 days, others in 2 months, 3, or 4. They
also become visible at variable times and show themselves to the
light having the blend (of fire and water) which they always will
have."
The work on Generation is equally interesting. The earlier sections
deal with the differences between the male and the female seed, and
the latter is identified with the vaginal secretion. Purely embryological
SECT. I] EMBRYOLOGY IN ANTIQUITY 57
discussion begins at Section 14, where it is stated that the embryo is
nourished by maternal blood, which flows to the foetus and there
coagulates, forming the embryonic flesh. The proof alleged for this is
that during pregnancy the flow of menstrual blood ceases; therefore
it must be used up on the way out. In Section 15 the umbiHcal cord
is recognised as the means by which foetal respiration is carried on.
Section 1 7 contains a fine description of development with a very
interesting analogy. "The flesh", it is said, "brought together by the
spirit, TO TTvevfia, grows and divides itself into members, hke going to
like, dense to dense, flabby to flabby, humid to humid. The bones
harden, coagulated by the heat." Then a demonstration experiment
follows : "Attach a tube to an earthen vessel, introduce through it some
earth, sand, and lead chips, then pour in some water and blow through
the tube. First of all, everything will be mixed up, but after a certain
time the lead will go to the lead, the sand to the sand, and the earth
to the earth, and if the water be allowed to dry up and the vessel
be broken, it will be seen that this is so. In the same way seed and
flesh articulate themselves. I shall say no more on this point".
Here again was an attempt at causal explanation, rather than
morphological description, in complete contrast to the later work of
Aristotle.
Section 22 contains a suggestive comparison between seeds of
plants and embryos of animals, but the identification of stalk with
umbihcal cord leads to a certain confusion. Perhaps the most inter-
esting passage of aU is to be found in Section 29. "Now I shall speak",
says the unknown Hippocratic embryologist, "of the characters
which I promised above to discuss and which show as clearly as
human intelligence can to anyone who will examine these things
that the seed is in a membrane, that the umbilicus occupies the
middle of it, that it alternately draws the air through itself and then
expels it, and that the members are attached to the umbilicus. In a
word, all the constitution of the foetus as I have described it to you,
you will find from one end to the other if you wiU use the following
proof Take 20 eggs or more and give them to 2 or 3 hens to incubate,
then each day from the second onwards tiU the time of hatching,
take out an egg, break it, and examine it. You will find everything
as I say in so far as a bird can resemble a man. He who has not
made these observations before will be amazed to find an umbihcus
in a bird's egg. But these things are so, and this is what I intended
58 EMBRYOLOGY IN ANTIQUITY [pt. ii
to say about them." We see here as clearly as possible the beginnings
of systematic embryological knowledge, and from this point onwards,
through Aristotle, Leonardo, Harvey and von Baer, to the current
number of the Archivf. Entwicklungsmechanik, the line runs as straight
as Watling Street.
In Section 30 there is an important passage in which the author
discusses the phenomena of birth. "I say", he says, "that it is the
lack of food which leads to birth, unless any violence has been done;
the proof of which is this ; — the bird is formed thus from the yolk
of the egg, the egg gets hot under the sitting hen and that which is
inside is put into movement. Heated, that which is inside begins to
have breath and draws by counter-attraction another cold breath
coming from the outside air and traversing the egg, for the egg is
soft enough to allow a sufficient quantity of respiration to penetrate
to the contents. The bird grows inside the egg and articulates itself
exactly like the child, as I have previously described. It comes from
the yolk but it has its food from, and its growth in, the white. To
convince oneself of this it is only necessary to observe it attentively.
When there is no more food for the young one in the egg and it has
nothing on which to live, it makes violent movements, searches for
food, and breaks the membranes. The mother, perceiving that the
embryo is vigorously moving, smashes the shell. This occurs after
20 days. It is evident that this is how things happen, for when the
mother breaks the shell there is only an insignificant quantity of
liquid in it. All has been consumed by the foetus. In just the same
way, when the child has grown big and the mother cannot continue
to provide him with enough nourishment, he becomes agitated,
breaks through the membranes and incontinently passes out into the
external world free from any bonds. In the same way among beasts
and savage animals birth occurs at a time fixed for each species
without overshooting it, for necessarily in each case there must be
a point at which intra-uterine nourishment will become inadequate.
Those which have least food for the foetus come quickest to birth
and vice versa. That is all that I had to say upon this subject."
The theory underlying this passage evidently is that the main food
of the fowl embryo is the white and that the yolk is there purely for
constructional purposes. Had the author not been strongly attached
to this erroneous view he could not have failed to notice the un-
absorbed yolk-sac which still protrudes from the abdomen of the
SECT. I] EMBRYOLOGY IN ANTIQUITY 59
hatching chick, and if he had given this fact a little more prominence
he could hardly have come to enunciate the general theory of
birth which appears in the above passage. Moreover, had he been
acquainted with the circulation of the maternal and foetal blood in
viviparous animals, he could hardly have held that there was less
food in a given amount of maternal blood at the end of development
than at the beginning. At any rate, his attempted theory of birth
was a worthy piece of scientific effort, and we cannot at the present
moment be said to understand fully the principles governing incuba-
tion time (see p. 470).
The treatises on food and on flesh, trepl Tpo(f>rj<i and irepl aapKcovy
are both late additions to the Hippocratic corpus, but contain points
of embryological interest. Section 30 of the former contains some
remarks on embryonic respiration, and Section 3 of the latter has a
theory of formation of nerves, bones, etc. by difference of composition
of glutinous substances, fats, water, etc. Section 6 supports the view
that the embryo is nourished in utero by sucking blood from the
placenta, and the proof given is that its intestine contains the
meconium at birth. Moreover, it is argued, if this were not so, how
could the embryo know how to suck after it is born?
1-4. Aristotle
After the Hippocratic writings nothing is of importance for our
subject till Aristotle. It is true that in the Timaeus Plato deals with
natural phenomena, eclectically adopting opinions from many pre-
vious writers and welding them into a not very harmonious or logical
whole. But he has hardly any observations about the development
of the embryo. The four elements, earth, fire, air, and water, are,
according to him, all bodies and therefore have plane surfaces which
are composed of triangles. Applying this semi-atomistic hypothesis
to the growth of the young animal, he says, "The frame of the entire
creature when young has the triangles of each kind new and may
be compared to the keel of a vessel that is just ofT the stocks ;^ they
are locked firmly together and yet the whole mass is soft and delicate,
being freshly formed of marrow and nurtured on milk. Now when
the triangles out of which meats and drinks are composed come in
from without, and are comprehended in the body, being older and
weaker than the triangles already there, the frame of the body gets
the better of them and its newer triangles cut them up and so the
6o EMBRYOLOGY IN ANTIQUITY [pt. ii
animal grows great, being nourished by a multitude of similar
particles." This is as near as Plato gets to embryological speculation.
His description has a causal ring about it, which is in some contrast
with the predominantly teleological tone of the rest of his writings ;
for instance, only a few pages earlier he has been speaking of the
hair as having been arranged by God as "a shade in summer and a
shelter in winter". It is also true that Plato may have said more
about the embryo than appears in the dialogues. Plutarch mentions
various speculations about sterility, and adds, "Plato directly pro-
nounceth that the foetus is a living creature, for that it moveth and
is fed within the bellie of the mother".
But all this was only the slightest prelude to the work of Plato's
pupil, Aristotle. Aristotle's main embryological book was that
entitled Trepl ^mcov yeveaeco'i, On the Generation of Animals, but embryo-
logical data appear in irepl ^axop, The History of Animals, irepl ^wmv
fjLopicov, On the Parts of Animals, Trepl dva7rvofj<i, On Respiration, and
Trepl ^Mcov Ktvrjcr€(o<i, On the Motion of Animals. All these were written
in the last three-quarters of the fourth century B.C.
With Aristotle, general or comparative biology came into its own.
That almost inexhaustible profusion of living shapes which had not
attracted the attention of the earlier Ionian and Italo-Sicilian philo-
sophers, which had been passed over silently by Socrates and Plato,
intent as ever upon ethical problems, but which had been for cen-
turies the inspiration of the vase-painters and other craftsmen
{(^coypdcjioL), was now for the first time exhaustively studied and
reduced to some sort of order. The Hippocratic school with their
"Coan classification of animals", which Burckhardt has discussed,
had indeed made a beginning, but no more. It was Aristotle who
was the first curator of the animal world, and this comparative out-
look colours his embryology, giving it, on the whole, a morphological
rather than a physiological character.
The question of Aristotle's practical achievements in embryology
is interesting, and has been discussed by Ogle. There is no doubt
that he diligently followed the advice of the author of the Hippocratic
treatise on generation and opened fowl's eggs at different stages
during their development, but he learnt much more than the un-
known Hippocratic embryologist did from them. It is also clear
that he dissected and examined all kinds of animal embryos, mam-
malian and cold-blooded. The uncertain point is whether he also
SECT, i] EMBRYOLOGY IN ANTIQUITY 6i
dissected the human embryo. He refers in one place to an "aborted
embryo", and as he was able to obtain easily all kinds of animal
embryos without waiting for a case of abortion, it is likely that this
was a human embryo. Ogle brings forward six or seven passages
which all contain statements about human anatomy and physiology
only to be explained on the assumption that he got his information
from the foetus. So it is probable that his knowledge of biology was
extended to man in this way, as would hardly have been the case if
he had lived in later times, when the theologians of the Christian
Church had come to very definite conclusions about the sanctity of
foetal as well as adult life.
The Trept i^wcov ^eveaeoo<;^ the first great compendium of embryology
ever written, is not a very well-arranged work. There are a multitude
of repetitions, and the order is haphazard, so that long digressions
from the main argument are common. The work is divided into
five books, of which the second is much the most important in the
history of embryology, though the first has also great interest, and
the third, fourth, and fifth contain much embryological matter mixed
up among points of generation and sexual physiology.
Book I begins with an introduction in which the relative significance
of efficient and final causes is considered, and chapters i to 7 deal
with the nature of maleness and femaleness, the nature and origin
of semen, the manner of copulation in different animals and the
forms of penis and testes found in them. Chapter 8 continues this,
and describes the different forms of uterus in different animals, speaks
of viviparity and oviparity, mentions the viviparous fishes (the
selachians) and draws a distinction between perfect and imperfect
eggs. Chapter 9 discusses the cetacea; 10, eggs in general; and 11
returns to the differences between uteri. In chapter 12 the question
is raised why all uteri are internal, and why all testes are not, and
in chapter 13 the relations between the urinary and the genital
systems are discussed. Copulation now receives attention again, in
14 with regard to Crustacea, in 15 with regard to cephalopoda, and
in 16 with regard to insecta. After this point the argument Hfts
itself on to a more theoretical plane, and opens the question of
pangenesis, into which it enters at length during the course of
chapters 17 and 18, refuting eventually the widely-held view that
the semen takes its origin from all the parts of the body so as to be
able to reproduce in the offspring the characteristics of the parent.
62 EMBRYOLOGY IN ANTIQUITY [pt. ii
The nature of semen receives a long discussion; it is decided at last
that it is a true secretion, and not a homogeneous natural part (a
tissue) nor a heterogeneous natural part (an organ) nor an unnatural
part such as a growth, nor mere nutriment, nor yet a waste product.
It is here that the theory is put forward that the semen supplies the
"form" to the embryo and whatever the female produces supplies
the matter fit for shaping. The obvious question has next to be
answered, what is it that the female supplies? Aristotle concludes
in chapters 19 and 20 that the female does not produce any semen,
as earlier philosophers had held, but that the menstrual blood is the
material from which the seminal fluid, in giving to it a form, will
cause the complete embryo to be produced. This was not a new idea,
but had already been suggested by the author of the Hippocratic
ire pi yovi]^. What was quite new here, was the idea that the semen
supplied or determined nothing but the form. Chapters 21 and 22
are rather confused ; they contain more arguments against pangenesis,
and considerations upon the contrast between the active nature of
the male and the passive nature of the female. Chapter 23, which
closes the first book, compares animals to divided plants, for plants
in Aristotle's view fertilise themselves.
Book II opens with a magnificent chapter on the embryological
classification of animals, showing Aristotle, the systematist, at his
best — his classification is reproduced in Chart I. But the chapter
also includes a brilliant discussion of epigenesis or preformation,
fresh development or simple unfolding of pre-existent structures,
an antithesis which Aristotle was the first to perceive, and the sub-
sequent history of which is almost synonymous with the history of
embryology. The question in its acutest form was not settled until
the eighteenth century, but since then it has become clear that
there were elements of truth in the opinion which was the less true
of the two. Chapter 2 is not so important, though it has some
interesting chemical analogies; it compares semen to a foam, and
suggests that it was this foam, like that of the sea, which gave
birth to the goddess Aphrodite^. But chapter 3 returns to the
high level of speculation and thought found in the opening part of
the book, for it deals with the degree of aliveness which the embryo
has during its passage through its developmental stages. Aristotle
^ To the Greeks all natural foams possessed a generative virtue, and a Zeus Aphrios
was worshipped at Pherae in Thrace.
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64 EMBRYOLOGY IN ANTIQUITY [pt. ii
does not here anticipate the form of the recapitulation theory, but
he certainly suggests the essence of it in perfectly clear terms. This
chapter has also an interest for the history of theological embryology,
for its description of the entry of the various souls into the embryo
was afterwards made the basis for the legal rulings concerning
abortion. This chapter also discusses embryogeny as a whole, as does
the succeeding one. Chapter 5 is a digression into the problem
of why fertilisation is necessary by the male, but it has also some
curious speculations as to what extent the hen's egg is alive, if it is
infertile. The main thread is resumed in chapters 6 and 7, two very
fine ones, in which embryogeny and foetal nutrition are thoroughly
dealt with, but dropped again in the last section, chapter 8, which
is devoted to an explanation of sterility. This ends the second book.
The third book is chiefly concerned with the application of the
general embryological principles described in the previous book to
the comparative field, and the fourth book contains a collection of
minor items which Aristotle has not been able to speak of before.
But if the work as a whole tails off in a rather unsatisfactory
manner, its merits are such that this hardly matters. The extra-
ordinary thing is that, building on nothing but the scraps of specula-
tion that had been made by the Ionian philosophers, and the
exiguous data of the Hippocratic school, Aristotle should have pro-
duced, apparently without effort, a text-book of embryology of
essentially the same type as Graham Kerr's or Balfour's. It is even
very possible that Aristotle was unacquainted with any of the Coan
school, for, though he often mentions Democritus, Anaxagoras,
Empedocles and even Polybus, yet he never once quotes Hippo-
crates, and this is especially odd, for Aristotle is known to have
collected a large library. Probably Hippocrates was only known to
Aristotle as an eminent medical man; if this is so, Aristotle's achieve-
ments are still more wonderful.
The depth of Aristotle's insight into the generation of animals has
not been surpassed by any subsequent embryologist, and, con-
sidering the width of his other interests, cannot have been equalled.
At the same time, his achievements must not be over-estimated.
Charles Darwin's praise of him in his letter to Ogle (which is too
well known to quote) is not without all reservations true. There is
something to be said for Lewes as well as Piatt. Aristotle's con-
clusions were sometimes not warranted by the facts at his disposal,
SECT, i] EMBRYOLOGY IN ANTIQUITY 65
and some of his observations were quite incorrect. Moreover, he
stood at the very entrance into an entirely unworked field of know-
ledge ; he had only to examine, as it were, every animal that he could
find, and set down the results of his work, for nobody had ever done
it before. It was like the great days of nineteenth-century physiology,
when, as the saying was, "a chance cut with a scalpel might reveal
something of the first importance".
As has already been said, Aristotle regarded the menstrual blood
as the material out of which the embryo was made. "That, then, the
female does not contribute semen to generation", says Aristotle, "but
does contribute something, and that this is the matter of the cata-
menia, or that which is analogous to it in bloodless animals, is clear
from what has been said, and also from a general and abstract survey
of the question. For there must needs be that which generates and
that from which it generates, even if these be one, still they must
be distinct in form and their essence must be different; and in those
animals that have these powers separate in two sexes the body and
nature of the active and passive sex also differ. If, then, the male
stands for the effective and active, and the female, considered as
female, for the passive, it follows that what the female would con-
tribute to the semen of the male would not be semen but material
for the semen to work upon. This is just what we find to be the case,
for the catamenia have in their nature an affinity to the primitive
matter." Thus the male dynamic element {t6 appev iroLn^TtKov) gives a
shape to the plastic female element {to OrjXv TradrjTiKov). Aristotle
was right to the extent that the menstrual flow is associated with
ovulation, but as he knew nothing of the mammalian ovum, and
indeed, as is shown in his embryological classification, expressly denied
that there was such a thing, his main menstruation theory is wrong.
Yet it was not an illegitimate deduction from the facts before him.
These views of Aristotle's about the contribution of the female to
the embryo are in striking contrast with certain conceptions of a
century before which were probably generally held in Greece. There
is a most interesting passage relating to them in the Eumenides of
Aeschylus, when, during the trial scene, Apollo, defending Orestes
from the charge of matricide, brings forward a physiological argu-
ment. "The mother of what is called her child", Apollo is made to
say, "is no parent of it, but nurse only of the young Hfe that is sown
in her (jpo(f)6<; 8e Kv^iajo<i veoairopov). The parent is the male, and
66 EMBRYOLOGY IN ANTIQUITY [pt. ii
she but a stranger, a friend, who, if fate spares his plant, preserves it
till it puts forth."
There is evidence that this doctrine was of Egyptian origin, for
Diodorus Siculus says, "The Egyptians hold the father alone to be
the author of generation, and the mother only to provide a nidus
and nourishment for the foetus". Whether this was so or not, the
influence of such a doctrine must have been tremendous. We know
that the conception of the female sex as playing the part of farm-land,
i.e. of woman as a field in which grain was sown, was widespread
in antiquity; Hartland and S. A. Cook have collected examples of
it from Vedic, Egyptian and Talmudic sources. A late echo of it is
to be found in Lucretius, where he refers to "Venus sowing the field
of woman" —
atque in eost Venus ut muliebria conserat arva.
Nor would resemblances between mothers and children suffice to
kill this belief, for plants may diflfer slightly according to the soil
in which they are planted. Such an idea would be the natural
foundation for the practice — also widespread in antiquity — of putting
captured males to death, and retaining the females as concubines.
On such a theory no fear would be entertained of corrupting the
race with alien blood in this way. The whole matter affords an
excellent illustration of the way in which an apparently academic
theory may have the most far-reaching effects on social and political
events, and Aristotle, far from being remote from practical affairs
as he examined his viviparous fishes and made marginal notes on
his copy of Empedocles, is seen to be labouring at their very root.
The embryo, then, took its origin from the menstrual blood, on
which, and in which, the seminal essence operated to produce it.
But the perplexing question of the order of formation of the parts
remained unsettled, though it had already been opened by earlier
thinkers. What they had not done was to put the question, as it
were, into the form of a motion ; they had not grasped the existence
of two main alternatives, one of which would have to be chosen
before any further progress could be made. This is just what Aristotle
did. "There is considerable difficulty", says he, "in understanding
how the plant is formed out of the seed or any animal out of the
semen. Everything that comes into being or is made must (i) be
made out of something, (2) be made by the agency of something,
SECT, i] EMBRYOLOGY IN ANTIQUITY 67
and (3) must become something. Now that out of which it is made
is the material; this some animals have in its first form within them-
selves, taking it from the female parent, as all those which are not
born aHve but produced as a scolex or egg; others receive it for a
long time from the mother by sucking, as the young of all those
which are not only externally but also internally viviparous. Such
is the material out of which things come into being, but we now
are enquiring not out of what the parts of an animal are made, but
by what agency. Either it is something external which makes them
or else it is something existing in the seminal fluid and the semen;
and this must either be soul or a part of a soul, or something con-
taining soul." Aristotle concludes that there is no external shaping
influence, but only something or other contained in the embryo itself.
To this extent he was wrong, for the influence of the proper physico-
chemical environment on the growing embryo is as important as its
physico-chemical internal constitution (later he modified his views
on this). But now he goes on to deal with the main question, and
says, "How then does it (the shaping influence) make the other parts?
All the parts, as heart, lung, liver, eye, and all the rest, come into
being either together or in succession, as is said in the verse ascribed
to Orpheus, for there he says that an animal comes into being in
the same way as the knitting of a net. That the former is not the
fact is plain even to the senses, for some of the parts are clearly
visible as already existing in the embryo wliile others are not; that
it is not because of their being too smaU that they are not visible is
clear, for the lung is of greater size than the heart, and yet appears
later than the heart in the original development". This passage
demonstrates that Aristotle had opened hen's eggs at different stages,
and was well acquainted with the appearances presented there as
early as the third day. He goes on to set forth a further alternative.
Agreeing that a continuously new formation of parts takes place, and
not merely an unfolding of parts already present in the semen or the
menstrual blood, is this brought about by a chain of creations or by
one original creation? In other words, does the heart come into being
first, and then proceed to form the liver, and then the liver go on
to form the lungs, or do they simply appear one after the other without
such a creative inter-relationship? Aristotle argues against the former
view on the ground that if one organ formed another, the second
one would have to resemble the first in some way, which is not the
5-2
68 EMBRYOLOGY IN ANTIQUITY [pt. ii
case. His words on this subject cannot be condensed. "But neither
can the (formative) agent be external and yet it must needs be one
or other of the two. We must try then to solve this difficulty, for
perhaps some one of the statements made (already) cannot be made
without qualification, e.g. the statement that the parts cannot be
made by what is external to the semen. For if in a certain sense they
cannot, yet in another sense they can." (Thus Aristotle does some
justice to the environment.) "It is possible, then, that A should
miove B and B should move C, that, in fact, the case should be the
same as with the automatic machines shown as curiosities. For the
parts of such machines while at rest have a sort of potentiality of
motion in them, and when any external force puts the first of them
into motion, immediately the next is moved in actuality. As, then,
in these automatic machines the external force moves the parts in
a certain sense (not by touching any part at the moment but by
having touched one previously) , in like manner also that from which
the semen comes or in other words that which made the semen, sets
up the movement in the embryo and makes the parts of it by having
touched first something though not continuing to touch it. In a way
it is the innate motion that does this, as the act of building builds
a house. Plainly, then, while there is something which makes the
parts, this does not exist as a definite object, nor does it exist in the
semen at the first as a complete part." This notion of the setting
in motion of a wound-up clock is substantially modern and underlies
the physico-chemical analysis of the developing embryo. It is really
striking to find Aristotle using the machine analogy in order to explain
himself, for he, of all biologists, emphasised the final cause in natural
operations. However, he soon returns to a more vitalistic attitude
in the succeeding section. "But how is each part formed?" he says.
"We must answer this by starting in the first instance from the
principle that, in all products of Nature or art, a thing is made by
something actually existing out of that which is potentially the same
as the finished product. Now the semen is of such a nature and has
in it such a principle of motion, that when the motion is ceasing
each of the parts comes into being and that as a part having life or
soul. . . .And just as we should not say that an axe or other instrument
or organ was made by the fire alone, so neither shall we say that foot
or hand were made by fire alone . . . .While, then, we may allow that
hardness and softness, stickiness and brittleness, and whatever other
SECT, i] EMBRYOLOGY IN ANTIQUITY 69
qualities are found in the parts that have life and soul, may be caused
by mere heat and cold yet, when we come to the principle, Xoyof,
in virtue of which flesh is flesh and bone is bone, that is no longer so ;
what makes them is the movement set up by the male parent, who
is in actuality what that out of which the offspring is made is in
potentiaUty . This is what we find in the products of art ; heat and
cold may make the iron soft or hard, but what makes a sword is the
movement of the tools employed, this movement containing the
principle of the art. For the art is the starting-point and form of
the product; only it exists in something else (i.e. potentially in the
mind of the artist), whereas the movement of nature exists in the
product itself, issuing from another Nature (i.e. the parent) which
has the form in actuaHty."
Thus Aristotle, evidently influenced by his doctrine of "form" and
"matter", decided against preformation and pictured at one and
the same time the unformed catamenia as containing a kind of clock-
work m.echanism which, once set in motion, would inevitably produce
the finished embryo, and also as an inchoate substance on which
the seminal essence should act like a swordmaker producing a sword
according to the motions of a natural art. These two ideas are not
completely reconciled in Aristotle, and a consideration of artificial
fertiHsation would have provided a test case, had he been able to
know of the experiments of Delage and Loeb. For, on his second
theory, butyric acid would transmute a sea-urchin's egg into butyric
acid and not into a sea-urchin: while, on his first theory, the egg
would make the sea-urchin irrespective of what influence it was that
swung the starting-handle.
Aristotle has a good deal to say about the theory of recapitulation,
as it was afterwards to be called. He thought there was no doubt
that the vegetative or nutritive soul existed in the unfertilised material
of the embryo, "for nobody", as he says, "would put down the un-
fertilised embryo as soulless or in every sense bereft of life (since
both the semen and the embryo of an animal have every bit as
much life as a plant) and it is productive up to a certain point. ... As
it develops it also acquires the sensitive soul in virtue of which an
animal is an animal. . . . For first of all such embryos seem to live
the life of a plant, and it is clear that we must be guided by this in
speaking of the sensitive and the rational soul. For all three kinds
of soul, not only the nutritive, must be possessed potentially before
70 EMBRYOLOGY IN ANTIQUITY [pt. ii
they are possessed actually". These passages show very clearly the
line of thought contained in the recapitulation theory, as do the
following. "For an animal does not become at the same time an
animal and a man or a horse or any other particular animal", i.e.
the more general appears first and the more particular later. "For
the end is developed last, and the peculiar character of the species
is the end of the generation in each individual", i.e. the embryo
attains the point of being definitely not a plant before it attains that
of being definitely not a mollusc but a horse or a man. Aristotle
concludes that the diflferent sorts of souls enter the embryo at different
stages of development, just as the shape of the embryo gradually
approximates to whatever adult shape it is destined to conform to.
Aristotle continues to discuss the central problems of embryology,
but now in a way which presents features of directly physico-chemical
interest. "When the material secreted by the female in the uterus
has been fixed by the semen of the male (this acts in the same way
as rennet acts upon milk, for rennet is a kind of milk containing vital
heat, which brings into one mass and fixes the similar material, and
the relation of the semen to the catamenia is the same, milk and the
catamenia being of the same nature) , when, I say, the more solid
part comes together, the liquid is separated off from it, and as the
earthy parts solidify, membranes form all round it; this is both a
necessary result and for a final cause, the former because the surface
of the mass must solidify on heating as well as on cooling, the latter
because the foetus must not be in a liquid but separated from it."
Later on, he also says, "The reason is similar to that of the growth
of yeast, for yeast also grows great from a small beginning as the more
sohd part liquefies and the liquid is aerated. This is effected in
animals by the nature of the vital heat, in yeasts by the heat of the
juice contained in them".
These remarkable passages contain the first reference to enzyme
action ever made in a discussion on embryology. The solidification of
the outer crust is of course Hippocratic, as we have already seen. The
part about the amnios is unfortunate ; for the facts are exactly contrary.
"The heart is first differentiated", says Aristotle, "as is clear not
only to the senses (for it is so) but on theoretical grounds. For when-
ever the young animal has been separated from both parents it must
be able to manage itself, like a son who has set up house away from
his father." This is good observation. "The heart is the principle
SECT, i] EMBRYOLOGY IN ANTIQUITY 71
and origin of the embryo", says Aristotle. This conception of cor
primum vivens, ultimum moriens (a phrase never used by Aristotle
himself), has henceforward a long and tortuous history, which has
been described by Ebstein and others.
Aristotle goes on to describe the membranes of the mammalian
foetus with its umbilical cord: "The vessels join on to the uterus
like the roots of plants and through them the embryo receives its
nourishment. This is why the embryo remains in the uterus"; not
as Democritus thought, so that it might be moulded into the maternal
shape. The embryo "straightway sends off the cord like a root to
the uterus ". He carefully notes, as if the conception of axial gradients
was existing deep down in his mind, that the cephalic parts of the
embryo are formed first. "The greater become visible before the less ",
he says, "even if some of them do not come into being before them.
First the parts above the hypozoma" (a term more or less corre-
sponding to "diaphragm") "are differentiated and are superior in
size, the part below is both smaller and less differentiated. This
happens in all animals in which exists the distinction of upper and
lower, except in the insects." Aristotle gives as his explanation of
this a teleological argument: "This is why the parts about the head
and especially the eyes appear largest in the embryo at an early
stage, while the parts below the umbilicus, as the legs, are small;
for the lower parts are for the sake of the upper and are neither
parts of the end, nor able to form it".
Embryonic growth is thus described by Aristotle: "The homo-
geneous parts (tissues) are formed by heat and cold, for some are
put together and solidified by the one and some by the other ....
The nutriment oozes through the blood-vessels and the passages in
each of the parts, like water in unbaked pottery, and thus is formed
the flesh or its analogues, being solidified by cold, which is why it
is dissolved by fire. But all the particles given ofT which are too
earthy, having but little moisture and heat, cool as the moisture
evaporates along with the heat, so they become hard and earthy in
character, as nails, horns, hoofs, and beaks, and therefore they are
softened by fire but none of them is melted by it, while some of them,
as egg-shells, are soluble in liquids. The sinews and bones are formed
by the internal heat as the moisture dries, and hence the bones are
insoluble by fire like pottery, for like it they have been as it were
baked in an oven by the heat in the process of development. . . . The
72 EMBRYOLOGY IN ANTIQUITY [pt. ii
skin, again, is formed by the drying of the flesh, Hke the scum upon
boiled substances; it is so formed not only because it is upon the
outside, but also because what is glutinous, being unable to evaporate,
remains on the surface". Here is a splendid collection of mechanical
processes, but Aristotle is careful to add: "As we said, all these things
must be understood to be formed in one sense of Necessity, but in
another sense not of Necessity but for a Final Cause".
Concurrent growth and differentiation, the former being temporally
sequent to the latter, he thus describes: "The upper half of the body,
then, is first marked out in the order of development ; as time goes
on the lower also reaches its full size in the sanguinea. All the parts
are first marked out in their outlines and acquire later on their colour
and softness or hardness, exactly as if Nature were a painter producing
a work of art, for painters too first sketch in the animal with lines
and only after that put in the colours". Aristotle had some difficulty
about the eyes; he noted that they were disproportionately large
in early bird embryos, but he seems to have thought that they shrunk
absolutely as well as relatively during further development. It takes
him a great deal of ingenuity to invent a teleological explanation for
this quite imaginary fact.
The food which the embryo derives from the mother, according
to Aristotle, is of two distinct kinds, nutritious, formative, or creative,
TO OpeTTTLKov, and that which is concerned with simple increase of
size, TO et? fxeyedo'; itolovv tijv eirihoaiv. This distinction is difficult to
understand, and, though it would be attractive to interpret the former
as vitamines and the latter as fats, proteins and carbohydrates, that
would probably be putting too much of a strain on our belief in
Aristotle's insight. He has much to say of the placenta, and ascribes
to it its correct function. He combats the idea that foetal nutrition
is maintained by uterine paps, alleging against it the fact that all
embryos are enclosed in membranes. He discusses birds' eggs in
great detail, referring to infertile or "wind-eggs" and to the action
of heat during incubation. He considered that the embryo was
formed from the white exclusively, and only got its nourishment
from the yolk, which was a backward step in view of what had already
been said by the Hippocratic embryologist. He knew of the whiteness
of the yolk when first formed in the oviduct and of the yellow colour
of the layers of yolk added to it in its passage down that tube, but
he held that the yellow colour was "sanguineous", and therefore hot,
SECT, i] EMBRYOLOGY IN ANTIQUITY 73
while the white was cold. He held also that the bird embryo always
developed at the pointed end; no doubt, as Piatt has suggested,
Aristotle belonged to Swift's class of "Little-endians", and must have
always opened them at that end, in which case he would find the
embryo there, for the yolk always swims embryo uppermost. He
knew also that the yolk liquefied during the first week of develop-
ment, and that it grew larger, but he did not guess the right reasons
of these phenomena. He knew the arrangements for embryonic
development in the dolphins and ovo viviparous sharks. He takes
a strong line over spontaneous generation; "nothing", he says,
"comes into being by putrefying, but by concocting". And so in
many other passages where detailed observation is joined with acute
reasoning. So far only the treatise on the generation of animals
has been under consideration. But in the irepl ^oiwv, also, there are
many embryological data, and it is strange that those detailed
observations upon the developing fowl embryo, which demonstrate
more than anything else Aristotle's wonderful powers of observation,
are not contained at all in the Generation of Animals, but in the History
of Animals. He takes animals one by one in order, and in each case
deals with their generative peculiarities, such as their mode of
hatching, their incubation period, their fertility, etc. For instance,
he correctly relates how cartilaginous fish embryos possess a yolk-sac
like bird embryos, but no allantois. In his account of the fowl he is
unusually precise.
Most of the sixth book is occupied with the account of the genera-
tion of birds and fishes, and the seventh treats also very fully of that
of man. But in both cases it is a description that is given; more
theoretical considerations are left to the book on generation, and for
this reason the latter work is the more interesting. From a general
point of view the History of Animals has a more wonderful wealth
of material in it than the book on generation, but, at the same time,
it also indulges in much more extravagant stories, such as those of
the "kindly and gentle dolphin" and the equine Oedipus, and to
that extent the austerity of the book on generation charms us more.
Other treatises also mention embryology. The irepl ^(owv fioployvj On
the Parts of Animals, has a passage in which the appearance of the
third-day chick embryo is described, and refers to observations on
the lack of pigment and of distinct medullary canals in bones in
foetal life. The small work entitled Trepl dvairvoTj^;, On Respiration, also
74 EMBRYOLOGY IN ANTIQ^UITY [pt. ii
refers to the heart as the first organ to be formed, and so as the seat
of the soul. But these minor sources contribute little to the progress
of the science, and it is upon the great work On the Generation of Animals
that Aristotle's well-deserved fame as an embryologist will always rest.
If I have devoted a very large space to an account of Aristotle's
contributions to embryology, it is, firstly, because they are actually
greater in number than those of any other individual embryologist,
and secondly, because they had so profound an influence upon the
following twenty centuries. Embryology from the third century B.C.
to the seventeenth century a.d. is meaningless unless it is studied in
the light of Aristotle.
His outstanding contributions to embryology may be put in the
following way:
1. He carried to their logical conclusion the principles of the
observation of facts suggested by the unknown Hippocratic
embryologist, and added to them a discipline of classification
and correlation of facts which gave embryology a quite new
coherence.
2. He introduced the comparative method into embryology, and
by studying a multitude of living forms was able to lay the
foundation for future science of the various ways in which
embryonic growth can take place. Thus he knew of oviparity,
ovoviviparity, and viviparity, and one of his distinctions is
substantially the same as that known to modern embryology
between holoblastic and meroblastic yolks.
3 . He distinguished between primary and secondary sexual charac-
teristics.
4. He pushed back the origin of sex-determination to the very
beginning of embryonic development.
5. He associated regeneration phenomena with the embryonic
state.
6. He reaUsed that the previous speculations on the formation of
the embryo could be absorbed into the definite antithesis of pre-
formation and epigenesis, and he decided that the latter alterna-
tive was the true one.
7. He put forward a conception of the unfertilised egg as a com-
plicated machine, the wheels of which would move and
perform their appointed function in due course when once the
master-lever had been released.
SECT, i] EMBRYOLOGY IN ANTIQUITY 75
8. He foreshadowed the theory of recapitulation in his specula-
tions on the order in which the souls came to inhabit the
embryo during its growth, and in his observation that
universal characteristics precede particular characteristics in
embryogeny.
9. He foreshadowed the theory of axial gradients by his observa-
tions on the greater and more rapid development of the cephalic
end in the embryo.
10. He allotted the correct functions to the placenta and the
umbilical cord.
11. He gave a description of embryonic development involv-
ing comparison with the action of rennet and yeast, fore-
shadowing thus our knowledge of organic catalysts in
embryogeny.
But there was another side to the picture. Aristotle made three
big mistakes, and here I do not refer to any matters of detail, in
which it would not have been humanly possible to be more than
very often right, but rather to general notions, such as the eleven
correct ones.
They were as follows :
1. He was incorrect in his view that the male supplies nothing
tangible to the female in the process of fertilisation. To say
that the semen gave the "form" to the inchoate "matter" of
the menstrual blood was equivalent to saying that the seminal
fluid carried nothing in it but simply an immaterial breath
along with it. Aristotle did not envisage the existence of
spermatozoa.
2. He was entirely wrong in his teaching about the scolex. The
caterpillar is not, as he supposed, an egg laid too soon, but has
already passed through the embryonic state.
3. He was misled by some observations on castrated animals and
so did not ascribe to the testis its true function.
Such mistakes as these, however, were not nearly so important as
the solid ground gained by his correct answers. They were always
open to experimental test, even though the authority of his name
precluded it until the Renaissance. But there was one aspect
of his embryological work which was to exercise an unfortunate
influence on the subsequent progress of the science, namely, his in-
sistence on teleological explanations. He was always seeking for
76 EMBRYOLOGY IN ANTIQUITY [pt. ii
final as well as efficient causes. "The ancient Nature-philosophers
did not see that the causes were numerous ; they only saw the material
and efficient causes and did not distinguish even these, while they made
no enquiry at all into the formal and final causes." "Democritus",
he says, "neglecting the final cause, reduces to Necessity all the
operations of Nature. Now they are necessary it is true, but yet
they are also for a final cause, and for the sake of what is best in
each case."
Now in Aristotle this was all to the good. A metaphysician as well
as a scientific worker, he was able to use the concept of purposiveness
as a heuristic aid, but he never rested upon it. The trouble was
that he introduced it into the discussion at all. It is an interesting
speculation to consider what would have happened if the first great
biologist had not brought final causes into his teaching ; perhaps the
subsequent history of biology, and science as a whole, would have
been very different. For final causes irresistibly led to the theological
blank alleys into which men's thoughts were ushered and there left
to grope till the end of the Middle Ages.
Perhaps Aristotle would not have made so many great discoveries
if he had been more of a Democritus. For teleology is, like other
varieties of common sense, useful from time to time; e.g. Harvey told
Boyle that he was led to certain important considerations by meditat-
ing upon the final cause of the valves in the veins ; and every biologist
acts in the same way at the present time. But the important thing
is not to give the last word to teleology. And those attractive shady
places which Aristotle, guided by his genius, quickly passed through
on his perpetual journeys into the hot sunlight of research and specu-
lation were so many traps for those who followed him. He himself
knew how to change rapidly from metaphysician into physicist and
back again, how to bow politely to the final cause and press on with
the dissection; but the later Peripatetics had no knowledge of this art,
nor had the Patristic Doctors, nor the mediaeval Aristotelians; who
all remained sleeping quietly in the shade of the will of God. He knew
very well from the sea (to use Bacon's metaphor at last) the look of
the Circe country of teleology, but he never visited it for long at a
time^ being an authentic Odysseus, unlike so many later heads, who,
following the example of Plato, "anchored upon that shore" and,
dropping their hooks to the sound of plain-song, there rode, never
to hoist sail again.
SECT, i] EMBRYOLOGY IN ANTIQUITY 77
1-5. The Hellenistic Age
Aristotle died in 322 e.g. From that year until 1534, the date of
the birth of Volcher Goiter, first in time of the Renaissance embryo-
logists, embryology has very little history.
The founder of the stoical philosophy, Zeno of Gitium, was born
some twenty years before the death of Aristotle. "Pious and mag-
nanimous as Stoicism was in the field of conduct", says AUbutt,
"creating or nourishing that elevation of mind which distinguished
the nobler Roman of the Empire, yet in Rome, as in England, its
natural science was of no account. The spirit of it was indeed rather
alien than akin to science. The mind of the Porch which called itself
'practical' was reluctant to all 'speculation', natural science in-
cluded." The Stoics regarded the four quahties of cold, hot, wet,
and dry, as ultimate, instead of the earth, fire, air, and water of the
Peripatetics and their predecessors, Plutarch, in his summary of
philosophic opinions already mentioned, has some passages relating
to their views on the development of the embryo. "The Stoicks say",
he relates, "that the foetus is fed by the fecundine and navell; where-
upon it is that midwives presently knit up and tie the navell string
fast, but open the infants mouth, to the end that it be acquainted
with another kind of nourishment." And elsewhere, "The Stoicks
say that it is a part of the wombe and not an animall by itselfe. For
like as fruits be parts of trees, which when they be ripe do fall, even
so it is with an infant in the mother's wombe. . . . The Stoicks are
of opinion that the most parts are formed all at once ; but Aristotle
saith the backbone and loines are first framed like as the keele in
a ship." But to which of Zeno's successors, Gleanthes, Ghrysippus,
Grates or the rest, these sayings are to be attributed, is not known.
The Epicureans also had opinions on these subjects. They thought
that the foetus in utero was fed by the amniotic liquid or the blood,
and they also beheved, in contradistinction to the Peripatetics, that
both male and female supplied seed in generation, as is shown by
the lines of Lucretius :
usque adeo magni refert, ut semina possint
seminibus commisceri genitaliter apta
crassaque conveniant liquidis at liquida crasso.
But much more important than the teaching of these philosophers
was the rise of what might be called the scientific faculty of the great
78 EMBRYOLOGY IN ANTIQUITY [pt. ii
University of Alexandria. That seat of learning, perhaps the most
glorious, after Athens, of any in antiquity, and greater than its con-
temporary rival Pergamos, was important because all the traditions
of earlier times were united in it like a bundle of strands coming
together to form a rope. Democritean atomism. Peripatetic science
and metaphysics, Goan biology, Coan and Cnidian medicine, above
all, Athenian mathematics and astronomy, all were gathered in
the fMovaelov of Alexandria under the benevolent dynasty of the
Ptolemies. The link between the Alexandrian biologists and the
school of Aristotle was Straton of Lampsacus, who, though apparently
not making any contribution to embryology himself, must have
brought the knowledge of generation gained by Aristotle to Alex-
andria as he sailed south across the Mediterranean to be the tutor
of Ptolemy Philadelphus. The link between Cos and Alexandria was
Diodes of Carystus, who was the last of the Hippocratic school and
also a pupil of Philistion of Locri. Diodes has a certain importance
in the history of embryology; for Oribasius refers to him as the dis-
coverer of the punctum saliens in the mammalian embryo, "on the
ninth day a few points of blood, on the eighteenth beating of the
heart, on the twenty-seventh traces of the spinal cord and head ". He
thus showed that the early development of chick and mammal was
very alike. Plutarch also tells us that he occupied himself with the
question of sterility. He described the human placenta, as well as
embryos of twenty-seven and forty days, and he held that both male
and female contribute seed in generation. Cnidian medicine in-
fluenced Alexandria through Chrysippus of Cnidus — not the Stoic —
whose embryological doctrine seems to have been that the embryo had
only a vegetative soul until birth or hatching.
All these influences were fruitful, for they produced the two
greatest physiologists of ancient times, Herophilus of Chalcedon and
Erasistratus of Chios. These two, who were contemporaries during
the third century B.C., experimented much and wrote voluminously,
but all except fragments of their writings have been lost, and can
now only be pieced together out of the books of Galen, as has been
done by Dobson. Allbutt has well described the differences between
them, such as the predilection of Herophilus for the humoral patho-
logy and pharmacy, and the greater interest taken by Erasistratus in
atomistic speculations. "Herophilus", says Plutarch, "leaveth to
unbome babes a mooving naturall, but not a respiration, of which
CHART II
Co[N-re-MPoR.A(e.r EveNfrs and
8o EMBRYOLOGY IN ANTIQUITY [pt. ii
motion the sinewes be the instrumental! cause, but afterwards they
become perfect Hving animall creatures, when being come forth of
the wombe, they take in breath from the aire."
Herophilus described the ovaries and the Fallopian tubes, but did
not advance further than Aristotle towards correct sexual physiology
in this respect. We gather that he made many dissections of embryos
from the testimony of Tertullian, though this may not be worth much.
Moreover, he called the outer membrane of the brain, chorion, after
the membranes which surround the embryo. He gave a correct de-
scription of the umbilical cord, except that he assigned to it four vessels
instead of three, carrying blood and breath to the embryo. The
veins, he thought, communicated with the venae cavae, and the
arteries with the great artery running along the spine. Herophilus
also occupied himself much with obstetrical matters, and wrote a
treatise on them, fiaicoTtKov. Together with Erasistratus he denied
that there were any diseases special to women other than those
attendant on their special sexual functions, but the greatest contribu-
tion which he made to biology was the association of the brain with
the intellect^, for even Aristotle had made the he^rt the seat of the
mental individual.
Erasistratus did not study embryology as much as did Herophilus,
but a passage in Galen throws an interesting light on his notions of
embryonic growth. "The heart", says Galen, "is no larger at first
than a millet seed, or, if you like, a bean. Ask yourself how it could
grow large otherwise than by being distended and receiving nutri-
ment throughout its whole extent, just as we have shown above that
the seed is nourished. But even this is unknown to Erasistratus, who
makes so much of Nature's Art. He supposes that animals grow just
like a sieve, a rope, a bag, or a basket, each of which grows by the
addition to it of materials similar to those out of which it began to
be made." This is only one instance out of many in which Galen, the
teleologist, finds fault with Erasistratus, the mechanistic philosopher.
During the period when the biological school of Alexandria was
at its height, that city became an important Jewish centre. Two
centuries later it was to produce Philo, but now the Alexandrian
Jews were writing that part of the modern Bible known as the Wisdom
Literature. In books such as the Wisdom of Solomon, Ecclesiasticus,
Proverbs, etc., the typical Hellenic exclusion of the action of gods
^ This was not absolutely new: Alcmaeon had held the same view (see Burnet).
SECT, i] EMBRYOLOGY IN ANTIQUITY 8i
in natural phenomena is clearly to be seen. There are two passages
of embryological importance. Firstly, in the book of Job (x, 9-1 1),
Job is made to say, "Remember, I beseech thee, that thou hast
fashioned me as clay; and wilt thou bring me into dust again?
Hast thou not poured me out as milk, and curdled me like cheese?
Thou hast clothed me with skin and flesh, and knit me together with
bones and sinews". This comparison of embryogeny with the making
of cheese is interesting in view of the fact that precisely the same
comparison occurs in Aristotle's book On the Generation of Animals, as
we have already seen. Still more extraordinary, the only other
embryological reference in the Wisdom Literature, which occurs in
the Wisdom of Solomon (vii, 2), also copies an Aristotelian theory,
namely, that the embryo is formed from (menstrual) blood. There
the speaker says, "In the womb of a mother was I moulded into
flesh in the time of ten months, being compacted in blood of the
seed of man and the pleasure that came with sleep". Perhaps
it is no coincidence that both these citations can be referred back
to Aristotle, and, in the second case, even to Hippocrates; perhaps
the Alexandrian Jews of the third century B.C. were studying Aristotle
as attentively as Philo Judaeus studied Plato a couple of hundred
years later.
The Alexandrian school was directly responsible for the introduc-
tion of Greek medicine and biology into Rome, through the physician
Cleophantus, who seems to have been particularly interested in
gynaecology. At the end of the second century B.C. and the beginning
of the first, Rome received the first and greatest of her Greek
physicians, Asclepiades of Parion, who brought atomism with him.
He was thus the Hnk between Epicurus and the methodistic school
of physicians, and may have been a potent influence upon Lucretius.
Again, Alexander Philalethes provides the link between Cleophantus
and Soranus. Soranus lived in Rome from about a.d. 30 tifl just
after the end of the first century, and so twenty years before the
birth of Galen.
Of aU the ancient writers on embryology, Soranus is the one whose
works were in later times most widely appropriated, mutilated,
furbished up, quoted from rightly and wrongly, and generally upset.
Allbutt, Barbour and Singer give accounts of the way in which this
process went on, and the whole question has given rise to a con-
siderable literature. (See Lachs, Ilberg, Sudhov, etc.) It lasted
82 EMBRYOLOGY IN ANTIQUITY [pt. ii
right into the Middle Ages, and was particularly vehement in the
case of the treatise on gynaecology, Trepl ywaiKeLwv Tradotv. This was
translated into Latin under the name of Moschion, then back into
Greek and finally back into Latin again. It is largely obstetrical,
but it shows an advanced knowledge of embryology, and especially
an accurate idea of the anatomy of the uterus. (See Plate II.)
The other writers of this period are unimportant embryologically.
Among the Greeks, Aelian wrote a De natura animalium, in which
he spoke of eggs, but without adding anything to our knowledge of
them; Nicander in his Theriaca refers to mammalian embryos, and
alleges that they breathe and eat through the umbilical cord; and
Oppian has a few unsystematic remarks about the embryos of various
animals. Junius Columella's work on husbandry contains two chapters
on eggs, but he was not much interested in the theoretical aspect of
development. In Aulus Gellius we have the cheese analogy appearing
in conjunction with obscurantist views about the powers of the number
seven. It is not generally known that a clear statement of the pre-
formationist or " Entfaltung " theory of embryogeny occurs in Seneca's
Quaestiones naturales, where there is the following passage: "In the
seed are enclosed all the parts of the body of the man that shall be
formed. The infant that is borne in his mother's wombe hath the
rootes of the beard and hair that he shall weare one day. In this
little masse likewise are all the lineaments of the bodie and all that
which Posterity shall discover in him". Perhaps this notion was
derived by Seneca from the Homoeomereity of Anaxagoras, for a
discussion of which in relation to embryology, see Cornford. "Hair
cannot come out of not-hair, nor flesh out of not-flesh", said
Anaxagoras.
The Natural History of Pliny, that "voluminous, industrious, un-
philosophical, gullible, unsystematic old gossip", as Singer justly
calls him, contains little of embryological importance, although he
devotes many sections to eggs, and what there is comes straight from
the fountain-head, Aristotle. As, for example, "All egs have within
them in the mids of the yolk, a certain drop, as it were of bloud,
which some thinke to be the heart of the chicken, imagining that,
to be the first that in everie bodie is formed and made ; and certainlie
a man shall see it within the verie cggc to pant and leape. As for
the chick, it taketh the corporall substance, and the bodie of it is
made of the white waterish liquor in the egge, the yellow yolke
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SECT, i] EMBRYOLOGY IN ANTIQUITY 83
serves for nourishment; whiles the chick is unhatched and within the
egge, the head is bigger than all the bodie besides; and the eies
that be compact and thrust together be more than the verie head.
As the chick within growes bigger, the white turneth into the middest,
and is enclosed within the yolke. By the 20 day (if the eggs be stirred)
ye shall heare the chick to peepe within the verie shell; from that
time forward it beginneth to plume and gather feathers ; and in this
manner it lies within the shell, the head resting on the right foot,
and the same head under the right wing, and so the yolke by little
and little decreaseth and faileth". But the best way to illustrate
Pliny's embryology is to copy out some of his index, as follows :
The Table to the first Tome of Plinies Naturall Historie.
Egs diverse in colour 298
Egs of birds of 2 colours within the shell ibid.
Egs of fishes of i colour ibid.
Egs of birds, serpents, and fishes, how they differ ibid.
Egs best for an hen to sit upon 299
Egs hatched without a bird, onely by a kind heat ibid.
Egs how they be marred under an hen ibid,
wind-egs, called Hypenemia 300
how they be engendred 301
wind-egs, Zephyria ibid.
Egs drawne through a ring ibid.
Egs how they be best kept ibid.
The Table to the second Tome of Plinies Naturall Historie,
Egs of hens and their medicinable properties 351
yolke of hens egs, in what cases it is medicinable 352
Egs all yolke, and without white, be called Schista ibid,
skinne of an Hens egge-shell, good in Physicke ibid.
Hens Eggeshell reduced unto ashes, for what it serveth ibid,
the wonderfull nature of Hens Eggeshels ibid.
Hens Egges, all whole as they be, what they are good for 353
the commendations of Hens Egges, as a meat most medicinable ibid.
Hens Egge, a proper nourishment for sicke folks, and may go
for meat and drinke both ibid.
Egge-shels, how they may be made tender and pliable ibid,
white of an Egge resisteth fire ibid,
of Geese Egges a discourse 354
the serpents egge, which the Latines call Anguinum, what it
is, and how engendred 355
This last item exhibits Pliny at his worst. It is worth quoting, apart
from its intrinsic value, for it shows to what depths embryological
knowledge descended within four hundred years after Aristotle col-
lected his specimens on the shores of the lagoon of Pyrrha, and talked
with the fishermen of Mitylene. "I will not overpasse one kind of
eggs besides, which is in great name and request in France, and
whereof the Greeke authors have not written a word ; and this is
6-2
84 EMBRYOLOGY IN ANTIQUITY [pt. ii
the serpents egg, which the Latins call Anguinum. For in Summer
time yerely, you shall see an infinit number of snakes gather round
together into an heape, entangled and enwrapped one within another
so artificially, as I am not able to expresse the manner thereof; by
the means therefore, of the froth or salivation which they yeeld from
their mouths, and the humour that commeth from their bodies,
there is engendred the egg aforesaid. The priests of France, called
Druidae^, are of opinion, and so they deliver it, that these serpents
when they have thus engendred this egg do cast it up on high into
the aire by the force of their hissing, which being observed, there
must be one ready to catch and receive it in the fall again (before
it touch the ground) within the lappet of a coat of arms or souldiours
cassocks. They affirme also that the party who carrieth this egg away,
had need to be wel mounted upon a good horse and to ride away
upon the spur, for that the foresaid serpents will pursue him still,
and never give over until they meet with some great river betweene
him and them, that may cut off and intercept their chace. They ad
moreover and say that the only marke to know this egg whether it
be right or no, is this, that it will swim aloft above the water even
against the stream, yea though it were bound and enchased with a
plate of gold." But one must not be too severe upon Pliny, for he
and his translator, Philemon Holland, provide an entertainment
unequalled anywhere else.
To some extent the same applies to Plutarch of Chaeronea, who
lived about the same time. Plutarch's writings, inspired as they were
throughout by the desire to commend the ancient religion of Greece
to a degenerate age, represent no milestone or turning-point in the
history of embryology, yet there is a passage in the Symposiaques, or
Table-questions which bears upon it. The third question of book 2
is "Whether was before, the hen or egg?" "This long time", says
Plutarch, "I absteined from eating egges, by reason of a certaine
dream I had, and the companie conceived an opinion or suspition
of me that there were entred into my head the fantasies and super-
stitions of Orpheus or Pythagoras, and that I abhorred to eat an
egge for that I believed it to be the principle and fountaine of genera-
tion." He then makes the various characters in the dialogue speak
to the motion, and one of them, Firmus, ends his speech thus, "And
^ For further information about the serpent's eggs of the Druids, see Kendrick; they
were probably fossil echinoderms.
SECT, i] EMBRYOLOGY IN ANTIQUITY 85
now for that which remaineth (quoth he and therewith he laughed)
I will sing unto those that be skilfull and of understanding one holy
and sacred sentence taken out of the deepe secrets of Orpheus, which
not onely importeth this much, that the cgge was before the henne,
but also attributeth and adjudgeth to it the right of eldership and
priority of all things in the world, as for the rest, let them remaine
unspoken of in silence (as Herodotus saith) for that they bee exceeding
divine and mysticall, this onely will I speake by the way; that the
world containing as it doth so many sorts and sundry kinds of living
creatures, there is not in manner one, I dare well say, exempt from
being engendred of an egge, for the egge bringeth forth birdes and
foules that fiie, fishes an infinit number that swimme, land creatures,
as lizards, such as live both on land and water as crocodiles, those
that bee two-footed, as the bird, such as are footlesse, as the serpent,
and last of all, those that have many feet, as the unwinged locust.
Not without great reason therefore is it consecrated to the sacred
ceremonies and mysteries of Bacchus as representing that nature
which produceth and comprehendeth in itselfe all things". This
emphatic passage looks at first sight as if it was a statement of
the Harveian doctrine omne vivum ex ovo. But the fact that no
mammals are mentioned makes this improbable. Firmus then sits
down and Senecius opposes him with the well-worn argument that
the perfect must precede the imperfect, laying stress also on the
occurrence of spontaneous, i.e. eggless, generation, and on the fact
that men could find no "row" in eels. Three hundred years later,
Ambrosius Macrobius handled the question again (see Whittaker),
and the progress in embryological knowledge could be strikingly
shown by the difference in treatment. It would be an interesting
study to make a detailed comparison of them.
1-6. Galen
Another fifty years brings us to Galen of Pergamos, second in
greatness among ancient biologists, though in spite of his multi-
tudinous writings he does not quite take this high rank in embiyology.
That knowledge of the development of the foetus was at this time
specially associated with Peripatetic tradition appears from a remark
of Lucian of Samosata, Galen's contemporary. In the satire called,
The Auction of the Philosophies, Hermes, the auctioneer, referring to the
Peripatetic who is being sold, says, "He will tell you all about the
86 EMBRYOLOGY IN ANTIQUITY [pt. ii
shaping of the embryo in the womb". But Galen was now to weld
together all the biological knowledge of antiquity into his voluminous
works, and so transmit it to the Middle Ages.
Most of Galen's writing was done between a.d. 150 and 180. Out
of the twenty volumes of Kiihn's edition of 1829, l^^s than one is
concerned with embryology, a proportion considerably less than in
the case of Aristotle. Galen's embryology is to be found in his
Trepl (f)V(nKcov Svvdfiecov, On the Natural Faculties, which contains the
theoretical part, and in his On the Formation of the Foetus, which con-
tains the more anatomical part. There is also the probably spurious
treatise et ^mov to Kara <yaa-Tp6<i, On the Question of whether the Embryo
is an Animal.
It is important to realise at the outset that Galen was a vitalist
and a teleologist of the extremest kind. He regarded the living being
as owing all its characteristics to an indwelling Physis or natural
entity with whose "faculties" or powers it was the province of
physiology to deal. The living organism according to him has a kind
of artistic creative power, a t6xvv> which acts on the things around
it by means of the faculties, Swd/xei's, by the aid of which each part
attracts to itself what is useful and good for it, rb oUelov, and
repels what is not, to aXXorptov. These faculties, such as the "peptic
faculty" in the stomach and the "sphygmic faculty" in the heart,
are regarded by Galen as the causes of the specific functions or
activity of the part in question. They are ultimate biological cate-
gories, for, although he admits the theoretical possibility of analysing
them into simpler components, he never makes any attempt to do
so, and evidently regards such an effort as doomed to failure, unlike
Roux, whose "interim biological laws" are really conceived of as
interim. "The effects of Nature", says Galen, "while the animal is
still being formed in the womb are all the different parts of the body,
and after it has been born an effect in which all parts share is the
progress of each to its full size and thereafter the maintenance of
itself as long as possible." Galen divides the effects of the faculties
into three. Genesis, Growth, and Nutrition, and means by the first
what we mean by embryogeny. "Genesis", he says, "is not a simple
activity of Nature, but is compounded of alteration and of shaping.
That is to say, in order that bone, nerve, veins, and all other tissues
may come into existence, the underlying substance from which the
animal springs must be altered; and in order that the substance so
SECT, i] EMBRYOLOGY IN ANTIQUITY 87
altered may acquire its appropriate shape and position, its cavities,
outgrowths, and attachments, and so forth, it has to undergo a
shaping or formative process. One would be justified in calling this
substance which undergoes alteration the material of an animal, just
as wood is the material of a ship and wax of an image." In this
remarkable passage, Galen expresses modern views about chemical
growth and chemical differentiation.
Galen then goes on to treat of embryogeny in more detail. "The
seed having been cast into the womb or into the earth — for there is
no difference — ", he says (see p. 65), "then after a certain definite
period a great number of parts become constituted in the substance
which is being generated; these differ as regards moisture, dryness,
coldness and warmth, and in all the other qualities which naturally
derive therefrom", such as hardness, softness, viscosity, friability,
lightness, heaviness, density, rarity, smoothness, roughness, thickness,
and thinness. "Now Nature constructs bone, cartilage, nerve, mem-
brane, ligament, vein, and so forth at the first stage of the animal's
genesis, employing at this task a faculty which is, in general terms,
generative and alterative, and, in more detail, warming, chilHng,
drying and moistening, or such as spring from the blending of these,
for example, the bone-producing, nerve-producing, and cartilage-
producing, faculties (since for the sake of clearness these terms must be
used as well) .... Now the peculiar flesh of the liver is of a certain kind
as well, also that of the spleen, that of the kidneys and that of the
lungs, and that of the heart, so also the proper substance of the brain,
stomach, oesophagus, intestines and uterus is a sensible element, of
similar parts all through, simple and uncompounded. . . . Thus the
special alterative faculties in each animal are of the same number
as the elementary parts, and further, the activities must necessarily
correspond each to one of the special parts, just as each part has its
special use. . . . As for the actual substance of the coats of the stomach,
intestine, and uterus, each of these has been rendered what it is by
a special alterative faculty of nature; while the bringing of these
together, the combination therewith of the structures that are in-
serted into them, etc. have all been determined by a faculty which
we call the shaping or formative faculty; this faculty we also state
to be artistic — nay, the best and highest art — doing everything for
some purpose, so that there is nothing ineffective or superfluous, or
capable of being better disposed."
88 EMBRYOLOGY IN ANTIQUITY [pt. ii
Thus the alterative faculty takes the primitive unformed raw
material and changes it into the different forms represented by the
different tissues, while the formative faculty, acting teleologically
from within, organises these building-stones, as it were, into the
various temples which make up the Acropolis of the completed
animal. Galen next goes on to speak of the faculty of growth. "Let
us first mention", he says, "that this too is present in the foetus
in utero as is also the nutritive faculty, but that at that stage these
two faculties are, as it were, handmaids to those already mentioned,
and do not possess in themselves supreme authority."
Later on, until full stature is reached, growth is predominant, and
finally nutrition assumes the hegemony.
So much for Galen's embryological theory. But before leaving the
treatise On the Natural Faculties, it may be noted that he ascribes a
retentive faculty to the uterus as well as to the stomach, and explains
birth as being due to a cessation of action on the part of the retentive
faculty, "when the object of the uterus has been fulfilled", and a
coming into action of a hitherto quiescent propulsive faculty. This
wholesale allotting of faculties can obviously be made to explain
anything, and is eminently suited to a teleological account such as
Galen's. It was not inconvenient as a framework within which all
the biological knowledge of antiquity could be crystallised, but it was
utterly pernicious to experimental science. Fifteen hundred years later
it received what would have been the death-blow to any less virile
theory, at the hands of Moliere in his immortal Malade Imaginaire :
Bachelirius. Mihi a docto doctore
Demandatur causam et rationem quare
Opium facit dormire
A quoi respondeo
Quia est in eo
Virtus dormitiva
Cujus est nature
Sensus assoupire.
Chorus. Bene, bene, bene, bene respondere.
Dignus, dignus est entrare
In nostro docto corpore.
Bene, bene, respondere.
But to return to Galen. The book on the formation of the embryo
opens with a historical account of the views of the Hippocratic writers
SECT, i] EMBRYOLOGY IN ANTIQUITY 89
with whom Galen was largely in agreement. It goes on to describe
the anatomy of allantois, amnios, placenta, and membranes with
considerable accuracy. The embryonic life consists, it says, of four
stages: (i) an unformed seminal stage, (2) a stage in which the tria
principia (a concept here met with for the first time) are engendered,
the heart, liver and brain, (3) a stage when all the other parts are
mapped out and (4) a stage when all the other parts have become
clearly visible. Parallel with this development, the embryo also rises
from possessing the life of a plant to that of an animal, and the
umbilicus is made the root in the analogy with a plant. The embryo
is formed, firstly, from menstrual blood, and secondly, from blood
brought by the umbilical cord, and the way in which it turns into
the embryo is made clearer as follows: "If you cut open the vein
of an animal and let the blood flow out into moderately hot water;
the formation of a coagulum very like the substance of the liver will
be seen to take place". And in effect this viscus, according to Galen,
is formed before the heart.
Galen also taught that the embryo excreted its urine into the
allantois, and was acquainted with foetal atrophy. He gave a fairly
correct account of the junction of the umbilical veins with the
branches of the portal vein, and the umbilical with the iliac arteries,
of the foramen ovale, the ductus Arantii and the ductus Botalli. He
maintained that the embryo respired through the umbilical cord,
and said that the blood passed in the embryo from the heart to the
lungs and not vice versa. The belief that male foetuses were formed
quicker than female ones he still entertained, and explained as being
due to the superior heat and dryness of the male germ. He also
associated the male conception with the right side and the female
with the left and asserted that the intra-uterine movements are sooner
felt in the case of the male than in the case of the female. Dry foods
eaten by the mother, he thought, would lead to a more rapid develop-
ment of the foetus than other kinds.
In this account of the Galenic embryology I have drawn not only
upon the book on the formation of the foetus, but also upon his
v7r6fMV7]/jba, Commentary on Hippocrates, his Trepl alricov av/jLTTTco/naTcov,
On the Causes of Symptoms, and his book Trepl %peta? tmv fjuoplcor, On the
Use of Parts. It is this latter work that had the greatest influence on the
ages which followed Galen's Hfe. In the course of seventeen books, he
tries to demonstrate the value and teleological significance of every
go EMBRYOLOGY IN ANTIQUITY [pt. ii
structure and function in the human and animal body, and to show
that, being perfectly adapted to its end, it could not possibly be other in
shape or nature than what it is. At the conclusion of this massive work
with all its extraordinary ingenuity and labour, he says, "Such then
and so great being the value of the argument now completed, this
section makes it all plain and clear like a good epode — I say an epode,
but not in the sense of one who uses enchantments (eVwSat?) but as
in the melic poets whom some call lyric, there is as well as strophe
and antistrophe, an epode, which, so it is said, they used to sing
standing before the altar as a hymn to the Gods. To this then I
compare this final section and therefore I have called it by that
name". This is one of the half-dozen most striking paragraphs in
the history of biology ; worthy to rank with the remarks of Hippo-
crates on the " Sacred Disease". Galen, as he wrote the words, must
have thought of the altar of Dionysus in the Athenian or Pergamene
theatre, made of marble and hung about with a garland, but they
were equally applicable to the altar of a basilica of the Christian
Church with the bishop and his priests celebrating the liturgy at it.
What could be more charged with significance than this? At the
end of the antique epoch the biology of all the schools, Croton,
Akragas, Cos, Cnidus, Athens, Alexandria, Rome, is welded together
and as it were deposited at the entrance into the sanctuary of
Christendom. It was the turning-point, in Spengler's terminology,
between ApoUinian civilisation and Faustian culture. Galen's words
are the more extraordinary, for he himself can hardly have foreseen
that the long line of experimentalists which had arisen in the sixth
century B.C. would come to an end with him. But so it was to be,
and thenceforward experimental research and biological speculation
were alike to cease, except for a few stray mutations, born out of
due time, until in 1453 the city of Byzantium should burst like .a
ripe pod and, distributing her scholars all over the West, as if by
a fertilising process, bring all the fruits of the Renaissance into being.
SECTION 2
EMBRYOLOGY FROM GALEN TO
THE RENAISSANCE
2-1. Patristic, Talmudic, and Arabian Writers
We are now at the beginning of the second century a.d. The next
thousand years can be passed over in as short a time as it has taken
to describe the embryology of Galen alone. The Patristic writers,
who on the whole were careful to base their psychology on the
physiology of the ancients, had little to say about the developing
embryo. Most of their interest in it was, as would naturally be
expected, theological; Tertullian, for instance, held that the soul was
present fully in the embryo throughout its intra-uterine life, thus
denying that kind of psychological recapitulation which had been
suggested by Aristotle. "Reply," he says in his De Anima, "O ye
Mothers, and say whether you do not feel the movements of the
child within you. How then can it have no soul? " These views were
not held by other Fathers, of whom St Augustine of Hippo {De
Immortalitate et de quantitate ahimae) may serve as a representative, for
he thought that the embryo was "besouled" in the second month
and "besexed" in the fourth. These various opinions were duly
reflected in the law, and abortion, which had even been recom-
mended theoretically by Plato and defended practically by Lysias
in the fourth or fifth century B.C., now became equivalent to homicide
and punishable by death. This fact leads Singer to the view that
the Hippocratic oath is late, perhaps early Christian. The late Roman
law, which, according to Spangenberg, regarded the foetus as not
''Homo'", not even '' Infans'\ but only a ''Spes animantis'\ was
gradually replaced by a stern condemnation of all pre-natal infanti-
cide. "And we pay no attention", said the Bishops of the Quinisext
Council, held at Byzantium in 692, "to the subtle distinction as to
whether the foetus is formed or unformed." Other authorities, follow-
ing St Augustine, took a more liberal view, and the canon law as finally
crystallised recognised first the fortieth day for males and the eightieth
day for females as the moment of animation, but later the fortieth
day for both sexes. The ''embryo informatus" thus had no soul, the
92 EMBRYOLOGY FROM GALEN [pt. ii
^'^ embryo formatus" had, and as a corollary could be baptised.
St Thomas Aquinas was of opinion that embryos dying in utero might
possibly be saved : but Fulgentius denied it. As for the ancient belief
that male embryos were formed twice as quickly as female ones, it
lingered on until Goelicke took the trouble to disprove it experi-
mentally in 1723.
Clement of Alexandria, in his book \6<yo<i TrporpeTTriKO'i Trpo?
"EX\.7]va'i, has some remarks to make on embryology, but adds nothing
to the knowledge previously gained. He adopts the Peripatetic view
that generation results from the combination of semen with menstrual
blood, and he uses the Aristotelian illustration of rennet coagulating
milk. Lactantius of Nicomedia, who lived about the date of the
Nicene Council (a.d. 325) perpetuated the deeply-rooted association
of male with right and female with left in his book On the work of
God, De opificio Dei. He also maintained that the head was formed
before the heart in embryogeny, and seems to have opened hen's
eggs systematically at different stages, so that to this extent he was
a better embryologist than Galen. St Gregory of Nyssa, as we have
already seen (p. 20), evolved a neo-vitalistic theory which he ap-
plied to the growth of the embryo.
Late Latin writers, other than the theologians, do not say much
about it. There is a passage in Ausonius, however, which describes
the development of the foetus {Eclog. de Rat. puerp.) but it is almost
wholly astrological. Elsewhere he says:
juris idem tribus est, quod ter tribus; omnia in istis;
forma hominis coepti, plenique exactio partu,
quique novem novies fati tenet ultima finis.
Idyll II (Gryphus ternarii numeri), 4-6.
(The power of 3, in 3 times 3 lies too,
Thus 9 rules human form and human birth,
And 9 times 9 the end of human life.)
But this is probably a late echo of the Pythagoreans rather than
an early prelude to Leonardo da Vinci and the mathematisation of
nature.
That great mass of Jewish writings known as the Talmud, which
grew up between the second and sixth centuries a.d., also contains
some references to embryology, and certain Jewish physicians, such
as Samuel-el-Yehudi, of the second century, are said to have devoted
SECT. 2] TO THE RENAISSANCE 93
special attention to it. The embryo was called peri habbetten (fruit of
the body), ]a2n ns. It grew through various definite stages:
(i) golem (formless, rolled-up thing), nbu, 0-1-5 months.
(2) shefir meruqqdm (embroidered foetus), api» T'Dit.
(3) ^ubbar (something carried), imi?, 1-5-4 months.
(4) walad (child), n*?!, 4-7 months.
(5) walad shel qaydmd (viable child), so'^^p '7tri'?i, 7-9 months.
(6) ben she-kallu khaddshdw (child whose months have been com-
pleted), rirnn I'^rir ]n.
The ideas of the Talmudic writers on the life led by the embryo
in utero are well represented by the remark, "It floateth like a nut-
shell on the waters and moveth hither and thither at every touch"
ms o*» "rtr ':'SDn niia TUNb las •'^lan n»n n'?i rrch ity'^s •'sn lasi
And the classical passage, "Rabbi Simlai lectured: the babe in its
mother's womb is like a rolled-up scroll, with folded arms lying
closely pressed together, its elbows resting on its hips, its heels against
its buttocks, its head between its knees. Its mouth is closed, its navel
open. It eats its mother's food and sips its mother's drink: but it
doth not excrete for fear of hurting"
bv rT* niioi "rsipa'!^ Q^ith las "'yan n»n n'^in rxh ''V^b's^^ •'in tJ^m
ittNtr na» nmtyi n'?sis las:^ n»» '?2isi mns "nuai miio rsi rsin ^■'n i"?
It was thought, moreover, that the bones and tendons, the nails,
the marrow in the head and the white of the eye, were derived from
the father, "who sows the white", but the skin, flesh, blood, hair,
and the dark part of the eye from the mother, "who sows the red".
This is evidently in direct descent from Aristotle through Galen, and
may be compared with the following passage from the latter writer's
Commentary on Hippocrates: "We teach that some parts of the body
are formed from the semen and the flesh alone from blood. But
because the amount of semen which is injected into the uterus is
small, growth and increment must come for the most part from the
blood". It might thus appear that, just as the Jews of Alexandria
were reading Aristotle in the third century B.C., and incorporating
IujILIBRAR Y
V<^XN4?AI
94 EMBRYOLOGY FROM GALEN [pt. ii
him into the Wisdom Literature, so those of the third century a.d.
were reading Galen and incorporating him into the Talmud. As for
God, he contributed the life, the soul, the expression of the face, the
functions of the different parts. This participation of three factors in
generation, male, female, and god, is exceedingly ancient, as may
be read in Robertson Smith. Some Talmudic writers held that
development began with the head, agreeing with Lactantius, and
others that it began at the navel, agreeing with Alcmaeon. Weber
has given an account of the Talmudic beliefs about the infusion of
the soul into the embryo. They do not seem to have embodied any
new or striking idea.
Although the Talmud contained certain references of embryo-
logical interest, the first Hebrew treatise on biology was not composed
till the tenth century, when Asaph Judaeus or Asaph-ha-Yehudi
wrote on embryology about a.d. 950. His MSS. are exceedingly
rare, but, according to Gottheil's description, they contain several
sections on embryology. Steinschneider has given another descrip-
tion of them. For further details on the whole subject of Jewish
embryology see Macht.
Arabian science, so justly famed for its successes in certain branches,
was not of great help to embryology. Abu-1-Hasan ' Ali ibn Sahl ibn
Rabban al-Tabari, a Moslem physician who flourished under the
Caliphate of al-Mutawakldl about a.d. 850, wrote a book called
The Paradise of Wisdom, in which an entire part was devoted to
embryology, all the more interesting as it is a mixture of Greek and
ancient Indian knowledge. Browne gives a description of it. Ibn
Rabban's contemporary, Thabit ibn Qurra, is also said to have
written on embryology. The great Avicenna, or, to give him his
proper name, Abu 'Ali-1-Hasan ibn 'Abdallah ibn Sina, who lived
from 978 to 1036, devoted certain chapters of his Canon Medicinae to
the development of the foetus, but added nothing to Galen. His
contemporaries, Abu-1-Qasim Maslama ibn Ahmad al-Majriti and
Arib ibn Said al-Katib, a Spanish Moslem, wrote treatises on the
generation of animals, but neither has survived.
What was alchemy doing all this time? It was engaged on many
curious pursuits, but among them the interpretation of embryonic
development was not one. Alchemical texts before the tenth century
do make reference to eggs from time to time, but it is safe to say
never with any trace of an interest in the development of the embryo
SECT. 2] TO THE RENAISSANCE 95
out of them. One example taken from Berthelot's collection will
suffice; it comes from the "6th book of the Philosopher" (Syriac).
To make water of eggs
Take as many eggs as you wish, break them and put the whites in a
glass flask, place this in another vessel and surround it with fresh horse-
dung up to the neck of the vessel. Leave it so for 15 days changing the
dung every 5 days. Then distil the liquid in an alembic and taking a
pound of the distillate add lime of eggs 2 ozs. Shake well and distil again.
Do this 4 times. Take then of elixir of arsenic, 2 parts, of sulphur i part,
of pyrites and magnesia, each i part. Pound in a mortar and add to the
final distillate from the eggs. Do this for 7 days always working in the
sunlight, once at sunrise, once in the middle of the day, and once at
sunset. When this has been done, dry the mixture, pound it, and set it
aside.
I could only find one reference to the embryo in a hen's egg among
the vast number of alchemical directions of this time, and then only
as a constituent of the egg which must be discarded. As we shall see,
it is not until after the time of Paracelsus that the notion of applying
chemical methods to eggs or embryos arises at all.
2-2. St Hildegard: the Lowest Depth
Not long after the death of Avicenna, St Hildegard was born.
She lived from 1098 to 1180, and was Abbess successively of Disi-
bodenberg and Bingen in the Rhineland. Her treatises on the world,
which are an extraordinary medley of theological, mystical, scientific
and philosophical speculation, have been described in detail by
Singer, and, though in the books. Liber Scivias and Liber Divinorum
Operum simplicis hominis, there is little of embryological interest, yet
she does give an account of development and especially of the entry
of the soul into the foetus.
This is illustrated in Plate HI taken from the Wiesbaden Codex B
of the Liber Scivias. The soul is here shown passing down from heaven
into the body of the pregnant woman and so to the embryo within
her. The divine wisdom is represented by a square object with its
angles pointing to the four corners of the earth in symbol of stabihty.
From it a long tube-Hke process descends into the mother's womb
and down it the soul passes as a bright object, "spherical" or "shape-
less", illuminating the whole body. The scene shows the mother in
the foreground lying down ; inside her there are traces of the foetal
membranes; behind this ten persons are grouped, each carrying a
96 EMBRYOLOGY FROM GALEN [pt. ii
vessel, into one of which a fiend pours some noxious substance from
the left-hand corner. St Hildegard describes and expounds the scene
as follows: "Behold, I saw upon earth men carrying milk in earthen
vessels and making cheeses therefrom. Some was of the thick kind
from which firm cheese is made, some of the thinner sort from which
more porous cheese is made, and some was mixed with corruption
and of the sort from which bitter cheese is made. And I saw the like-
ness of a woman having a complete human form within her womb.
And then by a secret disposition of the most high craftsman, a fiery
sphere having none of the lineaments of a human body possessed the
heart of the form and reached the brain and transfused itself through
all the members. . . . And I saw that many circling eddies possessed
the sphere and brought it earthward, but with ever renewed force
it returned upwards and wailed aloud, asking, 'I, wanderer that I
am, where am I?' 'In death's shadow.' 'And where go I?' 'In the
way of sinners.' 'And what is my hope? ' ' That of all wanderers.' . . .
As for those whom thou hast seen carrying milk in earthen vessels, they
are in the world, men and women alike, having in their bodies the
seed of mankind from which are procreated the various kinds of
human beings. Part is thickened because the seed in its strength
is well and truly concocted and this produces forceful men to whom
are allotted gifts both spiritual and carnal.. . .And some had cheeses
less firmly curdled, for in their feebleness they have seed imperfectly
tempered and they raise offspring mostly stupid, feeble, and use-
less, . . . And some was mixed with corruption . , . for the seed in that
brew cannot be rightly raised, it is invalid, and makes misshapen
men who are bitter distressed and oppressed of heart so that they
may not lift their gaze to higher things. . . .And often in forgetfulness
of God and by the mocking devil a mistio is made of the man and
the woman and the thing born therefrom is deformed, for parents
who have sinned against me return to me crucified in their children".
We have already traced the wanderings of the cheese-analogy,
which, beginning fresh with Aristotle, was taken to Alexandria and
incorporated in the Wisdom Literature, and so found its way to the
Arabic of 'Ali ibn a'1-Abbas al-Majusi, or Haly-Abbas, as he was
known in the West, a Persian. His Liber Totius appeared in Latin
in 1523, but had been translated much earlier, at Monte Cassino
between 1070 and 1085, by Constantine the African, who called it
Liber de Humana Natura, and gave it out to be his own work. Thus
PLATE III
AN ILLUSTRATION FROM THE LIBER SCIVIAS OF ST HILDEGARD OF BINGEN
(Wiesbaden Codex B) showing the descent of the soul into the embryo {ca. 1 150 a.d.).
SECT. 2] TO THE RENAISSANCE 97
St Hildegard obtained it, and worked it up into one of her visions.
At this point embryology touched, perhaps, its low-water mark. But
a great man was at hand, destined to carry on the Aristotelian
tradition and to add to it much of originality, in the shape of Albertus
of Cologne. Before speaking of him, however, a word must be said
about that very queer character, Michael Scot (i 178-1234), who,
according to Gunther, "appeared in Oxford in 1230 and experi-
mented with the artificial incubation of eggs, having got an Egyptian
to teach him how to incubate ostriches eggs by the heat of the Apulian
sun". That "muddle-headed old magician", as Singer rightly calls
him, was not the man to profit by it, but the point is interesting,
especially as an Egyptian is mentioned. Haskins, in his curious
studies of the scientific atmosphere of the court of the Emperor
Frederick II of Sicily, has shown Scot, newly arrived fi"om his
alchemical studies in Spain, assisting that very learned and unor-
thodox monarch in his artificial incubation experiments.
2-3. Albertus Magnus
Albertus Magnus of Cologne and Bollstadt was born in 1206,
and died in 1280, six years after his favourite disciple, St Thomas
Aquinas. The greater part of his life was spent in study and teaching
in one or other of the houses of the Dominican friars, to which he
belonged, though for a time he was Bishop of Regensburg. Albert
resembles Aristotle in many points, but principally because he pro-
duced biological work with, as it were, no antecedents. Just as
Aristotle's contributions to embryology were preceded by no more
than the diffuse speculations of the Ionian nature-philosophers, so
Albert's came immediately after the dead period represented by the
visions of St Hildegard. In many ways, Albert's position was much
less conducive to good work than Aristotle's.
Albert follows Aristotle closely throughout his biological writings,
quoting him word for word in large amounts, but the significant
thing is that he does not follow him slavishly. He resembled Aristotle
in paying much attention to the phenomena of generation, as a rough
computation shows, Aristotle devoting 37 per cent, of his biological
writings to this subject, and Albert 31 per cent., to which Galen's
7 per cent, may with interest be compared. Albert is extremely
inferior to Aristotle, however, in point of arrangement; for Aristotle,
although some of his books, such as the De Generatione Animalium,
98 EMBRYOLOGY FROM GALEN [pt. ii
are sufficiently confused and repetitive, does yet succeed in infusing
a clarity and incisiveness into his style. Albert, on the other hand,
allows his argument to wander through his twenty-six books De
Animalibus in the most complex convolutions, so that the sections on
generation and embryology are found indiscriminately in the first,
sixth, ninth, fifteenth, sixteenth, and seventeenth. In Book i he gives
a kind of summary or skeleton of his views on the embryo. These
follow Aristotle fairly closely; thus, he accepts the AristoteHan classi-
fication of animals according to their manner of generation, and
thinks still that caterpillars are immature eggs ; he derives the embryo
from the white, not the yolk, and he explains why soft-shelled eggs,
being imperfect, are of one colour only. But there are new observa-
tions; for instance, he describes an ovum in ovo, which he has seen,
calling it a natura peccatis, and he speaks definitely of the seed of
the woman, thus departing from Peripatetic opinion, and adopting
the Epicurean view. The female seed, he thinks, suflfers coagulation
like cheese by the male seed, and to these two humidities there must
be added a third, namely, the menstrual blood (corresponding to
the yolk in the case of the bird). "When these three humidities
therefore have been brought into one place, all the similar members
except the blood and fat are formed from the two humidities of which
one generates actively but the other passively. But the blood which is
attracted for the nutriment of the embryo is double in virtue and
double in substance. For a certain part of the blood is united with
the sperm in such a way that it takes on some of the virtue of the
seed because a certain part of the spermatic humour remains in it
and from this are begotten the teeth and for this reason they grow
again if they are pulled out at an age near the time of sperm-making
and do not grow again at an age remoter from this, at which the
virtue of the first generating principle has vanished from the blood.
But another part of the blood is of twofold or threefold substance
and from the thick part of the blood itself is generated the flesh.
And this flows in and flows out and grows again if rubbed away.
From the watery part of the same blood or of the nutritive humour
are generated the fat and oil and this flows in and out more easily
than the flesh itself, but other parts of the blood are its refuse and
impurities and are not attracted to the generation of any part of the
animal, but having been collected until birth are expelled with the
embryo from the uterus in the foetal membranes, like the remnants
SECT. 2] TO THE RENAISSANCE 99
in the hen's egg after the chick has hatched. There is a similar virtue
in the liver and heart of animals which organs after the animals are
born form the flesh and fat from food in accordance with its twofold
substance, and expel the refuse as we said before,"
In the sixth book, Albert contradicts Aristotle's opinion that male
chick develops out of the sharp-ended egg, and one hopes that he
is going to say there is no relationship between egg-shape and sex,
but no, he goes on to say that the Aristotelian statement rested on a
textual error (in which he was quite wrong), so that really Aristode
agreed with Avicenna in saying that the males always develop from
the more spherical eggs because the sphere is the most perfect of figures
in solid geometry. These errors had a most persistent life : Horace has
a passage in which they appear —
longa quibus facies ovis erit, ilia memento
ut suci melioris, et ut magis alma rotundis
ponere: namque marem cohibent callosa vitellu.m.
(When you would feast upon eggs, make choice of the long ones ; they
are whiter and sweeter and more nourishing than the round, for being
hard they contain the yolk of the male.)
They were finally abolished by two naturalists, Giinther and Biihle,
who took the trouble to disprove them experimentally in the eigh-
teenth century. Albertus refers here to artificial incubation: "For
the alterative and maturative heat", he says, "of the egg is in
the egg itself and the warmth which the bird provides is altogether
external [extrinsecus est amminiculans] since in certain hot countries
the eggs of fowls are put under the surface of the earth and come
to completion of their own accord, as in Egypt, for the Egyptians
hatch them out by placing them under dung in the sunlight".
Next he speaks of monsters and of the modes of corruption
of eggs which he divides into four: (i) decomposition of white,
(2) decomposition of yolk, (3) bursting of the yolk-membrane,
(4) antiquitas ovi. "And from the second cause it sometimes happens ",
he says, "that in the corruption of the humours certain igneous
parts are carried blazing to the shell of the egg and distribute them-
selves over it so that it shines in the dark like rotten wood; as
happened in the case of that egg^ which Avicenna said he saw in
the city called Kanetrizine in the country of the Gorascenes." Albert
^ See on this subject Zach.
7-2
lOO EMBRYOLOGY FROM GALEN [pt. ii
is inclined to think that astrological influences may have an effect
on foetal life, but he treats the suggestion with considerable scepticism,
although he believes that thunder and lightning kill the embryos of
fowls (a popular belief to which Fere tried not long ago to give a
scientific foundation), and he regards the embryo of the crow as
especially susceptible, though on what grounds he does not say.
The fourth chapter of the first tractate of the sixth book contains
Albert's description of development of the chick, and is extremely
interesting. He makes two principal mistakes: {a) he describes a
quite non-existent fissure in the shell by which the chick may emerge,
{b) he maintains that the yolk ascends after a day or two into the
sharp end of the egg, adducing as the reason that there is found
there more heat and formative force than elsewhere. On the other
hand, he correctly describes {a) the pulsating drop of blood on the
third day, and {b) he identifies it with the heart with its systolen et
dyastolen sending out the "formative virtue" to all the parts of the
growing body. He notices [c) that the differentiation of the chick at
first proceeds rapidly and later more slowly. But the most notable
characteristic of Albert's embryology is the way in which he is
hampered by his inability to invent a technical terminology. Singer
has studied the way in which anatomical terms, such as "syrach",
etc., came into use, but whatever the causes were which produced
them, they did not operate much in Albert's mind. He represents
the point beyond which embryology could not advance, until it
had created a new set of terms. This is well illustrated by the
following passage:
"But fi'cni the drop of blood", he says, "out of which the heart is
formed, there proceed two vein-like and pulsatile passages and there is in
them a purer blood which forms the chief organs such as the liver and
lungs and these though very small at first grow and extend at last to the
outer membranes which hold the whole material of the egg together. There
they ramify in many divisions, but the greater of them appears on the
membrane which holds the white of the egg within it [the allantois]. The
albumen, at first quite white, is changed owing to the power of the vein
almost to a pale yellow-green tint [palearem colorem]. Then the path of
which we spoke proceeds to a place in which the head of the embryo is
found carrying thither the virtue and purer material from which are
formed the head and the brain, which is the marrow of the head. In
the formation of the head also are found the eyes and because they are of
an aqueous humidity which is with difficulty used up by the first heat
they are very large, swelling out and bulging from the chick's head. A short
SECT. 2] TO THE RENAISSANCE loi
time afterwards, however, they settle down a little and lose their swelling
owing to the digestive action of the heat — and all this is brought about by
the action of the formative virtue carried along the passage which is
directed to the head, but before arriving there is separated and ramified
by the great vein of the albumen-membrane, as may be clearly seen by
anyone who breaks an egg at this time and notes the head appearing in
the wet part of the egg and at the top of the other members. For what
appears first in the making of a foetus are the upper parts because they are
nobler and more spiritual being compacted of the subtler part of the egg
wherein the formative virtue is stronger. When this happened one of the
aforementioned two passages which spring from the heart branches into
two, one of them going to the spiritual part which contains the heart and
divides there in it carrying to it the pulse and subtle blood from which the
lungs and other spiritual parts are formed, and the other going through
the diaphragm \dyqfracmd\ to enclose within it at the other end the yolk
of the Qgg, around which it forms the liver and stomach. It is accordingly
said to take the place of the umbilicus in other animals and through it food
is drawn in to supply the flesh for the chick's body, for the principle of genera-
tion of the radical members of the chick comes from the albumen but the food
from which is made the flesh filling up all the hollows is from the yolk."
After ten days, Albert goes on to say, all the constituent organs
are mapped out and the head is greater then than the rest of the
body put together. He observes that the yolk liquefies early in
development and that slimy concretions are present in the allantoic
fluid later on (uric acid). But the passage quoted does demonstrate
that before further progress could be made some better name must
be found than "the interior membrane to which the first vessel
proceeds" for a given structure.
Albert, however, was accomplishing a good work. One of his best
amplifications of Aristotle was his description of the relationship
between yolk and embryo in fishes. Just as his words about the chick
demonstrate that he must have opened hen's eggs at different stages
during incubation, so his words about fish eggs show that he must
have dissected and examined them also. Thus (Book vi, tractate 2,
chap, i) he says, "Between the mode of development [anathomiam
generationis] of birds' and fishes eggs there is this diflference ; during
the development of the fish the second of the two veins which extend
from the heart does not exist. For we do not find the vein which
extends to the outer covering of the eggs of birds which some wrongly
call the umbilicus because it carries the blood to the outside parts,
but we do find the vein which corresponds to the yolk vein of birds,
for this vein imbibes the nourishment by which the limbs increase.
102 EMBRYOLOGY FROM GALEN [pt. ii
Therefore the generation of the fish embryo begins from the sharp
end of the egg like that of birds and channels extend from the heart
to the head and eyes and first in them appear the upper parts.
As the growth of the young fish proceeds the yolk decreases in
amount being incorporated into the members and it disappears en-
tirely when development is complete. The beating of the heart, which
some call panting, is transmitted through the pulsating veins to the
lower part of the belly carrying life to the inferior members. While
the young fish are small and not yet fully developed they have veins
of great length which take the place of the umbilicus, but as they grow
these shorten till they contract into the body by the heart as has
been said about birds. The young fish are enclosed in a covering
just like the embryos of birds, which resembles the dura mater and
beneath it another containing the foetus and nothing else, while
between the two there is the moisture rejected during the creation of
the embryo". Albert also described ovoviviparous fishes but it is more
difficult in that case to tell whether he had himself seen and dissected
them. He notes also the prodigality of nature in producing so many
marine eggs only destined to be eaten.
In Books IX and xv he treats of the Galenic views on generation
and insists again that there is a seed provided by the female. In
Book XVI he gives his opinions about the animation of the embryo,
quoting the views of the ancients as given in Plutarch, e.g. Alexander
the Peripatetic, Empedocles, Anaxagoras, Theodorus and Theo-
phrastus, the Peripatetics, Socrates, Plato, the Stoics, Avicenna, and
Aristotle, "who saw the truth", but— and it is interesting to notice
it — never the Christian Fathers, whose writings must have been well
known to him. In discussing the Aristotelian views he compares the
menstrual blood to the marble and the semen to the man with a
chisel in his hand.
On the question of epigenesis and preformation, he follows
Aristotle almost word for word, using the same analogies, such as
the "dead eye" and the sleeping mathematician. Here his scho-
lasticism comes out clearly, for in rejecting altogether the theory
that one part being formed then forms the next part, he says, not
that A would have to be in some way like B, but is not, as Aristotle
had, but simply "^Generans et generatum, est simul esset et non esset,
quod omnino est impossibile''^ — a high-handed and very unscientific
manner of settling the question. In conformity with his theology and
SECT. 2] TO THE RENAISSANCE 103
in contradistinction from Aristotle he makes the vegetative and
sensitive souls arrive automatically into the embryo but the rational
soul only by a direct act of God.
His mammalian embryology presents some points of interest. He
follows Hippocrates ("Ypocras") in an account of the co-operation
of heat and cold in member-formation, and he holds very enlightened
views about foetal nutrition, "It appears therefore that the embryo
hangs from the cord and that the cord is joined with the vein and
that the vein extends through the uterus and has blood running
through it to the foetus like water through a canal. Round the embryo
there are membranes and webs as we have seen. But those who think
that the embryo is fed by little bits of flesh through the cord are
wrong and lie, because if this were the case with man it would happen
also with other animals and that it does not do so anybody can find
out by investigation [per anathomyani].'"
Finally, it is typical that in Book xvii Albert repeats what he has
already said in Book vi about the generation of the hen out of the
tgg all over again with slight changes, but he adds the significant
biochemical remark that "eggs grow into embryos because their
wetness is like the wetness of yeast". The importance of Albert in
the history of embryology is clear. With him the new spirit of in-
vestigation leapt up into being, and, though there were many years
yet to pass before Harvey, the modern as opposed to the ancient
period of embryology had begun. Albert's writings were often
copied and printed in the next few centuries, and even as late as 1601
De Secretis Mulierum, an epitome of his books on generation, was
published. In some sense, it still is, as it forms the backbone of the
little book Aristotle's Masterpiece, of which thousands of copies are sold
in England every year. The copy of the De Secretis in the Caius College
Library has written across the title-page in faded ink "Simulacra
sanctitas, duplex iniquitas, Nathan Emgross, Nov. 20. 161 3." But in
spite of Mr Emgross, Albertus, rightly called Magnus, has had the
happy fate of being beatified both by the Church and by science.
2-4. The Scholastic Period
St Thomas Aquinas (i 227-1 274) incorporated the Aristotelian
theories of embryology into his Summa Theologica especially under
the head De propagatione hominis quantum ad corpus. There are some
striking passages, such as "The generative power of the female
104 EMBRYOLOGY FROM GALEN [pt. ii
is imperfect compared to that of the male; for just as in the crafts,
the inferior workman prepares the material and the more skilled
operator shapes it, so likewise the female generative virtue provides
the substance but the active male virtue makes it into the finished
product". How admirably this expresses the dominating sentiment
of the Middle Ages! Aristotle might make a distinction between
matter and form in generation, but the mediaeval mind, with its
perpetual hankering after value, would at once enquire which of the
two was the higher, the nobler, the more honourable.
St Thomas' theory of embryonic animation was complicated. He
had a notion that the foetus was first endowed with a vegetative
soul, which in due course perished, at which moment the embryo
came into the possession of a sensitive soul, which died in its turn,
only to be replaced by a rational soul provided directly by God,
This led him into great difficulties, for, if this scheme were true, it
was difficult to say that man generated man at all; on the contrary
he could hardly be said to generate more than a sensitive soul which
died before birth, and, on this view, what was to happen to original
sin? As Harris has put it, Plato had said that the intellect was the
man, using the body as a boatman uses a boat. Averroes had said
precisely the opposite, namely, that the essence of humanity was in
the body, and that the intellect was something extrinsic, not limited
to the individual, but common to the race. Aristotle had taken the
middle position, and given a soul to plants and animals, but, in
doing so, he had made it into a vital rather than a psychological
principle. The task of combining this -^vxv with the anima of the
Fathers was what scholastic philosophy had before it. No wonder
that St Thomas' account of embryonic animation was open to
criticism. An echo of it appears in a poem of Jalalu'd-Din Rumi
( 1 207-1 273), the greatest of the Persian Sufi poets, and an exact
contemporary of St Thomas Aquinas :
I died from mineral and plant became:
Died from the plant, and took a sentient frame;
Died from the beast, and donned a human dress;
When by my dying did I e'er grow less?
Duns Scotus (i 266-1 308) objected to St Thomas' theory on the
grounds already mentioned, and he himself abandoned the vegetative
and sensitive souls altogether in his De Rerum Principio. This solution
SECT. 2] TO THE RENAISSANCE 105
was no better than that of St Thomas, for, agreeing with the latter
as Duns did that the rational soul was not an ordinary form "educed "
from the "potentiality" of the material, but rather an ad hoc creation
of God, injected by divine power into the embryo at the appropriate
moment, it was difficult to see how the spiritual effects of Adam's
fall could be transmitted to the men of each generation. It was as
if only acquired characteristics were inherited. But the further course
of theological embryology need not be pursued here ; it runs in every
century parallel with true scientific embryology, and it is not my
purpose to do more than take a glance at its progress from time to time.
In the Speculum Naturale, which was written about 1250, by Vincent
of Beauvais, the embryology of Constantine the African appears
again, and the embryology of Aristotle, Galen, and the scholastics
is to be found in Dante Alighieri (i 265-1 321), who dealt with the
subject in his Convivio, and especially in the Divina Commedia. In
Canto XXV of the Purgatorio, Statins (the personification of human
philosophy enlightened by divine revelation) is made to speak to
the poet thus: "If thy mind, my son, gives due heed to my words
and takes them home, they will elucidate the question thou dost ask.
Perfect blood which is in no case drawn from the thirsty veins, but
which remains behind like food that is removed from table, receives
in the heart informing power for all the members of the human body,
like the other blood which courses through the veins in order to be
converted into those members. After being digested a second time
it descends to the part whereof it is more seemly to keep silence than
to speak, and thence it afterwards drops into the natural receptacle
(the uterus) upon another's blood ; there the one blood and the other
mingle. One is appointed to be passive, the other to be active
according to the perfect place whence it proceeds (the heart). And
being united with it, it begins to operate, first by coagulating it, and
then by vivifying that to which it has given consistency, so that there
may be material for it to work upon [e poi avviva, Cib che per sua materia
fe' constare]. The active power having become a (vegetative) soul like
that of a plant — only differing from it in this, that the former is in
progress while the latter has reached its goal — thereafter works so
much that it moves and feels like a sea-fungus and as the next stage
it takes in hand to provide with organs the faculties which spring
from it. At this point, my son, the power which proceeds from the
heart of the begetter is expanded and developed, that power in which
io6 EMBRYOLOGY FROM GALEN [pt. ii
Nature is intent on forming all the members, but how from being
an animal it becomes a child, thou seest not yet, moreover this is
so difficult a point that formerly it led astray one more wise than thou
[Averroes], so that in his teaching he separated the active 'intellect'
from the soul because he could not see any organ definitely appro-
priated by it. Open thy heart to the truth and know that as soon
as the brain of the foetus is perfectly organised, the Prime Mover,
rejoicing in this display of skill on the part of Nature, turns him
towards it and infuses a new spirit replete with power into it which
subsumes into its own essence the active elements which it finds al-
ready there, and so forms one single soul which lives and feels and
is conscious of its own existence. And that thou mayst find my saying
less strange, bethink thee how the heat of the sun passing into the
juice which the grape distils, makes wine".
Having said this. Statins, Virgil and Dante pass on to the seventh
ledge in Purgatory. It is interesting to see how Dante emphasises
the dynamic teleological side of Aristotle and practically speaks of
the soul enfleshing itself and arranging organs for its faculties. The
reference to Averroes is explained by the fact that Averroes was a
Traducianist, and held that all the soul was generated by man at
the same time as the body, whereas both St Thomas and Dante, as
Creationists, held that each fresh soul was a special creation of God
inserted by him into the brain of the embryo. The mention of Dante's
contemporary, Mondino de Luzzi (1270-1326), brings us to the more
practical aspects of embryology at this period. Mondino is the most
outstanding figure among the Bolognese anatomists of what is really
the first period of the revival of biology. After him, as we shall see,
biology languished for a couple of centuries until the advent of such
men as Ulysses Aldrovandus in the sixteenth century, and Singer has
shown that this was probably due to the fact that anatomy professors
did not dissect in person. A fortiori embryotomy was infrequent.
But Mondino's Anathomia, published in 13 16, contained statements
about the organs of generation which were rather important. He
retains the notion of the seven-celled uterus, which had been intro-
duced by Michael Scot, but he adopts a reasonable compromise
between the opinions of Galen and Aristotle on the physiology of
embryo formation. The distance between him and Leonardo da
Vinci (1452-1519) would, however, be estimated rather at five or six
centuries than at the century and a quarter that it actually was.
SECT. 2] TO THE RENAISSANCE 107
2-5. Leonardo da Vinci
Leonardo was not alone among the artists of the Renaissance in
his anatomical interests, for Michael Angelo, Raphael, Diirer,
Mantegna, and Verrochio all made dissections in order to increase
their knowledge of the human body. But he penetrated more curiously
into biology than they did, and he will always remain one of the
greatest of biologists, for he first introduced the quantitative outlook.
In this he was some four hundred years before his time.
Leonardo's embryology is contained in the third volume of his
notebooks, Quaderni d' Anatomia, published in facsimile by the ad-
mirable labours of three Norwegian scholars, Vangensten, Fohnahn
and Hopstock, in 191 1. His notebooks are a remarkable, and, indeed,
charming miscellany of anatomical drawings, physiological diagrams,
architectural and mechanical sketches and notes such as "Shirts,
hose, and shoes", "Go and see Messer Andreas", "get coal", "the
supreme fool (is the) necromancer, and enchanter".
His dissections of the pregnant uterus and its membranes are
beautifully depicted, as can be seen from the figures which are here
reproduced (Plate IV). He was acquainted with amnios and chorion,
and he knew that the umbilical cord was composed only of vessels,
though he seems to have thought the human placenta was cotyle-
donous. There is one drawing which the editors suppose to represent
the developing hen's egg, but I do not feel that this ascription is likely.
Indeed, Leonardo worked with eggs much less than with mammalian
embryos, though there are references to the former. "See how birds
are nourished in their eggs", he says in one place, to remind himself,
perhaps, of possible experiments, and, elsewhere, "Chickens are
hatched by means of the ovens of the fireplace". Again, "Ask the
wife of Biagino Crivelli (was she the Lucrezia Crivelli, whose portrait
Leonardo painted?) how the capon rears and hatches the eggs of the
hen when he is inebriated", a subject recently reopened by Lienhart.
"You must first dissect the hatched egg before you show the difference
between the human liver in foetus and adult." Leonardo perpetuates
a persistent error in the note, "Eggs which have a round form
produce males, those which have a long form produce females".
Concerning the mammalian foetus, he says, "The veins of the
child do not ramify in the substance of the uterus of its mother but
in the placenta which takes the place of a shirt in the interior of the
io8 EMBRYOLOGY FROM GALEN [pt. ii
uterus which it coats and to which it is connected but not united by
means of the cotyledons". Thus in one sentence Leonardo falls into
a mistake in saying that the human placenta is cotyledonous, but
at the same time asserts a fact which it took all the ingenuity of the
seventeenth century to prove to be true, namely, that the foetal
circulation is not continuous with that of the mother, for the placenta
is only connected to the uterine wall and not united with it. "The
child", Leonardo goes on to say, "lies in the uterus surrounded with
water, because heavy things weigh less in water than in the air
and the less so the more viscous and greasy the water is. And then
such water distributes its own weight with the weight of the creature
over the whole body and sides of the uterus." The tendency towards
quantitative and mathematical explanations is apparent at once.
Further notes are, "Note how the foetus breathes and how it is
nourished through the umbilical cord and why one soul governs two
bodies, as you see the mother desiring food and the child remaining
marked (by a given amount of growth) because of it. Avicenna
pretends that the soul generates the soul and the body the body.
Per errata^'. The child, says Leonardo, secretes urine while still in
utero, and has excrement in its intestines; at four months it has chyle
in its stomach, made perhaps from menstrual blood. But it has no
voice in utero, "when women say that the foetus is heard to weep
sometimes within the uterus, this is rather the sound of some flatus . . . ".
Nor does it breathe there (on this point Leonardo contradicts him-
self). "The child does not respire within the body of its mother
because it lies in water and he who breathes in water is immediately
drowned." "Breathing is not necessary to the embryo because it is
vivified and nourished by the life and food of the mother." Nor does
the embryonic heart beat. To us the statement that there is no
respiration in the uterus is obviously false, but we mean by the word
tissue respiration, whereas in Leonardo's time pulmonary respiration
was intended; he was therefore perfectly right in denying that the
embryo breathed, as certain anatomists before him had asserted.
His only reference to the soul runs thus: "Nature places in the
bodies of animals the soul, the composer of the body, i.e. the soul
of the mother, which first composes, in the womb, the shape of man
and in due time awakens the soul which shall be the inhabitant there-
of, which first remains asleep and under the tutelage of the soul of
the mother which through the umbilical vein nourishes and vivifies
PLATE IV
vf Hi^l
-'<fft''*'\»-t
O V^xf^rtl
V-..,'.'.^/ I »-^ '•'
•V 'j'.'/** •»T^'"(' ''♦'
A PAGE FROM LEONARDO DA VINCI'S AxNATOMICAL NOTEBOOKS
{QUADERNI D' ANATOMIA), ca. 1490 a.d.
SECT. 2] TO THE RENAISSANCE 109
it". This is not very revolutionary. But Leonardo was the first
embryologist to make any quantitative observations on embryonic
growth ; he defined, for instance, the length of a full-grown embryo
as one braccio and the adult as three times that. "The child", he
says, "grows daily far more when in the body of its mother than
when it is outside of the body and this teaches us why in the first
year when it finds itself outside the body of the mother, or, rather,
in the first 9 months, it does not double the size of the 9 months
when it found itself within the mother's body. Nor in 18 months
has it doubled the size it was 9 months after it was born, and thus
in every 9 months diminishing the quantity of such increase till
it has come to its greatest height." Here Leonardo touches on one
of the most modern quantitative aspects of embryology, and one
almost expects to see him exemplify his words with a graph until
one remembers with a shock that he lived two centuries before
Descartes and five before Minot. His numerical data may also have
included figures about the relative sizes of the parts, and the germ
of the line of research so successfully pursued by Scammon in our
own times may be found in the note "The liver is relatively much
larger in the foetus than in the grown man". Other quantitative
notes concern the length of the embryonic intestines as in the laconic
"20 braccia of bowels" and the statement that "the length of the
umbilical cord always equals the length of the foetal body in man
though not in animals".^
He said little about heredity, but in one place he mentions a case
of sexual intercourse between an Italian woman and an Ethiopian,
the outcome of which assured him that blackness was not due to the
direct action of the sun and that the "seed of the female was as potent
as that of the male in generation". Finally, the best instance of the
wideness of his thought appears in the note, "All seeds have an um-
bilical cord which breaks when the seed is mature. And similarly
they have matrix and secundines as the herbs and all the seeds
which grow in shells show". We have met this idea before in Hippo-
crates of Cos, and we shall find it again in Nathaniel Highmore.
It is no coincidence that pictures of weights and cogs and pulleys
stand side by side in Leonardo's notes with anatomical drawings of
the embryo. As Hopstock says, "Leonardo arrives at the conclusion
that there is but one natural law which governs the world. Necessity.
^ Leonardo would have enjoyed Fog's statistical study of 8000 umbUical cords (1930).
no EMBRYOLOGY FROM GALEN [pt. ii
Necessity is Nature's master and guardian, it is Necessity that makes
the eternal laws". If Aristotle is the father of embryology regarded
as a branch of natural history, Leonardo is the father of embryology
regarded as an exact science.
2-6. The Sixteenth Century: the Macro- Iconographers
After such a man, the writings of his contemporaries, such as the
mythical Johannes de Ketham, Alessandro Achillini and Gabriele de
Gerbi, appear beyond description inferior. De Ketham's embryology
has been described by Ferckel. De Gerbi included in his Liber
Anatomiae corporis humani et singulorum membrorum illius a section
entitled De Generatione Embrjonis, but there is nothing to be said
about it except that it is a verbose compilation of the views of
Aristotle and Galen taken from Avicenna. The work of Nolanus in
1532 presents certain points of interest, but it is of little importance.
Petrus Crescentius in his work on husbandry of 1548 mentions
artificial incubation in ovens, but rather as a lost art. About this
time also Hieronymus Dandinus Cesenas, a Jesuit, wrote a treatise
on Galen's division of organs into white and red, those proceeding
from the semen and those proceeding from the blood: it is cited by
Aldrovandus, but I have not been able to consult it.
The most remarkable feature of the first half of the century was
the encyclopaedic group of zoologists which now arose. Thus Belon
and Rondelet, whose well-illustrated catalogues of animals were
appearing from 1550 onwards, did a good service to comparative
embryology in figuring the ovoviviparous selachians and viviparous
cetacea. Gesner belongs to this group. All of them reproduce thin
versions of Aristotle, when they speak of generation as such, and this
is what differentiates them from Ulysses Aldrovandus, of whom I
shall speak presently. Figs. 3 and 4 show Rondelet's pictures of a
viviparous dolphin and an ovoviviparous selachian.
But the end of the twilight period was now at hand, for, within
thirty years after the death of de Gerbi in 1505, four great
embryologists were born as well as the greatest anatomist of any age,
Andreas Vesalius (1514), of whom I shall say no more, for he had
no opportunities for dissecting human embryos, and took hardly
any interest in foetal development. But in 1522 Ulysses Aldrovandus
was born, and in the following year Gabriel Fallopius, in 1530
Julius Caesar Arantius and in 1534 Volcher Goiter. Only three
SECT. 2]
TO THE RENAISSANCE
III
more years bring us to the birth of Andreas Laurentius and of
Hieronymus Fabricius ab Aquapendente, the teacher of William
Harvey.
The senior member of this group, Ulysses Aldrovandus, was the
first biologist since Aristotle to open the eggs of hens regularly during
Fig. 3. A (viviparous) dolphin: from Rondelet's De piscibus marinis of 1554.
their incubation period, and to describe in detail the appearances
which he found there. In his Ornithologia, published at Bonn in 1597,
he set out to describe all the known kinds of birds, discussing in
turn not only their zoological and physiological characteristics, but
Fig. 4. An (ovoviviparous) shark: from Rondelet's De piscibus marints of 1554.
also their significance as presages and for augury, their mystical
meaning, their use as allegories and for eating, and finally all the
legends respecting them, Generositas, Temperantia, Liberalitas, aquilae
one finds. Beginning with the eagle, he proceeds to the vulture,
the owl, the bat (the only viviparous bird!), the ostrich, the harpy (!),
112 EMBRYOLOGY FROM GALEN [pt. ii
the parrot, the crow, and so to the fowl. Side by side with a
reference to the famous poem of Prudentius {Multi sunt Presbyteri,
translated by J. M. Neale) about the steeple-cock, we find an
excellent account of the generation of the chick in the c:gg. The book
is illustrated sumptuously, but unfortunately there is only one picture
of embryological interest, namely, a chick in the act of hatching.
In Aldrovandus' embryology there is much discussion of Aristotle
and Galen, but traces of an independent spirit abound. Pliny's view
that the heart was formed in the white is "exploded", and Aldro-
vandus says that it is formed on the yolk-membrane. He refutes the
opinion of Galen also that the liver is first formed, in connection
with which he says, "In order that I might bring to an end this
controversy between the philosophers and the physicians I followed
with the keenest curiosity and diligence the incubation of 22 hen's
eggs, opening one each day; thus I found Aristotle's doctrine to be the
truest. And because apart from the fact that these matters are most
worthy of being looked into they provide also the greatest pleasure
and entertainment I have thought it well to describe them as clearly
and briefly as possible".
Aldrovandus also contradicts Albertus, and propounds a new
theory, namely, that the spiritualia (the organs in the thorax) are
formed from the seed of the cock {ex maris semine sunt). This seed
he aflfirms to be present in the egg, and he identifies it with the
chalazae, thus anticipating Fabricius ab Aquapendente, but not
going quite so far, and explicitly opposing Gaza, who had said not
long before that the chalazae were simply congealed water. Aldro-
vandus' admiration for Aristotle is extreme, and, though he differs
from him about the chalazae, he defends the Aristotelian opinion
that the chick was made from the white but nourished from the yolk.
His argument for this is new, however; it is that, during incubation,
the latter liquefies but the former hardens; now in all digestion
liquefaction takes place, and in all growth hardening, therefore, etc.
This argument is a great deal more cogent than most of those which
were current between 1550 and 1650. He goes out of his way to
castigate Albertus for saying that the yolk moves up into the sharp
point of the egg, for experience assures him that it does not, "as I
have observed by cutting open an egg after one day's incubation".
A striking instance of his powers of observation was his description
of the "egg-tooth" of embryonic birds, a discovery made anew in
SECT. 2] TO THE RENAISSANCE 113
the nineteenth century by Yarrell and Rose. The chick was perfect
in form, according to him, on the tenth day.
The peculiarity of Aldrovandus lies in the fact that he incorporated
so many elements into one book, and was able to produce a collection
of chapters in which good scientific observation sat at the closest
quarters with literary allusion and semi-theological homily. So well-
proportioned a mixture as the Ornithologia is not often found. As a
final instance three consecutive paragraphs may be mentioned, in
the first of which he discusses Plutarch's arid problem about the
priority of egg or hen, next he makes some very reasonable remarks
about teratology, suggesting that monsters come from yolks which are
physico-chemically abnormal in some way, while in the third he
expresses strong scepticism concerning the tale that the basilisk is
sometimes hatched out from a hen's egg — ''Ego ne jurantibus quidem
crediderim'\ he says. This last notion is found in the fourteenth-
century poem of Prudentius alluded to above, and appears again in
the Miscellaneous Exercitations of Caspar Bartholinus the younger,
whose second chapter is devoted to showing "That the basilisk
hatcheth not from the egg of the hen", a conclusion which has been
amply confirmed in the light of subsequent experience. Bartholinus
gives a bibliography of this curious legend.
Aldrovandus and his disciple Volcher Coiter the Frisian, as he
described himself, were alike in not suffering from the prevailing
vice of the age, verbosity. Colter's Externarum et Internarum princi-
palium humajii corporis partium tabulae et exercitationes, which appeared
at Nuremberg in 1573 — a beautifully printed book — contained a
brief section entitled De ovorum gallinaceorum generationis primo exordio
progressuque et pulli gallinacei creationis ordine. His Latin style betrays
his German origin, for the constructions are very Teutonic, although
the meaning is always perfectly clear. Coiter says, "In the year
1564 in the month of May at Bologna, being instigated by that
excellent professor of philosophy outstanding in varied sciences and
arts. Doctor Ulysses Aldrovandus, and by other doctors and students,
I ordered 2 broody fowls to be brought and under each of them
I caused 23 eggs to be placed, and in the company of these persons
I opened one every day so that we could see firstly the origin of
the veins and secondly what organ is first formed in the animal".
What follows is practically a repetition of the facts available in
Aristotle, but described with much greater clearness than either
114 EMBRYOLOGY FROM GALEN [pt. ii
Aristotle or Aldrovandus had been able to bring to the matter.
On the third day, he saw the globulus sanguineus which in vitello
manifeste pulsabat, and so solved his first problem. He decides that
the first organ to be formed is the heart, and quotes Lactantius'
experiments. He explains the large size of the eye as due to the fact
that the most complicated part of the body needs the longest time
for its manufacture. He correctly describes the various membranes,
and the faeces subviridies in the intestines at hatching. Once he
contradicts Aristotle, maintaining that on the tenth day the body as
a whole is larger than the head, and once he contradicts Albertus,
denying that any yolk can be found in the stomach at hatching. He
concludes his tractate by a succinct and clear account of the opinions
of Aristotle and Hippocrates about embryonic development. His
importance is that he drew the attention of scientific thinkers to the
problems arising out of the hen's egg, and assisted in the formation
of that iconographic phase in embryology which was later to find
its climax in the plates of Fabricius, and its close in Harvey's Exer-
citations.
Gabriel Fallopius, who belongs to this time, must be mentioned
as the discoverer of the organs which bear his name, but his services
to embryology were only indirect. A. Benedictus, who was now
growing old, and Caesar Cremonius, who was still young, may be
remembered as the principal upholders of pure Aristotelianism at this
time. Realdus Columbus also wrote on the embryo. B. Telesius,
in his De Natura Rerum of 1565, studied the hen's egg and suggested
that the parts of animals were formed by the pressure of the uterus
acting as a mould: he was thus the middle term between Galen
and Buffon.
Julius Caesar Arantius has already been referred to. His De
Humano Foetu was an important book, but, though it appeared in
1564, just at the time when the macro-iconographic school was at
its height, it dealt with a rather different field and cannot be con-
sidered as a constituent of that group. He begins by relating that a
pregnant woman was killed by an accident at Bologna a couple of
years before, so that he had an opportunity of testing whether the
opinions about certain points in generation, which he had formed
on a priori grounds during the previous fifteen years, were true or
not. In the first place, he found on dissection that the placenta was
not cotyledonous, and he spoke thus of its formation: "Blood flows
SECT. 2] TO THE RENAISSANCE 115
out from the spongy substance of the uterus and this blood growing
in bulk forms a soft and fungus-like mass of flesh, rather like the
substance of the spleen, which adheres to the surface of the uterus
and transmits to the foetus in proportion as it grows the nourishment
for it which reaches the uterus in the form of blood and spirits".
Then, going on to discuss the functions of the jecor uterinae, as
he calls the placenta (with what justice may be seen by turning to
Section 8-5), he devotes a chapter to De vasorum umbilicalium origine,
and, contradicting Hippocrates, Galen, Erasistratus, and Aetius, says
that the maternal and foetal blood-vessels do not pass into each
other by a free passage. "This is repugnant to sense", he writes,
"and as may be seen by ocular inspection, these vessels do not reach
the inner membrane of the uterus, for the substance of the placenta
is placed between their ramifications and the proper substance of
the womb." He was thus the first to maintain that the maternal
and foetal circulations are separate, but he naturally did not, and
could not, speak of circulations, since he lived before Harvey. Nor
could he have satisfactorily proved his point with the means then
at his command, and, as we shall see, it was to take another century
before the proof was given. Apart from this valuable contribution
to embryology, Arantius gave some admirable anatomical descrip-
tions of the foetal membranes.
Hieronymus Fabricius ab Aquapendente, the pupil of Fallopius,
has always been given an important place in the history of embryo-
logy by those who have written on him. As one comes upon him
in the process of tracing out that history itself, however, he does
not take such a high place. With the statement, for instance, that
"Fabricius carried embryology far beyond where Goiter had left
it and elevated it at one bound into an independent science" I
find that I cannot agree. Embryologists who called themselves
that and nothing else did not appear till the end of the eighteenth
century, and it seems to me doubtful whether the anatomical ad-
vances in embryology made by Fabricius are not counterbalanced
by the erroneous theories which he invented at the same time. His
De Formatione Ovi et Pulli pennatorum, and his De Formato Foetu of 1604
show far more scholasticism and mere argumentativeness than is to
be found in Goiter, and are remarkable for their bulk. Fabricius
seems to have had a genius for exsuccous and formal discussions. He
spends much time, for example, in taking up the problem of whether
8-2
ii6 EMBRYOLOGY FROM GALEN [pt. ii
the yolk of the hen's egg is more earthy than the white, and looking
at it from all possible angles. He disagrees at last with Aristotle
and decides that the white is the more earthy. Bones, he says, are
white, but also very earthy. The albumen is colder, stickier, and
heavier than the yolk, "sequitur, terrestrius esse^\ And this particular
example is the more flagrant because the actual matter of it is
fundamentally physico-chemical. But, in addition, he introduced a
number of grave errors and misleading theories into embryology, so
that subsequently Harvey had to spend a large part of his time
refuting them. Fabricius was, indeed, a good comparative anatomist,
and it is upon that ground that he deserves praise: his plates, some
of which are reproduced herewith, were far better than anything
before and for a long time afterwards. He dissected embryos of man,
rabbit, guinea-pig, mouse, dog, cat, sheep, pig, horse, ox, goat, deer,
dogfish, and viper, a comparative study which had certainly never
been made previously.
In his first tractate he begins by dealing with a question not unlike
that of how the sardines got into the tin, i.e. how the contents got
into the hard-shelled egg. He rejects Aristotle's idea that the egg
is formed in the oviduct by a kind of umbilicus, and ascribes its
growth there to transudation through the blood-vessels. He marks
a definite advance upon Aristotle when he says that silkworms and
other insects are born into their larval state from an egg, though he
still terms the chrysalis an egg, and therefore holds that they are
generated twice. Then follows his discussion of what part of the egg
the chick comes from. The chalazae, he says, are not semen, for the
semen is not present at all in the fertilised egg. His argument sounds
peculiar when he says that both the white and yolk of the egg are
the food of the embryo, for neither of them is absent at the end of
incubation, therefore neither of them is its material. Hippocrates
had said, "^ex luteo gigni, ex albo nutriri''; Aristotle had said, "ex
albo fieri, ex luteo nutrirV\ The latter was the view generally held
in the sixteenth century, as may be gathered from Ambrosius
Calepinus' dictionary, Scaliger's Commentary on Aristotle, and the
treatise on the soul of Johannes Grammaticus.
Fabricius now says both nourish, neither makes. This distinction
between food and building-materials seems to us unnecessary, but
it had a great influence on later thought. Fabricius devotes much
time to proving, as he thinks, that albumen and white are of the
PLATE V
ILLUSTRATIONS FROM FABRICIUS AB AQUAPENDENTE'S
DE FORMATIONE OVI ET PULLI, 1604
SECT. 2] TO THE RENAISSANCE 117
same nature, and adduces the fact that "in cooking the white hardens
first, whether the egg be boiled or poached, but the yolk hardens
also if the heat is more", comparing the heat of the kitchen to the
innate heat of the chick. "But you will say", he goes on, "if the
albumen and the yolk are the food of the chick in the egg, what
then must we decide the material of the chick to be, since we have
already said that the semen is not present in the eggs. You will
find this material from an enumeration of the parts of the egg — there
remains only the shell, the two membranes, and the chalazae; —
nobody will assign the membranes or the shell as the material of
the chick, therefore the chalazae alone are the fitting substance out
of which it can be made." Having discovered this truth by the
infallible processes of logic, Fabricius brings all kinds of arguments
forward to support it; he adduces the three nodes in the chalazae
as the precursors of brain, heart, and liver; tadpoles, he thinks,
resemble significantly the chalazae, being "armless legless spines".
The eyes are transparent, so are the chalazae, therefore the latter
must give rise to the former. The liver is formed as soon as the heart
but is practically invisible as it does not palpitate. One of his most
gratuitous errors was the suggestion, now newly introduced, that the
heart (and other organs) of the foetus has no proper function, no
munus publicum, but beats only in order to preserve its own life.
Then there is a considerable section called De Ovorum utilitatibus,
which almost does for the hen's egg what Galen's De Usu Partium
did for the human body, and in which such questions as Why
the shell is hard and porous? and Why there are any membranes
in the egg? are taken up and answered with an elaborate display
of common sense. The influence of Galen is perceptible in a passage
about a liver-like substance being formed if blood is freshly shed into
hot water, in the usual terminology of formative faculties, and in the
division of fleshes into white and red, though the former is not
specifically derived fi"om the semen nor the latter from the menstrual
blood. The human placenta is described as cotyledonous, and need-
less confusion is caused by the doctrine that the "liquors, humours,
or rather, excrements, around the foetus, are two in number, sweat
and urine, the former in the amnios, the latter in the allantois".
But the drawings and illustrations of Fabricius' work are beautiful
and accurate — so much so, indeed, that it will always remain
a mystery how the man who figured the early stages of the
ii8 FROM GALEN TO THE RENAISSANCE [pt. ii
development of the chick as Fabricius did, showing the blood-
vessels radiating from the minute heart, should have been able
to propound the thesis that the chalazae were the material of the
embryo.
The other biologist to whom Harvey was most indebted was
Andreas Laurentius of Montpellier, whose Historia Anatomica (printed
with his other works in 1628) contained a whole book (viii) devoted
to embryology, but which presents us with nothing except a com-
mentary on Hippocrates and Aristotle. The only evidences of life
are furnished by two polemics, one of which was against Simon
Petreus of Paris, who had propounded some new views about the
foetal circulation. Laurentius gave also a table showing the changes
which occur in the heart and lungs of the foetus at birth.
It was about this time that the embryological observations of that
many-sided genius, Hieronymus Cardanus, began to attract atten-
tion. His main thesis was that the limbs of the embryo were alone
derived from the yolk, while the rest of the body came from the
white. This was a well-meant attempt to mediate between the two
traditions headed respectively by Aristotle and Hippocrates, but the
arguments in support of it were not even remarkable for ingenuity.
Constantinus Varolius treated of the formation of the embryo in a
book which appeared in 1591, but very inadequately. He had
certainly opened hen's eggs, and describes the fourth-day embryo
as forma minimi faseoli. But nearly every one of his marginal
headings begins with the word Cur, and this tells its own story,
for the didactic style rarely hides genuine works of research. Johannes
Fernelius, a rather earlier worker, in his De Hominis Procreatione fol-
lowed Aristotle and Galen in nearly all particulars, and made no
real contribution to embryology. On its practical obstetrical side,
the sixteenth century produced some remarkable compilations of
ancient gynaecological writings. The first of these was that of Caspar
Wolf, which was published at Ziirich in 1566, and, after having
been enlarged by Caspar Bauhin in 1586, subsequently formed the
backbone of the most important and famous one, namely, that of
Israel Spach (Strassburg, 1597). Although these composite text-
books represented no real embryological progress, they yet showed
that great interest in development was alive, an interest which,
though doubtless utilitarian in its origin, could hardly fail to lead
to advances of a theoretical nature. (See Fig. 5.)
Fig. 5. Illustration from W. H. Ryff's Anatomia of 1541.
120 FROM GALEN TO THE RENAISSANCE [pt. ii
The obstetrical literature intended for midwives is also of great
interest. It was about this time that the first popular guides to their
subject began to appear, founded not upon mere superstition and
the remnants of ancient knowledge derived in roundabout fashion
through Syriac and Arabic, but either upon a careful study of Galen
and Aristotle, or upon the results of dissections and living speculation.
The principal representative of the former class is that of Jacob
Rueff, which appeared in 1554 and was called De Conceptu et Genera-
tione Hominis. Although written in Latin, it was evidently a popular
work, for the illustrations given in it are such as would naturally
be incorporated in such a book. It is the illustrations which give it
its importance, and I reproduce them in Fig. 6. I think they show
very clearly what the general ideas were at this period about mam-
malian embryology, and thus afford us a precious insight into what
was in the minds of such writers as Riolanus the elder, Mercurialis,
Saxonia, Rondeletius, Venusti, Holler and Vallesius. There are many
points which their expositions of foetal growth and development leave
vague, and without Rueff it would be difficult or impossible to picture
in what manner they imagined it to go on. Rueff 's text follows Galen
and Aristotle with fidelity, as does theirs — with the exception of a few
minor ideas not quite consonant with this.
In (a) of Fig. 6 Rueff portrays the mixture of semen and menstrual
blood in the womb, or, as he loosely refers to it, of both seeds,
coagulating into a pink egg-shaped mass surrounded with a fine
pellicle, {b) shows the same mass in the uterus and wrapped round
with the three coats, amnion, chorion, and allantois — a lamentable
but interesting misrepresentation of the facts. Then in {c) it is shown
that upon the surface of the yolk-like mass of semen and blood
appear "three tiny white points not unlike coagulated milk", these
being the first origins of the liver, the heart, and the brain. Next {d)
shows the first blood-vessels springing from the heart, four in number,
and distributing themselves over the surface of the mass. It is plain
that Rueff must either have opened hen's eggs himself and seen the
early growth of the blastoderm or have been told about it by some
observer such as Goiter or Aldrovandus. He could not have copied
his pseudo-blastoderm pictures from their works, for in 1554 none
of them had appeared, and, as far as I know, there were no similar
illustrations in existence at that time.
After this point the pictures grow even more fanciful, and, in (^),
Fig. 6. Illustrations from Jacob Rueff's De Conceptu et Generatione Hominis of 1554
(arranged by Singer) showing the Aristotelian coagulum of blood and seed in the uterus.
122 EMBRYOLOGY FROM GALEN [pt. ii
the first outline of the cranium is seen taking shape in the upper
part of the "egg". In (/) the blood-vessels have suddenly assumed
the outline of a human being, and in (g) the finished product is seen.
Rueff gives what seems to be a mnemonic in hexameters :
iniectum semen, sex primis certe diebus
est quasi lac : reliquisque novem sit sanguis ; at inde
consolidat duodena dies; bis nona deinceps
effigiat; tempusque sequens producit ad ortum
talis enim praedicto tempore figura consit.
Rueff gives some excellent diagrams of the foetus in utero with
relation to the rest of the body, and the various positions which are
familiar to obstetricians. His teratology is less happy, for he attri-
butes the production of monsters to the direct action of God, though
he does venture upon a few speculations concerning "corrupt seed".
But his principal significance for this history is that, in his picture
of the yolk-like mass of mixed semen and blood and the pseudo-
blastoderm upon it, he throws a good light on the conceptions of
the time.
Rueff 's book was subsequently translated into English, and had
many editions as The Expert Midwife.
The principal representative of the second class of popular books
of this period is that of Euch. Rhodion, or Rosslein, which was
translated into English, and published as his own work, by Thomas
Raynold, " physition ", in 1 545, under the title of The Byrth ofMankynde
otherwyse named The Woman" s Book (cf. d'Arcy Power). It was the first
book in the English language to contain copper engravings. They
were variants of the traditional Soranus-Moschion figures. The
Rosslein-Raynold book pays less attention to Galenic theory than does
that of Rueff, and includes much better drawings of actual dissec-
tions. Another famous obstetrical book was that of Scipio Mercurius;
for further information here see Spencer.
The minor embryologists of the sixteenth century included among
them Ambroise Pare, the founder of modern surgery. His teaching
on generation involved nothing original, but it seems to have been
Galenism interpreted by a very intelligent and well-balanced, un-
speculative mind. The three-bubble theory appears in him very
clearly; thus, we read, "The seed boileth and fermenteth in the
womb, and swelleth into three bubbles or bladders" — the brain, the
SECT. 2] TO THE RENAISSANCE 123
liver, and the heart. Fare's illustrations are copied wholesale from
Vesalius and Rueff, without acknowledgment. The last author to
take the three-bubble theory quite seriously was A. Deusingius, who
wrote in 1665, after Harvey. Others who deserve a mention, but
no more, were Severinus Pinaeus, L. Bonaciolus and Felix Platter.
None of them made any advance, and the illustrations of the former's
De Virginitatibus notis graviditate et partu were almost ludicrous.
Hieronymus Capivaccius, F. Licetus, J. Costaeus and V. Cardelinus,
who wrote in 1608, were the last true supporters of the ancient
theories, such as that the male embryo was twice as hot and developed
twice as quickly as the female.
SECTION 3
EMBRYOLOGY IN THE SEVENTEENTH
AND EIGHTEENTH CENTURIES
3-1. The Opening Years of the Seventeenth Century
iEmilius Parisanus, a Venetian, now dealt with embryology in the
fourth, fifth, and sixth books of his De Subtilitate. They were entitled
as follows: "(4) Of the principles and first instruments of the soul
and of innate heat, (5) Of the material of the embryo and of its
efficient cause, (6) Of the part of the animal body which is first
made, and of the mode and order of procreation". Parisanus is
very wordy, but he has the merit of giving many quotations from
the lesser known authors, and providing (as a rule) accurate refer-
ences. He held that the spleen was formed in all development before
the heart, and that neither heart nor lungs moved in utero. With
regard to the controversy over the function of white and yolk, he
was in agreement with Fabricius, but he firmly opposed the view
that the chalazae were the first material of the chick, as much, it
must be confessed, because of the opinion of Aristotle as from his
own observation. Nevertheless, his own observations were note-
worthy, and he will always be remembered for his discovery of the
fact that the heart of the chick begins to beat some time before any
red blood appears in it.
Parisanus was the last of the macro-iconographic group of sixteenth-
century embryologists. Their labours established the fundamental
morphological facts about the developing embryo; the first great
step in the history of embryology. But there were numerous errors
in their work, and Harvey, who occupies a terminal or boundary
position, was destined to correct them. He marks the transition from
the static to the dynamic conception of embryology, from the study
of the embryo as a changing succession of shapes, to the study of it
as a causally governed organisation of an initial physical complexity,
in a word, from Goiter and Fabricius to Descartes and Mayow.
Iconography did not die : on the contrary, the improvement of the
microscope gave it new life, and the micro-iconographic school
emerged with its principal glory, Malpighi.
SECT. 3] THE SEVENTEENTH CENTURY 125
Harvey sums up the work of the macro-iconographic period in the
historical introduction contained in Ex. xiv of his De Generatione
Animalium. I give it in full in the beautiful seventeenth century
English into which Harvey's Latin was translated under his guidance
by the physician, Martin Llewellyn,
"We have already discovered the Formation, and Generation of
the Egge; it remains that we now deliver our Observations, con-
cerning the Procreation of the Chicken out of the Egge. An under-
taking equally difficult, usefull, and pleasant as the former. For
Nature's Rudiments and Attempts are involved in obscurity and
deep night, and so perplext with subtilties, that they delude the most
piercing wit, as well as the sharpest eye. Nor can we easier discover
the secret recesses, and dark principles of Generation than the method
of the fabrick and composure of the whole world. In this reciprocal
interchange of Generation and Corruption consists the ^Eternity
and Duration of mortal creatures. And as the Rising and Setting
of the Sun, doth by continued revolutions complete and perfect
Time; so doth the alternative vicissitude of Individuums, by a
constant repetition of the same species, perpetuate the continuance
of fading things.
"Those Authors which have delivered any thing touching this
subject, do for the most part tread a several path, for having their
Judgements prepossessed with their own private opinions, they pro-
ceed to erect and fashion principles proportionable to them.
"Aristotle of old, and Hieronymus Fabricius of late, have written
so accurately concerning the Formation and Generation of the Foetus
out of the Egge, that they seem to have left little to the industry
of Posterity, And yet Ulysses Aldrovandus hath undertaken the
description of the Pullulation or Formation of the chicken out of the
Egge, out of his own Observations ; wherein he seems rather to have
directed and guided his thoughts by the Authority of Aristotle, than
by his own experience.
"For Volcherus Goiter, living at Bononia at the same time did
by the advice of the said Aldrovandus (whom he calls Tutor) dayly
employ himself in the opening of Egges sat upon by the Hen, and
hath discovered many things truer than Aldrovandus himself, of
which he also could not be ignorant. Likewise iEmilius Parisanus
(a Venetian Doctor) despising other mens opinions hath fancied A
new procreation of the Chicken out of the Egge.
126 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
"But because somethings, (according to our experience) and those
of great moment and consequence, are much otherwise than hath
been yet delivered, I shall declare to you what dayly progress is made
in the egge, and what parts are altered, especially about the first
dayes of Incubation; at which time all things are most intricate,
confused, and hard to observe, and about which authors do chiefly
stickle for their own observations, which they accomodate rather to
their own preconceived perswasions (which they have entertained
concerning the Material and Efficient Causes of the generation of
Animals) than to truth herself.
"Aldrovandus, partaking of the same error with Aristotle, saith
(which none but a blind man can subscribe to) that the Yolk doth
in the first dayes, arise to the Acute Angle of the Egge ; and thinks
the Grandines to be the Seed of the Cock; and that the Pullus is
framed out of them, but nourished as well by the yolk as the white ;
which is clean contrary to Aristotle's opinion, who conceived the
Grandines to conduce nothing to the fecundity of the egge. Volcherus
Goiter delivers truer things, and more consonant to Autopsie, yet his
three Globuli are meer fables. Nor did he rightly consider the
principle from whence the Foetus is derived in the Egg. Hieronymus
Fabricius indeed contends, that the Grandines are not the seed of
the cock, and yet he will have the body of the Chicken to be framed
out of them (as out of its first matter) being made fruitful by the seed
of the cock. He likewise saw the Original of the Chicken in the Egge;
namely the Macula, or Cicatricula annexed to the membrane of the
Yolke but conceived it to be onely a Relique of the stalk broken off,
and an in-firmity of blemish onely of the Egge, and not a principle
part of it. Parisanus hath plentifully confuted Fabricius his opinion
concerning the Chalazae or Grandines, and yet himself is evidently
at a loss in some certaine circles and points of the Principle parts
of the Foetus (namely the Liver and the Heart) and seems to have
observed a Principium or first Principle of the Foetus, but not to
have known which it was, in that he saith, that the Punctum Album
in the Middle of the Circles is the Cocks Seed out of which the Chicken
is made. So that it comes to pass that while each of them desire to
reduce the manner of the Formation of the Chicken out of the Egge
to their own opinions they are all wide from the mark."
Before discussing how Harvey put them right, however, there are
a number of other matters to be mentioned. Parisanus' work was
SECT. 3] AND EIGHTEENTH CENTURIES 127
published in 1623, and twenty-five years were to elapse before
Harvey's Exercitations were to be put before the learned world by
George Ent. In that time not a few events of importance for the
history of embryology took place.
It will be convenient to speak first of Adrianus Spigelius, whose
De Formato Foetu appeared in 1 63 1 . In this book the plates of the
gravid uterus which had been prepared some years before for Julius
Casserius were now published. They had more influence than
Spigelius' text, perhaps, in contributing to the permanent fame of
his book.
He gives for the most part straightforward anatomical descriptions,
but he returns to the notion of a cotyledonous placenta in man, and
he combats Arantius' opinions about the placenta. Arantius had said
that the function of the jecor uterinae was to purify the blood-
supply to the foetus, a thoroughly modern idea, but Spigelius opposes
this on two grounds, firstly, because the foetus has its own organs
for purifying blood, and secondly, because, if Arantius was right,
the placenta would always be as red as blood, but this is not the case
in such animals as the sheep. Spigelius himself thought that the
placenta was for the purpose of preventing severe loss of blood at
birth, as would be the case if the embryo was joined to the mother
with only one big vessel and not a great many little ones.
However, Spigelius upholds the view, taken by Rufus of Ephesus
and by Vesalius, that the allantois contains the foetal urine, which
has to be separated from the amniotic liquid in which the embryo
is, because it would corrode the embryonic skin [ne cuti tenellae
aliquod damnum urinae acrimonia inferret). This passage is interesting,
as showing biochemical rudiments. The first discussion of the
vernix caseosa, or sordes, as he calls it, appears in Spigelius, who,
however, hazards no guess as to its nature. He is happy in his
refutation of Laurentius, who had affirmed that the foetal heart did
not beat in utero, and he shows some advance on all previous writers
save Arantius in declaring that the umbilical vessels take vital spirits
away from the foetal heart, not exclusively to it. He gave, moreover,
the first denial of the presence of a nerve in the umbilical cord, and also
made the first observation of the occurrence of milk in foetal breasts
at birth (for the endocrinological explanation of this see Section 15).
Finally, he abolished at last the notion that the meconium in the
foetal intestines argued eating in utero on the part of the embryo.
128 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
Riolanus the younger, the correspondent and almost exactly the
contemporary of Harvey, was Professor in Paris and published his
Anthropographia in 1618. As he was a keen advocate of the ancient
views, his section on the formation of the foetus has little importance.
Yet it contains the first known instance of the use of the lens in embryo-
logy, the germ of that powerful instrument which was to lead in
due course to so many discoveries. "In aborted embryos", said
Riolanus, "the structure is damaged and can often not be properly
seen, even when you make use of lenses [conspicilid] which make
objects so much bigger and more complicated than they ordinarily
seem."
The De Formatrice Foetus of Thomas Fienus, Professor at Louvain
and a friend of Gassendi, published in 1620, is interesting because
it is the middle term between Aristotle and Driesch. As the title-
page informs us, he sets out to demonstrate that the rational soul is
infused into the human embryo on the third day after conception.
This by itself would not be very attractive, but the most cursory
inspection shows that Fienus' interests were not at all theological.
He divides the book up into seven main questions, (i) What is the
efficient cause of embryogeny? He concludes that it is neither God,
nor Intelligence, nor anima mundi (influence of Neo-platonism here
as on Galileo). (2) Is it in the uterus or in the seed? In the latter,
says Fienus, adding a list of authorities who agree with this view —
Haly-Abbas, Gaietanus, Zonzinas, Turisanus, Fernelius, Vallesius,
Peramatus, Saxonia, Carrerius, Zegarra, Mercurialis, Massaria, and
Archangelus, ^' solus Fabio Pacio utero imprudenter adscribit'' (!). (3) Is
it heat? Fienus nearly decided that it was, and, if he had done
so, would have shown a modern mind, but no, he gave his
opinion against it, saying, "the process (of development) is so divine
and wonderful that it would be ridiculous to ascribe it to heat, a
mere naked and simple quality". After weighing various other
alternatives in questions (4), (5) and (6), he asks whether it is
^'^ anima seminis post conceptum adveniens'' (7), and concludes that it is.
It is here that he becomes really interesting, for he quotes with
approval certain writers, e.g. Alexander Aphrodisias [Organicum corpus
esse organicum ab anima et anima praeexistere organizationi) , Themistius
{Anima fabricatur architecturaque sibi domicilium et accommodatum instru-
mentum) and Marsilio Ficino in his commentary on Plato's Timaeus
[Priusquam adultum sit corpus, anima tota in illius fabrica occupatur), and
SECT. 3] AND EIGHTEENTH CENTURIES 129
then maintains with them that the soul is the principle which
organises the body from within, arranging an organ for each of
its faculties and preparing a residence for itself, not merely allowing
itself to be breathed into a being which has already organised
itself "The conformation of the foetus is a vital, not a natural,
action", he says. He develops this idea in the remainder of the
book; according to him, the seed first coagulates the menstrual
blood into an amorphous cake, taking three days to do so, after
which, the rational (not vegetative or sensitive) soul (entelechy),
which has entered the uterus with the seed, finding a suitable
mass of shapeless material, enters into it and begins to give it a
shape. Fienus was attacked by several writers, and published a
defence of his views.
Later writers on the same subject included Fidelis, Teichmeyer,
Albertus, de Reies, Torreblanca and de Mendoza. The Spanish
influence here is perhaps significant. Hieronymus Florentinus, who
adopted the same standpoint as Fienus in 1658 was forced to recant it.
In 1625 Joseph de Aromatari, a Venetian, included in his epistle
on plants the first definite statement of the preformationist theory
since Seneca, but he did not develop the idea. He had noted that
in bulbs and some seeds the rudiments of many parts of the adult
plant can be seen even without glass or microscope, and this led him
to suggest that probably in all animals as well as plants a similar
thing was true. "And as for the eggs of fowls", he said, "I think the
embryo is already roughly sketched out in the egg before being formed
at all by the hen [quod attinet ad ova gallinarum, existimamus quidem
pullum in ovo delineatum esse, antequam formatur a gallina].'' This sug-
gestion did not begin to bear its malignant fruits till the time of
Swammerdam and Malpighi.
Johannes Sinibaldi's Geneanthropia might be mentioned as belonging
to this time. It was a compilation of facts relating to the generation
of man, but it expressly excluded from its field any discussion of the
embryo. It is no more important for our subject than the queer
Ovi Encomium of Erycius Puteanus, another of Gassendi's friends, which
has already been referred to (p. 8).
3-2. Kenelm Digby and Nathaniel Highmore
Much more significant was the controversy between Sir Kenelm
Digby and Nathaniel Highmore. In 1644, Sir Kenelm, whose in-
I30 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
triguing personality will be sufficiently familiar to anyone even slightly
acquainted with seventeenth-century England, and whose biographic
details may be found in John Aubrey, published a work with the
following title: Two treatises, in the one of which, The Nature of Bodies,
in the other, The Nature of Man's Soule is looked into, in way of discovery
of the Immortality of Reasonable Soules. It was inscribed in a charming
dedication to his son, and consisted, in brief, of a survey of the whole
realms of metaphysics, physics, and biology from a very individual
point of view.
One of Sir Kenelm's principal objects in writing was apparently
to attack the old terminology of " qualities " in physics and "faculties "
in biology. To say, as contemporary reasoning did, that bodies were
red or blue because they possessed a quality of redness or blueness
which caused them to appear red or blue to us, or again, to say that
the heart beat because it was informed by a sphygmic faculty, or,
to take the famous example, that opium sent people to sleep because
it contained in it a dormitive virtue, appeared mere nonsense and
word-spinning to Digby, "the last refuge of ignorant men, who not
knowing what to say, and yet presuming to say something, do often
fall upon such expressions".
Digby, like Galileo and Hobbes, wished to explain all phenomena
by reference to two "virtues" only, those of rarity and density,
"working by means oflocall Motion". Chapters twenty-three, twenty-
four, and twenty-five contain his opinions and experiments in embryo-
logy. He begins by opening the question of epigenesis or preforma-
tion, practically for the first time since Albert the Great. "Our main
question shall be", he says, "whether they be framed entirely at
once, or successively, one part after another? And if this latter way,
which part first?" He declares for epigenesis, but after a manner
of his own, refuting "the opinion of those who hold that everything
containeth formally all things". "Why should not the parts be made
in generation", he asks, "of a matter like to that which maketh them
in nutrition? If they be augmented by one kind of juyce that after
severall changes turneth at the length into flesh and bone; and into
every sort of mixed body or similar part whereof the sensitive creature
is compounded, and that joyneth itself to what it findeth there already
made, why should not the same juyce with the same progresse of
heat and moisture, and other due temperaments, be converted at
the first into flesh and bone though none be formerly there to joyn
SECT. 3] AND EIGHTEENTH CENTURIES 131
it self unto?" He gives a clearly deterministic account of develop-
ment. "Take a bean, or any other seed and put it in the earth, and
let water fall upon it; can it then choose but that the bean must swell?
The bean swelling, can it choose but break the skin? The skin
broken, can it choose (by reason of the heat that is in it) but push
out more matter, and do that action which we may call germinating?
Can these germs choose but pierce the earth in small strings, as they
are able to make their way? . . . Thus by drawing the thrid carefully
along through your fingers, and staying at every knot to examine
how it is tyed ; you see that this difficult progresse of the generation
of living creatures is obvious enough to be comprehended and the
steps of it set down; if one would but take the paines and afford
the time that is necessary to note diligently all the circumstances in
every change of it. . . . Now if all this orderly succession of mutations
be necessarily made in a bean, by force of sundry circumstances and
externall accidents ; why may it not be conceived that the like is also
done in sensible creatures, but in a more perfect manner, they being
perfecter substances? Surely the progresse we have set down is much
more reasonable than to conceive that in the seed of the male there
is already in act, the substance of flesh, bone, sinews, and veins, and
the rest of those severall similar parts which are found in the body
of an animall, and that they are but extended to their due magnitude
by the humidity drawn from the mother, without receiving any
substantiall mutation from what they were originally in the seed.
Let us then confidently conclude, that all generation is made of a
fitting, but remote, homogeneall compounded substance upon which
outward Agents, working in the due course of Nature, do change it
into another substance, quite different from the first, and do make it
lesse homogeneall than the first was. And other circumstances and
agents do change this second into a third, that third, into a fourth;
and so onwards, by successive mutations that still make every new
thing become lesse homogeneall than the former was, according to
the nature of heat, mingling more and more different bodies together,
untill that substance bee produced which we consider the period of
all these mutations." This passage is indeed admirable, and well
expresses the most modern conception of embryonic development,
that of the ovum as a physico-chemical system, containing within
itself only to a slight and varying degree any localisation answering
to the localisation of the adult, and ready to change itself, once the
9-2
132 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
appropriate stimulus has been received, into the completed embryo
by the actions and reactions of its own constituents on the one hand
and the influence of the fitting factors of the environment upon the
other. Digby has not received his due in the past; he stands to
embryology as an exact science, much in the same relationship as
Bacon to science as a whole.
"Generation is not made", he says, "by aggregation of like parts
to presupposed like ones; nor by a specificall worker within; but by
the compounding of a seminary matter with the juice which accrueth
to it from without and with the steams of circumstant bodies, which
by an ordinary course of nature are regularly imbibed in it by degrees
and which at every degree doe change it into a different thing ..." (see
p. 317). "Therefore to satisfie ourselves herein, it were well we made
our remarks on some creatures that might be continually in our power
to observe in them the course of nature every day and hour. Sir lohn
Heydon, the Lieutenant of his Majesties Ordnance (that generous
and knowing Gentleman, and consummate Souldier both in theory
and practice) was the first that instructed me how to do this, by
means of a furnace so made as to imitate the warmth of a sitting hen.
In which you may lay severall eggs to hatch, and by breaking them
at severall ages you may distinctly observe every hourly mutation
in them if you please." Sir Kenelm then goes on to describe the
events that take place in the incubating egg, which he does very
accurately, though briefly. In vivipara, he says, the like experiments
have been made, and the like conclusions come to by "that learned
and exact searcher into nature. Doctor Harvey" — these he must
have learnt of by word of mouth, for Harvey's book had not at that
time been published. As regards heredity, he adopts a pure theory
of pangenesis, and has more to say about it than any other writer
of his time. He is sure that the heart is first formed both in ovipara
and vivipara, "whose motion and manner of working evidently ap-
pears in the twinckling of the first red spot (which is the first change)
in the egge".
Sir Kenelm Digby not only anticipated the outlook of the physico-
chemical embryologist, but he also foreshadowed with considerable
accuracy Wilhelm Roux's definition of interim embryological laws.
"Out of our short survey", he says, "of which (anserable to our weak
talents, and slender experience) I perswade myselfe it appeareth
evidently enough that to effect this worke of generation there needeth
SECT. 3] AND EIGHTEENTH CENTURIES 133
not to be supposed a forming virtue or Vis Formatrix of an unknown
power and operation, as those that consider things suddenly and in
grosse do use to put. Yet in discourse, for conveniency and shortnesse
of expression we shall not quite banish that terme from all commerce
with us; so that what we mean by it be rightly understood, which is
the complex assemblement, or chain of all the causes, that concur
to produce this effect, as they are set on foot to this end by the great
Architect and Moderatour of them, God Almighty, whose instrument
Nature is : that is, the same thing, or rather the same things so ordered
as we have declared, but expressed and comprized under another
name." Thus Sir Kenelm admits that it is allowable to speak of the
"complex assemblement" of causes, as if it were one formative virtue,
and this corresponds to Roux's "secondary components" or interim
embryological laws. But that the portmanteau generalisations can
be resolved into ultimate physico-chemical processes, Digby both
believes and spends two entire chapters in trying to show. Digby
has been one of the two seventeenth-century Englishmen most under-
estimated in the history of biology, but his place is in reality a very
high one. How far he was in advance of his time may be gauged
from the work of his contemporary Sperlingen, whose book of 1641
was thoroughly scholastic and retrograde.
His Treatise on Bodies evoked several answers. Undoubtedly the
most interesting from the progressive side was that of Nathaniel
Highmore, who will always be well remembered in embryological
history. Highmore's The History of Generation came out in 1651, so
that Harvey must have known of it, and it is one of the puzzles of
this period why Harvey did not make any mention of it in his work,
especially as J. D. Horst in a letter to Harvey refers to Highmore as his
pupil. Harvey replying in 1655 said he had not seen Highmore for
seven years. Highmore's title-page expressly states that his book is
an answer to the opinions of Sir Kenelm Digby. But before dis-
cussing in what the answer consisted, we may look at the plate which
is bound in immediately after the dedication (to Robert Boyle).
It is interesting in that it shows again the idea initiated by Leonardo,
namely, that all growing things, plants as well as animals, have an
umbilical cord, and in that the drawings of the chick embryos and
eggs are more quaint than accurate (Plate VI).
Highmore first describes the Aristotelian doctrine of form and
matter, and then censures both it and the extensions of it with their
134 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
"qualities", etc., much as Digby himself had done. "Some of our
later philosophers have showed us that those forms w'^*^ they thought
and taught to bee but potentially in the matter, are there actually
subsisting though till they have acquired fitting organs, they manifest
not themselves. And that the effects which were done before their
manifestation (as the forming and fashioning of the parts wherein
they are to operate) can rise from nothing else than from the Soul
itselfe. This likewise I shall leave to the Readers enquiry, and shall
follow that other way of introducing Forms, and Generation of
creatures (as well animals as vegetables) which gives Fortune and
Chance the preheminency in that work." He then describes Sir
Kenelm's opinions, quoting from him in detail, and dissents from
them mainly on the ground that they do not sufficiently account
for embryogeny, as it were, from a technical point of view. That they
subvert the "antique principals of philosophy" does not worry
Highmore, but in his view their detailed mechanisms do not explain
the facts, a much more serious drawback. Highmore is himself by
way of being an Atomist, and it is because embryology was first
treated by him from an atomistic standpoint that he derives his
importance. "The blood, that all parts may be irrigated with its
benigne moisture, is forc'd by several channels to run through every
region and part of the body; by which meanes every part out of
that stream selects those atomes which they finde to be cognate to
themselves. Amongst which the Testicles abstract some spiritual
atomes belonging to every part, which had they not here been
anticipated, should have been attracted to those parts, to which
properly they did belong for nourishment. . . . These particles passing
through the body of the Testicles, and being in this Athanor cohobated
and reposited into a tenacious matter, at last passe through infinite
Meanders through certain vessels, in which it undergoes another
digestion and pelicanizing." Highmore objects, therefore, more to
Digby 's theory of pangenesis than to his description of embryogenesis.
He goes on to give a long description of the development of the chick
in the egg, mentioning in passing that the albumen corresponds to
the semen and the blood of vivipara and the yolk to their milk.
"Fabritius, who hath taken a great deal of pains in dissections. . .
supposes the chick to be formed from the chalazae, that part which
by our Women is called the treddle. But this likewise is false, for
then every egge should produce 2 chickens, there being one treddle
PLATE VI
l^: j:
a
9 pp ^^^E
3"
■^ ^
ta
0y: 6^^-
0a: ^f
Tat-. 1^
(P^: 6.
ta.
ILLUSTRATION FROM NATHANIEL HIGHMORE'S HISTORY OF GENERATION, 1651
SECT. 3] AND EIGHTEENTH CENTURIES 135
at each end of the egg, which serve for no other end than for Hga-
ments to contain the yolk in an equilibrium, that it might not by
every moving of the egg be shakt, broke, and confused with the white."
Highmore was the first to draw attention to the increase of brittleness
which takes place in the egg-shell during incubation, and he holds
still to the Epicurean view that the female produces a kind of seed,
though he thinks that the chick embryo is nourished in the early
stages by the amniotic liquid.
Perhaps the most interesting reply to Digby from the traditional
angle was that of Alexander Ross. In his Philosophicall Touchstone he
upheld the Galenic view that the liver must be first formed in genera-
tion, for the nourishment is in the blood and the blood requires a liver
to make it : ergo, the liver must be the earliest organ. Such arguments
could dispense with observations. Ross also mentions Digby's sug-
gestion that the "formative virtue" was only a bundle of natural
causes, but he claims that the notion was an old one in school-
philosophy, being included in the phrase causa causae^ causa
causati.
3-3. Thomas Browne and the Beginnings of Chemical Em-
bryology
There are references to embryology in Sir Thomas Browne's
Pseudodoxia Epidemica, or Inquiries into very many vulgar Tenents and
commonly received Truths, which was published at this time. The
twenty-eighth chapter of the third book contains a number of
difficult problems in the embryology of the period, in most cases
stated without any solution. "That a chicken is formed out of the
yolk of the Egg was the opinion of some Ancient Philosophers.
Whether it be not the nutrient of the Pullet may also be considered ;
since umbilical vessels are carried into it, since much of the yolk
remaineth after the chicken is formed, since in a chicken newly
hatched, the stomack is tincted yellow and the belly full of yelk
which is drawn at the navel or vessels towards the vent, as may be
discerned in chickens a day or two before exclusion. Whether the
chicken be made out of the white, or that be not also its aliment,
is likewise very questionable, since an umbilical vessel is derived unto
it, since after the formation and perfect shape of the chicken, much
of the white remaineth. Whether it be not made out of the grando,
gallature, germ, or tred of the egg, as Aquapendente informeth us,
136 EMBRYOLOGY IN THE SEVENTEENTH [pt. 11
seemed to many of doubt; for at the blunter end it is not discovered
after the chicken is formed, by this also the white and the yelk are
continued whereby it may conveniently receive its nutriment from
them both.. . .But these at last and how in the Cicatricula or little
pale circle formation first beginneth, how the Grando or tredle, are
but the poles and establishing particles of the tender membrans
firmly conserving the floating parts in their proper places, with many
observables, that ocular Philosopher and singular discloser of truth,
Dr Harvey hath discovered, in that excellent discourse of generation,
so strongly erected upon the two great pillars of truth, Experience,
and Reason.
"That the sex is discernable from the figure of eggs, or that cocks
or hens proceed from long or round ones, experiment will easily
frustrate.. . .Why the hen hatcheth not the egg in her belly? Why
the egg is thinner at one extream? Why there is some cavity or
emptiness at the blunter end? Why we open them at that part?
Why the greater end is first excluded [cf p. 233]? Why some eggs
are all red, as the Kestrils, some only red at one end, as those of
kites and buzzards? Why some eggs are not oval but round, as
those of fishes ? etc. are problems whose decisions would too much
enlarge this discourse." And elsewhere, "That (saith Aristotle) which
is not watery and improlifical will not conglaciate; which perhaps
must not be taken strictly, but in the germ and spirited particles;
for Eggs, I observe, will freeze, in the albuginous part thereof".
Again, "They who hold that the egg was before the bird, prevent
this doubt in many other animals, which also extendeth unto them;
for birds are nourished by umbilical vessels and the navel is manifest
sometimes a day or two after exclusion.. . .The same is made out
in the eggs of snakes, and is not improbable in the generation of
Porwiggles or Tadpoles, and may also be true in some vermiparous
exclusions, although (as we have observed in the daily progress of
some) the whole Magot is little enough to make a fly without any
part remaining. . . . The vitreous or glassie flegm of white of egg will
thus extinguish a coal."
These citations show Sir Thomas to have been more than simply
the supreme artist in English prose which is his common title to
remembrance. In picking his way carefully among the doubtful
points and difficult problems which previous embryologists had pro-
pounded but not answered, he usually managed to give the right
SECT. 3] AND EIGHTEENTH CENTURIES 137
answer to each. But in addition to this, he was also an experimentahst,
he had made both anatomical and physical experiments on eggs,
and he was prepared to put any disputed point to the test of "ocular
aspection", if this could be done. His experimental contributions
to embryology come out more clearly in his Commonplace Books which
were published by Wilkin in 1836.
"Runnet beat up with the whites of eggs seems to perform nothing,
nor will it well incorporate, without so much heat as will harden
the tgg. . . . Eggs seem to contain within themselves their own
coagulum, evidenced upon incubation, which makes incrassation of
parts before very fluid.. . .Rotten eggs will not be made hard by
incubation or decoction, as being destitute of that spirit or having
the same vitiated. . . . They will be made hard in oil but not so easily
in vinegar which by the attenuating quality keeps them longer from
concoction, for infused in vinegar they lose the shell and grow big
and much heavier then before. ... In the ovary or second cell of the
matrix the white comes upon the yolk, and in the later and lower
part, the shell is made or manifested. Try if the same parts will give
any coagulation unto milk. Whether will the ovary best?... The
whites of eggs drenched in saltpeter will shoot forth a long and hairy
saltpeter and the egg become of a hard substance. Even in the whole
egg there seems a great nitrosity, for it is very cold and especially
that which is without a shell (as some are laid by fat hens) or such
as are found in the egg poke or lowest part of the matrix, if an hen
be killed a day or two before she layeth. . . . Difference between the
sperm of frogs and eggs, spawn though long boiled, would not grow
thick and coagulate. In the eggs of skates or thornbacks the yolk
coagulates upon long docoction, not the greatest part of the white. . . .
In spawn of frogs the little black specks will concrete though not the
other. ... In eggs we observe the white will totally freeze, the yolk, with
the same degree of cold will grow thick and clammy like the gum of
trees, but the sperm or tread hold its former body, the white growing
stiff that is nearest to it."
The only conclusion that can be drawn from these remarkable
observations is that it was in the " laboratory " in Sir Thomas' house
at Norwich that the first experiments in chemical embryology were
undertaken. His significance in this connection has so far been quite
overlooked, and it is time to recognise that his originality and genius
in this field shows itself to be hardly less remarkable than in so many
138 EMBRYOLOGY IN THE SEVENTEENTH [pt. 11
others. To have occupied himself with the chemical properties of
those substances which afford the raw material of development was
a great step for those times, but it was not until some twenty-five
years later that Walter Needham carried this new interest into the
mammalian domain, and made chemical experiments there.
3-4. William Harvey
The Latin edition of William Harvey's book on the generation of
animals appeared in 1651, and the English in 1653. The frontispiece
of the former which is reproduced as the frontispiece of this book is
a very noteworthy picture, and derives a special interest from the
fact that on the egg which Zeus holds in his hands is written, "^x
ovo omnia'\ — a conception which Harvey is continually expounding
(see especially the chapter, "That an egg is the common Original
of all animals"), but which he never puts into epigrammatic form
in his text, so that the saying, omne vivum ex ovo, often attributed
to him, is only obliquely his.
The De Generatione Animalium was written at different times during
his life, and not collected together for publication until George Ent,
of the College of Physicians, persuaded Harvey to give it forth about
1650. As early as 1625 Harvey was studying the phenomena of
embryology, as is shown among other evidences by a passage in his
book where he says, "Our late Sovereign King Charles, so soon as
he was become a man, was wont for Recreation and Health sake,
to hunt almost every week, especially the Buck and Doe, no Prince
in Europe having greater store, whether wandring at liberty in the
Woods and Forrests or inclosed and kept up in Parkes and Chaces.
In the three summer moneths the Buck and the Stagge being then
fat and in season were his game, and the Doe and Hind in the
Autumme and Winter so long as the three seasonable moneths con-
tinued. Hereupon I had a daily opportunity of dissecting them and
of making inspection and observation of all their parts, which liberty
I chiefly made use of in order to the genital parts". Nor was Harvey
less diligent in examining the generation of ovipara. John Aubrey,
in his Brief Lives, says, " I first sawe Doctor Harvey at Oxford in 1642
after Edgehill fight, but I was then too young to be acquainted with
so great a Doctor. I remember that he came often to Trin. Coll.
to one George Bathurst, B.D. who kept a hen in his chamber to
SECT. 3] AND EIGHTEENTH CENTURIES 139
hatch egges, which they did dayly open to discerne the progress
and way of generation". Aubrey mentions a conversation he had with
a sow-gelder, a countryman of Httle learning, but much practical
experience and wisdom, who told him that he had met Dr Harvey,
who had conversed with him for two or three hours, and "if he had
been", the man remarked, "as stiff as some of our starched and
formall doctors, he had known no more than they". Harvey seems
also to have learnt all he could from the keepers of King Charles'
forests, as several passages in his book show. Nor was the King's own
interest lacking. "I saw long since a foetus", he says, "the magnitude
of a peasecod cut out of the uterus of a doe, which was complete
in all its members & I showed this pretty spectacle to our late King
and Queen. It did swim, trim and perfect, in such a kinde of white,
most transparent and crystalline moysture (as if it had been treasured
up in some most clear glassie receptacle) about the bignesse of a
pigeon's Ggge, and was invested with its proper coat." And, again —
"My Royal Master, whose Physitian I was, was himself much
delighted in this kinde of curiosity, being many times pleased to be
an eye-witness, and to assert my new inventions".
Harvey's book is composed of seventy-two exercitations, which
may be divided up for convenience into five divisions. In Nos. i
to 10 he speaks of the anatomy and physiology of the genital organs
of the fowl, and the manner of production of eggs. Nos. 11 to 13
and also Nos. 23 and 36 deal with the hen's egg in detail, describing
its parts and their uses, while in Nos. 14 to 23 the process of the
"generation of the foetus out of the hen egge'' is described. The
greater part of the book, comprising Nos. 25 to 62, as well as Nos. 71
and 72, is theoretical, and treats of the embryological theories held
by Aristotle on the one hand, and the physicians, following Galen,
on the other, instead of which it propounds new views upon the
subject. Finally, Nos. 63 to 70, as well as the two appendices^ or
"particular discourses", are concerned with embryogenesis in vivi-
parous animals, especially in hinds and does.
It will be best to refer to certain details and main points of interest
in Harvey's discussions, before trying to assess his principal contribu-
tions to the science as a whole. Harvey is the first, since Aristotle,
to refer to the "white yolk" of birds. "For between the yolk", he
says, "which is yet in the cluster and that which is in the midst of
the eg when it is perfected this is the difference in chief, that though
140 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
the former be yellowish in colour and in appearance, yet its con-
sistence representeth rather the white, and being sodden, thickeneth
like it, growing compact and viscous and may be cut into slices. But
the yolk of a perfect eggc being boiled groweth friable and of a more
earthy consistence, not thick and glutinous like the white." All of
Harvey's observations on the formation of the egg in the oviduct
contained in this chapter are interesting, and may with advantage
be compared with the studies of Riddle upon the same subject, where
the chemical explanation will be found for many of Harvey's simple
observations. Harvey's controversy with Fabricius on the question
of whether the egg is produced with a hard shell or only acquires
its external hardness upon standing in the air, which follows im-
mediately on the above citation, is interesting. "Fabricius seemeth
to me to be in errour, for though I was never so good at slight of
hand to surprise an egge in the very laying, and so make discovery
whether it was soft or hard, yet this I confidently pronounce that the
shell is compounded within the womb of a substance there at hand
for the purpose, and that it is framed in the same manner as the
other parts of the egg are by the plastick faculty, and the rather,
because I have seen an exceeding small egge which had a shell of
its own and yet was contained within another egge, greater and
fairer than it, which egge had a shell too."
Harvey was the first to note that the white of the hen's egg is
heterogeneous, in the sense that part of it is much more liquid than
the rest, and that the more viscous part seems to be contained in an
exceedingly fine membrane, so that if it is sliced across with a knife,
its contents will flow out. He also set right the errors of Fabricius,
Parisanus and others, by showing that the chalazae were neither the
seed of the cock nor the material out of which the embryo was formed,
and, most important of all, by demonstrating that the cicatricula
was the point of origin of the embryo. He denied, as against popular
belief, that the hen contributed anything to the developing egg but
heat, "For certain it is that the chicken is constituted by an internal
principle in the egge, and that there is no accession to a complete
and perfect egge by the Hennes incubation, but bare cherishing and
protection; no more than the Hen contributeth to the chickens
which are now hatched, which is only a friendly heat, and care, by
which she defendeth them from the cold, and forreign injuries and
helpeth them to their meat". Whether future work will still affirm
SECT. 3] AND EIGHTEENTH CENTURIES 141
that nothing is given to the egg by the hen except heat is beginning
now to be in doubt, if the results of Chattock are correct.
In the description of the development of the embryo in the hen's
egg, which remains to this day one of the most accurate, Harvey
says with regard to the spot on the yolk, which had, of course, been
seen and mentioned by many previous observers, "And yet I con-
ceive that no man hitherto hath acknowledged that this Cicatricula
was to be found in every egge nor that it was the first Principle of
the Egge". His description of the beginning of the heart, that
"capering bloody point" or "punctum saliens'\ is too famous to
need more than a reference. He thought that the amniotic liquid
was of "mighty use", "For while the embryos swim there, they are
guarded and skreened from all concussion, contusion, and other out-
ward injuries, and are also nourished by it".
Thus he made no advance on the opinion which had for long been
held, namely, that the amniotic liquid or colliquamentum served
for sustenance. "I believe", he says, "that this colliquamentum or
water wherein the foetus swims doth serve for his sustenance and
that the thinner and purer part of it, being imbibed by the umbilicall
vessels, does constitute and supply the primo-genital parts, and the
rest, like Milk, being by suction conveyed into the stomack and there
concocted or chylified, and afterwards attracted by the orifices of
the Meseraick Veins doth nourish and enlarge the tender embryo."
His arguments for this are, ( i ) that swallowing movements take place,
and (2) that the gut of the chicken is "stuft" with excrement which
could hardly arise from any other source. He was thus led to divide
the amniotic liquid into two quite imaginary constituents, a purer
and "sincerer" part, which could be absorbed straight into the blood
without chylification, and a creamless milky part which could not
be treated so simply.
"About the fourth day", says Harvey, "the egg beginneth to step
from the life of a plant to that of an animall." "From that to the
tenth it enjoys a sensitive and moving soul as Animals do, and after
that, it is compleated by degrees and being adorned with Plumes,
Bill, Clawes and other furniture, it hastens to get out." These and
other passages which deal with the forerunner of the theory of re-
capitulation are interesting, but we have already met essentially the
same idea in Aristotle. Harvey contributed nothing new to it. The
first point on which he went definitely wrong was the statement that
142 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
he made that the heart does not pulsate before the appearance of
the blood. No doubt his lack of microscopical facilities or of the
desire to use them affords the reason for this error, but it was a very
unfortunate one, for it was to a large extent upon it that he formulated
his doctrine "the life is in the blood". For example, he says, "I am
fully satisfied that the Blood hath a being before any other part of the
body besides, and is the elder brother to all other parts of the foetus ".
The yolk, Harvey thought, supplied the place of milk, "and is
that which is last consumed, for the remainder of it (after the chicken
is hatched and walks abroad with the Henne) is yet contained in its
belly". He thus ranged himself with Alcmaeon and Abderhalden.
All his remarks about the relationships of yolk and white in nutrition
are worth consideration; in noting, for instance, that the yolk is
the last to be consumed, he comes very near to anticipating the
knowledge of the succession of energy-sources which we now possess
(see Section T"]). "In that Physitians affirme, that the Yolke is the
hotter part of the ^gg&, and the most nourishing, I conceive that they
understand it, in relation to us, as it is become our nourishment,
not as it doth supply more congruous aliment to the chicken in the
tggt. And this appeares out of our history of the Fabrick of the
chicken ; which doth first prey upon and devoure the thinner part of
the white, before the grosser; as it were a more proper diet, and did
more easily submit to transmutation into the substance of the foetus.
And therefore the yolke seems to be a remoter and more deferred
entertainment than the white; for all the white is quite and clean
spent, before any notable invasion is made upon the yolke." A com-
parison between these simple facts and our knowledge of embryonic
nutrition is most interesting (see Section 6-9).
In connection with Minot's distinction of the periods of embryonic
growth, it is curious that Harvey says, "And now the foetus moves
and gently tumbles, and stretcheth out the neck though nothing of
a brain be yet to be seen, but merely a bright water shut up in a small
bladder. And now it is a perfect Magot, differing only from those
kinde of wormes in this, that those when they have their freedom
crawle up and down and search for their living abroad, but this
worm constant to his station, and swimming in his own provision,
draws it in by his Umbilicall Vessels".
Sometimes Harvey confesses himself puzzled by problems which
could only be solved by chemical means, yet it does not occur to
SECT. 3] AND EIGHTEENTH CENTURIES 143
him that this is the case. For instance, he enquires why heat will
develop a chick out of a good egg but will only make a bad one worse.
"Give me leave to add something here", he writes, "which I have
tried often; that I might the better discerne the scituation of the
foetus and the liquors at the seventeenth day to the very exclusion.
I have boiled an eggc till it grew hard, and then pilling away the
shell and freeing the scituation of the chicken, I found both the
remaining parts of the white, and the two parts of the yolk of the
same consistence, colour, tast, and other accidents, as any other stale
egge, thus ordered, is. And upon this Experiment, I did much ponder
whence it should come to passe that Improlifical eggs should, from
the adventitious heat of a sitting Henne, putrifie and stink; and yet
no such inconvenience befall the Prolifical. But both these liquors
(though there be a Chicken in them too, and he with some pollution
and excrement) should be found wholesome and incorrupt; for that
if you eat them in the dark after they are boyled, you cannot dis-
tinguishe them from egges that are so prepared, which have never
undergone the hen's incubation." Harvey was never afraid of trying
such tests on himself; in another place, for example, he says, "Eggs
after 2 or 3 days incubation, are even then sweeter relished than stale
ones are, as if the cherishing warmth of the hen did refresh and
restore them to their primitive excellence and integrity". "And the
yolke (at 14 days) was as sweet and pleasant as that of a newlaid
cgge, when it is in like manner boyled to an induration." Another
matter on which Harvey set Fabricius right was on the question
whether at hatching the hen helps the chicken out or the chicken
comes out by itself. The latter was the belief held by Harvey, who
said of Fabricius' arguments on this point that they were "pleasant
and elegant, but not well bottomed".
On the great question of preformation v. epigenesis, Harvey keenly
argued in favour of the latter view. "There is no part of the future
foetus actually in the egg, but yet all the parts of it are in it poten-
tially. ... I have declared that one thing is made out of another two
several wayes and that as well in artificial as natural productions,
but especially in the generation of animals. The first is, when one
thing is made out of another thing that is pre-existent, and thus a
Bedstead is made out of Timber, and a Statue out of a Rock, where
the whole matter of the future fabrick was existent and in being,
before it was reduced into its subsequent shape, or any tittle of the
144 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
designe begun. But the other way is when the matter is both made
and receiveth its form at the same time. ... So Hkewise in the Genera-
tion of Animals, some are formed and transfigured out of matter
already concocted and grown and all the parts are made and dis-
tinguished together per metamorphosin, by a metamorphosis, so
that a complete animal is the result of that generation; but some
again, having one part made before another, are afterwards nourished,
augmented, and formed out of the same matter, that is, they have
parts, whereof some are before, and some after, other, and at the
same time, are both formed, and grow. . . . These we say are made
per epigenesin, by a post-generation, or after-production, that is
to say, by degrees, part after part, and this is more properly called
a Generation, than the former. . . . The perfect animals, which have
blood, are made by Epigenesis, or superaddition of parts, and do
grow, and attain their just future or ciKfir} after they are born. . . . An
animal produced by Epigenesis, attracts, prepares, concocts, and
applies, the Matter at the same time, and is at the same time formed,
and augmented.. . .Wherefore Fabricius did erroniously seek after
the Matter of the chicken (as it were some distinct part of the egg
which went to the imbodying of the chicken) as though the genera-
tion of the chicken were effected by a Metamorphosis, or trans-
figuration of some collected lump or mass, and that all the parts of
the body, at least the Principall parts, were wrought off at a heat
or (as himselfe speaks) did arise and were corporated out of the
same Matter." Nothing could be more plain than Harvey's teaching
on epigenesis, so that he has precedence over Caspar Wolff on this
matter.
On the relation between growth and differentiation Harvey has
some valuable things to say. The term "nutrition" he restricted to
that which replaces existent structures, and the term "augmenta-
tion" or "increment" to that which contributes something new. That
process which led to greater diversity of form and complexity of shape
he called "formation" or "framing". "For though the head of the
Chicken, and the rest of its Trunck or Corporature (being first of
a similar constitution) do resemble a Mucus or soft glewey substance;
out of which afterwards all the parts are framed in their order; yet
by the same Operatour they are together made and augmented, and
as the substance resembling glew doth grow, so are the parts dis-
tinguished. Namely they are generated, altered, and formed at once,
SECT. 3] AND EIGHTEENTH CENTURIES 145
they are at once similar and dissimilar, and from a small similar is
a great organ made." Harvey was thus very certain that the processes
of growth in size and differentiation in shape went on quite con-
currently, though he had no inkling of changes in the relative rapidity
of each process. On this point he goes further than Fabricius.
Fabricius thought that growth was a more or less mechanical process,
taking its origin from the properties of elementary substances, but
that differentiation was brought about by some more spiritual or
subtle activity. "Fabricius", says Harvey, "affirmes amisse, that
the Immutative Faculty doth operate by the qualities of the elements,
namely. Heat, Gold, Moisture, and Dryness (as being its instruments)
but the Formative works without them and after a more divine
manner; as if (forsooth) she did finish her task with Meditation,
Choice, and Providence. For had he looked deeper into the thing,
he would have seen that the Formative as well as the Alterative
Faculty makes use of Hot, Cold, Moist, and Dry, (as her instruments)
and would have deprehended as much divinity and skill in Nutrition
and Immutation as in the operations of the Formative Faculty her
self." "I say the Concocting and Immutative, the Nutritive and
Augmenting Faculties (which Fabricius would have to busie them-
selves only about Hot, Cold, Moist, and Dry, without all knowledge)
do operate with as much artifice, and as much to a designed end, as
the Formative faculty, which he affirms to possess the knowledge
and fore-sight of the future action and use of every particular part
and organ." Thus although in nearly every respect Harvey makes an
advance on Fabricius, yet here he is retrograde, for, in the former's
thought, the growth process at least had struggled towards a deter-
ministic schema; with Harvey this movement is rigidly suppressed.
"All things are full of deity" {Jovis omnia plena), said he, "so also in
the little edifice of a chicken, and all its actions and operations, Digitus
Dei, the Finger of God, or the God of Nature, doth reveal himself"
There can be no doubt that Harvey's leanings were vitalistic. In
the following passage, he argues against both those who wished to
deduce generation from properties of bodies (like Sir Kenelm Digby)
and the Atomists ; in other words, against the outlook of those types
of mind which in later times were to build up biophysics and bio-
chemistry. Aubrey notes that Harvey was "disdainfull of the chymists
and undervalued them".
"It is the usual error of philosophers of these times", says he, "to
146 EMBRYOLOGY IN THE SEVENTEENTH [pt. 11
seek the diversity of the causes of parts out of the diversity of the
matter from whence they should be framed. So Physicians affirm,
that the different parts of the body are fashioned and nourished by
the different materials of blood or seed ; namely the softer parts, as
the flesh, out of a thinner matter, and the more earthy parts as the
bones, out of grosser and harder. But this error now too much
received, we have confuted in another place. Nor are they lesse
deceived who make all things out of Atomes, as Democritus, or out
of the elements, as Empedocles. As if (forsooth) Generation were
nothing in the world, but a meer separation, or Collection, or Order
of things. I do not indeed deny that to the Production of one thing
out of another, these forementioned things are requisite, but Genera-
tion her self is a thing quite distinct from them all. (I finde Aristotle
in this opinion) and I my self intend to clear it anon, that out of
the same White of the Egge (which all men confesse to be a similar
body, and without diversity of parts) all and every the parts of the
chicken whether they be Bones, Clawes, Feathers, Flesh, or what
ever else, are procreated and fed. Besides, they that argue thus
assigning only a material cause, deducing the causes of Natural
things from an involuntary or casual concurrence of the Elements,
or from the several disposition or contriving of Atomes ; they doe
not reach that which is chiefly concerned in the operations of nature,
and in the Generation and Nutrition of animals, namely the Divine
Agent, and God of Nature, whose operations are guided with the
highest Artifice, Providence, and Wisdome, and doe all tend to some
certaine end, and are all produced, for some certaine good. But these
men derogate from the Honour of the Divine Architect, who hath
made the Shell of the Egge with as much skill for the egge's defence
as any other particle, disposing the whole out of the same matter
and by one and the same formative faculty." But although these are
Harvey's theories, it is significant that in his preface he says, "Every
inquisition is to be derived from its causes, and chiefly from the
material and efficient", thus expressly excluding formal and final
considerations. Certainly, as far as his practical work went, he was
unaffected by them, and in the case of the egg-shell, for example,
Harvey was not the man to say, "it is present for the protection of
the embryo", and then to do or say nothing more. Such an explana-
tion, though he might gladly accept it, was no bar to further explora-
tion both by way of experiment and observation.
SECT. 3] AND EIGHTEENTH CENTURIES 147
Harvey not only follows Aristotle in his good discoveries and true
statements about the egg, but also, unfortunately, in his less useful
parts, as, for example, when he devotes several pages to the dis-
cussion of how far the egg itself is alive, and whether there is any soul
in subventaneous or unfruitful eggs. He decides that there is only
a vegetative soul. On the other hand, he admirably refutes the
opinion of those physicians — who were not few in number — who
declared that the foetal organs were all functionless during foetal life.
"But while they contende", he says, "that the mother's Blood is
the nutriment of the foetus in the womb, especially of the Partes
Sanguineae, the bloody parts (as they call them) and that the Foetus
is at first, as if it were a part of the mother, sustained by her blood
and quickened by her spirits, in so much that the heart beats not
and the liver sanguifies not, nor any part of the Foetus doth execute
any publick function, but all of them make Holy-Day and lie idle;
in this Experience itself confutes them. For the chicken in the egge
enjoyes his own Blood, which is bred of the liquors contained within
the egge, and his Heart hath its motion from the very beginning,
and he borroweth nothing, either blood or spirits, from the Hen,
towards the constitution either of the sanguineous parts or plumes,
as those that strictly observe it may plainly perceive." We have already
seen how the Stoics in antiquity believed that the embryo was a part
of the mother until it was born ; from this idea the transition would
be easy to the belief that all the organs in the embryo were functionless
and dependent on the activity of the corresponding ones in the
maternal organism.
One of Harvey's most important services to thought lay in his
abolishing for good the controversy which had gone on ever since
the sixth century B.C. about which part of the egg was for nutrition
and which for formation. He had the sense to see that the distinction
was a useless and baseless one — "There is no distinct part (as we
have often said) or disposed matter out of which the Foetus may be
formed and fashioned. . . . An egge is that thing, whose liquors do
serve both for the Matter and the Nourishment of the foetus.. . .Both
liquors are the nourishment of the foetus."
As regards spontaneous generation, Harvey considered that even
the most imperfect and lowest animals came out of eggs. "We shall
show", he writes, "that many Animals themselves, especially insects
do germinate and spring from seeds and principles not to be discerned
148 EMBRYOLOGY IN THE SEVENTEENTH [pt. 11
even by the eye, by reason of their contract invisible dimensions (like
those Atomes, that fly in the aire) which are scattered and dispersed
up and down by the winds ; all which are esteemed to be Spontaneous
issues, or born of Putrefaction, because their seed is not anywhere
seen." Unfortunately, he never returned to this subject, for, as he
himself informs us in another place, all the papers and notes in his
house in London were destroyed at the time of the Civil War, so that
what he had written on the generation of insects irretrievably
perished.
Another point on which Fabricius had been in error was the ap-
pearance of bone and cartilage in the embryo. According to him,
"Nature first stretcheth out the Chine Bone, with the ribbes drawn
round it, as the Keel, and congruous principle, whereon she foundeth
and finisheth the whole pile". This armchair conceit Harvey was
easily able to destroy by a mere appeal to experience, but by ex-
perience also he came upon a fact less easily to be explained, namely,
that the motion of the foetus began when as yet there was hardly any
nervous system. "Nor is it less new and unheard of, that there should
be sense and motion in the foetus, before his brain is made; for the
Foetus moves, contracts, and extends himself, when there is nothing
yet appears for a braine, but clear water." On the basis of this
paradox Harvey may be said to be the discoverer of myogenic con-
traction, but he already could claim that distinction, for the first
heart-beats are accomplished long before there are any nerves to the
heart, as he himself points out. "We may conclude from this fact",
he remarks, "that the heart and not the brain is the first principle
of embryonic life", and he gives instances of physiological actions
not under the conscious control of the individual, such as the reflexes,
as we should call them, of the intestinal tract, and the emetic action
of infusion of antimony which cannot be tasted much and "yet there
passeth a censure upon it by the Stomack" and a vomit ensues. Thus,
twenty-five years before Francis Glisson, Harvey had formulated,
from embryological studies, the view that irritability was an intrinsic
property of living tissues.
Both Harvey and Fabricius were very puzzled about the first
origin of the blood. "What artificer", says Harvey, "can transform
the two liquors into blood, when there is yet no liver in being?"
It was to be a long time before this question was answered by Wolflf 's
discovery of the blood islands in the blastoderm, and, even now, the
SECT. 3] AND EIGHTEENTH CENTURIES 149
chemistry of the appearance of haemoglobin is one of the most obscure
corners of chemical embryology. The older observers explained it by
considering the yolk to be akin to blood and ready to turn into it
at the slightest inducement.
Another problem which neither Fabricius nor Harvey did any-
thing to solve was the nature of the air-space at the blunt end of
the egg. "Fabricius recounts several conveniences arising from it,
according to its several magnitudes, which I shall declare in short,
saying, It contains aire in it, and is therefore commodious to the
Ventilation of the egge, to the Respiration, Transpiration, and Re-
frigeration, and, lastly, to the Vociferation of the Chicken. Where-
upon, that cavity is at the first very little, afterwards greater, and
at last greatest of all, according as the several recited uses do require."
As regards the placenta, Harvey took the side of Arantius and
denied any connection between the maternal and foetal circulations.
"The extremities of the umbilicall vessels", he said, "are no way
conjoined to the extremities of the Uterine vessels by an Anastomosis,
nor do extract blood from them, but are terminated in that white
mucilaginous matter, and are quite obliterated in it, attracting
nourishment from it." "Wherefore these caruncles may be justly
stiled the Uterine Cakes or Dugs, that is to say, convenient and
proportionate organs or instruments designed for the concocting of
that Albuginous Aliment and for preparing it for the attraction of
the veins." From this it would appear that Harvey regarded the
uterine milk as the special secretion of the placenta, conveyed to the
foetus through the umbilical cord. The nature of the uterine milk
is still very imperfectly understood (see Section 21). Its discovery is
usually attributed to Walter Needham, but various remarks in this
chapter (Ex. lxx) seem to show that Harvey was well acquainted
with it. In later times, it was regarded by some (Bohnius and
Charleton in 1686, Zacchias in 1688 and Franc in 1722) as the sole
source of foetal nourishment. Mercklin spoke of it in 1679 as ''materia
albuginea, ovique albo non absimili". Harvey often calls the placenta
the uterine liver, no doubt only for this reason, but the remarkable
appropriateness of the term was to become apparent in Claude
Bernard's day. As regards the matter of the continuity of the maternal
and foetal circulations, he criticises van Spieghel. "There came forth
a book of late", he says, "wrote by one Adrianus Spigelius, wherein
he treateth concerning the use of the umbilicall arteries and doth
150 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
demonstrate by powerfull arguments that the Foetus doth not receive
its Vital Spirits by the arteries from the Mother, and hath fully
answered those arguments which are alledged to the contrary. But
he might also as well have proved by the same arguments that the
blood neither is transported into the Foetus from the mother's veines
by the propagations of the umbilicall veins which is made chiefly
manifest by the examples drawn from the Hen-Egge and the Caesarean
Birth."
The least satisfactory parts of Harvey's book are the Exercitations
Lxxi and lxxii on the innate heat and the primigenial moisture.
Here he becomes very wordy and highly speculative, and gives us
little but a mass of groundless arguments. He devotes many pages
to proving that the innate heat is the blood and to drawing distinc-
tions between blood and gore, the one in the body, the other shed.
In one place he speaks of the processes of generation as so divine
and admirable as to be "beyond the comprehension and grasp of
our thoughts or understanding". Two centuries previously Frasca-
torius had said precisely the same thing about the motion of the heart,
and it was ironical that the very man who let the light in on cardiac
physiology should in his turn despair of the future of our knowledge
of embryonic development.
Harvey did not say much about foetal respiration, and his few
remarks are contained in one of the "additional discourses". He is
puzzled exceedingly by the question. But he comes very near indeed
to the truth when he says, "Whosoever doth carefully consider these
things and look narrowly into the nature of aire, will (I suppose)
easily grant, that the Aire is allowed to animals, neither for refrigera-
tion, nor nutrition sake. For it is a tryed thing, that the Foetus is
sooner suffocated after he hath enjoyed the Aire, than when he was
quite excluded from it, as if the heat within him, were rather inflamed
than quenched by the aire". Had Harvey pursued this line of thought,
and looked still more narrowly into the nature of air, he might have
anticipated Mayow. He does say that he proposes to treat of the
subject again, but he never did.
The mainspring of Harvey's researches on the does and hinds can
be realised by a reference to Rueff's figures in Fig. 6. According
to the Aristotelian theory, the uterus after fertile copulation would
be full of blood and semen ; according to the Epicurean theory (held
by the "physitians") it would be full of the mixed semina. If this
SECT. 3] AND EIGHTEENTH CENTURIES 151
coagulated mass exists, said Harvey, it ought to be possible to find
it by dissection, and this was what he tried to do. It soon became
plain, as may be read in Ex. lxviii, not only to Harvey but to the
King and the King's gamekeepers, that no such coagulum existed,
^t^^^^^jr. S>'^'^^^ ^>^T*-^^J-^ r "^
^^^ A--r> >^r'-/f >^'^^ S" ^,-^^ ^ , ■
x> f.^^ ^^^^^/s S^^ . rr- f
Fig. 7. Manuscript notes of Dr William Harvey.
and the result was made still more certain by means of segregation
experiments which the King carried out at Hampton Court. Ac-
cordingly there was nothing to be done but to abandon all the older
theories completely, and have recourse to some sort of hypothesis
in which an aura seminalis, an "incorporeal agent" or a "kinde
of contagious property" should bring about fertilisation. This was
152 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
a perfectly sound deduction from Harvey's experiments, and did not
then appear anything like so unsatisfactory as it does now, for Gilbert
of Colchester was not long dead, the "lodestone" was beginning
to be investigated by the virtuosi, and even such extravagances as
Sir Gilbert Talbot's Powder "for the sympatheticall cure of wounds"
were only with difficulty distinguishable from the real effects of
magnetic force. Harvey's idea of fertilisation by contagion has recently
been in a sense revived by the work of Shearer (see Section 4*2).
But to Harvey himself the subject of the action of the seed was
hid in deep night, and he confessed that, when he came to it, he
was "at a stand". Some very interesting light is thrown upon his
mind in this connection by a copy of the De Generatione Animalium
annotated by himself, and now in the possession of Dr Pybus, by
whose courtesy and by that of Dr Singer, who has transcribed the
notes, I have been enabled to study it. It was given by Harvey to
his brother Eliab, whose name it still bears. The notes, which are
on the fly-leaves, are written in much the same way as those famous
ones which Harvey used for his lectures at the College of Physicians in
London, and which have been reproduced in facsimile. There is the
same mixture of Latin and English, and the same signs, such as WI, to
denote thoughts claimed as original. A page is reproduced in Fig. 7.
For the most part, the notes are uninteresting and nothing but a
confusion of Aristotelian terms. But one page is concerned with the
mode of action of the seed, and here we can, as it were, see Harvey's
mind wrestling with this most difficult of problems. He sees that
odour and the sense of smell may give a clue. That his thoughts on
this point were doomed to frustration as soon as eggs and spermatozoa
were discovered does not detract from the interest of the struggle.
Quod facit semen fecundum
What makes the seed fertile is on the analogy of an injection. In fact,
the injection causes disease in many cases, and that from a distance, both
by another. . .and by the same. . . A Venereal (?) disease corrupts coitus
with a woman in whose uterus is the poison.
They do not [or do not yet ?] come forth in actuality but lie dormant
as in fuel [? fomite]. Again, rabies in dogs lies dormant for many days
on my own observation W4. Again, smallpox for days. Again, the genera-
tive seed, just as it (passes) from the male, lies dormant in the woman as
infuel(?).
Or else like a . . . , like light in stone . . . , the pupil in the eye, in sense
motion, ... in the body.
SECT. 3] AND EIGHTEENTH CENTURIES 153
Like ferment, vapour, odour, rottenness ... by rule.
Or like the smell given off by flowers.
Like heat, inflammation (?) A in chalk (heat ?) both the wet form
Like what is first ... in the art of cooking . . . principles of vegetation and
propagation. A Dormice by hibernating. . .cleansing by water and all
kinds of lotions, again for insects, as for their seeds as well (?). Or when
a soul is a god present in nature, that is divine which it brings about
without an organic body by means of law.
See Aristode Marvels concerning odours and smells given off. Whether
on sense and everything that can be smelt gives off something and so
the objects of disperses (?) what is not without heat, or by destroying . . .
sense. attracts to itself
A Amongst inflammable (objects are) fire, naphtha, paper
A WI manus et odore car . . . anatomia manair. ...
A Anat . . . post 4°"^ poras. otium inclinente die rursus quod prius et
olefrere vid. . . . Galen. ...
A Mr. Boys spainel in Paris lay all ye third night and morning in
getting dogg. Whelping dogg's sent (scent) are a stronger sent,
vesting in vestigio alios ord . . . gr . . . lepris odore lepris esse
libidine esse. Hors, the mare, hors, the cow, a bull per mutta
millsa.
A ... si lepra fracedo in farioli fader cupidinitus. Dogg ye otter in aquas
fracedo vasorum ex sulpore?
Just as Aristotle put much of his best embryological work into his
Historia Animalium and not into the work with the appropriate title,
so Harvey has some admirable observations on the embryonic heart
scattered through his De Motu Cordis et Sanguinis in Animalibus. Turning
now to consider Harvey's influence on embryology, we must admit
that it was in certain respects reactionary.
1 . He did not break with Aristotelianism, as a few of his pre-
decessors had already done, but on the contrary lent his authority
to a moribund outlook which involved the laborious treatment of
unprofitable questions.
2. His opposition to atomism and to "chymistry" precluded any
close co-operation between his followers and those of the Descartes-
Gassendi tradition.
3. Fabricius had elaborated a vitalistic theory of differentiation,
but had allowed growth to be "natural" or mechanical. Harvey,
however, made both growth and differentiation the results of an
immanent spirit, a sort of divine legate.
But these failings are far outweighed by his positive services. It
must always be remembered that he had no compound microscope.
154 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
and had to rely, like Riolanus, on "perspectives", or simple lenses
of very low power.
1. There can be no doubt that the doctrine omne vivum ex ovo
was a tremendous advance on all preceding thought. Harvey's
scepticism about spontaneous generation antedated by less than a
century the experiments of Redi. It is important to note that he was
led to his idea of the mammalian ovum by observations on small
conceptions surrounded by their chorion and no bigger than eggs,
for the true ovum itself was not discovered until the time of de Graaf
and Stensen.
2. He identified definitely and finally the cicatricula on the yolk-
membrane as the spot from which the embryo originated.
3. He denied the possibility of generation from excrement and
from mud, saying that even vermiparous animals had eggs.
4. He discussed the question of metamorphosis (preformation) and
epigenesis, and decided plainly for the latter, at any rate for the
sanguineous animals.
In addition to these achievements, there are others, perhaps less
striking, but equally important.
5. He destroyed once and for all the Aristotelian (semen-blood)
and Epicurean (semen-semen) theories of early embryogeny. This
was perhaps the biggest crack he made in the Peripatetic teaching
on development; but, in spite of it, Sennertus, van Linde and
Sylvius adhered to the ancient views, and Cyprianus, in 1700,
had the distinction of being the last to support them in a scientific
discussion, though Sterne, as late as 1 759, referred to them in a way
that shows they still lived on in popular thought.
6. He handled the question of growth and differentiation better
than any before, anticipating the ideas of the present century.
7. He settled for good the controversy which had lasted for 2200
years as to which part of the egg was nutritive and which was forma-
tive, by demonstrating the unreality of the distinction.
8. He set his predecessors right on a very large number of detailed
points, such as the nature of the placenta.
9. He made a great step forward in his theory of foetal respiration,
though here he did not consolidate the gain.
10. He affirmed that embryonic organs were active, and that the
embryo did not depend on external aid for its principal physiological
functions.
SECT. 3] AND EIGHTEENTH CENTURIES 155
But all these titles to remembrance, great as they are, do not
account for the pecuhar fascination of Harvey. A little of it is perhaps
due to his imaginative style, which comes out clearly in Martm
Llewellyn's English version. A word of censure is due to Willis
for transmuting it in his translation into the dull and pedestrian
style of 1847. None who reads the 1653 edition of Harvey can ever
forget such metaphors as this, "For the trunck of the body hitherto
resembles a skiff without a deck, being in no way covered up by
the anteriour parts"; or the vigour of diction which promotes such
remarks as, "In a hen-egge after the tenth day, the heart admits no
spectators without dissection"; or again, "For while the foetus is yet
feeble. Nature hath provided it milder diet and solider meats for its
stronger capacity, and when it is now hearty enough, and can away
with courser cates, it is served with commons answerable to it. And
hereupon I conceive that perfect eggs are not onely party-coloured,
but also furnished with a double white"; or, lastly, "An egge is, as it
were, an exposed womb ; wherein there is a substance concluded, as
the Representative and Substitute or Vicar of the breasts".
In this connection, it would be a pity not to quote from the verses
which Llewellyn prefixed to his translation of Harvey's book. After
describing the controversies that followed the De Motu Cordis he wrote
A Calmer Welcome this choice Peice befall,
Which from fresh Extract hath deduced all,
And for Belief, bids it no longer begg
That Castor once and Pollux were an Egge :
That both the Hen and Houswife are so matcht,
That her Son born, is only her Son hatcht;
That when her Teeming hopes have prosp'rous bin.
Yet to conceive, is but to lay, within.
Experiment, and Truth both take thy part:
If thou canst 'scape the Women ! there's the Art.
Live Modern Wonder, and be read alone,
Thy Brain hath Issue, though thy Loins have none.
Let fraile Succession be the Vulgar Care;
Great Generation's Selfe is now thy Heire.
Curiously enough, the "calmer welcome" which Martin Llewellyn
hoped for actually happened. Harvey's book was so well reasoned
and based on such good observations that it produced only two
156 EMBRYOLOGY IN THE SEVENTEENTH [pt. 11
answers, and they were of little importance. Janus Orcham took
exception to Harvey's finding no seed in the uterus and suggested
that it had vaporised like a steam, but his Aristotelian leanings were
promptly detected and castigated by Rallius. Matthew Slade, taking
the pseudonym of Theodore Aides, published in 1667 his Dissertatio
epistolica contra D. G. Harveium, which was, in his own words, "a
detection of one or two errors in that golden book on the generation
of animals of William Harvey, greatest of physicians and anatomists".
The errors were purely anatomical, and ab Angelis defended Harvey
against Slade's attack, claiming that the "errors" were not errors
at all. A manuscript work of Slade's appears to be extant.
Harvey's influence was evidently speedily felt by his contemporaries.
Strauss soon wrote a rather poor book on the bird's egg in imitation
of him. But the best instance is that in 1655, very soon after the
publication of Harvey's book, William Langly, "an eminent senator
and physician of Dordrecht", made a great many experiments on
the development of the hen's egg. Buffon says that he worked in
1635, i.e. before Harvey, but this is not the case, for in his observa-
tions which were published by Julius Schrader in 1674 the later date
is given several times. Langly mentions Harvey more than once,
and evidently followed his example in careful observation, for his
text is concise and accurate and his drawings very noteworthy.
Julius Schrader included Langly's work in a composite volume
containing a well-arranged epitome of Harvey's book on generation
and some observations of his own on the hen's egg. The book was
dedicated to Matthew Slade and J. Swammerdam. On the practical
side Schrader added nothing memorable to Harvey and Langly, but
it is noteworthy that the mammalian embryo was throughout these
centuries more popular material than that of the chick. Out of fifty
embryologists between Harvey and Haller, the names of Langly,
Schrader, Malpighi and Maitre-Jan practically exhaust the list of
those who studied the egg of the hen. This rather unfortunate orienta-
tion of mind doubtless sprang from the strong influence of medicine,
and especially obstetrics, on seventeenth and eighteenth century
embryology.
3-5. Gassendi and Descartes: Atomistic Embryology
Harvey's death took place in 1657. The following year saw the
publication of Pierre Gassendi's Opera Omnia, and thus brought in
SECT. 3] AND EIGHTEENTH CENTURIES 157
an entirely new phase in embryology. Together with Rene Descartes'
treatise on the formation of the foetus, Gassendi's De generatione
animalium et de animatione foetus marks a quite different attitude to the
subject. Harvey had adopted a rather contemptuous position about
the "corpuscularian or mechanical philosophy", which was then
coming in, and had expected even less help from it in the solution
of his problems than from his equally despised "chymists". Gassendi
now set out to show that the formation of the foetus could be explained
on an atomistic basis: and, using the Galenic physiology and the
new anatomy as a framework, he set forth his theory in full. As we
read it through at the present day, however, we cannot avoid the
confession that it was not a success. In spite of his frequent
quotations from Lucretius and his persuasive style, it does not
carry conviction. The truth of the matter was that the time was
not ripe for so great a simplification. The facts were insufficiently
known, and that Gassendi is not quite as interested in them as he is
in his theory is shown by the circumstance that he only mentions
Harvey once.
Gassendi examines in turn the Aristotelian and the Epicurean
doctrines of embryogeny and rejects them both, the former on the
ground that the change from tgg to hen is too great and difficult
for anything so shadowy and ghost-like as a "form" to accomplish,
and the latter because it leaves no room for teleology. He therefore
adopts as the basis of his system atomism + preformationism, al-
leging that all the germs of living things were made at the creation,
but that they come to their perfection as atomic congregations in an
atomistic universe. Thomas' monograph is a valuable help to the
study of this very interesting thinker.
At exactly the same time, Descartes was speculating on the same
subject. Added to his posthumous De Homine Liber (1662) is a treatise
on the formation of the foetus. He may also have written a work
On the generation of animals, for a manuscript with that title was found
among his papers after his death, and was believed to be in his
handwriting. There is evidence, however, that it is not his, and though
it was published in Cousin's edition of his works, we may safely
neglect it, agreeing, in the words of that editor, that it is "a fragment
in which very mediocre and often quite false ideas struggle to light
through the medium of a style devoid alike of clarity and of grandeur ".
It must be admitted, however, that even his main treatise is very
158 EMBRYOLOGY IN THE SEVENTEENTH [pt. 11
confused. It suffers from containing in its earlier part a great deal
of matter which really belongs to the physiological text-book which
immediately preceded it. Thus it begins abruptly in the middle of
a disquisition on the error of attributing bodily functions to the soul.
Before long, however, it warms to its theme, and a conception of
growth is outlined. "When one is young, the movement of the little
threads which compose the body is less slow than it is in old age,
because the threads are not so tightly joined one to the other, and
the streams in which the solid particles run are large, so that the
threads become attached to more matter at their roots than detaches
itself from their extremities, so that they grow longer and thicker,
in this way producing growth." The fourth part of the book is called,
strangely enough, a Digression, in which the formation of the animal
is spoken of. The mixture of seeds is then described, and a theory
of the formation of the heart is attempted by means of an analogy
with fermentation. The explanation is unconvincing, but has a cer-
tain interest as showing chemical notions beginning to permeate
biological thought. However, Descartes' way of looking at develop-
ment was thoroughly novel, as is illustrated by the following citation.
"How the heart begins to move.. . .Then, because the little parts
thus dilated, tend to continue their movement in a straight line, and
because the heart now formed resists them, they move away from it
and take their course towards the place where afterwards the base
of the brain will be formed, they enter into the place of those that
were there before, which for their part move in a circular manner
to the heart and there, after waiting for a moment to assemble
themselves, they dilate and follow the same road as the aforemen-
tioned ones, etc." Descartes, in fact, with premature simplification,
was trying to erect an embryology more geometrico demonstrata.
That he failed in the attempt was as obvious to his contemporaries
as it is to us — "We see", said Garden, "how wretchedly Descartes
came off" when he began to apply the laws of motion to the forming
of an animal". In doing so, he was many years before his time;
Borelli had done all that could be done at that period in that direc-
tion, and, significantly enough, he left embryology alone. The rest
of Descartes' book is exactly like the citations which have been given,
only applied to each organ and part in turn; he practically uses the
traditional teaching as a scaffolding in which to interweave his
mechanical theory, and he discovers no new facts.
SECT. 3] AND EIGHTEENTH CENTURIES 159
But in the history of embryology these men and their writings have
a very great significance. Impressed by the unity of the world of
phenomena, they wished to derive embryology as well as physics
from fundamental laws. This attempt, which resulted in a Galen-
Epicurus synthesis on the one hand and a Galen-Descartes synthesis
on the other, must be regarded as a noble failure. Its authors did not
realise what a vast array of facts would have to be discovered before
a mechanical theory could with any justice be applied to explain
them. Gassendi and Descartes were like the Ionian nature-philo-
sophers, propounding general laws before the particular instances
were accurately known. Their ineffectiveness arises from the fact
that they did not themselves appreciate this, and consequently
worked out their idea in a prolix detail, the whole of which was
inevitably doomed to the scrap-heap from the very beginning. But
the spark was not to die ; and if anywhere in this history we are to
find the roots of physico-chemical embryology, we must pause to
recognise them here.
Much less well known, but not without interest, was the Dissertatio
de vita foetus in utero of Gregorius Nymmanus, which appeared in the
same year as the second edition of Descartes' book, 1664. Nymmanus
writes with a very beautiful Latin style, and expresses himself with
great clearness. His proposition is, he says, "That the foetus in the
uterus lives with a life of its own evincing its own vital actions, and
if the mother dies, it not uncommonly survives for a certain period,
so that it can sometimes be taken alive from the dead body of its
mother". In supporting this thesis, Nymmanus answers the argu-
ments of those who had held that the lungs and heart of the foetus
were inactive in utero. Fabricius, Riolanus and Spigelius all proved,
says Nymmanus, that the mother and the foetus by no means neces-
sarily die at the same time. "The essential life", he says, "is the soul
itself informing and activating the body, the accidental life is the acts
of the soul which it performs in and with the body." Though the
foetus cannot be said to have life in the latter sense, it can in the
former. The foetus, says Nymmanus, prepares its own vital spirits
and the instruments of its own soul; there is no nerve between it and
its mother. If, he says, the foetal arteries got their sphygmic power
from the maternal heart, they would stop pulsating when the umbilical
cord was tied, but this is not the case. The pulse of the embryo is
therefore due to the foetal heart itself. Galen, says Nymmanus, was
i6o EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
aware of this, but did not understand the meaning of it. Again, the
foetus in utero moves during the mother's sleep, and vice versa. Nym-
manus' dissertation is an interesting study in the transition from
theological to scientific embryology which took place all through the
seventeenth century, and may be followed in the writings of Varan-
daeus, de Castro, Dolaeus, Hildanus, Scultetus, Ammanus, Augerius
and Garmannus. The problem of animation-time, a more meta-
physical aspect of the same question, was still being handled, but
less attention was being paid to it than formerly. Honoratus Faber's
De Generatione Animalium of 1666 does not belong to its period. Its
author, a Jesuit, proceeds in scholastic fashion to lay down four
definitions, three axioms, one hypothesis, and seventy-seven pro-
positions, in the last of which he summarises his conclusions. He is
interesting in that he displayed a disbelief in spontaneous generation,
thereby anticipating Redi, and he is careful to mention the work of
Harvey, but nevertheless his treatise is of little value. His chief
importance is that he is an epigenesist, and therefore demonstrates
to us how the true opinion was becoming accepted, when Malpighi's
brilliant observations and bad theory sent it out of favour, and pre-
pared the way for the numerous controversies of the following
century.
3-6. Walter Needham and Robert Boyle
It was in 1666 also that the following appeared in the Philosophical
Transactions of the Royal Society :
A way of preserving birds taken out of the egge, and other small f actus' s:
communicated by Mr. Boyle.
When I was sollicitous to observe the Processe of Nature in the Forma-
tion of the Chick, I did open Hens Eggs, some at such a day, and some
at other daies after the beginning of the Incubation, and carefully taking
out the Embryo's, embalmed each of them in a distinct Glass (which is
to be carefully stopt) in Spirit of Wine; Which I did, that so I might have
them in readinesse to make on them, at any time, the Observations, I
thought them capable of affording; and to let my Friends at other seasons
of the year, see, both the differing appearances of the chick at the third,
fourth, seventh, fourteenth, or other daies, after the eggs had been sate
on, and (especially) some particulars not obvious in chickens, that go
about, as the hanging of the Gutts out of the Abdomen, etc. How long
sfecT. 3] AND EIGHTEENTH CENTURIES 161
the tender Embryo of the Chick soon after the Punctum saliens is
discoverable, and whilst the bodie seems but a little organized Gelly,
and some while after that, will be this way preserv'd, without being
too much shrivel'd up, I was hindred by some mischances to satisfie
myself; but when the Faetus's, I took out, were so perfectly formed
as they were wont to be about the seventh day, and after, they so
well retained thjeir shape and bulk, as to make me not repent of my
curiosity; And some of those, which I did very early this Spring, I can
yet shew you.
Boyle said in conclusion that he sometimes also "added Sal
Armoniack, abounding in a salt not sowre but urinous".
In the same year that Nymmanus' book appeared, Nicholas
Stensen, that great anatomist, later a Bishop, who was also to all
intents and purposes the founder of geology, published his De musculis
et glandulis specimen, in which Goiter's observations on the vitelline
duct and the general relations between embryo and yolk in the hen's
tgg were made again and confirmed. About this time also Deusingius
described his case of abdominal pregnancy, and was thus the first
anatomist to draw attention to this phenomenon.
In 1667 Stensen published his Elementorum myologiae specimen, in
which he described the female genital organs of dogfishes. He
demonstrated eggs in them and affirmed that the "testis" of women
ought to be regarded as exactly the same organ as the "ovary" or
"roe" of ovipara. At the time he carried the suggestion no further,
but it was an extremely fruitful one, and it is surprising that it did
not create more interest, for it was exactly what Harvey had been
looking for. Nothing obvious having been found in the uteri of King
Charles' does, and the conviction yet being very strong that viviparous
conceptions really came from eggs, Stensen's minute ova supplied
the fitting answer to the question. Thus Harvey and Stensen between
them substituted the modern knowledge of mammalian ova for the
ancient theory of the coagulum all in the space of fourteen years. The
other event for which the year 1667 is remarkable is the De Formato
Foetu of Walter Needham. Needham was a Cambridge physician
who went to Oxford to study in the active school of physiological
research which such men as Christopher Wren, Richard Lower, John
Ward and Thomas Willis were making famous. His book on the
formation of the embryo, written later (and dedicated to Robert
Boyle), after he had been in practice in Shropshire for some time.
i62 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
is important because it is the first book in which definite chemical
experiments on the developing embryo are reported, and also
because it contains the first practical instructions for dissections of
embryos.
Sir Thomas Browne had, as we have already seen, made experi-
ments of a chemical nature on the constituents of birds' eggs and of
the eggs of amphibia, but he did not analyse them after any develop-
ment had been allowed to take place. He may therefore be regarded
as the father of the static aspect of physico-chemical embryology,
while Walter Needham may be regarded as the founder of the
dynamic aspect. The practical difficulties of these pioneers of animal
chemistry may be seen in such a book of practical instructions as
Salmon's General Practise ofChymistry of 1678. They had no satisfactory
glassware, no pure reagents, the methods of heating were incredibly
clumsy, and there was no means of measuring either heat or atmo-
spheric pressure.
In the review of Needham's book which is to be found in the
Philosophical Transactions of the Royal Society for September 1667 there
occurs the sentence, "These humors (the amniotic, allantoic, etc.)
he saith, he hath examined, by concreting, distilling, and coagulating
them; where he furnishes the Reader with no vulgar observations".
What were these observations ? They are to be found in the chapter
entitled "The nature of the humours":
"I now proceed to speak of this other nutritive liquor round about
the urine itself which latter is plainly separated by the kidneys and
the bladder. These liquors also proceed from the blood and seem
similar to its serum but yet they are different from it. For when fire
is applied to them in an evaporating basin [cochlea] they do not
coagulate, as the blood-serum always does. Indeed, not even the
colliquamentous liquid of the egg itself coagulates in this manner,
although it is formed from juices which are evidently liable to coagu-
lation— in the same way humours differ from themselves before and
after digestion, filtration, and the other operations [mangonial of
nature. All, when distilled, give over a soft and clear water [mollem
et lenem] very like distilled milk. This property is common to the liquor
of the allantoic space, along with the rest. Because when the salts
are not yet made wild and exalted the serum of the blood remains
still quite soft and does not give proof of a tartaric or saline nature.
Indeed, the first urine of an infant is observed by nurses to be not at
SECT. 3] AND EIGHTEENTH CENTURIES 163
all salt, but in older animals, when I distilled it in an alembic, I
seemed to observe a little volatile salt at the small end [in capitello].
Coagulations attempted by acids happened differently in respect of
the different humours. For when I poured a decoction of alumina
into the liquor of the cow's amnios it exhibited a few rather fine
coagulations but they were clearly white. The allantoic juice, how-
ever, was precipitated like urine. Spirits of vitriol and vinegar
brought about less results than alumina in each case. Spontaneous
concretions I found also in the later months; these I discovered in
both places. They are more frequent and larger, however, within
the allantoic membrane."
From the above excerpt, which contains the account of all that
Needham did on the chemical composition of the embryonic liquids,
it can be seen that he treated the whole matter more dynamically
than Browne. He was the first to describe the solid bodies in the
amniotic fluid (see Jenkinson) and his chemical experimentation was
all pioneer work.
His book has other merits, however. In the first chapter, he refutes
the theory which Everard had propounded, that the uterine milk was
identical with the contents of the thoracic duct, conveyed by lym-
phatic vessels to the uterus from the lac teals of Aselli, instead of
elsewhere, and he shows that arteries must be the vessels bringing
the material to the womb. The second chapter deals with the placenta
"where he giveth a particular account of the double Placenta or
Cake, to be found in Rabbets, Hares, Mice, Moles, etc., and examines
the learned Dr Wharton's doctrine, assigning a double placenta to at
least all the viviparous animals, so as one half of it belongs to the
Uterus, the other to the Chorion, shewing how far this is true, and
declaring the variety of these Phaenomena. Where do occur many
uncommon observations concerning the difference of Milk [uterine]
in ruminating and other animals, the various degrees of thickness of
the uterin liquor in oviparous and viviparous creatures". He de-
scribes the human placenta very correctly indeed. "The use of the
placenta is known to be to serve for conveighing the aliment to the
foetus. The difficulty is only about the manner. Here are examined
three opinions, of Curvey, Everhard, and Harvey. The two former
do hold that the foetus is nourished only from the Amnion by the
mouth ; yet with this difference, that Curvey will have it fed by the
mouth when it is perfect, but whilst it is yet imperfect, by filtration
i64 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
through all the pores of the body, and by a kind of juxtaposition : but
Everhard, supposing a simultaneous formation of all the instruments
of nutrition together and at first, and esteeming the mass of bloud
by reason of its asperity and eagerness unfit for nutrition, and rather
apt to prey upon than feed the parts, maintains, that the liquor is
sucked out of the amnion by the mouth, concocted in the stomack,
and thence passed into the Milky Vessels even from the beginning.
Meantime they both agree in this, that the embryo doth breath but
not feed through the umbilicall vessels. This our Author undertakes
to disprove; and having asserted the mildness of, at least, many
parts of the bloud, and consequently their fitness for nutrition, he
defends the Harveyan doctrine of the colliquation of the nourishing
juyce by the Arteries and its conveyance to the foetus by the
veins."
In the third chapter Needham gives the first really comparative
account of the secondary apparatus of generation, enunciating the
rather obvious rule that in any given case the number of membranes
exceeds the number of separate humours by one. He affirms that
all the humours are nutritive save the allantoic. It had previously
been held that all fish eggs were of one humour only, but he points
out that a selachian egg has its white and yolk separate. He gives
the results of his chemical experiments at this point, and suggests
that the noises heard from embryos in utero and in ovo may be due
to the presence of air or gas in the amniotic cavity, thus forming a
link between Leonardo and Mazin. In his fourth chapter he deals
with the umbilical vessels and the urachus, and here he claims priority
over Stensen for the discovery of the ductus intestinalis in the chick,
referring to Robert Boyle, Robert Willis, Richard Lower and Thomas
Millington, to whom, he says, he showed the duct before Stensen
published his observations on it. The fifth chapter is concerned with
the foramen ovale, and the arterial and venous canals, and with the
foetal circulation in general. The sixth is about respiration or "bio-
lychnium", and in it Needham writes against the conception of a
vital flame, alleging cold-blooded animals, etc., in his favour, but
here he takes a retrograde step, for he argues that the use of the lungs
is not for respiration but to "comminute the bloud and so render
it fit for a due circulation". "The seventh and last chapter contains
a direction for the younger Anatomists, of what is to be observed in
the dissection of divers animals with young, and first, of what is
SECT. 3] AND EIGHTEENTH CENTURIES 165
common to the viviparous, then, what is pecuHar to severall of them,
as, a sow, mare, cow, ewe, she-goat, doe, rabbet, bitch, and a woman,
lastly, what is observable in an Egg, skate, salmon, frog, etc. All is
illustrated with divers accurate schemes."
The subsequent course of chemical embryology in the seventeenth
century may be put in a very few words. Marguerite du Tertre
incorporated in her obstetrical text-book of 1677 the results of some
similar experiments to those of Needham. "If you heat the (amniotic)
liquor", she says, "it does not coagulate, and if you boil it it flies
away leaving a crass salt like urine, but if you heat the serosity of
blood, it solidifies as if it were glue." The same observation was re-
corded by Mauriceau in 1687, who concluded, with some common
sense, that, as there was so little solid matter present, the liquid could
not be very nutritive; and by Case in 1696, who said, "In this juice
the plastic and vivifying force resides, for although to our eyes it looks
in colour and consistency like the serum of the blood, yet it is abso-
lutely \toto coelo] different; for if a little of the former is slowly evaporated
\si in cochleari super ignem defines] no coagulation will ever appear."
Lister said this once more in 171 1, but with Boerhaave's work of 1732
the subject entered a new phase.
In 1670 Theodore Kerckring published an adequate work on foetal
osteology, and, two years later, de Graaf and Swammerdam,
making full use of the opportunities afforded them by the invention
of the microscope, described in detail the ova of mammalia, thus
demonstrating the truth of Stensen's suggestion of some years before.
It is important to note that these workers mistook the "Graafian
follicles" for the eggs — a mistake which was not rectified till the time
of von Baer. Stensen himself published not long after an account
of these eggs also, but he was by then too late to gain the priority
of demonstration. Portal's claim that Ferrari da Grado, who lived
in the fifteenth century, was the true discoverer of mammalian ova
has been disproved by Ferrari; and, although it is true that Volcher
Goiter described what we now call the Graafian follicles, he did not
recognise in any way their true nature.
De Graaf 's discovery was confirmed in 1678 by Caspar Bartholinus,
and, in 1674, by Langly, whose original observations had been made,
so it was said, in 1657, the year of Harvey's death. If this is true,
Langly has the priority of observation, Stensen of theory and de
Graaf of demonstration.
i66 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
3-7. Marcello Malpighi: Micro- Iconography and Preforma-
tionism
In the year 1672, Marcello Malpighi, who had for many years
previously been working on various embryological problems with the
aid of the simple microscope, published his tractates De Ovo Incubato
and De Formatione Pulli in Ovo. In spite of its great importance, there
is not much to be said about it, for it is anything but a voluminous
work. The plates in which Malpighi represented the appearances he
had seen in his examination of the embryo at different stages are
beautiful, and some of them are reproduced. Description of the embryo
was now pushed back into the very first hours of incubation, and it
is interesting to note that Malpighi could not have done his work
without Harvey, whose name he mentions on his first page, and who
pointed out the cicatricula as the place where development began,
and therefore, as Malpighi must have reasoned, the place where
microscopic study would be very profitable. Now for the first time
the neural groove was described, the optic vesicles, the somites, and
the earliest blood-vessels.
Malpighi opened the modern phase of the controversy preformation
versus epigenesis by supporting the former view. Embryogeny, he
held, is not comparable to the building of an artificial machine, in
which one part is made after another part, and all the parts gradually
"assembled", but takes place rather by an unfolding of what was
already there, like a Japanese paper flower in water. He was led
to this belief by the fact that development goes on after fertilisation
as the tgg passes down the oviduct, and in the most recently laid
eggs gastrulation is already over, so that in his researches he could
never find an absolutely undivided egg-cell. It is curious to note
that he says his experiments were done "mense Augusti, magno vigente
calore'\ so that more than a usual degree of development would
have taken place overnight. Had he examined the cicatriculae in
hens' eggs before laying, he would very probably not have formed
this theory, and the epigenesis controversy would have been settled
with Harvey. Another influence which was unfavourable to the
epigenetic position was that it was Aristotelian, and therefore un-
fashionable. Yet Malpighi's view was much more sensible than many
which succeeded it, for he did not maintain a perfectly equal swelling
up of all parts existing at the start, but rather an unequal unfolding,
SECT. 3]
AND EIGHTEENTH CENTURIES
167
a distribution of rate of growth at different times and in different
regions of the body. Thus he says, "Now, as Tully says, Death truly
belongs neither to the living nor to the dead, and I think that some-
thing similar holds of the first beginnings of animals, for when we
enquire carefully into the production of animals out of their eggs,
we always find the animal there, so that our labour is repaid and we
see an emerging manifestation of parts successively, but never the
first origin of any of them".
Ti£ JCr
I>e Cue .
^^'
#1? J
Ti>-ia.
Ti^ lOI.
Fig. 8. Malpighi's drawings of the early stages of development in the chick embryo.
What had been an unfounded speculation for Seneca in antiquity
and for Joseph de Aromatari and Everard in late times was now set
upon an apparently firm experimental basis by Malpighi.
It is most instructive to note the difference in the attitudes of
Langly and Schrader respectively towards the preformation question.
Langly has no doubts about it, nor has Faber; they both follow
Harvey and epigenesis unquestioningly, but Schrader, although he
believes in epigenesis on the whole, is not at all certain about it.
His friend, Matthew Slade, he says, brought the epistle of Joseph
de Aromatari to his attention, and what with that and the unexplained
observations of Malpighi on the pre-existence of the embryo, he is
not willing to deny all value to preformationist doctrine. Others
were bolder. It was immediately seized upon by Malebranche, the
1 68
EMBRYOLOGY IN THE SEVENTEENTH [pt. n
Streeter of his age, who, in his Recherche de la Verite of 1672, reaHsed
its philosophical possibilities, and gave it a kind of metaphysical
sanction. That mystical microscopist, Swammerdam, made use of
it as an explanation of the doctrine of original sin. In a remarkably
short space of time it was a thoroughly established piece of biological
theory.
Malebranche refers to it in his Recherche de la Verite in the chapter
where he treats of optical illusions and emphasises the deceitfulness
and inadequacy of our senses. "We see", he says, "in the germ of a
fresh Qgg which has not been incubated an entirely formed chicken.
We see frogs in frogs' eggs and we shall see other animals in their
B^-xh-
Fig. 9. Malpighi's drawings of the chick embryo's blood-vessels.
germs also when we have sufficient skill and experience to discover
them. We must suppose that all the bodies of men and animals which
will be born until the consummation of time will have been direct
products of the original creation, in other words, that the first females
were created with all the subsequent individuals of their own species
within them. We might push this thought further and belike with
much reason and truth, but we not unreasonably fear a too premature
penetration into the works of God. Our thoughts are, indeed, too
gross and feeble to understand even the smallest of his creatures."
Malebranche, who was a priest of the Oratory of the Cardinal de
Berulle, took an ardent interest in the scientific life of his time — for
example, in a letter to Poisson, the Abbe Daniel wrote, "Reverend
Father, M. Malebranche has written to me saying that he has in-
stalled an oven in which he has hatched eggs. He has already opened
PLATE VII
I.XVI-
lu Ouc
XVI.
ILLUSTRATIONS FROM MALPIGHI'S DE OVO INCUBATO OF 1672
Showing the early stages of the development of the chick, somites, area vasculosa, etc.
SECT. 3] AND EIGHTEENTH CENTURIES 169
some and has been able to see the heart formed in them and beating,
together with some of the arteries" (Blampignon).
Swammerdam's support for preformation came from a different
angle. He had been investigating insect metamorphosis, and, having
hardened the chrysalis with alcohol, had seen the butterfly folded
up and perfectly formed within the cocoon. He concluded that the
butterfly had been hidden or masked {larvatus) in the caterpillar, and
thence it was no great step to regard the Qgg in a similar light. Each
butterfly in each cocoon must contain eggs within it which in their
turn must contain butterflies which in their turn must contain eggs,
and so on. Before long, Swammerdam extended this theory to man.
"In nature", he said, "there is no generation but only propagation,
the growth of parts. Thus original sin is explained, for all men were
contained in the organs of Adam and of Eve. When their stock of
eggs is finished, the human race will cease to be."
In 1684 Zypaeus reported that he had seen minute embryos in
unfertilised eggs, and there were other similar claims. ''^Hinc recentiores
physiologV\ said Schurigius in 1732, ^^ hominem in ovulis delineatum
quoad omnes partes in exiguis staminibus ante conceptionem existere
statuunt"
Swammerdam cannot be regarded simply as one of the principal
pillars of the preformation theory. His own embryological researches,
which were made chiefly on the frog, were remarkable in many
ways. He was the first to see and describe the cleavage of the egg-
cell and later segmentation. He said that there was a time during the
development of the tadpole when its body consisted of granules
{greynkens or klootkens), but as these grew smaller and much more
numerous they escaped his penetration. Leeuwenhoek also saw these
cells, and his account was published long before Swammerdam's,
but his observations on the rotating embryos oi Anodon and the eggs
of fleas were equally interesting.
3-8. Robert Boyle and John Mayow
In 1674 John Mayow, a young Oxford physician, published his
tractate, De Respiratione Foetus in Utero et Ovo, which was included as
one of the parts of his Tractatus Quinque medico-physici in that year.
Mayow was the first worker to realise that gaseous oxygen, or, as he
termed it, the " nitro-aerial " vapour, was the essential factor in the
burning of a candle and the respiration of a living animal. His work
170 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
was forgotten until Beddoes drew attention to it in 1 790, but since then
many have praised it and Schultze makes him the equal of Harvey.
The reason why he became interested in embryology is given in
the opening sentences of his work. "Since the necessity of breathing",
he says, "is so essential to the sustaining of life that to be deprived
of air is the same as to be deprived of common light and vital spirit,
it will not be out of place to enquire here how it happens that the
foetus can live though imprisoned in the straits of the womb and
completely destitute of air." He first of all gives an account of the
opinions held about foetal respiration and the umbilical cord. He
says that he disagrees (i) with the view that the embryo breathes
per OS while it is in the womb, for there is no air in the amnion and
the suctio infantuli proves nothing; and (2) with the view pro-
pounded by Spigelius that the umbilical vessels existed to supply
blood to the placenta for the nourishment of the latter. If this were
the case, he says, the membranes in the hen's egg could not be formed
before the vitelline vein, as they are, and in cases of foetal atrophy
the placenta would always die and be corrupted too, which does not
happen. Nor does he support the view of Harvey (3) that the
umbilical vessels supply blood for the concoction and colliquation
of the food of the foetus, for why should not the embryonic body
prepare its own nutritious juice before birth just as it does afterwards.
He further thinks the theory (4) that the umbilical vessels are for
carrying off surplus foetal nourishment quite untenable and as little
likely as the theory (5) that they exist for the object of allowing a
foetal circulation — for this could just as well be accomplished through
the vessels which exist in the embryonic body.
Mayow decides therefore for the opinion of divino sene Hippo-
crate and Everard that the umbilicus is a respiratory mechanism,
carefully dissociating himself, however, from the hypothesis of
Riolanus that the umbilical cord with all its windings is so arranged
to cool the blood passing through it. He then says, "We observe,
in the first place, that it is probable that the albuminous juice exuding
from the impregnated uterus is stored with no small abundance of
aerial substance, as may be observed from its white colour and frothy
character [Needham's uterine milk]. And in further indication of
this, the primogenial juices of the egg, which have a great resemblance
to the seminal juice of the uterus, appear to abound in air particles.
For if the white or the yolk of an tgg be put into a glass from which
SECT. 3] AND EIGHTEENTH CENTURIES 171
the air is exhausted by the Boyhan pump these liquids will imme-
diately become very frothy and swell up into an almost infinite
number of little bubbles and into a much greater bulk than before — a
sufficiently clear proof that certain aerial particles are most intimately
mixed with these liquids. To which I add that the humours of an
tgg when thrown into the fire, give out a succession of explosive
cracks which seem to be caused by the air particles rarefying and
violently bursting through the barriers which confined them. Hence
it is that the fluids of the egg are possessed of so fermentative a nature.
For it is indeed probable that the spermatic portions of the uterus
and its carunculae are naturally adapted for separating aerial particles
from arterial blood. These observations premised, we maintain that
the blood of the embryo, conveyed by the umbilical arteries to the
placenta or uterine carunculae, brings not only nutritious juice, but
along with this a portion of nitro-aerial particles to the foetus for
its support, so that it seems that the blood of the infant is impregnated
with nitro-aerial particles by its circulation through the umbilical
vessels quite in the same way as in the pulmonary vessels. And there-
fore I think that the placenta should no longer be called a uterine
liver but rather a uterine lung". These splendid words, informed by
so much insight and scientific acumen, show that, by the time of
Mayow, chemical embryology had definitely come into being. He died
at the early age of thirty-six, and we may well ponder how different
the subsequent course of this kind of study would have been if he had
lived a little longer.
The second part of Mayow's treatise is concerned with respiration
in the hen's egg during its development, and it may be noted that
his observations on the air contained in the liquids before develop-
ment probably account for the facts which have been reported at
one time and another concerning an alleged anaerobic life of embryos
in early stages. Mayow is wrong in supposing that the gas which he
pumped out from white and yolk was purely "nitro-aerial", but he
shows the greatest good sense in his reminder that the amount of
nitro-aerial particles required by embryos must be comparatively
small owing to their small requirement for "muscular contraction
and visceral concoction". His remarks on the effect of heat on the
developing egg are not so clear as the remainder of the treatise, but
he seems to mean that the heat will disengage the nitro-aerial particles
from the liquids, and so aid in respiration, an idea which was later
172 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
used by Mazin. His fundamental mistake here was that he failed to
realise that the egg-shell was permeable to air; and this vitiates all
his reasoning about the respiration of the egg. "It will not be ir-
relevant", he says, "to enquire here whether the air which is con-
tained in the cavity in the blunter end of every egg contributes to
the respiration of the chick." He first notes that the cavity in question
lies between two membranes and not between the shell-membrane
and the shell as Harvey himself had supposed ; and then he goes on
to say that he disagrees with the opinion of Fabricius, who had asserted
that the air in the air-space serves for the respiration of the chick.
His reasons are (i) that there would not be enough therein for the
needs of the embryo which would use it, as it were, in one gulp,
and (2) that the air in it cannot pass through the inner membrane,
an error into which he was led by observing that, if an egg-shell
with its contents removed and its air-space intact, was put into a
vacuum, the air-space would swell up until it was as big as the egg
itself. Mayow sees now what had escaped the attention of all previous
observers, namely, that the egg-contents are not "rarefied or ex-
panded, but are on the contrary condensed and forced into a nar-
rower space than before". Such a condensation could, he thinks,
take place in four ways, (a) by an increase in propinquity of discrete
particles, (b) by a subsidence of motion on the part of a congregation
of particles into rest, (c) by the extraction of some subtle spirit from
amongst the particles, and, (d) by a decrease in elasticity on the part
of some elastic substance previously present. We should at the present
time choose the third alternative as being the truest, in view of the
loss of water and carbon dioxide which the egg suffers as it develops,
but Mayow chose the fourth, thinking it probable that the "air
distributed among the juices of the egg loses its elastic force on account
of the fermentation produced among these juices by incubation".
Now since the egg-contents are compacted into smaller bulk by the
process of incubation, a vacuum would be created somewhere if
Nature had not, with her customary prudence, inserted a small
amount of air into the air-space which might in due course expand
and avoid this. His proof for this was an inaccurate observation;
he thought he saw, in eggs at a late stage, when the contents were
removed, the air-space collapse to the normal size which it occupies
in unincubated eggs. He expressly says that his theory does not depend
upon the conception of horror vacui, but that, by the compressive
SECT. 3] AND EIGHTEENTH CENTURIES 173
action of the imprisoned air, the fluids of the egg would be forced into
the umbiHcal vessels, and the particles composing the embryonic body
packed more tightly together. "The internal air appears to perform
the same work as the steel plate bent round into numerous coils by
which automata are set in motion."
With this ingenious but erroneous supposition Mayow concludes
what is undoubtedly the first great contribution to physiological or
biophysical embryology. His views on foetal respiration were soon
generally accepted, as the writings of Zacchias, Viardel, Pechlin and
John Ray show, but Sponius as late as 1684 was asserting that the
lungs of the foetus were functional in utero, absorbing from the
amniotic liquid the nitro-aerial particles which P. Stalpartius sup-
posed the placenta to be secreting into it. It is interesting to note
that by Mayow's own air-pump method Bohn found nitro-aerial
particles in the uterine milk in 1686, and Lang found them in the
amniotic liquid in 1 704. The problem had by then arrived at a stage
beyond which it could not progress in the absence of quantitative
methods.
The year 1675 saw the publication of Nicholas Hoboken's useful
treatise on the anatomy of the placenta, and of the English edition
of P. Thibaut's Art of Chymistry. I mention the latter here, because
of a reference to the special conditions of embryonic life which is
found in it. As yet no real help was being given to embryology by
contemporary chemistry.
The Magistery and Calx of Egg-shells.
Obs. 2. That you must use the eggshells of hens and not of ducks, geese,
or turkeys because that hens eggshells easier calcin'd being thinner by
reason that a hen is a more temperate animall; waterfowl are hotter and
by reason of their heat do concoct and harden their eggshells more than
other fowl ; and from thence it comes that you must have a greater quantity
of your Dissolvant, employ more heat, and spend more time to calcine
the eggs of waterfowl than those of hens.
About this time also Francis Willoughby published his famous
book on birds, an attempt to bring Aldrovandus up to date, in which
a good picture is given of the embryological knowledge of the time,
although no new observations or theories are given. Another con-
temporary review is that of Barbatus.
In 1677, spermatozoa were discovered, as announced by Hamm
and Leeuwenhoek in the Philosophical Transactions of the Royal Society,
174 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
though Hartsoeker afterwards claimed that he had seen them as
early as 1674, but had not had sufficient confidence to publish his
results. There is a reference to this in the letters of Sir Thomas
Browne, who, writing to his son, Dr Edward Browne, on December 9,
1679, said, "I sawe the last transactions, or philosophicall col-
lections, of the Royal Society. Here are some things remarkable, as
Lewenhoecks finding such a vast number of little animals in the melt
of a cod, or the liquor which runnes from it ; as also in a pike ; and
computeth that they much exceed the number of men upon the whole
earth at one time, though hee computes that there may bee thirteen
thousand millions of men upon the whole earth, which is very many.
It may bee worth your reading".
At the same time as these events were taking place, Robert Boyle,
at Oxford and London, was engaged in carrying out those experi-
ments in chemistry which led him before long to write his Sceptical
Chymist. It is not generally known that in this work, which appeared
in 1680, and which set the key for the whole spirit of subsequent
physico-chemical research, Boyle has a reference to embryology, and,
curiously enough, in connection with a point which, although it is
easily seen to be of the highest importance, has been quite overlooked
by the commentators upon him. One of the main things he was
trying to urge was that, until some system could be proposed which
would give a means of quantitative estimation of the constituents of
a mixture, no further progress would be made. He was asking, in
fact, that chemistry should become an exact science, and his demand
is only veiled by the unfamiliarity of his language. His preference
for the "mechanical or corpuscularian" philosophy was mainly due
to his realisation that, unless chemistry was going to start measuring
something, it might as well languish in the obscurity to which
Harvey would have willingly relegated it. Thus he says, "But I
should perchance forgive the Hypothesis I have been all this time
examining (that of the alchemists), if, though it reaches but to a
very little part of the world, it did at least give us a satisfactory account
of those things which 'tis said to teach. But I find not that it gives
us any other than a very imperfect information even about mixt
bodies themselves; for how will the knowledge of the Tria Prima
discover to us the reason why the Loadstone drawes a Needle, and
disposes it to respect the Poles, and yet seldom precisely points at
them? how will this hypothesis teach us how a Chick is formed
SECT. 3]
AND EIGHTEENTH CENTURIES
175
in the Egge, or how the seminal principles of mint, pompions, and
other vegetables, can fashion Water into various plants, each of them
endow'd with its peculiar and determinate shape and with divers
specifick and discriminating Qualities? How does this hypothesis
shew us, how much Salt, how much Sulphur, how much Mercury must be
taken to make a Chick or a Pompion? and if we know that, what
principle is it, that manages these ingredients and contrives, for
instance, such liquors as the White and Yolke of an Egge into such
a variety of textures as is requisite to fashion the Bones, Arteries,
Veines, Nerves, Tendons, Feathers, Blood and other parts of a Chick;
and not only to fashion each Limbe, but to connect them altogether,
after that manner which is most congruous to the perfection of the
Animal which is to consist of them? For to say that some more fine
and subtile part of either or all the Hypostatical Principles is the
Director in all the business and the Architect of all this elaborate
structure, is to give one occasion to demand again, what proportion
and way of mixture of the Tria Prima afforded this Architectonick
Spirit, and what Agent made so skilful and happy a mixture?"
Boyle's instance of the magnetic needle pointing nearly, not exactly,
at the north, and his use of the expressions "how much, how many,
proportion, way of mixture", indicate that he was moving towards
a quantitative chemistry, and by express implication a quantitative
embryology. Elsewhere he says that he thinks the Tria Prima will
hardly explain a tenth part of the phenomena which the "Leucip-
pian" or atomistic hypothesis is competent to deal with. Thus,
although Boyle made few experiments or observations on embryos,
he occupies a very important position in the history of embryology.
During the last two decades of this century, the Oxford Philo-
sophical Society were occupied on a good many occasions with
problems relating to embryology. It is extremely interesting to note,
in connection with what we have just seen in Boyle, that John
Standard of Merton College reported on February 10, 1685, "the
following obbs. concerning ye weight of ye severall parts of Henn's
eggs ; done with a pair of scales which turned with \ a grain.
ozs. dr.
A henn's egg weighed 2
The skin weighed
The shell
The yolk
The white
grns.
15
16
4
Loss in weighing
[ujf LIBRARY
176 EMBRYOLOGY IN THE SEVENTEENTH [pt. 11
ozs. dr. scr. grns.
Another raw egg of the same sort ... 2 i 2 13
Another
The former egg boiled
Lost in boiling
The skin
The shell
The yolk
The white
2 I I 19
.2 I I 18
• - - - 15
13
- I 2 19
- 5 - 7
I 2 - 13
Loss in weighing 5
Another early quantitative observation was that of Claude Perrault
who found about 1680 that developing ostrich eggs lost one-ninth
of their weight in five weeks. The Oxford Philosophical Society,
however, preferred as a rule to consider more unusual things, such
as "the egges of a parrot hatched in a woeman's bosome, a hen egg
figur'd like a bottle, a hen egg that at the big ende had a fleshie
excrescence, another hen-eg, monstrous, a suppos'd cocks egg, and
the eggs of a puffin, an elligug, and a razor-bill". Mention of these
different kinds of eggs reminds us that the systematic collection and
classification of eggs had been begun some years before by Sir Thomas
Browne (as may be seen in John Evelyn) and by John Tradescant.
About this time R. Waller made some noteworthy observations on
the "spawn of frogs and the production of Todpoles therefrom",
extending the work begun by Swammerdam not long before.
Mauriceau now gave a description of the phenomenon of sterile
foetal atrophy. The century fittingly closes with Michael Ettmiiller's
ponderous treatise, in which all the embryological work of the
seventeenth century is summarised with considerable accuracy. He
supported the moribund menstruation theory of embryogeny with
the argument that animals do not menstruate because they are more
prolific than men, and therefore all their blood is required for genera-
tion. Garmann's Oologia curiosa, which appeared in 1691, is worth
mention also, as a review of the knowledge of the time. But that his
work was what the booksellers' catalogues describe as "curious" is
shown by the following chapter-headings: De ovo mystico, rnpthico,
magico, mechanico, medico, spagyrico, magyrico, pharmaceutico.
3-9. The Theories of Foetal Nutrition
During the course of the seventeenth, and the first quarter of the
eighteenth, century, many theories were propounded concerning
foetal nutrition. It is convenient to classify them.
SECT. 3] AND EIGHTEENTH CENTURIES 177
I. That the embryo was nourished by the menstrual blood.
Beckher, 1633.
Plempius, 1644. (He did not deny that the umbilical cord was
functional, but insisted that the blood passing through it was
menstrual.)
In 1 65 1 Harvey's work was published.
Sennertus, 1654.
Seger, 1660.
van Linde, 1672.
F. Sylvius, 1680.
Cyprianus, 1700.
II. That the embryo was nourished by its mouth.
{a) By the amniotic liquid.
(A) In addition to the umbilical blood.
Harvey, 1651.
W. Needham, 1667.
de Graaf, 1677.
C. Bartholinus, 1679.
van Diemerbroeck, 1685.
Ortlob, 1697.
D. Tauvry, 1700.
Linsing, 1701.
PauH, 1707.
Barthold, 1717.
S. Middlebeek, 17 19.
Teichmeyer, 17 19.
Gibson, 1726.
(B) Alone; the umbilical blood being regarded as un-
necessary or of minor importance,
Moellenbroeck, 1672.
Cosmopolita, 1686.
Everardus, 1686.
P. Stalpartius, 1687.
Bierling, 1690.
Case, 1696. (Case thought the embryo arose entirely
out of the amniotic liquid like a precipitate from
a clear solution.)
Berger, 1702.
These persons referred as their principal experi-
mental basis to cases in which embryos had been
born without umbilical cords, e.g. of those of:
Rommelius, 1675 (in Velsch).
Valentinius, 1 7 1 1 .
178 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
(b) By the uterine milk or succum lacteo-chylosum.
Mercklin, 1679.
Drelincurtius, 1685.
Bohnius, 1686.
Zacchias, 1688.
Tauvry, 1694.
Franc, 1722.
Dionis, 1724.
III. That the embryo was nourished through the umbiHcal cord
only.
{a) By foetal blood (the circulations distinct).
Arantius, 1595.
Harvey, 1651.
W. Needham, 1667.
F. Hoffmann, 1681. (He proved the point by injection
long before Hunter, who is stated by Cole to have been
the first to demonstrate this.)
Ruysch, 1 70 1.
Snelle, 1705.
Falconnet, 171 1.
It is to be noted that Bierling, P. Stalpartius, Berger,
Barthold, and Charleton, who supported the discon-
tinuity theory of the circulations, were all upholders
of the theory of foetal nourishment per os, so that their
reasons for doing so were not those on account of
which we agree with Hoffmann and Needham at the
present time.
{b) By maternal blood (the circulations continuous).
Laurentius, 1600.
de Marchette, 1656.
Rallius, 1669.
Muraltus, 1672.
Blasius, 1677.
Veslingius, 1677.
Hamel, 1700.
de Craan, 1703.
Lang, 1704.
van Home, 1707.
Freind, 171 1. (Freind's Emmenologia deserves a special
mention. He proved by a calculation that the amount
of blood passing through the umbilical cord would be
sufficient for the needs of the embryo. This is a parallel
to Harvey's famous calculation about the circulation
of the blood. He also quotes some experiments of
SECT. 3] AND EIGHTEENTH CENTURIES 179
Rayger and Gayant, who injected a blue dye into the
foetal circulation and found it again in the maternal.
Therefore he regards it as continuous.)
Mery, 171 1. (Mery combated Falconnet's view of the
separate circulations. He said that he had not himself
tried Falconnet's experiment, but that some students
had, and could not repeat it.)
Aubert, 1 7 1 1 . (Narrative of a case in which the um-
bilical cord had not been tied at the maternal end and
the mother had nearly bled to death through it.)
Nenterus, 17 14.
Wedel, 1 71 7.
Bellinger, 171 7. (Bellinger believed that the maternal
blood was transformed by the embryonic thymus gland
into proper nourishment for itself, after which it was
secreted into the mouth by the salivary ducts and so
went to form meconium without the necessity for de-
glutination. Heister's comments on this extraordinary
theory are worth reading. Perhaps Bellinger was in-
debted to Tauvry for his idea of the importance of the
thymus gland. Tauvry had drawn attention in 1700
to its diminution after birth.)
de Smidt, 17 18.
Dionis, 1724.
(c) By menstrual blood.
Plempius, 1644.
(d) By uterine milk.
Ent, 1687.
Camerarius, 17 14. {Opinio conciliatrix!)
F. Hoffmann, 1718.
{e) By the amniotic fluid.
Vicarius, 1700.
Goelicke, 1723.
IV. That the embryo was nourished by pores in its skin.
Deusingius, 1660.
Nitzsch, 1 67 1.
Stockhamer, 1682.
This was suggested on the ground that in the earlier stages of
development there is no umbilical cord. In 1684 St Romain argued
against it on the ground that, if it were true, the embryo would
dissolve in the amniotic liquid.
i8o EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
During this period also there were continued disputes about the
origin of the amniotic liquid, van Diemerbroeck and Verheyen con-
sidered that it could not be the sweat of the embryo, for the embryo
was always much too small to account for it, and, moreover, Tertre
had described cases where the secundines had been formed with the
membranes but in the absence of the embryo. Dionis affirmed that,
whatever it was, it could not be urine, for urine will not keep good
for nine days, a fortiori not for nine months. Drelincurtius put
forward a theory that the embryo secreted it from its eyes and mouth
by crying and salivating, while Bohn and Blancard derived it from
the foetal breasts. Lang, Berger and Gofey criticised this notion
without bringing forward anything constructive, and Gofey was in
his turn annihilated by D. Hoffmann, who with Nenter and Konig
supported the modern view, namely, that it was a transudation from
the maternal blood-vessels in the decidua. The question was com-
plicated further by the alleged discovery by Bidloo in 1 685 of glands
in the umbilical cord, and by Vieussens in 1 705 of glands on the
amniotic membrane. J. M. Hoffmann and Nicholas Hoboken sup-
ported the view that these were the important structures. There the
problem was left during the eighteenth century, various writers
supporting different opinions from time to time, and it is still under
discussion (see Section 22).
Very early in the eighteenth century (1708) there appeared a
work by G. E. Stahl, van Helmont's most famous follower, which
struck the keynote of the whole century. Stahl's Theoria Medica Vera,
divided as it was into Physiological and Pathological sections, be-
longed in essence to the a priori school of Descartes and Gassendi.
It differed from them profoundly, of course, for, instead of trying to
explain all biological phenomena, including embryonic develop-
ment, from mechanical first principles, it started out from first
principles of a vitalistic order, and, having combined all the archaei
into one informing soul, it sought to show how the facts could
be perfectly well explained on this basis. But the spiritual kinship
of Stahl with Descartes and Gassendi is due to an atmosphere
which can only be called doctrinaire, and which was common
to them all. Like the methodist school of Hellenistic medicine,
they subordinated the data to a preconceived theory, during which
process any awkward facts were liable to be rather submerged than
subordinated.
SECT. 3] AND EIGHTEENTH CENTURIES 181
In 1722 Antoine Maitre-Jan published his book on the embryology
of the chick, the only one on this subject between Malpighi and
Haller. It was an admirable treatise, illustrated with many drawings
which, though not very beautiful, were as accurate as could be
expected at the time. Perhaps its most remarkable characteristic
is its almost complete freedom from all theory — Maitre-Jan says
hardly a word about generation in general, and is far from putting
forward a "system" in the usual eighteenth-century manner. He
contents himself with the recital of the known facts, including those
added by his own observations. He gives no references, and writes
in an extremely modern and unaffected style.
The only traces of theoretical presupposition which can be found
in him are Cartesian, for he speaks of the activity of ferments in
blood-formation. He is an epigenesist, and long before Brooks, he gives
the right explanation of Malpighi's error, affirming that the hot
Italian summer was responsible for some development in Malpighi's
eggs before Malpighi examined them. Maitre-Jan's book must have
been accessible both to Buffon and Haller, so it is difficult to see why
they should have perpetuated Malpighi's mistake till nearly the end
of the century.
In technique, Maitre-Jan was pre-eminent. He was the first
embryologist to make practical use of Boyle's suggestion regarding
"distilled spirits of vinegar" for hardening the embryo so that it
could be better dissected. He also used "weak spirits of vitriol";
after treating blastoderms with it, he said, "I saw with pleasure an
infinity of little capillary vessels which had not appeared to be there
before". He made a few chemical experiments also, noting that
vinegar would coagulate egg-white, and estimating quantitatively
the difference in oil-content of different yolks — though for this he
gives no figures.
His theory he relegated to an appendix entitled Objections sur
la generation des animaux par de petits vers. There were sixteen of
them, but the most cogent one was that, as little worms had been
found under the microscope in pond-water, vinegar, and all kinds
of liquids, there was no reason to suppose that those in the semen
were in any essential way connected with generation. For his time,
this argument was an excellent one, and was open to no demur
save on the ground of filtration experiments which had not yet been
made (see p. 215).
i82 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
About this time there was some controversy over the circulation
of blood, the foramen ovale, etc., in the embryo. From 1700 to 1710,
Tauvry and Mery were engaged in a polemic on this subject, and
the latter also corresponded with Duverney, Silvestre and Buissiere
in a controversy which recalls that of Laurentius and Petreus a
hundred years before. Nicholls wrote later on the same subject.
Daniel Tauvry was interesting, however, for other reasons. He was
an epigenesist, and wrote vigorously against the view that the soul
constructed during embryogeny a suitable home for itself.
Nine years later two books appeared, which form very definite
landmarks in the history of embryology. One was Martin Schurig's
Embryologia, and the other the Elementa Chymiae of Hermann Boer-
haave.
The former, however, gave to the world no new experiments or
observations ; it was the first of what we should now call the typical
"review" kind of publication. Schurig saw that he was living at
the end of a great scientific movement following the Renaissance,
and set himself accordingly for many years to compile large treatises
on definite and restricted subjects, taking care to give all references
with meticulous accuracy, and to omit no significant or insignificant
work. His Spermatologia was the first to appear (in 1720), and it was
followed in 1723 by Sialologia (on the saliva), Chylologia (1725),
Muliebria (1729), Parthenologia (1729), Gynaecologia (1731) and Haema-
tologia (1744). His Embryologia was the last but one of the series. In
it he treated compendiously of all the theories which had been
advanced about embryology during the immediately preceding two
centuries, and his chapters on foetal nutrition and foetal respiration
throw a flood of light on to the "intellectual climate" in which
Harvey and Mayow worked, providing, as it were, the perishable back-
ground of their immortal thoughts. Schurig's bibliography is a very
striking part of his book, extending to sixteen pages, and including five
hundred and sixty references; it was the first attempt of its kind.
3-10. Boerhaave, Hamberger, Mazin
Hermann Boerhaave was a more prominent figure, a Professor at
Leyden for many years, and renowned for his encyclopaedic learning
on all subjects remotely connected with medicine. His Elementa
Chymiae, which became the standard chemical book of the whole
period, demonstrates throughout the exceedingly wide outlook of its
SECT. 3] AND EIGHTEENTH CENTURIES 183
author, and contains in the second volume what must be regarded
as the first detailed account of chemical embryology. I reproduce
here the relevant passages in full because of their great interest.
It will be noted that they are cast in the form of lecture addresses,
as if they had been taken down direct from the lectures of the
Professor, a fact which gives them a peculiar charm when it is
remembered how many great men must have listened to them, among
them Albrecht von Haller and Julien de la Mettrie. In considering
what follows, it should be noted that Boerhaave's interest is bio-
logical all the time, and that he does not treat the liquids of the egg,
as nearly all the chemists before him had done, as substances of
curious properties indeed, but quite remote from any question re-
lating to the development of the embryo. Another interesting point
is that he deals only with the white, and hardly mentions the yolk;
this is perhaps to be explained by the Aristotelian theory that the
embryo was formed out of the white, and only nourished by the
yolk {ex alb 0 fieri, ex luteo nutriri), a theory which was still alive, in
spite of Harvey, in the first half of the eighteenth century. If this
was what was at the bottom of Boerhaave's mind, then it is obvious
that the egg-white would be to him the liquid inhabited more par-
ticularly by the plastic force. This, then, is what he has to say about
the biochemistry of the egg.
Op. Chem. in Animalia. [Processus log.] The albumen of a fresh egg is not
acid, nor alkaline, nor does it contain a fermented spirit. The white of a fresh
egg, separated from the shell, the membranes, and the yolk, I enclose in
clean glass vessels, and into each of these I pour different acids, and shake
them up, mixing them, and no sign of ebullition appears however I treat
them. Therefore I lay these vessels aside. Now in these other two vessels
I have two fresh portions of albumen, and I mix with them in one case
alkaline salt and in the other volatile alkali. You see they are quiet without
any sign of effervescence. Now behold a remarkable thing, in this tall
cylindrical vessel is half an ounce of the albumen of an egg and two drams
of spirits of nitre, in this other vessel is half an ounce of egg-white, together
with four and a half ounces of oil of tartar per deliquium both heated
up to 92 degrees. Pray observe and behold, with one movement I pour
the alkaline albumen into the acid albumen, with what fury they boil up,
into what space they rarefy the mass, so that they stream out of the vessel
although it is ten pints in size [decupli capace] . They have scarcely changed
their colour. But when the effervescence has abated how suddenly they
return to the limits of space occupied before. But now if more egg-white
is heated to 100 degrees in a retort [cucurbita] an insipid water containing
i84 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
no spirit is given off. If egg-white is applied to the naked eye or naked
nerve it does not give the smallest sense of pain, and scarcely affects the
smell; nothing more inert and more insipid can be put on the tongue. It
appears mucous and viscid to the touch, not at all penetrable. Hence in
the fresh white of an egg there is no alkali or acid, or both together. It is
indeed a thick, sticky, inert, and insipid liquor, yet from this truly vital
liquid at a heat of 93 degrees within the space of 2 1 days the chick grows
in the incubated egg from a tiny mass hardly weighing a hundredth of a
grain into the perfect body of an animal, weighing an ounce or more.
We have learnt therefore of a liquid distinct from all others, from which by
inscrutable causes fibres, membranes, vessels, entrails, muscles, bones,
cartilages, and all the other parts, tendons, ligaments, the beak, the claws,
the feathers, and all the humours can be produced — and yet in this liquid
we find softness, inertia, absence of acid, alkali, and spirit, and no ten-
dency to effervesce. Indeed, if there were the slightest effervescence in it,
it would certainly break the eggshell, therefore we see from how slow and
inactive a mass all the solid and fluid parts of the chick are constructed.
And yet this itself is rendered absolutely useless for forming the chick by
greater heat. It scarcely bears 100 degrees with good effect but at a less
temperature never brings forth a chick, for under 80 degrees will not
suffice. But by a heat kept between these limits, there is brought about so
marvellous an attenuation of the mucous inactivity that it can exhale a
great part through the shell of the egg and the two membranes, the yolk
and chalazae alone remaining along with the amniotic sac. For the yolk,
the uterine placenta of the chick, takes little part in the nourishment.
Meanwhile Malpighius has shown that this albumen is not a liquid of a
homogeneous kind, as the blood-serum flowing through the vital vessels
is, but that it is a structure composed of numerous membrane-like and
distinct small saccules, filled with a liquid of their own, in the same way
as in the vitreous humour of the eye.
[Processus 1 1 1 .] Exploration of the egg-white with alcohol. In this trans-
parent vessel is the albumen of an egg, and into it, as you perceive, I
gently pour the purest alcohol, so that it descends down the sides of the
vessel and reaches the albumen. I do this deliberately and with such
solicitude that you may see the surface of the albumen which, touching
the alcohol, holds it up, being immediately coagulated, while the lower
part remains liquid and transparent. As I now gently shake them together,
it appears evident that wherever the alcohol touches the albumen a con-
cretion is formed. Behold now, while I shake them up thoroughly together,
all the egg-white is coagulated. If alcohol previously warmed is employed
in this experiment, the same result is brought about but more rapidly.
It appears therefore that the purest vegetable spirits immediately coagulate
the plastic and nutrient material.
[Processus 112.] The fresh albumen of an egg is broken up by distillation.
These fresh eggs have been cooked in pure water till they became hard.
I now take the shining white, separating off all the other things, and break
it up into small pieces. I put these, as you see, into a clean glass retort
SECT. 3] AND EIGHTEENTH CENTURIES 185
[cucurbita] and I duly cover it by fitting on an alembic and add a receiver.
By the rules of the (chemical) art I place the whole retort in a bath of
water and I apply to it successive degrees of fire until the whole bath is
boiling. No vaporous streaks [^strid] of spirits are given off but simple
water in dewy drops and this in incredible quantity, more than nine-tenths.
I continue so with patience until by the heat of boiling water no more
drops of this humour are given off. Then this water shows no trace of oil,
salt, or spirit ; it is perfectly transparent and tasteless, except that it eventu-
ally grows rather sour. It is odourless, save that towards the end it gives
off a slight smell of burning. It shows absolutely no sign of the presence
of any alkali, when I test it in every way, as you can see for yourselves ;
nor does it reveal any trace of acid, when tried how you will. Here you
see pounds of this water, but in the bottom of the now open retort see,
I beg of you, how little substance remains. Behold, there are fragments
contracted into a very small space in comparison with the former quantity.
They are endowed with a golden yellow colour, especially where they have
touched the glass, but yet they are transparent after the manner of coloured
glass. When I take them out I find them very light, very hard, quite fragile,
and breaking apart with a crack, smelling slightly of empyreuma, with
a taste rather bitter from the fire, and without any flavour of alkali or
acid. This is the first part of the analysis. Now I take these remaining
fragments in a glass retort [retortam] in such a way that two-thirds remain
over. I put the retort into a stove of sand, first arranging a large receiver.
Then thoroughly luting all the joints I distil by successive grades of fire
and finally by the highest which I call suppressionis. There ascends a spirit,
running in streaks [^striatim] fat and oily, and at the same time, volatile
salts of solid form everywhere on the walls of the vessel, rather plentiful
in proportion to the dried fragments but small in proportion to the whole
albumen before the water had been removed from it. Finally an oil appears
besides the light golden material mixed with the first, black, thick, and
pitchy. When by the extreme force of the fire this oil is finally driven forth,
then the earth in the bottom, closely united with its most tenacious oil,
swells up and is rarefied and rises right up to the neck of the retort so that
had the retort been overfull it would have entered into the neck and
clogged it up, even causing it to burst, with danger to the bystanders. The
operation is to be continued till no more comes out. That first spirit, oily
and fatty, is clearly alkaline by every test, as you may tell from the way it
effervesces when acid is poured on it. If we rectify it we resolve it into
an alkaline volatile salt, an oil, and inert foetid water. The salt fixed to
the walls is completely alkaline, sharp, fiery, oily, and volatile; and the
final oil is specially sharp, caustic, and foetid. The black earth which
remains in the retort is shiny, light, thin, and fragile, foetid from the final
empyreumatic oil, and soft because of it. If then it is burnt on an open
fire, it leaves a little fixed earth which is white, insipid, tasteless, and
odourless, from which scarcely any salt can be extracted, but only a very
heavy dusty powder \^pollinein\.
Cf. the dry distillation of egg-white by Pictet & Cramer in 1919.
i86 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
[Processus 113.] The fresh albumen of an egg will putrefy. Sound eggs kept
at 70° for some days will become foetid and stink. . . .We have learnt then
that this is the nature of the material which will shortly be changed into
the structure, form, and all the parts of the animal body. Repose and a
certain degree of heat produce that effect in that material. We observe
therefore the spontaneous corruption and change of the material, and what
is extremely remarkable, if an impregnated egg is warmed in an oven [in
hypocaustis] to a heat of 92 degrees it employs these attenuated parts
changed by such a heat to nourish, increase, and complete the chick for
21 days. But in this chick nothing alkaline, foetid, or putrid is found,
hence observe, O doctors [medici] , the remarkable manifestations of nature
^by repose and a certain degree of heat a thick substance becomes thin,
a viscous substance becomes liquid, an odourless substance becomes foetid,
an insipid substance becomes sour and extremely acrid and bitter to the
taste, a soothing substance becomes caustic, a non-alkali becomes alkaline,
a latent oil becomes sweet and putrid. Let these results be compared with
the observations of Marcellus Malpighius on the incubated egg, and we
shall observe things which shall surprise us. I took care to investigate only
the albumen of the egg first of all, separating the other parts off where
possible, for the albumen alone forms the whole of the material which
proceeds to feed [in pabulum] the embryo. The other constituents of the
egg only assist in changing the albumen, so that when it is changed, it
miay be applied to forming the structure of the chick.
Boerhaave's treatment of these subjects has only to be compared
with that of Joachim Beccher, who wrote in 1 703, to show how
thoroughly modern in outlook it is. Beccher's Physica Subterranea
contains a whole section devoted to the growth of the embryo, but
it is extremely confused and very alchemical in its details. The
advance made in the thirty years between Beccher and Boerhaave
was immense, but, if the biochemistry of development advanced so
fast, its biophysics was not far behind, as is shown by the work of
G. E. Hamberger and J. B. Mazin.
Hamberger's most important contributions, contained in his Physio-
logia medica of 1 75 1 , were his quantitative observations on the water-
content of the embryo and its growth-rate, in which he had no fore-
runners, Hamberger showed "that there are much less solid parts
in the foetus than in the adult. The cortical substance of the brain of
an embryo loses 8694 parts in 10,000 on drying but in the adult it only
loses 8096 and that of the cerebellum from 81 parts is reduced to 12.
The maxillary glands of the embryo lose out of 10,000 parts 8469, the
liver 8047, the pancreas 7863, the arteries 8278 and even the cartilages
lose four-fifths of their weight, decreasing from 10,000 to 8149I ". The
SECT. 3] AND EIGHTEENTH CENTURIES 187
corresponding figures for the adult were: liver 7192, and heart 7836.
These figures do not widely diflfer fi:-om those obtained in recent times.
J. B, Mazin published his Conjecturae physico-medico-hydrostaticae de
respiratione foetus in 1737 and his Tractatus medico-mechanica in 1742.
In the first of these works Mazin supports what is essentially Mayow's
theory of embryonic respiration, without, however, mentioning
Mayow more than once. It had not been popular since 1700, though
Pitcairn had defended it. Mazin put the liquids of eggs under an
air-pump, and observing that air could be extracted from them
affirmed that the air was hidden in them and that the embryo could
therefore respire. He spoke of "aerial particles" in the amniotic
liquid, and discussed the respiration of fishes in connection with this.
The specific gravity of the embryo also interested him, and he did
a great deal of calculation and experiment on it. His most interesting
passage, perhaps, is that in which he mentions the "Eolipile" of
the Alexandrians, the primitive form of the steam-engine, and says
that just as the heat of the fire makes the water boil, so the heat of
the viscera makes the amniotic liquid boil, giving off respirable
vapours. The time-relations of this analogy are interesting, for in
1705 Thomas Newcomen had succeeded in making a steam-engine
which worked with considerable precision, and the question of steam-
power was widely discussed. Possibly Mazin was acquainted with
the Marquis of Worcester's Century of the Names and Scantlings oj
Inventions, which had been published in 1663, and which had con-
tained an aeolipile or "water-commanding machine". England was
the centre of this movement and other countries employed English-
men as engineers; Humphrey Potter, for instance, erected a steam-
engine for pumping at a Hungarian mine in 1 720.
As for the discovery of oxygen, it was near at hand, and Scheele in
1 773 and Priestley in 1 774 were soon to supply the knowledge without
which Mazin could not proceed further.
In his second book, Mazin reported many quantitative observations
on the specific gravity of the embryo. He found that it diminished
as development proceeded, being to the amniotic liquid as 282 to
274 in the fourth month and as 504 to 494 in the fifth month.
Another instance of the way in which experimental physical ques-
tions now began to come in is afforded by the work of Joseph Onymos,
whose De Matura Foetu of 1 745 spoke of the specific gravity of the
embryo at different stages of development.
i88 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
These writers, together with Haller himself, and J. C. Heffter
who handled problems of embryonic rate of growth contribute to
one of the best, because most quantitative, aspects of eighteenth-
century embryology.
3*11. Albrecht von Haller and his Contemporaries
Boerhaave's greatest pupil was Albrecht von Haller. Like O. W.
Holmes, at Harvard, Haller occupied a "settee" rather than a "chair",
at Gottingen, and taught not only physiology but also medicine
and surgery, botany, anatomy and pharmacology. Nor did he
merely deal with so many subjects superficially; in each case he
published what amounted to the best and most complete text-book
up to then written. Haller was made Professor in 1736, and for
many years worked at Gottingen, devoting much of his time to
embryological researches, which, with those of his opponent Wolff,
stand out as the greatest between Malpighi and von Baer. In 1 750
he published a series of dissertations and short papers on all kinds
of physiological subjects, which would have been the direct ancestors
of the modern compilations of groups of experts, had they been more
systematically arranged. The volume on generation repays some
study. The contributions relevant to the present discussion had been
written at various times during the previous seventy years, and may
be summarised as follows :
IV. Christopher Sturmius, De plantarum animaliumque generatione.
(First published 1687.) In this paper Sturmius argues on
behalf of the preformation theory "which in our times
does not lack supporters", quoting Perrault, Harvey and
Descartes. He contents himself with countering arguments
which had been urged against it, as, {a) spontaneous
generation, {b) annual recurrence of plants, {c) insect
metamorphosis, {d) generation without copulation.
V. Rudolf Jacob Camerarius, Specimen experimentorum physiologico-
therapeuticorum circa generationem hominis et animalium. The
most interesting thing about this is that Camerarius
mentions the observations of D. Seiller, a sculptor, who
had ascertained that the body is five times the size of the
head in the embryo but seven and a half times the size of it
in the adult. This is in the direct line between Leonardo
and Scammon.
SECT. 3]
AND EIGHTEENTH CENTURIES
189
XV. Philip Gravel, De Super Joetatione. (First published 1738.)
XVIII. Adam Brendel, De embryone in ovulo ante conceptum praeexistante.
(First published 1703.) Brendel "stands for the Graafian
hypothesis''. Unfortunately, he was also a preformationist
and believed that every limb, organ, and function existed
not potentially but actually in the unfertilised Qgg before
its passage down the Fallopian tube.
XXII. Camillus Falconnet, Non est fetui sanguis maternus alimento.
(First published 171 1.) This is the first of the French
contributions to the book; they are all very markedly
shorter than the German ones and much less heavily
ornamented with irrelevant quotations. Falconnet is con-
cerned to prove that the maternal and foetal circulations
are separate, and he describes in an admirably concise
manner an experiment in which he bled a female dog to
death, after which, opening the uterus, he discovered that
the embryonic blood-vessels were full of blood although
those of the mother had none in at all. Arantius was there-
fore justified. Falconnet was soon confirmed by Nunn.
XXIII. Jean de Diest's Sui Sanguinis solus opifex fetus est (first
published 1735) was written to prove a similar point. He
refers to the experiment of Falconnet and the injections
of F. Hoffmann, and criticises Cowper's experiment in
which mercury had been injected into the umbilical vessels
and found in the maternal circulation, on the grounds that
mercury is so "tenuous and voluble" that it might pass
where blood could not pass normally. He also objects to
the view that the foetus is nourished by the amniotic liquid.
XXIV. Francis David Herissant, Secundinae fetui pulmonis praestant
officia, et sanguine materno fetum non alitur. (First published in
1 741.) An excellent paper, in which the respiratory function
of the placenta is proved by the observation that the foetal
blood-vessel leading to the placenta is always full of dark
venous blood, while that leading away fi-om the placenta
is light and arterial [floridiori coccineoque colore, ut ipsemet
observavi]. Herissant adduces also the cases of acephalic
monsters, such as that of Brady, which could not possibly
have drunk up any amniotic fluid, and yet were fully formed
igo THE EIGHTEENTH CENTURY [pt. ii
in all other respects. He concludes that the umbilical cord
serves for respiration and nutrition.
XXV. After these three French workers, there is a great drop to
Johannes Zeller, whose Infanticidas non absolvit nee a tortura
liberal pulmonum infantis in aqua subsidentia (first published
1 691) is a long-winded discussion of the floating lung test
in forensic medicine. His memory deserves a word of obloquy
for his vigorous insistence upon torture and death for in-
fanticide even during puerperal insanity. Perhaps it was
Zeller who called forth the noble answer of de la Mettrie
to this inhumanity in his Man a Machine.
XXVI. Zeller's De Vila Humana ex June pendenle (first published
1692) is no better, though at the time, perhaps because
of its striking title, it was famous. It deals with the ligation
of the umbilical cord at birth.
This completes the list of the papers published by Haller in his
1750 collection. He retired from the Gottingen chair three years
later, and in 1757 the first volume of his Elemenla Physiologiae was
published, probably the greatest text-book of physiology ever written.
It appeared only by slow degrees, so that it was not until 1766 that
the embryological section was available. This volume contains
a discussion of a mass of literature, most of which had arisen
during the preceding twenty-five years, for, although many of the
names mentioned by Haller occur also in Schurig, yet many are
quite new.
Haller himself published in 1 767 a volume of his collected papers
on embryology, most of which were concerned with the developing
heart of the chick, which he worked out very thoroughly, in collabora-
tion with Kuhlemann. (Kuhlemann had already done for the
sheep what Harvey had done for the doe.) He made a beginning
with the quantitative description of embryogeny, and one of his
tables showing the changing lengths of the bones is reproduced here-
with (Fig. 10). He was a convinced preformationist, a fact which was
largely due to his researches on the hen's egg, where he observed that
the yolk had a much more intimate connection with the embryo
than had previously been supposed. Since the whole yolk was part of
the embryo, as it were, the preformation theory seemed to him to
fit the facts better than epigenesis.
DIES.
5EXTUS.
Septimus
OCTAVUS
NONUS
Decimus
Undeci-
MUS
DuODfi-
CIMUS
Decimus
Iertius
Decimus
Quart us
Decimus
QyiNTUs
Decimus
SEXTys
Decimus
Septimus
Decimus
Octavus
Decimus
NdNUs
.Vigesi-
l| MUS
VlGESl-
MUS
Primus
VlGESI-
MUS Se-
ICUNDUS
TIBIA.
FEMUR.
8
10^5
8i
20
14
a4.prox.
1 8- prox.
30
22i
41?
3'l
43j
3^;
48
37
^5.prox
+4. ptox.
7^
?o
84*
6i9
9.i
«?;
ioo|
7»i
103
7U
iioi
78J
113
83
«'7|
83f
PARS
OSSEA
TIBI^.
o
o
10
1 1
I7i
30
43. prox
39**
73
74
70
7?
PARS
OSSEA
FEMORIS,
o
o
100
I2i
24
3 0. prox.
29
48
53
52
72
CUBUS
TlBiiE.
i"4rWo
8000
I38?4
27000
72301
8o^24.prox.
110^92
itf<?379
5.00149
5513^8
7650^1
1017575
1098712
13242^0
1422897
1628405
CUBUS
FEMORIS.
512
2744
5832
loiCi
29791
305^8
50653
85284
125600
257505
320047
378040
44<^^75
483736
571787
^76381
Fig. 10. Facsimile of a table in A. von Haller's Elementa Physiologiae of 1766, containing some of his
observations on the growth in length and weight of embryonic bones in the chick.
192 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
Haller went further than Schurig, in that he usually gave an opinion
of his own after summarising those of other people, but his views were
by no means always enlightened, and the atmosphere of Buffon is,
on the whole, more congenial to us than that of Haller. Haller,
for example, believed that the amniotic liquid had nutritious pro-
perties, and that the nutrition of the embryo in mammalia was
accomplished first of^ all per os and afterwards per umbilicum. He denied
that the placenta had any respiratory function, and, indeed, his
whole teaching on respiration was retrograde. He mentions, how-
ever, an experiment of Nicolas Lemery's, in which it had been
found that indigo would penetrate the shell of a developing hen's
tgg from the outside. Consequently, air might do so too, and
Vallisneri had shown that, if an egg was placed in boiled water under
an air-pump, the air inside would rush out through the shell and
appear in the form of bubbles.
Haller was much more progressive in holding the origin of the
amniotic liquid (according to him a subject of extraordinary diffi-
culty— " solutionem non promittam'") to be a transudation from the
maternal blood-vessels. He followed Noortwyck in asserting the
separateness of the maternal and foetal circulations in mammalia.
He opposed the existence of eggs in vivipara — "We may conclude
from all this", he said, "that the ovarian vesicles are not eggs and
that they do not contain the rudiments of the new animal". But he
accepted it in the restricted sense that the embryonic membranes
resembled an egg, thus: "If we call an egg a hollow membranous
pocket full of a humour in which the embryo swims, we may admit
the opinion of the older authors who derive all animals from eggs
with the exception of the tiny simple animals of which we have
already spoken. It was in this sense that Aristotle and Empedocles
before him, said that even trees were oviparous. This has also been
confirmed by the experiments of Harvey on insects, fishes, birds, and
quadrupeds".
Haller's most original work was in connection with the growth-
rate of the embryo ; here he struck out, for once, into entirely new
country. "The growth of the embryo in the uterus of the mother is
almost unbelieveably rapid. We do not know what its size is at the
moment of its formation, but it is certainly so small that it cannot
be seen even with the aid of the best microscopes, and it reaches in
nine months the weight of ten or twelve pounds. In order to clear
SECT. 3] AND EIGHTEENTH CENTURIES 193
up this speculation, let us examine the growth of the chick in the
egg. We cannot in this case either measure its size at the moment
when the egg is put to incubate but it cannot be more than j^ in.
long, for if it were, it would be visible, and yet 25 days later it is
4 ins, long. Its relation is therefore as 64 to 64 millions or i to i
million. This growth takes place in a singular manner, it is very rapid
in the beginning and continually diminishes in speed. The growth
on the first day is from i to gi^^ and what Swammerdam calls a
worm grows in one day from one-twentieth or one-thirtieth of a
grain to seven grains, i.e. it increases its weight by 140 or 240 times.
On the second day the growth of the chick is from i to 5, on the third
day, from i to not quite 4, on the fifth day from i to something
less than 3. Then from the sixth to the twelfth day, the growth each
day is hardly from 4 to 5, and on the twenty-first day it is about
from 5 to 6. After the chick has hatched, it grows each day for the
first 40 days at an approximately constant rate, from 20 to 2 1 on each
day. The increase of the first twenty-four hours is therefore in relation
to that of the last twenty-four hours as 546I to 5 or 145 to i. Now
as the total increase in weight in the egg is to that of the whole
growth period (up to the adult) as 2 to 24 ozs., all the post-embryonic
growth is as i to 12, i.e. it is to the growth of one day alone early
in incubation as i to 7|.. . .The growth of man, like that of the
chick, decreases in rapidity as it advances. Let us suppose that a
man, at the instant of conception, weighs a hundred-thousandth of
a grain and that a one-month old embryo weighs 30 grains; then
the man will have acquired in that time more than 300,000 times
the weight that he had to begin with. But if a foetus of the second
month weighs 3 ozs. as it approximately does, he will only now have
acquired 48 times the weight he had at the beginning of the period.
This is a prodigious decrease in speed, and at the end of the ninth
month he will not weigh more than about 105 ozs., which is not
more than an average increase of 15 per month. A child three years
old is about half the size of an adult. If then the adult weighs
2250 ozs. the three-year old child only weighs 281 ozs., which is an
eighth of the adult weight. Now from birth to 3 years he will grow
from 105 to 281 or as 5 to 14, but in the following 22 years he will
only accumulate 2250 ozs. or eight times what he had at 3 years.
The growth of a man will therefore be in the first month of intra-
uterine life as I to 300,000, in the second as i to 48, in each of the
N E I 13
194 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
others as i to 15. In the first 3 years of extra-uterine life his growth
will be from 164 to 281 and in the succeeding 22 years from 281 to
384, and the growth of the first month to the last will be as 300,000
to ^% or 136,800,000 to 28, or 4,885,717 to i. The whole growth of
man will consequently be as 108,000,000 to i."
In spite of the rather unfamiliar language in which these facts are
described, and the theory of the growth of the heart which Haller
subsequently put forth to explain them, they remain fundamental
to embryology. Their quantitative tone is indeed remarkably modern.
In my opinion, when all the voluminous writings of Haller are care-
fully searched through, nothing more progressive and valuable than
these figures can be found. Haller and Hamberger stand thus
between Leonardo on the one hand and Minot and Brody on the
other. That they stood so much alone is only another indication of
the extraordinary reluctance with which the men of past generations
assented to the truth contained in Robert Mayer's immortal words,
"Eine einzige Zahl hat mehr wahren und bleibenden Wert als eine
kostbare Bibliothek von Hypothesen".
Of development as a whole, Haller spoke thus, " In the body of the
animal therefore, no part is made before any other part, but all are
formed at the same time. If certain authors have said that the animal
begins to be formed by the backbone, by the brain, or by the heart, if
Galen taught that it was the liver which was first formed, if others have
said that it was the belly and the head, or the spinal marrow with the
brain, adding that these parts make others in turn, I think that all these
authors only meant that the heart and the brain or whatever organ
it was, were visible when none of the other parts yet were, and that
certain parts of the embryonic body are well enough developed in the
first few days to be seen while others are not so until the latter part of
development; and others again not till after birth, such as the beard
in man, the antlers in the stag, the breasts and the second set of teeth.
If Harvey thought he descried an epigenetic development, it was
because he saw first a little cloud, then the rudiments of the head, with
the eyes bigger than the whole body, and little by little the viscera
being formed. If one compares his description with mine, one will see
that his description of the development of the deer corresponds
exactly with mine of the development of the chick. If, more than
twenty years ago, before I had made many observations upon eggs
and the females of quadrupeds I employed this reasoning to prove
SECT. 3] AND EIGHTEENTH CENTURIES 195
that there is a great difference between the foetus and the perfect
animal, and if I said that in the animal at the moment of conception
one does not find the same parts as in the perfect animal, I have
realised abundantly since then that all I said against preformation
really went to support it". The reasons for this change of opinion
become no clearer as Haller's writings are more assiduously read,
and, as Dareste says, why he should have made it, will always
remain a mystery.
The emboitement aspect of preformation presented no difficulties to
Haller. "It follows", he said, speaking of the generation of Volvox,
"that the ovary of an ancestress will contain not only her daughter,
but also her granddaughter, her great-granddaughter and her great-
great-granddaughter, and if it is once proved that an ovary can
contain many generations, there is no absurdity in saying that it
contains them all,"
The following passage is interesting. "We must proceed to say
what is the efficient cause of the beautiful machine which we call
an animal. First of all let us not attribute it to chance, as Ofrai
[is this Julien Offi-ay de la Mettrie? Haller had a habit of using
Christian names, e,g, Turberville for J. T. Needham] would have
us do, for although he pretends that all animals come from earth,
he is not attached to the ancient opinion, and nobody now believes
what Aelian says, namely that frogs are born from mud. . . , Vallisneri
has found the fathers and mothers of the little worms in galls, a quest
of which Redi despaired, and Redi in his turn has made with exacti-
tude and precision those experiments which Bonannus, Triumphet,
and Honoratus Faber had only sketched out imperfectly. Moreover,
no seed, no clover. . . . This was the received opinion but in our century
a proscribed notion has been revivified and some great men have
pretended that there are little animals which are engendered by an
equivocal generation, without father and mother, and that all the
viscera and all the parts of these animals do not exist together, but
that the nobler parts are formed first by epigenesis and that then
the others are formed little by little afterwards." This is an admirable
illustration of how spontaneous generation and epigenesis were bound
up together, "M. Needham", Haller goes on to say, "does not admit
an equivocal generation but he does admit epigenesis, and a corporeal
non-intelligent force, which constructs a body from a tiny little germ
furnishing the necessary matter for it. He says that there are only
13-2
196 EMBRYOLOGY IN THE SEVENTEENTH [pt. 11
the primitive germs which were made at the original creation and
that germs organised Hke animals do by no means pre-exist, for if
they did, molae uterinae, encysted tumours, and the like, could not
come into being." Haller then goes on to describe Needham's
experiments with meat broths, etc., and objects to his "system",
largely on the ground that "blind forces without any intelligence
could hardly be able to form animals for ends foreseen and ready
to take their places in the scheme of beings". He considers that
Needham's theories are completely disproved by experiments such
as those of Spallanzani, though, curiously enough, he does not quote
the latter author in this connection. I shall return to this later.
"Nobody", he goes on to say, "has upheld epigenesis more than
M. Wolff, who has undertaken an examination to demonstrate that
plants and animals are formed without a mould out of matter by a
certain constant force which he calls 'essential' [in his Theoria
Generationis] .... I have indeed seen many of the phenomena which
he describes, and it is certain that the heart seems to be formed out
of a congealed humour and that the whole animal appears to have
the same consistency. But it does not follow that because this
primitive glue which is to take on the shape of the animal does not
appear to possess its structure and all its parts, it has not effectively
got them. I have often given greater solidity to this jelly by the use
merely of spirits of wine and by this means I saw that what had
appeared to me to be a homogeneous jelly was composed of fibres,
vessels, and viscera. Now surely nobody will say that the vis
essentialis of the spirit of wine gave an organic structure to an un-
formed matter, on the contrary it is rather in the removal of trans-
parency and the accession of greater firmness to the extremities, as
well as the making of a more obvious boundary to the contour of
a viscus that one could see the structure of a cellular tissue, which
was ready to be formed but which the transparency had previously
hidden and the wetness not allowed to be circumscribed by lines. . . .
Finally, to cut a long story short, why does this vis essentialis,
which is one only, form always and in the same places the parts of
an animal which are so different, and always upon the same model,
if inorganic matter is susceptible of changes and is capable of taking
all sorts of forms? Why should the material coming from a hen
always give rise to a chicken, and that from a peacock give rise to
a peacock? To these questions no answer is given." This was the
SECT. 3] AND EIGHTEENTH CENTURIES 197
case because Wolff was not a theorist, but rather an experimentalist;
his writings are marked by their abstention from the discussion of
speculative points. The above passage is very interesting. It reminds
us of the great difficulties with which the embryologists of this epoch
had to contend. Serial section cutting was unknown, the staining
of thin layers and reconstruction were unheard of; even the hardening
of the soft embryonic tissues was only just discovered, as is indicated
by Haller above. Hertwig has excellently discussed the advances in
embryological technique which took place during this and the fol-
lowing century. It is true that dyes were beginning to be used, as
some instances already given demonstrate, and as is seen from the use
of madder in the staining of bones, which began about this time, and
was later much used by the Hunters. Heertodt's Crocologia is im-
portant in this connection. Heertodt, by injecting saffron into the
maternal circulation, found it afterwards in the amniotic fluid, and
his experiment was cited by Haller in support of that theory of the
origin of the liquid. But the most important advance in technique
was the progress in artificial incubation. The art, though lost through-
out the Middle Ages and the seventeenth century, was now to be
revived.
During this period much work was done on it. As far back as
1 600, de Serres had mentioned some experiments of this nature, but
they were not successful. "The chicks", he said, "were usually born
deformed, defective or having too many legs, wings, or heads, nature
being inimitable by art." Birch, in his History of the Royal Society, also
refers to it. "Sir Christopher Heydon [a relative of Digby's Sir
John?] together with Drebell, long since in the Minories hatched
several hundred eggs but it had this effect, that most of the chickens
produced that way were lame and defective in some part or other."
Antonelli states that similar trials were made at the court of the
Grand-duke Ferdinand II at Florence about 1644, Thomas Bartho-
linus gives a like account with reference to the contemporary court
of King Christian IV of Denmark, and Poggendorff and Antinori
relate that the Accademia d. Cimento, inspired by Paolo del Buono,
made trial of artificial incubation between 1651 and 1667.
But the most famous of all the attempts to make artificial as suc-
cessful as natural incubation, were those of Reaumur, whose book
De I' art defaire eclore les poulets of 1749 achieved a wide renown. He
devotes many chapters to a detailed description of incubators of very
198 EMBRYOLOGY IN THE SEVENTEENTH [pt. 11
various kinds : but he nowhere gives any indication of his percentage
hatch. It was probably low. He speaks also of the ^^funestes effets'^
of the vapours of the dung on the developing embryos, without, how-
ever, furnishing any foundation for an exact teratology. In the second
volume he describes those experiments on the preservation of eggs
by varnishing them, which caught the imagination of Maupertuis
and were held up to an immortal but by no means deserved ridicule
by Voltaire in his Akakia. For the details of this amusing but
irrelevant issue see Miall and Lytton Strachey.
After Reaumur, there were numerous continuations of the kind of
work which he had done, in particular by Thevenot, La Boulaye,
Nelli, Porta and Cedernhielm. Much the most interesting of these
was the work of Beguelin, who attempted to incubate eggs with
part of the shell removed so as to form a round window. He was not,
however, successful in the carrying out of this very modern idea.
Probably the most peculiar investigation made in this field at this
time was that of Achard, who is mentioned in a passage of Bonnet's.
"Reaumur did not suspect in 1749", says Bonnet, "that someday
one would try to substitute the action of the electric fluid for his
borrowed heat. This beautiful invention was reserved for M. Achard
of the Prussian Academy who excels as an experimentalist. He has
not so far succeeded in actually hatching a chick by means of so new
a process, but he has had one develop up to the eighth day, when
an unfortunate accident deranged his electrical apparatus." Bonnet
goes on to say that this substitution of electricity for heat gives him
hope that by electrical means an artificial fertilisation will one day
become possible.
The references to these experiments and to those of many minor
investigators will be found in Haller. By the beginning of the nine-
teenth century a great mass of literature had developed on the
subject, and it had become possible to hatch out eggs more or less
successfully from furnaces, though the losses were still great. Early
in the nineteenth century Bonnemain and Jouard referred to the
large number of monsters produced, and in 1809 Paris wrote,
"During the period that I was at College, the late Sir Busick Har-
wood, the ingenious Professor of Anatomy in the University of
Cambridge, frequently attempted to develope eggs by the heat of
his hotbed, but he only raised monsters, a result which he attributed
to the unsteady application of the heat".
PLATE VIII
SECT. 3] AND EIGHTEENTH CENTURIES 199
This is the most convenient place to mention theological embryo-
logy once again. Its place in the eighteenth century was small, and
in the nineteenth, with the recognition that whatever the soul is, it
is not a phenomenon, it altogether disappeared from serious general
discussion. F. E. Cangiamilla's Embryologia Sacra, however, ran through
several editions between 1700 and 1775. Cangiamilla {Panorm.
Eccl. Can. Theol. et in toto Sicil. regno contra haereticam pravitatem
Inquisitore provinciali) deals very frilly with the time of animation,
quoting a host of writers such as St Gelasius, St Anselm, Hugh of
St Victor and Pico della Mirandola. His mind retains a quite
mediaeval conformation, as the following curious passage illustrates :
'^ Quot non foetus abortivos ex ignorantia obstetricum et matrum excipit
lafrina, quorum anima, si Baptismate non fraudaretur, Deum in aeternam
videret, esset decentius tumulandum! " His instructions for the baptism
of monsters are also very odd. But theological embryology probably
reached its climax in the report of the Doctors of Divinity at the
Sorbonne on March 30, 1733, in which intra-uterine baptism by
means of a syringe was solemnly recommended. This is included
in Deventer's book, and has been referred to by Sterne and Spencer.
For other aspects of these tracts of thought see Nicholls and his
anonymous antagonist. But Cangiamilla and his colleagues — Gerike,
Kaltschmied, etc. — are only of decorative importance to our present
theme, and for fuller information regarding them, reference must be
made to the treatise of Witovski. It is interesting to note that as
late as 1913, 182 days was fixed as "perfection-time", whatever that
may be, by Moriani.
3*12. Ovism and Animalculism
We must now return to the beginning of the century in order to
pick up the thread of the main trend of thought. By 1720 the theory
of preformation was thoroughly established, not only on the erroneous
grounds put forward by Malpighi and Swammerdam, but on the
experiments of Andry, Hartsoeker, Dalenpatius and Gautier, who
all asserted that they had seen exceedingly minute forms of men,
with arms, heads, and legs complete, inside the spermatozoa under
the microscope. Gautier went so far as to say that he had seen a
microscopic horse in the semen of a horse (he gave a plate of it)
and a similar animalcule with very large ears in the semen of a
donkey; finally, he described minute cocks in the semen of a cock.
200 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
Haller remarks gently that he has searched for these phenomena in
vain. Vallisneri asserted the same kind of thing about the mammahan
ovum, though he admitted that, in spite of long searching, he had
never seen one. Besides the main distinction between prefer mationists
and epigenesists, then, there arose a division among the former
group, so that the ovists regarded all embryos as being produced
from smaller embryos in the unfertilised eggs, while
the animalculists regarded all embryos as being
produced from the smaller embryos provided by
the male in his spermatozoa. The animalculists
thus afforded a singular example of a return to
the ancient theory mentioned by Aeschylus in the
Oresteia (see p. 65). Their most conspicuous ex-
ample was Nicholas Andry, who pictured each
c^gg as being arranged like the Cavorite sphere in
which H. G. Wells' explorers made their way to the
moon, i.e. with one trap-door. The spermatozoa,
like so many minute men, all tried to occupy an
egg, but as there were far fewer eggs than sperma-
tozoa, there were, when all was over, only a few
happy animalcules who had been lucky enough
to find an empty egg, climb in, and lock the door
behind them.
The whole controversy was intimately bound
up with the question of spontaneous generation,
for, whatever the case might be in the higher
animals, if it were true that the lower ones could arise de novo
out of slime, mud, or meat infusion, for instance, then their parts
at least must have been made by epigenesis, and not in any other
way, for it could hardly be held that a homogeneous infusion had
any structure of that kind. And if epigenesis could occur in the lower
animals, then the thin end of the wedge had been driven in, and it
might occur among the higher ones as well. It was in this way that
the spontaneous generation controversy came to have a peculiar
importance for embryology in the eighteenth century. Driesch has
essayed to make the generalisation that all the supporters of epigenesis
were vitalistic in their tendencies, while those who adhered to the
preformation theory were not. But there are too many exceptions
to this rule to make it of any use. In so far as there is truth in it.
Fig. 1 1 . Hartsoeker's
drawing of a human
spermatozoon.
SECT. 3] AND EIGHTEENTH CENTURIES 201
it doubtless arose from the fact that, in epigenesis, a continual pro-
duction of new organs and new relationships between organs already
formed would seem to require an immanent formative force of some
kind, such as the vis essentialis of Wolff; while, on the preformation
hypothesis, where embryogeny was little more than a swelling up
of parts already there, it could be explained as simply as nutrition.
The failure of the "short-cut" mechanical philosophers such as
Gassendi and Descartes thus led to preformationism just as much as
to epigenesis. A remark of Cheyne's throws much light on this
question, for in 17 15 he wrote, unconsciously following Gassendi's
line of thought, "If animals and vegetables cannot be produced
from matter and motion (and I have clearly proved that they can-
not), they must of necessity have existed from all eternity". Pre-
formationism was thus the only resource if the universal jurisdiction
of the mechanical theory of the world was to be retained. Stahl and,
later, Wolff, saw no point in retaining it, and carefully joined
together what Descartes had, with equal care, put asunder.
The original discoveries of de Graaf and Stensen were extended
by Tauvry in 1690 to the tortoise, and by Lorenzini in 1678 to the
Torpedo', so that the eighteenth century began with an excellent
basis for ovistic preformationism. The greatest names associated with
this school were Swammerdam, Malpighi, Bonnet, v. Haller, Winslow,
Vallisneri, Ruysch and Spallanzani. But there were many others,
some of whom did valuable work, such as Bianchi, Sterre, Teichmeyer,
Weygand, Perrault, Vercelloni, Vidussi, Bussiere, Fizes and Cosch-
witz. The treatises of Imbert and Plonquet were written from this
point of view, as was the bright little dialogue of de Houpeville.
J. B. du Hamel asserted that he could see the chick embryo in the
Ggg before fertilisation, and Jacobaeus made a like affirmation in the
case of the frog.
On the other side, that of animalculistic preformationism, the
contestants were fewer. Their greatest names were Leeuwenhoek,
Hartsoeker, Leibnitz and the cardinal de Pohgnac. In England
the physicians Keil and Cheque supported this position, in France
Geofroi and the obstetrician la Motte, in Germany Withof and
Ludwig, and in Belgium Lieutaud. De Superville wrote in favour
of it in the Philosophical Transactions of the Royal Society, and an anony-
mous Swedish work of some fame supported it. To the argument
of Vallisneri that the existence of so many animalcules must be an
202 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
illusion, since Nature could hardly be so prodigal, the animalculists
retorted by instancing such observations as that of Baster, who had
taken the trouble to count the eggs of a crab and had found that
they amounted to 12,444. James Cooke later elaborated a theory
of a world of the unborn to which the spermatozoa could retire
between each attempt to find a uterus in which they could develop —
this avoided Vallisneri's argument. "All those other attending
Animalcula, except that single one that is then conceived, evaporate
away, and return back into the Atmosphere again, whence it is very
likely they immediately proceeded; into the open Air, I say, the
common Receptacle of all such disengaged minute sublunary bodies;
and do there circulate about with other Semina, where, perhaps,
they do not absolutely die, but live a latent life, in an insensible or
dormant state, like Swallows in Winter, lying quite still like a stopped
Watch when let down, till they are received afresh into some other
Male body of the proper kind, to be again set on Motion, and ejected
again in Coition as before, to run a fresh chance for a lucky Con-
ception ; for it is very hard to conceive that Nature is so idly luxurious
of Seeds thus only to destroy them, and to make Myriads of them
subservient to but a single one." But Cooke's attractive hypothesis,
published in 1762, came too late, as Punnett says, to save the
animalculists.
On the experimental side. Garden and Bourguet came forward
with descriptions of little men inside the animalcules, thus confirming
the work of Gautier and Hartsoeker. It is fair to add, however, that
Garden held quite enlightened views of the mutual necessity of egg
and spermatozoon. La Motte maintained that the egg (which he
identified with the Graafian follicle) was too big to go down the
Fallopian tube, and Sbaragli, another writer on the animalculist
side, agreed with him.
Leeuwenhoek, it must be admitted, indulged in assertions no less
fantastic than those of his followers. He said there were spermatic
animalcules of both sexes, as one could see by a slight difference near
their tails, that they copulated, that the females became pregnant
and gave birth to little animalcules, that young and feeble ones could
be seen, that they shed their skins, and, finally, that some had been
observed with two heads. Haller, who made good use, on the whole,
of his strong vein of scepticism, characterised all these remarks as
"only conjectures". (See Fig. 12.)
SECT. 3]
AND EIGHTEENTH CENTURIES
203
As for the supporters of epigenesis, they were few, but they
included Descartes, de Maupertuis, Antoine Maitre-Jan and John
Turberville Needham. Von Haller affords some evidence against the
identification of epigenesis with vitaHsm and preformation with
mechanism, for he says, "Various authors have taught that the parts
of the human body are formed by a mechanism depending on general
laws (i.e. laws not simply of biological jurisdiction) or by the virtue
of some ferment, or by rest and cold making crusts out of the different
juices, or in other ways. All these (mechanical) systems have some
resemblance to that of M. Wolff". Haller also speaks always of
Wolff's vis essentialis as "blind". Minor writers on the epigenetic side
were Tauvry, Welsh,
Dartiguelongue, Bou-
ger, Drelincurtius and
Mazin. After 1 750
C. F. Wolff brought
an abiding victory to
their opinion.
Some maintained a
quite independent po-
sition, such as Buffon,
who welded together
an epigenetic theory
of fertilisation with a ^^S- 12. Dalenpatius' drawings of human spermatozoa.
preformationist theory of embryogeny. Pascal (not the great Jan-
senist) put forward the chemical view that fertilisation consisted of
a combination between the acid semen of the male and the "lixivious "
semen of the female, no doubt because in chemistry acids were
regarded as male and alkalies female. Claude Perrault and Connor
also suggested that the formation of the embryo was a fermentation
set up in the egg by the spermatic animalcule. In this they were
following the example of van Helmont, who had originally suggested
such a theory. In 1 763 Jacobi discovered how to fertilise fish eggs
with milt; a practical matter which had a good deal of influence on
biological theory. Launai alone still held to the Aristotelian con-
ception of form and matter.
There is no need here to do more than glance at the spontaneous
generation controversy itself, for it has always been well known in
the history of biology, especially in connection with the subsequent
204 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
work of Pasteur. J. T. Needham's books, New Microscopical Dis-
coveries of 1745 and Observations upon the generation, composition, and
decomposition of animal and vegetable substances of 1 750, exercised a con-
siderable influence. They were written after the French fashion
(Needham had been educated at Douai) very concisely, and with
some brilliance of style, and it is hardly true to say, as Radl does, that
their experimental foundation was meagre. That it was inadequate
was proved definitely as events turned out by Spallanzani. De
Kruif 's account of the controversy is false and misleading, especially
in its estimate of Needham who is much more truly described in
the words of Louis Pasteur (see also Prescott).
Needham's case rested upon the statement that, if meat broth was
placed in a sealed vessel and heated to a high temperature so that
all life was destroyed in it, it would yet be found to be swarming
some days later with microscopical animals. All depended, therefore,
upon the sureness with which the vessel had been sealed and the
efficacy of the heat employed to kill all the animalcules initially
present, and, in the ensuing controversy, Needham lost to Spallanzani
entirely on a question of technique. It may be remarked here, with-
out irrelevance, that the problem is still unsolved; for all that was
proved by the experiments of Spallanzani was that animals the size
of rotifers and protozoa do not originate spontaneously from broth,
and all that was proved by those of Pasteur was that organisms the
size of bacteria do not originate de novo in that way. The knowledge
which we have acquired in recent years of filter-passing organisms,
such as the mosaic disease of the tobacco-plant, and phenomena such
as the bacteriophage of Twort and d'Herelle, has reopened the whole
matter, so that of the region between, for example, the semi-living
particles of the bacteriophage (lO"^^ gram) and the larger sized
colloidal aggregates (io~^^ gram) we know absolutely nothing. The
dogmatism with which the biologists of the early twentieth century
asserted the statement omne vivum ex vivo was therefore, like most
dogmatisms, ill-timed.
But to dwell further on this would be a digression. The important
point was that Spallanzani's victory was a victory not only for
those who disbelieved in spontaneous generation, but also for those
who believed in the preformation theory of embryogeny. By
1 786, indeed, that viewpoint was so orthodox that Senebier, in his
introduction to an edition of Spallanzani's book on the generation
SECT. 3] AND EIGHTEENTH CENTURIES 205
of animals and plants, could treat the epigenesists as no better than
atheists.
Spallanzani's views on embryology were largely drawn from his
study of the development of the frog's egg. Here he went far beyond
Bosius, but, in spite of many careful observations, he thought he saw
the embryo already present in the unfertilised ova. This led him to
claim that amphibia ought to be numbered among viviparous
animals. His principal step forward was his recognition of the
semen as the actual agent in fertilisation on definite experimental
grounds — the narrative of his artificial insemination of a bitch
is too famous to quote; he said it gave him more intellectual satis-
faction than any other experiment he had ever done. This demon-
stration finally disposed of the aura seminalis which Harvey had
found himself obliged to adopt on the grounds of his dissections on
does. Curiously enough Spallanzani never convinced himself that
the spermatozoa themselves were the active agents.
3-13. Preformation and Epigenesis
Of all the preformationists Charles Bonnet was the most theoretical.
He was an adherent of that way of thinking mainly on the theoretical
ground that the organs of the body were linked together in so intimate
a manner that it was not possible to suppose there could ever be a
moment when one or two of them were absent from the ranks. "One
needs", he said, "no Morgagni, no Haller, no Albinus to see that
all the constituent parts of the body are so directly, so variously, so
manifoldly, intertwined as regards their functions, that their relation-
ship is so tight and so indivisible, that they must have originated all
together at one and the same time. The artery implies the vein,
their operation implies the nerves which in their turn imply the
brain and that by consequence the heart, and every single condition
a whole row of other conditions." Bonnet compared epigenesis to
crystal-growth in which particles are added to the original mass
independently of the plan or scheme of the whole, i.e. in opposition
to the growth of an organism, in which particles are added on only
at certain places and certain times under the guidance of "forces de
rapport". Przibram has recently discussed the question of how far
such a comparison is admissible, but, in Bonnet's time at any rate,
it became very famous. Bonnet made reference to Haller's discovery
of the intimate relationship between embryo and yolk as evidence
2o6 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
for his theory. The embryo begins, according to him, as an exceedingly
fine net on the surface of the yolk, fertilisation makes part of it beat
and this becomes the heart, which, sending blood into all the vessels,
expands the net. The net or web catches the food particles in its
pores, and Bonnet supposed that, if it were possible to abstract all
the food particles at one operation from the adult animal, it would
shrivel and shrink up into the original invisible web from which it
originated.
Bonnet was no more afraid of the emboitement principle than was
Haller; indeed, he called it "one of the greatest triumphs of rational
over sensual conviction". Many of his arguments were reproductions
of Haller's, and he says in his preface that he had written his book
some time before Haller's papers on the chick appeared, but then,
finding his own views confirmed by the more experimentally founded
ones of Haller, he determined to publish what he had set down. Thus
in one place he says, " I shall be told, no doubt, that the observations
on the development of the chick in the tgg and the doe in the maternal
uterus make it appear that the parts of an organised body are formed
one after another. In the chick for instance it has been observed that
during the early part of incubation the heart seems to be outside
the animal and has a very diflferent form to what it will have. But
the feebleness of this objection is easy to apprehend. Some people
wish to judge of the time when the parts of an organised body
begin to exist by the time when they become visible to us. They
do not reflect that minuteness and transparency alone can make
these parts invisible to us although they really exist all the
time".
Bonnet was therefore what might be called an " organicistic pre-
formationist", for his objection to epigenesis lay in the fact that it
apparently did not allow for the integration of the organism as a
whole. His mistake was that he assumed the capacities of the adult
organism to be present all through foetal life, whereas the truth is
that they grow and differentiate in exactly the same way as the
physical structure itself does. Bonnet's philosophical position, which
has been analysed by Whitman, seriously contradicts the generalisa-
tion of Driesch that all the epigenesists were vitalists and all the pre-
formationists mechanists. For Bonnet an epigenetic and a mechanical
theory were one and the same; he hardly distinguished, as Radl
says, between Descartes and Harvey; and it was just the neo-vitalistic
SECT. 3] AND EIGHTEENTH CENTURIES 207
idea of the organism as a whole that he could not fit in with epigenesis.
Needham and Wolff were undoubtedly epigenesist-vitalists, and
Bonnet was undoubtedly a preformationist-vitalist, but Maupertuis
was equally clearly an epigenesist-mechanist.
G. L. Leclerc, Comte de Buffon, the most independent figure in
the controversy, stood alone as much because of his erroneous experi-
ments as because of his originality of mind. As has so often been
observed, Buffon was not really an experimentalist at all: he was a
writer, and preferred other people to do his experiments for him.
The volume on generation in his Histoire Naturelle begins with a very
long historical account of the work that had been done in the previous
centuries on embryology. At the beginning of the section on repro-
duction in general he said, "The first and most simple manner of
reproduction is to assemble in one body an infinite number of similar
organic bodies and to compose the substance in such a manner that
every part shall contain a germ or embryo of the same species and
which might become a whole of the same kind with that of which
it constitutes a part". Such an idea resembles the ancient atomistic
speculations, and is explicated by W. Smellie, the obstetrician, who
translated Buffon into English, as follows: "The intelligent reader
will perceive that this sentence, though not very obvious, contains
the principle upon which the whole theory of generation adopted
by the author is founded. It means no more than that the bodies
of animals and of vegetables are composed of an infinite number
of organic particles, perfectly similar, both in figure and substance,
to the whole animal or plant of which they are the constituent parts ".
This conception explains Buffon's curious attitude to the preformation
question. An embryo was preformed in its germ because all the parts
of the germ were each a model of the animal as a whole, but it was
also formed by epigenesis because, the sexual organs being first
formed, all the rest arose entirely by a succession of new origins.
Buffon's "organic, living, particles" bear some resemblance to the
"biogen molecules" which later generations were to discuss, and he
says that an exactly similar but simpler structure is present in dead
matter.
In his discussion of former theories he resolutely rejects the em-
boitement aspect of preformationism, giving various calculations to
show its impossibility and maintaining that "every hypothesis which
admits an infinite progression ought to be rejected not only as false
2o8 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
but as destitute of every vestige of probability. As both the vermicular
and ovular systems suppose such a progression, they should be ex-
cluded for ever from philosophy". He completely destroys the theory
which the ovists and animalculists had set up in order to explain
resemblance to parents, namely, that, although the foetus might
originate either from egg or spermatic animalcule originally, it was
moulded into the form of its parents by the influence of the maternal
organism during pregnancy. This field, which was more than once
disturbed by the contestants during the course of the century, re-
ceived systematic attention from time to time by medical writers.
There was a memorable dispute on this point between Turner and
Blondel, whose polemics, written in an exceedingly witty manner,
are still very pleasant and amusing to read. Blondel was the sceptic
and Turner the defender of the numerous extraordinary stories which
passed for evidence on this subject. It is interesting to note that
Turner believed in the continuity of foetal and maternal blood-vessels.
Krause and Ens later supported the opinions of Turner, while Okes,
in a Cambridge disputation, argued against them.
Buflfon's sixth chapter, in which he relates the progress of his own
experiments, is unfortunate, in that his main result was to discover
spermatozoa in the liquor folliculi of ovaries of female animals. The
explanation of how he came to make such an enormous mistake
has never been satisfactorily given, and it was not long before the
truth of the observation was questioned by Ledermuller. It led him
naturally to the assertion that the ovaries of mammalia were not egg-
producing organs but animalcule-producing organs, and to the view
that the beginning of embryonic development lay in the fusion of the
male with the female spermatic animalcules — a curious revival of
Epicureanism. But it is to be observed that he does not mean one
male animalcule with one female animalcule, but rather all with all,
in a kind of pangenesis. "All the organic particles", he says, "which
were detached from the head of the animal will arrange themselves
in a similar order in the head of the foetus. Those which proceeded
from the backbone will dispose themselves in an order corresponding
to the structure and position of the vertebrae". And so on for all
the organs. The fact that for the organs common to both sexes a
double set of animalcules will thus be provided does not give Buffon
any difficulty and is fully admitted by him. Accordingly he could
only agree to the aphorism omne vivum ex ovo in the sense of
SECT. 3] AND EIGHTEENTH CENTURIES 209
Harvey, namely, as referring to the egg-shaped chorion of vivipara,
and definitely not in the sense of de Graaf and Stensen, namely, in
the modern sense. "Eggs", he says, "instead of being common to
all females, are only instruments employed by Nature for supplying
the place of uteri in those animals which are deprived of this organ.
Instead of being active and essential to the first impregnation, eggs
are only passive and accidental parts, destined for the nourishment
of the foetus already formed in a particular part of this matrix
by the mixture of the male and female semen." Biology at this period
was still labouring under the disadvantage of being without the cell-
theory, and therefore unable to distinguish between an egg and an
egg-cell.
In spite of his leanings towards epigenesis, Buffon repeats precisely
the error of Malpighi. "I formerly detected", he says, "the errors
of those who maintained that the heart or the blood was first
formed. The whole is formed at the same time. We learn from actual
observation that the chicken exists in the egg before incubation. The
head, the backbone, and even the appendages which form the
placenta are all distinguishable. I have opened a great number of
eggs both before and after incubation and I am convinced from the
evidence of my own eyes that the whole chicken exists in the middle
of the cicatrice the moment the egg issues from the body of the hen.
The heat communicated to it by incubation expands the parts only.
But we have never been able to determine with certainty what parts
of the foetus are first fixed, at the moment of its formation." The
experiment of taking a look at the cicatrices of eggs on their way
down the parental oviduct is so obvious that Buffon must have thought
of it, and it would be really interesting to know what factor in the
intellectual climate it was that made him regard such an observation
as not worth attempting. His observations on the embryo itself were
good and, in some ways, new; thus he noticed that the blood first
appears on the "placenta" or blastoderm, and for the first few days
seems hardly to enter the body of the embryo. He gave an extremely
good account of the whole developmental process in the chick and
in man, and his opinions on the use of the amniotic liquid and the
functions of the umbilical cord were very advanced.
J. T. Needham, however, spoke very clearly in favour of epi-
genesis, though he himself did no embryological experiments. His
Idee sommaire of 1776, written against Voltaire, who had called him
N E I 14
2IO EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
a Jesuit and who had drawn materialistic inferences from his writings,
contained the following passage: "The numerous absurdities which
exist in the opinion ofpre-existent germs together with the impossibility
of explaining on that ground the birth of monsters and hybrids, made
me embrace the ancient system of epigenesis, which is that of Aristotle,
Hippocrates, and all the ancient philosophers, as well as of Bacon
and a great number of savants among the neoteriques. My observa-
tions also led me directly to the same result". Needham's embryology
is mostly contained in his Observations nouvelles sur la Generation of
1750. He was explicitly a Leibnitzian and postulated a vegetative
force in every monad.
Needham was not the only thoroughgoing epigenesist of this
period. Maupertuis, whose Venus Physique was published anony-
mously in 1746, came out very clearly on the side of epigenesis.
"I know too well", he said, "the faults of all the systems which I
have been describing, to adopt any one of them, and I find too
much obscurity in the whole matter to wish to form one of my own.
I have but a few vague thoughts which I propose rather as thoughts
to be examined than as opinions to be received, and I shall neither
be surprised nor think myself aggrieved if they are rejected. It seems
to me that both the system of eggs and that of spermatic animal-
cules are incompatible with the manner in which Harvey actually
saw the embryo to be formed. And one or the other of these systems
seems to me still more surely destroyed by the resemblance of the
child, now to the father and now to the mother, and by hybrid
animals which are born from two different species. ... In this ob-
scurity in which we find ourselves on the manner in which the foetus
is formed from the mixture of two liquors, we find certain facts which
are perhaps a better analogy than what happens in the brain. When
one mixes silver and spirits of nitre with mercury and water, the
particles of these substances come together themselves to form a
vegetation so like a tree that it has been impossible to refuse it the
name." This was the Arbor Dianae, which played a great part in
these embryological controversies of the eighteenth century. It has
a great interest for us, for it was perhaps the first occasion on which
a non-living phenomenon had been appealed to as an illustration
of what went on in the living body. It is true that Descartes long
before had said that the movements of the living body were carried
out by mechanisms like clocks or watches, and that they resembled
SECT. 3] AND EIGHTEENTH CENTURIES 211
the statues in certain gardens which could be made to perform un-
expected functions by the pressure of a manipulator's foot on a
pedal, but these instances were all artificially constructed mechanical
devices, whereas the Arbor Dianae was a natural phenomenon quite
unexplained by the chemists of the time, and the lineal forerunner
of Lillie's artificial nerve, and Rhumbler's drop of chloroform. We
know now that its formation is a simpler process than anything
which occurs in the developing embryo, but the course of research has
made it undeniably clear that the same forces which operate in the
formation of the Arbor Dianae are at work also in the developing
embryo. To this extent Maupertuis is abundantly justified, and
Driesch's comments on him are not in agreement with the facts.
"Doubtless many other productions of a like kind will be found",
Maupertuis goes on, "if they are looked for or perhaps if they are
looked for less. And although they seem to be less organised than
the body of most animals, may they not depend on the same mechanics
and on similar laws? Will the ordinary laws of motion suffice, or
must we have recourse to new forces? These forces, incomprehensible
as they are, appear to have penetrated even into the Academy of
Sciences at Paris, that institution where so many opinions are weighed
and so few admitted." Maupertuis goes on to speak of the contem-
porary deliberations on the subject of attraction. "Ghymistry", he
says, "has felt the necessity of adopting this conception and attractive
force is nowadays admitted by the most famous chymists who have
carried the use of it far beyond the point which the astronomers
had reached. If this force exists in nature, why should it not take
part in the formation of animals?" Maupertuis was thus an epi-
genesist and a mechanist at the same time. His opinions have an
extremely modern ring, and his only retrograde step was in suggesting
that the spermatic animals had nothing else to do except to mix the
two seeds by swimming about in them. But that legacy of ovism
was common all through the eighteenth century, and thirty years
later Alexander Hamilton could say, "From the discovery of Animal-
cula in semine masculino by Leeuwenhock's Glasses, a new Theory was
adopted which is not yet entirely exploded".
But the real middle point and fulcrum of the whole period lay in
the controversy between von Haller and Caspar Friedrich Wolflf, the
former at Gottingen and the latter at St Petersburg in the Academy
of the Empress Catherine. Kirchhoflf has described this polemic.
14-2
212 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
Wolff's Theoria generationis, which was a defence of epigenesis on
theoretical and philosophical grounds, written in a very formal,
logical, and unreadable manner, appeared when he was only twenty-
six years old, in 1759. Leibnitz, as Radl points out, had borrowed
from the earlier preformationists the conception of a unit increasing
in bulk in order to become another kind of unit; but Wolff, following
Needham, borrowed from Leibnitz the idea of a monad developing
into an organism by means of its own inherent force, and to this he
joined the Stahlian notion of a generative supra-physical force in
nature. On the practical side, Wolff's work was indeed of the highest
importance. If the embryo pre-exists, he argued, if all the organs
are actually present at the very earliest stages and only invisible to
us even with the highest powers of our microscopes, then we ought
to see them fully formed, as soon as we see them at all. In other
words, at the moment at which any given organ comes into view, it
ought to have the form and shape, though not the size, of the same
organ when fully completed in the embryo at birth. On the other
hand, if this is not the way in which development goes on, then one
ought to be able to see with the microscope one shape changing into
another shape, and, in fact, a series of appearances, each one different
from that which had immediately preceded it, or, in other words,
a series of advancing adaptations of the various parts of the primitive
embryonic mass. WoliT chose as his first test case the blood-vessels
of the blastoderm in the chick, for he saw that at one moment this
apparatus was in existence, while the moment before it had not been.
His microscopical researches led him to the conclusion that the homo-
geneous surface of the blastoderm partially liquefies and transforms
itself at these points into a mass of islands of solid matter, separated by
empty spaces filled with a colourless liquid but afterwards with a red
liquid, the blood. Finally, these spaces are covered with membranes
and become vessels. Consequently it was obvious that the vessels had
not been previously formed, but had arisen by epigenesis.
Haller replied to this new experimental foundation for epigenesis
without delay, for he was working on the development of the chick
at the same time, and held closely to the opposite theory. We have
already seen what his one and only argument against Wolff was. He
used it time after time in all its possible variations, maintaining stoutly
that the chick embryo was so fluid in the early stages that Wolff had
no right to deny the presence of a given structure simply because
SECT. 3] AND EIGHTEENTH CENTURIES 213
he could not see it. Haller's explanation of Wolff's results was that
the blood-vessels had been there all the time but that they had not
become visible until the moment at which Wolff saw the islands
forming. "After I had written the above", said Haller, "M. Wolff
made new objections against the demonstration. Instructed by new
researches, he denies absolutely that the yolk-membranes, which he
makes two in number, exist before incubation. He pretends that they
are new and that they are born at the beginning of incubation, and
consequently that the continuity of their vessels with the embryo
does not in the least prove that in the body of the mother the yolk
received vessels from the foetus. I have compared the observations
of this great man with my own and I have found that the yolk
never has more than one pulpy and soft membrane, part of which
is what I have called the umbilical area, and that the fine exterior
membrane does not belong to the yolk but to the inner part of the
umbilical membrane. ... I do not believe that any new vessels arise
at all, but that the blood which enters them makes them more
obvious because of the colour which it gives them, and so by the
augmentation of their volume, they become longer."
Wolff replied by another extensive piece of work, which he called
De Formatione Intestinorum, and which appeared in one of the publica-
tions of the Russian Academy for 1768. It ruined preformationism.
In it he demonstrated that the intestine is formed in the chick by
the folding back of a sheet of tissue which is detached from the ventral
surface of the embryo, and that the folds produce a gutter which in
course of time transforms itself into a closed tube. The intestine,
therefore, could not possibly be said to be preformed, and from this as
starting-point, Wolff went on to propose an epigenetic theory which
applied the same process to all organs. It is interesting to note that
the facts brought forward by Wolff have never been contradicted, but
have been used as a foundation to which numberless morphological
embryologists have added facts discovered by themselves. It is
noteworthy that, although Wolff's second general principle, that of
increasing solidification during embryonic development, led to no
immediate results, it has been abundantiy confirmed since then (see
Fig. 221). His observations on the derivation of the parts of the early
embryo from "leaf-like" layers were even more important, and acteA
as a very potent influence in the work of Pander and von Baer.
It happened, however, that Haller had much the greater in-
214 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
fluence in the biological world at the time, so that Wolff's conceptions
did not immediately yield fruit in any general advance. Looking
back over the second half of the seventeenth and the first two-thirds
of the eighteenth century, it is remarkable how little theoretical
progress was made in view of the abundance of new facts which were
discovered. Punnett, in an interesting paper, has vividly brought this
out. "The controversy between the Ovists and Animalculists had
lasted just a century", he says, "and it is not uninteresting to reflect
that the general attitude of science towards the problem of generation
was in 1775 niuch what it had been in 1675. When the period opened,
almost all students of biology and medicine were Preformationists
and Ovists; at its close they were for the most part Ovists and Pre-
formationists." Ovism sprang in the first instance from de Graaf's
discovery of the mammalian egg, which gave a new and precise
meaning to Harvey's aphorism. Preformationism, already old as a
theory, acquired an apparent factual basis in the work of Malpighi
and Swammerdam, and allied itself naturally with ovism. With
Leeuwenhoek and his spermatozoa, animalculism came upon the
field. The main outlines of the battle which went on between the
two viewpoints have already been drawn, but it is worth remembering
that there were independent minds who were impressed by the obvious
facts of heredity and found it difficult to call one sex essential rather
than the other. Among these Needham and Maupertuis might be
counted, and among the lesser men, James Handley with his Me-
chanical Essays on the Animal Oeconomy of 1730 ought to receive a
mention. Though fond of theological arguments he upheld the
common-sense attitude against ovists and animalculists alike — "We
dissent in some things", he said, "both from Leeuwenhoeck and
Harvey. . . . Both the semen and ova (notwithstanding all that can
be said) we believe to be a causa sine qua non in every Generation".
But what finally killed animalculism was the discovery in so many
places of small motile living beings, flagellates, protozoa, large
vibrios. It was difficult to maintain in the face of this new evidence
that the spermatozoa were essential elements in generation, though
the seminal fluid itself might very well be, as of course was Spallan-
zani's opinion. The preformation theory was what was holding up
further progress, and when Wolff's arguments prevailed in the very
last years of the eighteenth century, the way was open for the
recognition of the true value of the spermatozoa.
SECT. 3] AND EIGHTEENTH CENTURIES 215
The otherwise unknown physician d'Aumont, who wrote the
article on "Generation" in Diderot's famous Encyclopaedia, brought
this out in an interesting way, for himself an ovist, he summarised
the arguments, which, in 1757, were destroying the animalculist
position, and reducing rapidly the number of its adherents.
1. Nature would never be so prolific as to produce such millions
of spermatic animalcules, each one with its soul, unnecessarily.
2. The spermatic animalcules of all animals are the same size, no
matter how large the animal is: how, therefore, can they be
involved in its generation?
3. They are never found in the uterus after coitus, but only in
the sperm (?).
4. How do they reproduce their kind?
5. What evidence is there that they are any different from the
animalcules (of similar shape, etc.) which are to be found in
hay infusion, scrapings from the teeth, etc. ? Nobody supposes
that these have any relation to reproduction.
3-14. The Close of the Eighteenth Century
The last forty years of the century were not marked by any great
movement in a fruitful direction for morphological embryology, an
iconographic wave of some merit due to Albinus, W. Hunter, Tarin,
Senffj Rosenmuller, Danz and Soemmering excepted ; and it was not
until 181 2 that J. F. Meckel the younger translated Wolff's papers
into German. This was one of the principal influences upon Pander
and von Baer. In his introduction, Meckel describes how Wolff's
work had been disregarded, and points out that Oken, writing in
1806, had apparently never even heard of it. In the very early
years of the nineteenth century morphological embryology received
a great impetus, however. One of the most interesting figures of
the new period was de Lezerec, a Breton, whose father had been
in the Russian naval service. The son, as a Russian naval cadet, no
doubt stimulated by the writings of Wolff, who had lived at St Peters-
burg, used to incubate eggs on board ship. He eventually left the
sea, studied medicine at Jena, and wrote an excellent dissertation
on the embryology of the chick in 1 808, which Stieda has recently
brought to light. He then went to Paris, and, taking a medical
appointment at Guadeloupe, was lost to science. Very much more
important was the work of Pander in 181 7 and von Baer in 1828,
2i6 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
but it belongs to the present period, and I shall not treat it historically.
For data on von Baer, see Kirste, Addison and Stieda. It is interesting
to note, however, that the recapitulation theory, which was first clearly
formulated by von Baer, was already taking shape in various minds
during the closing years of the eighteenth century. Lewes has thus
described the thesis of Goethe's Morphologie, written in 1795: "The
more imperfect a being is the more do its individual parts resemble
each other and the more do these parts resemble the whole. The more
perfect a being is the more dissimilar are its parts. In the former case
the parts are more or less a repetition of the whole, in the latter
case they are totally unlike the whole. The more the parts resemble
each other the less subordination is there of one to the other : and
subordination is the mark of high grade of organisation".
William and John Hunter belong also to the end of the century.
The former, in his book on the anatomy of the gravid uterus, proved
finally and completely the truth of the view that the maternal and
foetal circulations are distinct. His injections left no shadow of doubt
about the matter, and the way was clearly opened up for the study
of the properties of the capillary endothelial membranes separating
the bloods, a study which is still vigorously proceeding, especially
in its physico-chemical aspect (see Section 21). There was a quarrel
between the brothers over the priority of this demonstration. John
Hunter's Essays and Observations also contain material important for
embryology. His drawings of the chick in the &gg were very beautiful,
and are still in the archives of the Royal College of Surgeons. He
adopted Mayow's theory of the office of the air-space, and anticipated
von Baer's theory of recapitulation much as did Goethe. "If we were
capable of following the progress of increase of the number of parts
of the most perfect animal as they were first formed in succession,
from the very first to its state of full perfection, we should probably
be able to compare it with some one of the incomplete animals
themselves, of every order of animals in the creation, being at no
stage different from some of the inferior orders. Or, in other words,
if we were to take a series of animals, from the more imperfect to
the perfect, we should probably find an imperfect animal corre-
sponding with some stage of the most perfect." It is impossible not
to reflect on the curious course which was taken by the essence of
the idea of recapitulation in the history of embryology. As Aristotle
first formulated it, it was as much bodily as mental, but all his sue-
SECT. 3] AND EIGHTEENTH CENTURIES 217
cessors until the eighteenth century a.d. treated it as a psychological
rather than a physiological or morphological theory, and lost them-
selves in speculations about the vegetative, sensitive, and rational
souls. Yet the other aspect of the theory was only asleep, and was
destined to be of the greatest value as soon as investigators began to
direct their attention more to the material than to the spiritual
aspect of the developing being.
Hunter did not absolutely reject preformationism, but regarded it
as holding good for some species in the animal kingdom ; he therefore
attached no philosophical importance to it.
Although Wolff's work did not lead to the immediate morphological
advances which might have been expected, it was in many ways
fruitful. It produced J. F. Blumenbach's Uber den Bildungstrieb of
1789, a work which elaborated the Wolffian vis essentialis into
the nisus formativus, a directing morphogenetic force peculiar to
living bodies. It is interesting to note that Blumenbach passed through
an exactly opposite succession of opinions to that of Haller, i.e. he
was first attracted by preformationism, but, being convinced by
Wolff's work, abandoned it in favour of epigenesis. Blumenbach
compares his nisus formativus with the force of gravity, regarding
them as exactly similar conceptions and using them simply as
definitions of a force whose constant effects are recognised in
everyday experience. Blumenbach says that his nisus formativus
differs from Wolff's vis essentialis because it actively does the shaping
and does not merely add suitable material from time to time to a
heap of material which is already engaged in shaping itself. Wolff
was still alive at this time, but he did not make any comment on
Blumenbach, though he might very well have said that Blumenbach
had misunderstood him, and that their forces were really alike in
every particular. Both Blumenbach and Wolff were mentioned by
Kant in the Critique of Judgement where he adopted the epigenetic
theory in his discussion of embryogeny.
A word must be said at this point about the opinions of the
eighteenth century on foetal nutrition. At the beginning of it, there
was, as has been shown, a welter of conflicting theories; and though,
later on, writers on this subject were fewer, the progress made was
no more rapid. In 1802 Lobstein was supporting the view (which
had been defended by Boerhaave) that the amniotic liquid nourished
the embryo per os, although Themel had shown forty years before
CHART III
igooB^
1600
1900
SECT. 3] THE EIGHTEENTH CENTURY 219
that this could be at most the very slightest source of material, from
a study of acephalic monsters. These workers had obviously learnt
nothing from Herissant and Brady, who had been over precisely the
same ground fifty years before. On the other hand. Goods and
Osiander reported the birth of embryos without umbilical cords, so
that the solution of this question became, in the first year of the
nineteenth century, balanced, as it were, between the relative
credibility of two kinds of prodigy. Nourishment per os was defended
by Kessel, Hannes and Grambs, and was attacked by Vogel, Bern-
hard, Glaser, Hannhard and Reichard. The idea lingered on right
into the modern period, and as late as 1 886 von Ott, who was much
puzzled about placental permeability, decided that a great part in
foetal nutrition must be played by the amniotic liquid. WeidHch,
a student of his, fed a calf on amniotic liquid for some days, and as
it seemed to get on all right, he reported the amniotic liquid to have
nutritive properties. The appeal to monsters was still resorted to at
the end of the nineteenth century, for Opitz, in order to negative von
Ott's conclusions, drew attention to a specimen in the Chemnitz
Polyklinik in which the oesophagus of a well-nourished normal infant
was closed at the upper third without the development of the body
having been in any way restricted. The fuller possibilities of bio-
chemistry itself have sometimes been exploited in favour of the ancient
theory of nourishment />^r os\ thus Kottnitz in 1889 collected some
data about the presence of peptones and protein in the human
amniotic liquid with this object in view. That the foetus swallows
the liquid which surrounds it towards the end of gestation in all
amniota, can hardly be disputed, and as there are known to be
active proteolytic enzymes in the intestinal tract, no doubt some of
the protein which it contains is digested — but to maintain that any
significant part is played in foetal nutrition by this process has
become steadily more and more impossible since 1600.
But to return to the eighteenth century; all was not repetition;
occasionally somebody brought forward a few facts. Thus the de-
glutition of the amniotic Hquid was discussed by Flemyng in 1 755
in a paper under the title " Some observations proving that the foetus
is in part nourished by the amniotic liquor". "I believe", he said,
"that very few, if any at all, will maintain now-a-days with Claudius
de la Courvee and Stalpartvan-der-Wiel, that the whole of its nourish-
ment is conveyed by the mouth." But he himself had found white
220 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
hairs in the meconium of a calf embryo with a white hide. Both
Aides and Swammerdam had found the same thing, but Aides did
not think it of any significance, and Swammerdam merely remarked
that the calf must lick itself in utero.
More interesting was W. Watson's "Some accounts of the foetus
in utero being differently affected by the Small Pox". This was the
earliest investigation of the permeability of the placenta to patho-
logical agents. "That the foetus", said Watson, "does not always
partake of the Infection from its Mother, or the Mother from the
Foetus, is the subject of this paper." Two of his cases, he said, "evince
that the Child before its Birth, though closely defended from the
external Air, and enveloped by Fluids and Membranes of its own, is
not secure from the variolous Infection, though its Mother has had
the Distemper before. They demonstrate also the very great Subtility
of the variolous Effluvia". But other cases "are the very reverse of
the former, where though from Inoculation the most minute portion
of Lint moisten'd with the variolous Matter and applied to the slightly
wounded Skin, is generally sufficient to propagate this Distemper;
yet here we see the whole Mass of the Mother's Blood, circulating
during the Distemper through the Child, was not sufficient to pro-
duce it. . . . From these Histories it appears that the Child before its
Birth ought to be consider'd as a separate, distinct Organization;
and that though wholly nourish'd by the Mother's Fluids, with
regard to the Small Pox, it is liable to be affected in a very different
Manner and at a very different Time from its Mother". Doubtless
the modern explanation of Watson's discordant results would be that!
in one case there were placental lesions, destroying the perfect barrier
between the circulations, and in others there were not.
In the last year of the century (but the seventh of the Republic)
Citizens Leveille & Parmentier contributed an interesting paper to I
the Journal de Physique in which they observed the increase in size]
of the avian yolk on incubation and spoke of a current of water yolk-
wards (see Fig. 225).
3* 15. The Beginning of the Nineteenth Century
At the beginning of the new century a fresh influence came in!
with the work of Lamarck, though it did not have such a great effect
on his contemporaries as on later generations. Its relations with]
biochemistry are so remote that there is no need to deal in any detail
SECT. 3] AND EIGHTEENTH CENTURIES 221
with it here, but Lamarck's opinions on embryology may perhaps
be given in the words of Cuvier, written in 1836.
"In 1802 he pubUshed his researches on living bodies, containing
a physiology peculiar to himself, in the same way that his researches
on the principal facts of physics contained a chemistry of that char-
acter. In his opinion the egg contains nothing prepared for life
before being fecundated, and the embryo of the chick becomes
susceptible of vital motion only by the action of the seminal vapour;
but, if we admit that there exists in the universe a fluid analogous
to this vapour, and capable of acting upon matter placed in favour-
able circumstances, as in the case of the embryon, which it organises
and fits for the enjoyment of life, we will then be able to form an
idea of spontaneous generations. Heat alone is perhaps the agent
employed by nature to produce these incipient organizations, or it
may act in concert with electricity. M. de Lamarck did not believe
that a bird, a horse, nor even an insect, could directly form them-
selves in this manner; but, in regard to the most simple living
bodies, such as occupy the extremity of the scale in the different
kingdoms, he perceived no difficulty; for a monad or a polypus are,
in his opinion, a thousand times more easily formed than the embryo
of a chick. But how do beings of a more complicated structure, such
as spontaneous generation could never produce, derive their existence?
Nothing, according to him, is more easy to be conceived. If the
orgasm, excited by this organizing fluid, be prolonged, it will aug-
ment the consistency of the containing parts, and render them
susceptible of reacting on the moving fluids which they contain, and
an irritability will be produced, which will consequently be possessed
of feeling. The first efforts of a being thus beginning to develope
itself must tend to procure it the means of subsistence and to form
for itself a nutritive organ. Hence the existence of an alimentary
canal. Other wants and desires, produced by circumstances, will
lead to other efforts, which will produce other organs : for, according
to a hypothesis inseparable from the rest, it is not the organs, that is
to say, the nature and form of the parts, which give rise to habits
and faculties ; but it is the latter which in process of time give birth
to the organs. It is the desire and the attempt to swim that produces
membranes in the feet of aquatic birds; wading in the water, and
at the same time the desire to avoid getting wet, has lengthened the
legs of such as frequent the sides of rivers; and it is the desire of flying
222 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
that has converted the arms of all birds into wings, and their hairs
and scales into feathers. In advancing these illustrations, we have
used the words of our author, that we may not be suspected either of
adding to his sentiments or detracting any thing from them."
If the latter part of the eighteenth century did not produce the
move forward in the morphological direction which might have been
expected from the work of Wolff, a remarkable amount of work was
accomplished on the chemical side. This mass of work did not spring
from any one source, it was not due to a great discovery on the part
of one man, but rather it came about that, as the technique of
chemistry itself improved, a number of otherwise undistinguished
investigators, such as Dehne, Macquer and Bostock, applied physico-
chemical methods to the embryo, though it is true that among the
names are those of certain great chemists, such as Scheele and
Fourcroy. The results of this movement were summarised in the work
of J. F.John, whose Chemische Tabellen des Tierreichs appeared in 1814.
With this date I propose to bring my historical assessment to an
end. The work that was done in physico-chemical embryology
after 181 4 will be considered in the appropriate sections dealing
with the problems of the present time; for Gobley, as an example,
who gave the name to the substance still called vitellin, was working
only a dozen years after the date of the publication of John's
Tabellen.
In this translation of the Tables, I have made one alteration only.
John groups together a number of data which are contained in von
Haller's Elementa Physiologiae, and attributes them to that great man.
But actually they were obtained by earlier investigators and only
came to John through the medium of Haller and Fourcroy — I have
therefore allotted them to their true originators.
EXCERPTS FROM J. F. JOHN'S CHEMISCHE TABELLEN OF 1814
Substance or liquid
investigated Composition
Amniotic liquid (man) It contains a substance which can be
precipitated with tincture of gall,
phosphate of lime and muriatic salts
„ It is salt
„ It is sweet
,, It coagulates on boiling
Investigator
Date
Rhades
1753
Schrader
1674
Vieussens
1705
Rhades
1753
Roederer
1750
Barbati &
1676
Heertodt
SECT. 3]
AND EIGHTEENTH CENTURIES
223
EXCERPTS FROM J. F. JOHN'S CHEMISCHE TABELLEN OF 1814 {cont.)
Substance or liquid
investigated
Amniotic liquid (man)
Cheesy material, given
off into the amniotic
liquid by the body of the
foetus (man)
Embryonic tissue-juice
(man)
Amniotic liquid (cow)
Amniotic and allantoic
liquids (cow)
Composition
It is miscible with water
It is coagulable by tincture of gall
It is coagulable by alcohol
It is coagulable by alumina
It is coagulable by spirits of nitre
Free mineral alkali, water, albumin-
ous substance, common salt
Much water, very little common
salt, fire-stable alkali, phosphoric
acid, some earth, and oxyde of
iron
Much water, a lymphatic coagulum,
common salt, salmiac, a trace of
phosphate of lime
Sp. g. 1-005. Albuminous matter,
soda, muriate of soda, phosphate of
lime, the rest is water
Animal slime, and a characteristic
fatty material, or rather an albu-
minous material tending to fat, car-
bonate of lime
It contains hydrofluoric acid
Water, much sulphate of soda,
phosphate of lime and talc, an
animal substance soluble in water,
insoluble in spirits of wine, and
not forming a combination with
tannic acid, a crystalline amniotic
acid
The liquid of the allantois is very
different quantitatively in the dif-
ferent periods of pregnancy, as also
in the qualitative aspect of its com-
position. First it is crystalline and
colourless, then it gets yellowish,
and finally a dark reddish-brown.
But it remains watery all the time
and never has the property possessed
by the amniotic liquid, of becoming
at last quite slimy even to the point
of showing fibres in it. During the
last months the hippomanes appear
in it, these are soft and yet tough.
The quantity of this liquid is much
greater at the end than at the be-
ginning. Alcohol precipitates from
it a very large amount of a reddish
substance; sulphate of baryta, tar-
taric acid, and carbonate of lime
give a large precipitate. These re-
agents do not change the amniotic
fluid at all. 1000 gm. Uq. allant.
gave 20-25 gm. solid residue,
1000 gm. liq. amnii gave lo-i i gm.
solid residue
Investigator
Date
Longfield
1759
Rhades
1753
Spielmann
1753
Tauvry
1690
Langly
1674
Gmelin &
1796
Ebermaier
van den Bosch
1792
Scheele
Vauquelin &
Buniva
Vauquelin &
Buniva
Berzelius
Buniva &
Vauquelin
Dzondi
1807
1806
224
EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
EXCERPTS FROM J. F. JOHN'S CHEMISCHE TABELLEN OF 1814 {cont.)
Substance or liquid
investigated
Blood of embryo (man)
Blood of embryo (rabbit)
Foetal urine (man)
Meconium (man)
Meconium (cow)
Eggs (wild birds)
Air-space
Shells
Shell-membranes
Egg-white
Yolk
Shell-membranes
Shell
Composition Investigator
Soda, much serum, and some leathery Fourcroy
fibrous threads, which made up
only ^ grain out of 3 gros 6 grains
of cruor . They were jelly-like in con-
sistency. No phosphoric acid. It
differed from the blood of an adult
(i) in not giving a red flush when
shaken up with air, (2) in not clot-
ting in air, (3) in the fibres being
more jelly-like
Does not coagulate in the cold but Fourcroy
gives rise to a red serum tending
towards brown. It was not as solid
as usual except when heated, then
it went grey though the supernatant
liquor was red
It is odourless and colourless and of Fourcroy
a slimy nature
Water f , ^r> spirituous extract similar Bay en
to gall, a black residue dissolving
partially in water to give a yellow
colour. He holds it to be a milky
excrement
Contains true gall-like substances
Date
1790
1803
Does not contain air of different com-
position from atmospheric air
Phosphate of lime, animal glue, and
some combustible substance which
escapes with a sulphurous smell
from shells when they are softened
in acid. Ferrous particles. Some-
times some common salt. An egg,
which weighed 2 ozs. 2 scruples 15
grains, had white which weighed
10 qentchen 2 scruples, yolk ^ oz.
\ scruple, and shell and membranes
2 drachms 5 grains
An animal material insoluble in acids
6 qentcfwn 2 scruples 7 grains lost
practically 6 qentchen in drying, it
contains no caustic salts, the ash is
an earthy insipid dust
Albuminous matter, water, muriate
of soda, phosphate of lime, and
sulphur
Albuminous matter, oil, yellow pig- Adet
ment
From 60 eggs, 5^ ozs. oil Dehne
Albuminous matter with much Adet
oxygen
Carbonate of lime, phosphate of lime, Adet
and very oxydised albuminous
matter
Buniva & —
Vauquelin
Hehl 1 796
von Wasserberg 1 780
von Wasserberg 1 780
von Wasserberg 1 780
Adet
SECT. 3]
AND EIGHTEENTH CENTURIES
225
EXCERPTS FROM J. F. JOHN'S CHEMISCHE TABELLEN OF 1814 {cent.)
Substance or liquid
Composition
investigated
Eggs (domestic hen)
Shell
Investigator Date
Shell-membranes
A fine earth and a gelatinous material
True lime, containing perhaps phos-
phoric acid
\ oz. of pulverised clean shell, di-
gested with spirits of wine, gave i \
grains of an extract which smelt and
tasted rancid. The same amount of
shell gave i scruple of a yellow
watery extract which tasted salt
Carbonate and phosphate of lime,
traces of a jelly, which can be used
as gum. Phosphoric acid can be
had from the ash
Carbonate and phosphate of lime,
bitter earth and iron, a jelly which
can be used as gum
Carbonate of Hme 72 parts, phos-
phate of lime 2, jelly 3, water and
loss 23
Carbonate of lime 89-6 parts, phos-
phate of lime 5-7, animal substance
4-7, traces of sulphur. As a hen lays
130 eggs in six months and as an
egg weighs on an average 58-117
grams, 7486-226 grams of solid must
be used for egg-production in that
time, i.e. since the shells would
weigh 64-685 gm., 7333-793 gm.,
14 pounds 15 ounces 7 gros 8
grains. The secretion of the lime is
probably accomplished by means
of the kidneys
Carbonate and phosphate of lime,
and jelly
Very much carbonate of lime, very
little phosphate. Traces of phos-
phate of iron, earthy carbonates,
rnuriates, albuminous and gela-
tinous substance to hold it together.
I cannot find any uric acid in it, as
Vauquelin says is there, nor is he
right in saying that the sulphur is
in the shell — it is in the membranes
only, and under the form of sul-
phuric acid
Consist of an animal material
Have the properties of the fibrous
part of blood
A jelly-like material, soluble in hot
water
An animal substance with traces of
phosphate of lime, carbonate of
lime, muriates, and a sulphurous
body
An albuminous substance containing
traces of sulphur and soluble in
caustic potash
Macquer
Leonhardi
Neumann
Berniard
Hatchett
1 781
1780
1800
Merat-Gaillot —
Vauquelin 1 799
Fourcroy
John
1811
Macquer
Jordan
Fourcroy
John
178:
1811
Vauquelin —
15
226 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii
EXCERPTS FROM J. F. JOHN'S CHEMISCHE TABELLEN OF 1814 {cont.)
Substance or liquid
investigated
Chalazae
Egg-white
Yolk-membrane
Yolk
Egg (Snipe, Tringa vanellus)
Shell
Composition
An agglutinative substance insoluble
in water, apparently like dried tra-
gacanth gum
A white lymphatic transparent sticky
slimy material
Soda, albuminous matter, water,
sulphur
Water, albuminous matter, with
some free alkali, phosphate of lime,
muriate of soda, and sulphur
Contains benzoic acid
Water 80 parts, uncoagulable sub-
stance 4-5 parts, albuminous matter
15-5 parts, traces of soda, sul-
phuretted hydrogen gas, and ben-
zoic acid
Contains sulphur
Water, albuminous matter, a little
jelly, soda, sulphate of soda, muriate
of soda, phosphate of lime, oxyde
of iron (?)
An oxydised albuminous substance
Apparently an albuminous substance
Consists of a lymphatic material and
a fatty oil
Water, oil, albuminous matter, jelly
Water, oil, albuminous matter, jelly,
phosphates of lime and soda, with
other salts
Water, oil, albuminous matter
Water, a mild oil, albuminous matter,
a colouring matter which is perhaps
iron
Water, a yellow mild oil, traces of free
(phosphoric?) acid, a small amount
of a reddish-brown material, not
fatty, and soluble in ether and warm
alcohol, a jelly-like substance, a
great deal of a modified albuminous
substance, and sulphur
Egg (lizard, Lacerta viridis)
Yolk
White
Egg (fish, salmon)
Is composed of the same constituents
as that of the hen, but the dark
green pigment and the dark brown
splashes are probably oxyde of iron
A yellow oil, an albuminous material,
and salts
Diflfers from that of fowls in being
granular and greasy when hardened
by boiling
420 grains contained of pure dry
albuminous matter 26 grains, of a
viscous oil 18 grains, insoluble al-
buminous matter 102 grains, mu-
nvestigator
John
Date
Macquer
—
Jordan
—
Fourcroy
—
Proust
Bostock
—
Scheele
John
Fourcroy —
John —
Macquer 1781
Thomson —
Hatchett —
Jordan —
Fourcroy —
John
i
John
John
John
John
SECT. 3] AND EIGHTEENTH CENTURIES 227
EXCERPTS FROM J. F. JOHN'S CHEMISCHE TABELLEN OF 1814 {cont.)
Substance or liquid
investigated Composition Investigator Date
riate of soda and sulphuric alkali
28 grains, jelly, phosphate of lime,
and oxyde of iron 2 grains, water
242 grains
Egg (fish, Cyprinus barbiis) Contains a substance dangerous for Crevelt —
man, the nature of which is unknown
Egg (insect, Locusta viridissima, and migratoris)
Shell An animal combustible substance John —
and phosphate of lime
Contents Albuminous matter, a yellow fluid John —
fatty oil, a little jelly and a charac-
teristic substance, acid, phosphates,
and sulphuric alkali
The most interesting of the investigators in this table is Dzondi,
whose work in 1806 was the first in which definite chemical charac-
teristics were systematically followed throughout embryonic develop-
ment. It is surprising that so long a time should have elapsed between
Walter Needham and John Dzondi: no less than 139 years.
After 1 8 14 events were to move so rapidly in the world of science
that it would not be possible to follow all the embryological work
that was done, and at the same time maintain the proper proportion
between the historical part of this book and the other parts. The
eighteenth century was the period during which the chemical side
of embryology began to differentiate and split itself off from the rest.
After 1 8 14 it pursued a course of its own, the individual tracks of
which I shall mention under their appropriate heads. But another
century had yet to pass before the value of the physico-chemical
approach to embryology could become generally recognised, and we
are ourselves only at the very beginning of this new period.
A certain contrast may appear between the critical treatment
which I have given to the investigators whose work I have been
discussing, and the saying of William Harvey's — "all did well",
which stands prefixed to this Part of the book. Yet history without
criticism is a contradiction in terms, and the praise and dispraise,
which I have tried to allot as accurately and justly as I could, is,
as it were, technical, rather than spiritual. All the workers who have
been mentioned, and others besides them who left no special marks
on their time, are worthy of our respect and of our fullest praise,
for they preferred wisdom before riches and, according to their
several abilities and generations, diligently sought out truth.
15-3
PART III
GENERAL CHEMICAL EMBRYOLOGY
All things began in order, so shall they end, and
so shall they begin again, according to the ordainer
of order and the mystical mathematicks of the city
of heaven.
Sir Thomas Browne.
PRELIMINARY NOTE
There have already been certain reviews of work in chemical embryo-
logy as a whole, among which those of Paechtner and Schulz are
the most valuable. The former dealt almost exclusively with the
chemistry of the egg from a static viewpoint, and only devoted a
short section to the metabolism of the embryo during its develop-
ment, while the latter, though dealing specifically with embryonic
metabolism, gave hardly more space to it than Paechtner. In both
cases the discussion was little more than a catalogue of references,
and in neither case was the literature anything like complete, in-
cluding, indeed, less than a tenth of the relevant citations.
The first review of chemical embryology was written by Grafe in
19 10, but, though he outlined several valuable ideas, it is now of
small importance. Good information may, however, be found in
Aron's monograph on the chemistry of growth and on the mammalian
side there are Harding and Murlin. Other, less satisfactory, reviews
are by Cazzaniga and Steudel. Finally there is, of course, an
immense amount of work which can be found in no review, for
investigators have followed the counsel of Godlevski (1910): "Un-
sere Kenntnisse hinsichtlich der chemischen Zusammensetztung der
Eier noch lange nicht ausreichend sind, so waren weitere Forschungen
auf diesem Gebiete auch aus dem Grunde sehr erwunscht weil sie
den Ausgangspunkt fiir die Physiologic des embryonalen Stoff-
wechsels welcher bisher gleichfalls nur sehr wenig untersucht wurde,
bilden mussen".
Every effort has been made to give an accurate and complete
presentation of the data in the Tables of this book and of the
experimental conditions under which they were obtained, but
investigators should always consult in addition, whenever possible,
the relevant original memoirs referred to in the Bibliography.
SECTION I
THE UNFERTILISED EGG AS A
PHYSICO-CHEMICAL SYSTEM
I -I. Introduction
In giving an account of the present state of our knowledge about the
chemical constitution of the egg-cell and the food-material which is
accumulated around it or inside it, I shall not follow a strictly logical
order of exposition, according to the phyla of systematic biology.
I have judged it best to begin with the egg of the hen, for not only
is it the most familiar and the best known of all eggs, but it is also
the one which has been most thoroughly investigated biochemically.
It should be remembered that the two main morphological divisions
of the egg, (a) the egg-cell itself and {b) its coverings, appear in
protean modifications throughout the animal kingdom. The former
may be a simple cell with its ooplasm, nucleus, nucleolus, etc., as in
the echinoderms, and no covering at all save its cell-membrane, or
at the other extreme it may be swollen up with food-material or yolk
to the prodigious proportions of the avian egg-cell. The membrane
again may be a thin coat of investing cells such as the tunicate egg
possesses, or it may be the jelly of the amphibian egg, or, again, it
may be the complex arrangement of egg-white, chalazae, shell-
membranes, and shell, which is present in the bird's egg. All imagin-
able degrees of richness in yolk are present in the egg-cells of animals,
and upon this fact depend the various kinds of cleavage which they
show: alecithic eggs, on the one hand, such as those of most inverte-
brates, having a holoblastic form of development in which the whole
egg participates in cleavage; and yolk-rich eggs, on the other hand,
such as those of most vertebrates, having a meroblastic development,
only a localised part of the egg undergoing cleavage, the rest remaining
as a sac full of yolk until it is finally absorbed.
1-2. General Characteristics of the Avian Egg
After the historical introduction which has been given, it should be
unnecessary to remark on the general arrangement of the bird's egg.
We have with Harvey referred to it as an exposed, and, as it were,
detached uterus, and with Fabricius ab Aquapendente we have
SECT, i] THE UNFERTILISED EGG 233
enumerated the parts of the typical avian ovum. Fig. 13, however,
shows the general disposition of parts diagrammatically.
First, as to size and shape. The size and shape of the egg were shown
by Curtis in 191 1 and by Surface in 191 2 to be due partly to the
structure of the oviduct, which very probably may be considered an
inherited character, as was claimed by Newton. D'Arcy Thompson's
discussion of the mechanics of egg-formation in birds, in his Growth
" White or Milky Yolk
\ Germinal Di^c .Pander's Nucleus
Shell -Membranes ^ ^ ' t- _ , ^
Shell
Latebra
Chalazae ^^^^^^'^ ' "^ ...^'^ ^
(Treadles, HailstOTie^r^^^::;^^^ I ^ ^^^^^^'^ ^, \
y, ^^=^^^^^— 1-______-^^^^^'^ Chalazae
/■ «r~ I ^; ^
Vitelline Membmne Haloes or ^
Layers of Yellow Yolk ^Whi te
Fig. 13. Diagrammatic representation of the hen's egg. The chalazae were called by
Tredern Ligamenta albuminis. Bartelmez gives a discussion of the factors governing
the angle which the embryonic axis makes with the axis of the egg as a whole. The
yolk is not a perfect sphere but lengthened along the main axis. The egg-white
is divisible into three layers which increase in density from without inwards. The
chalazae, as Berthold was the first to find, are not present in reptilian eggs.
and Form, will be famiHar, but some biologists, such as Horwood,
have taken exception to his conclusions about the physical influences
which shape the egg. Ernst's well-known experiment was the starting-
point of these discussions ; she caused hens to lay on a surface of wet
sand and charcoal, and so, observing the process, found the blunt end
to be blackened. This was in agreement with many other observers,
such as V. Nathusius; Landois; Jasse; Konig-Warthausen and Erd-
mann; and d'Arcy Thompson accordingly described the hen's egg
as moving down the oviduct blunt end forwards, the pointed end
owing its form to the peristaltic compression of the oviduct. Unfor-
234 THE UNFERTILISED EGG AS A [pt. iii
tunately all observers agree (Purkinje; von Baer; Coste; Kiitter;
Taschenberg; Wickmann and Patterson for the hen, Blount and
Patterson for the pigeon, Kiitter for the hawk, and Wickmann for
the canary) that the pointed end passes first down the oviduct. It
appears that the egg must turn right round in the act of being laid,
and Bartelmez, indeed, has seen this occur. Curtis has shown that
the shape of the egg depends to some extent upon its size and this
biometric observation was afterwards confirmed by Pearl & Curtis.
Many abnormalities have been reported in eggs. They need merely
be mentioned here with their authorities, thus :
General.
Hargitt.
Henneguy.
Bartelmez.
Eggs containing masses of tissue^ more or less organised.
von Nathusius.
Gliicksmann.
Benjamin.
Dwarf eggs.
Pearl & Curtis.
Fere.
Benjamin.
Ovum in ovo.
Herrick.
Chidester.
Weimer.
Rosnatovski.
Camerano.
Pearl & Curtis.
Racah.
Benjamin.
Roberts & Card.
Hilden.
Double and triple-yolked eggs.
Hargitt.
Parker. '
Patterson.
Glaser.
Curtis.
Inadequate shell.
Riddle & King.
Dwarf or absent yolk (ovum centennium^).
Mercier.
Szuman.
Bugnion.
Gelabert.
^ See Sir Thos. Browne, Pseudodoxia Epidemica, Bk iii, ch. 7, "Of the basilisk". The eggs
of Chelonia also, according to Deraniyagala, are sometimes laid without yolks.
SECT, i] PHYSICO-CHEMICAL SYSTEM 235
It is interesting in this connection that Riddle has traced the
occasional production of eggs with deficiency of white and shell but
not of yolk, to a lack of the thymus hormone which he has called
"Thymovidine". Feeding with desiccated thymus removed com-
pletely these effects. "The whole of the data", he said, "seem to
demonstrate the presence in the thymus of a substance having a highly
specific action on the oviduct of birds — and presumably on that of all
those vertebrate animals which secrete egg-envelopes." The syndrome
involved eggs with normal yolks but hardly any shell or albumen,
frequent reduction of normally paired ovulations to single ovulations,
diminished fertility, and restricted hatchability of the eggs. "Though
not necessary to the life of the individual", said Riddle, "thymo-
vidine would seem to be essential to the perpetuation of those verte-
brate species whose eggs are protected by egg-envelopes. Such
animals were the ancestors of mammals and thus mammals could
hardly have come into existence without the thymus." These con-
siderations are of much interest in view of other speculations on the
evolutionary aspect of chemical embryology, e.g. Section 6-6. They
also suggest that the mammalian thymus is now a vestigial organ.
The air-space, the shell and the white of the normal egg need no
special remark at present, but the yolk is a more complicated structure.
Around a central core of "white" or "milky" yolk the yellow yolk is
secreted in the ovary of the hen in concentric layers, which form the
appearance of "haloes" in the finished egg, and which show up es-
pecially clearly when the hen is fed on Sudan III or some other non-
toxic dye which has a selective staining action on fat. The white yolk
in the centre is continued in a flask-like shape (the latebra) up to the
surface of the yolk underneath the germinal disc, and is then con-
tinued in a very thin layer all round the exterior of the yolk under-
neath the vitelline membrane. The white yolk is thus the first
nourishment of the embryo. It is not certain whether there are
also layers of white yolk between the concentric layers of yellow yolk,
for they have never been analysed chemically, and Balbiani main-
tains that they only differ from the yellow layers by having less yellow
pigment. The differences between the true white yolk and the yellow
yolk are, as will be seen later, far more profound. Balfour &
Foster, in their Elements of Embryology of 1877, described the yellow
yolk as consisting histologically of spheres of from 25 to loo/x in
diameter, filled with numerous minute highly refractive granules and
236
THE UNFERTILISED EGG AS A
[PT. Ill
very susceptible to crushing and rough treatment. After boihng, the
spheres assume a polyhedral form. The granules seen within them must
consist of protein, for they are not soluble in ether or alcohol. On the
other handjthe white yolk elements are vesicles smaller than the globules
of the yellow yolk, being about 4 to 75 /n across, with a highly refractive
body, often as small as i [x, in the interior of each. These vesicles are
sometimes collected together into much larger vesicles. They observed
also underneath the blastoderm or the germinal disc a number of
large vacuoles filled with fluid — large enough, in fact, to be seen with
the naked eye. The histology of yolk has been reviewed by Dubuisson,
and at one time many papers were published on it, e.g. those of
Virchow. They cannot be considered in detail here.
1-3. The Proportion of Parts in the Avian Egg
Of the weight of the whole egg, the shell takes up about 10 per
cent., the albuminous white 50 per cent, and the yolk 30 per cent,
in round numbers. These relationships have been determined by a
multitude of investigators, whose results are drawn up in Table i .
Table i . Distribution of the parts in the egg,
Italic figures represent dry weight only.
Average
egg weights
Shell
White
Yolk
Investigator
Species
(gm.)
(%)
(%)
(%)
and date
Hen, Polish i
43-6
IO-34
—
—
van Hamel-Roos ('.890)
,, Polish ii
56-3
9-75
—
—
5>
,, Holland (Zwol.)
43-8
9-21
—
—
5>
„ Holland (Tiel)
62-6
970
—
—
,,
Duck
5688
9-72
49-50
40-78
Miinster Ag. Sta. (1900)
Hen
—
9-39
57-50
33-11
Drechsler (1896)
Duck
—
10-38
58-28
31-34
,,
Hen
—
lo-i
59-7
302
Plimmer (1921)
Duck
—
I i-i
51-0
37-9
,,
Hen
56-70
10-9
57-00
Langworthy (190 1-2)
Duck
13-7
—
35
Goose
157-5
14-2
—
—
J>
Turkey
13-8
—
—
5J
Guinea-fowl
—
16-9
—
—
53
Plover
—
9-6
—
—
Hen
50-50
10-89
58-42
30-69
Lebbin (1900)
,,
11-47
5833
30-20
Welmanns (1903)
Goose
158-9
11-30
53-40
3530
Segin (1906)
Duck
67-74
11-40
53-20
35-40
Liihrig (1904)
Hen
52-50
12-70
54-40
32-90
,,
Hen (various breeds)
55-58
10-21
55-63
34-15
von Czadek (191 7)
Hen
10-00
54-38
35-62
Rose (1850)
,,
—
—
33-82
66-i8
J,
Hen (various breeds)
—
10-47
5607
33-46
Carpiaux (1903)
Hen
—
—
54-70
35-30
Lehmann (1850)
»
—
—
3230
67-70
Prout (1859)
>j
—
—
37-30
62-70
Poleck (1850)
PLATE IX
YOLK OF HEN'S EGG AT THE TIME OF LAYING
Stained by Kossa's method for the detection of calcium phosphate. The considerable
variations in the vitelline globules may be noted. Magnification, 6xD: prepared and
microphotographed by Dr V. Marza.
I
I
SECT. l]
PHYSICO-CHEMICAL SYSTEM
237
Table i {cont.)
Averaee
egg weights
Shell
White
Yolk
Investigator
Species
(gm.)
(%)
(%)
(%)
and date
Hen, Leghorn
58-81
9-21*
—
—
Murray (1925)
Hen (various breeds)
50-65
1000
60-00
30-00
Iljin (1917)
„
—
—
30-0
6o-o
>>
Hen
60-5
12-40
56-20
31-40
van Meurs (1923)
,,
560
12-50
55-50
32-00
j>
55
^5-37
27-50
22-30
50-00
Voit (1877)
Nidicolous birds
Gull
—
—
76-20
2380
Tarchanovf (1884)
Starling ...
—
—
79-20
20-80
JJ
Robin
—
—
76-60
23-40
5>
Nightingale
—
—
75-00
2500
JJ
Canary ...
—
—
73-60
26-40
JJ
Thrush ...
—
—
74-40
2560
J J
Raven
—
—
8000
20-00
Corncrake
—
—
85-90
14-10
JJ
Dove
—
—
78-30
21-70
JJ
Nidifugous birds
Plover
—
—
61-40
38-60
JJ
Quail
—
—
60-00
40-00
JJ
Hen
—
—
64-30
35-70
JJ
Guinea-fowl
—
—
54-50
45-50
JJ
Turkey ...
—
—
5830
41-70
JJ
Duck
—
—
58-30
41-70
JJ
Goose
—
—
47-40
52-60
)J
Pigeon
17-78
(grains)
6-75
71-10
22-15
Glikin (1908)
Hen
764-5
10-13
64-51
25-21
Davy (1863)
—
—
wig
55-00
JJ
Jay
127-3
5-03
68-11
26-86
JJ
—
—
1280
30-40
JJ
Hedge-sparrov^'
34-5
5-79
72-46
21-45
JJ
—
—
12-20
58-10
3 J
Golden-crested wren
143
4-90
71-05
24-05
) )
—
—
9-90
43-60
,,
Robin
38-8
5-43
70-33
24-22
,,
»
73-8
2-71
75-20
22-o8
J,
8-20
16-50
J J
Missel-thrush
124-6
5-13
81-30
12-84
J J
Starling
115-1
7-12
78-62
14-25
Pigeon
2780
8-05
73-95
17-94
,,
—
11-30
39-30
"
Hen
(gm-)
57-57
11-6
56-8
31-6
Hartung (1902)
»>
11-9
550
33-1
Voit (1881)
,,
60-18
Fere (1896)
Duck
73-90
—
—
JJ
Hen
5838
10-30
89
70
Pott & Preyer (1882)
Rhea
707-0
10-45
89
55
Rozanov (1926)
Goose
1330
12-72
63-14
36-86
Hepburn & Katz (1927)
Duck
80-3
10-58
61-92
38-08
)j
Hen
889
61-41
29-7
Baudrimont & de
St Ange (1846)
Dwarf hen ...
—
16-88
47-40
35-71
Sacc (1847)
Hen
49-92
—
Pott (1879)
jj
11-2
88
8
Atwater & Bryant ( 1 906)
±0-11.
■f- All Tarchanov's figures exclude the shell weight.
238
THE UNFERTILISED EGG AS A
[PT. Ill
Table i
{cont.)
Weight of
egg-con-
%of
Shell
White
Yolk
Investigator
Species
tents (gm.)
whole
(%)
(%)
(%)
and date
Hen
• 57-12
87-94
12-06
56-22
31-72
Friese (1923)
Goose
• 137-38
8595
14-05
5323
32-72
Duck
781 1
88-05
11-95
50-03
38-02
Turkey
• 92-93
89-23
10-77
56-72
32-51
Dove
21-88
90-40
9-60
72-35
18-05
Seagull
4040
91-16
8-84
64-28
26-88
Plover
• 25-40
91-54
8-46
50-71
40-83
Guinea-fowl
41-14
83-40
16-60
44-02
39-38
Pheasant
. 27-03
90-12
9-88
52-79
37-33
Blackbird ...
6-98
91-69
8-31
59-46
3233
Starling
2-98
90-27
9-73
Canary
1-82
95-05
4-95
58-59
36-26
Weight of
egg-con-
Shell
White
Yolk
Investigator
Species
1
ents (gm.)
(%)
(%)
(%)
and date
Plover ( Vanellus crist
atus)
22-75
7-5
27-5
68-0
Bauer (1893-5)
Hen {Gallus domestict
is)
48-0
120
55-0
33-3
»>
Guinea-fowl {Meleag
ris gallopavo)
73-0
10-9
58-5
29-5
Swallow (Hirundo ru.
stica)
1-36
11-8
54-0
19-3
Partridge [Perdrix ci
nerea)
13-1
12-2
50-0
37-0
Sparrow {Passer dom
esticus)
2-7
—
—
—
Thrush (Turdus
? ) ...
6-4
14-4
56-0
37-0
Duck (Anas) [doubl
E]
101-6
9-6
49-3
35-5
The above data were all obtained without any ad hoc investigation of the probable
errors involved in weighing eggs and parts of eggs. An elaborate study by M. R. Curtis
in 1 9 1 1 gave the following results on Gallus domesticus :
Actual
weight in gm. %
56-04 100
33-22 59-26
16-31 29-14
6-28 ii-i8
0-23 0-42
Whole egg
Albumen
Yolk
Shell and membranes
Error
But though this is the case with the egg in its natural state, the solid
matter is concentrated much more in the -yolk than in the white,
so that, as the analyses of Poleck and Iljin, for instance, show, for
dry weight the conditions are exactly reversed. The egg-white may,
indeed, be regarded as the principal reservoir of water for the embryo
which develops on dry land, and this is a point which will be dis-
cussed later (see Section 6-6). The eggs of different breeds of hen
vary to some extent in the relative weights of shell, white and yolk;
but, although it is difficult to lay down any general rule, these varia-
tions do not greatly exceed the variations due to factors connected with
the individual hen. Iljin's lightest shells make up about 7 per cent,
of the G,gg weight and the heaviest not more than 11-5 per cent.
SECT. I] PHYSICO-CHEMICAL SYSTEM 239
It is certain that there are constant differences between the eggs of
different breeds, but as a whole these are quite outweighed by
individual differences, and only appear when extended statistical
studies are undertaken. The eggs of other birds, however, do not fall
within these limits. Langworthy, for example, has shown that, in
the duck's egg, the shell may account for as much as 14 per cent,
of the whole weight. A similar result was found for the turkey and
the goose, while the guinea-fowl's egg has a shell of nearly 1 7 per
cent, of the whole weight. The wide series of Friese, shown in Table i,
seems to indicate that the larger the egg the more shell it has to
have : thus the canary's egg weighing just under 2 gm. has 4 per cent,
while the goose's egg which weighs 137 gm. has 14 per cent. Heinroth,
and Groebbels & Mobert, among others, have collected a great
many data of this kind for all varieties of bird, but their papers must
be referred to for the figures. Thus the fertilised embryo starts its
development on the surface of a mass of food only slightly diluted
with water, and surrounded by a further and much wetter supply.
This is reflected well by the work of Bellini, who found that the yolk
of the hen's egg was seven times as viscous as the white at the begin-
ning of development. (Alb. 3-4 units, yolk 28-5 units.)
A good deal of work has been done on the variability of the
weights of the parts of the egg within a given species of fowl. Thus
Jull found that egg weight is the least variable factor, albumen
weight slightly more variable than egg weight, yolk weight con-
siderably more variable than albumen weight, and shell weight the
most variable. It would seem, therefore, as if a compensatory process
takes place during egg-production, the largest yolks having the
smallest whites, since the weights of the entire eggs do not vary as
much as the weights of the components. On the other hand, the
smaller eggs contain the highest percentage of albumen and shell
and the lowest percentage of yolk. Jull also studied closely the
seasonal variations, which may be quite considerable, finding that
the component parts of the egg contribute in different degrees at
different times of the year towards the total egg weight. The question
as to which part of the egg is mainly responsible for large or small
eggs is still debated, for Curtis concluded from his observations that
it is the egg-white, while Atwood found many indications contrary
to this. Statistical studies on the egg of the tern have been made by
Rowan, Parker & Bell; Rowan, Wolff, Sulman, Pearson, Isaacs,
240
THE UNFERTILISED EGG AS A
[PT. Ill
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SECT. l]
PHYSICO-CHEMICAL SYSTEM
241
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16
242 THE UNFERTILISED EGG AS A [pt. iii
Elderton & Tildesley; and by Watson, Watson, Pearson, Karn,
Irwin & Pearson.
The division of birds into the two classes of nidicolous (those
which hatch as squabs) and nidifugous (those which hatch as downy,
feathered and active chicks) has been shown to extend to the com-
position of their eggs by several investigators. Davy found that the
eggs of the nidicolous birds had thinner and more fragile shells,
which took up a less proportion of the weight of the whole egg than
the shells of nidifugous birds. Thus the wren's egg-shell weighs only
5 per cent, of the whole egg weight, while the hen's weighs lo per
cent. Da\y's figures show very clearly that the main reservoir of
solid is the yolk and the main reservoir of water is the egg-white.
Tarchanov carried the matter further, and observed that the yolks
of nidicolous birds always formed a smaller proportion of the total
amount of material inside the egg than in nidifugous birds. Thus,
for the former class the egg-white accounts for about 78 per cent,
of the egg and the yolk for 22 per cent., while, in the latter class,
the egg-white accounts for about 55 per cent, and the yolk for 45.
All these differences are probably related to the shorter incubation-
time of the nidicolous eggs; and, as will be seen later, there are not
wanting indications that the yolk of these is less tightly packed with
food-material and more rich in phosphatides.
1*4. The Chemical Constitution of the Avian Egg as a whole
The composition of the egg as a whole is further considered in
Table 2, where it is noticeable that the analyses of water and ash
have not been significantly improved upon between 1863 (the date
of the first analysis, Payen's) and the present time. The later figures
for protein and fat are, however, much the more reliable. It should
be observed that there is an approximately equal quantity of fat and
protein at the disposal of the embryo, though the former is, of course,
in the yolk, and the latter is preponderantly in the egg-white. This
protein-fat equality is by no means the rule in all eggs, and, as we
shall see later, the eggs of fishes depart widely from it. There appear
to be only small differences between the eggs of different kinds of
birds in protein content. At one time it was thought that the duck's
egg was particularly rich in fat, on the authority of Commaille's
analyses, but Liihrig has since then brought it into line with all the
others. It does seem, however, to have a considerably higher per-
SECT.
I]
PHYSICO-CHEMICAL SYSTEM
243
centage of mineral substances than the rest. The dry-weight figures
merely demonstrate again the approximate equality of the protein
and fat.
Before we proceed to consider the parts of the tgg in separation,
the question of individual and racial differences must be taken up
Table 3. Individual differences between hen's eggs,
Malcolm's figures (1902). Averages of individual hens.
Breed unknown
Italian hens (fed on maize and barley)
From one hen
From one hen
Iljin's figures (19 17).
Houdan...
Orpington
Plymouth Rock
Rhode Island ..
itty acids
Nitrogen
P.O5
27-79
300
1-49
29-45
2-62
—
28-50
2-65
1-36
28-66
2-67
1-53
31-89
2-68
1-40
28-96
3-12
I 52
29-11
2-87
1-51
30-08
2-67
1-57
30-12
2-82
1-50
29-56
2-84
1-65
31-04
2-57
1-39
31-57
2-55
1-42
3043
2-61
1-44
30- 1 2
2-75
1-48
30-26
2-75
1-51
In
% dry weight
Fat
Lecithin P
P2O5
56-79
0-33
1-13
56-68
0-57
I-IO
57-17
0-71
I-I3
59- 1 6
062
I-IO
Table 4. Race differences in hen's eggs.
Leveque & Ponscarme's figures.
Mean
% of egg d.
"y weigh
weight
gm.
dry weight
A
>
of whole
A
.P2O5
N in
r
^
i> 111
egg
Shell
White
Yolk
in yolk
white
Andalusia ...
64-7
9-14
7-05
16-32
2-91
18-26
Berrichonne
66-3
8-06
8-18
14-10
3-04
18-42
Bressane ...
60-8
8-41
8-00
13-15
2-88
18-55
Coucou de Rennes
60-3
IO-66
7-41
17-07
3-02
18-50
CreveccEur
62-5
8-86
8-95
12-73
2-93
18-14
Dorking ...
52-4
9-09
9-47
15-57
2-99
^^'^.
Faverolles ...
563
884
8-31
14-80
2-90
18-46
Houdan
57-8
8-95
8-o6
13-76
2-99
18-48
La Fleche ...
62-2
9-20
823
14-70
302
18-53
Minorca
63-6
8-99
8-17
14-98
2-96
18-60
Orpington
57-3
9-03
7-75
15-23
2-86
18-36
16-2
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246 THE UNFERTILISED EGG AS A [pt. iii
again. Malcolm maintained in 1902 that, although there were un-
doubtedly differences between the eggs of different breeds of hen,
they did not exceed the amount of variation between individual eggs
from hens of the same breed. His results are shown in Table 3.
Thus, although the feeding was carefully controlled, the eggs
from one hen might show a difference of i'i4gm. of fat, while
between two eggs from different breeds the difference might be only
0-13 gm. His conclusions were supported in the main by Carpiaux;
Leveque & Ponscarme ; von Czadek ; Iljin ; and Willard, Shaw,
Hartzell & Hole; who made very long studies of a considerable
number of breeds. Some of the figures obtained by von Czadek
and Iljin are given in Table 5. Von Czadek studied the Sulmtal,
Minorca, Orpington, Rhode Island, Faverolle, and Wyandotte
races, together with an Italian and a Rhineland breed. His outside
values for egg weight, for instance, were 43 gm. and 75 gm. —
a considerable difference — but the former was from the Rhineland
hen and the latter from the Minorca variety. The span showed great
variations, thus an egg weighing 55 gm. might be a heavy Orpington
or a rather light Faverolle or a medium weight Italian. The only
breed which stood well out of the range of individual differences was
the Minorcas which laid very heavy eggs. Certain instances have
shown, however, that remarkable agreement may exist between work
done on eggs of widely different breeds. Thus the classical work of
Plimmer & Scott on the phosphorus metabolism of the developing
chick was confirmed very strikingly by Masai & Fukutomi, who
worked in Japan. Here the correspondence was almost numerical.
But on the other hand there is evidence that eggs of different breeds
differ not only in their gross characteristics, but also as regards more
subtle properties; thus Moran has demonstrated that eggs from
different breeds of hen vary very greatly in their resistance to cold,
so that the viability is different, and Needham, working on the inositol
metabolism of the embryo, observed differences between the embryos
from Black and White Leghorn hens. Physico-chemical differences
between breeds of silkworm eggs are enumerated by Pigorini.
The individual differences between eggs may be equally important.
Benjamin has shown that there are numerous variable factors which
modify the constitution of the egg. The amount of yolk, egg-white,
and water, as well as the thickness of the shell, vary according to
the season, diet, age (Riddle) and general condition of the bird
SECT, i] PHYSICO-CHEMICAL SYSTEM 247
in question. Nor are such comparatively slowly changing factors
the only ones which bring about differences between individual
eggs ; the time the egg takes to pass down the oviduct, for instance,
will materially affect the amount of albumen it contains, and
such variable quantities as the blood-sugar level (Riddle) and the
level of cholesterinaemia in the parent animal will exercise their
effects upon the resulting egg. Again, the length of time elapsing
between the laying of the egg and the beginning of incubation will
have a marked efTect, for a certain amount of water will evaporate
from the egg-contents through the shell, and just how much does so
will depend on the humidity of the surrounding atmosphere. The
process of water-absorption by the yolk (Greenlee) from the white
will also be affected by these conditions, so that the embryo at
the initiation of its incubatory development may find a remarkably
inconstant set of circumstances in its immediate environment. More-
over, a certain amount of development always takes place in the egg
after fertilisation as it passes down the oviduct, so that the embryo
has already gastrulated by the time that the egg is laid by the hen.
It was the ignorance of these facts which led Malpighi, as we have
already seen, to his erroneous conclusions, for if he had known of
the phenomenon of "body-heating", as it is called by the poultry-
farmer, he would not have put forward the preformation-theory, and
the eighteenth century would have been spared the trouble of getting
rid of that embryological phlogiston. Thus no two eggs are ever
exactly the same age, and as there is reason to believe that enzymic
action begins in the yolk, if not in the white, very shortly after
fertilisation, this fact makes it additionally difficult to get precise figures
for the constitution of the unincubated egg. Then the position of the
egg in the clutch (whether first or second) in pigeons may, according
to Riddle, make a difference of 9-15 per cent, in yolk weight. It
may be concluded that nothing short of the greatest caution must
be employed in the material which is used for chemico-embryological
researches on the hen's egg. The individual hens should be marked,
and the eggs produced by them should be noted, their food should
be constant in composition and the breed used should be not only
single in any one series of experiments, but also, if possible, genetically
pure. It is very greatly to be wished that standard hens could be
obtained, such as the standard rats necessary for feeding experiments,
and much further work, with a proper statistical backing, is needed
248 THE UNFERTILISED EGG AS A [pt. iii
on the range of individual and racial variations in all the properties
of eggs.
The effect of the diet of the hen on the chemical composition of
the egg has been studied by various workers, notably by Terroine
& Belin. Except in certain respects, it showed a remarkable fixity
of composition :
Table 6.
Ordinary Corn and potato Hemp seed
mixed ration almost ration
ration free from fats (fatty)
White in % of total weight ... 56-7 54-3 —
Yolk in % of total weight ... 31-3 34-0 33-2
Shell in % of total weight ... 11-4 lo-g —
White
Water % 87-8 87-4 87-4
Ash% 0-49 — —
Yolk
Water % 49-9 50-33 50-99
Ash% 1-48 — —
Total nitrogen % ... ... 2-67 — —
Total fatty acids % 28-4 26-6 2655
Unsaponifiable fraction % ... 1-85 — 2-08
Cholesterol % i-i8 1-58 i-ii
Lecithin P % — 0-425 0-434
Thus, although the character of the substances stored for the use
of the embryo can be varied considerably, as will be seen later, the
balance of them cannot. But the question is probably rather com-
plicated, for it has been shown by Dam that by feeding hens on a
ration rich in cholesterol, the cholesterol content of eggs can be in-
creased from 501 to 615 mgm. per cent, of the wet weight or roughly
by 22 per cent, of the original value. In another instance the
cholesterol rose from 476 to 560 mgm. per cent. This would not be
in disagreement with Terroine & Belin's figures, but it would be
a very desirable thing to make a detailed study of the limits of
variation of all the constituents of the egg, and to find out exactly
how different in chemical composition an egg can be from the normal
while retaining its hatchability. Klein regards the cholesterol output
of the hen in its eggs as showing a synthesis of that substance in the
parent body. Leveque & Ponscarme have stated that it was not
possible to show any effect on the eggs in eleven breeds of hen by
minor variations in the diet; and this was amply confirmed by Gross.
The ingenious and partially successful attempt of Riddle and
Behre & Riddle to make hens preserve their own eggs by feeding
them with hexamethylenetetramine, sodium benzoate, and sodium
SECT, i] PHYSICO-CHEMICAL SYSTEM 249
salicylate, may here be mentioned. Starting out from this practical
suggestion the work led to the discovery of a number of specific
effects of substances such as quinine on egg size and yolk size. Thus
Riddle & Basset found that alcohol markedly reduces yolk size in
pigeons, Riddle & Anderson found that quinine reduces egg size,
yolk size and albumen size but has no effect on the protein/fat ratio
of the egg, while Behre & Riddle found that the diminution of
albumen size under quinine bore more on the solids than on the
water and involved considerable reduction of the protein.
The elaborate investigations on the egg of the tern, already
mentioned, led to a significant correlation between abundance of
food and size of egg, and it is certain that the size of the hen's egg is
affected by its diet since the work of Atwood. There seems also to be
a seasonal fluctuation, the weight of the eggs increasing from July
to February and decreasing from March to June. These seasonal
fluctuations appeared distinctly in Atwood's data, and explain the
results of Curtis and of Fere. Rice, Nixon & Rogers and Riddle
found a definite relation between the amount of food consumed and
the number of eggs produced, both of these factors varying exactly with
the seasonal variation in the egg size. Fluctuations of a regular kind
seem even to occur each month, according to Hadley who observed
such changes in egg weight and number. According to Curtis the size
of the eggs increases as the laying bird matures, in the case of the hen,
and Pearson has observed similar variations in the case of the sparrow.
The genetics of egg production have been studied by Pearl and
Benjamin.
The relation between the egg weight and the chick weight at
hatching has been studied by Halbersleben & Mussehl and by
Iljin. The former workers found a quite consistent relation within
one breed between the weight of the egg before incubation and the
weight of the chick at hatching, the latter averaged 64 per cent, of
the former. After thirty-five days of post-natal life, however, the
slight advantage possessed by the chicks from the heavier eggs had
altogether disappeared. They also noted that, other things being equal,
chicks hatched from the more pigmented eggs (browner) weighed
slightly more than those hatched from the less pigmented ones.
Abnormally large and abnormally small eggs did not hatch as well
as those of medium weight. Iljin collected a great many figures but
his text contains no statistical analysis.
250 THE UNFERTILISED EGG AS A [pt. iii
Stewart & Atwood reported that chicks hatched from pullet eggs
were neither so large nor so vigorous as those hatched from the eggs
of hens two or three years old. Whether there is here a direct effect
on the chick of the age of the hen, or whether the effect is indirect,
due to the small size of the egg, may be well questioned.
What relations exist between the chemical constitution of the egg
and the percentage "hatchability" are at present obscure, owing
perhaps to the comparative crudity of our estimation methods. The
work of Pearl & Surface indicated definitely that differences in the
hatchability of eggs are determined by or associated with innate
differences in the individual hens which laid them, that these dif-
ferences are probably inherited, and that variations within rather
wide limits in certain environmental factors, e.g. the temperature,
during incubation, are of secondary importance in determining the
death or the hatching of the embryo. Hatchability of embryos would
appear then to be, like fecundity, a heritable character. The experi-
ments of Lamson & Card confirmed the conclusions of Pearl &
Surface, but although some physico-chemical mechanism is un-
doubtedly at work, these statistical studies gave no hint as to its
nature.
Dunn determined to probe further into it. In his first paper he
argued that if hatchability was associated with constitutional vigour,
it should show a correlation with such a value as the chick mortality
in the first three weeks of post-natal life. Experimentally this was
not the case, e.g. post-natal mortality remained the same, although
in two instances the pre-natal mortality was on the one hand ex-
tremely high (20-39 per cent, hatchability) and on the other hand
extremely low (80-100 per cent, hatchability). It therefore seemed
likely that mortality before and after hatching is determined by quite
different factors. The more specific influences operating in embryonic
life must doubtless be looked for in the physico-chemical constitution
of the unincubated egg.
Hays & Sumbardo, in a search for such influences, were able to
exclude statistically fresh weight, length, diameter, specific gravity,
shell thickness, outer and inner shell-membrane thickness, porosity
and imbibition of water from 25 per cent, salt solution. Other factors
which have been excluded are percentage of protein in the diet of
the laying hen (Rosedale), percentage of yolk-pigment (Benjamin),
evaporation rate of the egg (Dunn), yolk-fat percentage (Cross), egg-
SECT, i] PHYSICO-CHEMICAL SYSTEM 251
fat percentage (Cross), yolk-protein percentage (Cross), egg-protein
percentage (Cross), egg-phosphorus percentage (Cross), chick-
phosphorus percentage (Cross).
It appears, however, that the constitution of the egg-proteins may-
be influenced by the presence of unusual proteins in the diet of the
hen, and that this may influence hatchability. Pollard & Carr have
reported the results of feeding the following proteins to laying hens :
wheat, rye, corn, oats; kaffir, barley, peas, soya, hemp; buckwheat,
popcorn, sunflower seed.
The first group of four (all, of course, being fed alone) were very
efficient for the production of normal eggs; the second group (of
five) permitted the hens to lay eggs but the eggs were hardly hatch-
able at all, while the third group allowed of no eggs. Pollard &
Carr studied the egg-proteins in all cases and obtained evidence of
tryptophane deficiency in the second group, so that they concluded
that a minimum tryptophane content was essential for successful
development through hatching. It is unfortunate that their results
were never published in full.
The effect of sex on the chemical composition of the egg has been
discussed by Riddle. As is well known, in some, probably most,
animals, the male produces two kinds of spermatozoa which are not
equal in their prospective sex value, i.e. some which will give rise
to females and some which will give rise to males. In birds, on the
other hand, the dimorphism of the germs exists not in the spermatozoa
but in the egg-cells. The female produces two kinds of eggs of unequal
prospective sex value. Riddle found that pure wild species of doves
and pigeons were ideal material for studies on sex, since very abnormal
sex-ratios could easily be obtained from them, and his studies led
him to the view that sex was more a matter of metabolic level or
rate of protoplasmic activity than anything else. But what concerns
us here are the consistent differences which he was able to demon-
strate between male and female eggs.
Pigeons generically crossed, when not permitted to lay many eggs,
produce only males, but when made to lay many eggs produce first only
males, and eventually "under stress of overwork" only females. These
facts and their proper conditions having been ascertained previously
by extensive statistical investigations, the way lay open for the
chemical analysis of the two sorts of pigeon's eggs. 900 analyses
were made and more than 12,000 yolks weighed.
252
THE UNFERTILISED EGG AS A
[PT. Ill
Fig. 14, taken from Riddle, gives the differences diagrammatically.
A glance at it shows that the male-producing egg of the spring
contains less stored material than the female-producing egg of the
Sex conbrol and known correlations in pigeons
SPRING
AUTUMN
d cf cT d* cT ^ $ ^^ *? *$ ^9 •.>' ^ •-': "■
9 10 11 12 13 14
N? 1
N° <2
N° 3
N9 4
N? 5
N° 6
N? 7
N9 8
N° 9
N?10
.siji
N0I. shows comparabive size of eggs of Alba (A)
and Orienbalis (0)
Fig. 14.
autumn. The amount of water and ash present, on the other hand,
diminishes, and the rise indeed is mainly to be seen in the fat and
lipoid fractions and in the calorific value. Table 7 gives the figures
for one individual pigeon during 191 2. The differences are not
SECT. l]
PHYSICO-CHEMICAL SYSTEM
253
large, but they were invariably found. Another series of figures show-
ing the rise in calorific value during the course of the year and the
transition fi-om male to female eggs is given in Table 8. Here also
the increase is unmistakable. Within one clutch, also, the water-
content of the second egg is lower and the calorific value higher than
the first egg, which fits in very well with the fact that under normal
Date
May 26
28
June 7
15
26
July 3
5
15
17
23
25
2
4
13
25
15
17
29
I
Aug.
Sept.
Nov.
Dec.
Table 7. Effect of sex on pigeon's eggs.
Female Turtur orientalis x Streptopelia alba, no. 410 for 191 2.
% wet weight % dry
Analysed Weight t ^ ^ weight
or in- of Pro- Extrac- ale- Calories
cubated yolk Lipoid tein tives Ash Water sol. per egg Sex
An. 2-330 18-32 25-44 5-28 4-85 5701 72-65 7405 —
An. 2-660 17-54 25-63 5-25 2-62 54-82 72-45 8990 —
Inc. — — — — — — — — Male
Inc. — — — — — — — — Male
Inc. — — — — — — — — Male
An. 2-026 16-49 26-00 3-63 2-43 56-05 71-95 6714 —
An. 2-330 19-18 26-55 3-75 1-93 5522 72-27 7881 —
Inc. — — — — — — — — Male
Inc. — — _____ _ Male
An. 2-422 17-82 25-88 3-82 I -80 55-84 72-42 8061 —
An. 2-720 18-88 25-96 3-86 1-81 55-33 72-45 9296 —
Inc. _______ _ Male
Inc. — — — — — — — — Male
Inc. — — — — — — — — Male
Inc. — — — — — — — — Female
Inc. — — — — — — — — Female
Inc. — — — — — — — — Female
An. 2-700 21-40 — — — 55-45 73-17 9323 —
An. 2-715 21-63 — — — 55-39 73-02 9383 —
Table 8. Eggs from the same female Streptopelia risoria (1914).
Date
Weight of yolk Energy in cals.
June 6
i-oio
3358
8
0-970
3175
19
0-855
2807
21
I -000
3245
July 14
I-I45
3815
16
1-463
5008
Aug. 30
1-395
4812
Nov. 6
1-440
4837
8
1-720
5797
20
1-590
4906
22
1-780
6015
Dec. I
1-640
5614
3
1-820
6255
12
1-535
5302
14
1-690
5601
23
1-485
5266
25
1-718
5880
254 THE UNFERTILISED EGG AS A [pt. iii
conditions the first egg laid nearly always gives rise to a male and the
second to a female.
In Fig. 14 the line marked "developmental energy" implies that
a higher percentage of the male eggs hatch successfully than of the
female eggs. The data for length of life show the same curve. The
smaller eggs of both clutch and season are the eggs which give
positive results in strength and vigour tests, and the larger eggs are
those which are liable to display weakness. These facts are in entire
accord with the higher metabolic level which Riddle associates with
the small male eggs. It is interesting to note that Lawrence &
Riddle found consistently higher values for total fat and total phos-
phorus in the blood of female fowls than in that of male fowls, from
which they concluded that the metabolic differences between male
and female germs persist in the adult, and all these facts are in agree-
ment with the work of Goerttler and Baker on human and Smith on
crustacean blood-fat, and of Benedict & Emmes on sex differences
in basal metabolism. But for further discussion of the metabolic
theory of sex, the papers of Riddle must be consulted. Interesting
data on the hatchability, vigour, etc., of rotifer eggs are contained
in the paper of Jennings & Lynch, but these authors made no
chemical experiments.
To say, as Riddle does, that there are, as it were, two kinds of eggs
in some species, one male-producing, and the other female-producing,
may either be taken to mean that there are quantitative differences
between them or that their constituent substances are qualitatively
chemically different, or, thirdly, that the same substances in the same
quantities are differently distributed spatially and temporally. As will
be seen later in connection with the lipoids of mammalian egg-cells,
the second view finds supporters, and some such opinion is held
by Russo. Faure-Fremiet, in the course of his work on the egg of
Ascaris megalocephala, to which he applied every conceivable method,
examined a very large number of individual eggs in order to find
whether they separated at all chemically into two types. His method
was to centrifuge them separately, much as McClendon had done
with the frog's egg, and then to measure in mm. the thickness of
{a) the mitochondria layer, and {b) the fatty layer. Fig. 15 {a) taken
from his paper shows the frequency polygon which he constructed
on the basis of these results, the ordinate giving the number of eggs
measured, and the abscissa the thickness of the mitochondrial layer.
SECT. l]
PHYSICO-CHEMICAL SYSTEM
255
It is quite evident that there are not two modes on the line joining
the points, i.e. that there are not two types of eggs, but only one type.
Faure-Fremiet made very similar experiments, determining the gly-
cogen content of the eggs histo-colorimetrically with iodine solutions,
and there also the frequency polygon had but one mode (Fig. 15 (^)).
But this second case was based on an unsatisfactory method. In the
particular instance under investigation, neither mitochondria nor
30
20
12 3^5
30
20
\5
(«)
J :
<k 5 0 7 8
ib)
Fig. 15-
glycogen happens to be an entity which varies as between the two
kinds of eggs. Nevertheless, the plan of work was an interesting one,
and widely extended researches with it, using accurate chemical
methods, would be very desirable.
1-5. The Shell of the Avian Egg
Litde attention has been paid to the shell of the bird's egg from a
physiological point of view. The relevant analyses are given in
Table 9. There is some difference between the shells of different
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SECT. l]
THE UNFERTILISED EGG
257
birds, and it would be interesting, for example, to know why the
pheasant's contains such an unusually high percentage, of phos-
phorus, and why the herring gull's has so high a percentage of organic
substance. A certain interest attaches to the determinations of
Balland on ostrich eggs, some from a tomb
of the Hellenistic period and others from
modern ostriches, but the differences he
found were probably not very significant,
as the analysis of Torrance seems to give
values half-way between those of Balland.
Neither Balland, Torrance nor Wicke states
whether the ostrich used was the North
or South African variety, a complication
which might make a difference. Wicke
believed that the difference in shell-com-
position between different kinds of birds
was almost entirely dependent on their
usual foods.
The microscopic structure of the shell
was investigated by Nathusius in the 'sixties,
and since then little has been added to his
work. The shell consists of an outer layer of
crystals of calcium carbonate arranged with
their long axes perpendicular to the bound-
ing surface (Fig. 16), and an inner layer
composed of undifferentiated calcium car-
bonate (Herzog & Gonell). Kelly; Schmidt;
Meigen and Osawa have found that the
mineralogical form of the lime is invariably
calcite, no aragonite being present in any
bird's egg-shell. This has been confirmed
with X-ray analysis by Mayneord. The
ostrich, Emj>s europaea, is the only doubtful case, for Kelly identified
its egg-shell lime as conchite, but Torrance considers it to be calcite.
Prenant's review should be consulted for further details regarding
this interesting biochemical problem. Only one paper exists dealing
with the changes which the shell undergoes histologically during the
development of the chick; it will be considered in the section on em-
bryonic respiration, where the data we possess on the question of the
NEi 17
-^:- -;>.,«■ j
Fig. 16. a, Outer crystalline
layer; b, c, d, amorphous
layers; e, mamillae; /, shell-
membrane.
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PHYSICO-CHEMICAL SYSTEM
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26o THE UNFERTILISED EGG [pt. m
permeability of the shell and its membranes will also be dealt with.
There has been some controversy on the subject of whether the egg-
shell contains any elements of the secreting organ in it, like the decidua
of mammals. Von Baer thought not, but the presence of cellular
structures has been reported by von Hemsbach; Landois; and
Blasius.
The shell-membrane has been studied chemically by Liebermann
and Lindwall, who found that it consisted almost entirely of a protein,
the percentage composition of which agreed very closely with keratin
(Table loa). Krukenberg, alone, on the ground of its reactions, held
it to be a mucin. This ovokeratin, which contains four times as much
sulphur as the albumen of the egg-white, was found by Morner to
include 7 per cent, of cystine, but there are reasons for supposing
that this figure is much too low. Nothing is known of the part
played by ovokeratin in embryonic metabolism, but, in view of the
fact that calcium is transported from the shell to the embryo during
the period of ossification of embryonic cartilage, it is not impossible
that the sulphur or the cystine of ovokeratin may be made use of
in a similar manner to meet the need for sulphur and cystine
for the feathers. This will be discussed later under the head of
sulphur metabolism (Section I2'7). Morner considered that sulphur
must exist in the ovokeratin in other forms besides cystine, for that
amino-acid would not account for more than a third of what he found
was there. The amino-acid analyses of ovokeratin are placed in
Table 1 1 ; they are due to Abderhalden & Ebstein, and to Plimmer &
Rosedale, the former by isolation and the latter by the van Slyke
nitrogen distribution method. The arginine figure is rather high.
The strength of the shell is clearly an important biological factor :
according to Romanov its average thickness is 0*3 11 mm. giving a
breaking-strength of 4-46 kilos. The relation between shell-thickness
and breaking-strength is a straight line.
The physiological properties of the shell of the bird's egg have been
very insufficiently studied. In the last century there was a general
impression that the shell possessed a differential permeability and
that, while water and other liquids would readily go through the
egg-shell and its membranes from outside in, they would not easily
pass from the inside to the outside. It is difficult to find how this
idea originated; thus Ranke in 1872 attributed it to the younger
Meckel, and Ranke's own statement was subsequently copied down
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264 THE UNFERTILISED EGG AS A [pt. hi
wrongly by Schafer. However, Thunberg in 1902 conclusively
demonstrated the error of the belief, and showed experimentally
that water would pass through the membranes equally well both ways,
though he found that of the two the inner one was the less permeable.
In the case of water birds, there is evidence that the shell is absolutely
impermeable to water; Loisel, for instance, found that the eggs of
the grebe, Podiceps cristatus, and the duck, Anas domesticus, when
placed in distilled water absorbed not a trace of it, and gave out
no chloride to it over a period of many hours. The eggs of the
ordinary hen, on the other hand, increased considerably in weight
and allowed some chloride to pass out into the water. At the same
time, Loisel found that hen's eggs develop quite normally, at any
rate up to the seventh day, even if they are lying in water. This
experiment of Loisel's was confirmed by Lippincott & de Puy, who
succeeded in hatching chicks from eggs incubated while lying in | in.
of distilled water. The differences between these eggs and the controls
suggested that the eggs lying in the water not only failed to lose as
much water through evaporation as eggs incubated under ordinary
conditions, because of the limitation of the evaporating surface,
but actually absorbed some water. Trials with rhodamine red and
methylene blue demonstrated penetration by these dyes, extending in
the former case to vital staining of the embryo. It is known (Rizzo)
that the avian egg-shell has many pores (o-86 to 1-44, average 1-23
per sq. mm.).
As regards gases, the only paper is that of Hiifner. Hiifner placed
small pieces of egg-shell with their membranes in a diffusiometer,
and measured the rate at which gases passed through the obstacle.
He found that oxygen diffused through with most difficulty, then
nitrogen, then carbon dioxide, and, finally, hydrogen most easily.
It may be significant that carbon dioxide would thus appear to be
able to escape somewhat quicker than oxygen can enter. But under
normal atmospheric pressure the amount diffusing through the whole
egg-shell (goose) per second was 2-115 c.c. of oxygen and 0-503 c.c.
of carbon dioxide. The diffusion velocity was always proportional
to the partial pressure of the gases, and the removal of the inner
membrane made no difference at all, suggesting that the principal
barrier was the amorphous calcium carbonate layer. It would
be very desirable to repeat these observations with more modern
methods, and on a greater variety of eggs.
SECT. I] PHYSICO-CHEMICAL SYSTEM 265
1-6. The Avian Egg-white
The white of the egg is divisible into three portions which have
been studied separately by Romanov. The outermost and thinnest
layer makes up 39-8 % of the whole and has 1 1'6 % dry solid. The
middle layer accounts for 57-2 % and has 12-4% dry solid, and the
innermost, thickest, layer is only 3 % and has i4'5 % dry solid*. The
chalazae have only once been analysed separately (Liebermann),
when the elementary composition of their protein was ascertained.
Table 1 2 summarises the results of the investigators who have made
general analyses of the white. It is a very watery solution of
protein, containing only the most negligible traces of fats and
lipoids, but a great many water-soluble substances such as carbo-
hydrate in various forms, protein breakdown-products, choline,
inositol, etc. Natural egg-white, according to Rakusin & Flieher,
is a saturated solution of ovoalbumen (15-35 P^^ cent.). The
water-content does not vary much, but Tarchanov's analyses go
to show that the smaller eggs with short incubation-time are
wetter than the others. The proteins of the egg-white are believed
to be variable in number in the eggs of different birds. In that
of the hen, four are known, in that of the crow three, and in that
of the dove one only. The egg-white of the hen's egg contains two
albumens, ovoalbumen and conalbumen, and two glucoproteins,
ovomucoid and ovomucin.
It was at one time thought that there was a fifth, ovoglobulin.
Dillner studied it in 1885, and estimated that it made up 0-67 per
cent, of the egg-white and 6-4 per cent, of the total protein, but
Osborne & Campbell showed that it was simply a mixture of the
others in different proportions. This had already been made probable
by the results obtained by Corin & Berard, who were able to separate
the ovoglobulin into two or three constituent proteins having
several different coagulation temperatures (57*5°, 67°, 72°, 76° and
82° C.) and other special characteristics.
Hofmeister was the first to prepare crystalline ovoalbumen, and
he published several papers on it. Other workers confirmed his dis-
covery, such as Gabriel ; Harnack ; and Bondzynski & Zoja, but
Hopkins & Pincus showed that the albumen so crystallised only
accounted for half the protein present in the egg-white. Part of the
missing protein was found by Osborne & Campbell to be in the
* A large number of concentric rings can be seen in egg-white coagulated in situ,
according to Remotti.
266
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268 THE UNFERTILISED EGG AS A [pt. iii
Table 13. Distribution of proteins in avian egg-white
1
Species
Hen
Passeres (singing birds)
Turdus iliacus
Turdus piliaris ...
Anthus pratensis ...
Anthus cervinus ...
Garrulm infaustus
Corvus cornix
Corvus nionedula ...
Zygodactyli (2-toed)
Dryocopus martins
Accipitres (birds of prey)
Strix aluco
Falco gyrfalco
Falco peregrinus ...
Falco aesalon
Falco tinnunculus...
Astur palumbariiis
Buteo lagopus
Pandion haliaetiiis
Pullastrae (doves)
Columba livia
Gallinae (fowls)
Gallus domesticus
Meleagris gallopavo
Grallae (marsh birds)
Charadrius apricarius
Haematopus ostralegus
Tringa alpina
Phalaropus hyperboreus
Totaniis glareola . . .
Actitis hypoleiicos
Limosa lapponica
Numenius arquatiis
Numenius phaeopus
Fulica atra
Lamellirostres (ducks)
Anser segetum
Fuligula marila ...
Fuligula fuligula
Oedemia fusca
Clangida glaucion
Somateria mollissima
Mergus senator . . .
% wet
weight
ovo-
mucoid
% of total protein
in eg5-white
ovo-
mucoid
10-5
7-6
12-5
ovo-
mucin
1-78
1-85
2-33
2-09
1-56
1-73
1-58
2-40
1-47
1-53
1-26
1-55
1-28
1-45
2-II
1-25
0-83
I "49
i-i6
1-40
1-05
1-03
1-32
1-31
1-26
1-71
1-54
1-57
2-o6
1-45
1-40
1-56
2-00
1-67
OVO-
albumen
80
80
80
Investigator
and date
Osborne & Campbell
(1909), r^
Komori (1920)
Needham (1927)
Morner (19 12)
— Morner (1912)
12-50
18-25
15-90
SECT. I] PHYSICO-CHEMICAL SYSTEM 269
Table 13 [cont.].
% wet
weight
ovo-
mucoid
% of total protein
in egg-white
Species
ovo- ovo- ovo-
mucoid mucin albumen
Investigator
and date
Steganopodes (pelicans)
Phalawcrocorax carbo
Phalarocrocorax graculus ...
0-2I
0-46
2-o8 — —
Mdrner (191 2)
Longipennes (swallows)
Larus cantis
Lesiris crepidata ...
Sterna macrura ...
Sterna hirundo
1-76
1-28
1-36
1-53
15-00 — —
J5
Pygopodes (divers)
Podiceps cristatus
Colymbus arcticus
2-04
1-88
— — —
J)
form of the two glucoproteins and the other albumen, while part of
it was accounted for by the fact that the yield of the crystallising
process is not great. Ovomucoid was originally discovered by
Neumeister, who called it "pseudopeptone", and first studied by
Salkovski and Zanetti.
The investigations of Osborne & Campbell, whose memoir is the
best on this subject, give no very definite indication of the pro-
portions in which these proteins make up the protein fraction of
egg-white, but they put ovoalbumen at about 80 per cent., and
ovomucin at about 7 (see Table 13). Later, Komori estimated that
ovomucoid accounted for about 10-5 per cent, of the proteins, and
in 1927 I obtained a figure of 7-6 per cent, for the same con-
stituent. Morner, in his extensive study of ovomucoid in numbers
of birds' eggs, obtained results from which higher figures emerge on
calculation, namely, from 10 to 20 per cent. The only exceptions
were the pelicans, which seemed to have very little ovomucoid. The
most probable relationship between the proteins is as follows:
ovoalbumen 75, ovomucoid 15, ovomucin 7 and conalbumen 3 per
cent., but these values are only very approximate, and further work on
this point is much to be desired. Leaving out ovomucin, Wu & Ling
found that the proportions were as follows (for Gallus domesticus) :
ovoalbumen 78-3, ovomucoid 12-3 and conalbumen 9-4 per cent., or
1-34, 0-21 1 and o-i6i gm. per cent, respectively. Certain Russian
workers (Worms and Panormov) have described two proteins, anatin
and anatidin, in the egg-white of the duck's egg, and three, corvin,
270 THE UNFERTILISED EGG AS A [pt. iii
corvinin and corvinidin, in the egg of the crow. It is not certain,
however, to which of the well-known proteins of the hen's egg-white
these others correspond. Judging from the percentage composition
tables in Table lo a, the columbin of the dove's egg corresponds
to hen ovoalbumen and to duck anatinin, while duck anatin corre-
sponds to hen ovomucin, but in the absence of definite information
the question must be regarded as unsettled, and would repay further
investigation.
The minimal molecular weight of ovoalbumen, according to
Cohn, Hendry & Prentiss, is 33,800 (Marrack & Hewitt suggest
43,000), and its percentage composition is seen in Table 10 a;
the best analyses are probably those of Osborne & Campbell,
who give an account of its general properties. It has been further
analysed by several workers who have determined the proportions
of its constituent amino-acids, and whose results are seen in
Table 11. The hydrolyses of Osborne, Jones & Leavenworth;
Osborne & Gilbert, and of Abderhalden & Pregl were all done by
acid, but those of Hugounenq & Morel and Skraup & Hummel-
berger were alkaline, the former using baryta. The figures agree
accordingly, and all that can be said of them is that for purposes of
calculation the amounts of amino-acids must be taken as minimum
in each case. Attention may also be drawn to the less complete
analyses of Chapman & Petrie and Hugounenq & Galimard and to
the analysis of mixed egg-white proteins by Plimmer & Rosedale,
using the van Slyke technique. The large amounts of hexone 4Dases
found by them contrast with those found by the remaining workers,
using direct isolation, and if this is not due simply to difficulties of
technique it may lead us to expect a high content of hexone bases
in conalbumen and ovomucin when they come to be analysed.
In Table 10 a the results obtained by Gupta on the hydrolysis pro-
ducts of ovoalbumen are given (see also Rudd). It is noticeable in
them, as in the analyses of ovoalbumen itself, that they contain
a high proportion of sulphur, though not so much as ovomucoid.
The spontaneous evolution of hydrogen sulphide by egg-white
on standing has long been known, and was made the subject
of a paper in 1893 by Rubner, Niemann & Stagnita, who found
that 100 gm. of egg-white gave off when boiled with water 10-7 mgm.
of HgS. Hausmann later decided that its source must be some labile
sulphydryl grouping in the ovoalbumen molecule. In 1922 Harris
SECT. I] PHYSICO-CHEMICAL SYSTEM 271
observed that raw egg-white was quite non-reactive towards the
nitroprusside test for sulphydryl groups, but that immediately upon
coagulation by heat it became vividly reactive, and gave an intense
purple colour. This change only took place in conditions where
denaturation of the protein was involved, and Harris suggested that
this treatment might unmask a thiopeptide linkage or some similar
arrangement which by hydrolysis or keto-enol transformation would
give rise to an active sulphydryl group in the resulting metaprotein
molecule. Later, Harris found that only 14 per cent, of the sulphur
in ovoalbumen could be accounted for as cystine, so that some un-
known sulphur compound must be present in considerable quantity,
and an exactly similar finding was later reported by Osato for the
egg-membrane protein of the herring. The cystine recoverable from
serum albumen, on the other hand, accounted for 86 per cent, of
the sulphur there. The possibilities of these facts with relation to the
metabolism of the embryo have not yet been explored. Philothion,
according to de Rey-Pailhade, exists in the egg-white of the hen
but not in that of the duck.
The principal investigation of ovomucoid is that of Morner. He
had previously discovered that percaglobulin, a protein extracted
from the unripe ovarial fluid of the perch {Perca fluviatilis) would
precipitate ovomucoid from its solution. With this reagent he made
an examination of a wide variety of birds' eggs, in order to study
the distribution of ovomucoid. By direct estimation he found it to
be present in all the eggs he studied, but it seemed to exist in two
sharply distinguished forms, one which would give a precipitate with
percaglobulin, and another which would not. Thus the hen [Gallus
domesticus) with i -46 per cent, of ovomucoid gave a highly positive
reaction, but the hawk {Astur palumbarius) with i -45 per cent, gave
none at all. Preparations of ovomucoid from the two varieties of
egg-white (see Table 10 a) did not show up the existence of two ob-
viously different chemical individuals, and it was concluded that the
preparations were in each case mixed with a small amount of the
other substance. Moreover, of the egg-whites which gave a positive
reaction with percaglobulin, some contained ovomucoid precipitable
with Esbach's reagent (e.g. Clangula glaucion and Somateria mollissima)
and others contained an ovomucoid which could not be so pre-
cipitated (e.g. Gallus domesticus and Podiceps cristatus). It is quite
uncertain how many of the effects observed by Morner are due to
272
THE UNFERTILISED EGG AS A
[PT. Ill
physical and colloidal rather than to chemical differences, and the
whole question should be reinvestigated. There seemed to be no
special significance in the distribution of the ovomucoid which was
precipitable by percaglobulin ; thus it was present in the accipitres,
grallae, lamellirostres, longipennes and pygopodes, but not in the
passeres, zygodactyli, pullastrae and steganopodes. As for the fowls,
it was present in the eggs of the hen and pheasant, but not in
those of the guinea-fowl. Morner was inclined to agree with Milesi's
view that ovomucoid did not exist as such in the natural egg-white
at all.
Table 14. Variations in properties of avian egg-white.
Coagulation point
Consistency and
of the
egg-white m
colour of coagulum
degrees Fahrenheit
Nidifugous
Hen
Hard white
160
Nidicolous
Pigeon
Pretty firm, bluish
189
Jay
Soft, white, translucent .
184
Missel thrush
Soft, transparent
188
StarHng
Firm white
195
Robin
Soft white
187
Hedge-sparrow
Pretty firm, greenish,
transparent
168
Golden-crested wren
Soft, bluish, semi-trans
parent
"
As has already been observed, Sir Thomas Browne was one of
the first to note that the coagulated egg-white of the gull's egg was
quite different in consistency and translucency from that of the hen's
egg. In 1863 Davy collected some data on these points, which are
shown in Table 14, and Tarchanov devoted much time to the
question in the 'eighties of the last century. He found that the whites
of many kinds of eggs would not coagulate in the ordinary way on
boiling, but either remained liquid and transparent or else set to a
watery translucent jelly. This he called " tataeiweiss ", and as he went
on to examine the distribution of this property he found that it was
associated with the hatching quality of the bird in question. Thus
all nidifugous birds, whose chicks are born fully feathered ("downy")
and soon leave the nest, had eggs with ordinary egg-white, but all
nidicolous ones, whose chicks are hatched as "squabs" or naked and
weak, and have some development yet to complete, had eggs with
uncoagulable or transparent egg-white. Thus the sand-martin, linnet,
finch, thrush, canary, crow, dove, rook, nightingale, robin, starling
(roughly passeres and pullastrae), all had tataeiweiss] while the hen,
SECT, i] PHYSICO-CHEMICAL SYSTEM 273
duck, goose, guinea-fowl, partridge and corncrake had ordinary
white. This classification agreed roughly with Davy's high and low
coagulation points for the egg-white, and corresponded on the whole
to Morner's two classes, the former having ovomucoid not precipit-
able with percaglobulin and the latter having the precipitable
variety, but to this there were some exceptions; thus the plover's
ovomucoid could be so precipitated, but its egg-white was tata
and it yet produced fully-feathered chicks. It was, however, the only
exception to Tarchanov's generalisation, (It should be explained
that the word tata was derived from the name of Tarchanov's small
daughter.) Tarchanov found that tata egg-white was about 3 per
cent, richer in water than the other kind, a conclusion which Morner's
later analyses did not confirm. He also said that it was alkaline to
litmus, but became less so as the tata eggs developed. This agrees
with the later classical work of Aggazzotti on the reaction of the egg-
white of the hen's egg during its development. Tarchanov reported
that tata egg-white could be made to coagulate at ordinary tempera-
tures by the addition of a little potassium sulphate, and that it would
itself coagulate if the temperature was raised well above the boiling-
point of water. It was, he said, more easily digested by enzymes, it
putrefied more readily, and during development it changed into a
form resembling ordinary egg-white. He made some studies on its
secretion by the oviduct of these birds, and was the first to perform
the experim.ent of putting a ball (in his case a lump of amber) at
the top of the oviduct and seeing it emerge at the bottom with layers
of egg-white and a shell secreted around it. The change during
development from tata to ordinary egg-white Tarchanov found he
could imitate by bubbling carbon dioxide through the original white,
after which it would coagulate in the usual way. On the other hand,
he found that if he soaked normal hen's eggs in a 10 per cent, solu-
tion of alkali the white took on the properties of tata egg-white, and
became just like the glassy material in the sand-martin's egg. He
suggested some relation between these phenomena and the alkali-
albuminate of Lieberkiihn, but did little to determine its chemical
relationships. He was unable to get any development in the case of
hen's eggs soaked in alkali.
In 1 89 1 Zoth took up the whole question of tataeiweiss once
more. He was led to do so on account of some researches which he
had been making on the effect produced on serum-clotting by various
274 THE UNFERTILISED EGG AS A [pt. iii
concentrations of alkali, and which showed that the clot could vary
very greatly in its properties, from opacity to almost perfect trans-
parency, for instance. Tarchanov had decided that the transparent
coagulum of the nidicolous egg-whites was not to be identified
with that produced by sodium or potassium albuminate, but Zoth
succeeded in showing that the differences were not sufficient to dis-
tinguish them. Zoth fully confirmed Tarchanov's finding that ordinary
egg-white could be made to pass over into nidicolous egg-white by
treating it with i o per cent, potash in the cold for ten days, and was
able to explain all the differences between tataeiweiss and alkali
ovoalbuminate as due to variations in the amount of alkali present,
or rather the amount of cation as compared to anion. It is most
unfortunate that we have no detailed ash analyses of the egg-whites
of nidicolous birds, for, as will later be seen, the egg-white of the
hen has rather more total anion than total cation, and this relation-
ship might be expected to be even more strongly marked in the case
of nidicolous egg-white, perhaps^ indeed, as much as to counter-
balance the excess of cation over anion in the yolk. There can be no
doubt, however, that the egg-whites of nidicolous birds are relatively
richer in alkali than are those of others, and it is this, combined with
their different water and total ash content, which causes the albumen
to coagulate differently from those of others. Thus, if 5 c.c. of filtered
egg-white from a fresh hen's egg be put in each of three small
Erlenmeyer flasks, 2 c.c. of water added to A, 2 c.c. of 0-89 per cent.
KOH to B, and 2 c.c. of a mixture of equal parts 0-89 per cent. KOH
and 0-66 per cent. NaCl to C, the coagulum in A will be the usual
white, thick, solid and opaque mass, while the other two will be
transparent like tataeiweiss, slightly opalescent, more or less liquid, and
Cmore opalescent than B. It would be interesting to reinvestigate the
whole question anew in the light of recent knowledge and technique.
Another curious effect was noted by Melsens and Gautier. Melsens
found that, if a stream of carbon dioxide, hydrogen, nitrogen or
oxygen, was passed through dilute egg-white, or if it was shaken
violently, a precipitate of fibrous membranous shreds was formed.
Gautier observed that about i -5 per cent, of the protein was thus
changed; he filtered it off and determined its elementary composition,
which showed nothing remarkable. He concluded that a protein
which he called " ovofibrinogen " existed in the egg-white, and even
suggested that an " ovo thrombin " was present to turn it into "ovo-
SECT, i] PHYSICO-CHEMICAL SYSTEM 275
fibrin". He apparently thought that the ovofibrin was incorporated
without change into the substance of the embryo. The subject has
not received any attention since the time of Gautier, but it is probable
that this phenomenon is explained by the work of Young; Dreyer &
Hanssen and others, on the high instability of protein solutions.
Peptones were reported by Reichert to exist in fresh egg-white.
Wu & Ling have recently studied the coagulation of ovoalbumen
by strong mechanical agitation. The fact that conalbumen is not
coagulable by such means gave them a method of estimating it in
egg-white. Thus they obtained the following figures for Gallus
domesticus egg-white:
Nitrogen
Total N (ovoalbumen + conalbumen 4- ovomucoid) 1-71 gm. %
After shaking (conalbumen + ovomucoid) ... 0-372 ,,
After shaking and heating (ovomucoid) ... 0-211 ,,
Coagulation of ovoalbumen by shaking was not separable into two
stages (denaturation and agglutination) like that by heat or alcohol.
The isoelectric point of the protein was the most favourable for
shaking coagulation (/?H 4-8) and the Q^^q of the reaction was i-g.
Piettre has published a method for separating the proteins which
involves the use of acetone.
The relationships between the avian egg-white proteins have been
the subject of some interesting immunological work. The earliest
investigators who crystallised ovoalbumen found that perfect fresh-
ness was necessary, for at room temperature the crystallisable protein
gradually turns into a non-crystallisable one. Bidault & Blaignan
found that this process could be arrested by placing the ^gg at 0°.
Sorensen & Hoyrup suggested that the protein formed was conalbu-
men and wished to look upon the latter as a product of ovoalbumen.
Hektoen & Cole, however, first showed that though ovoalbumen
was distinct from the serum albumen of the hen immunologically,
conalbumen was not, and then went on to demonstrate that during
the loss of crystallisable ovoalbumen which takes place as the egg
ages, there was no corresponding increase in conalbumen. We must
therefore look upon the latter as probably identical with the serum
albumen of the adult: and perhaps only present in the ^gg owing to
the inefficiency of the oviduct.
The analyses of ovomucoid and Eichholz's ovomucin, as well as
the fragmentary one of conalbumen, will be found in Tables i o a
and 1 1 . Willanen found that ovomucoid was much more susceptible
18-2
276 THE UNFERTILISED EGG AS A [pt. hi
to hydrolysis by pepsin than by trypsin (see later under enzymes and
antitrypsin). For its properties see the papers of Morner and Neu-
mann. Both the glucoproteins have twice as much sulphur as ovo-
albumen. Their carbohydrate content has been the subject of a great
amount of discussion and experimental work. Berzelius was the
first to draw attention to certain similarities between the breakdown-
products of sugars and proteins when acted upon by boiling acids.
In 1876 Schiitzenberger asserted that the ovoalbumen molecule
contained a carbohydrate group, basing his views on positive results
with Trommer's test after total hydrolysis. In later years a number
of workers supported the view that the carbohydrate was glucose,
using in different cases methods of varying reliability, e.g. Kruken-
berg in 1885, Hofmeister and Kravkov in 1897, and Blumenthal ;
Blumenthal & Mayer and Mayer in 1898 and 1899. Spencer and
Morner, however, failed to get any evidence of a carbohydrate
group after hydrolysis, and reported their negative results in 1898.
Weiss, about the same time, thought he could identify a methyl
pentose among the hydrolysis products, but he was never confirmed.
Seemann was the first to announce that the carbohydrate was glucos-
amine, and his discovery was quickly confirmed by Frankel and
Langstein. These later workers began to attempt quantitative estima-
tion of the sugar, and their figures are given in Table 15. Pavy,
using the then recently discovered osazone technique, made a study
of a variety of proteins, and showed, as might be expected, that the
yield from ovoalbumen was always greatly less than from ovomucoid.
Eichholz obtained glucosazone from ovoalbumen, ovom.ucoid and
ovomucin, but not from either serum albumen or casein. On the
whole, it is most likely that ovoalbumen contains extremely
little glucosamine, and the figures recorded in the literature for
this are probably due to contamination with ovomucin. This is the
view of Osborne, Jones & Leavenworth, for neither they nor
Osborne & Campbell obtained any glucosamine from their very
carefully purified ovoalbumen. Komori has prepared from ovo-
mucoid, and Frankel & Jellinek from ovoalbumen, polysaccharide-
like substances which they regard as the prosthetic group containing
all the glucose. Following this up Levene & Mori have prepared a
trisaccharide containing glucosamine and mannose from egg-white.
Ovoalbumen contains 0-26% of this substance, coagulated egg-white
1*9%, and ovomucoid 5*1%. According to Levene & Rothen the
SECT. l]
PHYSICO-CHEMICAL SYSTEM
277
molecule consists of four trisaccharides each containing one molecule
of glucosamine and two of mannose.
Table 15. Ovoalbumen and ovomucoid glucosamine content.
Hen {Callus domesticus)
Hofmeister (1898)
Seemann (1898) ...
Langstein (1900)
Willanen (1906) ...
Pavy (1907)
Samuely (1911) ...
Neuberg & Schewket (1912) ...
Zeller (1913)
Needham (1927)
Abderhalden, Bergell & Dorpinghaus
Neuberg (i 901) ...
Blumenthal & Mayer (1900) ...
Izumi (1924)
Tillmans & Philippi (1929)
Turtle ( Thalassochelys corticata)
Takahashi (1929)
Bywaters (1909) Seromucoid
Pavy (1899) Ovomucoid
(1904).
Ovoalbumen
/o
90
IO-5
Trace
3-4
3-7
Ovomucoid
%
15-0
34-9
I9-5-22-3
21-7
29-4
24-0
33-7
II-5
7-4
6-2
Glucose % by hydrolysis with
Osseomucoid ...
Tendon mucoid
Ovoalbumen ...
Serum globulin
10 % KOH
I2-0
12-3
4-6
33
2-6
2-8
5 % HCl
15-9
21*7
lo-B
II-6
2-5
2-8
The free carbohydrate of the egg-white has also received a great
amount of attention from an early date. In 1846 Winckler isolated
a quantity of a sugar (4 grains) from the egg-white of the hen's egg,
and identified it as lactose. ''Physiologists", he said, "will be able
to tell me whether this is of importance for the embryo or whether
it was some abnormality." The observation has not since been re-
peated, and it is in the highest degree unlikely that any lactose was
ever in an egg, unless the diet of the hen was a very unusual one.
Budge and Aldridge soon were at work on this subject, the former
concluding that the carbohydrate was glucose, but suggesting that
it might form a disaccharide during development, and the latter
making no concession to Winckler. The presence of glucose was
afterwards abundantly established by the work of Barreswil ; Leh-
mann ; Meissner ; Salkovski ; and Pavy. Later many quantitative
estimations were made, and these are collected together in Table 16.
The older figures for free carbohydrate may be regarded as fairly
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28o THE UNFERTILISED EGG AS A [pt. iii
trustworthy, but not for combined sugar, in view of the demonstra-
tion of Holden that all copper-reducing methods are seriously inter-
fered with by the presence of amino-acids and protein breakdown
products. No method at present in use gives satisfactory results in
those conditions, but the most reliable is that of Hagedorn &
Jensen. No estimations of total carbohydrate in egg-white alone at
present exist, but there is a single figure for glycogen due to Sakuragi.
Morner found no evidence of fructose, pentoses or maltose.
A curious phenomenon : the fluorescence of egg-white has been
reported by van Waegeningh & Heesterman, but it only occurs if
the egg is not perfectly fresh, and is therefore probably not physio-
logical.
I '7. The Avian Yolk
The vitelline membrane was investigated by Liebermann in il
who found that it consisted almost exclusively of keratin. This he
purified, and, having freed it from ash, made an elementary analysis
of it, which is shown in Table loa. Some experiments which demon-
strate the peculiarities of the vitelline membrane have been devised
by Osborne & Kincaid. They found that a fresh yolk floated into
distilled water, o-g per cent. NaCl solution, or glycerol, behaved
exactly like a red blood corpuscle in that it swelled up and burst
in the former, and shrank to a corrugated globe in the latter, while
in the isotonic salt solution it remained unchanged. But with other
treatment, nothing took place which corresponded to haemolysis. If
the yolk was put into lo per cent. NaCl solution, it did not shrink,
as had been expected, but swelled up, owing to the penetration of
the saline and the consequent osmotic pressure due to the dissolving
of the vitellin in the saline. This showed at once the scleroprotein
nature of the membrane and its impermeability to vitellin even when
in solution. The membrane is also impermeable to phosphatides and
fats dissolved in ether, for if a yolk is put into ether it sinks and swells,
so that the upper pole is distended by an accumulation of deeply
pigmented ether. But until the yolk bursts, as it eventually does,
not a trace of pigment or other substance passes out into the ether,
and the same results were found with chloroform and carbon disul-
phide. In alcohol, on the other hand, there is no swelling, for the
alcoholic solution of phosphatides and other bodies can pass out
SECT, i] PHYSICO-CHEMICAL SYSTEM 281
through the keratin membrane. It would be very interesting to make
a more extended study of the osmotic properties of the vitelline
membrane (see in this connection Section 5*6).
The yolk of the egg was investigated earlier in the modern period
than the white. We may pass directly, excluding the curious analysis
of the eggs of Struthio casuarius by Holger in 1822, to the papers of
Gobley, which appeared from 1846 to 1850, and which, with those
of Valenciennes & Fremy from 1854 to 1856, still remain models
of embryo-chemical work. "John, a German chemist," said Gobley,
"appears to have been the first to occupy himself with serious
researches on the yolk of the egg. The chemists who preceded him
considered it as made up only of water, albumen, oil, gelatine, and
colouring matter; such was the opinion of Macquer, Fourcroy, and
Thomson. John concluded from his experiments, which he published
in 181 1, that the yolk was composed of water, a sweet yellow oil,
traces of a free acid which he thought was phosphoric acid, and a
small amount of a brownish red substance, soluble in ether and
alcohol. Besides these he found gelatine, sulphur, and a great deal
of a modified albuminous substance." Gobley referred also to the
work of Prout, of Chevreul, of Berzelius and of Lecanu, who dis-
covered the presence of cholesterol in yolk in 1829.
Gobley himself found in the yolk nearly all the substances which
we now know to be there. His own list of them ran as follows :
1 . Water.
2. An albuminous matter, "vitellin",
3. Olein.
4. Margarin.
5. Cholesterin.
6. Margaric acid.
7. Oleic acid.
8. Phosphoglycerilic acid.
9. Lactic acid.
10. Salts such as chloride of sodium, chloride of potassium, chlorhydrate
of ammonium, sulphate of potash, phosphate of lime, and phos-
phate of magnesia.
11. A yellow colouring matter and a red colouring matter.
12. An organic substance containing nitrogen, but which is not al-
buminous.
Most of the constituents of egg-yolk may be recognised under this
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284 THE UNFERTILISED EGG AS A [pt. iii
old-fashioned terminology. Gobley made many quantitative observa-
tions on the various substances, and his figures are given in Table 17,
which sums up all the analyses that have been made of the yolk
in the eggs of birds. The original discovery of vitellin was made by
Dumas & Cahours, but Gobley was the first to make an extended
study of it. His elementary analysis is given in Table 10 a. He knew
that it contained both sulphur and phosphorus. Gobley was able to
isolate oleic and margaric (palmitic) acids from the fat fraction of
the yolk, but, unlike Planche twenty-five years before, he got no
stearic, and Kodweiss, one year later, reported its presence under
the impression that it had not been found before. Gobley, however,
was easily able to repeat Lecanu's discovery of the presence of
cholesterol, and made a remarkable examination of the lipoids.
"These viscous materials", he said, "appear to have been considered
by John as not being of a fatty nature at all. They form the most
interesting part of the yolk; they contain all the phosphorus which
exists there in considerable quantity." He analysed the glycero-
phosphoric acid which he obtained from the lipoid, which he named
"lecithin", and speculated as to the significance which it might have
for the growth of the embryo. He also recognised that fatty acids
and nitrogen were present in the viscous matter.
Ten years later Valenciennes & Fremy made a further examina-
tion of the yolks of a large variety of eggs with special reference to
\itellin. It was they who discovered substances very similar to vitellin
in the eggs of reptiles and fishes; these they named the ichthulins.
As regards the eggs of birds, they contented themselves with con-
firming the results of the previous in\'estigators, but they regarded
vitellin as having practically the same constitution as fibrin, on the
grounds of elementary composition only. At the same time, they held
it to be a different compound because it would not, like blood fibrin,
decompose hydrogen peroxide.
If Table 1 7 is examined, it will be seen that the yolk is much drier
than the white in all birds' eggs examined, having only about 50 per
cent, of water as against the 85 per cent, of the latter. On the other
hand, the percentage of fatty substances and lipoids is much higher,
being just about double the amount of protein, whether related to
wet weight or to dry. It is noticeable from the analyses of Tarchanov
that the yolks of eggs from nidicolous birds having a short incubation
time are about 10 per cent, richer in water than yolks from the eggs
SECT. I] PHYSICO-CHEMICAL SYSTEM 285
of nidifugous birds. This must imply that the greater requirement
for nutrient material in the latter case has, as it were, packed the
fat tighter into the yolk. Exactly the same relationship is brought
out from the figures of Spohn & Riddle, who compared the pigeon
which hatches out as a squab with the hen which hatches out as a
fully-feathered chick. Spohn & Riddle's analyses are the only com-
plete ones we have for a nidicolous egg, and bear clearly the same
relationship, for there is less protein and less fat, relatively, in the
pigeon's egg than in the hen's. The ash content and the amount of
non-nitrogenous extractive substances seem, however, to be slightly
higher in the latter case. Langworthy's figures were all obtained
from the eggs of nidifugous birds, and they show a great similarity
among themselves. More delicate consideration, of course, reveals
differences according to breed in the hen's egg, e.g. the figures of
Pennington and his collaborators, but these are of a comparatively
minor order.
The most interesting analyses are those of Spohn & Riddle. They
compared the egg of the jungle-fowl, which is supposed to have been
the evolutionary ancestor of the domestic hen, with averaged figures
for hen's eggs of various breeds, and, as is evident, there was a very
close agreement. They also analysed the white yolk as distinct from
the yellow yolk of the hen's tgg. When the yolk begins to be formed
in the ovary of the hen, it is white and not yellow, and not until
the critical point in its maturation is reached, when its growth-rate
completely changes, does it begin to store lipochrome pigment. This
change in growth-rate, which has been observed by other workers
as well as Riddle (e.g. Walton), will be dealt with in more detail
in the appendix on maturation. Von Hemsbach, in a paper on the
milky or white yolk of the birds, in 1851, suggested that the corpus
luteum of mammals corresponded to the yellow yolk of birds, and
that the mammalian ovum having been shed out of the ovary into
the Fallopian tube and uterus, the fats and lipochrome pigment
were laid down in the Graafian follicle instead of around the white
*'ovum". Von Hemsbach also supported the view already mentioned
that the shells of avian and amphibian eggs corresponded to the
decidua of mammals. He laid stress on the work of Zwicky and
Gobel, who had investigated the pigments of yolk and corpus luteum,
and had thought them to be identical. This subject will be referred
to again under the head of pigments.
LliRARYUo
^i>^^?AS5f4^
286 THE UNFERTILISED EGG AS A [pt. iii
In the fresh egg, as laid, the white yolk occupies a central position,
and is surrounded by concentric layers of yellow yolk. But as a kind
of cylindrical prolongation of the white yolk reaches to the surface
of the vitellus underneath the blastodisc or germinal spot, the white
yolk must be considered the first food of the embryo, and, until its
composition was determined, it was not possible to say what sort
of nutrient environment the embryo possessed in the very early days
of development, although the composition of the yellow yolk would
give this for the later period. The histological differences between
white and yellow yolk had been known for a long time (see Purkinje ;
His ; Leuckart ; Klebs ; Dursy ; Strieker ; and Virchow) but Riddle
and Spohn & Riddle were the first to approach the subject chemically.
Their figures showed that the white yolk much the more nearly
approximated to the contents of invertebrate eggs with holoblastic
cleavage, and living undifferentiated tissue generally. Instead of 45 per
cent, of water, the white yolk had 86 per cent., instead of 15 per cent,
of protein, it had only 4, and instead of 25 per cent, of fat it had
only 2. Thus in its water-content, it was much more like {a) egg-
white and {b) the young embryo itself than like ordinary yolk, while
instead of having twice as much fat as protein it had twice as much
protein as fat. These data are extremely interesting in view of the
facts that are known about the sources of energy made use of by the
embryo during its development. Although by far the greatest pro-
portion by weight of substance combusted during embryonic life is
fat, yet, in the early stages, the embryo undoubtedly gets its energy
preponderantly from protein and carbohydrate (see the whole of
Section 7). The percentage of non-nitrogenous extractives did not
differ much between white and yellow yolk in the experiments of
Spohn & Riddle, but it would be very interesting to know the
relative amounts of carbohydrate, and analyses to discover this should
certainly be done. Again, the yellow yolk contained eight times less ash
than the white yolk, a finding which acquires considerable significance
from the fact that, if the ratio inorganic substance/organic substance
in the embryonic body is plotted, it is seen to descend steadily
from the beginning of development (see Fig. 249). Moreover, as
Mendeleef has shown, early embryonic cells contain twice as much
electrolyte as those of later stages (see Section 5"8). The amount of
phosphatide in the yellow yolk, furthermore, was ten times that in
the white, a significant difference; for, as Plimmer & Scott have shown,
SECT. I] PHYSICO-CHEMICAL SYSTEM 287
one of the main functions of the phosphatide is in furnishing phos-
phorus for the embryonic bones during the period of ossification, a
requirement which is not present in the earher stages of the develop-
ment. The histochemical work of Marza, who compared the white
and yellow yolk following the method of Romieu, is in agreement
with this, for he found the elements of the yellow yolk to be richer
than those of the white. (See Plate X.)
1-8. The Avian Yolk-proteins
As regards the protein, vitellin (Tables 10 a and 11), several interest-
ing points are to be observed. The best elementary analyses of ovo-
vitellin are probably those of Osborne & Campbell. After its discovery
by Dumas & Cahours, Gobley, and Valenciennes & Fremy, it was
studied by Hoppe-Seyler, and now for the first time with special refer-
ence to its position in the classification of the proteins. Virchow had some
time before then suggested that the yolk-platelets, familiar to histolo-
gists, contained lecithin, and there had been some doubt as to their
nature. Valenciennes & Fremy had opposed the view that they were
crystals, basing their view on Sennarmont's work, but Radlkofer and
Hoppe-Seyler returned to the crystal theory. Hoppe-Seyler believed
that vitellin contained no phosphorus, but that what appeared in the
analyses was due to contamination with lecithin. This view was sup-
ported also by his assistant, Diakonov, who contributed to the Med.Chem.
Untersuchungen one of the earliest investigations of phosphatide. But at
the same time Miescher obtained from the yolk of the hen's egg a sub-
stance containing a great deal of phosphorus, and possessing certain
of the properties of a protein. This he believed to be nuclein. "It is
interesting ", he said, "in relation to the origin of nuclear substance,
that the nutrient yolk contains ready-formed nuclein in significant
quantity." At this time, then, the proteins of the yolk were believed
to be ovoglobulin (for so Hoppe-Seyler called the vitellin of the
earlier workers) and Miescher's nuclein. Miescher himself identified
his nuclein as a constituent of the white yolk of the histologists, but
he noted that the hen's egg seemed to have no xanthine in it.
Lehmann, Schwarzenbach and others, however, did not agree with
this classification, and regarded vitellin as a mixture of albumen
and casein. They did so not on the grounds of its containing phos-
phorus, but because they found that rennin would completely
coagulate it from its pure solution. But this attitude did not prevail.
288 THE UNFERTILISED EGG AS A [pt. iii
and the word " nucleovitellin " became general, until Kossel in 1886
found that, if vitellin was really a nuclein, it differed from all other
such substances by giving no trace of xanthine after acid hydrolysis.
On the other hand, true nuclein, he found, was present by the tenth
day of development. Hall and Burian & Schur, Bessau and von
Fellenberg confirmed this absence of purines from the fresh egg. In
more recent times, Sendju and Mendel & Leavenworth have found
exceedingly small amounts of true nucleoprotein (2 and i*6 mgm.
per cent, respectively wet weight) in the hen's egg (by purine bases),
and Plimmer & Scott, and Heubner & Reeb have done the same (by
phosphorus analysis) . Shortly after Kossel's work, Milroy found that
vitellin gave a biuret test though no Millon, and materially differed
in nitrogen and phosphorus content from any of the nucleoproteins,
while, at the same time, Miescher admitted that he could not isolate
any purine bases from his "nuclein" in the hen's egg. Levene
& Alsberg next investigated the manner of breakdown of vitellin,
finding the substance they named " paranuclein " after digestion with
pepsin, and "avivitellic acid" after hydrolysing with ammonia. The
elementary composition of these substances is given in Table 10 a,
from which it could be seen that the increasing phosphorus content
implied the presence of phosphorus as an important constituent of
the original molecule. Six years later Levene & Alsberg ascertained
the amino-acid distribution (see Table 11). They pointed out the
significance of the high proline figure, in view of the task of haemo-
globin synthesis which the young embryo has before it. Abderhalden
& Hunter and Hugounenq undertook a like investigation in the same
year, from which a striking similarity between the amino-acid dis-
tribution in vitellin and casein came to light, especially as regards
the high proportion of leucine and glutamic acid. They drew atten-
tion to the similarity in physiological requirements as between the
"erste Nahrung" of chick and mammal. The historical associations
of this discovery have already been referred to (see p. 53). It was
at this time that Neuberg, and Blumenthal & Mayer reported the
existence of glucosamine in the vitellin molecule, two observations
which stood together in isolation, until in 1929 Levene & Mori
isolated from egg-yolk the same trisaccharide which they found to
be present in ovoalbumen and ovomucoid and which has been re-
ferred to above.
It was not until the paper of Bayliss & Plimmer in 1906 that the
PLATE X
^
/ kj^EHHHH^^^HHI
' '^ "
HEN'S EGG I MM. IN DIAMETER, NOT YET LIBERATED FROM THE OVARY
Stain, haemalum-eosin: magnification, gxA: prepared and microphotographed by
Dr V. Marza. The stratification of the yolk into white and yellow is beginning.
SECT, i] PHYSICO-CHEMICAL SYSTEM 289
nature of vitellin really became clear. They subjected casein
and vitellin to the action of trypsin, and studied the time taken under
varying conditions for the phosphorus to be split off into soluble
form. Ovovitellin, they found, was much more slowly digested than
casein, for after 36 days only half of its phosphorus had been made
soluble, whereas after 2 or 3 days a large percentage of the casein
phosphorus had gone into solution in inorganic form, and most of
the rest was present in water-soluble organic combination, i per
cent, soda, however, would bring aU the phosphorus of casein into
solution in 24 hours. BayUss & Plimmer concluded that ovovitellin
and casein were both phosphoproteins, as distinguished from nucleo-
protein, where the phosphorus would be present in the prosthetic
group and not in the protein itself Plimmer & Scott later found that
ovovitellin behaved in the same way to soda. This reaction served
to distinguish between phosphoproteins and nucleoproteins, for all
the latter, it was found, were stable to alkali and easily split by
acids. From the phosphorus distribution in the unincubated hen's
egg, Plimmer & Scott concluded that vitelHn accounted for at
least 30 per cent, of the phosphorus, and this led them on to their
investigation of the changes which take place in the different
phosphorus fractions during the development of the embryo.
The distribution of phosphorus-containing compounds in egg-yolk,
as Plimmer & Scott found, makes a very different picture from that of
any other tissue. Their summary is shown in the accompanying table
(18). It would be extremely interesting to investigate the phosphorus
distribution in the white yolk, which at present is altogether uncharted.
Table 18. Phosphorus in per cent, of the total phosphorus.
Hen
Ox
Ox
Ox
egg-yolk
testis
thymus
pancreas
Lecithin P ...
61-4
33-7
14-3
"•5
Total water-soluble P
9-5
435
41-3
43-5
Water-soluble inorganic P
None
27-9
2I-I
23-0
Nucleoprotein P
1-6
22-8
44-4
41-9
Phosphoprotein P ...
27-5
None
None
31
Total protein P
29-1
22-8
44-4
45-0
The third of these fractions i
includes the phosph
orus of all unstable water-soluble corn-
pounds.
Steudel, Ellinghaus & Gottschalk have recently found that vitellin
behaves towards pepsin exactly like casein. The rate of increase of
titratable COOH groups during the digestion far exceeds that of
NH2 groups, reaching a maximum about the fourth hour. The
19
ago
THE UNFERTILISED EGG AS A
[PT. Ill
Table 19.
Per cent, of dry weight
Total N
P
Fe
N/P rati
mer's figures.
Ovovitellin ...
ovolivetin ...
Ovoalbumen ...
Casein ...
... 15-29
15-12
••• 15-51
... 15-30
i-o
o-i
o-i
—
15-3/1
151-0/1
155-0/1
Levene & Alsberg's figures.
Avivitellic acid ... 13-13
Swigel & Posternak's figures.
0-57
Swigel & Posternak's figures.
Hydrolysis of ovotyrine b^ (% )
Pyruvic
H3PO4 acid NH3 Arginine Histidine Lysine
12-00 1-60 4-90 0-62 0-70 0-75
1-32/1
Ovotyrine a^ ...
... 10-87
13-76
None
1-75/1
Ovotyrine ^^ ...
... 11-33
12-55
None
2-00/1
Ovotyrine /Sj ...
... 10-92
12-09
3-31
2-00/1
Ovotyrine 71 ...
10-70
7-90
None
3-00/1
/-serine
7-90
Table 20. Nitrogen distribution.
Plimmer's figures (1908).
Per cent of dry weight
Ovoalbumen
Casein
Ovovitellin .
Ovolivetin .
Total
N
15-51
15-30
15-29
15-12
Amide
N
1-34
1-52
0-84
0-75
Humin
N
0-29
0-22
0-25
0-32
Diamino
N
3-30
330
3-84
3-29
Mono-
amino N
10-58
10-36
10-26
10-76
linkages must break, therefore, between a carboxyl group and pro-
line, tr-yptophane, histidine or arginine.
Weyl ; v. Moraczevski ; and Gross were the first to describe the
properties of the egg-yolk proteins, but the standard account is that
of Plimmer, who in 1908 identified two yolk-proteins, ovovitellin,
and ovolivetin. Ovovitellin, according to his analyses, contained
1*0 per cent, of phosphorus, but ovolivetin only o-i per cent. He was
usually able to isolate far more of the former than the latter, but in
some experiments the yield seemed to be nearly equal. Livetin was
SECT. l]
PHYSICO-CHEMICAL SYSTEM
291
soluble in water as well as 10 per cent, salt solution, but it cannot be
ovoalbumen or any of the egg-white proteins, for it is not coagulated
by ether. Plimmer suggested that possibly livetin was vitellin with
the majority of the phosphorus-containing parts split off from it.
Tables 19, 20 contain Plimmer's figures for these two proteins.
Table 21.
Tryptophane.
May & Rose
Folin & Looney
(1922), (%)
(1922), (%)
Casein
i'5
i'54
Lactalbumen
2-4
Gliadin
1-05
1-14
Glutenin
1-8
1-68
Edestin
1-5
1-4
Gelatin
Ovovitellin
1-74
—
Ovoalbumen
i-ii
1-23
Free tryptophane (% of whole egg)
von Fiirth
& Lieben
Ide (1921)
(1922)
Whole egg-contents
0-359
—
White
0310
—
Yolk
1
0-437
"otal tryptophane.
(% of proteins)
(% of proteins)
Whole egg-contents
2-74
—
White
2-83
2-0
Yolk
2-51
(ro'ofegg)
Whole egg-contents
—
0-23
Ovovitellin has been the subject of recent investigations by Swigel
& Posternak. They found that it broke up into three polypeptides
which they call ovotyrine a^, ^^ and y^. The properties of these are
listed in Table 19. It was found that ovotyrine ^ contained all the
iron in the original compound, and that it could be split up into
ovotyrine /Sj and ovotyrine ^2 the second of which again contained
all the iron. All these derivatives were laevorotatory, and showed
considerable resemblance to the lactotyrines which the same workers
had previously isolated from casein. They identified their ovotyrine ^
with the avivitellic acid of Levene & Alsberg, and they stated that
an enzyme was present in the fresh yolk which would, on standing
at 37° C. for 10 days, double the yield of preformed ovotyrine jS.
19-2
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294 THE UNFERTILISED EGG AS A [pt. iii
Hydrolysis of ovotyrine ^ revealed the presence of large amounts
of /-serine, an amino-acid which had not previously been found in
ovovitellin (see e.g. Plimmer & Rosedale's analyses). Some pyruvic
acid and ammonia being given off as well, Swigel & Posternak
calculated that, supposing these arose from breakdown of serine,
there would have been sufficient serine present initially to combine
with for all the phosphorus. They therefore suggested that the main
phosphorus-containing unit of ovovitellin was serine-phosphoric acid.
Cohn, Hendry & Prentiss consider the minimal molecular weight
of vitellin to be 192,000, i.e. much higher than ovoalbumen.
Kay & Marshall have also studied the yolk-proteins. They have
prepared purer samples of vitellin and livetin than those of any
previous worker, and have been able to free the former almost
entirely from contamination with ovolecithin. Their vitellin is a true
phosphoprotein containing i -3 per cent, of phosphorus and hydrolysed
by I per cent, soda, though not by the phosphatase of the kidney.
Their livetin is a pseudo-globulin, containing only the slightest
traces of phosphorus (less than 0-05 per cent.). The yolk of the
fresh egg contains no albumen. Vitellin, hydrolysed with dilute
ammonia, gives a vitellinic acid containing about 10 per cent, of
phosphorus. Kay has estimated the cystine, tryptophane and tyrosine
in vitellin and livetin (Table 11); in the latter protein they are dis-
tinctly high in amount, a fact of some importance in embryonic
nutrition. The relative amounts of vitellin and livetin in yolk would
appear to be of the order of 3-6 to i for the hen and 3-8 to i for the
duck, calculating from their nitrogen content. Kay regards livetin
as identical with Gross' protein. The yolk of a fresh egg would
contain from 600 to 900 mgm.
1*9. The Fat and Carbohydrate of Avian Yolk
The fatty acids of the yolk have been much investigated since the
time of Gobley and Kodweiss, but little has been added to our know-
ledge of them. Paladino found olein, palmitin and stearin to be
present. Analytical details are in Table 22.
A large part of the study of phosphatides, under the generic name
of lecithin, has been made on that obtained from the yolk of the egg;
thus the work of Diaconov, who showed it contained no neurine,
Strecker, who discovered the presence of choline, Bergell ; Cousin ;
Laves & Grohmann; Laves; Wintgen & Keller; Erlandsen; Stern
SECT. I] PHYSICO-CHEMICAL SYSTEM 295
& Thierfelder; Frankel & Bolaffio (whose egg-yolk neottin was only
a mixture of sphingomyelin and cerebrosides), McLean; Serono &
Palozzi; Eppler; Riedel; Wilson, and Trier, who prepared amino-
ethylalcohol from it, all comes under this heading. In McLean's
book will be found a review of it. Certain aspects of it, however, are
important here ; for instance, the question of the presence of very
unsaturated acids in ovolecithin. McLean in 1909 found stearic and
oleic acids in it, but Cousin was able to isolate linolenic and palmitic
as well, and Riedel; Hatakeyama; and Levene & Rolf obtained
linolic and arachidonic acids. In another paper Levene & Rolf
showed that the lecithin, carefully freed from kephalin, contained
only palmitic, stearic, and oleic acids : saturated and unsaturated
molecules being present in equal proportions. Again, Stephenson in
1 9 1 2 found an acid in the phosphatide fraction from egg-yolk, which
had 20 carbon atoms and 6 or 8 unsaturated linkages. Although
the proportion of unsaturated acids in egg-yolk is generally agreed
to be small, yet it may be of importance for the young embryo if
it passes through a period in the early developmental stages before
it has the power of desaturating the ordinary fatty acids. Evidence
which suggests this will be presented later (Section 1 1 • i ) .
The nitrogenous radicle in ovolecithin is largely choline, but
difficulty was at first experienced in obtaining a theoretical yield
on hydrolysis; thus Moruzzi got only 77 per cent, in 1908 and
McLean only 65 per cent, in 1909. This was accounted for, however,
when it was found that amino-ethyl alcohol was also present. The
two bases together make up all the nitrogen in the molecule. Erlandsen
was the first to question the view that lecithin alone accounted for
the phosphatide fraction, but he was not himself able to isolate any-
thing else. Later workers (Levene & West and Stern & Thierfelder),
however, found that kephalin is also present in yolk, and it would
probably be in the kephalin molecule that the unsaturated fatty acids
would be present. Analyses of kephalin from the yolks of fowls are
given in Table 10 ^. McLean in 1909 isolated from egg-yolk a third
phosphatide which resembled cuorin, but it is very doubtful whether
this was a true chemical individual. Sphingomyelin has also been
found in egg-yolk by Levene (191 6), and lignoceric as well as hydroxy-
stearic acid was present in it.
All these substances exist in the yolk in close association with
the proteins. Hoppe-Seyler it was who first observed that, after
296 THE UNFERTILISED EGG [pt. iii
prolonged extraction of the yolk with ether, a considerable proportion
of the phosphatides still remained behind, and could be extracted
with alcohol. It was thought for a long time that the phosphatides
and the vitellin were in chemical combination which was broken
by the alcohol, but since the paper of Fischer & Hooker in 1 9 1 6 the
general opinion has been that this combination is only physical.
Stern & Thierfelder isolated traces of the cerebrosides, phrenosin and
kerasin, from egg-yolk in 1907.
The neutral fats and the lipoids of the yolk are variously affected
by the nature of the fats in the food of the fowl. Henriques & Hansen,
who were the first to investigate this subject, found that, if food con-
taining very unsaturated acids was fed to the laying hens, the neutral
fats in the eggs were affected, but not the fatty acid components of
the lecithin. Their figures are shown in Table 22. When the food
consisted of barley, pea or rice, the iodine number of the neutral
fats in the egg varied round about 77, but hemp or linseed sent it
up to about no, although no matter what the food was the iodine
number of the fatty acids in the phosphatide fraction remained con-
stant at 75 or so. Henriques & Hansen also found that the iodine
number of the fluid fatty acids of the neutral fat was normally 107-5,
and that the fluid and solid fatty acids of the phosphatide fraction
were 151-3 and 98-9 respectively. The former accounted for 64-3 per
cent, of the lecithin fatty acids. The experiments of Henriques &
Hansen have been repeated and confirmed by Belin and by Terroine
& Belin. The last-named workers, together with McCollum, Halpin
& Drescher, some years later reported that the lecithin fatty acids
would vary, as well as the neutral fatty acids, with the diet of the
hen. Their figures, which are given in Table 22, certainly show a
variation in the iodine numbers of both fractions. All these workers
recognised the presence of unsaturated acids in the yolk fat, and
Henriques and Hansen's figures came between the theoretical values
for oleic and linolic acids.
Work was continued along these lines by McGlure & Carr. Using
pigeons, they found that the fat content of the eggs could only be
altered slightly by feeding rations high and low in fat.
% fat in the eggs
Cocoanut fat ... ... ... 4-0
Beef tallow ... ... ... ... 6-75
Average of all fat diets ... ... 4-96
Average of all non-fat diets ... 4-81
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298
THE UNFERTILISED EGG AS A
[PT. Ill
Again, during the fat feeding, tiie saponification value of the egg-
fat was 176 (166-190) and during the rest of the time 173. The iodine
value was in the former case 70-5 and in the latter 70-8.
Table 24. Lipoid in egg-yolk.
Lecithin
0/
0/
/o /o
Wet weight Dry weight
Laves
Hen
8-90
16-80
Manasse
5»
9-41
17-80
Parke
»
i0'70
18-40
Barro
9-16
—
Glikin
3J
io*i6
17-36
"
Pigeon
13-11
28-76
Glikin' s figures.
Total
fatty
P,05 in Lecithin
Lecithir
Dry
acids
% of in %
in%
substance
% dry
fatty of fatty
dry
%
substance
acids acids
weight
Pigeon (yolk)
45-75
65-07
3- 16 35-73
23-37
45-63
6289
3-88 38-42
24-16
Turtledove (yolk)
17-95
44-06
4-10 46-65
20-55
Starling (whole egg) ...
28-26
5-67 64-44
18-21
Hen (yolk)
54-31
—
— —
13-71
Thrush
Cat ...
Rabbit
Guinea-pig
Lecithin in % dry
weight at birth or hatching
8-18
5-06
4-91
3-79
Some suggestive investigations on the biological significance of
ovolecithin were made b-y Glikin in 1 908, whose figures are shown in
Table 24. Choosing the pigeon as a typically nidicolous bird, and
the hen as a typically nidifugous one, he was able to show, using a
variety of extraction methods, that the yolk of the former was con-
siderably richer in lecithin than the latter, the former containing
about 29 per cent, dry weight, and the latter about 17. The further
but rather fragmentary observations which he made on the starling
and the turtledove confirmed this relationship. It is interesting that
Tso informs us that certain small Chinese breeds of hen produce very
small eggs (scarcely 40 gm.) and that these contain a much higher per-
centage of lipoids than ordinary eggs though an equivalent percentage
of protein. He concluded that lecithin, one of the most essential
yolk-constituents, was specially concentrated in nidicolous yolks and
SECT, i] PHYSICO-CHEMICAL SYSTEM 299
was associated with the property of early hatching or birth. Thus
he compared the thrush (nidicolous) with the guinea-pig, which is
born almost ready to eat green food and hardly passes through a
lactation stage; in the body of the former 8 gm. per cent, lecithin dry
weight was found, in the latter only 4. The new-born cat and rabbit
occupied intermediate positions. It is interesting to note that Glikin's
figures bear out those of Tarchanov on the question of water-content
of yolks from the two types of birds.
Tornani affirmed in 1909 that a difference in lecithin-content was
observable between fertilised and unfertilised eggs. But as he gave
no figures in support of his contention, it has not been treated with
much respect by subsequent workers.
The carbohydrate of the yolk has been the subject of only a very
few researches compared with that of the white. The figures which
have been obtained are shown in Table 16, and it will be seen that
in no case has the total carbohydrate been estimated, and only
in one case the glycogen. After Claude Bernard's isolation of gly-
cogen from the yolk, a persistent belief grew up that considerable
amounts of this substance were present there ; this was apparently
based on the description by Dareste in 1879 of "amyloid bodies" in
the yolk which gave microchemically a strong blue colour with
iodine. Dastre immediately pointed out that the occurrence of starch
there was highly improbable, and that if any glycogen was there it
should give a wine-red colour; he himself, however, could find
neither. But he did not succeed in suppressing the rumour, for
Virchow, and later Schenk, supported Dareste, though nothing has
been heard of this yolk-constituent since 1897, and Sakuragi's
analysis revealed the presence of only 2-2 mgm. per cent, of glycogen.
Bierry, Hazard & Ranc asserted in 191 2 that they could obtain a
great increase of carbohydrate after hydrolysing the yolk with
hydrofluoric acid under pressure, but this would not imply, as they
seemed to think, that glycogen was present, for all kinds of other
compounds such as proteins (Gross' protein for instance) might yield
glucosamine under such treatment. They identified glucosamine in
the hydrolysate. On the other hand, Diamare, who hydrolysed with
acetic acid, could only obtain faint traces of combined glucose in
the yolk. He dialysed both white and yolk, and in both cases was
able to estimate the free sugar, but in the case of the yolk very little
combined glucose seemed to be present. Further studies on this
300 THE UNFERTILISED EGG [pt. iii
subject should be undertaken, for the methods of Diamare and Bierry
ahke were of questionable reliability. Diamare, however, went rather
further into the matter than other investigators, and, thinking that
the yolk glucose might only be present there owing to an inflow from
the white, examined the ovarian eggs, in which he found glucose in
much the same proportion as in the yolks of laid eggs. He does not
state whether the ovarian eggs were yellow or white, and, as he
frequently gives his results in the form of grams of glucose without
mentioning the weight of the fresh material, it is impossible to calcu-
late the percentage (see also Tillmans & Philippi) .
We have already seen that cholesterol was identified in the yolk
of the hen's egg by very early workers such as Gobley. In 1908
Menozzi and in 191 5 Berg & Angerhausen sho\ved that egg cholesterol
was identical with that from milk and bile. It is certainly present
in the unincubated yolk both free and in esterified form with fatty
acids. Serono and Palozzi investigated a substance from egg-yolk in
191 1 which they called "lutein" but which turned out to be nothing
but a mixture of cholesterol esters. Other investigators have estimated
the amount of free and combined cholesterol in the unincubated egg,
and their figures are given in Table 25.
Table 25. Cholesterol-content of hen^s egg.
Mgm.
per whole egg
Investigator and date
Parke (1866)
Mendel & Leavenworth (1908)
Mueller (1915) ...
Ellis & Gardner (1909)...
Thannhauser & Schaber (1923)
Cappenberg (1909)
Dam (1929)
Schonheimer (1929)
Cholesterol Cholesterol
(free) esters
378 -
215-9 24-2
489 -
173 54-2
296 —
337 —
Total
248-3
240-1
600-6
227
180
Free in %
of total
89-92
806
76-8
88-87
Many Other substances have been found to be present in the egg at the
beginning of development, e.g. choline, alcohol, creatine, creatinine,
inositogen, lactic acid, plasmalogen (Stepp, Feulgen & Voit, 1927),
etc. These will be mentioned as occasion arises during the succeeding
sections of the book. Allantoin is not present (Ackroyd).
The yolk of the hen's egg also contains vitamines, pigments, and
a variety of enzymes, but these will be dealt with under their respec-
tive sections. As Langworthy has shown, it may also contain very
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302 THE UNFERTILISED EGG AS A [pt. hi
various substances derived from the diet of the hen, and these, if
they are odorous or possess taste may very easily betray their presence
(e.g. the Swedish "Schareneier" described by Hansson). Table 23
gives the figures which are available for the nitrogen and Table 26
for the phosphorus distribution. These summaries bear out on a
detailed basis what has already been said.
I -10. The Ash of the Avian Egg
The ash of the yolk and the white of the hen's egg has been in-
vestigated by several workers, and a study of it reveals certain inter-
esting features. If Table 27 be examined, it will be seen that, in the
yolk as well as the white, potassium has almost invariably been found
to be present in greater amount than sodium. This is one of the
characteristics of the egg-cell, as will be seen later when the eggs of
other animals are considered. The yolk is also marked by the very
high percentage of phosphorus, most of which is, in accordance with
other evidence, in organic combination. The calcium is also mainly
in the yolk, as is the iron, but not the magnesium. If now the amounts
of metallic and acidic ion be calculated out in millimols and milli-
equivalents per cent, wet weight, it is found that in both yolk and
white there is an uneven balance, but while in the former case there
is much more anion than cation (anion/cation ratio above unity), in
the latter case the exact reverse holds, and the anion/cation ratio is
somewhat below unity, about 0-55. In the white, therefore, some of
the potassium and sodium must be combined with the proteins, as
ovoalbumenates, etc.* However, the excess of cation over anion in
the white is not so considerable as the excess of anion over cation
in the yolk, and, bearing in mind also the much higher percentage
of solid in the yolk than in the white, it would be expected that the
anion/cation ratio of the whole egg would be greater than unity,
and would approach that of the yolk. The facts show that this is,
indeed, the case, for the average anion/cation ratio calculated from
the results of all observers for the whole egg is 2-3, as against 2-8
for the yolk alone and 0-54 for the white alone. This was first noted
by Garpiaux. Forbes, Beegle & Mensching expressed it simply thus :
Cubic centimetres normal solution
per 1 00 gm. dry weight egg
Total acid ... ... ... 120-28
Total base ... ... ... 39*42
Excess acid over base ... ... 8o'86
* In both white and yolk, of course, the inorganic ash is basic.
Duck
Hen ...
302*
Table 27. Aili of the avian
Lsh%
% of total ash
1/cight
K
Na
Mg
Ca
Fc
SO,
PO, c
■543
6-5
4-05
1-17
B-5
-
-
79-8 -
•437
U"5
0-76
i-i-2
D-7
-
-
79-1 —
•725
ma
I-J2
116
81
-
-
Bi-2 15-2
Mgm./loo gm. wet weight
K Na Mg Ca
Whole
2-59 2-72 0-75 3-28
)-33 048 0-79 3-5
Fe SO, PO,
.,.„. . , Total cation Total anion
Millicquivalents . '■ ^ , * ^ Anion/
K Na Mg Ca Fe SO, PO. CI mols equiv. mob equiv. ratio Investigator and date
3 — 2-59 2-72 1-50 6-56 — — 30-9 — 9-34 ,j.j7 ,0-3 jo-j 231 Polcck (1850)
y - 3-33 0-48 1-58 7-u - - 35-7 - 8-t ,2-39 ,,-9 jj.7 ,.-88 Rose (1850)
65 738 4-5 0-9. 1-66 7-0 — _ 34-95 7.38 9-79 /.,■„ ,9.03 ^.^j 3.00 Bialascewicz (1926)
3-32 50-3 S'll" 173 204
93 — 3-8 604 ]o8 443 8-8 0-34
004 636 302 443
4-6 — o-o8 19-08
S-j/ 9-42 s2-;a
Plimmer & Lowtidnt (1927)
Vaughan & Bills (t878)
Delezeime & Foumeau (1918)
Carpiaux {1908)
Buckner & Martin (
Normandy
Bourbonnais
)125 17-5 273 fo
)I30 15-4 23-8 Trace
3-590 23-55 ao-7 0-96
0-720 a7-tiG i'2-i 2-7
0-634 — — 24-B 25
o-Goo 22-1 20-63 1-41 I
2-9
368
6-1
35-7
G-9
ti-46
25-25
3-,6
15-5
435
-
27-6
3'-3
76
5-6,
14-8
0-52 2-9 1
89
5-13
13-5
4-87
While
49
356
53
0-24 o-.e
■ 2
5-1I
3-8
o-Bi 052
—
-
—
0-65 0-40
3-56 5-3
5-11 3-8
23-84
27-27
23-50
28-37
9-28
970
10-29
11-72
7-17 77-0^ 0-C25 Champion & Pellet (1876)
(not very reliable)
7-77 17-84 0-G29
4-76
0-9 4-4 9-39 s-s8
S-72 0-58 Poleck (1850)
3-51 4-12 035 Rose(iB5o)
- - - Voit(,877)
4-82 ssf c-55 lljin (1917)
— — — Prout (1822)
— — — — 4-25' —
523 4-l'8 3-79 1-24 8-72 -
345 '0-9 '-"8 2-2 13-62 —
129-a 52-5
85-5 4-25
Go- 16 3-6
84-6 —
64-7 1-8
48-4 3-7
.■58 6-6
!-48 9-0
2-4 9-6
,2-48
13-81
12-69
Average ... 0-54
40-8
- - 139-8
Straub & Hoogcrduyn (1929)
Poleck (1850)
Rose (1850)
Voit (1877)
Carpiaux (1903)
lljin (1917)
Prout (1822)
Straub & Hoogerduyn (1929)
*Kreis & Studinger.
SECT, i] PHYSICO-CHEMICAL SYSTEM 303
This is probably the most interesting consideration that emerges
from Table 27, but it may also be noted that the ash-content of the
white is just about half that of the yolk, a relation which would
practically be reduced to equality if the phosphorus in the yolk was
not taken into account.
The presence of certain chemical elements of lesser biological im-
portance has been announced from time to time in a group of papers
which have some interest, although it is difficult to see, as yet, what
their importance is for the development of the embryo. Fluorine has
been estimated by Tammann and by Nickles, copper by Dhere,
boron by Bertrand & Agulhon, manganese by Bertrand & Medigre-
ceanu, iodine by Bonnanni and by von Fellenberg, lead by Bishop.
These elements appear to be normal constituents of the egg. The
iron-content can be artificially increased by feeding iron-rich rations
to the hen, and iodine can also be introduced into the egg in this way,
as has been done by Bonnanni, Kreis and others, but Hofmann found
that though iron and iodine would enter the egg thus, it was impossible
to get copper to do so. In just the same way Ricci found it difficult
if not impossible to get As or Hg into the hen's egg by feeding sub-
toxic doses to the hen. The normal copper-content of the hen's egg
cannot be varied like its iron-content. The importance of iron in
the formation of haemoglobin is obvious, and the little that is
known about this process will be discussed in the section on pigments
in the embryo. Wassermann made a histochemical examination
of the egg-yolk and vitelline membrane for iron, and found a
relationship between the embryonic blood-islands and the iron of
the yolk.
Some of the other data in Table 28 call for comment, Tammann's
1-13 mgm. per cent, fluorine in the fresh yolk works out at a quantity
of 0-2 mgm. per egg, and, as Zaleski found 0-23 per cent, fluorine
in the bones of the chick at hatching, o-o8 mgm. fluorine would be
required in the egg at the beginning, or less than half of what is
actually there. Zaleski's figure, however, is old, and may be too low.
It would appear, on the whole, as if the greater part of the fluorine,
iodine, copper, zinc, lead, aluminium, silicon and manganese is
localised in the yolk, and the greater part of the boron and arsenic in
the white. In view of the importance which we now attribute to these
less common elements as catalysts in living tissues, this distribution
may be found to have considerable significance.
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3o6 THE UNFERTILISED EGG AS A [pt. iii
The figures which have been obtained by those investigators
who have examined the iron-content of eggs are seen in Table 29.
All found a great deal more iron in the yolk than in the white,
as might have been expected from the earlier micro-chemical
researches of Tirmann and Kobert. This kind of work had been
originated by Schmiechovski, and was continued later by Wasser-
mann in the interesting paper already referred to. Schmiechovski
found iron histochemically throughout the yolk, but considered that,
in the white or milky yolk, it was confined to the megaspheres.
Table 29* Iron in hen's eggs.
Italic figures indicate dry weight data.
FcaOs gm. % wet weight
Without iron-rich diet With iron-rich diet
, ' ^ r ^ ^
Egg-white Egg-white
Egg- plus Egg- plus
white Yolk yolk Shell white Yolk yolk Shell Investigator and date
•0024 -0088 -0047 -0272 -0040 'oogs -0059 -0272 Loges & Pingel (1901)
— — -0046 — — — -0040 — Kreis (1900)
•0057 -026 •0165 — — — — ^ Lebbin (1900)
■03 -05 -03 _____
•001 12 -00995 '00425 — — — — — Hartung (Mar. 1902)
•00087 -01106 -00451 — — — — — ,, (May 1902)
— — — — -00208 -01621 -00729 — ,, (June 1902)
Trace -0108 — — — — — — Bunge (1892)
— -0121 -0018 — — '0175 -0032 — Hofmann (1901)
— -;r -0057 — — — — — Boussingault (1850)
— -063 -og§ — — — — — Leveque & von
Tschermak (191 3)
None 'OI43 — — None '0143 — — Elvehjem, Kemmerer,
Hart, & Halpin (1929)
Wassermann, using both the ammonium sulphide and the Berlin
blue methods, decided that it was present in both kinds of yolk,
but that it was not confined to those special elements in the white
part. In fact, it was very much more abundant in the white than
in the yellow part. This finding has never been corroborated by
chemical analysis, but, if it is, it will have considerable importance,
in view of the time at which haemoglobin is most vigorously manu-
factured by the embryo. For a further discussion of these questions
see the section on pigments.
i-ii. General Characteristics of Non- Avian Eggs
With this we may conclude the discussion of what is known
about the typically terrestrial egg, that of the bird. Now
SECT. I] PHYSICO-CHEMICAL SYSTEM 307
aquatic species far outnumber the terrestrial ones; as Spenser
put it:
O ! What an endlesse Worke have I in hand
To count the sea's abundant progeny,
Whose fruitfull seede farre passeth those on land,
And also those which wonne in th' azure sky:
For much more eath to tell the starres on hy,
Albe they endlesse seeme in estimation,
Than to recount the sea's posterity.
So fertile be the flouds in generation,
So huge their numbers, and so numberlesse their nation.
It might therefore be supposed that a much greater space would
have to be devoted to their eggs than what has already been taken
up, but this is not the case, for the bird's egg has been so convenient
a material for research that the knowledge we have of it outweighs
that of the eggs of all other animals put together. Indeed, the data
about the eggs of other groups are very fragmentary, so that much
caution has to be used in making comparisons, and general relation-
ships are much more difficult to enunciate. Van Beneden's classical
memoir may be recommended as an account of the morphology of
the eggs which are to be mentioned, and it is hardly necessary to
refer to Balfour's book on comparative embryology.
If Table 30 is examined, and compared with Table 2, it will at
once be seen that the percentage composition of eggs of other classes
of animals differs markedly and in very definite ways from the egg
of the hen. The case of reptiles may first be taken, as being less remote
than others. The reptilian egg seems to be distinctly drier than that
of the bird, by about 20 per cent., and much more variable in its
fat/protein ratio. For, while in all birds' eggs that have been in-
vestigated, the amount of protein, whether related to dry or to wet
weight, is about the same as that of fat, the reptilian egg shows big
variations from this rule. In the eggs of the tortoise and lizard, for
example, there is twice as much protein as fat, while in those of the
grass-snake, studied by Galimard, there is three times as much fat
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which have also been found to contain a great deal more protein
than fat. Thus, instead of the 40 per cent, protein and the 40 per
cent, fat which make up the dry substance of the hen's egg, Faure-
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312 THE UNFERTILISED EGG AS A [pt. m
Fremiet & Dragoiu found the dry material of the egg of the frog to
consist of 60 per cent, protein and only 14 per cent. fat. And this has
been the experience of all those who have analysed amphibian eggs.
Next to the hen's egg, the eggs of fishes have probably received
the most attention. The recorded analytical figures for fish eggs are
often deceptive, for many analyses have been made of salted fish
roes and egg-preparations such as caviar, but the greatest care has
been taken not to include in Table 30 results which might have been
vitiated in that way. The question is complicated by the fact that
analyses of the purified constituent egg-substances prepared from
preserved material may well be admitted into consideration, for,
except in certain cases, they would probably not undergo much
change during the process of preservation. As in the case of reptiles
and amphibia, the fish egg is characterised by its predominance of
proteins as the food of the growing embryo. It should be remembered
that, in all these comparisons, the yolk of the hen's egg is a more
proper standard of reference than the whole egg-contents, in which
case the differences become even more remarkable. This generalisa-
tion appears at all points; thus, the brook-trout, a fresh-water fish,
has 30 per cent, of protein and only 9 per cent, of fat, and the herring
has 26 per cent, as against 3 per cent. In Table 31 the protein/fat
ratios are collected together, and the difference emerges there with
great clearness. Though it is obvious that fishes vary considerably
among themselves as to the fat-content of their eggs, yet all of
them have more protein than fat. The only fishes in Table 31
which come near to being exceptions to this rule are the sturgeon
and the dogfish, both of which have an unusually high amount of
fat in their eggs (a fact which accounts for the superiority of Russian
caviar over other varieties, for the former is made chiefly from the
eggs of sturgeon) . Besides this general difference between the egg of
the bird and that of the fish, there are many others, but they concern
the chemistry of the individual components rather than the rough
constitution of the egg as a whole, and will, therefore, be dealt with
later on. If Table 31 be further studied, it will be seen that, as far as
can be known at present from the few analyses of crustacean and
cephalopod eggs, the superior proportion of protein over fat holds
good there also. Curiously enough, the only analysis we have for
a gastropod egg gives a picture more resembling the egg of the hen,
comparatively equal amounts of fat and protein being present.
SECT. l]
PHYSICO-CHEMICAL SYSTEM
313
The position of affairs may perhaps be summarised by saying that
it is only the birds which have been successful in producing an egg
really well stocked with fat, though the reptiles clearly show an
approximation to this achievement. Does this mean that the storage
of fat in the egg is particularly associated with terrestrial embryos?
The facts and arguments to be brought forward in later chapters (see
Sections 7-7, 9-15 and 1 1-8) make this hypothesis a very likely one, but,
as Table 31 shows, the silkworm (the only representative of terrestrial
Table 31. Protein/ fat ratio in various eggs.
VERTEBRATA
Amniota
Aves
Hen (yolk)
„ (whole egg)
0-450
1-035
Reptilia
Anamnia
Lizard
Grass-snake
Tortoise
1-450
0-298
2-271
Amphibia
Pisces
Frog
Cod
Sturgeon
Herring
Carp
Trout
Dogfish
Salmon
INVERTEBRATA
2-6lO
12-550
1-937
8-333
9-101
3-518
1-050
2-400
ECHINODERMATA
Echinoidea
Sea-urchin
2-370
MOLLUSCA
Cephalopoda
Gastropoda
Octopus
Limpet
5-750
1-245
Arthropoda
Crustacea
Arachnida
Insecta
Crab
King-crab
Silkworm
2-709
3-166
2-139
Annelida
Polychaeta
Sabellaria
3-162
arthropods) does not seem to have succeeded in storing fat in its
egg to any great extent. It might, of course, be argued that this was
one of the factors which prevented the insects attaining any con-
siderable size and rivaling reptiles and mammals for the possession
of the land. The mammals gave up the heavy fat storage in the egg
when they invented viviparity and the fully developed placenta. In
314 THE UNFERTILISED EGG AS A [pt. iii
this connection the monotreme egg would be a chemical study of
great interest, and it is characteristic of the exasperating fragmenta-
tion of this field of work that all we know about the monotreme egg
is that its membrane seems to have the properties of a keratin. The
suggestion that the metabolism of the fowl, operating on a con-
tinuously high level of energy turnover, would naturally tend to fill
up the eggs with fat, and is associated with the well-known higher
temperature of the avian body (Wetmore) may not be without value,
but any special emphasis on fat metabolism in adult birds is precluded
by the statements in Schulz's review.
It is very significant that as animals became more complicated and
more adaptable to varied surroundings, higher, in fact, in the taxo-
nomic scale, they loaded their eggs to a greater extent with yolk. Since
the extra material was usually fatty acids, this process appears
strikingly in Table 31 . The effects of the yolk have long been familiar
to embryologists, and have been best described, perhaps, in a passage
by Milnes-Marshall. "The immediate effect of a large amount of
yolk", he said, "is to retard mechanically the processes of develop-
ment, but the ultimate result is to shorten them. This paradox is
readily explained. A small egg, such as that of Amphioxus, starts its
development rapidly, and in about eighteen hours gives rise to a
free-swimming larva, capable of independent existence, with a
digestive cavity and a nervous system already formed ; while a large
egg such as that of the hen, hampered by the great mass of yolk
by which it is distended, has, in the same time, made very little
progress. From this time onwards, however, other considerations
begin to tell. Amphioxus has been able to make this rapid start owing
to its relative freedom from yolk, but now this freedom becomes a
retarding influence, for the larva, containing within itself but a very
scanty supply of nourishment, must devote much of its energies to
hunting for and to digesting, its food, and hence its further develop-
ment will proceed more slowly. The chick embryo on the other hand
has an abundant supply of food in the egg itself and has no occasion,
therefore, to spend its time searching for it, but can devote its whole
energies to the further stages of its development. Hence, except in
the earliest stages, the chick develops more rapidly than Amphioxus
and attains its adult form in a much shorter time. The tendency of
abundant yolk to lead to shortening or omission of the ancestral
history, is well known. The embryo of forms well provided with yolk
SECT, i] PHYSICO-CHEMICAL SYSTEM 315
takes short cuts in its development, and jumps from branch to branch
of its genealogical tree instead of climbing steadily upwards. Thus
the little West Indian frog, Hylodes, produces eggs which contain a
larger amount of yolk than those of the ordinary English frog. The
young Hylodes is consequently enabled to pass through the tadpole
stage before hatching, and to attain the form of the frog before leaving
the c:gg\ the tadpole stage is, in fact, only imperfectly recapitulated,
the formation of gills, for instance, being entirely omitted."
The more yolk, then, the longer the embryo can remain an embryo
before having to face the external world, and the more preparations
it can make for that event. It is probable that this question is
intimately bound up with the penetration of fresh-water surroundings
by the originally marine forms. "It has long been noticed", said
Milnes-Marshall, following the classical exposition of Sollas, " that
marine animals lay small eggs whereas their fresh-water allies
lay eggs of much larger size. The eggs of the salmon or trout are
much larger than those of the cod or the herring, and the crayfish,
though only a quarter the length of the lobster, lays eggs of
actually larger size. The larger size of the eggs of the fresh-water
forms appears to be dependent on the nature of the environment
to which they are exposed. Considering the geological instability
of the land as compared with the ocean, there can be no doubt that
the fresh-water fauna is, speaking generally, derived from the
marine fauna, and the great problem with regard to fresh-water life
is to explain why it is that so many groups of animals which flourish
abundantly in the sea should have failed to establish themselves in
fresh water. Sponges and Coelenterates abound in the sea, but their
fresh- water representatives are extremely few in number; Echino-
derms are exclusively marine ; there are no fresh-water Cephalopods,
no Ascidians, and of the smaller groups of Worms, Molluscs, and
Crustacea, there are many that do not occur in fresh water. Direct
experiment has shown that in many cases this distribution is not due
to the inability of the adult animals to live in fresh water, and the
real explanation appears to be that the early larval stages are unable
to establish themselves under such conditions. To establish itself in
fresh water permanently an animal must either be fixed, or else
be strong enough to withstand and make headway against the cur-
rents of the streams or rivers it inhabits, for otherwise it will in the
long run be swept out to sea, and this condition applies to larval
3i6 THE UNFERTILISED EGG AS A [pt. iii
forms equally with adults. The majority of marine invertebrates leave
the egg as minute ciliated larvae, which are quite incapable of holding
their own in currents of any strength. Hence it is only forms which
have got rid of the free-swimming ciliated larval stage, and which
leave the egg as organisms of considerable size and strength, that can
establish themselves as fresh-water animals. This is effected most
readily by the acquisition of yolk — hence the large size of the eggs of
fresh-water animals — and is often supplemented by special devices."
Here is an explanation for the well-known paucity of eggs in fresh-
water plankton. In certain cases it is possible to induce an embryo
to skip the larval stage which it should normally pass through. Thus
Child could abolish the free-swimming larval stage in the ascidian
Corella willmeriana, simply by removing the eggs from the parental
atrial chamber {p¥L j'^.) to normal sea-water (/>H 8-4).
Giard had also noticed the discrepancy in egg-size between closely
related marine and fresh-water forms, and had classed it among those
cases where like adults have unlike larvae ("Poecilogony"). The
classical instance is perhaps that of the shrimp Palaemonetes varians,
one variety of which {microgenitor) lives in the sea near Wimereux
and has eggs 0-5 mm. diam. (32 1 per female) and another of which
{macro genitor) lives in fresh water at Naples and has eggs 1-5 mm.
diam. (25 per female). Giard has reviewed this subject in a very
interesting paper. "Dans un groupe determine", he said {(Euvres
diverses, p. 18), "la condensation embryogenique va en croissant des
types marins aux types d'eau douce ou terrestres."
The correlated proposition, namely, that the fresh-water forms
generally lay fewer eggs than the marine ones, is illustrated by the
following instances collected by Carpenter:
No. of eggs laid per female per annum
A
Lamellibranchs ...
Gastropods
Fishes
Crustacea
Marine form
Ostrea edulis i ,800,000
Buccinum undatum 12,000
Haddock 9,000,000
Lobster 5,000
Fresh-water form
Uniopictorum 220,000
Anodonta cygnea 18,000
Average of many snails 100
Average of many limpets 6
Ovoviviparous pond-snails 15
Brook-trout 750
Crayfish 200
Another reason for the poverty of fresh-water fauna was suggested
by von Martens who pointed out that the fresh-water climate, with
its periods of desiccation and freezing, was much more severe than
SECT, i] PHYSICO-CHEMICAL SYSTEM 317
that of the sea. But even these two causes together cannot fully
account for the phenomenon, for there are many cases of individual
species which they will not cover; thus the Cephalopods, which hatch
out as minute but very active copies of their parents, i.e. which pass
their larval stage within the egg, and which should therefore be
immune from the disadvantage described by Sollas, never penetrated
into fresh water.
A third reason must be added to those of Sollas and of von Martens.
As will be shown in Sections 12 and 13 the marine invertebrate embryo
depends largely on the salts of the sea water for its supply of ash, and
therefore could not be expected to develop in a medium very poor
in inorganic matter. Colonisation of the fresh water could not occur,
then, until animals had begun to provide in each egg sufficient ash
to make one finished embryo. There seem to be few data concerning
the capacity of marine invertebrate eggs to develop in fresh water,
although the adult animals have been found often enough to ac-
custom themselves to a fresh-water environment (see the instances
given in Semper) . Many studies of the effect of hypotonic solutions
on marine embryos can, however, be called to mind, and in all the
cases the results are teratogenic.
The fate of the Cephalopods, it is interesting to note, is explained
by this third factor, for Ranzi has demonstrated the intake of the
salts in the sea water by the octopus egg.
As for the general statement that animals can afford their young
a better chance of survival by providing them with larger amounts
of yolk and therefore a longer incubation-period, there is a striking
parallel here with the seeds of leguminous plants which are packed
with nourishment. In the Origin of Species (6th ed. p. 56), Darwin
wrote, "From the strong growth of young plants produced from such
seeds as peas and beans when sown in the midst of long grass, it may
be suspected that the chief use of the nutriment in the seed is to
favour the growth of the seedlings, whilst struggling with other
plants growing vigorously all round".
It is interesting that the birds show an adaptation exactly similar to
the poecilogony of the invertebrates and fishes. Tree-nesting birds are
usually nidicolous, but the defenceless state of the newly-hatched squab
has brought it about that ground-nesting birds are usually nidifugous.
As Table 30 shows, the composition of the eggs of all animals other
than those of the frog, the silkworm, and certain fishes, is still, to
3i8 THE UNFERTILISED EGG AS A [pt. m
use a phrase of William Harvey's, "hid in obscurity and deep night".
It is as yet much too early to try to draw any conclusions from the
very fragmentary figures which are all that we have at our disposal,
and we may well admit that one of the most urgent needs of chemical
embryology is a much wider extension of our knowledge of the static
chemistry of the egg. This is a quite indispensable preliminary to the
investigation of the metabolism of the embryo in the lesser known
forms. The attempt has already once been made to link up in some
way the chemistry of the egg with what is known of the type of
embryonic development which takes place in it. Wetzel in 1907
analysed the eggs of a sea-urchin, a crab, a cephalopod, and an
elasmobranch fish. He pointed out that the eggs he studied
were examples of varying richness in yolk, of total and partial,
equal and unequal, superficial and discoidal cleavage, as well as
chemical systems. Taking the egg of Strongylocentrotus lividus as
his first case, he regarded it as typical of a class of alecithic
eggs, of a total and equal cleavage type, and he drew attention
to the fact that it was rich in water and in salts, but poor in
fatty substances, in nitrogen, and in phosphorus. Similarly, in the
case of the mollusca, where there is no very definite type of
development, the egg of Sepia could not stand as representative of
any wider class than the cephalopods, but, as far as it went, it
showed that the cephalopod egg was rich in nitrogen, poor in fat and
inorganic substances, with a moderate phosphorus and water-content.
The decapod Crustacea, to which Maia squinado belongs, have a
purely superficial type of cleavage, with no cell-multiplication in that
part of the egg which holds the yolk. Accordingly, the egg possessed
a moderate fat and water-content, a moderate ash, and much protein
and phosphorus.
The mammalian ovum is still as unknown chemically as it was
when Wetzel was writing, and it may be found to have a
constitution not unlike the alecithic echinoderm eggs. For the
eggs of birds (and of reptiles, which only differ from them in
having very little egg-white) Wetzel found a low protein and
water-content, a high proportion of fat and ash, and a large amount
of calcium and phosphorus. Here cleavage would only take place
at one isolated point on the surface of the mass of food-material.
In the amphibia, the richness of yolk, while much more significant
than in lower classes, does not reach the level of birds and reptiles.
SECT. I] PHYSICO-CHEMICAL SYSTEM 319
and this is duly reflected in the chemical composition by the moderate
water-content, the high proportion of protein which is yet only
double that of the fat. The case of the dogfish is again different, for
there the egg is rich in yolk and the cleavage is meroblastic; thus
the water is rather low, the fat rather high, the nitrogen very high,
and the ash and phosphorus moderate.
But these conclusions of Wetzel's, interesting though they are, can-
not really be assessed until a great deal more comparative work has
been done. They must rather be taken to represent the kind of cor-
relation we may hope for in the future. However, one of Wetzel's
generalisations may be accepted, if with some reserve. He pointed
out that the fat-content of eggs showed great variations, rising from
12 per cent, of the dry weight of the Sepia ^gg to 66 per cent, of the
dry weight of the (yolk of the) hen's tgg. Again, the nitrogen
gave very variable results, rising from 5-3 per cent, of the dry
weight in the (yolk of the) hen's ^gg to 6-9 per cent, in the egg
of the grass-snake, 1 2 per cent, in the egg of the dogfish, and even
in the case of the cod 14 per cent. On the other hand, the phosphorus-
content varied only between the (outside) limits of 2 • i per cent, for
the sea-urchin tgg and 3-6 per cent, for that of the grass-snake.
Wetzel, therefore, suggested that a distinction might be made, at any
rate, roughly, between those constituents of the egg which may serve
as sources of energy for the growing embryo, and those which in
no circumstances do so. Protein, fat, and carbohydrate would come
in the former class; phosphorus (for nucleoprotein) and cholesterol,
for example, would come in the latter class. The former would show
great variations among eggs of different species, the latter would not.
He thus supposed that one might be able to deduce, as it were, the
constitution of any given egg, if one knew what substances, and in
what proportions, were used by the embryo as combustible material
during its development, as well as the constitution of the newly born
or hatched organism.
From this standpoint Wetzel distinguished four types of substances
in the unincubated egg : ( i ) material for the embryo to burn during
the course of its development, (2) constituents of the finished proto-
plasm of the embryo, (3) constituents of the finished embryo, but
not for incorporation into the protoplasm itself, but into the para-
plasm (in Le Breton's terminology), (4) the protoplasm of the original
egg-cell. No aspect of chemical embryology needs attention more
320 THE UNFERTILISED EGG AS A [pt. iii
urgently than this, and the correlation of chemical constitution with
developmental type should offer a most attractive field for research.
But it is not only correlations of this type that lie hidden under
the enigmatic character of analytical figures. The water-content of
the eggs may have a powerful effect on the sex-ratio, for King found
in 191 2 that reducing the water-content of fertilised frog's eggs con-
siderably lowered the proportion of males, while increasing it by
means of treatment with dilute acid considerably raised the pro-
portion. A discussion of these facts in relation to genetics as a whole
will be found in the review of Huxley. It is probable that the effect
which delayed fertilisation has upon the sex-ratio is to be explained
by difference in water-content of the eggs. Hertwig was the first to
observe this delayed fertilisation phenomenon in some work which
he published in 1905, and since then it has many times been observed
not only for amphibia but also for trout (Kuschakevitsch; Huxley;
Mrsic) . Riddle has suggested that the mammalian egg may be subject
to such influences as it passes from ovary to uterus. He quotes van
der Stricht's histological work on the bat's egg during this process,
and points out that the swelling of the yolk-granules would indicate
an absorption of water. The exact degree of hydration of the mam-
malian egg might thus conceivably have an effect on the mammalian
sex-ratio.
Table 30 has several more important points which have not, so
far, been touched upon. It is interesting to follow in the figures of
Milroy the difference between the fish eggs which float at the surface
of the water during their development (pelagic ova), and those which
sink, or rather float, at lower and denser levels (demersal ova) —
the former have a water-content of about 90 per cent., the latter of
about 70 per cent. A knowledge of the chemical composition of fish
eggs throws a great deal of light upon their distribution in the sea,
and so indirectly upon ecological problems. Their fat-content, for
example, has been treated from this point of view by Polimanti, whose
work will be discussed in the section on the general metabolism
of the embryo; and the investigations of the specific gravity of fish
eggs, which are discussed in Section 5, have also an important
bearing upon these problems. Another point worth notice is the
approximately constant percentage of cholesterol in different eggs,
nearly always about 500 mgm. per cent, of the wet weight, a pro-
portion which, roughly speaking, holds for the egg of the hen as well.
SECT, i] PHYSICO-CHEMICAL SYSTEM 321
It would be as well to emphasise the fact that no principle of selec-
tion has been used in the preparation of Table 30, on the ground
that results such as those of Roffo & Correa on a Brazilian gastropod,
and McCrudden on fresh-water fishes, which seem obviously wrong,
may not be so at all. The estimation methods and analytical processes
which are by general consent judged most satisfactory at the present
time cannot be considered in any way final, and to have excluded
certain results on account of the technique employed in obtaining
them would not have been justifiable. Table 30 does not, therefore,
absolve investigators fi'om the duty of looking up the original papers
in such cases as touch them most closely, and forming an independent
judgment, according to the best opinion of the time, on the stress
which can be laid upon them. It is needless to say that I leave out
of account all doubtful figures in the generalisations made here.
I -12. Egg-shells and Egg-membranes
Very little is known about the relative proportions of yolk, white,
and shell, in the eggs of the lower animals, or rather, in most cases,
egg-contents and shell or surrounding membrane. Table 32 gives a
few figures. The discrepancy between the results of Ford & Thorpe,
on the one hand, and Wetzel, on the other, is very strange, especially
as they both used Scyllium canicula eggs, but it is probably due to
insufficiency of the statistical element. Ford & Thorpe's proportions
are more likely to be accurate.
Much work, however, has been done on the membranes and hard
coverings which invest the unincubated eggs of diflferent kinds of
animals. For instance, the gelatinous substance which surrounds the
undeveloped amphibian egg was examined chemically by Brande
in 1 810, who noticed that it absorbed water and was not precipitated
by tannin or by strong acids. Later work has shown that it consists
almost entirely of mucoprotein and water. Wetzel's figures for its
weight are shown in Table 32. Giacosa isolated mucin in a pure
state from it in 1882, and the figures which he obtained for its per-
centage composition are shown in Table 33. He was able to show
the presence of a reducing sugar on hydrolysis, but he could
isolate nothing else from the jelly, and therefore concluded that
it was pure mucin. The presence of glucosamine in the muco-
protein was afterwards confirmed by Hammarsten, by Schulz &
Ditthorn and by Wolfenden, who confirmed Giacosa's finding that
322
THE UNFERTILISED EGG AS A
[PT. Ill
Table 32.
In % of total egg-weight
Species
Egg-membranes
Whit(
Herring
Carp ...
Cod ...
Pike ...
2-4
3-7
4-4
4-1
—
Dogfish
Silkworm
5-4
26-9
8-87 (wet)
19-3
36-5
Trout ...
Octopus
25-97 (dry)
I3-57-20-29
86-0
—
Yolk Investigator and date
— Konig & Grossfeld (191 3)
3? 33
75-3 Ford & Thorpe (1920)
36-5 Wetzel (1907)
— Tichomirov (1882)
14-0
3J
Kronfeld & Scheminzki (1926)
Ranzi (1930)
Tomita's figures.
Marine turtle ( Thalassochelys cortica)
Weight
Shell
White
Yolk
Total
m gm.
2-0
13-5
18-9
34-4
%
5-8
39-2
55-0
Wetzel's figures.
I Frog {Rana temporaria)
Ovarial egg (no jelly)...
Egg with unswoUen jelly
Jelly alone
Swollen jelly ...
Water content of ovarial egg
jelly
„ „ egg and jelly
Empty dry jelly
Dry egg
Dry egg + dry jelly ...
Thus of dry weight egg
jelly
Weight in mg.
1-897
4-674
2-777
8-97
0-62
0-90
1-52
Melvin's figures.
Shell-weights of insects
Squash-bug {Anasa tristis)
Luna moth [Tropoeoa luna)
Cecropia moth {Sarnia cecropia) ...
Smartweed-borer {Pyrausta ainsleii)
52-5
78-65
67-48
59-28
40-72
% of total
weight of eggs
29-2
23-3
22-0
31-0
it was remarkably resistant to putrefaction, and studied the
effect of enzymes such as pepsin upon it. The resistance of
frog ovomucin to putrefaction was for long a puzzle to bio-
chemists, but it seems to be explained by the unwillingness of most
SECT, i] PHYSICO-CHEMICAL SYSTEM 323
bacteria to grow on pure proteins, and as the jelly contains no
enzymes of an autolytic character no protein breakdown products
are formed, and consequently no bacterial growth takes place. This
might be considered a protection of the developing embryo from
bacterial attack. It is very probable, moreover, that the mucoprotein
acts as a source of nourishment for the young tadpoles immediately
after hatching, for they invariably attach themselves to it after they
emerge from the egg-membrane, and hang on to it by their oral
suckers (for histological details consult Nussbaum and Lebrun). On
the other hand, development will readily proceed in the absence of
the jelly, for as Hluchovski has shown it is disintegrated by exposure
to ultra-violet light and may thus be removed without harming the
eggs.
The swelling which takes place in the gelatinous covering when
the eggs are shed into the water was studied as long ago as 1824
by Prevost & Dumas, who measured the size of the eggs at intervals
after they were laid. Their table is as follows :
Hours after laying
Diameter of egg (mm.)
0
2-5
1-5
5-0
2-5
6-3
3-5
7-1
4-5
7-2
5-5
7-1
6-5
7-3
They observed that dyes would pass through the jelly as soon as
it had swollen, but not before. Similar work by Wintrebert on
Discoglossus pinctus gave the following figures :
after laying
Diameter of egg (mm,
o-oo
2-5 X 2-3
0-03
3-0 X 2-7
016
3-3 X 3-0
0-66
5-8 X 3-2
800
5-OX4-6
As regards the mineralogical and morphological structure of the
egg-shells of the lower animals, a good deal is known, and for full
detail the reviews of Prenant and of Biedermann should be referred
to. The majority of reptile egg-shells have their calcium carbonate
in the form of calcite, as Kelly; Schmidtt, and Meigen have shown,
but the two first-named investigators discovered that the tgg-
sheUs of chelonia were of aragonite, and later Lacroix observed
a similar phenomenon in the case of certain saurians. The tgg-
-a
a
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a
plH lO
u
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d
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w JJ p
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to 0
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6^
326 THE UNFERTILISED EGG AS A [pt. m
membranes of snake's eggs which show all variations as to lime-content
(see Table 9) are, as Kelly has shown, composed of amorphous and
unstable calcium carbonate. The eggs of gastropods, such as Helix,
Ampullaria, Bulimus, Amphidromus, etc., are, as Turpin {Helix aspersa)
and Rose {Helix pomatia) , besides the workers mentioned above, have
demonstrated, like those of birds in having their lime in the form
of calcite. For a general theory explaining these differences see the
paper of Prenant.
The shells of eggs may also contain calcium phosphate. In the
hen and in birds generally there is very little, but the globules seen
in their egg-shells are believed to be calcium phosphate, though no
analysis has given a figure of more than i per cent, of this salt.
In other eggs, however, there may be more; thus Gmelin found
7-3 per cent, in the egg-shells of a tortoise, and Kelly noted its
presence also in those of Bulimus and Lophohelia, though she gives no
analytical figures.
It is interesting to note that the mineralogical form of lime in the
egg-shell may vary during the development of the embryo; thus
Kelly says that the shell of many full-grown mollusca is conchite,
while that of their respective embryos and eggs is calcite. Kelly found
that the organic substance was a remarkably constant proportion
of the shells of mollusca, reptilia and birds (see Table 9). Some egg-
coverings contain almost no water at all (birds), others have more
than the egg-contents, as has been shown for the trout's egg by
Kronfeld & Scheminzki (membrane 75 per cent., egg 66 per cent.).
By far the commonest substance of which egg-membranes are com-
posed is keratin, though this protein seems to take many forms, and
not to have exactly the same properties in different situations. The
earlier workers were content to assert the presence of it on the basis
merely of solubility tests. Thus in 1874 Schenk studied the egg-shell of
Raia quadrimaculata, and decided that it was 95 per cent, keratin after
the application to it of the protein colour reactions and an examina-
tion of its behaviour towards various solvents. The same conclusion
was arrived at by the same methods by Hussakov & Welker for the
egg-cases of Raia erinacea, and the Port Jackson shark, Heterodontus
philippi. The keratin of these egg-cases was insoluble in all solvents
except acid and alkali. They found that sulphur was present, but
no phosphorus, and they were unable to find any reducing sugar
after total hydrolysis. Irvine, using an optical test for chitin, found
SECT, i] PHYSICO-CHEMICAL SYSTEM 327
none in elasmobranch egg-cases. Krukenberg in 1885 decided that
the egg-case of Scyllium stellare was of a keratinoid nature, because
of its percentage composition, in which he found a marked amount
of sulphur. He observed the interesting fact that the egg-cases of
this fish, while still in the uterus of the parent animal, would dissolve
in pepsin and trypsin, while after they were laid they would not
dissolve in solutions of either enzyme. He also isolated tyrosine and
leucine firom the keratin of the egg-cases of Scyllium stellare. He made
very similar researches on the egg-cases of Scyllium canicula and
Myliobatis aquila, finding that they possessed rather different properties
and seemed to be of different constitution; thus on hydrolysis he
recovered a great deal of leucine and hardly any tyrosine from the
keratin of Scyllium canicula, while from the keratin of Myliobatis the
yields were precisely reversed. The latter substance was also con-
siderably more resistant to digestion than the former, and Krukenberg
considered that the former was not a keratin at all. He had already
decided (wrongly, as it turned out) that the shell-membrane of the
hen's egg was mucin, not keratin, and now he concluded that this
also applied to the egg-case oi Scyllium stellare, as well as to that ofLoligo
vulgaris, of which he made a separate examination. He thought it
possible also that the jelly which surrounds the egg in the ovo viviparous
selachians might be a mucin too, especially as, according to Schenk, it
was not precipitated by chromic acid, and he himself found that it was
extremely resistant to digestion by enzymes. This material has received
no further chemical investigation since the time of Krukenberg.
Other workers who identified the proteins of egg-membranes
by the aid of colour tests and solubility reactions were Leuckart,
who showed, as far as anything could be shown with such preliminary
methods, that the membranes of planarian eggs were of chitin, and
Yoshida & Takano and Jammes & Martin, who drew a similar con-
clusion about the coats of the eggs of Ascaris lumbricoides, which they
found were readily soluble in gastric juice or in any acid.^ The
case of the parasitic nematodes is of special interest, for the chitinous
membrane does not arise until after the fertilisation of the egg, being,
therefore, in a sense, analogous to the fertilisation membranes of
echinoderms. Whether the chitin is formed as it is required during
these early stages, or whether it is already present in the unfertilised
egg-cell in some soluble form, is uncertain. Faure-Fremiet in an
^ See also Campbell on the chitin of insect egg-membranes.
328 THE UNFERTILISED EGG AS A [pt. iii
attempt to throw light on this question, prepared pure samples of
chitin from the newly fertilised eggs ofAscaris megalocephala by boiling
them with strong potash, and identified the chitin chemically,
isolating glucosamine hydrochloride from it. Remembering that
Weinland showed that chitin is probably formed from glycogen
during insect metamorphosis, Faure-Fremiet estimated the glycogen
in the Ascaris eggs before and after fertilisation. Before fertilisation
there was an average amount of 20 gm. per cent, dry weight, but
afterwards only 4-67, the extreme values being 5-91 and 3-23, so
that no less than 17 per cent, of glycogen had disappeared. Estima-
tions of chitin in the egg-envelopes after fertilisation gave results of
between 8-3 and 10-7 per cent, dry weight of glucosamine (calculated
as glycogen) with an average of 9-23. The total glucose, then, in the
fertilised eggs was 12-83 to 15-08, as against 20-0 in the unfertilised
ones, a loss of 7 to 9 per cent. All the glucose lost, therefore, could
not have transformed itself into chitin, but must have had some other
destination, perhaps butyric and valerianic acid if Weinland's view is
correct. The eggs o^ Ascaris have also an " ovospermatic membrane",
but for the discussion of the significance of this reference should be
made to the memoir of Faure-Fremiet, and nothing is known about
it chemically. Their third membrane, the internal one, would seem
to be composed to a large extent of ascaristerol (see p. 352), for the
histological evidence demonstrates a collection of the ascaristerol
globules at the periphery of the cytoplasm. After fertilisation, Faure-
Fremiet found the saponification number of ascaristerol lowered from
199 to 145, from which he concluded that its constitution had been
slightly altered. Zavadovski has also described the egg-shells of many
nematodes. •
Neumeister, who found more than 5 per cent, of sulphur in the
shells of the reptiles, Calotes jubatus, Ptychozoon homalocephalus, and
Crocodilus biporcatus, concluded that they consisted of a true keratin,
and the reactions given by the egg-membrane protein of a mono-
treme, Echidna aculeata, led him to the same conclusion in that case
also. Table 9 gives the figures which he obtained for the calcium
and other constituents of some of these egg-shells, as well as the
very similar investigations of Wicke & Brummerstadt on Alligator
sclerops. From these fragmentary results, it would seem that the egg-
membrane protein is here keratin, and a quantity of calcium is
secreted into the membrane by the animal, varying in amount from
SECT, i] PHYSICO-CHEMICAL SYSTEM 329
90 per cent, to 10 per cent,, according to the species. Again, the
egg-membrane of the Brazihan gastropod studied by RofFo & Correa
is said, on the basis of qualitative tests only, to be a true keratin,
containing no reducing sugar and associated with no other sub-
stances, save 2*45 per cent, of ash. It contained calcium the amount
of which did not vary during development.
The transparent horny egg-membrane of the selachian Mustelus ^
laevis, which disappears half-way through the development of the ■
embryo, has also been investigated by Krukenberg, who compared
it with the egg-membrane of the grass-snake, Tropidonotus natrix. The
former resembled the shell-membrane of the hen's egg rather than
the true keratin of the Myliobatis egg-case. The latter seemed to have
some of the properties of elastin and some of those of keratin ; from
it he was able to isolate a reducing carbohydrate as well as glycine,
tyrosine and leucine.
Krukenberg was also one of the earliest workers to make quantita-
tive investigations on this subject. His figures for the protein of the
egg-shells of Murex trunculatus and the whelk Buccinum undatum, which
are given in Table 33, led him to make a new class of such substances,
the conchiolins. As no data exist for the sulphur content of most
of these proteins, it is impossible to say whether they are keratins
or not, and the whole subject needs re-investigation. About five years
later, Engel also investigated the egg-membrane protein of Murex,
and, obtaining 0-5 per cent, of sulphur from it, concluded, its other
properties taken into account, that it was a keratin. Engel also agreed
with Hilger, whose figures for the egg-membrane of the snake,
Coluber natrix (see Table 30), suggested an elastin as its principal
component. He had not been able to find any sulphur in it. About
the same time, Wetzel examined the conchiolin in_the_egg-shells of,
Mytilus edulis, and obtained from it, after hydrolysis, leucine, tyrosine,
glycine, Various hexone bases and ammonia, but no phenylalanine.
The first efforts at quantitative discrimination between egg-mem-
brane proteins were contented with ascertaining the elementary
composition ; thus von Fiirth analysed the protein of Loligo vulgaris
eggs in this way (39 per cent, glucosamine), and Verson, and later
Tichomirov, decided that the egg-shell of the silkworm, Bombyx mori,
was a keratin-like body (3-7 per cent, of sulphur), though, owing to
its unusual properties, they called it chorionin. Of these two last-
named analyses, it is probable that Tichomirov's is the more accurate.
330 THE UNFERTILISED EGG AS A [pt. m
for he was more careful to remove all the adhering silk than was
Verson, and Farkas' independent work agrees rather with his. It is,
at any rate, clear that the shell-substance of the silkworm's egg is not
chitin. According to Lavini the inorganic constituents of the silk-
worm egg-shell are potassium silicate, sulphate, and carbonate, to
the exclusion of all other salts.
The work of Pregl and of Buchtala in 1908 is perhaps the most
thorough investigation of the amino-acid distribution of an egg-
membrane protein. The figures they obtained are given in Table
34. The keratin of the egg-case' of Scyllium stellar e was the only
one of which they made a complete amino-acid analysis ; for that
of Pristiurus melanostoma and Scyllium canicula they only determined
the cystine content and large groups such as the monoamino-acid
nitrogen. Scyllium ovokeratin seemed to follow very closely in its
constitution the ovokeratin of the hen, according to the figures of
Abderhalden & Ebstein, which have already been discussed, but
separated itself off very sharply from it on account of its high tyrosine
content. The ovokeratin of the tortoise Testudo graeca, which had
been investigated two years previously by Abderhalden & Strauss,
was again different, having no tyrosine, but a very high percentage
of proline. As far as this work goes, it would seem right to con-
clude that, though the eggs of different species may use similar
proteins in their external membranes, the constitution of these proteins
may vary very considerably.
The work of Steudel & Osato, and of Osato, however, brought a
new factor into the problem. Their analyses of the egg-membrane
protein of the herring's egg, which are shown in Tables 34, 38 and 39,
gave results which differed from the usual keratin figures, but which
very closely approached the analyses which they were making at
the same time of the ichthulin of the herring's egg. Thus the amide
nitrogen (2-05 per cent.) was lower than any of the keratins, but
approximated instead to the i-8i per cent, of herring ichthulin. What
appeared to be the case on a general survey turned out to be certainly
so when the amino-acid distribution was examined, for the two sets
of figures almost exactly corresponded. The properties of the egg-
membrane protein and the minute amount of sulphur in it precluded
its classification as a keratin, and the fact that no reducing sugar
could be discovered among its breakdown products was convincing
evidence against its being a mucin. Osato suggested that it was
330*
Table 34. Distribution of amino-acids in egg-proteins.
Amide N ...
Glycine
Alanine
Valine
Leucine
Proline
Phenylalanine
Aspartic acid
Glutamic add
Serine
Tyrosine
Cystme ...
Hiiiudinc ...
Arginine
Lysine ...
Tryptophane
Humin
Unidentified
Total di-amino
Total mono-am
Non-amino N c
ii Scl
U
tl
Present 19'40 —
53-73
2-76
4.4 _ ^
^Jolie
O'bo
-
103
10-6
-
-
021
-
0'44
-
0-30
i-H'i
114
'•7
-
0-32
'3'5l'
l-o6
3-2
-
'■45
io-i6
0-29
3-7
—
—
9-B6
—
-
None
Present
Present
Present
Present
Present
50-7 —
2-56
9-04
Present
13-72
3-8o
0-37
4-i6
— None
6
3-8
3-66
-
Ii-ig
o-t
-
0-93
0-62
0-34
None
-
82
-
-
8-83
Present
'None
92
-
-
n-og
o-ig
O'SS
07
-
-
8-33
0-39
0-22
-
l-8l
2-05
7-56
-
-
-
—
■■77
—
—
—
6 1 -55
3.90
None
SECT, i] PHYSICO-CHEMICAL SYSTEM 331
simply an insoluble modification of ichthulin. As he pointed out,
industrial use has long been made of insoluble forms of proteins,
such as casein, and there was no reason why the egg-membranes
of certain eggs, at any rate, should not be insoluble modifications
of the proteins of their yolks. Steudel & Osato also suggested that
the ovomucoid of the egg-white of the hen might be a phylogenetic
reminiscence of the mucoprotein with which the amphibian egg is
surrounded. For a review of this work see Steudel.
The eggs of salps and tunicates are surrounded by a coat of very
much smaller cells which act as some sort of protection for the
developing embryo inside. Zavattari has demonstrated histochemi-
cally the presence of an abundance of glycogen in these test cells,
and believes that they have a nutritive function. If so, this would
be a third case where such an active participation of the shell or
case in embryonic metabolism would have been noted, the two others
being the abstraction of calcium from the shell of the hen's egg, and
the contribution of amino-acids by the egg-case of the silkworm.
A good deal is known about the osmotic and other properties of
the membranes of amphibian and fish eggs, but these are so intimately
associated with the physico-chemical processes taking place during
development that consideration of them will be postponed to Sec-
tion 5. It will suffice to mention here the experiments of Peyrega,
who found that the egg-cases of Scyllium canicula were permeable
to salt. He fitted up osmometers with small pieces of the case as
the membranes, and observed that it took about 20 days to establish
osmotic equilibrium with respect to solutions of sodium chloride
about as strong as sea water, when distilled water was put on the
other side. These egg-cases have also been shown by Needham &
Needham to be permeable to urea and ammonia.
1-13. Proteins and other Nitrogenous Compounds
The principal protein substance which is found to occur in the
eggs of all known animals closely resembles the vitellin of the
hen's egg. It has even been found, according to Chatton, Parat &
Lvov, in the food-reserves of infusoria. The early analyses of the eggs
of the pike by Vauquelin in 181 7, of the barbel {Cyprinus barbus) by
Dulong d'Astafort in 1827, and of the trout [Salmo fario and Cyprinus
carpio) by Morin in 1823, ^^^ to no more than the view that an
albuminous substance w£is present in them. But with the work of
332 THE UNFERTILISED EGG AS A [pt. iii
Gobley on the hen's egg, which has already been described, a more
solid basis for comparison was achieved, and Valenciennes & Fremy,
in a memoir which received a prize from the Academy of Sciences
and which was translated into English, proceeded to examine the
eggs of as many species as were available to them. Gobley's only
excursion into comparative chemical embryology had been a detailed
analysis of the carp's egg, published in 1850, but he had not been slow
to point out the differences between this analysis and that of the hen's
egg. His figures are shown in Tables 2, 30 and 33, where it will be
seen that he got a value of 15-76 per cent, protein (wet weight) for
the hen, and 14-23 per cent, for the carp, but 31-43 per cent, fat
for the hen and only 2-57 per cent, fat for the carp. The carp's egg
had, he found, about 10 per cent, more water than the yolk of the
hen's egg, but only a third of the lipoid substances.
Fremy & Valenciennes specially directed their attention to the
protein fraction, and attempted to discover whether the vitellin was
the same in all eggs. For the most part they relied on histological
appearances (the "dotterplattchen" were greatly discussed at this
time), but they also examined the solubility relationships of the
proteins from each egg, and in some cases subjected the purified
substances to elementary analysis. The figures they obtained for the
different compounds are all given in Table 33, and the eggs they
investigated in Table 35. They were able to isolate a number of
vitellin-like proteins, soluble in salt solution and precipitated
by the addition of water. They compared vitellin with fibrin,
and concluded that the two substances were almost identical, in
spite of slight differences in the analytical figures — "for bodies
of this nature", they said, "which are not crystallisable and
insoluble in water and which are therefore very difficult to purify,
where is the chemist who could answer for i per cent, of nitrogen in
an elementary organic analysis?" Ichthin, which they isolated from
fish eggs, differed from vitellin by not becoming an opaque mass
when placed for a long time in boiling water, and by giving a
violet instead of a blue colour when treated with boiling hydrochloric
acid. Ichthidin, another product offish eggs, differed from ichthin in
being soluble in water. Ichthulin, the third member of the group,
differed from the others in not being soluble in all dilutions of saline,
but in being precipitated from the aqueous extract by further ad-
dition of water. As for emydin, it closely resembled ichthin, and it is
SECT. l]
PHYSICO-CHEMICAL SYSTEM
333
not easy to see why Valenciennes & Fremy did not identify it with
that substance. The remaining egg-proteins, which they did not
further investigate, they referred to under the generic name of
albumen.
Table 35. Investigations
AvES Callus domesticus
Pisces
of Valenciennes & Fremy.
Vitellin
Elasmobranchs
Raia clavata
Ichthin
Torpedo martnorata
>j
Scyllium canicula
99
Galeus canis
jj
Alustelus laevis
99
Squatina angelus
99
Raia fullonica
>j
Raia rubus
j>
Teleosteans
Cyprinus carpio
Ichthidin and ichthulin
Labrax lupus
Ichthulin and ichthidin
Alugil chelo
Scomber scombrus
Pleuronectes maximus
Pleuromctes solea
Solea armorica
Unidentified species
of salmon
>)
eel
Albumen
Reptilia
Testudo mauritanica
Emydin
Cistudo europaea
jj
Unidentified species
of lizard
Vitellin
jj
grass-snake
)>
>>
viper
„ (?)
Amphibia
}>
frog
Ichthin
>>
newt
5>
Crustacea
jj
lobster
Albumen
Ar-achnida and Insecta —
—
Albumen
MOLLUSCA
—
Not albumen
The differences between the compositions which Valenciennes &
Fremy found for these substances are not great, and it is very doubtful
whether they are more than modifications of the same substance,
especially as these workers admittedly had great difficulty in ob-
taining pure preparations. But the problem of the identity of the
vitellins is not yet settled. The later investigations are all grouped
together in Table 33, and the differences between the preparations
can easily be seen to be small. The work of Plimmer & Scott
proved that ichthulin is a phosphoprotein closely allied to vitellin.
Among the more interesting observations must be mentioned
those of Levene & Mandel; Levene, and Walther, on ichthulic acid
obtained from the ichthulins of various fish eggs by digestion
334 THE UNFERTILISED EGG AS A [pt. iii
with pepsin and other methods. These with their very high
phosphorus content approach closely the " paranucleins " or
vitellic acids obtained from the vitellin of the hen's yolk by
Levene & Alsberg and others. Evidently there are several possible
stages of breakdown, for Walther's ichthulic acid only contains 2-8 per
cent, of phosphorus, while that of Levene has as much as 10-4.
Here, also, however, there are great variations; thus, while nearly
all the ichthulins studied have from o-6 to i -9 per cent, of phosphorus,
the preparation of Steudel & Takahashi from the herring's egg has
only 0-014 P^r cent. In the yolk of a dogfish egg, Zdarek found no
less than three proteins, the third of which may possibly correspond
with Konig & Grossfeld's albumen class.
In 1908 Alsberg & Clark claimed that phosphorus was quite absent
from the principal protein of the egg of an ovoviviparous selachian,
Squalus acanthias, but some twenty years later I re-examined the
question and obtained without difficulty o-6 per cent, from selachian
ichthulin (derived from the same species). This yolk also contains a
second protein, thuichthin, corresponding closely in properties and
constitution with the ovolivetin of the hen studied by Kay & Marshall
(see Tables 10 a and 33).
Gray has studied the properties of the ovoglobulin or ichthulin
of Salmo fario. If the yolks are poured into water, a dense white clot
is formed and the water becomes cloudy. The precipitate is soluble,
however, in acids, alkalies and neutral salts. When the egg-cell
dies, the egg becomes opaque, and this must certainly be due to the
precipitation of the globulin, for by placing dead white eggs in
normal sodium chloride solution they rapidly become clear and
resemble normal eggs, but regain their opacity when removed to
distilled water. The clearing process takes 15 minutes but the
precipitation takes i| hours. Evidently the dead protoplasmic
membrane can no longer retain in the egg the electrolytes necessary
for solution of the ichthulin.
Further work on the properties of teleostean ichthulin was
done by Runnstrom. 1-5 parts of egg "Pressaft" having been added
to 1-28 parts of water and the ichthulin precipitated, the effect
of various ions on its solubility was tried. The anions placed them-
selves in the order:
SON > I > NO, > SO. > CI > acetate.
SECT. I] PHYSICO-CHEMICAL SYSTEM 335
Thus for 2 c.c. of potassium chloride solution, 0-3 c.c. of distilled
water had to be added to get coagulation, but to 2 c.c. of KSCN
solution, as much as 6-4 c.c. The cations went as follows:
Ca > Mg > Sr > K and Na.
The egg-white of the dogfish egg was thought by Brande in 1810
to be identical with the jelly surrounding the egg of the frog, but
whether the former really consists of mucin and not albumen cannot
be definitely stated, for no work has since been done on it. However,
my wife and I, in our work on the eggs oi Scyllium canicula, frequently
observed a coagulation of the egg-white with acetic acid, which would
point to the latter possibility.
The proteins of the echinoderm egg have never been properly
investigated. Vies, Achard & Prikelmaier have estimated from cata-
phoresis experiments that the average isoelectric point of the Para-
centrotus lividus egg-proteins lies between 5-0 and 5-8 pH., but their
grounds for this figure are not free from criticism.
Vies & Gex, in some interesting experiments, have studied the
normal unfertilised sea-urchin's tgg spectrophotometrically. The
absorption spectrum of the normal egg has peaks or bands at
wave-lengths of 490, 395, 370, 315, and 230 Angstrom units, and a
marked trough between 260 and 240 A. This curve is very peculiar,
for on the one hand it shows much transparency in the ultra-violet
although most organic substances do not, while on the other hand
there is nothing at all corresponding to the bands of absorption
about A 275 which all proteins give. This absorption is brought
about by the cyclic amino-acids in the protein molecule, and it is
quite impossible that these should be altogether absent from the
egg-proteins of the sea-urchin. Vies & Gex considered various
technical possibilities which might explain these effects, but did
not think that any of them would account for what was perhaps the
most remarkable part of the investigation, namely, the finding that
on cytolysis ("white") a perfectly definite and clear absorption
spectrum for protein revealed itself In the intact egg, then, this
must be masked by something else. Speculation on the nature of
this mechanism would be easy, for all kinds of eflfects might be
responsible, e.g., formation of complexes, reduction equilibria, and
satisfaction in vivo but not in vitro of residual valencies in the protein
molecule. If this very interesting work should lead in the future
336 THE UNFERTILISED EGG AS A [pt. iii
to a revivification in a subtler form of the old biogen molecule
theory (though it is to be hoped that it will not), not only as regards
the egg-cell but as regards protoplasm in general, we shall at any
rate possess in the spectrophotometer a powerful means of studying
the untouched normal cell-interior.
Doubt exists with respect to the presence of reducing carbohydrate
in the ichthulin molecule. Levene & Mandel obtained minimal
quantities of laevulinic acid from their cod ichthulin, but this
finding was associated with the presence of purine bases. Six years
earlier Levene had been unable to find a trace of glucosamine in
cod ichthulin. Similar negative results were obtained by Steudel &
Takahashi on herring, and by Hammarsten on perch, ichthulin.
But the presence of glucosamine in notable amounts has been
reported for Torpedo ichthulin by Rothera, and for carp ichthulin by
Walther. While it is possible, and even probable, that ichthulins
from different fish eggs may vary much, it would be very desirable
to know to what extent this is the case, and a comparative study
of ichthulins is much needed. As we have seen Levene & Mori have
isolated a trisaccharide from avian vitellin.
Closely allied to the question of the presence of carbohydrate
groupings in the ichthulin molecule is the equally disputed problem
of the presence of purine bases in the undeveloped tgg. We have
already seen that Miescher's identification of nucleoprotein with
vitellin was quite erroneous, and have described how he was set
right by Kossel. For the hen's &gg, it is now fairly clear that nucleins
are present only in exceedingly small amounts at the beginning of
development, not exceeding, for instance, i or 2 per cent, of the total
nitrogen or phosphorus. But there has been more difficulty in de-
ciding what is the real state of affairs in the eggs of fishes and
aquatic invertebrates. Walther (carp), Hugounenq (herring), Linnert
(sturgeon), and Hammarsten (perch), all examined the ichthulin of
these eggs for nucleic acid, and all failed to find the least trace of it.
Henze, on the other hand, working with the whole tgg of the
cephalopod. Sepia officinalis, isolated considerable amounts of purines
together with no less than 1-15 gm. per cent, of a pentose. Tscher-
norutzki a little later found that 10 per cent, of the total phosphorus
of the herring's egg could be accounted for as nucleoprotein phos-
phorus, and the nucleoprotein itself amounted to i-ig gm. per cent,
dry weight. Masing; Tichomirov, and Needham & Needham reported
SECT, i] PHYSICO-CHEMICAL SYSTEM 337
quite similar results with the sea-urchin's egg, the egg of the silkworm
and the eggs of various Crustacea, echinoderms and an annelid. In
the sea-urchin egg purine bases were found accounting for 6 per cent,
of the total nitrogen as nucleoprotein nitrogen, while in the case of
Bombyx there were 20 mgm. per cent, dry weight. Again, Levene &
Mandel isolated from their ichthulic acid in 1907 0-344 P^^^ cent, of
guanine, 0-307 per cent, of adenine, 0-360 per cent, of uracil and
0-309 per cent, of thymine. Mandel & Levene were also able to
isolate nucleic acid from cod's eggs. It would certainly appear from
this evidence as if ichthulin and vitellin may be associated with small
quantities of nucleic acid. In this connection it is of interest that
Calvery has evidence that the chick embryo can synthesise "yeast-"
as well as animal nucleic acid. Steudel & Osato have also obtained
guanine and adenine from herring's eggs, but this was in the non-
protein nitrogen fraction, and there was therefore no evidence from
their work that any preformed nucleic acid was a constituent of the
egg. The most exhaustive investigation of the problem was that of
Konig & Grossfeld, who in 1913 set out definitely to clear up the
discrepancy. As perhaps might have been expected, they found that
they could isolate purine bases after hydrolysis from all the fish eggs
they studied, but only in small quantity; their results are shown in
Table 36. The question of nuclein synthesis by the developing
embryo will be discussed in relation to these findings in Section I0'3.
Table 36. Investigations of Konig & Grossfeld.
Total purine bases isolated
Mgm.%
dry weight
Herring
Carp
Cod ...
Pike ...
Sturgeon
0-408
I -060
2-440
0-014
0-230
But the exact relationship between the nuclein and the vitelHn
remains exceedingly obscure. It is possible that in one and the
same egg there may be more than one modification of vitelHn,
apart altogether from the insoluble form suggested by Steudel &
Osato. All the knowledge that we possess at the present time
338
THE UNFERTILISED EGG AS A
[PT. Ill
on this point is of an unsatisfactory histological nature, and any
discussion of it must inevitably include an unprofitable proportion
of guesswork. Thus, Jorgensen differentiated histologically between
two substances which seemed to be present in the unripe egg of
Patella vulgata, ergastoplasm No. i and ergastoplasm No. 2, one at
least of which was responsible for the formation of the vitelline
globules. Faure-Fremiet & Garrault identified ergastoplasm No. i
with the mitochondria, and ergastoplasm No. 2 with the fatty con-
stituents of the yolk. But if two forms of vitellin existed, one in loose
combination with a nuclein and the other free, the staining reactions
of histological elements mainly constituted by one or other of these
Table 37.
Phos-
Millon
Trypto-
Glucos-
Investigator
Protein
Iron
phorus
test
phane
Sulphur
amine
and date
Ichthulin
None
Much
Positive
Negative
Present
None
McCrudden (1921)
Albumen
Traces
Present
,,
,j
Much
,,
Ichthulin
None
A little
Negative
)>
None
>j
Albumen
,,
Much
,,
Present
Ichthulin
—
Present
—
—
None
Levene (1901)
,,
Present
J,
—
—
Present
Walther (1891)
^^
—
—
—
—
None
Hammarsten (1905)
"
—
Present
—
—
—
Valenciennes &
Fremy (1854)
,,
—
jj
—
—
>j
—
Gobley (1850)
substances would very likely differ, and it is possible that an explana-
tion on these lines may in the future correlate the chemistry with the
histology of the yolk. The vitellin question has been in a measure
reviewed by McCrudden, whose table (given in Table 37) illustrates
the difficulty of summing up the findings of investigators at all
succinctly.
The amino-acid analyses (Table 34) are rather more interesting.
We have data for the vitellins of the herring, the trout, the cod,
and the sturgeon among fishes, the frog among amphibia, the grass-
snake among reptiles, and Hemifusus tuba, a gastropod. To this may
be added amino-acid analyses of the mixed egg-proteins of the sea-
urchin egg and the eggs of the brook-trout and the giant salamander,
as well as the albumens of cod and sturgeon and the mucoprotein
of Hemifusus. If the fish ichthulin analyses of Iguchi or Hugounenq
be compared with those of Table 1 1 for the vitellin of the hen, no
very marked differences can be observed, although the predominancy
SECT, i] PHYSICO-CHEMICAL SYSTEM 339
of arginine and lysine over histidine, which is a constant feature of
the ichthulins, reaches greater values in the latter than in the case
of bird vitellin (see Table 38). Again, bird vitellin always shows
a notable proportion of proline and leucine, and this is also the case
with the vitellins of the lower animals (e.g. 10 per cent, of leucine
in gastropod vitellin, 19 per cent, in snake vitellin and 9 per cent,
in herring ichthulin), though the amount of proline is usually not so
great. The only instance of a real divergence between bird and other
vitellins would appear to be the glutamic acid content, which is
always high in the former, although this amino-acid is absent from
the latter.
Table 38. Hexone bases of yolk-proteins.
In gm. % original
In % total nitrogen protein
Investigator
Species Protein Hist. Arg. Lysine Hist. Arg. Lysine and date
Herring Ichthulin 2-45 I4"50 10-07 I'^S 6-33 7-40 Steudel & Takahashi (1923)
Egg-menibrane 3-99 14-41 7-51 2-09 6-35 5-55 Steudel & Osato (1923)
protein
Hen Vitellin — — — i-go 7-46 4-81 Osborne & Jones ( 1 909)
Herring Ichthulin 0-40 2-70 2-00 — — — Hugounenq (1904)
(clupeovin)
Sturgeon Ichthulin 0-47 0-97 o-oi — — — Konig & Grossfeld (1913)
Cod Ichthulin 0-55 0-54 0-02 — — — ,, ,,
Trout Ichthulin 0-54 0-41 o-oi — — — ,, ,,
Gastropod Ichthulin None 3-73 0-86 — — — Komori (1926)
Frog Vitellin 1-14 1-06 0-29 — — — Galimard (1904)
(ranovin)
Snake Vitellin 0-30 0-32 1-45 — — — ,,
If now Table 39 is considered, it will be seen that variations are
present in the general analysis of these proteins, but that they tend
to cancel each other out among the groups. Thus the mono-amino-
acid/di-amino-acid ratio is very constant indeed in different ichthulins,
although Rothera himself considered that he was dealing with two
entirely different proteins, the vitellin of the Torpedo egg and that
of the sturgeon. It is unfortunate that Komori's examination of
gastropod vitellin was confined to the estimation of the amino-acids
by isolation, and did not include a van Slyke determination of the
relative amounts of mono-amino and di-amino acids. In contra-
distinction to the ichthulins, the mixed egg-proteins studied by Russo
and Gortner show more variation, though the former's values for two
sea-urchin ^gg proteins agree well with the usual vitellin figure.
Masing, however, was not able to find any phosphoprotein phos-
340
THE UNFERTILISED EGG AS A
[PT. Ill
phorus in sea-urchin eggs, and Needham & Needham found only
very little. It is interesting to note that the ratio is subject to large
fluctuations among the keratins of the egg-cases. As for the albumens
which Konig & Grossfeld isolated from the eggs of the sturgeon and
the cod, they seem to approach in their composition, in so far as data
for the hexone bases permit one to form a conclusion, the ovoal-
bumen in the hen's egg. The 8 per cent, of tyrosine obtained from the
sturgeon ovoalbumen is, however, remarkable. The mucoprotein which
Komori found around the eggs of the gastropod Hemifusus tuba, and
which he partially analysed, is not sufficiently well characterised to
be compared except roughly with the mucoprotein of the amphibian
egg-jelly.
Table 39.
In % total nitrogen
Species
Torpedo {Torpedo marmorata)
Sturgeon
Dogfish (Scyllium stellare)
„ (Pristiurus melanostoma)
,, (Scy Ilium caniculd)
Hen
Herring ...
Sea-urchin
Brook-trout
Giant salamander
Hen
Protein
Ichthulin
Ovokeratin
Ichthulin
Egg-membrane
protein
Mixed egg-
proteins (total)
Mixed egg-pro-
teins (coag. only)
Mixed egg-
proteins
Vitellin (for i
comparison)
15-67
1609
i5-o8
14-33
14-23
1 6-43
14-09
ipz
8-49
849 1-26 6o-20
9-51
5-09
5-13
449
660
i-8i
0-99
0-56
0-14
0-24
0-21
63-60
79-66
66-45
64-19
73-70
61-77
25-10
27-65
15-78
28-78
30-75
2050
27-02
2-40
2-30
5-04
2-31
209
3-55
2-29
Investigator
and date
Rothera (1904)
Buchtala (1908)
Steudel & Takahashi
(1923)
Steudel & Osato
(1923)
Russo (1926)
7-33 2-05 — 62-11 25-91 2-40
— 284 45-70 17-30 2-64
— — — 62-20 29-80 209 ,,
1-82 — — 61-55 28-25 2-18 Gortner (1913)
2-25 —
1-63 S-55
53-73 29-35 1-83
67-10 25-10 2-67 Plimmer (1908)
The general distribution of nitrogenous substances in the eggs of
the lower animals is shown in Tables 40 and 41. Pigorini's investiga-
tion of the silkworm egg is suggestive, but his data about the
different protein fractions are insufficient to enable us to form any
judgment on their relation to those so well known in the bird's egg.
The very large amount of mucoprotein in the silkworm ovum is
certainly remarkable. In Table 41 are placed the few data which
we have on the relative amounts of protein and non-protein nitrogen
in different eggs, and the way the protein is divided between keratin,
albumen, and ichthulin or vitellin. Clearly enough there is great
variation, and a rough dichotomy into two groups, one in which the
SECT. I] PHYSICO-CHEMICAL SYSTEM 341
non-protein nitrogen accounts for from 14 to 35 per cent, of the total
nitrogen, and one in which it only accounts for less than 10 per cent,
of the total nitrogen. It is evident from the work of Konig & Gross-
feld that all the fishes examined belong to the first of these categories,
although within the group there are wide divergences, such as the
minute amount of albumen apparently present in the trout's egg
and the low non-protein nitrogen of the herring's egg. Good agree-
ment is to be noted between the results of Levene and Konig &
Grossfeld, who all worked on the cod; and, although nothing con-
cerning the non-protein nitrogen can be gathered from the figures of
Kensington and Hugounenq, their results do show general agreement
as regards the partition of nitrogen among the proteins. The only
reptile on whose eggs work has been done which could be incorporated
in the table is the grass-snake, and there, although no non-protein
nitrogen figures are available, it is interesting to note the very high
proportion of keratin.
Table 40.
Silkworm (Bomfryx: mon). (Pigorini, 1Q23.)
In % of total protein
A
Protein sol. in water but not
Protein sol. in Protein sol. in Protein sol. in coagulable by heat, and
distilled water 10 °„ salt sol. dilute alkalies yielding glucosamine on
(albumen) (vitellin) (nucleoprotein) hydrolysis (ovomucoid)
29-20 8-57 11-45 5090
The second principal group, consisting of those eggs which have
a relatively much lower percentage of non-protein nitrogen, contains
two members, the hen and the silkworm. The former may be said
with a high degree of probability to be characteristic of all
nidifugous birds, and perhaps of nidicolous ones also, but whether
the latter is at all representative of the centrolecithal insect eggs
may be considered doubtful. The sole insect egg which has been
investigated chemically, so far, is that of the silkworm, and until
more evidence is available the hen and the silkworm will have to
be placed together in this second group without comment. It is
significant that, in the hen's case, the percentage of albumen is
greater than in any other, a fact obviously referable to the large
amount of egg-white present in that egg. Finally, it is of interest
that the sea-urchin's egg seems to have a protein/non-protein nitrogen
ratio very like that of the fishes, but situated on the low protein
edge of their limits of variation.
342
THE UNFERTILISED EGG AS A [pt. iii
Table 41. Distribution of
Gm.
% wet
weight
Species
Water
Protein
(Nx
6-25)
Protein
(by diff.)
Protein
of egg-
mem-
brane Albumen
Ichthulin
Nitro-
gen
(direct)
Free
bases and
amino-
acids
Fat
Ash
Carp ...
66-15
2770
2997
363
16
43
4-432
9-91
2-48
1-40
Pike
6353
28-13
33-01
375
2-38
17-29
4-500
9-59
1-40
2-06
Trout
6385
27-81
30-81
1-76
0-15
24-33
4-450
457
3-71
1-63
Herring
69-22
26-32
2521
3-20
4-83
13-68
4-212
3-50
4-19
1-38
Cod
72-10
23-02
24-44
2-57
2-70
11-47
3-683
7-70
1-33
2-13
Salmon
—
—
—
—
—
-
—
—
—
—
Herring
65-00
28-ss
—
0-79
28-70
—
3-62
-
Sea-urchin (Strongylo-
centrotus lividus)
Silkworm
80-50
6-47
35-00
10-20
3-00
3-90
2-25
66-24
22-00
—
—
—
-
3-67
0-875
—
-
Grass-snake (Tropido-
notus natrix)
Cod
58-94
94-67
19-24
—
12-71
0-72
5-81
—
Fresh- water gar
5390
26-20
—
-
—
—
—
0-138
-
-
Hen (average results)
whole egg
Turtle (Thalassoclielys
corticata) yolk
—
—
11-81
0-45
6-13
5-23
2-91
0-500
0-033
—
-
* With so % mucoprotein and
Within the non-protein nitrogen fraction itself there are some
fragmentary data for the distribution, as may be seen from Table 42 .
Unidentified compounds usually account for from 20 to 35 per cent,
of the total non-protein nitrogen, and free amino-acids for approxi-
mately half of it. Among those identified by Steudel & Osato were
histidine, arginine, lysine and cystine. The ammonia may vary from
4 to 25 per cent., and the purine bases from 15 to 40 per cent.
As far as can be seen at present, the hen's egg seems to possess
the greater part of its non-protein nitrogen in the basic fraction.
The most interesting point brought out by the table is probably the
significant quantity of urea shown to be present by the analyses of
Steudel & Osato, amounting to no less than half of the total non-
protein nitrogen, and it is possible that a good deal of the unidentified
nitrogen of Konig & Grossfeld might be accounted for in this way.
The presence of nitrogenous excretory products in the undeveloped
egg, though at first sight paradoxical, is nevertheless undoubtedly a
fact in the case of some aquatic organisms. The hen's egg contains
hardly a trace of urea at the beginning of development but that of
a selachian fish contains a good deal (see Section 9- 1 1 ) .
SECT, i] PHYSICO-CHEMICAL SYSTEM 343
the nitrogen in eggs.
Gm. % dry weight (ash free)
^ A . ^
% of the total nitrogen Egg-
, — ^ ■ — \ Pro- mem- Free
Free tain Pro- brane Nitro- bases and
Protein Ker- Albu- Ichthu- amino- (N x tein (by pro- Albu- Ichthu- gen amino- Investigate
total atin men Hn acids 625) diflF.) tein men Hn (direct) acids Fat Ash and date
66-9 121 548 331 85-37 92-37 1119 50-64 13-65 30-54 7-64 — Konig & Gr
(1913)
70-96 11-35 721 52-4 2904 82-75 95-93 10-90 6-92 50-25 13-08 27-87 407 — „
85-20 15-7 0-49 790 1485 80-56 89-25 5-10 0-44 70-48 12-89 13-24 IO-7S —
86-15 12-70 9-15 543 13-9 89-52 85-75 10-88 16-43 46-53 14-33 "-Qi 14-25 —
68-41 10-50 i-oi 469 315 8933 94-84 9-97 10-48 44-51 14-29 29-88 5-55 — „
(loo-o?) — 11-70 880 — 87-80 — — 10-30 77-5 — — 4-50 7-50 Kensington (
(loo-o?) 2-7 973 — __ — — — — — — — Hugounenq
62-0 — — — 379 — _ — — — — — — — Russo (1926)
94-0 — 27-4 8-05 6-04* — — — — — — — — — Monzini(i9:
Pigorini (ic
96-0 — — — 3-98 65-25 — — — — — — — — Russo (1922)
(loo-o?) 66-0 3-74 30-2 — — — — — — — — — — Galimard (19
66-0 — — — 33-0 68-09 — — — — — — — — Levene (i89(
_ ___ — _________ Nelson & Gi
(1921)
96-4 406 49-8 42-5 366 — — — — — — — — — —
Q8-5 — — — 1-5 — — — — — — — — — Tomita (1921
chorionin in addition.
As is well known, these fishes have a special relation to this sub-
stance. In 1858 Stadeler & Frerichs isolated "kolossale Quantitaten
von Harnstoff" from the organs of plagiostomes, obtaining a solid
mass of urea nitrate when they added nitric acid to their final con-
centrates. One liver of an adult Scy Ilium canicula gave them 2 oz. of
urea, and similar high figures were reported for Acanthias vulgaris.
Teleostean fishes, however, and the cyclostome, Petromyzon planeri,
yielded practically no urea, at any rate not more than would be
present in mammalian tissues. Stadeler confirmed the selachian
results on Raia batis and clavata and on Torpedo marmorata and ocellata.
In 1 86 1 Schulze repeated and confirmed Stadeler's work on Torpedo,
and in 1888 Krukenberg published an extensive work on the subject,
in which he related his unsuccessful attempts to demonstrate urea
in the bodies of teleosts {Lophius piscatorius. Conger vulgaris, Acipenser
sturio), a cyclostome [Petromyzon fluviatilis and Ammocoetes) and a
cephalochordate (Amphioxus lanceolatus) , although he found large
amounts of it in the bodies of elasmobranch fishes [Scyllium stellare,
Mustelus vulgaris and laevis, Acanthias vulgaris, Squatina angelus, Torpedo
marmorata, Myliobatis aquila) and in the holocephalic Chimaera
344 THE UNFERTILISED EGG AS A [pt. iii
monstrosa. Particularly interesting were his experiments with eggs —
he isolated considerable amounts of urea from a 5 cm. embryo of
Mustelus laevis, and from the yolk of Scyllium stellare and Myliobatis
aquila eggs, but he could find none in the surrounding jelly or "white ".
An Ggg ofPristis antiquorum yielded 3920 mgm. per cent, (wet weight)
and a Torpedo ocellata egg 1 740 mgm. per cent. An Acanthias vulgaris
embryo 1 7 cm. long had 3360 mgm. per cent, in its muscles, 1800 mgm.
per cent, in its liver, and 2640 mgm. per cent, in its unused yolk.
Other work on urea in selachians was done by Grehant and by
Rabuteau & Papillon.
Table 42. Distribution of non-protein nitrogen in eggs.
% of total non-protein N (including purine N)
g-| z § iz g 2 -g I I |Z
w.« „ „ ^ 2^ c S "G 2 o op Investigator
Species HSo? cq <fe^ P U D U hUa and date
Herring — 198 — 44-3 359 — — — — — Konig & Grossfeld (1913)
Carp ... ... — 39-8 — 36-1 24-1 — — — — — >> >,
Sturgeon ... — 25-2 13-6 55-4 189 — — — — — ,, ,,
Herring ... ... 2060 244 67 21-6 — 519 None 18-3 — — Steudel & Osato (1923);
Steudel & Takahashi (1923)
Herring 1443 16-91 23-42 41-65 1802 — — — — — Yoshimura (1913)
Silkworm ... 440 — 4-44 54-30 34-60 — - — — 610 6-7 Russo (1922)
Hen (aver, figures) — 88-80 4-22 7-04 — None None Trace — — —
Fresh- water gar 299 92-00 — 4-02 — — — 4-0 — — Nelson & Greene (1921)
(not ripe)
More light, however, was thrown on the reasons for this richness
in urea when in 1897 Bottazzi working on the osmotic pressure offish
blood, found that the elasmobranchs differed fundamentally from
teleosts in being isotonic with sea water.
Serum
A
Selachians Torpedo marmorata —2-26°
Trygon violacea —2-44°
Teleosteans Charax pimtazzo —1-04°
Serranus gigas — i -03°
Bottazzi observed that the selachian osmotic pressure would corre-
spond to some 3-9 per cent, sodium chloride but laid no emphasis on
the fact that selachian blood did not contain anything like so much
ash. It was left for Rodier to show that the difference was made up
almost wholly by urea. Duval has since found that the salts alone
would only give an osmotic pressure of A — i-o6°. "High blood-
urea", as Smith says, "is a phyletic character of the orders Selachii
SECT, i] PHYSICO-CHEMICAL SYSTEM 345
and Batoidei", and its osmotic function was well shown by the
reciprocal relation between salts and urea which Smith found to
hold in selachian tissues and fluids.
Blood-urea
mgm. %
Smith (1929)
Selachians
Dogfish {Mustelus canis)
880
Denis (1913)
Selachians
Dogfish {Mustelus canis)
800
Sandshark {Carcharias littoralis)...
1000
Skate {Raia erinacea)
868
99
Teleosteans
Mackerel {Scomber scombrus)
86
Goosefish {Lophius piscatorius) ...
40
Flounder {Paralichthys dentalus)
46
In view of all these facts it is not surprising that Needham & Need-
ham in 1928 found about 5 mgm. of urea nitrogen present in the
Scyllium canicula egg at the beginning of development ; and 888 mgm.
per cent, of urea in the undeveloped Acanthias vulgaris egg. Gori, again,
found 7 10 mgm. in undeveloped Torpedo eggs. But since urea accumu-
lation is closely confined to elasmobranchs it is unlikely that the results
of Steudel & Takahashi and of Konig & Grossfeld can be interpreted
as being due to urea.
The presence of urea has also been reported in the undeveloped
eggs of "ants and flies" (in small quantities) by Fosse. Further
details would be desirable here.
There is reason to believe that nitrogenous substances other than
those already mentioned are present in certain eggs. Thus Yoshimura
and Poller & Linneweh isolated trimethylamine, tetramethylene-
diamine and choline from fresh herring eggs, and there is a certain
probability that fish eggs also contain betaine. As the characteristic
smell of fish is due to these amines and related substances, this is not
very surprising. Brieger is said to have found neuridine in fish eggs,
and Schii eking isolated spermine from echinoderm eggs in 1903.
Taurine and glycine were found in echinoderm eggs by Kossel &
Edlbacher.
Of the manner of formation of ichthulin in the maturation of the
ovum we know absolutely nothing. Paton & Newbigin concluded
from a very few analyses that the phosphorus was brought to the
ovaries from the muscle of the salmon as inorganic phosphorus, but,
in view of what is now known about the organic phosphorus com-
pounds of blood, this appears rather unlikely.
346 THE UNFERTILISED EGG AS A [pt. in
I '14. Fats, Lipoids and Sterols
Studies on the fatty substances of the undeveloped eggs of different
animals have resulted in much interesting information. There has
been, of course, a great body of histological work, and the yolks
of all kinds of eggs have been repeatedly subjected to microscopic
examination (for example, Kaneko's study on the silkworm); but,
in spite of many attempts, I have not succeeded in finding more than
a few hints in this literature which are of value to the chemical worker.
This subject has been dealt with in a general way by Ransom and
by Dubuisson, to whose papers those interested in the histological
aspects of yolk must be referred. Of the way in which the fat and
the protein are intermingled in the yolk we know practically nothing,
and it would be most desirable to investigate the yolk with the
methods which modern colloidal chemistry has developed. But that
the association between fat and protein indicated by the histological
evidence is not very close is shown by the interesting centrifugation
experiments of McClendon on the amphibian egg. If the egg of the
frog is centrifuged for five minutes under the right conditions, it
separates into three perfectly distinct layers, the upper one being
oily and yellow, the middle one translucent, colourless and proto-
plasmic, and the lowest one black, containing practically all the yolk.
By using a considerable number of eggs, McClendon was enabled
to obtain suflticient material for the chemical analysis of each layer.
The figures he obtained are shown in Table 43. It is evident from
a slight inspection of his results that the upper layer is composed
mainly of neutral fats and a little lecithin, and the middle layer of
water, salts and protein, with no fats or lipoids. The lowest and much
the largest layer is made up of the vitellin (ranovin or batrachiolin)
together with the major part of the lecithin. It is interesting that the
association between the phosphoprotein and the lipoid was the only
one that centrifuging could not break, for, as we have already
seen, the observation of a loose lecitho-vitellin combination in
the hen's egg is very old. McClendon found that mitotic figures
were all present in the middle layer, and that this centrifuging
produced a variety of monstrous embryos. He was led to regard
the protoplasm of the egg as constant in composition throughout,
but "anisotropic as regards its axes, in other words crystalline
in structure".
SECT. l]
PHYSICO-CHEMICAL SYSTEM
347
McClendon extended his observations to the egg of the sea-urchin,
Arbacia punctulata. Separated by centrifugal force, this egg divided
itself into four layers, as Lyon had already described, {a) a layer of
yolk bodies and red pigment granules extending from the centrifugal
end about half-way to the equator, {b) a layer of similar yolk bodies
but without the pigment granules, {c) a translucent fluid layer ex-
tending almost to the centripetal pole and containing the nucleus,
and finally {d) a very opaque layer or cap of minute volume, sitting
on the centripetal pole. When the crushed eggs were centrifuged,
the material separated into two layers, {a) and {b) being indistin-
guishable, centrifugal and containing the egg-membranes, and
[c) centripetal, {d) not being perceptible. McClendon analysed the
layers in the same manner as those of the frog's egg — the figures
are given in Table 43.
Table 43.
McClendon' s figures (1909).
In
the
/o
in the layers
%
, of
0
I"
1
"a
lay
i
ers
dry weight
phosphorus
A
ll
la
Is
/
t! 2
c t<
—1 1)
0
u
*-> tJ
a 0
& 2
1
Layers of centri-
o-o6
6
Upper centripetal
50
50
8o-o
4-0
8
8
Trace
0-34
1-4
0-41
—
fuged egg of
(fatty or oily)
Rana pipiens
016
16
Middle (proto-
plasmic)
82
18
7-5
II-5
60
21
Trace
0-05
i-o
0-37
~
~
0-78
78
Lower centrifugal
(yolky)
48
52
24-0
60
10
60
Trace
270
1-2
1-33
~
~
Layers of centri-
—
32-5
Centripetal (proto-
88
12
49-0
—
20
3I-0
2-
36
16-66
3-24
13-45
1-24
fuged egg of ^r-
plasmic)
bacia punctulata
—
67-5
Centrifugal (yolky)
79
21
38-2
—
10
51-8
2-
74
12-84
1-6
10-6
2-02
A short consideration of them shows that centrifugal force is not
nearly so successful in separating the egg of the sea-urchin into
chemically unlike layers as it is in the case of the frog. This fits in
perhaps with the long-established fact that centrifugal force inter-
feres far less with normal development in the sea-urchin's egg than
it does in the frog's egg (Morgan and Lyon). It was very noticeable
that, whereas the frog's egg separated out into layers of markedly
different water-content, this did not take place in the sea-urchin's
egg. In the case of the centrifuged frog's egg, again, there were big
differences between the phosphorus contents of the different layers,
but in that of the sea-urchin's egg this only applied to the residues
which were mainly protein. McClendon surmised that the inclusion
348 THE UNFERTILISED EGG AS A [pt. iii
of the membrane proteins in the centrifugal layer caused this
effect.
It is of course a fact of the first importance that normal develop-
ment can follow centrifugation and this will receive attention later
(see Section 3 and the Epilegomena) . I shall only mention here
as one of the best instances of this phenomenon, the work of
Schaxel on the axolotl egg. Here centrifugation caused atypical
discoidal cleavage which nevertheless resulted in a normally pro-
portioned embryo. Thus normal conclusions can follow abnormal
distribution of the so-called "organ-forming substances". For further
details of these experiments, see Morgan and Bertalanffy.
The early work of Gobley on the fat of the hen's and the carp's
egg has already been described. He isolated glycerophosphoric acid
from the latter, and pursued further his investigation of lecithin,
concerning which it is of interest to note that Sacc contested his claim
to have found organic alcohol-soluble phosphorus. Sacc believed
that the fats contained dissolved in them a quantity of inorganic
phosphorus. Gobley, however, was easily able to disprove this view
and to show the identity of carp's egg lecithin with brain lecithin.
Data which have accumulated since Gobley's time on the fatty
substances of the eggs of the lower animals are collected in Table 44,
and may be compared with those in Table 22. One of the most
striking differences between the hen's egg and other eggs is the
relatively low iodine value of the fatty acids of the former, both free
and combined in lipoids. The neutral fat of the hen's egg has an
iodine value varying roughly between 60 and 90, but for fish eggs
the figures vary from 90 to 150, and the same rule holds generally
of the lipoid fatty acids, for they average 60 in hen and 100 in
fish eggs. The saponification numbers, on the other hand, are much
the same throughout the two tables (from 170 to 200). The conclusion
might therefore be drawn that egg fats differ rather more as to the
number of unsaturated linkages in their acids, than as to the length
of their chains. Nevertheless, there are remarkable exceptions to
these generalities, the fatty acids of the echinoderm eggs, for example,
having enormous saponification and high Dyer numbers, and there-
fore presumably only very short chains of carbon atoms. Arbacia
is more remarkable in this than Asterias. Yet, though they are
exceptional in that respect, they have iodine numbers very like those
of fish-egg fats. Another point of interest is that the cholesterol/fatty
SECT.
I]
PHYSICO-CHEMICAL SYSTEM
349
acid ratio, as shown by expressing the cholesterol in percentage of
the fat present, is rather constant, never going below 4 and never
rising above 12. This may have some connection with the physical
Table 44. Data for fat fraction of eggs.
1
0
"o-tio a
<S
1
3
■3
a
^1
l^^l
0
J3
0?
£
0
0
C a
?f H ^ 5
^
<4-r
."2
."S
"S
- c
0
3
■$t
Qj cj.S .2
■3 c
0?
'0
'0
•s|
&3
.-a (u u
— 2
J5 4^
11
'S
2
0
3 u
p
Investigator
Species
^ ^
m c
Q^Z c
u^
Ji
fc
fe
cfiJ:
D^
and date
Amphibia
Frog {Rana tempor-
123-0
—
—
5-97
—
—
—
—
—
Faure-Fremiet & Dragoiu
aria)
(1923)
Fishes
Sturgeon ...
107-6
191-4
—
4-35
12-92
—
—
—
—
Konig & Grossfeld (191 3)
Trout
128-3
181-8
—
6-52
41-10
o-ig
0-19
—
—
„ ,,
Cod
148-4
1 76- 1
—
12-05
35-19
0-21
0-43
—
—
,, jj
Herring ...
123-1
230-6
—
6-94
43-61
—
—
—
,, J,
Carp
78-9
186-9
—
10-98
59-19
—
—
—
—
55 55
Pike
—
—
—
—
—
0-27
0-22
—
—
55 3J
Trout {Salmo fario)
108-6
219-8
—
6-23
37-50
—
—
—
1-7
Faure-Fremiet & Gar-
(132-9
189-8
Fatty acids of the ph
osphatide fraction'
rault (1922)
Carp {Cyprinus carpio)
—
140-0
■ —
5-99
60-00
—
—
—
3-36
55 55
(64-4
—
Fatty acids of the phosphatide fraction'
Dogfish
(55-88
—
Fatty acids of the phosphatide fraction^
Ponce (1924)
Shark (Lepidorhinus
—
—
—
—
—
—
—
17-3
—
Tsujimoto (1920)
kinbei)
ECHINODERMS
Sea-urchin {Arbacia
147-0
606-0
4-001
—
—
—
—
—
—
Page (1927)
punctulata)
Sea-urchin [Echinus
145-0
195-0
—
—
29-40
—
—
—
—
Moore, Whitley & Adams
esculentus)
(78-8
225-0
Fatty acids of the phosphatide fraction]
(1913)
Sea-urchin {Arbacia
—
—
—
—
50-00
—
—
—
—
Matthews (191 3)
punctulata)
Sea-urchin [Paracen-
150-0
—
—
—
—
—
—
—
—
Ephrussi & Rapkine
trotus lividus)
(1928)
Starfish (Asteriasgla-
112-5
318-8
3-778
—
—
—
—
—
—
Page (1927)
cialis)
POLYCHAETE
Polychaete worm
—
—
—
8-85
—
—
—
—
IO-6
Faure-Fremiet (1921)
{Sabellaria alveolata)
Nematode
Roundworm {Ascaris
—
—
—
5-0
—
—
—
—
80-0
Faure-Fremiet (191 3)
megalocephala)
State of the egg-cell, and will be referred to again (see Section 12-5).
The lipoids, expressed as lecithin in per cent, of the fat present, show
greater variations, but it is not possible to say at present what the
significance of these may be.
350 THE UNFERTILISED EGG AS A [pt. iii
The mention of squalene in Table 44 indicates the existence of an
egg-constituent, our knowledge of which is of very recent origin. In
1 906 Tsujimoto isolated from the liver oils of elasmobranch fishes a
saturated hydrocarbon of approximate formula C30H20, and in 191 6
published a further study of it. Its properties and constants are given
in Table 22. In 1920 he reported that he had been able to isolate
it from the egg-yolks of two elasmobranchs, Chlamydoselachus anguineus
and Lepidorhinus kinbei, where it made up no less than 13 per cent,
of the egg (wet weight) and, in another case, 1 7 per cent, at least of
the total fat fraction. There the matter rested until 1926, when
Heilbron, Kamm & Owens, taking up the question of its presence
in eggs once more, isolated it from the undeveloped yolks of Etmo-
pterus spinax, Lepidorhinus squamosus and Scymnorhinus lichia. In the
fully developed eggs of the first-named of these three, practically
none was present, indicating that it must either have been combusted
or absorbed during development. Further researches on the embryo-
logical significance of this compound are greatly required. It is
possible that some hydrocarbon of this sort may explain certain
obscure points in the chemistry of the egg, for instance, the oil
extracted by Dubois from the locust's egg {Acridium peregrinum). It
contained 1-92 per cent, phosphorus, and was present to the extent
of 4*5 per cent, of the wet weight of the egg, no small proportion.
Kedzie studied a similar oil which he obtained from the egg of the
American locust.
A question which is perhaps related to the general problem of the
egg-oils is that of the oil-globules of the yolks of some of the teleostean
fishes. In 1885 Agassiz & Whitman divided all pelagic eggs into
those which had the oil-globule and those which had not. But it
was soon found that this method of classification was valueless, for
the appearance of the globule is rather erratic; thus, although Lota
vulgaris (van 'S>2ivs\he\ie) , Brosmius (anon.) and Motella mustela (Brook)
were all found to have it, the common pike's egg does not have it
(Truman). Ryder first suggested that the oil-globule might have a
relation to buoyancy, but Prince, reviewing the whole subject a little
later, pointed out that this could hardly be so, for the salmonoid
fishes all have them, and yet their eggs never float. Moreover, out
of 22 teleost eggs with no globule, 17 are pelagic, while out of
24 teleost eggs which have globules, only 15 are pelagic. Ryder
replied to this by partially withdrawing his theory, and Mcintosh
SECT, i] PHYSICO-CHEMICAL SYSTEM 351
simultaneously showed that the eggs of the catfish, which are un-
doubtedly bottom ova, have large oil-globules. Another theory was
put forward by van Bambeke, who believed that the oil-globule was
a special form of yolk, and of a purely nutritional significance. Prince
criticised this view on the ground that the oil persists in the yolk
after the liberation of the embryo from the egg-membrane, and travels
beneath it as it swims about. This would not, however, negative the
possibility that the oil was used for larval rather than embryonic
nourishment. Van Bambeke' s claim that a protoplasmic thread
passes from the oil-globule to the germinal disc was almost completely
disproved by van Beneden. His and Miescher, examining the oil
histochemically, found that it only stained very slowly with osmic
acid, and therefore differed profoundly from the yolk, and, although
it was soluble in ether, it contained no more than a trace of phos-
phorus. It is remarkable that the oil has never been subjected to
a proper chemical examination, especially in view of the extensive
zoological literature on it. What we know of its properties faintly
hints, perhaps, that it may be a hydrocarbon like squalene, and the
whole question, indeed, holds out great possibilities for physiological
as well as chemical work. The oil must readily dissolve lipochromes,
for the pink pigment of the salmonoids is found in it. Prince's own
theory was that the globule was a constituent of ancestral significance,
a vestige from the time when, as Balfour showed, the teleostean yolk was
very much larger than it is now. The nutrition view is probably the best.
The lipoids and sterols of the eggs of the lower animals are very
little known, and their further study is much to be desired. Page in
1923 described a sterol — asteriasterol — which he isolated from the
eggs of Asterias forbesii and which turned out to be closely related to,
though not identical with, ordinary- cholesterol; the eggs of Arenicola
cristata, on the contrary, yielded a sterol absolutely identical with
the well-known substance as it occurs in mammals. Ten years pre-
viously, in a less accurate study, Matthews had failed to find any
cholesterol at all in the eggs of Asterias forbesii, though he had been
able to isolate some from those of Arbacia punctata. From the former
he got a jecorin-like substance, containing 10 per cent, of glucos-
amine, which was probably a mixture of kephalin, cerebrosides,
"protagon" and various carbohydrates. Page's later study of the
fats and lipoids of the echinoderm egg led to the conclusion that
(qualitatively) there was more kephalin in the eggs of Arbacia than
352 THE UNFERTILISED EGG AS A [pt. iii
in those of Asterias, and more lecithin in the eggs of Asterias than
in those of Arbacia. Asterias contains large amounts of soaps, and
its oil is present in much greater abundance than the oil of
Arbacia; moreover, it contains more sulphur compounds (sul-
phatides?) decomposable with potash than does the Arbacia egg.
Page, Chambers & Clowes made a study of the effects of various
cytolytic agents on the eggs of Asterias separated by microdissection
into their cortical and endoplasmic components. They used for this
purpose hypotonic sea water, digitonin and saponin, and found that
digitonin caused slow cytolysis of the cortical and rapid cytolysis of
the interior protoplasm when the two were isolated, whereas hypo-
tonic sea water caused slow cytolysis of the interior and rapid
cytolysis of the cortical protoplasm. If these results do not actually
demonstrate that the greater part of the asteriasterol is localised in
the outer and fertilisable parts of the egg, they at any rate suggest
a new method of investigation which may help to solve many similar
questions in the future. Runnstrom has studied the lipoids of the
echinoderm Qgg in relation to its coloured interference fringes and
its membrane properties.
Among the sterols existing in eggs must be mentioned a substance
which has long been known to occur in the ova of Ascaris, and which
has been called "ascarylic acid". Faure-Fremiet identifies it with
the droplets or crystals described in the egg oiAscaris by van Beneden.
It was isolated simultaneously by Faure-Fremiet from the eggs and
by Flury from the whole body of the nematode ; the former worker
found that it accounted for 22 per cent, of the dry material. Ascarylic
alcohol, ascarylic acid, or, as it would probably be best to call it,
ascaristerol, seems to exist in the egg-protoplasm in combination
with palmitic, oleic, and perhaps stearic acid in ester form. Faure-
Fremiet & Leroux studied its properties, and proposed the pro-
visional formula of C32Hg404 . Its saponification number was 199, and
its m.p. 82°, it did not give the cholesterol colour-reactions, and its
molecular weight was close to 511. Flury considered it to be related
to oenocarpol. Acaristerol seems to be strictly confined to the eggs,
for even the parietal cells of the ovary and uterus do not contain it,
as Faure-Fremiet showed by means of histochemical tests. Nor is it
present in the testes and spermatozoa. It may at present be classed
with the sterols, like asteriasterol. Ascaris eggs also contain o-i6 per
cent, dry weight of ordinary cholesterol.
SECT. l]
PHYSICO-CHEMICAL SYSTEM
353
Table 45. Distribution of phosphorus.
In % of the total P
f
_3
u*
Sh
0
u
i
0
"o
gfln
15 p-^
^Oh
Oh
0
\-i
^ U
^ JJ
. w
0
A
cx
1^
M)-?
£P3
0 3
3 fl
2 G
3
Investigator
Species
HDh
Hi
OI
-Si
7 -S
fl
0
and date
Bird
Hen
61-4
9-5
9-5
None
1-6
27-5
29-1
Plimmer & Scott (1909)
Amphibian
Frog (ovarian)
26-2
4-3
4-3
None
7-6
61-9
69-5
Plimmer & Kaya (1909)
Fishes
Sturgeon
28-8
i6'9
—
9-9
None
54-3
54*3
Plimmer & Scott (1908)
Herring
—
—
—
—
63-0
63-0
■>■) J3
Grey mullet
—
—
—
—
—
48-0
48-0
5» J>
Trout
26-0
—
—
—
—
34-6
—
Faure-Fremiet & Garrault
(1922)
Herring
33-2
—
—
—
—
66-8
—
Yoshimura (191 3)
Haddock
—
—
21-22
—
—
—
Milroy (1898)
Herring
—
—
—
—
lO-O
90-0
—
Tschernorutzki (191 2)
Salmon
37-8
i8-9
—
Traces
—
43-3
—
Paton (1898)
Herring
—
—
+
+
—
—
Steudel & Takahashi (1923)
ECHINODERMS
Sea-urchin {Strongylo-
43-0
33-1
—
—
—
—
23-8
Robertson & Wasteneys
centrotus purpuratus)
(1913)
Sea-urchin {Arbacia
—
—
—
—
Much
None
—
Masing (1910)
punctulata)
Sea-urchin [Arbacia
29-2
47-0
—
i-i
—
—
23-5
McClendon (1909)
punctulata)
Sand-dollar {Dendras-
6-09
46-95
17-55
29-4
32-0
I2-I
44-1
Needham & Needham ( 1 930)
ter excentricus)
Starfish {Patiria mini-
32-3
51-8
32-0
20-0
15-40
Trace
15-4
J> 59
ata)
Crustacea
Sand-crab (Emerita
28-2
6 1 -40
42-10
19-3
10-82
Trace
10-82
>J J>
analoga)
Brine-shrimp [Artemia
salina)
Gephyrea
5-9
56-4
38-3
18-1
37-9
None
37-9
)> >5
Gephyrean worm ( Ure-
chis caupo)
Nematode
27-4
56-90
40-60
16-3
15-80 Trace
15-8
5> J)
Roundworm {Ascaris)
25-6
—
—
20-0
54-4
—
54-4
Faure-Fremiet (191 3)
Miescher (1872) reported that in the salmon nearly all the phosphorus is in organic form.
* This fraction will include pyrophosphate P.
t This fraction will include guanidine phosphoric acids (arginine or creatine phosphate P).
+ "Present in some quantity."
The lipoids of the mammalian egg-cell have recently been the
subject of some work which is interesting, though, Uke all histo-
chemical studies, very difficult to appraise. Following on Russo's
claim to have found two different sorts of eggs in rabbits, varying
N E I 23
354 THE UNFERTILISED EGG AS A [pt. m
in their reactions to staining methods, Fels in 1926 confirmed this
difference for the human egg-cell, some specimens of which showed
a strong lipoid-reaction (Ciaccio and Smith-Dietrich methods) in the
nucleolus while others did not. Fels' illustration is certainly striking.
Leupold had already put forward the view that eggs whose nucleoli
were rich in lipoids produced females, and the remainder males, but
all the evidence, however, is against sex-dimorphism in the mam-
malian egg (see Parkes' review). These observations, together with
those of Pollak on the presence of Reinke's crystals in the egg of
Macacus rhesus, and similar work by Limon and" von Ebner (on
Cerrus capreolus), are all that we have on the chemical constitution
of the mammalian egg-cell.
Closely connected with the lipoids of the egg is the distribution
of phosphorus compounds in it and Table 45 gives what is known
upon this subject. It is interesting to see how the phosphoprotein
phosphorus varies, in some eggs being very large in proportion to
the total phosphorus, in others being almost insignificant. Masing
was wrong in saying that the echinoderm egg has none at all, for
Needham & Needham in 1929 observed quite a high percentage
in the &gg of the sand-dollar. It is significant in view of what
has already been said about the pre-eminence of birds in storing
fat in their eggs, that the hen's egg has 20 per cent, more
phosphorus in lipoidal form than any other egg investigated.
The fishes rank in this respect with the echinoderms and annelids,
little diflference being noticeable between alecithic and lecithic eggs.
Perhaps this famous distinction involves neutral fat rather than
lipoids.
It is to be noted from Table 45 that the inorganic phosphorus
content of eggs is very variable; in many cases almost none is present,
but the haddock's ^gg seems to have no less than 20 per cent, of the
total phosphorus in this form. About the same proportion is present
in the nematode egg, ii Ascaris can be taken as representative. Faure-
Fremiet was able to identify the calcium phosphate in the egg-cyto-
plasm with the "hyaline balls" described by van Beneden, using
various histochemical reactions (McCallum, Prenant, etc.). Pure
calcium phosphate, according to Faure-Fremiet, accounts for 0-4 to
0-6 per cent, of the dry weight of Ascaris eggs, an inconsiderable
amount in view of the share it takes in the appearance of the cyto-
plasm as a whole.
SECT, i] PHYSICO-CHEMICAL SYSTEM 355
1-15. Carbohydrates
The carbohydrates of the eggs of the lower animals have been less
investigated than anything else — a summary of our quantitative
knowledge concerning them is shown in Table 46. The presence of
glycogen in insect and mollusc eggs was noted by Bernard and in
those of arachnids by Balbiani. For the reasons mentioned above,
it is difficult to know how trustworthy the figures for carbohydrates
are, so bad have the methods been in the past. Faure-Fremiet & du
Streel's figure for the glycogen of the frog's tgg must surely be too
high, for most of the other workers are agreed on a value of about
2 gm. per cent, wet weight. In the case of animals other than
amphibia, the figures are too scattered to permit of any generaUsa-
tion: thus, though glycogen was not found in herring's eggs by
Steudel & Osato, Gori did note its presence in Torpedo eggs, and the
eggs of the reptiles Vipera aspis and Elaphis quadrilineatus, in addition
to free carbohydrate. Steudel & Osato pointed out that many histo-
logists such as Goldmann had published results concerning the fish
egg which might lead one to suppose that very large amounts of
glycogen were there. That this was not found by chemical methods
ought to induce, they felt, a more cautious attitude towards histo-
chemical work than was customary; indeed, much of what is called
glycogen histochemically can certainly not be glycogen. Greene's
carbohydrate figures for the eggs of the king-salmon Oncorhynchus
tschawytscha, from Cahfornian rixers, were of special interest, for,
throughout the maturation period, the carbohydrate content of the
egg remained the same. The duration of the fast did not affect it
at all.
Quahtative investigations of carbohydrate in eggs have been made
by Anderlini on the silkworm egg and by Konopacki, who observed
the presence of glycogen microchemically in the perivitelline fluid
of the frog's tgg. As has already been mentioned, a carbohydrate
group is undoubtedly contained in the mucoprotein of the amphibian
egg-jelly, and von Furth's analysis of the egg-cases of the squid Loligo
vulgaris showed that their protein also contained a carbohydrate, but
whether these play any part in the sugar supply for the developing
embryo remains an obscure point. Haensel found an amount of glucose
in the frog's tgg which is shown in Table 46, but he also tried the effect
of keeping the eggs in solutions of various mono- and di-saccharides,
23-2
356
THE UNFERTILISED EGG AS A
[PT. Ill
Table 46.
Mgm. % wet weight Mgm, % dry weight
. ' , , ' ^
6 ^ 4 ^
% y ^ h y c
S-o iJ ^ l-S.^ >■ Investigator
Species H-c'fa O h-o^ta O ^nd date
Reptiles
Turtle ( Thalassochelys corticata)
White ... ... ... — Trace — — — — Tomita (1929)
Yolk ... ... ... — 100 — — — — ,,
Tortoise {Testudo graeca) ... — 140 — — — — Diamare (1910)
Amphibia
Frog {Rana esc. and fuse.) ... — — 2,520 — — — Kato (1909)
,, ,, ... — — 1,100 — — — Athanasiu (1899)
Frog {Rana temp.) ... ... — — — — — 7810 Faure-Fremiet &
Dragoiu (1923)
Frog {Rana esc. and fuse.) ... — — 2,500 — — — Bleibtreu (1910)
Frog {Rana temp.) ... ... — — — — 193 — Gori (1920)
,, ,, ... ... — — 10,140 — — — P'aure-Fremiet & Vivier
du Streel (1921)
„ ,, ... ... 604 — — 1906 — — Needham (1927)
,, ,, ... ... — — 2,528 — — — Haensel (1908)
,, ,, ... ... — — 1,650 — — — Goldfederova (1925)
Fishes
Herring — 500 — — — — Steudel & Osato (1923)
King-salmon {Oncorhynchus — 96 — — — — Greene (1921)
tschawytscha)
Trout {Salmo fario) ... ... — — 340 — — — Faure-Fremiet &
Garrault (1922)
ECHINODERMS
Starfish {Asterias glacialis) ... — — 20 — — — Dalcq (1923)
Sea-urchin {Echinus esculentus) — — i ,360 — — 8980 Moore, Whitley &
Adams (1913)
Sea-urchin {Strongyloeentrotus 1360 — — 543° — — Ephrussi & Rapkine
lividus) (1928)
Insects
Silkworm {Bombyx mori) ... — — 1,110 — — — Pigorini (1922)
,, ,, ... — — 1,980 — — — Tichomirov (1882)
), „ ... — — — — — 3080 Vaney & Conte (1911)
Bee {Apis mellijica) ... ... — — 2,500 — — — Straus (191 o)
Cephalopod
Octopus {Sepia officinalis) ... — — None 3620 1000 None Henze (1908)
,, As glucose ... ... 2700 — — — — — ,,
,, As pentose ... ... 2380 — — — — — ,,
Polychaete
Polychaete worm (^aie/Zana — — 1,270 — — — Faure-Fremiet (192 1)
alveolata)
Nematode
Roundworm {Ascaris megalo- — — — — — 2105 Faure-Fremiet (1913)
cephala)
356*
Table 47. Ash content of eggs.
WllOLB EOU
H m I'ARTB
Pike (£j« lueiw
)
Sturgeon [Acitie
Sea-urdiilt [Arb
■jntlurw)
Starfuli LUUtia.
Sea water (Wo
ih Hole)
Sea waier {Clia
tnga cxpcdilion)
Sea- urchin (Sir
neylQcentrolus lividtu)
Dogfish {^p'""'
Spider-crab (A/
, cmicula) ...
no vtTTUCOsa) ...
Octopiu tStpia
Ifianalu)
Hen (6'<i//uj Join
'»""")■■• -
MotluK ( Volula
Dogfuh {Squaiin otonllUas)
Frog {Rana Umporaria)
l-rout (&ilmo fontimlii)
Salmon {Saimo ndar) ...
Wrauc (Labrax lupus) ...
Torpedo [Torptdo dctllala)
Dogfiah [Snilium canicula)
Spidcr-aab {Maia vnrucosi
Octopu) (Stpta ojficinalij)
a pmtuloia)
Gephyrcon worm {Sipuncutus nitdui) .
Lugworm [.irtnitola claparti""
Herring {Glupea hariagus)
Waicr-sol.
Salmon ..
Plaice
Dogfuh \Squatut acanthiai) (cgg-jclly)
Sawfish {Pritlii aniiquorum)
Mtlhi:quivalcnLa prcx
SO, PO, a
— 55-
Na Mg Ca
V»7
01Q7
413 <
.V»4
■1
325
'17
9-8o
300 10
—
I -05
4
107
—
0-99
46-6 - 5
i
1-04
z
1-04 16-29 ^fj^ ■6'65 ■iS-29
3-9B g-6i 11-51 1707 ^bs
— 10-79 ^3^ 4<'85
;&.Grossreld(i9i3) 2-08
'■«<> Page (1927)
a-27 — ■ O4-0 7S-3i 409 ia-37 „
0-004 5'-7 45-^2 Si-s^ 54*42 57-'i
— 4-64 — 34-a 54-03
47 b'^'b'^ 5^'^ Diiunar, ualUnger, vol. 1
■64 13-44 9"3i I3'5i Goblcy {1850)
- — — — Wetzel (1907)
— — 33-7
t' z 'U *;?
■54 — afj-g
— 3'-9 —
'"3 — 559
5 — —
7a — —
7-18
,8-,
11)
b-,!,
4-4
lo/i
10-31
13-93
3-3
11-33
'i-7«
6 — —
Silkworm iBombyx n
Turtle [Timloiiochew coTticata):
Whole egg
While
Yolk
>9'03 30-S8 Btalasccwicz (1926}
6i-oi 63ja RoBb & Correa (1927)
McCailum (1926)
Greig {1898)
Milroy [1898)
Perugitt (1879)
KrukeabcTS (1888}
Uf^UID OP TIIB '
Hen {Callus domaliats)
Frog {Rana lembotana)
Trout {Salmo/ontinatis)
Torpedo {Torptdo ocillata)
Spiaer-crab ^oifl wrrwfwfl) ...
— Sea-urchin (raracentntus lividus)
Octoput [Stpia qfficimlii)
Schroder (1909}
Karaahima (igag)
i?-j Bialasccwiti (i<
SECT, i] PHYSICO-CHEMICAL SYSTEM 357
to see if they would grow richer in glycogen. They all did; in fact,
he was able to double their glycogen content by this simple means
(glucose acted better than sucrose, sucrose than lactose, and lactose
than glycerol, though even the latter substance gave an effect) . These
curious observations have never been confirmed, and can hardly be
said to carry conviction as they stand. Diamare obtained discordant
results in his researches on the sugar of various eggs ; thus, he got a
rather low value for the free glucose of the egg of Testudo graeca, but
none at all, either free or combined, from the eggs of Scyllium catulus
or Torpedo marmorata. No explanation can be given for this fact. In
connection with carbohydrates, it should be remembered that viper
venom, which is in all probability a glucoside, has been shown by
Phisalix to be present in active form in the yolks of viper eggs.
I -16. Ash
We come now to the inorganic substances of eggs. Iron has been
shown to be present by microchemical tests in many eggs, such
as those of Limnaea, Tubifex, Rana esculenta (where it is massed
at the light ventral pole) and Pisidium, by the work of Schneider.
Dhere found traces of iron and copper in the eggs of Sepia. Warburg
found 0-02 to 0-03 mgm. iron per 100 mg. nitrogen in the unfertilised
sea-urchin €:gg', part of it seemed to be in ionic form and part not.
According to Wilke-Dorfurt, there are 4-8 mgm. per kilo iodine in
oyster egg-shells.
Ash analyses of eggs have been made by several workers, whose
results, it may be remarked, would have been more easily comparable
if they had expressed them in the same way, instead of in nine or
ten different ways, omitting in some cases the figures which would
enable them to be calculated into a form comparable with each
other. Table 47 summarises what is known about the distribution
of inorganic substances in eggs. It has entailed a good deal of calcula-
tion, for only one of the previous investigators expressed his results
in terms of millimols and milliequivalents, and unless this is done it
is impossible to gain any idea as to the relative preponderance of
cation and anion. The first thing which should be noted is the
fact that, when the salts are expressed in per cent, of the total
ash, potassium is always there in greater amount than sodium, and
nearly always to a greater extent than any other metal. This seems
to be quite characteristic of the ovum, though in other systems of
SECT. I] PHYSICO-CHEMICAL SYSTEM 357
to see if they would grow richer in glycogen. They all did; in fact,
he was able to double their glycogen content by this simple means
(glucose acted better than sucrose, sucrose than lactose, and lactose
than glycerol, though even the latter substance gave an effect) . These
curious observations have never been confirmed, and can hardly be
said to carry conviction as they stand. Diamare obtained discordant
results in his researches on the sugar of various eggs ; thus, he got a
rather low value for the free glucose of the egg of Testudo graeca, but
none at all, either free or combined, from the eggs of Scyllium catulus
or Torpedo marmorata. No explanation can be given for this fact. In
connection with carbohydrates, it should be remembered that viper
venom, which is in all probability a glucoside, has been shown by
Phisalix to be present in active form in the yolks of viper eggs.
i-i6. Ash
We come now to the inorganic substances of eggs. Iron has been
shown to be present by microchemical tests in many eggs, such
as those of Limnaea, Tubifex, Rana esculenta (where it is massed
at the Ught ventral pole) and Pisidium, by the work of Schneider.
Dhere found traces of iron and copper in the eggs of Sepia. Warburg
found 0-02 to 0-03 mgm. iron per 100 mg. nitrogen in the unfertiUsed
sea-urchin egg ; part of it seemed to be in ionic form and part not.
According to Wilke-Dorfurt, there are 4-8 mgm. per kilo iodine in
oyster egg-shells.
Ash analyses of eggs have been made by several workers, whose
results, it may be remarked, would have been more easily comparable
if they had expressed them in the same way, instead of in nine or
ten different ways, omitting in some cases the figures which would
enable them to be calculated into a form comparable with each
other. Table 47 summarises what is known about the distribution
of inorganic substances in eggs. It has entailed a good deal of calcula-
tion, for only one of the previous investigators expressed his results
in terms of millimols and milliequivalents, and unless this is done it
is impossible to gain any idea as to the relative preponderance of
cation and anion. The first thing which should be noted is the
fact that, when the salts are expressed in per cent, of the total
ash, potassium is always there in greater amount than sodium, and
nearly always to a greater extent than any other metal. This seems
to be quite characteristic of the ovum, though in other systems of
358 THE UNFERTILISED EGG AS A [pt. iii
the organism other relations are found; thus corpuscles and plasma
of some mammalian bloods have converse potassium/sodium ratios,
and, as a general rule, potassium preponderates in cells while sodium
preponderates in media. Of the anions PO4 usually takes up much
the greatest part, but SO4 may in certain cases equal it. In the
columns on the right of the table the total anion and total cation
are shown, in each case calculated as millimols and as milli-
equivalents, the former giving an idea of the total number of
molecules present, the latter of the total number of valencies.
Study of the anion/cation ratio expressed as milliequivalents per
cent, wet weight provides an important key to the constitution
of the egg, for it shows roughly to what extent anion or cation is
held in combination with protein or lipoid, or other organic sub-
stances. We have already seen that in the case of the hen's egg,
taking both yolk and white into account, the anion/cation ratio is
more than unity (Bialascewicz's figures give 2-17), showing that a
quantity of sulphur and phosphorus is in organic combination — a
conclusion which fits in admirably with all that we know of the
hen's egg from other sources. The same relationship is seen in the
figures of Konig & Grossfeld for the three fish eggs they investigated,
the pike, the cod and the sturgeon. On the other hand, the figures
of Page for two echinoderm eggs give ratios much less than unity,
demonstrating the organic combination of a good deal of the cation.
It may be noticed that the analyses of Dittmar and Page for sea
water give ratios in the very close neighbourhood of unity, as would
be expected, and indicate at the same time that the ratio cannot be
regarded as significant to less than o-og. From what has been said,
therefore, it might be concluded that the yolk-laden eggs of the
fishes, like that of the hen, have a ratio above unity, while the
alecithic echinoderm eggs have ratios much below it. But there
are exceptions to this generalisation. The ratio of unity for the
carp egg which is given by Gobley's results may perhaps be
neglected, owing to the date of the work (1850), and the similar
value obtained by Roffo & Correa on a gastropod egg may
also be regarded as suspicious because of the enormous amount of
sodium chloride that appears in their analysis. But the careful work
of Bialascewicz in 1926 does not altogether support the generalisa-
tion. His figures for the fish egg are in good agreement with those of
Konig and Grossfeld, but his anion/cation ratios for the echinoderms
SECT, i] PHYSICO-CHEMICAL SYSTEM 359
do not go below unity, though they approach it much more nearly
than do the fishes. Further work is needed to clear up this contra-
diction. In one case, however, Bialascewicz got a ratio below unity,
that of Arenicola claparedii, so that in a general sense his investigations
are not opposed to those of Page and Konig & Grossfeld. McCallum's
low ratio for the egg of the herring is difficult to explain, but
Perugia's analysis of the egg-jelly of ovo viviparous selachians fits in
well enough with the majority of the other evidence. Attention might
also be drawn to Bialascewicz's high ratio (15) for the eggs of the
octopus. Sepia, which would appear to be extraordinarily poor in
metallic ions (cf. p. 317, Section 13 and the Epilegomena).
Some further light is perhaps thrown on the inorganic composition
of eggs by Wetzel's figures for insoluble and soluble ash. He sub-
mitted the eggs of various animals to examination, with the following
results :
% dry weight
Species Total ash Insol. ash Sol. ash
Sea-urchin {Strongylocentrotus lividus) ... 9-7 2-4 7-2
S^iideT-cvah {Maia squinado) ... ... 4-12 0-27 3-8
Octopus {Sepia officinalis) ... ... ... 2-2 0'59 i'6
Tio^sh. [Scyllium canicula) ... ... ... 5-5 1-15 4-3
In all cases he found more soluble than insoluble salts, i.e., more
chlorides than sulphates and phosphates.
Table 48. Bialascewicz' s figures.
Concen-
Vol. of
tration
Vol. of
liquid
CI in I c.c.
Total CI
inter-
c.c. of
after
Deeree
ultra-
in ultra-
micellar
yolk
dilution
of
filtrate
filtrate
fluid per
Species
taken
(c.c.)
dilution
(mg.)
(mg-)
I c.c. yolk
Hen (yolk)
4-8
10
2-o8
I -08
1-080
—
>3
4-8
20
4-17
0-472
0-944
0-541
>>
4-8
30
6-25
0-306
0-918
0-569
4-8
40
8-33
0-222
0-888
o-537^
»J
4-8
50
10-40
0-195
0-975
(0-754)
It is very interesting, as Bialascewicz points out, that the mineral
composition of terrestrial and aquatic animals should be so alike.
The preponderance of potassium which is seen in the hen's tg^ does
not change as one passes to organisms laying their eggs in an environ-
ment containing far more sodium than potassium. Thus, although
36o THE UNFERTILISED EGG AS A [pt. iii
the normal sea water has twenty times as much sodium as potassium,
fish eggs often have quite twenty times as much potassium as
sodium. There would not appear to be in this connection any dif-
ference between homoio-osmotic and poikilo-osmotic aquatic animals.
It is also obvious from Table 47 that aquatic eggs often have very
much less salt in them than the ambient medium, and this would
be a special case of the phenomenon found in all marine animals,
and termed by Fredericq "Mineral hypotonicity". Bialascewicz
arranged the animals he studied in a list of ascending concentration
of metalHc ions as follows :
Metal gramions
Species
per litre
Octopus {Sepia officinalis) ...
o-oi6
Gephyrean worm {Sipunculus nudus)
0-064
Spider-crab {Maia verrucosa)
0-079
Wrasse {Labrax lupus)
0-091
Herring {Clupea harengus) (McCallum)
o-ioo
Dogfish {Scyllium canicula) ...
0-107
Sea-urchin {Arbacia pustulosa)
0-159
Sea-urchin {Paracentrotus lividus) ...
o-i8o
which would also be an ascending table of taxonomic groups, were
it not for the high metal content of the echinoderm eggs, which
exceed even the fishes.
There are other points concerning the relative amounts of salts
in the eggs which require mention. McCallum, who had for a long
time previously been studying the proportion of salts in the ash of
animals and parts of animals with reference to the composition of
sea water both now and in earlier geological epochs, made an
analysis of herring's eggs in 1926. He had previously differentiated
between palaeo-chemical salt ratios in bloods, namely, ratios re-
sembling that which pre-Cambrian sea water can be calculated to
have possessed, and neo-chemical salt ratios, namely, ratios resembling
the sea water of the present day. Thus Limulus polyphemus and Aurelia
flavidula, the king-crab and the medusa, which have always been
marine animals, now approach the modern sea in the composition of
their vascular fluids, but the lobster Homarus americanus, the selachian
Acanthias vulgaris, the frog, dog, and man, for instance, all have ratios
resembling the composition of the sea water at the appearance of
the protovertebrate form. He had also identified the kidney as the
organ responsible for maintaining the palaeo-ratios in the salts of the
blood. In order to explore the possibility of identifying a palaeo-ratio
SECT, i] PHYSICO-CHEMICAL SYSTEM 361
in the contents of the cell itself, he had recourse to eggs, and for those
of the herring obtained the following distribution :
Ratios on the basis of Na 100
Na K Ca Mg CI
100 216-7 ii'4 18-7 356-8
This stood in marked contrast not only with the vertebrate blood-
plasma but also with the Archaean sea water calculated for the
time at which life first began to appear in it, thus :
Vertebrate blood-plasma (dog)
6-6 2.8 0-7 139-5
Archaean sea water
100 100-250 10 0-05
But after extraction of the dried eggs with water in a Soxhlet
apparatus, the determination of the ratio of salts in the soluble part
gave results more like the ratio for the Archaean sea water:
100 219-9 5'6 1-6 359-2
McCallum therefore concluded that the soluble part of the ash of
the herring's egg exhibits a palaeo-chemical ratio. The bond shown
here between the metals and the organic substances is useful in
reminding us that even in fish eggs, where the anion/cation ratio is
well above unity, some of the metal as well as the acid radicles may
be united in organic combination.
The relation between the salts in the intermicellar fluid of yolk
and those in the dispersed phase itself has been studied by Bialasce-
wicz and by Vladimirov. Bialascewicz worked firstly with the yolks
of Torpedo eggs, but also with those of the hen and the trout. He
prepared series of mixtures of the yolk with diluents in different
concentrations, such as isotonic solutions of lithium sulphate and
lithium nitrate, or in some cases distilled water, and then, submitting
the mixtures to ultra-filtration, he estimated the ash and its com-
position in the filtrate and the residue. He first found that the
percentage of chlorine bound to the dispersed phase in the ooplasm
was practically independent of the degree of dilution, and from this
fact he was able to calculate the volume of the intermicellar fluid
of the yolk (see Table 48). For the hen's egg this was 0-549 c.cm, per
362 THE UNFERTILISED EGG AS A [pt. m
c.cm. wet substance, and for the egg of Torpedo ocellata a similar calcu-
lation, based on cryoscopic experiments, gave a value of 0-482. On
the basis of these figures, he proceeded to study the partition co-
efficient of each individual ion as between dispersed phase and inter-
micellar liquid. In Table 49 these partition coefficients are given;
they represent the ratio amount of ion in the continuous phase or inter-
micellar liquid j amount of ion in the dispersed phase. It will be noted
from Fig. 17 that as dilution of the original yolk goes on the
ratios in some cases change, but in others remain constant. Thus
the chlorine of the trout and the hen egg yolk remains constant
at 0-5 in the latter and 1-02 in the former case, showing that
Table 49. Bialascewicz's figures.
a
0
l-H
5
31
i
§1
si
34
Is
►3t
■f-s
SI
5
.2 ^
^1
11
•2 ~
K
0-722
I -000
0-890
0-768
I -000
0-870
0-967
I -000
0-945
o-8oo
Na
0-942
0-567
0-509
0-331
0051
—
0-080
0728
I- 000
I -000
Ca
0-093
0-391
0-274
0-169
0-321
0-760
0-474
0-696
0-505
I- 000
Mg
0-295
0-460
0-321
0-380
0-157
0-410
0-707
0-631
0-272
0-491
P
0-025
0-244
o-ioo
0-275
—
—
0-040
0-318
o-i86
—
CI
0-555
0-905
I -000
0-567
0-943
I -000
0-970
I- 000
I -000
0-766
it is very stably combined in the dispersed phase, though in different
proportions according to the animal. Thus there is considerably
more chlorine in the dispersed than in the continuous phase of the
yolk of the avian egg, while in the fish egg there is a very slight
excess of chlorine in the continuous phase. In all other instances,
however, both as regards the hen and the trout, the excess of ion is
in favour of the dispersed phase, the colloidal aggregates of which
may therefore be looked upon as reservoirs of ash. Nevertheless,
there is a good deal of the sodium combined in the continuous phase,
and not a little of the potassium, though here the trout differs from
the hen, for the potassium ratio is about 0-9 in the former case and
only 0-7 in the latter. All the other ions have lower ratios than
these; magnesium, calcium and phosphorus, for instance, are all
present to a much greater extent in the dispersed than in the con-
tinuous phase. These experiments show also exactly how firmly the
ions in the dispersed phase are bound there, and with what ease
they may be washed out into the ultra-filtrate. It is apparent from
SECT. l]
PHYSICO-CHEMICAL SYSTEM
363
S 2 °'^^
£ oT 0-8 -
it'-r
CO) 0-6-
3
« E
0-2
0-lb-.
3sl
Bialascewic^
-sSf.
Fig. 17 that the phosphorus, chlorine, and probably sodium in
the dispersed phase, are intimately united there, for, however
great the dilution of it, they do not increase in the ultra-filtrate.
Magnesium and calcium, on the other hand, show a comparative
readiness to pass out of the dispersed phase as the dilution is increased.
The behaviour of the potassium is the most pecuHar, for, as dilution
goes on, the calculated con-
centration of this ion actually
decreases, but as the decrease
is slight it is probably due to
experimental error, and it was
treated as such by Bialasce-
wicz himself. Thus, of the
ions bound to the dispersed i | o-s
phase, the cations sodium and
potassium, and the anions of §? 0-3^,
chlorine and (presumably)
phosphate, are firmly attach-
ed, while the cations calcium
and magnesium are not, and
can easily be washed out.
The high proportion of phos-
phorus in organic combination should be remembered here.
Bialascewicz also pointed out that the partition coefficient or ratio
followed with dilution a practically rectilinear course, so that some
idea of the ratio in the natural undiluted yolk might be obtained by
extrapolation. These figures so obtained are shown in Fig. 17, from
which it may be deduced that the ions follow the order phosphorus,
calcium, magnesium, chlorine, potassium, sodium, beginning with
the one most of which is in the dispersed phase and ending with
the one least of which is so distributed.
Fig. 1 8 shows another aspect of the passage of ash from dispersed
to continuous phase.
In succeeding papers Bialascewicz extended these researches to the
eggs of amphibia, some other fishes, Crustacea, molluscs, echinoderms
and annelids. He reported that the intermicellar liquid varied much
in its relative amount, accounting for from 20 to 63 per cent, of
the whole ooplasm. From the data in Table 50, however, there
does not seem to be a very close relation between the relative volume
extrapolated 2
values for undiluted
ooplasm
4 6
degree of dilution
Fig. 17.
364
THE UNFERTILISED EGG AS A
[PT. Ill
100
90
€1
-0%
70-
M,
Ca,
6
• — ■«..;«
of continuous phase and the percentage dry weight of the system. Bia
lascewicz's tables give
the concentration in
percentages of the prin-
cipal ions in the inter-
miceliar liquid of dif-
ferent eggs, and these
are conveniently sum-
marised in Fig. 19,
taken from his paper.
From this it is obvious
that all the eggs studied
have about the same
proportion of potas-
sium, but that the other
ions are rather variable.
There is much more
calcium, relatively, in
the continuous phase
of the yolk of the hen's
egg than in that of any
of the others except the
crustacean Maia verru-
cosa. Similarly, there is
more magnesium, relatively, in that of the frog than in any other egg.
A very interesting comparison may be made between the distribution
of ions in the continuous phase of the eggs and that in the serum of
Table 50. Bialascewicz's figures.
0-1 0-2 0-3
Concentration of the bhree elements in
the continuous phase (mgrnt per cc)
Fig. 18.
0-4
Continuous phase
cc. per cc.
In% of vol.
Egg
ooplasm
ooplasm
% dry weight
Scyllium canicula
0-83
17-0
—
Salmo fontinalis
0-79
20-8
—
Salmofario ...
—
41-5 (Faur^-Fremiet & Garrault)
Torpedo ocellata
0-41
59-0
—
Acanthias vulgaris
—
—
47-3 (Zdarek)
Arbacia pustulosa
0-82
17-8
—
Paracentrotus lividus ..
0-79
20-7
22-6 (Wetzel)
Rana temporaria
o-6o
39-9
42-6 (Kolb, Terroine, etc.)
Callus domes ticus
0-55
45-1
50-3 (Kojo)
Sepia officinalis
0-50
50-0
47-3 (Wetzel)
Maia verrucosa
0-37
63-2
43-6 (Wetzel)
SECT, l]
PHYSICO-CHEMICAL SYSTEM
365
the corresponding adult animals. Fig. 20, taken from Bialascewicz,
shows that the potassium preponderates in the former and the sodium
in the latter, while the other inorganic substances are more or less
equally distributed. As there is no difference in electrolyte con-
70-
-S 604
to
50
40
30i
20
10
:2
o
CO
I
O
o
o
^
to
I
^= Potassium ^ = Calciu
= Sod'rum
Fig. 19
^
= Magnesium
centration between the continuous ooplasm phases of fresh-water
and marine animals, one must conclude that salts do not
account for the properties possessed by the latter, and that crystal-
loidal organic compounds, such as taurine, urea and glycine, play
an important part in keeping up a high osmotic pressure. Thus
Bialascewicz found a concentration of 8-43 gm. per litre of urea in
366
THE UNFERTILISED EGG AS A
[PT. Ill
undeveloped Torpedo ocellata eggs, but none in those of Arbacia, Sepia,
or Maia.
Vladimirov occupied himself with the egg-white in the egg of the
bird. In connection with other investigations which dealt with the
Salmo fonblnalis
PcGassium
Sodium
Magnesium
Calcium
Torpedo ocellaba
Maia verrucosa
Continuous phase
of egg-yolk
Serum of adult animal
Fig. 20.
water metabolism of the egg (see Section 6-4) he measured the
electrical conductivity of the egg-white in the unfertilised ovum, using
the Kohlrausch and Holborn apparatus, and obtaining a value of
7*6 X io~^. By the aid of a dialysis method he calculated the
electrical conductivity of the intermicellar fluid of the egg-white,
allowing for the disturbing effect of so large a concentration of pro-
SECT. I] PHYSICO-CHEMICAL SYSTEM 367
tein. The result came out to 10-4 x io~^. If this work were repeated
for the yolk, interesting commentary on Bialascewicz's researches
would be possible. It agrees with the earlier measurements of
Bellini, who found the electrical resistance of the unincubated white
to be Q. 1 8-8 ohms. Much further work on such properties of the
yolks and egg-whites of a wide range of eggs is urgently needed, for
they must obviously be of the greatest importance to the developing
embryo. Such questions as the electrical conductivity of egg-cells
and developing embryos are very relevant here, but must be left
for consideration in Section 5.
As a conclusion to this discussion of the chemical constitution of
the egg, it may be admitted that great progress has been made in
our knowledge with respect to it during the last fifty years. But to
a discerning judgment, it remains none the less a matter for great
surprise that in view of our comparative ignorance of the chemical
architecture of the egg, we know as much as we do about the coming-
into-being of the chemical architecture of the finished embryo.
One further matter may be alluded to in this section. The com-
position of fossil eggs cannot be said to have much embryological
interest, but it is hard to exclude a mention of them. The only
analyses we have are those of ZoUer who worked on the fossil eggs
of Chincha Island, off the Peruvian coast, where seagulls have been
living and depositing guano from a very remote date. Zoller found
that "time, which antiquates antiquities, and hath an art to make a
dust of all things" had had that effect on these eggs and had reduced
their water content to 14-4 per cent. There was no urea or uric acid
present, although the protein had nearly all disappeared and had
given rise to ammonium salts. There was no trace of fat or of carbo-
hydrate, and the sulphur of the proteins had all turned into sulphate.
Water
%
14-4
Cholesterol
0-287
Phosphoric acid ...
0-045
Total nitrogen
945
Ammonia N
8-12
K
14-9
SOs
16-08
These figures make it only too clear that if palaeontology and bio-
chemistry enter into closer relations than exist at present, it will not
be by way of the chemical analysis of fossil eggs. More hopeful
approaches will be found in Section 9-15.
SECTION 2
ON INCREASE IN SIZE AND WEIGHT
2-1. Introduction
We have so far been considering the unfertilised egg-cell and its
reserves of nutrient material as a physico-chemical system, and we
must now proceed to summarise critically what is known about
the alteration the egg undergoes in passing into the state of the
finished embryo. Subsequent sections will take up the chemical
changes during this process in all their complexity, but first the
apparently simple phenomena of change of weight must receive con-
sideration. To this undertaking special difficulties are attached; for
example, the act of birth or hatching itself, important though it is
for the chemical embryologist as the term of his investigations, is yet
purely arbitrarily and conventionally chosen as such, and, as far as
the organism itself is concerned, may be relatively unimportant. The
age at which birth takes place varies in different animals consider-
ably, and may occur earlier or later in development, cutting across
cycles of growth at almost any point. However, the study of growth
in weight and alteration in shape, is an essential preliminary to the
study of the chemistry of the embryo. I do not propose to spend
any time in the discussion of definitions of growth.
The actual data which we have concerning pre-natal growth
will be found in Appendix i, where they have been placed in
the hope that a collection of them will be of assistance to chemical
embryologists. No previous assemblage of them has been made, and
they are to be found scattered all through the literature. Biochemists
have in the past been insufficiently careful to check their results on
embryos against normal tables of weight, length, age, etc.
The predecessors of this section are the chapters on growth in
d'Arcy Thompson's Growth and Form and Faure-Fremiet's La Cinetique
du Developpement. These authors gave a full criticism of the whole
subject, but without special reference to the development of the
embryo. Moreover, much has been done since they wrote, and their
treatment differs in various ways from what follows here.
PT. m, SECT. 2] ON INCREASE IN SIZE AND WEIGHT 369
It is obvious that the growth of an embryonic organism can be
measured in many ways besides that of increasing weight. Its en-
larging dimensions in various directions of space can be measured,
or its volume, or the quantity of various constituent substances. More
will later have to be said about the way in which these different
quantities may be thought of as fitting in together and changing
with age. But the simplest manner of representing growth will
probably always remain the measurement of the increase in weight
of the total mass, and it is this which is now to be considered. The
relation of this factor to the age and the length of the foetus is a
point of capital importance to the chemical embryologist in the
knowledge of his material. It is true that the data are fragmentary
enough, restricted as they are almost entirely to various mammals
and the chick.
2'2. The Existing Data
Cephalopods.
Octopus. A remarkably complete set of data for the embryonic
growth oi Sepia is given by Ranzi, and this is almost all we have as
regards invertebrate development.
Insects.
Silkworm. Luciani & LoMonaco have studied the curve of growth
through the successive moults in the larval condition, but, in spite
of their work and of many other researches on the silkworm larva
and ^gg, I cannot find any in which the increasing weight of the
embryo itself has been measured prior to hatching.
Fishes.
Trout. Weighings of fish embryos have been exceedingly few in
number, owing to the smallness of their size and the difficulty of
separating them from the yolk. Kronfeld & Scheminzki, however,
have made some estimations of the increase in weight of trout
embryos, and their figures, together with those of Gray, are shown
in Table i of Appendix i.
24
370
ON INCREASE IN SIZE
[PT, m
Amphibia.
Frog. In the case of amphibia, where the cleavage in the egg is
more or less inclusive of the yolk-laden portion, it is not possible to
obtain data for the weight of the embryo itself, for, before hatching,
although the protoplasm is constantly increasing at the expense
of the yolk, the two elements cannot be separated, and therefore
cannot be weighed in isolation. This appears in the figures of Faure-
Fremiet & Dragoiu ; Schaper ; Davenport ; and Bialascewicz, and
must always be taken into account when differences between species
in water-content and other constants are under consideration, for
much confusion may be caused by not distinguishing carefully
between yolk plus embryo and embryo alone.
Reptilia.
Snake. Bohr's very few figures on Coluber natrix are all that are avail-
able. (Appendix i. Table 2.)
Birds.
Chick. It is on this animal, as might be expected, that the greater
part of the work on embryo-
nic growth has been done.
Hasselbalch, in the course of
his work on the respiration
of the chick embryo, ob-
tained a regular series for a
race not given. These corre-
sponded well enough with
the earlier data of Falck (also
from an unknown breed),
which were the first to be
published, appearing in 1857.
Hasselbalch's curve is shown
in Fig. 21, in which for the
first time we see the usual
' ' embryo-placenta relation ' '
in the form of a weight of
extra-embryonic structures
larger than the embryo in the
earliest stages, but soon falling below it.^
relation between the two as follows:
4-0
C3.0
2'0
1«0
O Membranes
• Embryo
Hasselbalch calculated the
See also Fig. 521.
SECT. 2] AND WEIGHT 371
Wt. of embryo + wt. of membranes
Day
Wt. of embryo
8
9
ID
15
16
1-917
1-652
1-613
I -108
1-128
17
I-IIO
18
1-090
Other sets of weight data have been reported by Lamson & Edmond,
by Murray and by Needham, for White Leghorn chicks, and by
LeBreton & Schaeffer for chicks of an unspecified race. These are
all placed in Table 3 of Appendix i, where it will be seen that the
general agreement between them is good. The values obtained by
Murray; Byerly^ and Schmalhausen are probably the most accurate,
for the conditions were very carefully controlled. Hanan's values are
lower than all the others. It is unfortunate that Schmalhausen does
not state what breed of hen was used in his experiments, though he
does mention that it was not genetically pure. Some early measure-
ments by Welcker & Brandt are not included in the table, for they
do not appear to be trustworthy.
Other measurements which are useful are those of Edwards, who
has published a peaked curve showing the length of the primitive
streak during the first 50 hours of incubation. ^ Schmalhausen's work
on the growth of parts of the chick embryo will be dealt with later :
he was preceded by Falck, who measured and weighed various
organs but did not use enough material to make his figures valuable
to modern workers.
Mammals.
[a) Mouse. In 1923 LeBreton & Schaeffer published figures for the
embryonic growth of the mouse, but these were not very numerous.
The only other work on this subject is that of McDowell, Allen &
McDowell, probably the most accurate and satisfactory study of pre-
natal growth in any form that at present exists. Their figures are given
in Table 4 of Appendix i, and the curve obtained from them in
Fig. 22. This is drawn on arithlog paper, the ordinates in logarithmic
ruling giving the actual weights in gm., the abscissae in arithmetical
ruling giving the age and the number of the individuals. On each day
the range of the individual unclassified weights is shown by a vertical
line which is itself used as a base-line for the frequency distribution
of the classified individual weights. The number of cases in the
^ See also Fischel and Leva.
24-2
372
ON INCREASE IN SIZE
[PT. Ill
distribution is shown by the distance to the right of the vertical base-
Hnes, and can be judged by the frequency-scale at the bottom of the
WEIGHT
GRAMS
1 -000
•1000
•0100
•00100
•00010
•00001
1
e^.
J
.u^^
\-y^ ^^^
>
\^'
'^
^
>
, zz
Y
- k/^
T
V^Z ' -
/>H
V^
t
/
/
/
/
/
J
1/
.c;r.Al E np FRFniifNniF'=^
1 I 1 1 1 1 1 1 1 1 1 1 1 1
0 20 40 60
1
9 10 n 12 13 14 15
CONCEPTION AGE
Fig. 22.
16 17 18 19
chart. The means, weighted by the number of individuals in each
litter, are shown as dots on the vertical base-lines, and it is through
these, of course, that the "normal curve" would be drawn. The
continuous curve in the graph is one drawn to a formula which
SECT. 2]
AND WEIGHT
373
Foetus of albino rab
will be discussed later, in Section 2-4 (p. 393). The lay-out so
made reveals several interesting features; it appears, for example,
that there are always individuals on a given day which are equal
in weight to the mode of the day before. McDowell, Allen &
McDowell consider that this is evidence of a possible delay of as
much as 24 hours between copulation and fertilisation, but, whether
this is so or not, it certainly equates with exactly similar variations
found in the chick both in the early stages (primitive streak) and in
the later ones of organ-growth. Further, the modes and means are
generally close together, though less so at the beginning of develop-
ment than at the end, and the latter do not approximate to a straight
line. A glance at the graph also shows that the highest individual
weights on each day tend to form a curve parallel with that of the
means throughout development.
(b) Rat. Donaldson's comprehensive monograph of 191 5 includes
a discussion of the growth of the rat embryo, but much less work
has been done on this animal
than upon man, for in the latter 5
case the ad hoc labours of ob-
stetricians have often provided
much valuable material for the
biologist. However, Stotsen-
berg's work gave a good account
of the matter, and his figures
are reproduced in Table 5 of
Appendix i, and in Fig. 23.
They begin from the 13th day
after insemination, before which
weighing is difficult, and they
continue until birth, which takes
place at the 22nd day. This pre-
natal period would appear to
be one complete growth-cycle, if we may judge from the work of
Donaldson, Dunn & Watson on the post-natal growth of the rat.
Huber has studied the growth of the rat embryo in its earliest
stages prior to fixation to the uterine wall. He states that the egg-
cell of the rat approaches the uterine end of the oviduct while in
the two-cell stage, segmentation being slow and proceeding as the
transit takes place. Fig. 24, reproduced from his monograph, is a
16 17 18
Fig. 23.
374
ON INCREASE IN SIZE
[PT. Ill
photograph of a model of the oviduct with its contained eggs. By
reconstruction methods at a magnification of looo diameters of the
ova Huber was able to determine the volume changes during seg-
mentation as follows:
Age
bays
^ ,
Hours
Stage
Average vol.
(cubic mm.)
I
0
Pronuclear
0-000156
2
o
2-cell
0-000162
3
I
4-cell
0-000173
3
17
8-cell
0-000184
1 1 -cell
0-000210
There would, therefore, appear to be a certain increase in volume
during these very early stages, but as the specific gravity changes
are not known it is difficult to understand what it may imply. There
is at present a great gap in our knowledge of the embryonic growth
Fig. 24.
of the rat between the early point at which Ruber's studies end and
the later one at which those of Stotsenberg begin. Huber himself
suggested that the slow development of the ovum of the rat during
its passage down the oviduct was best accounted for by the lack of
any food-supply for an alecithic egg until fixation to the uterine wall
had taken place. As the whole embryonic period of the rat is only
22 days, it is of great interest that the first four days should involve
hardly any increase in size. This fact renders of no significance the
calculated weights of rat foetuses given by Donaldson, Dunn &
SECT. 2]
AND WEIGHT
375
Watson in their earlier paper, for, in assuming that embryonic growth
in the rat followed a quite similar course to that taking place in
man and the rabbit, they did not allow for the long time taken for
the rat egg to pass through the oviduct after fertilisation. Thus they
arrived at the result that the rat embryo of 15 days should probably
weigh 2-6 10 gm., whereas by direct measurement Stotsenberg found
that it only weighs o- 1 68 gm. Their calculated figures are consequently
not included in Appendix i.
(c) Guinea-pig. The most usually quoted work on the embryonic
growth of this animal used to
be that of Read, who used
a very indirect method of
measuring it. He weighed the
pregnant female every day be-
tween insemination and birth
and then each foetus with its
membranes and fluids, from
which data, assuming that
growth had taken place regu-
larly, the weight of one embryo
could be calculated. He con-
cluded that the guinea-pig
passes through two growth-
cycles during its intra-uterine
life. But no satisfactory conclu-
sion can really be drawn from
such figures, subject as they are
to all kinds of complicating
factors, and, like the earlier
ones of Minot on the guinea-
pig, obtained in the same in-
direct way, they are better
discarded. It is needless to point out that differences in the weight
of mother + embryo due to defaecation, filling of caecum, etc., may
amount to grams, while the weight of the embryo is still only
milligrams. In the absence of any other figures, they had their
importance, but in 1920 Draper made a complete study of the
embryonic growth of the guinea-pig. Together with the few frag-
mentary (but direct) figures of Hensen, and the careful work of
•
90
—
80
-
• /
• /
/ •
70
"
/ •
60
- E
.7
(
50
- C
tj
40
.«;
•/
A *
•
30
~
• /
• /
20
-
J
10
-
)
/
1
•
11
days,
1
0
1
10
20
30
40
50
60
70
Fig. 25.
376
ON INCREASE IN SIZE
[PT. Ill
Ibsen, and Ibsen & Ibsen, Draper's figures form the standard series,
and are shown in Tables 6 and 7 of Appendix i. As is generally
known, the guinea-pig differs from most other mammals in being
born much later in its life-span than is usual, so that its lactation
period is exceedingly short and it is able to eat green food a very
few days after its birth. This is reflected in its gestation time which
is relatively long.
20 '0
CO /
0 /
15«0
—~
-0/
CO
^
E
® /
a
'^ /
%.
0 /
a>
0 /
10*0
— c
^
a:>
K®/ -
^ • uv- of 'membranes
?
•
5«0
yA
IX, Age ab which wbs. of embryos &,
_>*x'^
membranes are equal
O'O
-1--^
i^;;i^
Aqe in days
15
20
25
30
35 40
Fig. 26.
45
50
55
60 65
During the 64 days of its development in utero, the guinea-pig
increases its weight to about 85 gm. and its length to 10 cm. This
process is shown in Fig. 25 taken from the figures of Draper. In
Fig. 26, which gives an enlarged view of the lower part of the growth-
curve, the increase in weight of the placenta, the membranes, and
adnexa, together with the amniotic and allantoic fluid, is also shown.
The extra-embryonic structures reach a more or less constant weight
about two-thirds of the way through development, but, as can be
seen from Table 5 of Appendix i, the values from which this curve
was drawn are very divergent. In comparing the growth of the
SECT. 2]
AND WEIGHT
377
embryo with the growth of the membranes, it is interesting to see
that for the first month the latter weigh much more than the former,
after which, for a certain period, they grow together at the same
rate. But soon the curves diverge, and the membranes hardly grow
any more, while the embryo continues to increase greatly in size.
Evidently when the membranes and placenta have reached a sufficient
size to meet the utmost further demands of the embryo they grow
no more. There can be little doubt that the size of the placenta
exercises an influence on the growth of the embryo, and is of the
highest importance from the point of view of embryonic nutrition.
The amniotic liquid bears the same relationship as regards weight
to the embryo as do the placenta and the membranes.
100
Fig. 27.
Fig. 27 shows Draper's curve for the length of the embryonic
guinea-pig.
Ibsen's work led to much the same conclusions as regards the
relations between embryo and adnexa as that of Draper. Ibsen
found that the number of foetuses in the uterus exerted an effect
on the growth-rate of each one, thus the larger the litter the slower
the rate of growth of the individual foetus. The early growth of the
placenta is more rapid than that of the foetus, but they reach the
same weight on the 25th day, after which the foetus outstrips the
placenta very soon. Placental weight and the weight of the mem-
branes towards the end of pregnancy are closely correlated with
uterine crowding, but this is not the case with the decidua basalis,
which corresponds to the maternal part of the placenta. Minot con-
sidered that the amniotic fluid of the cow and of man decreased in
378
ON INCREASE IN SIZE
[PT. Ill
amount after the middle of pregnancy, but this was not found to
be the case by Ibsen for the guinea-pig. Ibsen constructed from his
experimental data the interesting diagram shown in Fig. 28, which
shows the percentage of the whole system occupied by embryo,
placenta, decidua basalis, and amniotic fluid from the 20th day
onwards. The embryo does not rise in per cent, after the 55th day,
the placenta remains very much the same all through, the decidua
basalis is much smaller relatively at the end than at the beginning
and so is the amniotic fluid. Up to the 50th day Ibsen found no
correlation between foetus-weight and placenta-weight, but after-
wards there is undoubtedly such a correlation, evidently due to
crowding.
[d) Rabbit. Much less work has been done on this form than might
have been expected from its easy availability, but the figures of
Chaine (the standard ones) are given in Table 8 of Appendix i,
together with some early fragmentary ones of Fehling. Friedenthal
also gives a few data which are shown in Table 9.
{e) Dog. Liesenfeld, Dahmen & Junkersdorf made a thorough
study of (unfortunately only 5!) dog foetuses.
SECT. 2]
AND WEIGHT
379
(/) Sheep. As early as 1847 Gurlt made a study of the increase in
length of the foetus of the sheep, but Colin is the only investigator
who has ever determined the
growth in weight (see Fig. 29).
Gurlt's figures, which are quite
regular, are given in Tables 10
and II of Appendix i. Faure-
Fremiet & Dragoiu, in the
course of their extended work
on the growth and chemical
development of the embryo-
nic lung in the sheep, made
measurements of the growth
of that organ, but did not give
any data on the weights of their foetuses as a whole, a very unfortunate
omission in view of the incompleteness of the literature on this
subject.
[g) Pig. The only extensive figures in existence for the embryonic
growth of the pig are due to the careful work of Lowrey and of
Warwick. These are given in Tables 12 and 14 of Appendix i.
Lowrey's results will again be referred to in connection with the
growth of individual organs and parts in the embryo. LeBreton &
Schaeffer also measured and weighed a certain number of foetuses in
the course of their classical work on the behaviour of the chemical
nucleo-plasmatic ratio during
60 100
Days, Sheep (Colin)
Fig. 29.
embryonic development. Their
figures are shown in Table 1 3 of i
Appendix i. \
(Ji) Cow. The embryonic \
growth of the cow has been \
investigated by several workers -
whose results are shown in '
Table 15 of Appendix i.
Fig. 30, constructed from
Franck and Hammond, should
be compared with Fig. 28 for the guinea-pig.
{i) Man. The embryonic growth of man has been much studied,
and many thousands of embryos have been weighed. His's studies
have been the principal means of fixing the relation age/length,
Months, Cow (Hammond).
Fig. 30.
38o
ON INCREASE IN SIZE
[PT. Ill
and Balthazard also gives figures for this, which will be found in
Appendix i (Table i6). The earlier workers, Ahlfeld; Fehling;
Hennig; Legou; Faucon; and Michaelis all obtained valuable data,
but it was not until 1909 that a critical examination of them was
made by Jackson who analysed the figures of his predecessors, and
added a large number of new ones. His results gave a continuous
curve from the earliest stages till birth, which agreed with the
majority of the other investigators, but not perhaps with Hennig's
curve (he gave no figures),
which showed a very distinct
slackening of growth about
the sixth month, after which
the same rate was resumed.
If this phenomenon is real, it
may possibly be associated,
as Donaldson has suggested,
with a transition from one
growth-cycle to another, at
the end of the sixth month,
when the absolute weight
begins to rise so rapidly. On
the other hand, the mass of
data which Quetelet and
others after him have ana-
lysed regarding the growth
of man throughout life, would seem to show that there are three
growth-cycles only, one pre-natal one, one with its maximum at
5*5 years, and the third with its maximum at 16 years. Vignes'
S-shaped curve for human embryonic growth is shown in Fig. 31.
Bujard in 19 14 made a geometrical analysis of the early stages of
the human embryo.
Jackson measured the volume and weight of all the specimens in
his own collection, and for the early stages also the volumes of the
His-Ziegler models. His figures are given in Tables 17 and 18 of
Appendix i, and the curve which he constructed from his own data
as well as those of previous investigators is reproduced in Fig. 32.
The 1 6th table of the appendix shows the volumes of the His-
Ziegler models, and demonstrates that the human embryo, like all
others, is much exceeded in size by the yolk-sac during the earliest
3500
-
3398<jyP3405
31410^88
3000
-
im(p
2000
n
E
TO
i.
- C
1000
a3
'©
mcf
1
500
100
JH
no.
305
7
1 _. 1
260 280 300
1 1 i > f 1 1 1 ■
3 4 5
Months
Fig. 31-
.270 290 310
Days
SECT. 2]
AND WEIGHT
381
stages of development. Jackson, who adopted the Minot method
of measuring the growth-rate, concluded that the rate was 9999 per
a)
E
o
>
E
o
<4-
©
E
j3
CO
0)
(0
E
o^
c
X)
j:
3250
3000
2750
2500
2250
2000
1750
.1500
1250
1000
750
500
250
l:
•
: \
1
i
<l
/
/ !j
1.
1
AhlfelcL's dauta.
■ehling's da-bet
Jaxjkson's data,
L.egoas data,
Michael is' data
1
i
1 *
It
2.
3.
i
i
'/•
4
5.
L
1
!
1 1
1 1
! 1
J
/
i
1 /
/ /
/ /
1
t
I
1
i /
1
1 /
/ /
/ /
/
2//
''7 /
1
' /
/
-4-
'••
/
^
j^'
L^.
^
^
' •
0 50 75 1(
125 150 175 200 225 250 275
Age in Days
Fig. 32.
cent, for the first month, 74 for the second month, and 11 for the
third month. This was in general agreement with the point of view
taken by Miihlmann, and Jackson emphasised it further by showing
that, if the weight of the embryonic membranes and fluids was taken
382
ON INCREASE IN SIZE
[PT. Ill
into account, the growth-rate for the first month was 574,999 per
cent. What meaning can be attached to the enormous growth-rate
figures which always appear when the Minot method is used for very
young embryos must later be discussed.
Fig. 32, which collects together the data of many observers, shows
a considerable measure of unanimity between them. Ahlfeld's
figures are the only ones which show serious divergence, and they
were not taken into account by Jackson in his preparation of the
"normal curve". Fig. 32 shows also by points the volume of the
embryo at the different stages, but it does not differ much in value
from the weight in grams. The specific gravity of the foetus does not,
according to Jackson, remain precisely the same throughout develop-
SECT. 2] AND WEIGHT 383
merit, but changes from very slightly above i-o in the early stages
to 1-05 in the later ones. Probably this is associated with the pro-
gressive loss of water as the embryo develops.
One of the first to investigate quantitatively the growth of the
human embryo was Boyd in 1861, who studied the weights of all
the principal organs in embryos from 8*5 to 85 oz. He did not give
figures for individual embryos from which a curve could be con-
structed, but simply divided them into large groups such as "pre-
maturely-born", etc. Legou's data, already referred to, were worked
over again in 1903 by Loisel. Zangemeister, more recently, has
published figures for human embryonic growth — these are shown in
Table 17 of Appendix i.
Other data for embryonic growth in man will be found in the papers
of Fesser ; Toldt ; Meyer ; Heuser ; Bedu ; Sombret ; Arnoljevic ;
Stratz; Borri; Corrado; Balthazard & Dervieux; Ecker; Hamy;
Kolliker; Cruickshank & Miller; Browne; and Friedenthal. Scam-
mon & Calkins, who have made a great many measurements in
recent years, have constructed a three-dimensional isometric pro-
jection, from which the height, weight and age of a human embryo
may be read off if any one of them is known (see Fig. 33). The best
recent paper on the whole subject is that of Streeter.
Sandiford has shown that the weights and surface areas of foetuses
fall on straight lines when plotted on double log paper. For further
information on surface growth see Scammon & Klein.
(j) Whale. Some information on the embryonic growth of the
whale is contained in the papers of Harmer; Risting; Hinton; and
Mcintosh & Wheeler, but it mostly concerns increase in length.
2-3. The General Nature of Embryonic Growth
We may now turn to the theoretical aspect of the matter in the
attempt to find out what interpretation can be placed upon them.
We may in the first place take as a simple example of an embryonic
growth-curve the work on the growth of the chick (White Leghorn)
of H. A. Murray. Table 51 shows, firstly, the actual weights of the
embryos on each successive day, secondly, the amount gained in each
such 24-hour period, i.e. the amount of substance actually added on
to itself by the embryo during the lapse of the time in question. This
is known as the daily increment. In the next column the averages
of the daily increments are placed, and these figures, known as the
384 ON INCREASE IN SIZE [pt. hi
mid-increments, represent for each point which begins one period
and ends another how much substance the embryo is adding on to
itself between the times (a) half-way through the last period, and
(b) half-way through the period to come. In other words, the mid-
increments convert the daily increments into terms of the points
instead of the spaces between the points. If now the mid-increments
Table 5 1 . Growth of the chick embryo ( White Leghorn) .
H. A. Murray's figures.
Percentage
Daily
Mid-
growth-rate
.Age
Wet weight
Dry weight
increment
increment
of dry
in days
(mg-)
(mg.)
(dry weight)
(dry weight)
substance
5
221
"•75
"•85
6
423
23-6
19-4
15-7
66-5
7
735
43-0
30-8
25-1
58-4
8
1,189
73-6
44-3
37-5
50-8
9
1,817
ii8-i
68-2
56-2
47-5
10
2,661
186-3
102-5
85-3
45-7
II
3,750
288-8
160-7
131-6
45-6
12
5.105
449-5
241-0
200-8
44.7
13
6,839
690-5
409-4
325-2
47-1
14
8,974
1099-0
575-0
492-0
44.7
15
1 1 ,460
1674-0
686-0
630-0
37-6
16
14,390
2360-0
730-0
708-0
30-0
17
17,950
3090-0
797-0
763-0
24-7
18
22,030
3887-0
832-0
814-0
20-9
19
26,670
4719-0
are expressed as percentages of their actual weights of embryo at
their corresponding points in time, the last column or percentage
growth-rate is obtained. This last calculation is, as will be seen, the
only one so far made in the table which is open to serious criticism,
and it is associated with the name of C. S. Minot, who was the first
to propose it as a satisfactory measure of the growth-rate of an
organism. When these figures are plotted the curves shown in Fig. 34
appear. The actual growth of the embryo expressed in terms of its
SECT. 2]
AND WEIGHT
385
weight at any given moment gives a curve which rises steadily till
the observations cease without betraying much sign of any slackening.
But the increment curves, on the other hand, show an unmistakable
S-shape which is due to the fact that, for the earlier periods, the
— o AbsoLate weight gms. wet
• >> )> j> dry
— 0 Dajly incremenbl
^ « Mid » r^^-
Fig. 34-
weight gained each day is very little more on one day than the gain
on the previous day, while, towards the middle of development, the
daily increments and the mid-increments vary much more, each one
being considerably higher than the one before. On the other hand,
when the end of development is approaching, the increments each day,
though far higher in absolute amount than those which were made
in the early stages, do not differ so materially from one another, with
NEI 25
386
ON INCREASE IN SIZE
[PT. Ill
the result that the curve slackens off and enters a slowly-rising phase
again. It is possible, of course, to calculate the average daily incre-
ment for the whole developmental period (see Table 52), and the
figures so obtained have been made the basis of a comparison of
animals by Friedenthal. The fourth curve, that of the percentage
growth-rate, Minot's curve, as it may be called, begins at a high
level and continually descends, although in this instance there is a
slight kink on it midway through development, which may for the
moment be disregarded. All Minot curves begin at a high level and
descend as development proceeds. Now, in many cases, it may
happen that not only the increments but also the whole growth-
process itself slackens off towards the end of the period taken, in
which circumstance the curve relating weight of animal at any
given time to age will also have an S-shaped form. It has not
escaped the perspicuity of those who have considered these pheno-
mena that this S-shaped curve has a resemblance to the S-shaped curve
of an autocatalysed monomolecular reaction, and this likeness will
shortly be taken up at length.
Table 52. Average daily increments.
Friedenthal's figures.
gm.
gm.
Mouse
o-o8
Pig
14
Rat
0-24
Man
15
Ermine ...
03
Sheep ...
26
German marmot
1-2
Seal
30
Musk
1-5
Ass
53
Guinea-pig
2-0
Rhinoceros
90
Wolf
4
Stag ...
100
Puma
5-4
Horse ...
200
Bear
7-0
Hippopotamus . .
200
Lion
10
Camel ...
400
Roedeer...
II
Elephant
400
A more complicated example of the various types of growth-curves
is afforded by Fig. 35 taken from Brody. It shows the growth
throughout the life-span of the albino rat. The curve passing through
the circles shows the course of growth ; it is, in fact, the weight of
the whole animal at any given moment plotted against the age
at that moment. The strongly indented curve, passing through the
crosses, is the line showing increment in unit time. In the data of
Murray for the embryo chick the absolute growth-curve rises steadily.
SECT. 2]
AND WEIGHT
387
and has no slackening off or self-inhibitory phase; the increment
curve is therefore singly sigmoid. But here, when the absolute
growth-curve is itself sigmoid, the increment curve is symmetrically
sigmoid, rising to a maximum and then falhng away again to zero
during the second phase. Finally, the corresponding Minot curve is
shown by the line joining the triangles, and, as usual, it declines
throughout growth from an initially very high value.
n 70
30
«25
n)
u
CO
a»20
m
i.
OU
60
<0
2 40
■^30
0.20
gms.
200
cent
day
DaysO 10 20 30 40 50 60 70 80 90 100 HO 120 130 140 150 160 lyO^lB0 190 200 210 220
o 0 8 18 28 38 48 58 68 78 88 98 108 118 128138148 158 168 178 188198
% I Age
o
U
Fig- 35-
There are other ways, however, in which the subject of embryonic
growth-curves can be introduced. Ostwald's classical work on
growth in metazoa, which appeared in W. Roux's "Vortrage"
series in 1908, laid great emphasis on the value of knowing the
precise route through weight taken by an organism on its way
from egg-cell to finished embryo. In Fig. 36, taken from his mono-
graph, several different curves are shown relating time to weight.
At the time A, at hatching or birth, for instance, the weight of the
25-2
388
ON INCREASE IN SIZE
[PT. Ill
organism is exactly the same in all four cases, but the manner in
which the increase in weight has taken place is in the four cases
profoundly different. It is certainly quite clear that the chemical
embryologist, engaged in the attempt to understand the processes
which contribute to the final result, must pay detailed attention to
the path by which this final
result is arrived at. The four
different curves in Ostwald's
figure would imply four very
different sets of conditions with-
in the developing embryo. An
embryo which grew according
to Curve I would change very
rapidly in the beginning, and
afterwards change progressively
less rapidly as the curve became
asymptotic. The reverse of this
process would happen if the em-
bryo grew according to Curve ii,
for there the process continually increases in rapidity, and is pro-
ceeding at its fastest when the point A is reached. Curve iii, on the
other hand, being S-shaped, would seem to indicate the presence of
an autocatalytic process, for at first the growth proceeds faster and
faster, but later on, after the point of symmetry of the curve has
been reached and passed, slower and slower. Several such S-shaped
curves superimposed on one another make up Curve iv.
As far as is known, no growth takes place in the manner repre-
sented by Curve i, but rather in that of the other three curves,
though our present knowledge does not enable us to say definitely
which, except in certain cases. Ostwald's monograph should be
referred to by those who wish to see how he continued the discussion
of growth-curves, for it is probably the best presentation of the subject,
and it was certainly written from a much less doctrinaire point of view
than most of its successors.
The general interpretations of embryonic growth-curves may be
divided into several classes. They depend more than anything else,
as will be clearly seen, upon how the facts are expressed. One way
of expressing them led Minot to his "laws of cytomorphosis",
another led Robertson to his "autocatalytic master-reaction",
SECT. 2] AND WEIGHT 389
and, more recently, still other ways have been devised. The un-
prejudiced investigator cannot avoid a considerable measure of
scepticism in considering the claims of one way of expressing the
facts over another.
2-4. The Empirical Formulae
We may first direct our attention to those presentations of the facts
which do not carry with them any theoretical superstructure, but
aim simply at describing the data in as short a manner as possible.
The first of these "empirical formulae" was that of Roberts, who in
1906 pointed out that the growth of the human foetus could be
regarded as nearly proportional to the cube of the age ; thus, if the
weight in grams is W and the age in days T, the formula would be
W — T^. But this was only very approximate, and the curve it gave
did not fit the curve drawn through the experimental data with
any accuracy. Roberts, indeed, stated that his formula gave results
correct to "within an ounce at the third month". "Since the weight
of an embryo of the third month," was Meyer's remark, "according
to the best available evidence, is considerably less than an ounce, the
accuracy of Roberts' method must be fully apparent without further
comment."
Tuttle next introduced an equation in which arbitrary constants
were introduced, thus W = 50 {T — 2)^. Later still, Jackson, whose
work on the human embryo has already been mentioned, proposed
the formula :
where W is the weight in grams and T the age in days. This fitted
the experimental points much better than the formulae of Roberts
and Tuttle, but was still rather deficient, especially in the very
early period and the very late period. Henry & Bastien also pro-
posed
x^ + 2^xj> — 3q>'2 — i62y = o,
where x = months andj = kilos.
Duvoir has reviewed the other more or less practical rules which
have from time to time been proposed, such as Casper's rule that, from
the fifth month onwards, the age of an embryo in man can be found
by dividing the height in centimetres by 5. Balthazard & Dervieux
390 ON INCREASE IN SIZE [pt. iii
altered this formula to 5-6. Again, Mall's rule states that the number
of days embryo age is equal to the square root of the foetal total
length in centimetres x 100. Balthazard & Dervieux have also
evolved formulae relating foetal age to the length of the limb-bones,
e.g.:
L = femur length x 5-6 + 8 cm.
L = humerus length x 6-5 + 8 cm.
L = tibia length x 6-5 + 8 cm.
The use of empirical formulae in the description of human foetal
growth has been carried to its greatest refinement in the work of
Scammon & Calkins, whose formula,
n- 2-5/, L2
holds with great exactitude from the third month onwards. Another
of their formulae,
T = 2-134 X o-iZ X o-ooiiL^,
holds with rather less exactitude from 2-5 foetal months onwards.
In both these cases, T is the menstrual age in lunar months, L the
total or crown-heel length of the dead body in centimetres. They also
found that
W= (o-26L)3-i'>8 + 4-6,
3 108 ,
or Z, = 3-846 VW - 4-6,
where W is the weight of the dead body. From these equations, it
follows that
3 108/ 15S4 /
T= 2-134 + 0-3846 VW — 4-6 + 0-01627 VW — 4-6,
or T= 3-0 + ^•04.gVw — 0-012,
orW= 0-561 — 0-366 T X 0-061 T^.
The formula of Donaldson, Dunn «& Watson, for the post-natal
growth of the white rat- up to 80 days, W = a + bT + cT^, and after
80 days, W = a log T — bT — c, was of the same type as the other
equations mentioned, but it had the additional refinement of in-
cluding constants, a, b and c, which were variable according to sex
and other factors.
Murray, in his study of the chemistry of embryonic development,
SECT. 2]
AND WEIGHT
391
found that his series of chick embryo-weights could be accurately
described by the equation:
T3-6
1-496'
or W=KT^-\
where K = o-668.
This was not unlike the Balthazard-Dervieux formula for the
human embryo:
T= 19-4 X \^W.
23
Diojs 5
10 U t2 13 14
Incub&lion 2052
Fig- 37-
Murray plotted the log. wet weight against the incubation age, and
obtained a curve concave to the abscissa (see Fig. 37) corresponding
to the curve which McDowell, Allen & McDowell got for the mouse
embryo (see Fig. 22). He also found that the relation of log. weight
to log. age was a straight line as far as his series of weighings went,
and showed that the weighings of Hasselbalch and of Lamson &
Edmond fell on the same straight line (Fig. 38). Murray's formula
gave very good agreement with his figures, but these did not extend
further back than the fifth day of incubation. When, later, Needham
392
ON INCREASE IN SIZE
[pT. m
and Schmalhausen made weighings of embryos between the second
and the sixth day of incubation, it was found that Murray's formula
did not hold for these earlier stages. Fig. 39 taken from the paper
of McDowell, Allen & McDowell, illustrates this point. The broken
line is drawn to Murray's formula, and the dotted line is an extra-
polation of his curve which I made on the assumption that embryos
grew at the same rate before 5 days as between 5 and 7 days, i.e.
exponentially. The circles with dotted centres are the values experi-
mentally obtained by me, the dots are those obtained by Schmal-
hausen, and the cross within a circle is Murray's earliest figure.
4.6
4.2
>.?
3 J.4
Of
3
2.6
2.2
fi^
ft.
%^
H
»
t-
gjp
°
V^
t
0
A
^
8
\^
*^
"
^
0
X LSkiDSon i Ldmond (a^vep^oe of lo embrvos)
• H&ss2lbailch (sln^l? ojeigninos)
0 Single cuci§hin§s l^ken in the course
of other experiments in this series ■
»^
^
^
-"
'
1
^
1
0.65 0.70 0.75
0.85 0.90 . 0.95 1.00 1J)5
L05 incubation a^e (dsajs)
Fig. 38.
U5
1.20
1.25 UO
Murray's formula gives a line consistently above the experimental
points for this early period, and the exponential extrapolation is quite
at variance with them. But McDowell, Allen & McDowell evolved
an equation which fitted these early points (the solid line) as well
as all the later ones, as follows :
log W = 3-436 log {10 (r- 0-5)} + 7-626.
This new equation was based on an entirely different viewpoint
from that of previous workers. McDowell and his collaborators
regarded not "conception age" but "embryo age" as the right zero
hour to take in growth calculations. It had always been assumed
previously that conception or even insemination was the right
starting-point, and Brody & Ragsdale and Brody based their method
for finding age-equivalence in animals on this view, while Friedenthal
SECT. 2]
AND WEIGHT
393
had shown a similarity in relative growth-rates by plotting the log.
weights against log. conception ages. McDowell and his colla-
borators, on the other hand, suggested that the time of growth ought
rather to be calculated from the time at which the embryo first
begins to have an axis, i.e. from the primitive-streak stage. Thus the
major differences between the pre-natal growth of the guinea-pig,
WEIGHT
. INMGS.
■200
-180
•160
•140
-120
-100
• 80
■ 60
■ 40
- 20
DATA OF
• SCHMALHAUSEN
ONEEDHAM
e MURRAY
INCUBATION 1
AGE
FREQUENCIES
©85 077 ©88 © ©83
77 ffi200
Fig. 39-
the mouse, and the chick would be accounted for by the varying
times taken to get through the preliminary work of arrangement and
organisation. Processes such as gastrulation, according to this view,
would involve a law of growth so different from the later axial type
that no formula should be expected to cover the two. We have already
seen in the case of the rat's egg that some considerable time may
elapse between the time of fertilisation and that of fixation to the
uterine wall, during which the supply of nutrient material may be
394 ON INCREASE IN SIZE [pt. iii
very different from what afterwards obtains during embryonic develop-
ment. There is, therefore, much justification for the view of McDowell
and his collaborators. Brody himself had come nearly to this position
without recognising it, for, in a paper published in 1923, he had
pointed out that, during a short period in the early stages of growth
(or regeneration) the apparent observed speed seems to be slower than
would be expected. Thus the curve of the fitted equation cuts the
time axis not at zero, the beginning of growth, but a little later. He
advanced the explanation that Durbin had already applied to the
initial slow phase in the regeneration of tadpole tails, namely, that a
"cap of embryonic cells" was first formed, following in its growth
quite different laws from the subsequent process as a whole. "It is
suggested", said Brody, "that the apparent initial slow phase of
growth of the individual from the fertilised egg is due to a similar
qualitative growth."
(Estimated weights of eggs are shown in Table 53.)
McDowell and his collaborators proceeded to show that a similar
formula would fit very well the curves of growth for the guinea-pig
(Draper; Hensen; Ibsen & Ibsen), the mouse (McDowell, Allen &
McDowell) and the chick (Murray; Needham; and Schmalhausen).
For the mouse it was :
log W = 3-649 log {10 (^ - 7-2)} + 8-6587;
and for the guinea-pig it was :
log W = 3-987 log {10 {t - 12)} + 5-1839;
The significant thing about these empirical formulae is the deduction
of a certain time in each case from the conception age, thus 7-2 days
in the case of the mouse, 1 2 days in the case of the guinea-pig, and
0*5 day in the case of the chick (Allen & McDowell). The evidence
on which McDowell and his collaborators rested their case for this
shortening of the development time was drawn from various sources ;
thus, from their own histological observations they found that the
primitive streak in the mouse embryo appeared about the 7th day
of development, for 6-day embryos show no mesoderm, while 7-day
ones do, and usually the primitive groove as well. Sobotta's work is
in agreement with this estimate. Their estimate for 12 days as the
time taken for the embryo guinea-pig to reach the primitive-groove
stage was based on the generally accepted work of BischoflT and
Lieberkiihn, while, for the chick, Duval, whose illustrations are the
SECT. 2]
AND WEIGHT
395
usual "normaltafeln", shows the first appearance of the primitive
groove at the loth hour of development, an assessment which is
agreed to by Jenkinson and by Foster & Balfour (12th hour).
Table 53. Probable dimensions of egg-cells.
Weight
Diameter
in grams
in fi
(Friedenthal)
(Hartmann)
Gyclostomata
Amphioxus
...
o-oooooi
—
Lamprey ...
...
0-004
—
Pisces
Sturgeon ...
0*004
—
Pike
o-ooi
—
Amphibia
Frog
...
0-004
—
Reptilia
Crocodile
...
40-0
—
AVES
Hen
45-0
—
Aepyornis
I20000-0
—
Mammalia
Monotremata
Platypus
—
2500
Spiny anteater ...
o-i
3000
Marsupialia
Dasyurus
—
240
Opossum
O-OOI
150
Edentata
Armadillo
—
80
Cetacea
Whale
—
140
Insectivora
Mole
—
125
Hedgehog
—
100
Rodentia
Mouse ...
O-OOOOOOI
72
Rat
—
72
Guinea-pig
O-OOOOOI
80
Lagomorpha
Rabbit
0-000003
125
Carnivora
Dog
0-000003
140
Cat
—
125
Ferret
—
120
Ungulata
Horse ...
—
135
Sheep ...
—
120
Goat
—
140
Pig
0-000003
130
Cheiroptera
Bat
—
100
Lemurs
Tarsius ...
—
90
Primates
Gibbon ...
—
"5
Macacus
—
115
Gorilla
—
135
Man
0-000004
135
"The general course of pre-natal growth in the mouse, the guinea-
pig and the chick, can be expressed by straight lines relating the
logarithms of the weight and the age only when age is counted from
the beginning of the embryo proper." Such is the conclusion of
McDowell and his collaborators, and, though it may seem barren
396
ON INCREASE IN SIZE
[PT. Ill
in theoretical results, it is nevertheless based on sounder considerations
than the more ambitious
ones of other workers*. It
is probably legitimate to
assume that the laws of
growth before the formation
of the embryonic axis are
very different from what
they are afterwards. It is
also legitimate to assume
that the differences between
the velocity constants in the
three formulae are due to
the varying amount of or-
ganisation which has to go
on in each case before the ^
formation of the primitive <
groove. Fig. 40 shows the o
straight-line relationships ?
found to hold by McDowell, ^
Allen & McDowell in the 9
case of the guinea-pig, mouse ^
and chick, and Fig. 41
gives further examples, from
which further variants of
McDowell's formulae could
easily be calculated. Clearly
in embryonic growth log.
weight is always propor-
tional to log. time.
With respect to Fig. 39,
in which the weights of very
young chick embryos are
given, it should be noted
that the discrepancy would
naturally be expected to
occur only in the early
stages, for in the later ones the difference between conception age
and embryo age would be a smaller percentage of the total. The
* But see p. 427.
100
• n
r
t
I
7
1
1 o- ono
/
I 9 •
/ 7
> '-A
I
J
\
// /
^
// /i!
J
'■il P-
■h
7 VI'
•IOC
;
. L
/ i
rT
■//
//
/
•OK
s
// '
/ i
/ /
1
f UJ
ho J
h
h^
'§
c
HICK
•GO
•^
//
0
^
MURF
?AY
r/-
NEEDHAM
c
•
schmalhausem;
.1
•00010
I
GL
INFA
PIG
/
1
0
BSEN
♦ DRAPER
+ V.HENSEN
•00001
1
2 4 10 20 4060100 200 400 600
EMBRYO AGE IN }\J"^ OF DAY
Fig. 40.
SECT. 2] AND WEIGHT 397
averages for the early embryos reveal the difference by bending away
from the lines drawn on the basis of incubation or conception age.
Schmalhausen, continuing earlier work on the growth of bacilli and
protozoa, has also put forward empirical formulae for the embryonic
growth of the chick, but his equation
\^W= T,
while fundamentally the same as that of Murray, has no velocity
constant. Fig. 42, taken from Schmalhausen's paper, shows that
the cube root of the weight plotted against the age only gives an
approximately straight line. Schmalhausen has included in the same
figure the curves obtained by other methods; thus curve P' is the
Minot (percentage growth-rate) curve for the wet weight, and Ps'
for dry weight, while the curve marked log o- ip is the log. weight
plotted against the age. As we have already seen, in the case of
McDowell's figures for the mouse, and Murray's figures for the chick,
this value always gives a curve rising concave to the abscissa. The
curves P and Ps in Fig. 42 represent the absolute wet and dry weights
respectively.
Other empirical formulae have been proposed for growth-pro-
cesses from time to time.
Embryonic growth can be expressed roughly by exponential curves ;
thus:
W = wp\
where W is the mass of the embryo at time t, w the original mass,
and p a constant. Thus the equation of an exponential curve is one
in which the power is always changing. Janisch has given a dis-
cussion of the use of the exponential curve in all departments of
biology, and in it he shows how important this relation is in growth
phenomena.
The "law of compound interest "however, put forward byBlackman
in 1919 for the growth of plants, and which has been shown by
Luyet to be a special form of the exponential relation
W= w {i + ry,
has not so far been of any assistance in describing embryonic growth.
Another form of the exponential curve, the arithmetical progression
method, which gives the equation
log W - At,
398
ON INCREASE IN SIZE
[PT. Ill
where A is 3. characteristic constant, was used by Faure-Fremiet in
1922 for describing the growth of a Vorticella colony, but the con-
100
CHICK
Hasselbalch 1900
O Series (a)
D « (h)
0 Bohr& Hasselbalch 1900
• LamsonS^Edmond 1914
Ijin 1917
O LeBrebon8(Schaeffer1923
B Murray 1926
Schmalhausen 1926
^ Needham 1927
RAT
0 Stobsenberg 1915
MOUSE
® McDowell etc. 1927
<§> LeBreton&Schaeffer1923
GUINEA-PIG
O Draper 1920
SHEEP
® Colin 1888
PIG
O LeBrebon&.Scliaeffer1923
O Warwick 1928
TROUT
X Scheminski 1922
RABBIT
O Chaine 1911
« Lochhead&,Cramer1908
0-0
1 00 days
Fig. 41.
ditions there are too far removed from those of embryonic develop-
ment to make it worth while considering this aspect of the subject
in detail. The formula proposed by Faure-Fremiet for the growth
SECT. 2]
AND WEIGHT
399
of the epithelium of the foetal lung is, however, of greater
interest i
W = At^w + Bfiw + Ctw + Dw,
where w is the total weight of the lung at the time in question. But
here the number of arbitrary constants is so large that we reach the
point where the question naturally arises whether an empirical
/q^ 0,1 p
7 8 3 10 i1 12 13 1'^■ 15 16 17 18 19 20 21
Fig. 42.
formula is worth looking for at all. The more complicated it is,
the less valuable it is, in view of the fact that, in any case, it is
not intended to give us an idea of the basic factors underlying the
process.
2-5. Percentage Growth-rate and the Mitotic Index
We have so far been examining the results of those investigators
who have taken the curves obtained by simply plotting the weight
of the embryo each day during development against the time, and
who have endeavoured to find a correct mathematical expression
for them without a preconceived theory. We have now to consider
400 ON INCREASE IN SIZE [pt. iii
the work of those who have infused more theory into their treatment
of the experimental facts.
Before 1890 there was no regularity in the way in which experi-
mentalists examined their data on growth. But about that time
Minot began a long series of investigations on the growth of
animals, mainly the guinea-pig and the rat, in which he introduced
a new method, namely, the evaluation of the growth-rate by taking
it as the increment in per cent, of the weight of the animal at the
beginning of the period in question. Some workers, e.g. Preyer, had
already adopted this plan. The percentage growth-rate has always
been found to decline enormously as development proceeds, an
observation which led Minot to say that the embryo gets oldest
most quickly when it is youngest. This apparently paradoxical
statement drew a good deal of attention to his work at the time, and
his book. The Problem of Age, Growth, and Death, included many such
graphs showing how rate of senescence was greatest in the earliest
periods. One of these is reproduced here as Fig. 43.
Another contribution of Minot's was the conception of the "mitotic
index", or the number of mitosing cells per 1000 cells in a tissue.
He did not himself find time to do more than a few of these laborious
counts, but he gave the following figures, which showed that the
mitotic index declined with age:
Development of
rabbit foetus (days) Tissue
Mitotic index
7-5
Ectoderm
18
Mesoderm
17
Endoderm
18
lO-O
Ectoderm
14
Mesoderm
13
Endoderm
15
Blood
10
130
Spinal cord
Connective tissue
II
10
Liver
II
Skin
10
Excretory tissue
Muscle
6
6
These data lent weight to his principal conclusion, which was that
the younger the embryo the more rapidly it aged. Practically
nothing more was done along these lines until Olivo & Slavich in
1929 reported a large series of figures for the mitotic index of the
developing heart in the chick.
SECT. 2]
AND WEIGHT
401
Davs
Mitotic
period in
calculatec
development
index
hours
minutes
38 minu
2
22-5
19
42
—
3
21-2
20
55
—
5
169
26
1 1
15-7
7
150
29
24
94
9
8-3
53
II
IO-2
II
lO-O
44
3
4-8
13
7-0
70
15
57
15
4-4
100
33
40
17
3-7
117
39
2-5
19
2-8
155
55
4-2
21
1-2
363
7
1-8
10 (after hatching)
00
—
The fall in the mitotic index ran closely parallel with the fall in the
percentage growth-rate of the organ, as determined by a special
series of weighings. The time taken by one mitosis was calculated
from these data by Olivo & Slavich : it turned out to be constant at
38 minutes. But the intermitotic period grew longer and longer,
indicating that the later growth consists less than the former of
proliferation and more of increase in size of the cells already formed.
o
o\
( Males ]
Z 5 811 17 23 29 3S3« 45
75 80 105 120 135 150 165 180 185 210 dstye 241
Fig- 43-
For a long time Minot's way of looking at embryonic growth in par-
ticular and growth in general was universally adopted, e.g. by
Jenkinson, and even at the present time it is much used. But the
Minot curve is undoubtedly based on a fallacy, and it was not long
before a feeling that this was so began to arise. It was perhaps
intensified by the appearance of such estimates as that of
Muhlmann, who worked out the growth-rates in early stages of
N E I . 26
402 ON INCREASE IN SIZE [pt. iii
embryonic development in man as 3650 per cent, and above. It
was not unnatural to enquire whether the Minot growth-rate of the
original dividing egg-cell was even finite.
The dissatisfaction was voiced in 191 7 by d'Arcy Thompson, who
wrote as follows: "It was apparently from a feeling that the velocity
of growth ought in some way to be equated with the mass of the
growing structure that Minot introduced a curious and (as it seems
to me) an unhappy method of representing growth in the form of
what he called 'percentage-curves'. Now when we plot actual length
against time we have a perfectly definite thing. When we differentiate
this LjT we have dLjdT which is of course velocity, and from this
by a second differentiation we obtain d^LjdT^, that is to say, the
acceleration. But when you take percentages of jv, you are deter-
mining dyly and when you plot this against dx you have —-^ or
dx
— ^ or - . ^ , that is to say, you are multiplying the thing you wish
y.dx y dx
to represent by another quantity which is itself continually varying,
and the result is that you are dealing with something very much less
easily grasped by the mind than the original factors. Minot is of
course dealing with a perfectly legitimate function of x and y and
his method is practically tantamount to plotting logy against x^
that is to say, the logarithm of the increment against the time. [Cf.
log. weight-age curves.] This could only be defended and justified if it
led to some simple result, lOr instance if it gave us a straight line, or
some other simpler curve than our usual curves of growth". This
criticism was justified, for the Minot curve is certainly no simpler
than the untouched growth curves ; it merely falls instead of rising.
But d'Arcy Thompson did not point out the presence of a
definite fallacy in Minot's way of looking at growth, a physio-
logical rather than a mathematical one. This was grasped by Brody,
who has written as follows: "Minot's method for computing growth-
rates gives an exaggerated decline in the percentage rates of growth
with increasing age simply because an arbitrary unit of time, e.g.
a week, does not have the same significance at different ages. It is,
for example, entirely appropriate to express the gain in weight during
a week as a percentage of the weight at the beginning of the week
(Minot's method) for a 6-month old chicken, because the weights
(i.e. the number of cells or other reproducing units) at the beginning
SECT. 2] AND WEIGHT 403
and end of the week are nearly the same as compared to the gain in
weight. But to express the gain in weight during a week as a per-
centage of the weight of the body at the beginning of the week for
a 7-day old chick embryo would be quite fallacious. It would cor-
respond to expressing the gain in population in the U.S.A. from 1666
to 1927 as a percentage of the size of the population in 1666. The
growth of the population of the U.S.A. in 1927 is proportional to the
population in 1927 and not to the population in 1666. Similarly the
number of cells produced in a 7-day old chick embryo should be
functionally related to the number of reproducing cells (i.e. the
weight) of the chick at 7 days of age and not to the number of cells
at I day of age. In brief, growth is a continuous process and the
rate of growth at every instant should be functionally related to
the number of reproducing units at the given instant and not to
the number of reproducing units which existed in some relatively
remote past". In other words, Brody would prefer to ask not how
much 100 gm. of embryo add on to themselves during the im-
mediately succeeding period, but rather how many grams of those
1 00 gm. had been added on during a short preceding period. Murray's
modification of Minot's method, in which the mid-increments in-
stead of the daily increments are used as the basis for calculation,
goes some way to meet Brody's criticism, for it enquires how much
100 gm. of embryo add on to themselves during half the preceding
and half the following period, thus speaking in terms of a more
instantaneous measure. Brody himself has made use of a similar
amelioration. However, Brody's real point is that the fault lies in
choosing an arbitrary length of time interval for all stages of develop-
ment, in spite of the fact that they cannot possibly be equivalent for
the embryo.
Brody might say that the embryo cannot be regarded as having
been given an equal chance to accomplish its growth in each of the
daily periods throughout its development. On the other hand, it
might be argued that, though this is doubtless true as regards growth
in weight, it is not true with respect to the activities of the embryo
as a whole, which include many other processes, such as chemical
differentiation. Taking the embryo as a whole in all its activities,
the arbitrarily chosen and invariant period might be regarded as an
adequate one. As we shall see later, this is essentially the same
argument as that used by Murray against Robertson.
26-2
404
ON INCREASE IN SIZE
[PT. Ill
It may, however, be concluded that the Minot curve is only useful
provided no theoretical conclusions are drawn from it, and that it
is retained simply as a convenient method of comparing processes.
Brody's own theories will be discussed later.
As against the theory which Minot built up from his experiments
with growing animals, Murray has brought forward one convincing
-0.15 1
-0.14
n 4 «.
I
\
-0.12
'0.11
-0.10
-0.09
\
\
\
\
\
-0.08
^
-0.07
-0X?6
• -0.05
\;
>
\
\
-0.04
0.03
■0.02
K
X
X
^^
^^--^
^
'0.01
A^e
5 6 7 8 9 10 tl 12 13 14 15 16 17 IS 19 ^
0
Fk
?-44
argument. Minot's theory of cytomorphosis involved the following
propositions: (i) that the rate of growth depends on the degree of
senescence, (2) that senescence is at its maximum when development
begins, (3) that the rate of senescence decreases with age, and (4) that
death results from the differentiation of cells. But, as Murray says,
we have no real evidence to show us that the "degree of aliveness"
at any given moment is in any way connected with the velocity of
growth at that particular moment, or, more correctly, that the latter
SECT. 2] AND WEIGHT 405
value is a true measure of the former, "There are other and perhaps
more significant phenomena", said Murray, "than the growth rate,
which change with age."
Murray himself proposed the use of a variant of the usual Minot
curve by differentiating twice instead of once so as to get the ac-
celeration and not the velocity. Thus, after having found the per-
centage growth-rate by the equation
dt[_w\ t'
where K ^ 3-6, he went on to find the negative acceleration for each
day during embryonic growth :
dw
d
dt
dt
= - Kt^.
This value, so obtained, is, as it were, the negative increment of
the percentage growth-rate, and shows a regularly declining curve
(for the chick) as in Fig. 44. Such a curve must obviously suffer
from the same disadvantages as the curve from which it is derived,
and does not escape from Brody's criticism that the arbitrarily
chosen time-units are incommensurable at different developmental
stages.
In 1922 Przibram observed that in many cases of post-embryonic
growth the curve obtained by calculating according to Minot's
method was extremely like a regular hyperbola, but he did not find
that this was true for any example of embryonic growth. We shall
see later what further use of this idea has been made,
2-6. Yolk Absorption-rate
Another way of regarding embryonic growth (of a lecithic Ggg)
is to concentrate attention on the whole system, instead of upon
the growth of the embryo alone. It was in this way that Gray
treated the development of the trout embryo in the paper already
mentioned (p. 369 and Appendix i). He assumed that the rate of
growth of the embryo was proportional to the dry weight of the
embryo and to the dry weight of the remaining yolk. This idea had
already been introduced for the trout by Kronfeld & Scheminzki
(see p. 369 and Fig. 41) but Gray's figures were much more complete,
and showed very clearly a falling off of growth towards the end of
4o6 ON INCREASE IN SIZE [pt. iii
pre-natal life, when the yolk was becoming exhausted. Thus the wet
weight of an embryo plotted against the time gave an S-shaped
curve, which, however, was not symmetrical, for it had a point of
inflection after about 70 per cent, instead of 50 per cent, of the
development had been completed. This was of course well shown on
the increment curve, which was skewly bell-shaped. Assuming that
growth was proportional to the amount of yolk remaining as well
as to the size of the embryo already formed, Gray developed an
equation ^
(where x is the weight of the embryo at time t,yQ the total yolk in
the unfertilised egg, K^, the amount of yolk combusted by one gram
of embryo divided by the constant k in the equation
dx J
It = '"^'
X and y being weight of embryo and weight of yolk respectively)
which he considered accounted very well for the observed facts. He
deduced from it that there should be a period at the end of develop-
ment when the wet weight of the larva (the whole system, embryo
plus yolk) is decreasing, although the wet weight of the embryo itself
is still increasing. During the major part of development the wet
weight of both would increase, owing to the absorption of water
from outside. From the equations the maximum weight of the larva
should be reached when o-86 gm. of yolk is still unconsumed, and
in actual fact Gray found the peak at a point when i*io gm. \^as
yet remaining.
Another possibility used by Gray to test his hypothesis was that
as it was unlikely that the temperature coefficient of the growth
process would be the same as that of the catabolism going on, there
ought to be a measurable difference in the size of fishes raised at
various temperatures at the end of their development. Experiments
designed to reveal such differences gave the following results :
Mean weight of 100 embryos at
Temperature (°C.) the end of incubation (gm.)
15 i3-35±o-i6
ID i507±o-i8
so that the higher temperature not only accelerated the process, but,
by accelerating the combustion more than the storage, led to a
SECT. 2]
AND WEIGHT
407
smaller finished fish. These results are curious, for it is generally-
understood that temperature changes alter the rate at which a
growth-process goes on yet not the amount of end-product formed.
The work of Barthelemy & Bonnet on the frog is an exact parallel
to that of Gray on the trout, for these workers raised frog embryos
at different temperatures with the following results :
(P.E.G.)
Temperature
Dry wt. of 300
Dry wt. of 300
Dry wt. of embryos
(°C.)
eggs (gm.)
embryos (gm.)
Dry wt. of eggs
8
0-378
0-334
0-88
10
0-824
0-423
0-51
14
0-594
0-318
0-54
21
0-708
0-346
0-49
Temperature
Dry wt. of 70
Dry wt. of 70
Dry wt. of embryos
(°C.)
eggs (gm.)
embryos (gm.)
Dry wt. of eggs
8
0-092
0-079
0-85
10
0-128
o-ioi
0-79
14
0-097
0-082
0-84
21
0-107
o-o88
0-83
(P.E.C.)
If the second and third columns of this table are compared it will
be seen that in the first series the French workers did get results like
those of Gray, i.e. the higher the temperature the greater the com-
bustions and the less the storage, but that in the second series there
was no such effect to be observed. The Plastic Efficiency Coefficient
(P.E.C.) is the most convenient way of expressing this relation (see
Section 6- 1 o) . According to Gray it should change with temperature,
for assuming his trout embryos to have the same percentage composi-
tion, no matter what the temperature, those raised at 15° would con-
tain (each) 21*3 mgm. solid and consequently (since the eggs contain
43-4 mgm. solid) would have a P.E.G. of 0-50, while those raised
at 10° would contain (each) 24-2 mgm. solid and consequently would
have a P.E.C. of 0*56. I shall return to this subject in the section
on general metabolism of the embryo; here it is only necessary to
remark that the subject is clearly not yet settled and requires much
more attention than it has so far received. At the same time,
returning to the main theme, it must be remembered that Gray only
made use of these temperature phenomena as one of the supports for
his theory.
He drew another support from the fact that if his equations were
correct, the product obtained by multiplying the dry weight of yolk
by the dry weight of embryo should be at a maximum on the 71st
day of development. This he found to be actually the case, as is
4o8
ON INCREASE IN SIZE
[PT. Ill
shown in Fig. 45. And the growth-rate per gram of embryo (Minot
growth-rate divided by 100?) was also proportional to the amount
of yolk remaining.
Gray himself indicated a number of criticisms which might be
brought against his views. Thus there is little a priori reason for sup-
posing that the growth-rate of the embryo should be determined by
the amount of yolk in the yolk-sac, for apart from anything else, the
10
20 30 40 50 60 70
Fig. 45. Days of development.
80 90 100
amount and nature of the syncytium in the yolk-sac wall might be
a limiting factor. One would also expect that in the beginning when
the yolk is very large compared to the embryo, the yolk would be
present in excess, and would not exert any influence on growth-rate.
2-7. The Autocatakinetic Formulae
The workers mentioned so far might be divided into three groups,
those who have elaborated empirical formulae, those who have
adopted the Minot point of view, and those who have treated the
SECT. 2] AND WEIGHT 409
yolk plus embryo as one growth-system. Now a fourth and very
large group consists of those who have been greatly impressed
with the similarity which some empirical growth-curves show with
the curve for a monomolecular autocatalytic reaction. This manner
of looking at the subject is associated mainly with the name of
Brailsford Robertson, who has in many papers and in a book specially
devoted to the matter put it forward as the most fundamental ap-
proach to growth. Robertson was not, however, the first to notice
the likeness. As early as 1899 it had been referred to by Errera,
and Ostwald a little later. According to Monnier, Chodat of Geneva
paid some attention to it in 1904. "One may regard growth,
as M. Chodat has suggested", said Monnier, "as a complicated
chemical reaction in which the living cell is the catalyst and the
substances present are water, salts, and CO2." Four years later (in
1908 on May 9) Ostwald's monograph on growth was published in
the form of an inaugural dissertation, and only ten days later
Robertson's first paper appeared in the Archivf. Entwicklungsmechanik.
Ostwald had treated the question in a rather unmathematical manner,
but had fully explained the nature of his hypothesis ; Robertson, on
the other hand, gave the S-shaped curve a detailed mathematical
treatment. " The carrying out of a progressive development in
time has in animals a single characteristic type; the rapidity of
the process begins at a low value, increases with the continuance
of the action and falls off again at the end, in other words the type
of curve is S-shaped." This was as far as Ostwald went, but he did
not fail to point out that the S-shaped curve was identical with that
of an autocatalytic or an autocatakinetic reaction. An important
point to note is that Ostwald's curves were all curves of absolute
weight — he did not in any instance plot increment curves.
Robertson began by an explanation of the mathematical properties
of the autocatalytic curve. The differential equation characteristic
of the initial stages of an autocatalytic monomolecular reaction is
as follows : ,
-r = k-iX [a — x),
which expresses in mathematical symbols the fact that the velocity
of the transformation is, at any instant, proportional to the amount
of material which is undergoing change and to the amount of material
which has already undergone change. If, however, the reaction has
4IO
ON INCREASE IN SIZE
[PT, III
proceeded so far that the depressant effect of the products is measur-
able, then the previous equation becomes
— = k^x [a — x) — k^x^.
Now when the reaction has proceeded half-way to equilibrium, i.e.
when X = la, the equation becomes
X
or
log
log
A - X
A — X
= Ak{t- tj),
Amount transformed
and this is the well-known equation for the S-shaped curve, where
X represents the body-weight (not the increment) at time t, A repre-
sents the maximum of final weight which the organism is to reach,
^1 the time at which half this
maximum body-weight has
been attained, and K a con-
stant which has to be deter-
mined from a known value
of a; at a given time t. By
differentiating, it may be
found that dx/dt is at a maxi-
mum when X = \A — in other
words, that the rate of increase
in weight is greatest when
half the autocatalytic curve
has been passed through.
Robertson proceeded to apply
his calculation to the growth
of white rats, figures for which
had been reported by Donald-
son, Dunn, & Watson, with
excellent results over part of ^^^' 4^*
the curve, though not when the age amounted to more than lOO days.
At the time it seemed as if this was a most convincing example
of the value of the autocatalytic theory, but maturer consideration
showed that this lOO days was but a third of the possible life-span,
and the comparison of the two curves as made by Lotka, for in-
stance, does not look so impressive.
SECT.
2]
AND WEIGHT
411
Fig. 46, taken from Robertson's first paper, illustrates the relation
between the curves for the three chemical reactions, while in Fig. 47
is seen the theoretical and the experimental curve compared. In this
same first paper, Robertson applied his autocatalytic equation to
the growth of man, frog, a vine, and to certain organs. The frog
figures, which were those of Davenport, were the nearest approach
50
150 200
Age in days
Fig. 47-
300
to embryonic growth dealt with by Robertson, and they are given
in Table 54. They showed a good measure of agreement, but whether
it was right to say, as Robertson did in his summary, that "in all
probability cell growth or the synthesis of cytoplasm is an auto-
catalytic reaction" is a question that subsequent workers have not
by any means answered with a bald affirmative,
Robertson pointed out that his new interpretation of growth curves
fitted in very well with the views that had already been advanced
by Loeb. Loeb had suggested that the processes of cell-division and
412
ON INCREASE IN SIZE
[PT. Ill
growth were simply expressions of a more or less rapid return to
the chemical equilibrium between nucleus and cytoplasm which had
been temporarily shifted through the process of fertilisation. He
thought that there occurred in the sea-urchin's egg during the early
stages of its development a great synthesis of nuclear material, and
that this synthesis progressed, roughly speaking, all the more rapidly
the more nuclein was formed. The velocity of nuclear synthesis, said
Loeb, increases with lapse of time in geometrical progression.
Further, various observers, using the Q^k, formula, had concluded
that the growth-process had a "chemical temperature coefficient",
so Robertson felt fully justified in speaking of a "master-reaction"
of growth, and in thinking of it as autocatalytic — a reaction, which,
because it would be slower than any other, would act as the limiting
factor of growth, and would impress its own particular character
on the general appearance of the whole process from outside.
Table 54. Larval growth of frog [tadpole).
Robertson's figures
Davenport's
calculated ft-om
Days after
experimental
autocatalytic
hatching
figures (mgm.)
formula (mgm.)
I
1-83
1-64
2
3
4
2-00
2-03
—
—
5
3-43
3-90
7
8
5-05
6-00
9
10-40
9-OI
14
23-52
23-46
41
lOI-OO
110-90
81
1989-90
112-00*
* "Obviously another growth-cycle supervening", said Robertson
In his later writings, Robertson added further demonstrations of
the applicability of the autocatalysis equation to growth, but he also
added a large amount of extremely speculative considerations, such
as the "nutrient level", the "endogenous catalyser", etc., into which
we cannot here enter. Some of his suggestions, for example that the
autocatalyst is lecithin, may be regarded as now definitely out of
court. He showed, however, that the pre-natal growth in man (using
Zangemeister's figures) was susceptible of description in an auto-
catalytic curve with its early " autokinetic " phase (Robertson's
terminology), and its late "autostatic" phase. He also used, for the
SECT. 2] AND WEIGHT 413
last part of the curve, data on the weight of new-born infants, born
at different times, some earHer and some later than the normal. We
have, moreover, already seen that the curve constructed by Vignes
for human embryonic growth shows an S-shaped conformation.
In 1926 Robertson published a long paper in which he reviewed
the work which he and his collaborators had done on the subject
of the autocatalytic theory of growth. Here he showed that the fall
in the relative value of the velocity constant of (asymmetric) auto-
catalysis during the embryonic growth of the mouse followed almost
exactly the same curve as the fall in the chemical nucleoplasmatic
ratio as determined by LeBreton & Schaeffer (see Section 10-2). It is
not clear, however, what significance is to be attached to this finding,
especially as Crozier and Brown have shown that at least two velocity
constants must be postulated in a given cycle. Robertson himself
drew no theoretical conclusion from it.
Robertson regards the several growth-cycles distinguishable in the
life of an animal as being independent, in that they each have a
different catalyst. The first of these in the case of the mouse has an
equation of the type
where x is the growth attained at time /, A the maximum growth
attainable in the cycle under consideration, t the time required to
attain half the maximum growth and k a specific velocity constant.
B is another specific constant, an index of the asymmetry of the curve.
The second and third cycles have the formula
log-^-^ = A(^-0,
where the symbols are as before. He considers that skewness or
asymmetry originates probably in a progressive diminution of the
velocity constant as described above.
Robertson's suggestions were not allowed to go uncriticised.
Meyer, in his report of data for the growth of the human foetus, took
occasion to attack both them and the traditional Minot standpoint.
On the whole, Robertson's autocatalysis theory emerged with less
damage than Minot's cytomorphosis theory. Meyer brought into the
light what had been one of the most disturbing features of the Minot
method, namely, the absurd values which the percentage growth-
414 ON INCREASE IN SIZE [pt. iii
rate has in the earliest stages, e.g. five hundred and forty bilHon per
cent. But, as Meyer seems to have completely failed to understand
that Robertson and Minot were using different methods in calcu-
lating the "growth-rate", his criticism of Robertson was not of
any importance. On the other hand, he did draw attention to the
fact that growth-curves can be very misleading if it is not remembered
that, though in the early stages the absolute growth is minute, the
relative growth is enormous. "The weight of the impregnated human
ovum", said Meyer, "is approximately 0-005 ^§"^-5 ^.nd yet in-
vestigators in all seriousness indicate its weight on a short ordinate
reading in grams or even hundreds of grams. Little wonder, then,
perhaps, that Robertson, Ostwald, and Read have unwittingly as-
sumed that the curve of growth in man and mammals hugs the
abscissa for several months as the curve of autocatalysis does." This
was a good tilt at a common fallacy, but Meyer did not point out
that it could be remedied by using log. paper, and he left it quite
open to Robertson to reply that, even when strictly comparable
quantities were taken, the S-shaped curve or a succession of S-shaped
curves still resulted.
More serious criticisms than these have, however, been brought
against the Robertson method of treating embryonic growth. Luyet
has pointed out that it may suffer more than the other methods
from illusory difTerences in material. Again, Murray has written
as follows: "(i) The formula demands the introduction of three
separate constants which must be separately determined for every
set of figures collected. (2) The equation does not give the weight
as a function of age throughout life but only during an arbitrarily
selected part of the growth-cycle. For these two reasons the
equation as a practical simplification is not of great value. If
the equation were in such a form that knowing the species, the
age, the T°, and other environmental variables, one might calculate
the weight and growth-rate, it might be of use. But as it stands now,
it is necessary in each case to collect complete statistics and then find
a mathematical expression of the figures obtained. For instance, all
three constants in the equation for the growth of South Australian
males differ from the constants used in the equation for the South
Australian females. As one cannot extrapolate, the formula, like the
man with one talent, returns what it receives. In fact as it covers
only a section of the growth-curve, it yields less information than the
SECT. 2] AND WEIGHT 415
original data. Moreover, as a rational account this description of the
synthetic processes of growth is misleading, since (3) by this theory
the growth-rate is proportional to the increment gain in weight
regardless of the weight of the organism, or in other words, dis-
regarding the amount or concentration of the reacting substances.
(4) Figures for the growth of colonies as well as of individual organisms
are said to be described by autocatalytic equations and are classed
together. When growth is expressed in terms of percentage increase
in mass, however, the important distinction between phylogenetic
and ontogenetic growth is made evident ; for the former, after a short
latent period, in the presence of an experimentally modified environ-
ment proceeds at a constant rate [cf Richards], while the latter
does not. The S-shaped curve is the result of a limited and unre-
freshed culture medium. The individual organism, however, instead
of maintaining a constant growth-rate shows from the beginning
considerable negative acceleration. (5) The S-shaped curve is not
specific, for there are some physico-chemical processes not considered
to be autocatalytic which are described by a similar curve. Finally,
and this is the main objection, (6) chemical diflferentiation is not taken
into account by the autocatalytic theory, which is based on the con-
ception that there is some one master monomolecular reaction,
which, being the slowest of the chain of reactions concerned in the
phenomenon of growth, determines the velocity of the entire process.
As there is no direct way of measuring the product of the master-
reaction, the increase in the body-weight of the whole embryo is
taken to represent the product. In view of the marked changes in
chemical constitution which take place in the tissues with age there
is no reason to suppose, and in fact it is extremely unlikely, that the
total weight can be taken as an index of the amount or concentration
of any one chemical substance". The tendency that has existed in the
past to take the simple weight of the embryo as the sign par excellence
of its developmental stage and its "aliveness" is only another case
of the reluctance to judge by "ensemble", as Broca called it, instead
of by single indices, which was so long the bane of physical anthro-
pology. In this case, it is the increasing application of chemical
methods to the embryo which has shown the superficiality of regarding
mere weight as the pre-eminent factor. Lotka has drawn attention
to this point of view, and is inclined to compare embryonic growth
to the growth of a population such as Pearl & Parker have studied
4i6 ON INCREASE IN SIZE [pt. iii
in Drosophila. Janisch treats the S-shaped growth-curve as the re-
ciprocal of a catenary exponential curve. These presentations have
the advantage that they do not prejudge the issue from a physico-
chemical angle.
Exception to Robertson's views has been taken on quite other
grounds by Snell, who points out that Robertson's equation
^ = K^Ax - K^x"
dt ^ ^
and Crozier's modification of it
^ = (r^ + K,x) {A-x)
(where x is the concentration of the end-product at time t, A the
concentration of the substrate at time t, and K^ and K^ the velocity
constants of the forward and reverse reactions respectively) do not
take into account the fact that the system concerned is not a closed
one. All the time the embryo is growing it is also eating, i.e.
absorbing nutritive material, and in addition it is giving out
waste products. Accordingly these equations derived from the
law of mass action as we know it in the inorganic world do
not allow for the effect of increasing size on the concentration
of the reagents involved in growth. The equations hold true
only on the condition that the volume occupied by the resulting
substances remains constant, and since a growing organism is con-
stantly increasing in -volume, this condition is not met. "When a
chemical process is carried out in the laboratory," said Snell, "the
reagents are ordinarily dissolved in water or some similar solvent,
and the volume of the solvent is kept constant throughout the whole
process. To make the conditions of a laboratory process comparable
to those involved in the synthesis of new protoplasm the volume of
the solvent would have to be increased as fast as the amount of the
end-products is increased. As the solids of new protoplasm are
formed, they do not stay in the same little parcel of liquid occupied
by the old, rather they cause the liquids to expand with them. Hence
the volume occupied by the end-products of growth is proportional
to their amount, and the concentration of these products, instead of
increasing, remains constant. This is a very important difference,
for it is on the concentration and not the amount of reagents that
reaction velocity depends." Thus the equations of Robertson and
SECT. 2] AND WEIGHT 417
Crozier would only be true if the chick embryo, for instance, began
as a kind of watery ghost of dimensions equivalent to those it normally
has at hatching, and if development consisted in the gradual accu-
mulation of solid substances within it. This singular kind of pre-
formation certainly does not exist in reality. Snell developed another
equation, ^
(where A is the initial amount of substrate, x the amount of end-
product at time /, ex the corresponding concentration at time t,
Vq the volume of the organism at the beginning of the growth-
cycle, and iTi and K^ the velocity constants as before), which
he regards as the correct form for the representation of an auto-
catalysed monomolecular reaction in which the conditions are
similar to those in a growing animal, or in other words, where the
volume occupied by the reagents increases in proportion as the
end-product increases. Snell then showed that the curves obtained
by this equation did not resemble any of the empirical growth-
curves in the literature, and therefore concluded that there is no sound
basis for assuming that the master reaction is either monomolecular
or autocatalytic. Thus in the case of embryonic growth, where
X
— is almost constant, the curve approximates to that of a non-
autocatalysed monomolecular reaction, and again to nothing that
is given by any actual embryo.
In any case the only instance where a sigmoid curve has been shown
to fit the growth of an individual foetal organ is in the work of Faure-
Fremiet & Dragoiu on the lung of the embryo sheep.
Robertson's point of view was adopted in the earlier work of Brody
and his collaborators, who, however, introduced new viewpoints
into it. Instead of plotting the absolute weights of the embryo
against time, they plotted the increments per time-unit, thus ob-
taining curves similar to those in Fig. 34 above. Obviously, in
a case where the absolute growth-curve was S-shaped, these incre-
ment curves would be doubly S-shaped, rising to a maximum at
half-time and thereafter falling away. Or, put mathematically,
from the differential equation
I =Kx{A- x)
NEI 27
4i8 ON INCREASE IN SIZE AND WEIGHT [pt. iii
it follows that the velocity of change in an autocatakinetic system
progressively rises from zero hour to a maximum value when x = ^A
and afterwards constantly falls. When the data are plotted in this
way the existence of growth-cycles, whether real or not, comes out
much more clearly than when the absolute curves alone are used.
It is often said, however, that the increment curve emphasises small
fortuitous variations more than the absolute curve, and certainly
there are cases, notably the chick embryo itself, where the increment
curve shows up cycles which are but poorly shown on the absolute
weight curve. Brody & Ragsdale in their first memoir on this
subject dealt only with the growth of the cow, and concluded that
one complete growth-cycle was accomplished in the foetal condition.
A later paper considered the growth of the fowl, for which, on the
data of Card & Kirkpatrick for growth after hatching, two cycles
appeared, with maxima at 9 and 18 weeks respectively. For the
growth of the embryo, the data of Hasselbalch and of Lamson &
Edmond, re-arranged by Brody, gave two maxima also, but at slightly
different times, thus:
Lamson & Edmond 11-5 and 16-5 days
Hasselbalch 10-5 and 15-0 days.
Brody's figures for these curves are shown in Fig. 48. LeBreton &
Schaeffer,who subsequently published a series of chick embryo weights,
found maxima at 9 and 15 days, but in Murray's series there is no peak
at any time, except a doubtful one on the i6th day, and a plateau be-
tween the 1 2th and 15th days. Schmalhausen's data again, when the
increments are calculated, show in the case of both series peaks at
10 and 12*5 days, with an additional one, in the case of his 1927 series,
at 17 days.
In the face of this consensus of evidence, it is not altogether easy
to conclude with Murray that "Brody's rhythmic growth curves were
due to chance variations". It is true that Lamson & Edmond;
Hasselbalch; and LeBreton & Schaeffer used too few embryos
in their work, but even in Murray's own work, where about 650
embryos were used, there is an unexplained drop between the 17th
and 1 8th days, as well as a plateau between the 12th and i6th days,
both quite outside the probable error. Murray did not, it is true,
get the i8th-day drop in all his experiments. In Schmalhausen's
two series about 400 embryos were used.
Brody's chick (daily increment curves)
4 5 6 7 8 9 ion 1213141516171819 20 3 4 5 6 7 8 9 10111213141516171S
t t t t
Lamson 8c Edmonds data Hasselbalch
(«) {b)
Le Breton &. Schaeffer's chick
(daily increment curve)
^
7-
6-
5-
4-
3-
2-
i -
4 5 6 7 8 9 10 1112 13141516 17 18 1920
W.Legh. | |
(c)
Schmalhausen's chick (daily increment curves)
7-
N
i^
^
5 6 7 8 9 10111213141516171819 4 5 6 7 8 9 10111213141516171819
1926 Max.t t 1927 t t t
(d) (e)
Fig. 48.
27-2
420 ON INCREASE IN SIZE [pt. m
It may further be argued that weighing is a very simple process,
and it is, therefore, difficult to see why errors should arise which
should reflect themselves in these rhythmic curves. A greater degree
of scepticism would be justified if they were the results of a com-
plicated estimation method for a chemical substance. But, as it is,
these curves form perhaps the best evidence which at present exists
for the applicability of the Ostwald-Robertson view to embryonic
growth. In the absence of a really exhaustive statistical investigation
of the growth of the chick in the egg, these rhythmic curves must be
accepted for what they are worth. They would be more convincing
if all the workers had found peaks in the same places, but the
variation which exists is no argument against the reality of the
phenomenon in view of the fact that different breeds of hen were
used. Further work is greatly needed to clear up this question. If the
peaks on the daily increment curve do turn out to be real, it may be
possible to relate them to the peaks of normal mortality which Payne
and others have studied, and which will receive further consideration
later. (See Fig. 443, Section 18-2.)
The autocatalytic curve has also been found by Robertson to fit
the data of Stotsenberg already referred to for the growth of albino
rat embryos, and a peaked curve is obtained when the daily incre-
ments are plotted against time. But, as will be seen, Brody's ex-
ponential formula also fits these data, and it is probably right to
conclude, as McDowell and his collaborators do, that the figures are
not sufficiently good to allow us to distinguish between the two
formulae. They cannot be regarded as supporting, therefore, any
particular theory of embryonic growth.
2-8. Instantaneous Percentage Growth-rate
Brody introduced still another way of representing the facts. He
defined the "genetic growth constants" of animals as being the
same within each genetically identical group of animals, and as
corresponding to specific velocity constants and equilibrium constants
in chemical actions in vitro. It may be seen from Fig. 49 taken
from Brody's paper that the mature weight of the animal in
its life-span. A, is approached by successively decreasing gains in
weight after the point of inflection of the sigmoid curve has been
passed. The velocity of growth, therefore, declines in a geometrical
progression with age. The normal animal reaches, as Brody puts it.
SECT. 2]
AND WEIGHT
421
under a given set of favourable conditions (much more constant, of
course, in egg or uterus than outside), a mature weight which is
characteristic of its own species, just as the product of a chemical
reaction in vitro reaches under a given set of conditions a definite
equilibrium concentration characteristic of its kind. The mature
weight A was determined by Brody for a large range of animals
Fig. 49-
by a graphical method. Now, in this process of geometrical pro-
gression in which the increments in unit time are becoming pro-
gressively smaller, it is found that in each unit of time the gain made
in percentage of the gain made in the previous unit of time is a
constant. Thus, in the autostatic growth-phase of the rabbit, for
example, the gain is, during each month, 78 per cent, of what it was
during the previous month. Brody calculated out this constant, k
(simple growth persistency), for a great many animals. It corre-
sponds to the specific velocity constant in chemical equations.
422
ON INCREASE IN SIZE
[PT. Ill
Finally, B is the difference between the mature weight of the
animal and the weight the animal would have had at concep-
tion (a minus quantity) if the whole of growth was represent-
able by the curve for the autostatic or self-inhibiting phase. This
genetic growth-constant also was found for many animals by
Brody. From Fig. 50 it can be seen that, the higher the value of k,
the more rapidly the mature value is approached (pigeon > mouse
> rat > guinea-pig > sheep > pig > cow > man), and that the fact
can be equally well accounted for on the assumption that a substance
Yrs.cS^
Age (from blrbh) man
7 8 9 10 11 12
Mos.a'
60 64 68 72 76
12 16 20 24 28 32 36 40 44 48 52 66
Age (from conception) of animals
OoJ
Fig. 50.
is used up during growth, or on the assumption — perhaps more
likely in view of the work of Carrel and his collaborators on tissue
culture (reviewed by Pearl) — that during growth a growth-retarding
substance is produced according to the law of monomolecular change.
The paper of Brody, Sparrow & Kibler was concerned with age
equivalence. They showed that, with the aid of the formula pre-
viously established by Brody,
W=^ A- Be-""^
(where W is the weight at age t, A a. genetic growth-constant, the
mature weight, B another genetic growth-constant which increases
in value with increase in length of the processes preceding the point
SECT. 2]
AND WEIGHT
423
of inflection, and k a third genetic growth-constant, the fractional
decHne in the velocity of growth), it was possible to plot growth-
curves for all animals to the same base, and so to determine their
age-equivalence. Thus they found that i rat month was equivalent
to ii-gi cow months, and i guinea-pig gram to 509-1 cow grams.
They finally constructed a table (Fig. 51), in which the value of ^
was given for a great many animals, and a logarithmic graph, from
001
.02
Value of k.
.03 .M .05 06 -07 .C8 -09.1 Z
5 .6 7 .8 .9 10
CotVC.
Mos;
Fig. 51-
which can be read off the time in months (conception age) at which
the animal with the constant k in question will arrive at 10, 20, 30
or 90 per cent, of its mature weight.
Brody next considered the growth-constants during the autokinetic
or self-accelerating phase of growth. He subjected the methods
which had previously been used to represent growth to severe
criticism, part of which has already been referred to. Thus, in the
case of Minot's method, increments of growth are regarded as
being added on discontinuously at the terminal points of arbitrary
time periods, whereas growth is really a continuous process, and
I dt_
\ w
=-k
424 ON INCREASE IN SIZE AND WEIGHT [pt. iii
the mathematical expression for it must take account of its smooth
nature. Brody found the relationship between the relative rates of
growth k ^j^
W
to be k = log [R + i),
where R is Minot's percentage growth-rate. The latter can, therefore,
only be used when it does not exceed 10 per cent, for the period
under consideration, i.e. never in embryological work.
Brody also criticised the methods of Pearl, whose equation
dW _ k
dt t- a
does not take account of the fact that part of growth is self-accele-
rating, and also Pearl & Reed's modification of the original Robertson
equation. He maintained that the best way was to plot the log.
weight against the age, when, if the result is a straight line, the rate
of instantaneous growth, A;, must be a constant. This will not of
course be confused with the
fractional persistency con-
stant referred to above, for
the latter only refers to the
autostatic or self-inhibitory
phase. The autokinetic or
self-acceleratory phase is ^
clearly the more important ^
and interesting for em-
bryologists.
Figs. 52 a, b, taken from
Brody's paper, show the
data of various workers for
the wet weight of chick
embryos treated in the way
described, namely, the log.
weight plotted against the
age. It will be noted that a series of straight lines result, forming a
system concave to the abscissa and rising rapidly but more quickly
at first than later. The curve is thus the exact opposite of the Minot
Cms
,.:?
*«jy
6 10 12 14 16 Ifl 20
Incubation Age
Fig. 52 a.
Gms.
20
10
</^
o
I
-H
vy
I
I
^
^
/.
\
6
5
3
—
=
=
E
—^
?
V
%
h
—
—
—
—
—
1^
—
/
1
z
t
—
—
.
A
/.
/
?i
//
^
(
J
//
[■
/
/
V
-
—
—
—
^ .a
~
1 —
i^
—
1 — /
K/
i
—
—
—
—
—
—
' —
—
y.
/
j
5
'if
/
/
/
"l
^^
/
J
\
.3
.2
!
^^
A
/-
i
^
—
^
^
—
—
z
—
1
/
/-
—
r
i—
—
—
—
—
—
—
—
/
J
r^'
H
as
SQl
Da
ch
1
/A
B—
rtr:
r*
Days 0 2 4 6 6 10
12 14 16
Incubation Age
IS 20
Gms.,
22
20
19
16
14
12
10
8
6
4
2
0
10 12 14 16 15 20
Incubation Ag<^
Fig. 52 b.
426 ON INCREASE IN SIZE [pt. iii
curve, which falls so markedly during the same time. Thus the
growth-rate expressed in this manner also falls off as time goes on, or
rather rises less and less rapidly, becoming eventually asymptotic to
the mature value.
This curve is undoubtedly a great improvement on Minot's, for
it involves no arbitrary time period and depends on the differential
calculus, which has as its special province the evaluation of instan-
taneous change. An infinitesimally small period dt, and an infini-
tesimally small increase of weight dW, are the basis of its operation.
Then all the infinitesimal differences can be added together (i.e.
integrated). Thus: .j^
"^ _
becomes, if the number of dfs, and dW's is infinitely large [n) , when
integrated, W = Ae^*, for
I + ^y = e^\
A being the weight at the beginning of the whole period. If this is
turned into logarithms
log W =\og A ^ kt,
, _\ogW - log A
or k ,
and, as where growth is being considered A is, to all intents and
purposes, o, ^ ^
k = -^ — .
t
The instantaneous relative growth-rate for a unit time is the sum
of all the instantaneous rates during the given unit of time, and may
therefore be multiplied or divided, according to the time-unit in
which it is desired to express it.
The log. weight/age graph is, therefore, a measure of the instan-
taneous growth-rate, and the value of the constant k which can be
calculated from the last equation will give the slope of the straight, or
approximately straight, line. The log. weight/age graph could, of
course, have been plotted by Minot, but Brody's use of the differential
calculus was required to show that the slope of the curve gave an
instantaneous growth-constant. Thus, in the example given, Lamson
& Edmond's data, the constant is 56 from the 5th to the 8th day of
SECT. 2] AND WEIGHT 427
development (very steep slope), 36 from the 8th to the 13th day
(less steep slope), 24 from the 13th to the i8th day (still less steep),
and 25 from then onwards. The higher numerically the constant k,
the steeper the slope, and consequently the greater the instantaneous
growth-rate. In Figs. 53 a, b and Table 55 are shown most of the
weights and processes in the hen's egg whose constants have been
calculated by Brody. Each system grows at a rate peculiar to itself.
Murray, as we have seen, also plotted log. weight against age, but
he did not get a straight-line relationship ; on the contrary, the resulting
curve was concave to the age (abscissa) . McDowell, again, got a similar
concave curve for the pre-natal growth of the mouse, and there is much
point in his criticism of Brody 's work: "Brody draws a series of
straight lines through corresponding exponential curves and concludes
that growth-rate does not decline continuously but by abrupt drops
between periods of uniform rate. Since any curve can be approxi-
mated by a series of straight lines, the critical significance, both of
the specific number of straight lines, and of his general conclusions,
seems somewhat questionable"*.
Table 55 includes also a column in which the time taken for
the embryo or a corresponding entity to double its weight or amount
is shown. For, when the instantaneous percentage growth-rate is
constant, the time intervals between doubling of weights are con-
stant; therefore, from the expression
W - Ae^\
at a certain time
logs _ 0-695
k ~ k '
and, as k is found to be for the rat embryo 0-53 or 53 per cent., the
time required for it to double its weight must be — ^ or 1-3 days.
^ 0-53
Further, if growth in weight can be taken as a measure of the in-
crease in the population of cells in the body, a new cell-generation
is produced every 1-3 days on an average, and the cell-division
frequency is 1/1-3, i-^- 0-77 times per day. It is thus possible to
determine, as Brody says, the mean life of a mother cell before it
divides into two daughter cells.
* Nevertheless, McDowell himself admits a discontinuity between pre-axial and axial
growth, as we have seen on pp. 394 and 396.
n c
i
55
Mom
s.Gms
3.0
20
20
10
1.0
a
.8
5
.5
3
.3
2
.2
1 .1
.001 .01
DaysO
6 10 12 14 16 18 20
Incubation A^
Fig. 53«'
^1
8 S88 S '^'^^ ^
^ ':ind:^no'oO
430
ON INCREASE IN SIZE
[PT. Ill
Table 55. Instantaneous growth-rate {k).
Brody's figures.
Time in
Growth-
which the
Time
rate %
entity is
Entity in question
(days)
per day {k)
doubled {d)
Investigator
Chick
Wet weight .
5-8
56
1-2
Lamson & Edmond
,,
8-13
36
1-9
JJ 5>
13-18
24
29
5> 55
6-10
56
1-2
Hasselbalch
10-14
29
2-4
55
14-19
19
3-6
35
6-10
47
1-5
Murray
10-14
33
2-1
35
14-19
21
3-1
33
CO2 excretion
0-4
98
07
Atwood & Weakley
>>
4-14
31
2-2
35 35
5>
14-19
Pause
—
33 33
3)
19-21
31
—
33 55
0-16
36
I '9
Hasselbalch
5)
0-16
32
2-2
Murray
Urea excretion
5-7
76
—
Needham
,,
7-14
34
—
53
Glutathione content
6-9
54
—
Mvirray
5)
9-15
30
—
55
Total CO2 content
7-9
56
—
55
,,
9-16
37
— •
35
Chloride content
12-15
32
—
,j
Calcium content
12-16
85
—
Plimmer & Lowndes
Creatine content
14-21
28
—
Mellanby
Nitrogen content
6-9
60
—
Murray
jj
10-15
47
—
,,
15-20
23
—
Total solid content
5-10
57
—
,,
10-16
46
—
Ash content ...
10-14
43
—
,,
14-19
25
—
Calorific value
7-9
56
—
,,
10-15
47
—
Rat
Wet weight ...
13-22
53
1-3
Stotsenberg
Guinea-pig
Wet weight ...
17-20
100
0-7
Ibsen & Ibsen;
,,
20-35
25
2-8
Draper; Hensen
,,
35-52
9
7-8
S3 55
,,
52-70
5
15-1
33 33
Man
Wet weight ...
60-110
8
8-7
Streeter
,,
110-160
Not straight
55
,,
160-240
1-7
41-0
33
,
240-280
1-3
55-0
„
Brody himself did not omit to make suggestions as to possible cor-
relations between his abrupt breaks in growth-rate and other phe-
nomena known to be taking place during the embryonic development
of the chick. In the first place, he associated the breaks in the growth-
SECT. 2] AND WEIGHT 431
rates of carbon dioxide production at the 14th day with the change
in mode of respiration from aquatic to terrestrial which takes place
late in incubation. This is quite a convincing correlation, but his
suggestion that the first break (at four days), before which the
instantaneous growth-rate is about 100 per cent., and the Minot
growth-rate 1000 per cent., is associated with a general critical
period occurring at that time is not really so satisfactory. For
almost any process has its critical moments during develop-
ment— for example, the peak in protein metabolism at 8-5 days.
In cases where there is no a priori reason for assuming corre-
lations except the one fact that their peaks coincide or are
converse to each other, the utmost caution should be used in
so correlating them. Wholesale correlations of apparently unrelated
phenomena may be chemically misleading. Thus Brody cites
Tomita's peak in total lactic acid content at the 5th day (see Fig. 292)
as evidence of a critical period corresponding to the abrupt break
in his growth-rates of carbon dioxide production and to the peak
in Payne's mortality curve (see Fig. 443).
Brody is not the only investigator who has occupied himself with
the growth-rates of different chemical processes and amounts in
the embryo, but, before passing on to discuss these points, which
will lead naturally to the question of the growth-rates of parts of
embryos, a further word must be said about Brody's work.
At present it is not possible to tell much from the comparison of
embryos of different kinds, though it is obvious that an immense
field of research is opened up here for the comparative embryologist
of the future. Thus the equation for the development of the chick
embryo in weight according to Murray is W^=o-668^^^, corre-
sponding to instantaneous growth-constants of 0-47, 0-33, and 0-2 1
successively,* while the equation for Stotsenberg's rat embryo figures,
according to Brody, is W^ = 0-000065^°^^*, corresponding to a steady
rate of 53 per cent, per day instantaneous. On the steadiness of this
rate Brody says, "If there is no fallacy in this reasoning we have
reached a new and an extremely important conclusion. While all
investigators of the time relations of growth have reached the con-
clusion that the percentage growth-rate continuously and rapidly
declines with age, our conclusion is that the instantaneous per-
centage growth-rate remains constant for the relatively enormously
* A later value, due to Vladimirov & Danilina, is W=o-'^2^fi'^.
432 ON INCREASE IN SIZE [pt. iii
long period between 14 days and birth. The cause of this difference
in results is due to the fallacy in the method of analysis employed by
Minot". This is only true, subject to confirmation of the fact that
the foetal log. weight/age graph gives a straight line over definite
periods, and this is just what is not certain. Decision on the matter
cannot yet be made.
Another interesting point which emerges from Table 55 is the
long embryonic stage in the guinea-pig. The chick hatches when
its k is about 0-21 and the rat is born when its k is even higher— 0-53,
but the guinea-pig stays inside the uterus until its instantaneous
percentage growth-rate has dropped to 0-05. Brody succeeded, indeed,
in raising guinea-pigs by feeding them on hay and grain immediately
after birth, so that they tasted neither colostrum nor milk.
It is interesting, again, to note that there is only one break in the
instantaneous growth-rate of carbon dioxide production, whereas
there are at least two in the instantaneous growth-rate of wet weight.
This must mean either that the respiratory function develops at a rate
quite independent of the growth in mass, or that the weight of the
body cannot be taken as an index of the growth of the metabolising
tissues. This point will be referred to again, for it is of much importance
in chemical embryology. On the other hand, the respiration k does
show a break about the 17th or i8th day, which is duplicated in
the wet weight k, or, at any rate, in the log. weight curve con-
structed from Lamson & Edmond's data — for it is not so apparent
in those of Hasselbalch and of Murray. This may be associated, as we
have seen already is Brody's suggestion, with the change in form of
respiration occurring then (chorio-allantoic to pulmonary). There is
no doubt that some obscure events are associated with this late
stage in the chick, e.g. the mortality peak of Payne, which can be
greatly intensified if a certain lethal gene is present, and the sudden
immunity to implanted rat sarcomata (Murphy), which the chick
then acquires. Brody suggests that the chick embryo passes at this
stage through a "metamorphosis" similar to those hidden ones which
exist, according to Davenport, in the development of man.
The extremely small values of k for the embryonic period of man
are worth attention. The human embryo grows a great deal more
slowly than any other. Five months after conception the instan-
taneous percentage growth-rate is only 1-7 per day, while, during
the week preceding birth, the rat embryo grows at the rate of 53 per
SECT. 2]
AND WEIGHT
433
■ whole embryo (Murray
and Needham)
Bcalorlfic value (Murray)
Ddry solid (Murray)
Ocarbohydrate (Needham)
® protein (Murray
and Needham)
e fat (Murray)
cent, per day. The lowest rate of growth ever reached by the rat
after birth is 3 per cent, per day. Given percentage rates of growth,
therefore, do not signify equivalent stages of development irrespective
of the species of animal.
Calculation of the rates of growth for various processes and indivi-
dual components in the development of the embryo has also been
done by other investigators using the Minot method. In 1927
I calculated the percentage growth-rate for the total carbohydrate
content of the chick embryo; it fell from 56 to 22 per cent.
In Fig. 54 is shown the fall in
the Minot curves for the wet
weight of the whole embryo,
the calorific value, the dry
weight of the whole embryo,
the sugar, protein, and fat
content of the embryo. All of ^°
them fall, but we have here an
instance of the limited but real
value of the Minot curves,
which, although no absolute
conclusion can be drawn from
them, do show that the tissue
constituents and the dry weight
have a different behaviour from the wet weight. It can easily be
seen that they form a plateau between the loth and the 15th day,
during which they grow at a constant rate while the wet weight is
falling all the time. This plateau also appears on the curves for carbon
dioxide output calculated in the same manner as percentage growth-
rates from the figures of Bohr & Hasselbalch; Atwood & Weakley;
and Murray in 1927 by Brody. The plateau must be due to the fact
that the growth of dry substance is specially rapid during the middle
phase of development; it is then that the embryo makes the most
rapid strides from wetness to dryness. It is interesting to see that
the growth-rate of carbohydrate is never as high as some of the
others, and never drops so low. It is significant, moreover, that
on the 1 9th day the Minot growth-rate of the protein has dropped
below that of the whole body, while the growth-rate of fat remains
well above it. This is an illustration of the "relative" use of Minot's
method.
Fig- 54-
88
434
ON INCREASE IN SIZE
[PT. Ill
2-9. Growth Constants
Brody is not the only worker who has applied the differential
calculus to embryonic growth-curves. Teissier and Lambert &
Teissier suggested simultaneously that this should be done, but their
work was quite theoretical. However, Schmalhausen published in-
dependently at almost exactly the same time a paper in which it
actually was done. He criticised Minot's method of calculating
Per
cent
per
^"y k=.53
50
-
^
k- ^ -
0
0
\nW2-
IntVi
5 40
_
^
0
V.
^
0
i.
0,30
—
ct
JO
J3
(0
3
t.
0.
0>
I
JS20
-
C
0
c
'i3
Q)
0
0
w
U
JP
Q.
k=.11
c
ID
—
c.
0
k=-047
U=-031 ^
.^^
Rat,?,
— 1 1 h-
unmated
1 1 >==
l_
0
1 1
1
1 1
1 1 1 1
Dayso 10 20 30 40 50 60 70 so 90 100 no 120 iso uo i50 16O i70 18O
c
o
O
0 8
CO
18 28 38
58 68 78
Age
Fig. 55 a-
108 118. 128 138 U8 158
" mittlerer prozentualer Zuwachs " in arbitrary time-units from exactly
the same point of view as Brody. Thus, he says of the Minot method:
"the larger time intervals we take, the bigger the error will be. With
equal time intervals, the error will be bigger the bigger the rapidity
of growth, and this will in fact lead to altogether misleading figures
for the early periods". We need not follow Schmalhausen's reasoning,
which led him to adopt the calculus as a better assistance in studying
growth-curves, for we have already examined and approved the
SECT. 2]
AND WEIGHT
435
C JO
J3
a 10
ll
f
\
/
\
^
/
/
1
\
\
;A
^
c
r
c=—
,^
arguments of Brody. Schmalhausen speaks of the "wahre Wachs-
tumsgeschwindigkeit " instead of the instantaneous percentage
growth-rate, and of r instead of k. His equation relating the in-
stantaneous percentage growth-rate to the Minot growth-rate is
exactly the same as Brody's, and he points out that, when, according
to the old method, the growth-rate would be 700, the instantaneous
method would give a result of 207, though 50 per cent. (Minot)
would be equivalent to 40-5 (instantaneous). These figures might
have been read off from the
graph of relation given by
Brody, and it is surprising
that the two workers, one
at Kiev and the other in
Missouri, should have been
thinking on such very similar
lines. It is still more surprising
that embryologists had not
thought on such lines long
before. Schmalhausen and
Brody diverge, however,
upon one important point,
namely, the shape of the line
given when the log. weight is
plotted against the age, for
Schmalhausen regards it as a curve — just as Murray and McDowell
do — while, as we have seen, Brody lays great stress on the repre-
sentation of it by a series of straight lines having abrupt breaks between
them. Thus, the instantaneous percentage growth-rate, which with
Brody remains constant over certain definite periods, with Schmal-
hausen continually declines in value. In other words, Brody's diagram
which shows the instantaneous percentage growth-rate dropping
in a stepped formation from fertilisation to hatching is replaced in
Schmalhausen's work by a regular curve passing downwards to
become asymptotic to the abscissa (Fig. 55 h), just as the Minot curve
does, only, of course, plotted from a set of figures having a real
meaning as against the abstractions of Minot. This difference of out-
look leads naturally to very wide differences in conclusions; thus
Schmalhausen has nothing to say about critical points or hidden
metamorphoses. Having diverged from Brody in this direction,
28-2
10 n n 16
20 Zl
Age in days
P=wet weight;
Cv = instantaneous % growth-rate.
Fig. 55 *•
436
ON INCREASE IN SIZE
[PT. Ill
he proceeded a good deal further along it by observing that the
graph relating instantaneous percentage growth-rate to age was
practically identical with a rectangular hyperbola, and that there
was a simple relation between the values of r or Cv (Brody's K) and
the age, for the product of the two was always roughly equal to 300.*
Table 56. Embryonic growth: Schmalhauseri's '■^Wachstumskonstante"
{^^wahre WachstumsgeschwindigkeW^ x time).
Cvl
Cut
length
age
Investigator
Man
Whole embryo wet weight
193
369
Friedenthal
Mouse
95 5>
—
337-5
McDowell et al.
Rat
35 J5
441
518
Stotsenberg
Chick
J> »
—
318-5
Murray
35 55
518
321
Schmalhausen
Liver
329
Lung ...
—
321
Fore limb (whole period)
—
293
(3-18 days) ...
—
329
Hind limb (whole period)
—
347
(3-18 days) ...
—
395
Stomach ...
— ■
374
Brain
—
210
Lens
—
210
Whole eye (2-10 days) ...
—
317
,, (i 1-21 days) ...
—
99
Heart
—
276
Mesonephros (4-13 days)
—
224-5
Metanephros (7-17 days)
—
359
(17-21 days)
—
196
Ovary
—
145
Guinea-pig
Whole embryo wet weight
—
347
Draper
Trout
55 55
(30-51 days)
—
206
Kronfeld & ScJ
(51-99 days)
—
207
S3
Schmalhausen gives no explanation of the breaks in the cases of those organs which have
two values of Cvt, but calls attention to the fact that the organs of early differentiation
have low Cvt and vice versa.
If the curve obtained by plotting Cv (Brody's k) against time is a regular hyperbola,
then the product Cvt should be 300. If it exceeds this figure, the curve is descending and
becoming asymptotic less rapidly, i.e. the rate of growth (instantaneous) is not falling off
as rapidly as it will be if the product is less than 300 at any given moment.
This constant he calls the "Wachstumskonstante", and its values,
calculated by him for a number of embryonic processes, are seen in
Table 56. It is perhaps the least convincing part of his exposition,
for when during a certain series, e.g. the growth of the human
embryo, the constant Cvt oscillates between 899 and 93 as extreme
limits, one may legitimately doubt whether great stress can be laid
* Brody himself does not find this to be so.
SECT. 2] AND WEIGHT 437
on the average. Moreover, Janisch treats the same curve as a catenary
exponential one. And, ahhough the " Wachstumskonstante " for
the various organs and parts of the embryo show differences which
might well be regarded as characteristic for the tissue in question, it is
disturbing to find so wide a difference from the predicted value in
the case of the rat embryo, explained though it is by Schmalhausen
as due to variable factors in the food of the maternal organism. The
reason why 300 is the number to which these figures approach is,
of course, because, according to Schmalhausen's formula, the in-
crease of the embryonic weight can be expressed by the equation
W= k [atf,
where W is the weight, a the "Lineargrosse", t the time and k a
constant. This agrees with the hyperbolic nature of the Cvjt curve.
Table 57. Instantaneous percentage growth-rate [Chick).
Schmalhausen
Day of
Brody
j^
development
(Smoothed)
(Raw)
(Smoothed)
O-I
1-2
2-3
—
—
—
—
190
190
3-4
—
119
140
4-5
—
139
107
5-6
—
83
87
H
47
79
70
7-8
47
38
60
8-9
47
36
50
9-10
33
53
'^2
lO-II
33
22
38
11-12
33
^\
33
12-13
33
48
30
13-14
33
30
27
14-15
21
20
25
15-16
21
32
23
16-17
21
23
21
17-18
21
22
19
18-19
21
II
17
19-20
—
^l
15
20-21
—
16
12
for the equation of an equal-sided hyperbola is j; = 3/x. Schmal-
hausen does not derive his Cv directly, but calculates it in each case
from the Minot percentage growth-rate figures. It is instructive to
place side by side the instantaneous percentage growth-rates of Brody
and Schmalhausen for the chick embryo, as is done in Table 57.
That of the former has three constant periods, that of the latter
438 ON INCREASE IN SIZE [pt. m
shows a gradual decline, and the figures illustrate what has already
been said, namely, that, until we possess much better statistical data
than is actually the case, we cannot differentiate between the
Brody position and the Murray-McDowell-Schmalhausen position.*
As the matter is fundamental in view of the important theoretical
issues involved, the accumulation of more data is urgently to be
desired. It may be mentioned that Cohn & Murray, plotting log,
weight/age curves for the growth of embryonic heart cells in tissue
culture, obtained curves concave to the age axis and not straight lines.
Schmalhausen also studied the growth in length of the chick
embryo, calculating it from the weight by the formula L = VW.
The daily gain in length a he found to be variable around a constant
value of 1-47 for the first half of development and 2-00 for the second
half. But when the weights were corrected by the estimation of the
embryo's specific gravity (average for first half 1-025, average for
second half i -06) the corresponding daily gains in length worked out
at I -Go and 1-79. A further correction made necessary by the presence
of the feathers during the last half of development brought the figure
down to 1-64, so that throughout incubation the embryo apparently
grows in length at the same average rate. The duck embryo, ac-
cording to Schmalhausen, has a daily length increment of i*io mm.,
and this value he regards as constant for the species. He went on
to calculate a for the human embryo, using the weight data of
Friedenthal and Zangemeister, and for the embryo of the white rat,
using the data of Stotsenberg. All these results are shown in Table 58,
together with his further assessments of a calculated from the " Nor-
mal tafeln" of Minot & Taylor for the rabbit and Keibel for the pig.
The daily size ("Lineargrosse") increments of his own measurements
of separate organs and parts of the chick embryo are also given.
During the course of development the value of a rises and falls
according to the rate of growth. If the value a/ 1 is calculated, where
a is the daily increment in length and / the length of the part in
question at the beginning of the period, the absolute size of the
part will cease to affect the result, and the organs will be comparable
with themselves and with the whole embryo. When this is done, the
ratio is found to be fairly constant, rising as high as 10-3 per cent,
for the stomach and falling as low as 6-3 per cent, for the lens. These
figures are also given in Table 58. Schmalhausen concluded from
* Recent work by Byerly supports that of Brody.
SECT. 2]
AND WEIGHT
439
them that organs which reach a high state of differentiation early
grow the most slowly (brain and lens), while less differentiated
organs grow most quickly (liver and limb-buds). The growth of the
body as a whole is the average practically exactly of the rest, and it
is interesting to note that the organ which most nearly approaches
it is the heart. The heart would seem to grow in size at the same rate
as the entire body. But, as Schmalhausen says, this growth in size
seems to have no simple relation to the growth in weight as shown
by the percentage growth-rate.
Table 58. SchmalhauserC s values for a, i.e. daily increment in
size or '^ Lineargrdsse^\
Chick
Duck
Man
)>
Rat
Rabbit
Pig
Guinea-pig
Whole embryo (ist half)
(2nd half)
Brain
Lens
Spleen
Heart
Lung
Liver
Testis
Metanephros
Stomach
Fore limb
Hind limb
Pectoral muscles
Whole embryo
Millimetres
A
r
a
a/l
Investigator
1-6
Schmalhausen
1-64
7-65
05
6-50
o-i
6-30
o-i
6-62
03
7-50
0-29
7-82
0-55
883
0-09
8-82
0-32
ID- 10
0-64
10-30
0-44
7-33
0-76
885
0-43
909
I-IO
—
0"55
—
Friedenthal
0-55
—
Zangemeister
1-47
—
Stotsenberg
1-2
—
Minot & Taylor
i-i8
—
Keibel
0-75
—
Read
In a later paper Schmalhausen studied the relation between initial
weight and end weight in a number of animals, wishing to obtain
some means of comparing their " Wachstumsertrage " or Growth-
yields, on a basis independent of their size. He found that u, or the
mass of substance added on to itself by the organism between times
ti and tz, could be calculated by the formula
u =
t h *^i 0-4343
u
u post-
u
embryonic
embryonic
whole
period
period
life-span
2-2
17-2
19-4
1-8
15-3
I7-I
9-5
3-8
13-3
8-9
2-3
II-2
13-6
4-3
17-9
13-6
2-3
15-9
15-6
4-8
20-5
20-9
3-3
24-3
440 ON INCREASE IN SIZE [pt. iii
the initial weight being taken as unity {k = Cvt). His results for various
organisms show interesting differences, thus :
Sturgeon
Pike
Hen
Mouse
Rat
Guinea-pig ...
Pig
Man
Here we observe the effect of early hatching in the two aquatic
forms, which have the greater part of their growth still before them
at the time of leaving the tgg. The other figures demonstrate quanti-
tatively what is apparent to common sense, namely, that the em-
bryonic period is the time of greatest growth in terrestrial animals.
2-10. The Growth of Parts
We must now turn to the relative growth-rates of parts of the
embryonic organism. This is a field which has mainly been tilled
by anatomists, but it is of the greatest importance to the chemical
embryologist. For the increasing and decreasing intensities of physico-
chemical processes cannot be intelligently studied in the absence of
a knowledge of the distribution of the whole mass among the
different organs and tissues. The investigation of the relative
growths of endocrine glands, again, cannot but throw much light
on the development of the adult metabolism in the embryo.
D'Arcy Thompson sees an appreciation of this in the eighteenth-
century preformationists. "It was the apparently unlimited extent",
he says, "to which, in the development of the chick embryo, in-
equalities of growth could and did produce changes of form and
changes of anatomical structure that led Haller to surmise that the
process was actually without limits and that all development was
but an unfolding, an 'evolutio' in which no part came into being
which had not essentially existed before. In short the celebrated
doctrine of preformation implied on the one hand a clear recognition
of what, throughout the later stages of development, growth can do,
by hastening the increase in size in one part, hindering that of
another, changing their relative magnitudes and positions, and
altering their forms ; while on the other hand, it betrayed a failure —
SECT. 2] AND WEIGHT 441
inevitable in those days — to recognise the essential difference be-
tween these movements of masses and the molecular processes which
precede and accompany them and which are characteristic of another
order of magnitude."
The papers of Schmalhausen are of much importance in this
matter. Inspired by the views of His, who declared in 1874 that all
the development of shape could be ascribed to unequal growth in
various component parts of the embryo, he set himself to weigh and
measure a great number of these individual sections.
He first studied the relative growth-rates of the brain and
eye of the chick embryo, together with the liver, lung and stomach,
representing the organs of endodermal origin. In each case, he
calculated the % growth-rates and the percentages formed of the
weight of the whole body. For the organs of mesodermal origin,
he chose the heart, the mesonephros, the metanephros, the ovary,
and testis. These figures he treated in the same way. In many cases
his weights were not obtained directly but by reconstructing from
serial sections and then weighing, proper allowance being made for
complicating factors such as specific gravity. Fig. 56 shows one of
his graphs — it is specially interesting as showing the definite decrease
in weight which the mesonephros undergoes after the 15th day in
giving place to the metanephros or adult kidney. It also includes
% growth-rate curves for the fore and hind limbs. Lastly, he
ascertained the growth-rate of the feathers.
In general, he found that the changes in the growth-rates of organs
were synchronous. The percentage growth-rate (see Fig, 57) seemed
to have peaks in its descent, each one less marked than the pre-
ceding one. In each case, the growth-rate of every organ shows a
certain rise, but the amount of the rise differs in different cases — thus
the lung is the organ which is growing fastest about the 6th day, the
hind extremity about the loth day and the stomach about the 13th.
On the whole, the periods of depression of the growth-rate of the
majority of organs are from 7 to 9 days, from io| to ii| days, and
from 14 to 16 days. When the weights of individual organs, however,
were arranged plotted against weight of embryo, not age, the peaks
disappeared, as would be expected, for the total weight is the sum of
the weights of the organs. It would be interesting to plot the logs, of
Schmalhausen's organ-weights against age in order to obtain the
instantaneous growth-constants of Brody for each one. Schmal-
442
ON INCREASE IN SIZE
[PT.
Ill
hausen's general results, however, were as follows: on the 5th and
6th days, the growth-rate of all organs is falling, with the possible
exception of liver and hind limb. This continues till the beginning
of the 7th day, save that the eye and the lens may show a slight rise.
At the beginning of the 7th day, however, the growth-rates of all
organs rise, firstly the mesonephros, the liver and the lung, and, to a
^17
S. 16
x5 10
1.4
o^l3
o\i2
<n
clO
^ 9
c
m 8
2 7
(0 5
X3
E (E) = Fore limb
G = Gonad
M = Mesonephros
Abs. web
wei ghb
12 3 4 5 6?
9 10 11 12 13 14 15 16 17 18 19 20
Days
Fig. 56.
less extent, the hind limbs, then these are followed by the rest. After
a peak all fall until the 1 1 th day, when all again rise, only to fall
on the 14th, with the exception of the feathers, which maintain a rise.
Later a more gradual rise in growth-rate takes place throughout the
body, begun by the lens and liver and, to a less degree, by the eye
and the brain, and continued by the stomach and the mesonephros.
After the subsequent fall, only small variations take place, which are
found to be synchronous for groups of organs such as kidney-liver-
SECT. 2]
AND WEIGHT
443
stomach until the end of the embryonic period. It is certainly in-
teresting that organs so different in origin and nature as the eye and
the mesonephros should be similarly affected by spurts of growth
at various stages, and Schmalhausen concluded that this was an
argument against the hypothesis of specific organ-stimulating sub-
stances, the presence of which would from time to time cause more
0 1 Z 3 V 5 6 7 a 3 10'^ 1Z ^3 1'^ -75 16 17 13 13 20 Zi
Days
Fig. 57. C = brain; £; = hind limb; A" = whole body; Z-t = liver; Li = lens; Af=mesone-
phros; Oc = eye; P = lung; Af /n = metanephros ; 5^ = stomach; G= gonads.
rapid growth in one place of the embryo than in another. It looks
much more as if growth-promoting substances were passed round
in the embryonic circulation at certain definite intervals, and so
exercised an effect on a large number of different organs. In this
connection, the recent work on the growth-promoting substances of
egg-yolk should be borne in mind, and the experimentally deter-
mined cycles of varying permeability to fat-soluble and water-
soluble substances in the walls of the vitelline blood-vessels. One
relation which seems clear from Schmalhausen's work is that growth
444 ON INCREASE IN SIZE [pt. iii
of fore and hind limbs does not accomplish itself in the same spurts
as the viscera do, for during the 8th day depression in the growth-
rates of the latter the skeleton is growing vigorously, and during the
loth day peak it rather falls off. Very similar remarks apply to the
hind limb growth-rate. Schmalhausen concluded that very young
organs can respond to a given intra-embryonic environment by in-
crease in growth-intensity, while more differentiated organs can at
the same time respond by depressions in their growth-rates. "If one
and the same influence", he says, "can act in a stimulatory manner
on the growth of some parts or organs, and inhibitorily on the growth
of others, we can see how unequal growth can take place and hence
a change in form." It would also appear that the more development
goes on, the more different the rates of growth of different organs are.
Three factors seem to control the growth-rate of a single organ:
(i) the age of the embryo, (2) its own degree of differentiation, and
(3) growth-promoting substances or embryonic hormones present in
the circulation. Under (2) would be included the time of origin of
its "anlage" and the intensity with which its preliminary growth
would take place. These views are not compatible with Mehnert's
"laws of organogenesis", the main one of which was that the growth-
rate of an organ in the embryo was proportional to its degree of
development at the time in question. The only criticism that can be
levelled against Schmalhausen's work is that the number of embryos
employed was perhaps rather few.
In conjunction with Stepanova, Schmalhausen made further in-
vestigations on the growth of the embryonic skeleton in the chick.
Similar fluctuations in pre-natal growth-rates of parts have been
discussed as regards the primates by Schultz.
Schmalhausen has attempted to give an explanation of these
spurts in terms of metabolism. Summarised again there are, in the
case of the chick, three or four periods, in each of which the growth-
rate first rises and then falls, as follows:
Days
0—4 1st period, great fall from a high value
4-9 2nd period, rising to the 6th day then falling
9-12 3rd period, rising to the loth day then falling
12-15 4th period, rising to the 13th day then falling
15—21 5th period, rising to the 17th day then falling
He has suggested that these periods may partly correspond to the
periods which can be distinguished in the development of the chick
SECT. 2] AND WEIGHT 445
embryo, during which one type of chemical molecule is predominantly
burned to furnish energy for the growing organism. This subject will
be handled fully later (Sections 6-8, 7-7, and 9-5) ; here it suffices to say
that the beginning of development is in many ways closely associated
with an important carbohydrate metabolism, and the latter part
with the metabolism of fatty acids, while the intermediate part would
appear to have an association with catabolism of protein, in view
of the fact that the point of maximum protein catabolism occurs
when 8*5 days of development have been completed. These periods,
in Schmalhausen's view, may be identified with those in which he
finds spurts in the growth-rate. A certain amount of scepticism
about this identification would seem justifiable until we have
irrefragable proof that the spurts are more than chance variations
in a curve composed of too few data. His suggestions involve
the view that "Abbauprodukte" accumulate from time to time
in the developing embryo, and so hinder its growth (essentially
the same theory as those of Jickeli and of Montgomery) . Thus
his first depression of the growth on the 4th day corresponds to an
accumulation of lactic acid and ammonia (see further on for the
detailed references) and his second depression of growth on the
gth day corresponds to an accumulation of urea. Finally, his
third depression of growth about the 12th day corresponds to an
accumulation of uric acid. He admits that there is nothing chemical
which obviously coincides with the later depressions of growth, but
supposes that they depend on the decreasing excretory power of the
mesonephros. After the i6th day the metanephros would be under-
taking the duty of excreting waste products, and growth accordingly
begins again. Ingenious as these correlations are, they cannot be
said to be convincing, in view of the fact that many other processes
besides the excretion of waste products may be supposed to be exercis-
ing an eflfect on the growth-rate. More interesting is Schmalhausen's
attribution of great importance to the surface of the blastoderm,
the blastodermal capillaries, and the active surface of the excretory
organs. Measurement of these during the course of development
would throw a bright light on these problems. Schmalhausen did
himself compare the growth in weight of the embryonic kidneys
with the daily increment of the whole body, and, although the figures
were rather erratic, he felt able to conclude that, owing to the slow
growth of the mesonephros and metanephros, the excretory surface
446 ON INCREASE IN SIZE [pt. iii
was only just keeping pace with the growth of the embryo. In these
circumstances, it was not surprising to find now an accumulation
and now a flushing out of waste products from the embryonic body.
In a later paper, however, he modified considerably his views on this
subject.
As regards the growth of individual parts, Schmalhausen later
introduced several further expressions. "Homonomic growth", in
his terminology, means growth of an organism in which all the parts
and organs have the same growth-constant, "heteronomic growth"
— the more usual form — is the growth of an organism composed of
organs each with its own characteristic growth-constant. Then the
growth-directing force may exist either inside or outside the anlages of
the separate organs — in the chick it apparently does not exist inside —
and in the former case it would be called "autonomic growth", in
the latter " automorphic ", while, if the influence was directly the
growth of another organ, it would be termed "heteromorphic".
Schmalhausen found that, although the organs in the chick embryo
taken at any one moment had very different rates of growth, yet,
if they were all dated, as it were, from the time of formation of their
anlages, they showed very similar rates of growth. Thus an anlage
developing late would be growing much quicker than the whole body,
while, at the same time, if its instantaneous percentage growth-rate
curve was plotted, it would be found to have a shape very like that
of the organism as a whole. Thus organs can only be compared as to
their growth-rates if they are taken from their own particular origins
and not from the origin of the body as a whole. In homonomic growth,
of course, one is dealing with organs originating at the same time and
having identical growth-constants. In this case, the definite pro-
portions of the resulting organism can be deduced from those of the
anlages ; in other words, a kind of preformation holds good.
If Table 56 be again referred to, it will be seen that, on the whole,
it takes functioning organs longer to grow than functionless ones.
Thus the metanephros, which at first has a Cvt of 359, drops to 196
after it has begun to excrete actively by about the i6th day. In
Fig. 58 is shown the relation between the weights of the organs in the
embryo chick expressed as percentages of the total weight of the
body. The heart and mesonephros are seen to have their maximal
relative size very early in development, after which the former
declines slowly and the latter more rapidly. The first four days of
SECT. 2]
AND WEIGHT
447
development see also the maximal relative size of brain and lens,
but these fall very rapidly away from their pre-eminence. Towards
the end of the developmental period, the fore limb gains much in
importance, and about that time also the metanephros reaches a
maximal point of growth.
For further comparison further calculations are necessary. The
relative instantaneous percentage growth-rate could be obtained
from the equation ^ , 1
^ (^v _ log v-^ — log V
Cw log Wi — log w '
1
"\
\
/
\
zv
22
20
18
IB
n
n
10
/
\
\
\
I
\
/
\
s
'
/
A
\
,^
/
\ \
/
^
•^
^
1
\
/
\
\
1
/
\
s
H
1
1
f
^\
^A
\
/
1
1
""■--
^
^
8
6
**
I.
/u
^
L^
^
—
—
--
—
—
..^
^
^
— -^
^
::^
z
£
■ —
xC
— '
'^
=^
=;
N
^--M
"^
0 1 I 3 V J 6 7
9 10 Tl 12 13 n IS 16 1? 18 IS ZO 21
Days
Fig. 58. £ = fore limb; G = brain; ^=heart; Z,=lens;
jV= metanephros; f/ = mesonephros.
where Cv is the instantaneous percentage growth-rate for the organ
or part in question, v-^ and v the weights of the organ at the be-
ginning and end of the period in question, Cw the instantaneous
percentage growth-rate of the organism as a whole and w-^ and w
the weights of the organism as a whole at the beginning and end of
the period in question. But this would not take into account the
time of formation of the various anlages. More complicated expres-
sions have, therefore, to be found, but as they do not at present
seem to have any direct importance for the chemical embryologist,
a reference to the original paper must suffice. They involve the com-
putation of an "extensity factor" which is usually the same as the
448 ON INCREASE IN SIZE [pt. iii
constant a, already referred to, and an intensity factor which is the
corrected product of the instantaneous percentage growth-rate and
the time, i.e. Cvt. The relative extensity factor is obtained by
determining the time which is taken by the organism or the
organ to grow i mm. in length, thus reversing the process by which
a was originally found. The size of the anlage is also included, and
called the mass factor. By the aid of all this apparatus, Schmalhausen
compares organs on a common basis, i.e. the time taken for i mm.
increase in length to be made. Thus the extensity factor of the chick
embryo brain [r) is 1-27 and that of the duck embryo brain 1-26,
which means that the sizes of the respective organs are in their
earliest stages almost identical. But the relative instantaneous per-
centage growth-rate (intensity factor, k, or Cv) differs considerably,
for in the chick it is 1-87 and in the duck 2-01, which means that the
duck embryo brain grows distinctly more rapidly than that of the
chick embryo and finally attains a larger size. Again, for the stomach
of the chick embryo the extensity factor, r, is 0-244 ^^^ ^^^ the duck
0*324, but the intensity factor is 3-59 for the chick and 2-86 for the
duck, or, in unquantitative terms, the stomach is rather bigger to
start with (relatively) in the duck than in the hen, but the chick
stomach grows faster and reaches eventually a larger proportion of
the body.
This work on disproportionate or heterogonic growth led Schmal-
hausen into a field which had been in course of investigation by
Huxley and others. Schmalhausen was able to obtain Huxley's
formula from his own, and concluded with some justice that his own
were of fairly general validity and did not hold only for embryonic
growth. On the other hand, owing to the absence of a true extensity
factor in Huxley's formula, the latter could not be applied to the
embryo; for, although in post-embryonic growth-curves the ages of
all the organs can be taken as approximately identical with the age
of the animal, this is by no means the case in embryological work,
where the time of formation of the various " anlages " is of the greatest
importance. The investigations on heterogonic growth are not im-
mediately germane to the theme of this book, but they may at any
moment become very important for the chemical physiology of the
embryo, and it is necessary therefore to be aware of them. Schmal-
hausen's work is really an extension to the embryo of the conceptions
of Pezard and Champy, as worked out in recent years by Huxley
SECT. 2] AND WEIGHT 449
and his associates. A rich harvest awaits the investigator who dis-
covers the relation between chemical constitution and the differential
growth-ratios. Perhaps a fruitful line of work will develop from the
finding of Robb that the log. weight of an organ plotted against the
log. body-weight often gives a straight line. He has suggested that
organ-growth may depend on a kind of partition-coefficient, organs
competing, as it were, for the building-stones in the blood-stream,
and securing now a greater now a lesser proportion, according to
the changing permeability of their cell-walls.
The changes which occur in the chick's relative growth-rates of parts
at hatching have been studied by Latimer, who combined together
the data collected for pre-natal stages by Schmalhausen and those for
post-natal stages by various American workers. His results lead to
the conclusion that the organs and parts fall into three groups :
(i) Those in which no change in relative growth-rate is found,
e.g. liver, gizzard, feathers, ovaries.
(ii) Those which show a brief post-natal retardation, e.g. total
body-weight, brain and heart.
(iii) Those which show a marked post-natal acceleration, e.g.
kidneys and spleen.
As will be shown below, the brain and eyes, so prominent in the
embryo, fall consistently throughout life when expressed as per cent,
of the whole weight, while the gizzard, liver, kidneys, spleen and
heart have a maximum in early post-natal life.
Other work on relative sizes of parts has been done by Jenkinson
on embryonic trout, by Keene & Hewer on man, and by Jackson
who gives a graph (Fig. 59) showing the relative proportions in the
human embryo at different stages of its development, collected from
all the available data. Boyd (on man) and Welcker & Brandt (on
the chick, salamander and man) made earlier attempts at the same
thing, but the ages of their embryos were unknown. The graph
demonstrates the relatively large size of the brain in the early sizes,
and in many ways resembles the graph for the organs of the chick
given by Schmalhausen. "In general," says Jackson, "the period
of maximum relative growth passes in a somewhat wave-like manner
over the body from the head towards the foot. The head reaches
its maximum relative size about the 2nd month. In the trunk, the
upper portion, including the thorax and the upper abdominal
viscera, is relatively largest throughout the earlier half of foetal life.
NEi 29
450
ON INCREASE IN SIZE
[PT. Ill
The lower part of the abdomen becomes more prominent towards
the end of the foetal period, due chiefly to the rapid expansion of
the intestines at this time. The pelvis and lower extremities do not
reach their greatest relative size until early adult life, although the
upper extremities have reached their maximum relative size at birth.
It may also be noted that the organs lying dorsal to the body axis
grow at first far more rapidly than those ventral to the body axis, for,
20 40 60 80 100 120 140 160 180 200 220 240 260 280
Days
Fig- 59-
while in the 2nd month the former are three times the size of the
latter, at birth they are equal, and in the adult the latter are six
times the size of the former." Jackson's data on the growth of such
organs as the suprarenal gland should also be of much service to
chemical embryologists, and his paper as a whole is of great value,
as it summarises the results of all the earlier workers — Welcker &
Brandt; Brandt; Anderson; Lomer; Meeh; Liman; Thoma; Oppen-
heimer; Collin, Lucien & Beneke; Devergie; Schmidtt and Elsasser.
The general results of all the workers who have occupied themselves
with the weights of foetal parts are tabulated in Table 59, which
gives the point in development at which the maximum percentage
of the total body-weight is reached, what that percentage is, what it
becomes at birth and what eventually it is in the adult animal.
SECT. 2]
AND WEIGHT
451
Table 59. Relative maximal weights of constituent parts of the embryo.
Point at which
m ^ v I m 1 1 m 1 <i
% of total body-weight
Part or
LLL€XJ\.LLkL\jixL\. 13
attained in
At maxi-
At birth or
At adult
organ
development
mum
hatching
stage
Pig
Total viscera
1 5 (length in
38
160
8
(Lowrey)
Head
1 8 mm.)
30
22-0
6
Brain
i8
9
4-0
0087
Spinal cord
i8
1-87
0-33
004
Eyeball
86
1-15
04
o-oi
Heart
15
4-64
10
0-32
Lungs
86
3-9
20
07
Liver
25
15-9
31
1-38
Kidneys
58
2-59
I-OI
025
Mesonephros
15
120
—
Spleen
15
—
0-17
0-13
Pancreas
15
—
o-i6
0-14
Thymus
15
—
0-37
—
Thyroid
58
—
0026
0-004
Suprarenal
58
0-13
O-OIQ
0-005
Intest. and stomach
15
3-6
4-79
Man
Head
2 (months)
45
26-0
(Jackson *)
Trunk
I
65
40-0
—
Fore limb
10
10
lo-o*
Hind limb
10
20
200 i
72-4
Brain
2
20
135
—
Spinal cord
I
5
0-15
—
Heart
I
5
0-7
—
Liver
2-5
7-5
5-0
—
Lungs
Spleen
4
ID
33
0-4
2-0|
0-4 i
0-94
Thymus
10
0-3
0-3
—
Thyroid
ID
0-I2
0-I2
— .
Kidneys
7
i-o
1-05
Suprarenal
3
0-45
024
0-90
Chick
Heart
4 (days)
1-5
0-56
—
(Schmalhausen)
Mesonephros
4
o-6i
0014
—
Metanephros
16
0-39
023
—
Brain
4
300
2-6
—
Lens
6
0-17
0025
—
Fore limb
16
32
2-1
Stomach
20
3-59
3-56
—
Dogfish
Head
0-09 (wt. in
40-0
17-5
12
(Kearney t)
Skin (birth)
75-0 gm.)
11-3
I I -3
7
Skeleton
85
100
8-6
9
Muscles (adult)
630
45-0
63
Total viscera, (a) c.
o-i
19-0
I2-0
—
two maxima (b) c.
350-0
14-3
—
9-8
Brain
01
15-0
1-6
0-9
Spinal cord
01
1-76
0-5
0-17
Eyeballs
o-i
90
2-0
064
Heart
O-I
4-0
0-15
0-20
Pancreas
350-0
0-14
006
0-14
Liver
20-0
7-0
4-8
59
Spleen
350-0
0-38
0098
0-2
Rectal gland
01
01 05
0032
—
Mesonephros
1-8
4-8
i-i
0-38
Testes and ovaries
200-0
0-9
0-4
0-28
Stomach and intest.
350-0
5-5
2-9
4-0
* Including all previous work on man.
t "The various organs and parts in dogfish [Mustelus cams] embryos and adults show
relative growths strikingly similar to that which has been observed among the higher
vertebrates, including mammals and man".
29-2
452 ON INCREASE IN SIZE [pt. m
From this it can be seen that the organs which reach their maximum
relative weight early in development are the heart, spleen, pan-
creas, thymus, brain, spinal cord and head. The mesonephros also,
of course, reaches its maximum fairly soon and declines more or
less rapidly afterwards. The muscle masses, shown especially in the
figures for fore and hind limbs, increase steadily in relative weight
and reach their maximal relative size at or shortly before birth. The
suprarenal gland, the stomach, the lungs, and the thyroid are variable
in their point of maximum. But, as can be seen from the table, the
data on these matters are not very numerous, most of the attention
which has been given to the relative weights of parts and organs
having gone into the study of post-natal life. W. Schultze has made
an interesting investigation on the effect of hormones on the de-
veloping parts and organs in the tadpole.
It is interesting that the only conclusions to which Jackson would
commit himself were ( i ) that the embryo grows much faster in the
earlier stages than in the later, and (2) that, at any rate as far as
vertebrates were concerned, pre-natal growth is relatively much
greater at the cephalic than at the caudal end. These points had
already both been stated by Aristotle, and the whole advance lay
in giving them a quantitative backing. Jackson did not consider that
his figures supported the view of Preyer that those organs grow fastest
in the embryo which will afterwards first come into functional
operation.
A great mass of such data has since Jackson's time been collected
by Calkins & Scammon and many investigators working under their
influence. We heed not do more than mention their work on the
growth of the spinal axis in the human embryo, that of Scammon
on the height-weight index, Nafiagas on anencephalic embryos,
and Brody and Hammond on proportions in the cow, for these
and many others only indirectly concern us. But it is important to
note that for the human embryo Calkins & Scammon found that
from 3 months onwards the growth in length, girth and diameter
of the various external divisions of the body was directly proportional
to the growth in total body-length. While each dimension has its own
growth-rate with respect to the total body-length, this characteristic
rate does not alter during the period under consideration. All these
entities then may be expressed by the Calkins equation
D =^ aL±b,
SECT. 2] AND WEIGHT 453
where D is the dimension in question, L the total body-length, and
a and b constants. The constant ^ is a measure of the amount of
growth that has gone on prior to the period in question, and, as it is
negative for the limbs but positive for all head and neck measure-
ments, the conclusion is that while the latter have been growing
extremely rapidly before the period began the former have not. This
is as would be expected. The same conclusion emerges from the data
of Corrado on the weights of head, trunk and extremities as analysed
by Scammon.
Many organs have been examined by the investigators of this
school. The cerebellum, for instance, was found by Scammon &
Dunn to increase in absolute volume and weight first slowly and
then more rapidly during the foetal period; thus, its percentage
growth-rate rose for the first six months of pregnancy, only to fall
sharply afterwards. The pancreas, studied by Scammon, grows at
a rate very like that of the whole embryo, but the relative weight
of the organ with respect to total body-weight undergoes a
reduction from 0-3 per cent, at the 4th month to o-i per cent, at
birth. The uterus, on the other hand, passes through two definite
phases in pre-natal life. Until 7 months the organ shows a lineal
increase with respect to body-length which is comparable to that
of most lineal body-dimensions, but after 7 months it grows much
more rapidly. In early post-natal life, however, the organ goes
through an involution stage which has long been known, actually
decreasing in size by hypoplasia and hypotrophy until it reaches
the level it would have attained had the early foetal growth-rate
been continued. "This suggests", says Scammon, '^that the growth
of the uterus in the latter foetal months consists of a substratum of
typical foetal growth plus a secondary increment due to an extra
stimulus furnished by a hormone of placental or possibly ovarian
origin." Here is an excellent illustration of how an organ can act
as an index registering obscure physico-chemical changes in the
internal environment of the embryo. The other organ which
undergoes a reduction in size following birth in man is the
suprarenal gland, and it also has been investigated by Scammon.
But, unlike the uterus, its growth when observed in the foetal
period shows no increased intensity towards the time of birth, so
that the involution which occurs afterwards by degeneration of
the two inner cortical layers decreases its size far below what it
454 ON INCREASE IN SIZE [pt. iii
would have reached had it gone on growing at the same velocity
as before birth.
This leads on to the general problems raised by the growth of the
ductless glands in the embryo, problems of the greatest importance
in view of the regulating influence which the foetal endocrines
probably exercise. The pituitary gland, according to Covell, grows
proportionately to the total body-weight in human pre-natal life,
i.e. slowly till about the 5th month and more rapidly thereafter.
The thyroid also shows no outstanding variations from the normal
curve. The growth of the thymus, however, is characterised by high
variability, while the pineal gland grows at nearly the same rate as
the brain. The results of studies on the weight of these glands,
therefore, do not reveal any striking correlations, and they must be
supplemented by histological evidence. This will be presented in the
section on hormones.
Scammon; Scott; and Scammon & Kittleson have studied the
growth of the intestinal tract and the stomach in the human embryo.
While this work does not give us any help in evaluating the active
absorptive surface during embryonic life, its main conclusions are of
interest. Thus the growth of the gastro-intestinal tract follows the
law of antero-posterior gradient or direction, for the more cranial
portions grow relatively more rapidly in the early part of foetal life,
while the successive caudal portions show smaller amounts of growth
at the beginning and larger ones later. The number of crypts and
glands in the stomach mucosa seems to increase per sq. mm, very
regularly during the progress of foetal growth. Watkins' study of the
growth of arteries is also interesting, for it shows that the vessels
which supply the foetus only have a rate of growth similar to that
of the body as a whole, i.e. slow for a short time at first, and then
for a long time rapid, while the arteries which supply the placenta
as well as the embryonic body have a long period of slow growth
followed by a short period of rapid growth. These facts throw a
certain light on the metabolic needs of the developing organism.
Much valuable information is contained in the papers of Scammon
and Armstrong on the foetal growth of the eye, and in that of Noback
on the respiratory system, but it cannot be given here.
Davenport has recently considered the growth-curves of man in
the light of the work of Scammon and his associates, and Gunther
has written on these subjects especially in relation to sex.
SECT. 2] AND WEIGHT 455
2-1 1. Variability and Correlation
Enough has now been said about the growth of the parts and
organs of the whole considered in isolation, and we must consider
the relation between the growth-rate and two other factors, namely,
variability and correlation. The population of cells in the metazoal
embryo may no doubt be compared with the populations of
protozoa in cultures, but, whereas the functions of the whole in the
latter case are very limited, those of the whole in the former case
are highly complex. In other words, one may enquire to what
extent there is variability between different embryos of exactly the
same fertilisation age. Closely allied to this question is what is, to all
intents and purposes, its converse, namely, at what point in develop-
ment is the correlation coefficient greatest, i.e. at what point is the
swing of variation among embryos away from the mean least? It is
to be regretted that these enquiries have not been very deeply carried
on in embryology, but there are some rather significant observations
which need attention.
So far only the mean values for weights and measures of embryos
have been under consideration. But obviously no statistical study
of these is complete without a consideration of the amount of
variability among the individual cases from which the mean value
is derived. The variability coefficient is defined as the
standard deviation
X 100,
mean
the standard deviation being a measure of the spread of points
around the mean, i.e. a measure of the point upon the frequency-
curve where the change takes place between concave to the mean
and convex to it. Fig. 22 showing McDowell's points will explain
the meaning of this. It has long been known that the variability
coefficient decreases with age in man, and it is always stated that
it follows the changing growth-rate quite closely, but some con-
fusion has been caused in the past by a doubt as to what manner of
representing the growth-rate is being referred to. The fact is, how-
ever, that the variability coefficient follows the simple increment
curve. Thus, if for absolute growth a sigmoid curve holds good,
the greatest daily or monthly increment will occur as we have
seen at the middle of the period, and this peak will coincide
with a peak in the variability coefficient. This was found to hold in
456 ON INCREASE IN SIZE [pt. m
actual fact by Boas & Wissler, by Boas, and by Bowditch, who
studied exhaustively the growth of Toronto school-children. The
variability coefficient in that case followed exactly the curve of
yearly increment and reached an exactly simultaneous maximum at
the age of 15 years. The correlation coefficient behaved in the same
way. Boas & Wissler explained their results by saying that correla-
tions between measurements in one individual ought naturally to be
greater during periods of rapid growth than at other times, because
the variations in responsible factors will affect them all to an equal
extent. Variability is what governs correlation, so that would also
be expected to rise and fall in the same manner. But there is a
proviso that must be made here, for Boas & Wissler's use of the
term "growth-rate" is not the same as that of Brody, for instance.
Boas & Wissler mean by the time of greatest growth-rate the time
at which the largest increments are being added on to the organism
in unit time, i.e. the half-way point in the curve of the autocatalytic
equation. Brody means by the time of greatest growth-rate the time
at which the organism is adding on to itself the largest relative incre-
ments, i.e. the earliest stages of embryonic life, when the slope of
the log. weight/age curve is extremely steep, and the embryo doubles
its weight in an exceedingly small lapse of time. The variability
coefficient shows, therefore, that it is at any rate true to say that,
during the phase when the largest absolute increments are being
made, the widest variations from the mean tend to occur. This seems
very reasonable, but there is also evidence which shows that, when
the period of most rapid growth in Brody's sense is occurring, the
variability coefficient is also large.
Other examples are numerous. Buchem found a coefficient of
variability of 0-4 for the early stages of the embryo cow and o-i later.
Edwards found a variability coefficient of 0-1347 for unincubated
chick blastoderms, but of 0-1087 for those incubated 24 hours.
Jenkinson gives a graph exactly analogous to those of Boas & Wissler
computed from Roberts' measurements of English artisans. Jenkinson
also worked on the trout embryo (or rather the alevin, for his first
point was 2-3 weeks after hatching, by which time the yolk was not
completely absorbed). He found that there was a close general
agreement between weekly increment and variability coefficient for
the first 10 weeks after hatching, true not only for the growth and
variability of the body-length but also for some of the parts such as
SECT. 2]
AND WEIGHT
457
the diameter of the eye, the length of the head, and the length of the
caudal fin. Fig. 60, taken from Jenkinson, shows how, although the
curves do not run absolutely parallel, they certainly rise and fall
together. On the other hand, the correlation coefficients between
several pairs of organs show that in many cases — total length and
breadth of caudal fin, total length and length of anterior dorsal fin,
total length and length of head, head length and eye diameter —
there is a significant diminution in value during the time that there
70
a
.^ 60
iB
50
40
C
« 30
c
y 20
o
10
<
Tot
\
\
al lar
AX
/ae.
Total 1
lengt
h
At
\
\
^ ^^V,
\
\
\
V
V
J
^
\
*%
>
**
\
X^^^
■»
m
-,
.3 1'3 2'3 3*3 4*3 5-3 6*3 7'3 8*3 9*3 10-3
Weeks after hatching
Fig. 60.
is a decrease in growth-rate. Thus, when the growth-rate is highest,
the variations between individuals are greatest, but the correlation
coefficients between various organs or parts in the same individual
are also greatest. It is easy to see why this should be so, but Boas
has given a mathematical proof of the relation between these co-
efficients and the growth-rate. Expressed differently, it could be
said that the faster the growth-rate the more proportional the growth
but the greater the variation as between individuals.
Turning now to the early part of the embryonic period, the first
complete investigation was that of Fischel, who studied the individual
variations between duck embryos at the primitive streak stage. Von Baer
458
ON INCREASE IN SIZE
[PT. Ill
had already averred qualitatively as early as 1828 that variability
was much more pronounced in the earlier stages than in the later ones.
Kupffer & Benecke; Keibel & Abraham; and Assheton afterwards
drew special attention to it. Fischel, however, measured the length
of the embryos, and found that the older they were the more regularly
they agreed together. This led him to conclude that regulating
influences came into play during development which brought about
a more synchronous course of growth and differentiation, and made
6 7 8 9 10 n
•^ Number of Somites
Fig. 61.
the individual variations less and less obvious. Such a standpoint
would accord well with frequency-curves such as that of McDowell
and his collaborators for the mouse embryo (see Fig. 22), where
the range of weights on a given day is 4 or 5 times as large at the
beginning of development as it is at the end. His and Levi,
working on the development of the chick, came across the same
phenomenon.
Fischel divided the total length of the embryo into a number of
constituent lengths, e.g. from the cephalic to the caudal end of the
somites and from the extreme cephalic end to the anterior blastopore
("Darmpforte"). He distinguished 13 such lengths, and these he
measured in a large number of embryos, judging the age in each
case by the number of somites formed. As will be seen from Fig. 61,
the limits of variation considered as absolute maxima and minima
SECT. 2] AND WEIGHT 459
are much the same when the embryo has o somites as when
it has 20, but, in view of the increase in total length during this
time, it will be seen that the maximum variation is much less
important as time goes on. "The relative differences", said Fischel,
"are certainly less in the later stages than in the earlier ones."
Fischel's measurements of the lengths of the parts all showed the
same relation as regards variability, but, though the length of
the body is increasing regularly all through this period, the length
of the part between the anterior end and the ist somite remains
practically stationary, as does the length of the part between the
last somite and the posterior end of the embryo. In other words,
the increase in length is entirely due to growth of the middle region
in which the somites are being produced.* The size of the individual
variations can be large; thus an embryo may be more than 50 per
cent, longer than another one of the same stage. Fischel's examina-
tion of the work of other authors, such as that of Bonnet on the sheep
and of Keibel on the pig, induced him to suppose that very similar
effects were seen in mammalian embryos. Philiptschenko carried the
question into insect development by investigating an apterygote,
Isotoma cinera, and found that the older stages showed the greater
variability. On the other hand, Zuitin, who has studied the develop-
ment of Dixippus morosus from this point of view, found that, just as
in the birds and mammals, the earlier stages were the ones which
showed most variations from the mean. Schmalhausen has also con-
sidered the question on the basis of the figures he obtained in his
studies on the growth of the chick embryo, already referred to. He
points out that the early stages in embryonic development are those
when the anlages are being formed. At that time, every few hours,
as it were, are marked by the start off of one or more parts or organs
on the long declining hyperbola which represents their instantaneous
growth-rate. Thus a cross-section through an embryo in those early
periods would demonstrate, if some method of the future made it
possible, a series of growth-rates, some low, appertaining to the more
senior organs, some high, appertaining to the more junior ones.
Moreover, the mass of each anlage is different for each organ, so
that extremely complicated effects will be observed if the weights of
the organs are described in percentage of the total body-weight. Thus
* As Levi has shown, embryo size in the somite stage is very similar no matter what
the size of the adult bird, but soon the larger animals grow longer. Also the size of the
first somites of all sauropsida is between 5000 and 8000 /x^ surface.
460
ON INCREASE IN SIZE
[PT. Ill
10-0
the earlier the stage the more chance there will be for individual
differences in growth-rate to reveal themselves, while in later develop-
ment these will be equalised, adjusted, and compensated by a process
of self-regulation. Schmalhausen does not say, however, what this
process of self-regulation is, and its nature, indeed, offers one of the
most interesting problems in embryology. We shall refer to it again
in the chapter on hormones. Schmalhausen suggests that Philip-
tschenko's results might be explained by the extremely small time
elapsing between the formation of the various anlages in animals
with ultra-rapid incubation times such as some insects. Owing to the
high instantaneous growth-rates in the earliest stages, a minute
difference in time of formation of an anlage or a minute difference
in its initial size will make a very large difference between one
individual and another.
2*12. Explantation and the Growth-promoting Factor
We must now return to consider further the essential nature of
the embryonic growth process. Some reference has already been
made to Schmalhausen's findings
with regard to spurts of growth
and the extent to which all organs
are affected simultaneously by
the growth-impulse. But what is g ?•
this growth-impulse? What is its .£ e-o
essential nature? It is no longer ^ g.o
possible to say that we have no J ^
idea, for there are a number of
very significant observations re-
garding it arising from the study
of tissue culture in vitro outside
the living body. Ephrussi has
brought forward evidence sug-
gesting that growth-rate may be a genetic factor. The study of the
effect of temperature on the growth-process has also been fruitful to
a certain extent in answering this question, and its results will shortly
be described.
In the first place, it is natural to enquire whether the growth-rate
found to hold for the entire embryo growing under its normal con-
ditions would hold for parts of it isolated and growing in tissue
4 &
6 7 8 9 10 1112 13 14 15 16 17 18
Incababion age
Fig. 62.
SECT. 2]
AND WEIGHT
461
culture. Would small pieces of it be found to grow in tissue culture
proportionately to the growth of the organism as a whole, or is the
growth-rate a value absolutely dependent on the intactness of the
embryo? This question has found a definite answer in the re-
markable work of Cohn & Murray, who, following up a few pre-
cursory experiments of Carrel, removed hearts from chick embryos
from the 4th day onwards, made
1-0
Days 5
J I L
l—J I 1 ¥ J
dw
db
W
cultures of them in adult hen
plasma or plasma plus Ringer
solution, and determined their
rate of growth. This was done
by four methods : {a) subtract-
ing from the total area of tissue
after n days of cultivation the
area of the central portion,
{b) dividing the area of growth
made (total minus central por-
tion) by the size of the original
fragment, {c) dividing the area
of growth made by the size of q
the central portion at the time
of measurement, and [d) divid-
ing the area of growth made
by the square root of the area
of the central fragment. With
these methods and carefully
controlled conditions they found that the growth-rate of the cultures
of embryo heart fell in an exactly parallel manner to the Minot curve
previously found by Murray to hold for the chick. Figs. 62 and 63,
taken from Cohn & Murray's paper, show this fall, and may be
compared with Fig. 34 which shows the corresponding Minot curve
for the whole embryo. The kink in the heart culture curve may perhaps
be related to special changes occurring in the organ at the 9th day (see
Section 23). It must be remembered that the heart of the chick has
been stated by Schmalhausen to grow at the same rate as the whole
body, though, as a reference to Table 56 shows, the Schmalhausen
Cvt is 276 for the heart and 321 for the whole body. However, it
would not have been material which organ had been selected by
Cohn & Murray, for all organs have growth-rates either descending
6 7 8 9 10 11 12 13 14 15 16
Incobation age
Fig. 63.
462
ON INCREASE IN SIZE
[PT. Ill
in steps (see Fig. 55 a) as Brody would have it, or in a hyperbola
(see Fig. 55^) as Schmalhausen would have it. That Cohn & Murray
compared their results with the Minot curve does not matter, as
all percentage growth-rate curves, whether instantaneous or not,
fall towards the abscissa asymptotically. Cohn & Murray's achieve-
ment was to demonstrate that the growth-promoting impulse resides
to a large extent actually within the cells, and that its fading out
can be seen equally well in the
decreasing rapidity with which
isolated cells will grow in culture
as in the decreasing rapidity with
which the whole cell-population
of the embryo increases in the
egg. In other words, functional
changes taking place in the or-
ganism as a whole are mirrored x n
by similar changes in the in-
dividual cells.
Cohn & Murray also deter-
mined the latent periods in their
chick heart cultures, and were
able to show that, as the embryo
grew older, so the cells of its
heart took longer and longer to
accustom themselves to their in
vitro environment. The latent
1Q 7
IR^ ^
^ ,
17 V
t
Ti 7-
/
u t
J
«. '^ t.
^ -e
° 11 2: ^
^'^ ^7 ^^
in Y ^^
lU / ^^
q ^2 JJ.^
- 4 ,^
ft ~ /^ ^l
J' ^^^^
7 -^/^^"^
^2 J^^
_• ■^ -( )- "^ ^ "^""^r t^ "^
^ " _ _ _
Days 4 5 6 7
8 9 10 11 12 13 U 15 16 17 18
Incabation age
Fig. 64.
Heart-fragments in plasma, 0 ;
heart-fragments in plasma + Ringer, H-
period was defined as the time intervening between the incubation
of the cultures and the first appearance of cells protruding from the
peripheral margin. As Fig. 64 shows, this value markedly increases,
though not until after the 7th day. Unknown to them, Suzuki had
previously found just the same relationship between latent period and
age in the chick embryo. His figure (Fig. 65) is a beautiful illustration
of their conclusions. Growth-rate and latent period are two strangely
associated processes. "When older tissue", as Cohn & Murray put it,
"i.e. a heart fragment from a 16-day old chick embryo, is placed in a
fresh environment, it does not assume immediately a divisional velocity
typical of younger tissue. In our experiments the previous growth-
rate was not approached for 36 hours. In other words, a number of
cell-divisions (i.e. generations) were required for it to lose the habit
SECT. 2]
AND WEIGHT
463
100(—
of slow growth previously imposed upon it by its organised environ-
ment. This happening is an expression of a process of rejuvenescence
or dedifferentiation*. The tissue soon assumes a growth-rate deter-
mined by its environment, and as long as the environment can be
kept relatively stable, that is to say of uniform composition, pre-
sumably there will be no further change in divisional velocity."
As in the intact embryo, then, negative acceleration of growth
is greatest at the beginning of
life. On the other hand, the
latent period before growth in
vitro begins is greatest at the end
of embryonic life. It is likely,
therefore, that the factors which
determine the initiation of
growth and those which deter-
mine the extent to which it shall
take place in a given time are
not identical.
Olivo & Slavich and Oda &
Kamon, subsequently repeated
the experiments of Cohn &
Murray and as far as heart
fragments were concerned, con-
firmed them in every particular.
Nordmann also confirmed them
for liver cells. Oda & Kamon
found that the growth-rate in
culture was the more rapid the younger the embryo from which
the heart had been taken, and the latent period was the shorter.
The pulsatile activity of the heart explants was also more hardy the
younger the embryo. On the other hand, they did not obtain such
clear-cut results with the spleen which to some extent showed a
higher in vitro growth-rate the older the embryo from which it had
been taken. This doubtful point urgently requires re-investigation, for
it has become a commonplace of explantation research that growth-
rate in vitro follows growth-rate in vivo. Moreover, Allen has found
other differences between organs ; thus in saline, embryo heart will not
grow at all after the 8th day, intestine after the i ith day, and so on.
* This does not imply the acquisition of any pluripotence or totipotence.
5 10 15
Age of chrck giving the heart-cells
Fig. 65.
464 ON INCREASE IN SIZE [pt. iii
The experiments of the Cohn & Murray type were precisely the
converse?of those of Carrel & Ebeling who found that the growth-
rate of a standard 2-year-old strain of fibroblasts was affected by the
age of the hen from which the plasma culture medium was derived.
The older the donor the lower was the growth-index, the younger
the donor the higher, i.e. the faster the explant grew. Thus the growth-
promoting factor must be present both in the cell-protoplasm and
in the circulating blood plasma or "milieu interieur". This growth-
promoting factor was for long believed to be a hormone or even a
vitamine, but it now seems to be much more probably a special
collocation of the right nutrient substances, probably protein
breakdown-products.
Following upon the pioneer work of Ross Harrison, it was found
by Carrel; Carrel & Burrows; Fischer; Ebeling; and Carrel & Ebeling
that tissues would grow indefinitely in vitro on a medium composed
of one part adult plasma and one part embryonic tissue juice.
It was then soon found that it was the latter constituent which
furnished the necessary factors for cell nutrition and multiplication
(Carrel & Ebeling and Carrel). From this point there began an
extensive series of researches directed towards the identification of
the substance responsible for the effects produced by embryonic
tissue juice. Whatever it was, it was not species-specific, for duck
fibroblasts were found to grow well in chick embryo extract
(Fischer), and vice versa (Kaufmann), while rat tissues could be grown
in chick embryo extracts (Mottram), and rabbit embryo extracts
would stimulate chick tissues (Carrel & Ebeling) or human tissues
(Timofeivski & Benevolenskaia) .
Results which are interesting in this connection are those of Loisel
who substituted duck egg-white for the natural egg-white of the hen's
egg and found that the normal development of the intact chick embryo
up to the 4th day was not interfered with by these conditions.*
Carrel & Baker examined the fractions obtained from chick
embryo juice, and concluded that the growth-promoting factor was
associated definitely with the protein part. The precipitated proteins
of the embryo juice (acetone, carbon dioxide, alcohols, acetic acid,
etc.) never showed a greater growth-promoting power than the
original juice, but often nearly as much. Chemical examination
showed that the protein in question was a mixture of nucleoprotein
* Bouges also, by an injection technique, has replaced the yolk by yolk from other
breeds or even species, without interfering with normal development.
SECT. 2] AND WEIGHT 465
and a glucoprotein rather like mucin, but when these were prepared
in a pure state, they showed no growth-promoting action in tissue
cultures. Carrel & Baker tried a great variety of other substances,
especially proteins, such as pure egg-albumen, but always without
result. Ether extraction, they found, did not remove the growth-
promoting factor from the embryo juice proteins, so that it seemed
unlikely that it could be of lipoid al nature.
Some earlier experiments of Carrel & Ebeling had shown that
although pure amino-acids produced a slight stimulation of growth,
causing greater cell-migration, they did not produce an increase in
the mass of the tissue, as the protein fraction of embryo juice cer-
tainly did. This was confirmed by Carrel & Baker, who dialysed the
embryo juice in collodion bags of high permeability, or else ultra-
filtered it. They found that the dialysed juice did lose a certain
amount of its growth-promoting power, but they put this down to
denaturation of the proteins, in view of the fact that the amino-acids
separated in this way were practically without action on the growth
of fibroblasts in culture. Nor was there an enzyme in the juice which
produced amino-acids in the dialysing mixture more rapidly than
they could diffuse away. It is true that the liquid surrounding the
collodion bag and containing the diffusible amino-acids had a slight
effect on the increase in area of the explants, the rate being 1-28
times faster than in Tyrode solution, as against 10 times or upward
for the protein fraction. This small effect was put down by Carrel &
Baker to migration stimulus, and not to increase of mass of the explant.
When Carrel & Baker tried to restore the loss of activity of the protein
part by adding the amino-acids which had dialysed away, they
found it was impossible to do so — a fact which supported the view
that the proteins had been denatured by the dialysis. Thinking that
the concentration of amino-acids in the untreated juice might be too
small to show the presence of the growth-promoting factor, they
hydrolysed the juice protein with acids and with trypsin, but the
large concentrations of amino-acids so produced proved to be toxic
for the cells of the cultures. This toxicity had already been noticed
by Burrows & Neymann. Wright also worked with diffusates of
embryonic tissue juice and observed a perfectly equal number of
mitoses in explants of chick embryo heart, whether the medium used
contained embryonic tissue juice itself or the diffusate from it, e.g.
35 ± 5 in the former case and 32 ± 5 in the latter. In another
N E I , 30
466 ON INCREASE IN SIZE [pt. iii
instance the number of mitotic figures was: saline 2, extract 24,
diffusate 26. These results were interpreted by Carrel & Baker as
being due to a stimulatory action of amino-acids on migration, and
they stated that they observed similar phenomena themselves with
pure amino-acids, which, however, gave nothing approaching that
large extension of area seen in extracts with the protein fraction.
All amino-acids, according to Carrel & Baker, stimulate cell-
multiplication and migration, without increasing the mass of the
tissue, i.e. without causing growth.
In a succeeding paper Carrel & Baker put to the test the suggestion
that it was not the proteins as such in the embryonic tissue juice
which carried the growth-promoting factor, but rather their larger
split-products such as proteoses, broken off from them by the cells
of the tissue culture themselves. They found that if tissue juice from
embryos was digested with pepsin for 16 or 32 hours, the hydrolysate
was rather toxic for the fibroblast explants, but if the digestion was
only carried on for 3I hours, the presence of the growth-promoting
factor readily revealed itself, growth proceeding seven times as fast as
in Ringer solution. The large protein split-products which were
evidently responsible could be obtained just as well from peptic
hydrolyses of egg-albumen, and commercial fibrin, the latter especially
proving a rich source of growth-promoting activity. Chemical ex-
amination of the different fractions of the digests, compared with
tissue culture examination, showed that the substances responsible
were undoubtedly proteoses. Pepsin digests of rabbit brain were
found to be active. Commercial "peptones" were found to vary
in activity, according to the proportion of higher split-products
contained in them. Tryptic hydrolysis was not so satisfactory.
Carrel & Baker suggested that the function of the proteose is to
furnish a higher concentration of amino-acids to the cells than
could be obtained even from their saturated solutions, and to
supply them to the cells in this tightly packed, yet soluble and
diffusible form. Carrel & Baker's identification of proteoses as the
important substances was confirmed by Fischer & Demuth and by
Willmer.
All these facts led Carrel & Baker to conclude that there was no
justification for speaking of a "growth-promoting hormone". The
growth-promoting factor is probably no more than a right con-
junction of nutrient materials and the appropriate capacities for
SECT. 2] AND WEIGHT 467
making use of them. The particular position taken by unhydrolysed
embryo tissue juice is of course very important from the embryo-
logical point of view, and merits much further research.
Still later, Carrel & Baker went on to investigate the growth-
promoting activity of digests of pure proteins, fibrin, egg-albumen
and edestin. The proteins themselves, of course, had, from the earliest
days of tissue culture, been known to be more or less inert (Smyth;
Swezy). They now found that pure fibrin, when taken to pieces by
pepsin into its proteoses, had as powerful an effect as commercial
fibrin, so that the effect was not due to impurities such as blood
corpuscles. The split-products of pure egg-albumen, however, did
not show so large a growth-promoting action, and Carrel & Baker
found that it could be increased by the addition of pure glycine and
of pure nucleic acid. It was interesting that digests of vegetable
proteins such as edestin and gluten, showed a marked growth-
promoting activity. Carrel & Baker also showed that a medium con-
taining a maximum growth-promoting activity could be prepared
by digesting calf liver or pituitary with pepsin to the right extent,
and that a- and ^-proteoses were equally effective.
If now we go back some years we find that in 1921 Carrel &
Ebeling found that adult serum strongly inhibited the multiplication
of fibroblasts and epithelial cells in tissue culture. As already
mentioned, it was not long before they found that this property
clearly increased with the age of the animal providing the serum
(Carrel & Ebeling). Then in 1927 Carrel & Baker showed that this
action was due to changes with age in both the lipoid and protein
fractions of the adult serum, and that in all probability it was related
to the increase of antitrypsin : a significant finding if the hydrolysis of
the proteose by the explanted cells is the most fundamental mechanism
involved. Carrel & Baker showed that the growth of fibroblasts was
1-56 times as good in the young as in the old serum.
Brody, who was interested in this property of serum as an index
of senescence, constructed a chart showing the decline of its growth-
promoting power with age, and this is reproduced as Fig. 66.
Duration of life of fibroblasts grown in adult serum, plotted against
age of fowl giving the serum, declines with a k (instantaneous per
cent, growth-rate) of o-i8 and halves itself in 3-9 years. The
data from which this graph is derived were given to Brody by
Carrel.
30-2
468
ON INCREASE IN SIZE
[PT. Ill
After these demonstrations of the nature of the growth-promoting
factor, the assertions of other workers that a hormone is in reaHty
concerned do not seem very convincing. The paper of Heaton may
be consuked for experiments done from this point of view. The fact
that tissue cuhures of very early chick embryos are not successful
and the fact that observers are agreed on the necessity of keeping
the cells in numbers for successful cultivation led Wright to investigate
how it is that in nature the early stages of the chick are ever success-
fully passed through. Yolk from the hen's egg proves itself inactive
when added to tissue cultures, probably because, as Carrel & Baker
showed, yolk-lipoids inhibit growth in explants, but Wright found
that on dialysing the yolk a sub-
stance passed through the membrane ^
which behaved very similarly to the i;
dialysates from embryo tissue juice. 5
Thus in one experiment where the -^
yolk diffusate from 7-8-day eggs was —
tested on heart fragments of from E
10- 1 1 days' incubation, the number {£
of mitoses in the treated explant was "S
from 1 14 to 127, while in the control «^
it was only 14. Willmer has discussed
160
140
120
100
80
60
these observations in an interesting °
o
40
20
V
\
\Duration of life of
\
kfibroblasts in serut
n
N
»\
i.
1
s
,0
X,
^
■^L?
^
r«
^
Age
Fig. 66
6
n Years
10
review.
Fischer has elaborated a theory of J^
"desmones" or mutually stimulat- q
ing substances, and Burrows has
introduced the terms "archusia"
and "ergusia" for similar ideas. It
is not possible to review the work of these authors here. Burrows &
Jorstad did, however, point out that the growth of embryonic cells in
tissue culture depends on sufficient crowding and a certain stagnation
of the medium. For reasons not at all obvious to the bio-
chemist. Burrows & Jorstad identified archusia with vitamine B
and ergusia with vitamine A. Still less satisfactory is the work of
Carnot, of Roulet and of Carnot & Terriss who affirm, on very
slender evidence, that wounds of metazoal animals heal much quicker
when treated with embryo extract than when untreated. Carnot &
Carnot have also maintained that injection of foetal extracts into
SECT. 2] AND WEIGHT 469
unilaterally nephrectomised animals causes a much increased com-
pensatory hypertrophy of the remaining kidney.
Carnot has also studied the effect of chick and rabbit embryo
extracts on the growth of tadpoles, using glycerol extracts or the
powdered dry substance. There was no influence on metamorphosis,
but a great increase of growth, the experimental tadpoles being
two or three times as big as the controls. This recalls the older
observations of Springer who maintained that the eggs of Arbacia
punctulata and Asterias forbesii contained a substance inhibitory to
the growth of other eggs of the same species. The retardation
was noted in the early rather than the late embryos, and there
was a marked tendency for the eggs to stop development when they
arrived at the blastula stage. Subsequent work by Peebles indicates
that a growth-promoting factor (perhaps a proteose) may also
be found in echinoderm embryos. These observations are not very
convincing and perhaps the subject might be re-examined with
advantage.
Experiments which may, when extended further, throw some light
on these questions have been made by Skubiszevski, who transplanted
chick embryo tissues into adult hens. The complete failures were
more or less numerous at all stages of development, and so probably
represented errors of technique, but the conspicuous successes gave
a curve which was markedly peaked at the 3rd day of development.
The grafts grew well without antagonism for as much as 72 days
as mesodermal connective tissue, but in three cases (out of 1 78) they
resembled a sarcoma at the end of that time. This 3rd day peak
may be an important clue. Precisely similar work on grafts was done
by Uhlenluth, who transplanted the eyes and skin of larval sala-
manders into individuals of various ages. These transplanted organs
attained maturity not at the time when their original possessor took
on adult characteristics, but at the time when their new possessor
metamorphosed. They fell into step, as it were, with their new en-
vironment. The hormones and growth-promoting substances which
regulate such changes are still imperfectly understood, but Babak;
Gudernatsch; Swingle; Huxley & Hogben and many other workers,
have fully unveiled the importance which thyroxin has in amphibian
metamorphosis.
Another way of studying the essential nature of the growth-impulse
might be to plot the viability for in vitro tissue culture of individual
470 ON INCREASE IN SIZE [pt. iii
cells from embryos killed at a definite time. Perhaps it would be
found that the younger the embryo, the longer would its component
cells remain viable after its demise as a complete whole. Thus
Bianchini & Evangelisti found that in guinea-pig and rabbit foetuses
the cells of tissues were cultivatable as long as 78 hours after the
death of the whole body. And Bucciante, who incubated hen's eggs
for 6-12 days and then placed them at o or 15°, found that in
vitro cultivation was still possible for as long as 25 days afterwards
in the case of epithelial cells, though leucocytes and liver-cells only
retained viability for 3 or 4 days. Cells of other tissues occupied
various intermediate positions.
2-13. Incubation Time and Gestation Time
So far, the total time taken in embryonic growth has not been
considered at all. It was indeed said, when the work of Donaldson,
Dunn & Watson was being considered, that the act of birth or
hatching seems sometimes to have little influence on the course of
growth, but as Brody and Schmalhausen have shown, it more com-
monly aflfects profoundly the rate of increase of size. And it does set
a term to the embryonic period (I make no distinction between em-
bryonic and foetal period, as is the custom with some authors), forming
at least a convenient index to show whether the growth of the embryo
is being accelerated or retarded by external influences. It is therefore
important to know the normal incubation or gestation time for as
many animals as possible. These are collected together in Tables 60,
63 and 64. It can easily be seen that there is some difference between
the estimates of gestation and incubation times in the same animal
by different workers, but these discrepancies can only be cleared up
by more extended observations.
When we come to consider, however, the nature of the law which
must presumably govern the length of embryonic life we meet with
a remarkable degree of obscurity. Roughly speaking, the common-
sense rule that the larger an animal is, the longer its embryonic life
must be, is borne out by the figures. Thus in Table 63, where the
weight of the adult mammal is seen to vary between 0-014 kilo
and nearly 4000-0 kilos and the gestation time between 21 and 600
days, the larger mammals have the longest gestation times, although
there are several cases where animals of the same weight have
different gestation times (e.g. the pig and the deer) and animals of
SECT. 2]
AND WEIGHT
471
the same gestation time reach, when full-grown, very different weights
(e.g. the antelope and the hippopotamus). However, if the weight
when adult is plotted against the gestation time on double-log. paper,
a straight-line relation is obtained, individual points not lying very
far from the mean. The only reason for using double-log. paper
here is the convenience of getting all the data on to the same
Table 60. Przibram's figures.
Animal
Chimpanzee ( 7>o^/o^fe^ m^«r) ...
'M.acacus {Macacus siniciis)
Mandrill {Cynocephalus papio)
Uistiti (Callithrix jaccus) ...
L.emuT (Lemur catta)
Lion (Felis leo)
Puma {Felis concolor)
Ermine {Mustela erminea)
Wolf (Canis lupus)
Bear (Ursus arctus)
Seal {Phoca)
German marmot {Cricetus frummtarius) .
Mouse (Mus musculus)
Rat {Epimys rattus)
Rabbit {Lepus cuniculus) ...
Guinea-pig {Cavia cobaia)
Cow {Bos taurus) ...
Sheep {Ovis aries)
Goat {Capra hircus)
Stag {Cervus elaphus)
Roedeer {Cervus capreolus)
'EAand {Alces palinatus)
Camel {Camelus dromedarius)
Moschus {Moschus moschiferus)
Pig {Sus scrofa)
Hippopotamus {Hippopotamus amphibius)
Rhinoceros {Rhinoceros unicornis) ...
Horse (Eguus caballus)
Donkey {Equus asinus)
Elephant {Elephas indicus)
Kangaroo-rat {Hypsiprymnus cuniculus) . .
Opossum {Didelphys virginiana) ...
Gestation
Birth-
time (days)
weight (gm.)
260
1,000
160-210
480
177-210
480
84
33
144
240
105
1,000
92
500
^4
25
61
250
240
1,500
350
10,000
21
^5^
21
1-6
21
5
30
70
63
120
285
37,000
150
4,000
150
2,800
240-270
24,000
280-300
3,000
240-270
60,000
360-400
80,000
160
220
120
1,700
210-250
50,000
510-550
50,000
330-350
40,000-70,000
360-380
20,000
615-628
240,000
u
05
8
05
graph. The line does not meet the co-ordinates at the zero point;
it cuts the time scale, no matter what units are taken. This means
that no matter how small the mammal, an appreciable time
has to be taken in development, and in the case of a mammal
as small as a gnat, a surprisingly long gestation time would be
observed. Thus the mouse, which is 259,000 times as small as an
elephant, does not have an incubation period 259,000 times as
472
ON INCREASE IN SIZE AND WEIGHT [pt. iii
short, but only 31-6 times as short. A mammal as small as a
gnat, therefore, would probably have a gestation period of 8 or
9 days. The explanation of this must lie in the time requirement of
differentiation.
lOOOOpr
1000 -
100 -
10
TO
0-1
0-01
E
=\
V ®
«
\ ®
~
\ ®
V §
• Insectivora
© Rodents
-
e \ «
1
® Even toed ungulates
e Odd
-
e Carnivora
® Primates
V §
0 Cheiroptera
— CO
— _o
Z5
e \
9
■D
<
ir
®
\
— 0
j::
OS
1
'^ C C
U3 E §
cu 1 c
"^ ^ <-
e
e
0
3 \
® \ e
-
III
®
\
E
\°
Gest
ation-Time per
Kilo of Adult i
1 Days
\
Mil
I 1 1 INI
1 1
Ml!!l
1 1 1 Mill
1 1 Mini
K
0-05 0-1
1-0
10
Fig. 67.
100
1000 3000
Another way of looking at the data appears in column 10 of Table 63
and in Fig. 67. In order to eliminate the factor of weight, the
gestation time in days per kilo of adult animal is calculated, and this
-
/o
1 00000
/
-
o/
o / o
/
-
o A
10000
—
-
o /
o /o
°/o
1000
100
_ CO
-E
c
c
o /
/ °^
- a;
o /
AS
- S-
/ o
o / o
10
/
-
7
-
/ °
1
/
- CD
: /
/
1 1 1 1 1 II!
Days Gesta
1 1 1 M Ml
bion-Time
1 1 1 1 1 1 1 1
1 1 1 1 1 III
10
100
Fig. 68.
1000
10000
474 ON INCREASE IN SIZE [pt. iii
is then plotted on double-log. paper against the weight of the adult
animal in kilos. Evidently the relation is also linear in this case, but
the interesting thing is that the smallest animals take far the longest
time to construct unit weight. Thus the mouse performs the feat of
producing a kilo of mice in 1 790 days while the elephant produces
a kilo of elephant in 0-16 day. This must be due to the fact that
contained in i kilo of mouse there is a great deal more organisation
and differentiation than in i kilo of elephant, in other words that
the degree of heterogeneity is greater. Whether all mammals can
make unit quantity of differentiation in the same time is a question
one would like to have answered, but which seems to be at the
present time unanswerable.
So far, we have only considered the relation between adult weight
and gestation time. It would obviously be better to use birth-
weights for this purpose, but unfortunately only a few are known
(see Table 60 taken from Przibram). Nevertheless when the birth-
weight is plotted against the gestation time on double-log. paper,
a straight-line relation is found, except for a slight deviation in the
case of the heaviest animals ; this is shown in Fig. 68. The scattering
of the points is evidently considerable, but we may say that there is
some law which ensures that certain limits shall be held to. Thus if
an animal proposes to weigh 100 gm. at birth it must resign itself to
an incubation period of between 40 and 1 50 days, while if it is to
weigh I gm. it may be between 10 and 30 days in utero. Within
these wide limits individual species evidently have the power of
making drastic shortenings or lengthenings.
Thus gestation time alone may not be a very fundamental constant.
In the first place there are great differences in degree of development at
birth between such animals as the pig on the one hand and the rat on
the other, the former being born almost ready to assume complete
motor control of its musculature, the latter by no means ready to
do so ; the former able to see, the latter blind ; the former covered
with hair, the latter hairless. Any relation between weight and
gestation time can therefore only be approximate, and the law
governing it must be, as it were, elastic. Again the difference
between polytocous and monotocous animals will make itself felt,
and the large differences between the relative weights of new-
born and mother. The following table (Table 61), which has
been constructed from Franck's information, shows how large
these are:
SECT. 2]
AND WEIGHT
475
Table 6]
.
Total mass
Weight
of foetal
We
ght of
of one Weight of
tissue formed
mother
new-born mother
I :
X
I : X
^
1
K J
Y
Y
X
X
Man
19-1
191
Horse
14-6
14-6
Cow
15-5
15-5
Sheep
12-9
12-9
Dog
7-5
23-5
Cat
Rabbit
8-9
8-5
37-3
43-1
Pig ... .
8-2
980
® opening of eyes
^ appearance of eye-fissure
" " rodent teeth
And there is also the consideration that gestation time in some
animals must be arranged to suit the grazing season. This factor
would probably account for a good many of the divergences of
species from the line shown in Fig. 68.
Again, within the individual species, birth can apparently be
shifted to some extent backwards and forwards. Bluhm's work shows
that the opening of the eye, the
appearance of the ears, and
other marks of increasing differ-
entiation in the mouse, occur at
a fixed time after conception, so
that the smaller the birth-weight
the longer the time between birth
and the appearance of the mark
in question. This relation is illus-
trated by Fig. 69.
What governs the incubation
times of birds? The problem
has been much discussed, but
by far the best treatment of it in the literature at present is the book
of Bergtold. Of the 19,000 species of birds known, we have information
concerning the incubation periods of 625, and although most of the
facts are given in Table 62 Bergtold's book must be consulted for
the full material. The length of the incubation period varies more or
less with the size of the bird.
Fere long ago pointed out that the smaller the egg the smaller the
incubation time: thus:
Birth weight
Fig. 69.
Duck
Hen
Weight of
Days egg in gm. Ratio
25 739 I : 084
21 6018 I : 0-815
476
ON INCREASE IN SIZE
[PT, III
but this strict relation does not hold for many birds and is even
more elastic than with mammals. Thus the swift and the raven
have the same incubation period in spite of their different sizes,
while the kiwi and the hen are very similar in size but have quite
100
10 100
Weight of Adult Bird in Ounces
Fig. 70.
1000
different incubation periods. The lapwing, again, though smaller
than the woodcock, undoubtedly has a longer incubation period.
Nevertheless, when a broad view of the whole subject is taken, and
the incubation time is plotted against the adult weight on double-
log, paper, a definite trend does appear (see Fig. 70) and the same
100
1-0 lO'O 100
Weight of Egg in Ounces
Fig. 71.
kind of picture is obtained when the incubation time is plotted
against the egg-weight (see Fig. 71). The most interesting thing to
notice is the slope of these two lines, which is in both cases much
less considerable than in the mammalian graph of Fig. 68. In other
words, if the weight of any mammal is multiplied one thousand times,
the gestation period will be prolonged by about ten times, but if
SECT. 2]
AND WEIGHT
477
the weight of a bird is muhipHed one thousand times, the incubation
period will only be prolonged about four times. Similarly if the egg-
weight is increased by one thousand times, the incubation time is only
prolonged four times. No doubt this does not take us very far, but
it is always a step forward to have all the information on one graph.
Table 62. Bergtold's figures.
Weight of
Incubation
adult bird
Weight of
Order
Species
time (days)
(oz.)
egg (oz.)
Struthionidae
Ostrich
36-60
4000
48-60
Dromaeidae
Emu* ...
56-63
—
20
Spheniscidae
Emperor penguin
49
1440
16
Adelie penguin
37
42
4-5
Diomedeidae
Albatross
60
224-288
—
Phaethontidea
Yellow-billed tropic bird
28
14
1-4
Apterygidae
Kiwi ...
42
60-65
14-20
Pelicanidae
Pelican
28
512
White pelican
29
240
—
Ardeidae
Great blue heron
28
96-128
Common heron
25-28
64
Loon ...
29
5'7
Black-crowned night heron ...
24
—
1-2
Ibididae
Wood ibis
21
144-192
—
Anatidae
Domestic duck
27
128
Shoveller duck
28
17
Mallard duck ...
26-28
2-8
Pekin duck
30
—
2-3
Grey wild goose
28
160
Greater snow goose ...
29
80-104
—
Canadian goose
28
128-224
—
Domestic goose
28
—
6
Whistling swan
35-40
192-304
—
Cathartidae
Calif ornian vulture ...
29-31
320
II
Falconidae
Gyrfalcon
28
84
—
Prairie falcon ...
21-28
22-72
—
Western sparrow-hawk
21-28
5
—
European sparrow-hawk
29-30
5-6
05
Eastern sparrow-hawk
29-30
4
Honey-eater ...
21
32
—
American goshawk ...
28
47
—
Western red- tailed hawk
28
48-64
2
Buzzard
28
32-40
—
Red-shouldered hawk
28
32-48
—
Swainson's hawk
25-28
26-56
—
American rough-legged hawk
28
30-33
—
Golden eagle ...
25-35
160-184
—
Bald eagle
28-36
128-192
—
Cracidae
Globose currasow
28
114
8
Megapodidae
Mallee fowl ...
38-41
—
6-5
Phasianidae
Domestic turkey
28
—
3-2
Bobwhite
24
5-5-6-5
Scaled quail ...
21
7-8
—
* Haswell gives 84 days and 21 oz. for this bird.
478
ON INCREASE IN SIZE
[PT. Ill
Table 62. Bergtold's figures (cont.).
Weight of
Incubation
adult bird
Weight of
Order
Species
time (days)
(oz.)
egg (oz.)
Phasianidae
Grey partridge
24
12-13
—
Capercailzie ...
26
184
—
Dusky grouse ...
18-24
40-56
—
Ruffed grouse
24-28
18-40
—
Sage grouse
22
128
—
Wild turkey ...
28
160-288
—
Guinea-fowl ...
25-28
56
1-4
Ring-neck pheasant ...
24
36
1-2
Golden pheasant
21
20-24
10
Silver pheasant
26
—
'■5„
Reeves' pheasant
24
■ —
098
Domestic hen ...
21
64-80
I •9-2-1
RalHdae
American coot
14
16-20
—
Otididae
Great bustard
28
480
—
Charadriidae
European woodcock ...
20
8-27
—
American woodcock ...
20
5-9
—
Common snipe
20
3-8
—
Spotted sandpiper
15-16
1-53
—
Curlew
30
12-14
—
Killdeer
26-28
3-1
0-4
Mountain plover
27
05
Laridae
Lesser tern
14-16
2-0
—
Common tern
21-23
—
06
Columbidae
Band-tailed pigeon ...
18-20
12
—
Domestic pigeon
14-18
10
05
Passenger pigeon
14-16
12
—
Mourning dove
13-14
4-5-6-0
0-4
Cuculidae
Roadrunner ...
18,
II
—
Psittacidae
Cockatoo parrakeet ...
21
2-9
—
White cockatoo
21
21
—
Rose-breasted cockatoo
21
19
—
Blue and yellow macaw
20-25
37
—
Alcedinidae
Belted kingfisher
16-24
5-6
0-45
Strigidae
Long-eared owl
21
11
08
Barred owl
21-28
20-32
—
Screech owl ...
21-25
4-6
0-6
Eagle owl
21-24
112
—
Burrowing owl
21-28
6
—
Caprimulgidae
Western nighthawk ...
16-18
2-7
0-35
Trochilidae
Broad-tailed humming-bird ...
—
o-i
002
Picidae
Hairy woodpecker
14
3
—
Downy woodpecker ...
12
I '5
—
Williamson's sapsucker
14
1-6
—
Red-headed woodpecker
14
2-8
—
Lewis's woodpecker ...
14
3-8
—
Flicker
11-14
4-3
0-25
Tyrannidae
Kingbird
12-14
1-6
0-15
Arkansas kingbird
12-14
1-6
014
Say's phoebe ...
12
09
—
Alaudidae
Horned lark ...
11-14
1-2
—
Turdidae
Western robin
14
33
023
Eastern robin
14
006
SECT. 2]
AND WEIGHT
Table 62. BergtoWs figures (cont.).
479
Order
Species
Mimidae
Catbird
Bombycilidae
Bohemian waxwing ...
Troglodytidae
Western house-wren ...
Laniidae
White-rumped shrike...
Hirundinidae
Barn swallow ...
Tree swallow ...
Cliff swallow ...
Vireonidae
Warbling vireo
Red-eyed vireo
Sittidae
Rocky mountain nuthatch
Pigmy nuthatch
Corvidae
Long-tailed chickadee
Magpie
Long-crested j ay
Crow ...
Miniotiltidae
Yellow warbler
Myrtle warbler
Ovenbird
Redstart
Icteridae
Western meadow lark
Brewer's blackbird ...
Red-winged blackbird
Rusty blackbird
Bronzed grackle
Tanagridae
Western tanager
Fringillidae
House finch ...
Arkansas goldfinch ...
Pine siskin
English sparrow
Western vesper sparrow
Lark sparrow ...
Red-backed junco
Spurred towhee
Black-headed grosbeak
Chipping sparrow
Lazuli bunting
Incubation
Weight of
adult bird
Weight of
time (days)
(oz.)
egg (oz.)
12-13
1-4
0-06
10-16
2-2
—
10
0-5
—
15
20
—
11-13
14
—
0-05
0-06
12-14
—
0-07
12
0-5
—
12-14
0-07
13-14
12
0-65
038
11-14
0-4
—
17
53
— ■
17
16-18
4-0
0-6
10
035
0-04
12-13
0-45
—
12
— ■
0-09
12
—
0-05
15
14
te
0-2
0-18
10-14
I •6-3-0
—
13-16
2-2-5
3-8
12
i-i
—
14
066
0-08
12-14
0-47
—
13-14
0-43
—
12-14
I 05
0-09
11-13
0-9
009
12
095
0-07
11-12
0-7
—
12-13
1-5
—
10
1-3
OIO
10-12
—
0-05
12
—
0-07
Gurney's theory was that incubation time depended on longevity.
The view of Gadow — at first sight more acceptable — was that the
developmental period as a whole was uniform and the longer the
Qgg period the shorter the nest period. Yet this simply raises another
question, and while it is more difficult to determine the total develop-
mental period than the incubation period alone, the difficulty of
relating the preparatory period, whatever it is, to the causal factor,
still remains. No doubt the differences between nidicolous and
nidifugous birds are removed by this means. Glaus' theory was that
incubation period depended on egg-size, i.e. egg-weight, but this
48o ON INCREASE IN SIZE AND WEIGHT [pt. iii
as we have seen is only true within wide Hmits, for the ostrich and
the kiwi have equal incubation lengths, yet the ostrich's egg weighs
3I lb. while that of the kiwi weighs less than i lb. It is natural that
if incubation time depends to some extent directly upon egg- weight,
it should depend upon bird body-weight ; for as Huxley has shown,
the egg-weight varies closely with the body-weight, though the eggs
of large birds are not as large as they should be in proportion. Lastly,
Pycraft had a theory that incubation time depended on yolk-weight,
but as neither he nor anyone else accumulated any data with which
to test the hypothesis, and as it is not in any case a very attractive
one, it may be dismissed at once.
In Bergtold's view the body-temperature of the parent bird is the
important factor. It is likely a priori that the larger the bird the lower
its body-temperature, and a degree or two may make a big difference.
Bergtold gives in his book a long list of bird temperatures and it
certainly seems that the smaller the bird the higher the reading,
but unfortunately the data are as yet too few for it to appear whether
the exceptions noted above as destroying other theories are abolished
on this one. Bergtold's theory is complicated by various taxonomic
considerations, in which he supposes, following Sutherland, that the
higher a bird is taxonomically, the higher its temperature. As the
smaller birds (and mammals) are believed to be the most recent
palaeontologically, this may well be the case.
In favour of Bergtold's view are the experiments of Heinroth who
reported that the eggs of the Egyptian goose hatch in 28 days under
a common hen and in 30 days under a Muscovy duck. It is known
(see Fig. 83 a) that within narrow limits, the speed of embryonic
development in ordinary hen's eggs can be controlled by temperature
regulation. "The diminishing size of birds," says Bergtold, "ac-
celerated the metabolic rate, elevated the body-temperature, and
so shortened the incubation period." According to Bergtold the
scanty data of reptilian incubation times support his temperature
theory.
Returning now to the comparison between mammals and birds
which was raised by Figs. 68 and 71 in which the slope of their
weight/incubation-time lines was seen to be different, it is interesting
to plot the two on the same graph, as is done in Fig. 72. It can
now be seen that not only are the slopes different, but the
absolute values are also different, so that on the whole it takes less
31
482 ON INCREASE IN SIZE [pt. iii
time to make an equivalent birth-weight of bird than of mammal.
(The hatching-weights are here obtained by taking 75 per cent,
of the egg-weight in grams, the remaining 25 per cent, being
of course divided between shell-weight, weight of membranes
left behind, weight of water-vapour evaporated during incubation,
and weight of material combusted in the same period.) It is also
evident that the largest bird is, as regards birth-weight, 250 times
as small as the largest mammal. We are thus left with the following
three considerations (which apply wholly to birth-weight) :
(i) Although there are mammals as small as the smallest birds,
there are no birds as large as the largest mammals. In fact the largest
bird is only a little larger than the half-way point on the mammalian
line.
(2) The time required to make a given weight of bird is always less
than that required to make a given weight of mammal, as may be
roughly expressed by the following table:
Birds
Mammals
Birth- or hatching-
Incubation
Gestation
weight (gm.)
time (days)
time (days)
100,000
—
600
10,000
—
260
1,000
45
150
100
30
55
10
17
32
I
II
14
(3) The prolongation of the incubation time caused by raising
the hatching-weight a given amount is not so considerable as the
prolongation of the gestation time caused by raising the birth- weight
by the same amount.
The bird is therefore much more rapid than the mammal in its
development, and one may well ask whether this is not an adaptation
to life within the egg. In Section 9 and in the Epilegomena the
conception of the "cleidoic" egg will be developed, but without
forestalling those discussions, it may be said here that eggs such as
those of reptiles, birds and insects, with their isolation from their
terrestrial environment, quite unlike the close dependence of many
aquatic eggs upon the sea, are closed systems, characterised, as it
seems, by a definite type of metabolism in which protein breakdown
is suppressed and uric acid takes the place of urea and ammonia
as nitrogenous waste products. If, then, there are serious problems
confronting animals which make their embryos develop in closed
SECT. 2] AND WEIGHT 483
boxes, especially with regard to the disposal of incombustible waste,
is it not possible that their incubation time would naturally tend to
be shorter than that of beings such as mammals which can con-
veniently excrete their embryonic waste products through the
maternal kidneys? It is perhaps justifiable, therefore, to see in Fig. 72
the results of the closed-box system, development inside it being
adaptively hastened. It would be very interesting to have parallel
sets of data for insects and reptiles, and one might predict that they
also would take relatively shorter times than the mammals, but so
far I have not succeeded in finding any data from which graphs
could be constructed.
As for the comparatively mean size of the largest bird at hatching
compared with that of the largest mammal at birth, it has been
probably more than once suggested that eggs above a certain size
would begin to suffer from prohibitive mechanical difficulties. An
egg large enough to produce a bird as big as an elephant at birth
would require, either internal struts, which would be impracticable,
or else an extremely thick shell (see Friese's work, p. 239) which
would raise great difficulties with respect to gaseous exchange. It
is likely, therefore, that 100 days is the extreme limit to which ovi-
parous animals can prolong their incubation time (without hiber-
nating), as against the 600 or more which are possible to mammals.
Is this connected with the extinction of the Aepyornis?
It might well be asked at this point how it was that the extinct
reptiles attained their prodigious size if they were oviparous, and the
answer seems to be that for the most part they were not. Some form
of ovoviviparity was common, judging from the numerous finds of
small skeletons within the abdominal areas of the larger ones. Whether
these were really embryos or perhaps rather remnants of undigested
food is not yet, and probably never will be, certain, but the question
has been discussed by Fraas; Liepmann; van Straalen, and others
and the general opinion is that they should be regarded as embryos.
We may conclude that the relatively rapid development of birds
is an adaptation to cleidoic life, perhaps associated with the high
temperatures of birds.
It is interesting that hibernation of embryos is not unknown. The
best known case of this is probably the silkworm, the embryo of
which spends about 8| months in a more or less quiescent state, not
advancing to any extent with its development. Dendy reported in
31-2
484
ON INCREASE IN SIZE
[PT. Ill
1898 that the embryo oi Sphenodon, the tuatara Hzard, had an incuba-
tion period of 13 months, of which something hke 9 were spent in
a hibernatory state. Boulenger observed much the same thing in
the case of the European pond-tortoise, Emys orbicularis, which has
an apparent incubation period of no less than 23 months. Still more
extraordinary is the case of some mammals which possess the power,
according to Reinhardt and Prell, of hibernating in the embryonic,
partly-completed, state (mole, roedeer, bear, badger, pinemarten,
and stonemarten) . And as for the insects, Regen has shown that the
eggs of a locust, Thamnotrizon apterus, laid in September, hibernate two
or three winters and finally hatch out in March. Hibernation, indeed,
is very common among insect embryos, the mosquito for instance
{Aedes flavescens) occupies 7 months in its ^gg (Hearle).
Table 63. Gestation times of mammals.
Species
s
.a
c
J5
1
3
i-t
u
E
C3
3
i
1-
3
c
0
c
1
3
S *-■
CQ S ca
c3
"Is
Marsupials
S3
0
m
ffi
0,
iS
>
Sg3
0 a
Opossum
—
—
—
8
—
—
13
• —
—
Kangaroo-rat
—
—
—
8-5
—
—
—
—
Small kangaroo
—
—
—
—
—
38
—
—
Large kangaroo
—
—
—
39
—
—
40
—
—
Insectivora
Mole
—
—
—
—
—
29
30
0-283
105-8
Ant-eater
—
■ —
—
190
—
—
—
—
—
Hedgehog
49
49
—
—
—
42
49
0-175
280-0
Cheiroptera
Bat
—
—
—
—
—
34
36
0-028
1280-0
Cetacea
Whale
—
—
365
360
315
—
—
—
—
Dolphin
—
—
300-360
—
—
—
—
Rodents
Mouse
21
21
—
21-23
—
24
25
0-014
1790-0
Rat
35
35
—
21
• — •
30
35
0-340
103-0
Rabbit
28
28
—
30
—
29
30
1-360
22-0
Hare
—
28
—
28-35
—
29
30
3-640
8-2
Squirrel
28
—
—
—
28
30
0-340
880
Beaver
—
119
—
42
—
"9
42
13-600
3-1
Marmot
—
35
—
42
—
35
German marmot
—
28
—
21
■ —
• —
—
—
Guinea-pig
—
—
—
63
—
64
—
—
Even-toed Ungulates
Sheep
147
147
—
150
—
150
150
29-450
5-1
Chamois
154
154
—
—
—
—
140
36-250
39
Gazelle
154
154
—
—
—
—
150
36-250
4-2
Red deer
266
168
— •
240-270
—
280
245
77-000
3-2
Gnu
—
— ■
—
—
—
—
238
226-200
1-05
Reindeer
—
—
—
280-300
—
232
210
126-500
1-7
SECT. 2] AND WEIGHT
Table 63. Gestation times of mammals (cont.),
485
Species £■ >>
Even-toed Ungulates 2w
Elk —
^ ^ c
Llama
Camel
Antelope
Giraffe
Ox
Bison
Roebuck
Zebra
Goat
Ibex
Odd-toed Ungulates
Horse
Ass
Pig
Hippopotamus
Rhinoceros
Elephant
Zebra
Boar
Tapir
Carnivora
Weasel
Otter
Polecat
Puma
Leopard
Marten
Dog
Wolf
Fox
Bear
Cat
Lynx
Panther
Jaguar
Lion
Tiger
Badger
Ermine
Ferret
Seal
Primates
Monkey
Baboon
Gorilla
Man
Chimpanzee
Macacus
Mandrill
Uistiti
Lemur
168
365
305
154
300
300
"9
505
670
63
70
63
210
56
42
168
305
154
300
"9
35
"63
56
63
70
63
210
56
63
70
— 240-270 —
— 330-360 —
— 360-400 —
— 180-210 —
>
266
328
t53
■ -?-
260
300
300
m S «
272-000
1 13-000
181-000
— 315
— 280
— 168
431 249-000
300 590-000
280 816-000
150
150-180
330-350
360-380
120
210-250
510-550
615-628
345-375
120
400
— 151
119
35
63
63
35
63
62
8-160
1-240
92
93
60
63
60
240
56
56
63
92
63
217
50
63
105
105
90
63
63
63
120
56
70
63
— 100
— no
— no
63 -
1-360
22-500
40-700
6-800
135-500
5-430
127-000
113-000
158-000
158-000
74
350
245
280 — —
280
260
160-210
177-210
144 —
2'ia
A^
O a
0-96
2-65
1-7
1-7
0-5
03
356
357
114
. 350
365
100
680-000
362-000
81-800
0-5
i-o
1-2
235
658
235
600
2715-000
1810-000
3625-000
0-09
0-3
O-ID
7-7
50-0
66-1
2-8
1-55
9-3
09
10-3
0-5
0-9
0-7
0-7
210
—
—
210
22-600
9-3
280
136-000
2 06
280
63-000
4-45
486
Man
ON INCREASE IN SIZE
Table 64. Gestation time and incubation time.*
[PT. Ill
Species
Albino rat
Norway rat
" . "*
Guinea-pig
Sheep
,, ...
Cow
Horse
Opossum
Rhinoceros ...
Elephant
Jackal {Anubis pavian)
Dog
Pig
Goat
Coyote {Canis
ockropus) ...
Raccoon {Procyon
californicus) ...
Coatis {Nasua ?)
Opossum {Didelphys
virginiana) ...
Chimpanzee
Mammals
Extreme
variation
Gestation time in days
280
272-5
272-2
272-2 (calc. from date of last
menstruation)
280-5 (conception calc.)
270
279-14 (primipara)
281-99 (multipara)
271-0
282-5 (fo'" male foetus)
284-5 (for female foetus both calc.
from last menstruation)
282-8 (for male foetus)
282-0 (for female foetus both calc.
from last menstruation)
272-6 (for male foetus)
267-5 (for female foetus both calc.
from one definite coitus)
279
21-6-22-64
Gestation time may be pro-
longed from I to 6 days if it is
simultaneous with suckling
21-0
23-5-25-5 (probably suckling)
64
146
145-153 —
275-291 (according to breed) 210-335
334-359 264-420
13 —
540 —
630 —
210 —
60 55-68
120 104-133
151 —
65 -
65
71
13
210-245
137-162
Birds
Investigator
Hippocrates
Leuckart
Lowenhardt
Hasler
Schlichting
Zollner
Ahlfeld
Robertson
— Siegel t
Hecker
Stotsenberg
King
Lantz
Miller
Draper
Schwarz
Sabatini [Brody
Franck-Albrecht and
Ewart
Heuser & Hartmann
Feldman
Heinroth
Schwarz
Asdell
Gander
— Borland & Hubeny
Species
Swan
Goose
Duck
Hen
* See also Tabulae Biologicae, vol. 6.
I Siegel's figures were obtained during the war on the basis of very accurate data (see
also Jolly) .
Incubation
time
in days
Investigator
42
Davy
35
j>
28
>}
21
„
SECT. 2]
AND WEIGHT
487
Table 64. Gestation time and incubation time (cont.).
Birds (cont.)
Incubation time
Species
in days
Investigator
Turkey
28
Davy
Guinea-fowl
30-31
?»
Partridge ...
27
Pheasant
23
Red grouse
23
Pigeon
14
Turtle-dove
14
Canary
13
Wren
••• ..• •
10
Martin {Hirundo urbica)
12-13
Meyer
Swift {Hirundo apus)
16-17
Eagle-owl [Bubo maximus)
21
Goshawk (Astur palwnbarins)
21
Sparrowhawk {Accipiter fringillarius)
21
Stockdove {Columba oenas)
17
Turtledove {Columba turtur)
16-17
Pheasant {Phasianus colchicus)
24-26
Cock-of-the-wood {Tetrao urogallus)
28
'Rldick gTonse {Tetrao tetrix)
21
Vheasant {Tetrao perdix) ...
21
Swan {Cygnus olor)
35-42
Wild duck {Anas boschas) ...
29
Humming-bird
12
Milne-Edwards
Hen
21
Duck
25
Cormorant ...
25
Guinea-fowl
25
Turkey
27
Goose
29
Peacock
31
Swan
42
Cassowary ...
65
Turkey
29
Evans
Guinea-fowl
25-26
3 J
Duck
28
33
Partridge ...
25
J3
Humming-bird
12
Schenk
Calcutta hen
27
jj
Peacock
31
3>
Stork
42
)5
Cassowary ...
65
J>
Goose
29
»
Duck
21
J»
Guinea-fowl
21
33
Blackbird {Turdus merula)
15
Evans*
'Redstart {Ruticilla phoenicurus)
14
j>
Robin {Erithracus rubecula)
13-5
)»
Sedge-warbler {Acrocephalus phragmitis) .
15
j>
Great tit {Parus major)
14-9
»
Pied wagtail {Motacilla lugubris) ...
14-5
>9
Swallow {Hirundo rustica) ...
155
»
Skylark {Alauda arvensis) ...
135
»
Common tern {Sterna fluviatilis) ...
21-5
»
Redshank ( Tetanus calidris)
23
»>
Stormy petrel {Proc
ellaria pelagica)
36
3
* According to Evans the data in Giglioli's report are not reliable.
488
ON INCREASE IN SIZE
[PT. Ill
Table 64. Gestation time and incubation time (cont.).
Species
Condor {Sarcorhamphus gryphns) ...
Buzzard {Buteo vulgaris)
Turkey
Hen
Duck
Pigeon
Rhea {Rhea americana)
Buzzard {Buteo buteo)
Kestrel {Cerchneis tinnunculus)
Pheasant {Phasianus colchicus)
Nightingale {Turdus philomelos)
Yellow-hammer {Emberiza citrinella sylvestris)
Chaffinch {Fringilla coelebs)
Goldfinch {Acanthis cannabina)
Robin {Erithracus rubecula)
Hedge-sparrow {Prunella modularis)
Lesser whitethroat {Sylvia curruca)
Sedge-warbler {Acrocephalus phragmitis) ...
Golden vulture {Gypaetus barbatus)
Common pigeon: ist egg of the clutch ...
„ ,, and egg of the clutch...
Birds (cont.)
Incubation time
54 days
31
26-29 „
19-24 „
29-32 „
17-20 „
Marine turtle {Thalassochelys corticata)
Python {Python molurus)
Loggerhead turtle {Caretta caretta)
Green turtle
Loggerhead turtle {Caretta caretta)
European tortoise {Emys europaea)
Tuatara {Sphenodon)
Crocodile ...
Alligator
Python {Python reticulatus)
Python {Python molurus)
Black snake
Fox snake ...
Corn snake
Yellow rat snake ...
Ring snake
Milk snake ...
King snake
Coral snake
Dogfish {Scyllium catulus) ...
,, {Scyllium canicula)
Ray {Raia batis)
Herring
Sturgeon {Acipenser stellatus)
Plaice
Flounder
Trout {Salmo fario)
American flounder {Pseudopleuronectes
americanus)
Japanese salmonoid {Plecoglossus altivelis)
Pike
Reptiles
29
30
28-5 „
25
13-5 »
12-5 „
13
II
14
12
II-5 »
12-5 „
53
16-42 „
16-89 »
47 days
2i months
64 days
8^ weeks
8i „
20-44 „
52
12
8i „
6-8 „
10 „
8* „
74-8i „
6-8 „
11 „
6 „
8 „
6-8 „
Fishes
157-178 days (av. 1 69) Bolau
.. 234-280 ,, (av. 261)
Investigator
Broderip
Anonymous
Vignes
Rozanov
Groebbels & Mobert
Schumann
Cole & Kirkpatrick
Tomita
Cunningham
Hildebrand & Hatsel
Bergtold
Dendy
Bergtold
Detmers
Bergtold
9-10 months
12
44-80 hours
6-15 months
3-7
205 days (2°)
82 „ (5°)
41 „ (10°)
26 „
10-25 days
9
Beard
Hoflfmann
Derjavin
Dannevig
Haempel
Scott
Nakai
Kvasnikov
SECT. 2] AND WEIGHT
Table 64. Gestation time and incubation time (cont.
Crustacea
Species
Wa.teT-t\ea. (Daphnia pulex)
Lobster
Amphipod {Gammarus chevreuxi) ...
489
Incubation time
75-85 hours
ID months
8-9 days
Insects
Cockroach (Periplaneta orientalis) ... ... 75 days
Tent-caterpillar moth {Malacosoma americana) 9-5 months
Sarcophagidae ... ... ... ... Less than a day
y{ous,e ^y {Musca domestica) ... ... 8-12 hours
Lepidoptera ... ... ... ... Several months
Phasmidae ... ... ... ... 2 years
Ant {Aphaenogaster fulva) ... ... ... 17-22 days
,, {Myrmica rubra) ... ... ... 23-24 ,,
Dragonflies ... ... ... ... 3 weeks
Oriental peach-moth {Laspeyresia molesta) 4 days
Butterfly [Diacrisia virgimca) ... ... 5-5 ,,
^■win&-\ouse {Haematopinus sui) ... ... 14 ,,
Beet Leafhopper {Eutettix tenellus) ... 10-50 days
Stonefly {Perlodes mortoni) 91 days
,, (Ptrla carlukiana) ... ... ... 59 ,,
,, {Nephelopteryx nebulosa) ... ... 20 ,,
Mayky {Siphlurus armatus) 15 weeks
,, [Ecdyurus venosus) ... ... ... 15 days
Molluscs
West African land-snail {Achatina variegata)
Periwinkle {Littorina littorea)
Worms
Roundworm {Ankylostoma duodenale)
40-60 days
6 days
-3 days
Investigator
Ramult
Allen
Ford & Huxley
Zabinski
Rudolfs
Imms
Fielde
Needham
Snapp & Swingle
Johannsen
Weber
Severin
Percival &Whitehead
Pycraft
Haves
Looss
Table 65. Time-relations of early development.
Amphibians
Wilson's figures (15° C.)
Species
Salamander {Amblystoma punctata)
Frog {Rana temporaria) ...
Small wood-frog {Chorophilus tri-
serratus)
Hertwig's figures (14° C.)
Fertili-
sation
to first
cleavage
(hrs.)
10
35
1-5
Cleavage
periods
(min.)
1st
no
75
30
2nd
100
60
40
Rana temporaria
Gastrulation
Formation of medullary plate
Closure of medullary folds
Appearance of tail-bud
Appearance of tail and gills
Appearance of tail-fin
Beginning of operculum ...
Gastru-
lation (hrs.)
3rd
100
50
30
Begin-
ning
60
42
End
78
50
13
Days after
fertilisation
1-3
4-2
5-5
6-8
8-6
no
13-4
Appear-
ance of
external
gills
(days)
19
14
35
Adult
(days)
100
70
30
490 ON INCREASE IN SIZE AND WEIGHT
Table 65. Time-relations of early development (cont.).
Molluscs
A. Richards' figures:
[PT. Ill
(Opisthobranch)
Planorbis
Time Intervals
from in
fertilisation minutes
Haminea virescens
, ' ,
Time Intervals
from in
fertilisation minutes
Fertilisation ... o
1st cleavage ... 90
2nd cleavage ... 165
3rd cleavage ... 240
4th cleavage ... 315
5th cleavage ... 375
E. G. Conklin's figures:
90
75
75
75
60
ASCIDIAN
o
85
136
195
245
285
Cynthia
85
51
59
50
40
Fertilisation
1st cleavage
2nd cleavage
3rd cleavage
4th cleavage
5 th cleavage
6th cleavage
7 th cleavage
8th cleavage (218 cells)
Hatched tadpole ...
Time Intervals
from in
fertilisation minutes
o
40
70
90
no
130
150
170
190
310
40
30
20
20
20
20="
20
20
120
* Beginning of gastrulation.
The inner significance of the length of embryonic life relative to
the life-span is most obscure. Some interesting remarks have been
made by Moulton, who has pointed out that over the whole life-
span, the chemical changes are much more intense in the earliest
periods, i.e. pre-natal and to a certain extent post-natal. This reminds
If
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492
ON INCREASE IN SIZE
[PT. Ill
US that Murray's law only applies to the embryonic period, and
must not be extended beyond it (see p. 548). A typical graph is
that shown in Fig. 73 where the composition of the whole human
being as regards water, nitrogen and ash, is considered. The point
of sudden cessation of intense chemical redistribution was termed
quite logically by Moulton "chemical maturity", and this point, he
found, bore a fairly constant relation to the total life-span, as appears
from the following table :
Table 66.
Moulton's figures:
Species
Man
Cow
Pig ...
Guinea-pig
Dog
Cat ...
Rabbit
Rat ...
Mouse
This, however, was the only relation which did show any constancy,
and very little can be deduced about an animal if only its gestation
time, or conversely, only its average length of life, is knov/n.
Even its composition at birth is not related simply to any of the
other variables. These facts, to which Moulton was the first to draw
attention, illustrate the truth of the statement just made, namely,
that the act of birth or hatching is a comparatively unimportant one
in the life of the individual. Probably the time at which it takes place
in the life-span has been much involved with the adaptations due
to different modes of life, while yet the underlying process of physico-
chemical maturation has remained unaffected.
Extremely few researches have been done on these problems. In
1926 I estimated the non-protein nitrogen in a variety of bird
embryos, of different incubation periods, with a view to ascertaining
whether the rhythm of chemical differentiation went on at a constant
rate in all cases. The curve shown in Fig. 74 is from the data
on non-protein-nitrogen of White Leghorn embryos. Similar data
were obtained for two other races of domestic hen (21 days), the
xt
J2
•< 0
0.
a>
C
0
«
>
<
tion age at maxi-
mum of 3rd
growth cycle
(Brody & Rags-
dale) (days)
60 ,_,
•3 ^
.2 "
g"S
ut» e
Part of life-span
passed at chemi-
cal maturity % of
total life-span
Constitution of
body at birth
A
^0
1
0?
a
'v
u
o5
<
285
80
5300
1285
4-4
82
14
3
285
25
850
435
4-6
76
18
4
120
20
200
345
4-6
82
13
3
64
7
145
114
4-6
78
17
4
61
17
261
4-3
82
14
3
60
1 1
—
160
39
83
13
3
31
10
185
—
84
13
2
25
4
86
75
4-5
88
10
2
20
4
62
86
II
3
SECT.
2]
AND WEIGHT
493
pigeon (i8 days), the guinea-fowl (25 days), the duck (26 days), the
partridge (27 days), and the turkey (28 days). The results were
related to weights of embryos equal to those of the chick embryo
on different days, and as can be seen, the values for the other embryos
fell uniformly on the chick curve. The length of incubation would
thus appear to be a tune played as it were, " adagio " in the turkey and
"allegro" in the pigeon.*
o
EI
D
m
H6N : MuK.ttU^Ko^
Partridse:
Hen : \NKUre Wyo>«Ulte
Pigeon
DUCK
Turkey
guinea-fowu
Hen • Ba/»*»««ei<Ur
% of total incubation time
Fig. 74-
The possible adaptive significance of incubation time has been
shown remarkably in a paper by Friedmann, who found that the
incubation times of the cowbirds, such as Agelaioides, Molothrus and
Tangavius, varied according to their degree of parasitism. M. afer,
which is very parasitic, has an incubation time of 10 days (the
* For this it was necessary to assume that the pre-natal growth-curves for wet weight
were alike, an assumption which was subsequently shown to be legitimate by Kaufmann.
The instantaneous percentage growth-rates of the pigeon and the hen do not begin to differ
until after hatching ; and this holds, according to her, for heart, liver and eye, as well as
for the whole body. The cells of the pigeon embryo, however, are smaller than those of
the chick by about 30%.
494
ON INCREASE IN SIZE
[PT. Ill
shortest known), M, bonariensis, less parasitic, takes 11-5, and M.
rufo-axillaris , still less so, takes 12-5 to 13 days.
No consideration of the questions involved in incubation and
gestation time would be complete without mention of the work of
Rubner. Rubner found that, if a graph is constructed having the
durations of pregnancy of different groups of animals as abscissae
and the respective birth-weights of their young as ordinates, the
resulting curve is quite smooth and regular. These facts, which have
already been discussed, are shown in Fig. 75, taken from Rubner.
A glance shows that the only exception among the animals which
Rubner chose is man, who develops very slowly, and does not attain
at birth more than a quarter of the weight he should if he resembled
other animals. Rubner considered the anthropoid apes to be more
like animals than man in this matter, but an observation of Heinroth's
makes this doubtful. It has often been pointed out that small
birth-weight is probably an adaptive feature of considerable ad-
vantage to the human female. The actual figures are as follows :
Table 67.
Period
in days
during
which
Gestation
the new-born
period
Weight of
doubles
ts weight
in days
Weight of
new-born
,.
*■ "^
(Thiel;
maternal
Weight of
in % of
(Abder-
Landois;
organism
new-born
maternal
Animal
(Bunge)
halden)
Khmmer)
in kilos
in kilos
weight
Horse
60
60
340
450
500
I I'D
Cow ...
47
47
285
450
35-0
8-5
Sheep
12
15
154
50
3-9
7-8
Man...
180
180
280
55
30
55
Pig ...
16
14
120
80
2-4
30
Dog ...
8
9
63
—
2-0
Cat ...
9
9
56
—
— .
—
Rabbit
6
6
28
—
—
—
Guinea-
pig
—
—
67
0-62
0-087
14-2
Mouse
—
—
21
0-02
0001 7
8-5
Average
7-6
Rubner studied the heat production of the new-born animals, and
found that the larger the birth-weight the smaller the number of
Calories put out by i kilo of body-weight per day. Thus for a new-
born animal weighing 50 kilos, i kilo of body- weight would produce
34*2 Calories, for one weighing 25 kilos the production would be
42-6 Calories, for one of 12-5 kilos it was 60 Calories, and for one
of 6-25 kilos it was 66-6 Calories. The explanation of this obviously
was that, since no supply of energy can be built up in toto into
SECT. 2]
AND WEIGHT
495
potential energy stored in the tissues, but must to some extent be
wasted in upkeep metabolism, a wastage would be expected in the
case now under consideration, and as the relative surface of the new-
born is larger the smaller it is itself, this wastage would be expected
to be greater. Rubner then assumed that the heat production
of the embryo was about seven-tenths of the new-born animal,
presumably on the basis of premature birth figures, but also on
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Fig. 75-
theoretical grounds drawn from the embryo's existence in a thermo-
static hydrosphere unaccompanied by much muscular or digestive
exertion. This assumption seems a dangerous simplification, in view
of the peak in basal metabolism which we now know to exist in
some animals before and in some animals after birth. However,
Rubner calculated the intra-uterine heat production as follows :
Birth-weight of
animal (kilos)
50-0
25-0
12-5
625
Cals. put out by
I kilo * per day
23-9
298
42-0
46-6
* I.e. seven- tenths of the post-natal figures.
496 ON INCREASE IN SIZE [pt. iii
Averaging the whole of development, Rubner divided each of these
figures by 2, and, taking the duration of appreciable development at
six-tenths of the whole (the first four-tenths being a negligible
quantity), he multiplied each by six-tenths of the total gestation time
in days, thus:
Birth-weight of
animal in kilos
50-0
25-0
12-5
6-25
Total
tion
Average no. of Calories
gesta- put out by i kilo of
time finished embryo
340
250
205
177
x6/io development
204 2631
150 2235
123 2583
106 2470
Average ... 2480
Thus I kilo of an imaginary animal (it would be very like a horse),
which weighed 50 kilos at birth, would eliminate 2631 Calories
during its whole gestation period, while i kilo of an imaginary
animal which weighed 6-25 kilos at birth would eliminate 2470
Calories, its relatively more intense heat output being compensated
for by its shorter gestation period. Rubner called this relation the
"fundamental law of intra-uterine developmental energy".
In the above discussion, we were dealing with imaginary examples,
but Rubner calculated out the figures for actual animals, as follows :
Average no. of Calories
put out by I kilo of
embryo throughout
Animal
Birth-weight
development
Horse
50-0
2028
Cow
35-0
1915
Sheep
3-9
2728
Pig
2-4
2210
Dog
2318
Average ... 2240
Now if we assess the calorific value of the formed kilogram
of finished embryo at 1504 Calories, 2480 plus 1504 Calories, i.e.
3984 Calories, are required for the formation of i kilo of embryo
during intra-uterine life in the higher mammals. Rubner spoke of
a "growth-quotient" in this connection, defined as follows:
Energy stored in the tissues x 100
Energy stored plus energy given off as heat *
In the case of pre-natal life, it was 1500/4000, i.e. about 38 per cent.
There is a certain contradiction here between this efficiency datum
SECT. 2] AND WEIGHT 497
and the conclusions of other workers (for which see the section on
energy relations), and it is quite certain that the efficiency changes
during development. Rubner's law of constant energy requirement
during gestation time does not equate well with the findings of
Brody, Moulton and others, but it must be remembered that the
"law" rests on a very inadequate basis, nothing more, in effect,
than a few rough calculations in which some doubtful assumptions
are involved.
Rubner emphasised the fact that the value of about 4000 Calories,
which seem to be required to make one kilo of tissue during em-
bryonic development, is lower than the corresponding value of about
4800 for post-natal life. From this it would appear that the efficiency
is greater before birth than afterwards, but this statement needs
much qualification. Rubner also showed that the energy consumed
per kilo in doubling the birth-weight is approximately the same for
different animals. Thus:
Energy consumption per
kilo in doubling
Animal the birth-weight, Cals.
Horse
Cow
Sheep
Pig
Cat
Rabbit
4>5i2
4^243
3>926
3,754
4>304
4,554
Average ... 4j2I5
Man ... ... ... 28,864
Robertson has shown that the generalised form of this rule would be
— = a. log X ■\- h,
X
where E is the energy-consumption, x the weight of the animal
and a and b constants which are the same for all species except
man.
But we have digressed already too far from the problem of growth.
Before taking up the effects of external agents on the growth-rate,
it will be worth glancing for a moment at the relations between
gestation time and the life-span. Buffon in the eighteenth century
and Flourens in the nineteenth maintained that the life-span was
five or six times the youth-period (see Lusk), but Weissmann showed
that there were too many exceptions to this rule to make it of any
N EI 32
498 ON INCREASE IN SIZE [pt« iii
use. Rubner's book contains a discussion of this matter, from which
it is to be concluded that, at present, there is no rule which covers
all the cases. [See also Hollis and Szabo.]
Rubner's "second law", in which he laid down that the total
amount of heat eliminated from birth to death in all the higher
mammals is the same, i.e. about 191,600 Calories per kilo, with
the exception of man, for which the value was 725,700 Calories
per kilo, has often been criticised adversely, and does not here con-
cern us. But it is to be observed that it contradicts the lack of relation
found by Moulton and others between simple gestation time and
life-span, in that, according to Rubner, a constant percentage of the
total heat eliminated during life has been eliminated at birth, i.e.
2500 out of 191,600 or 1-305 per cent. This proportion does not, of
course, hold in the case of man, but will be greater because of his
prolonged stay in utero.
Our immediate aim must now be to examine the effects of various
physical influences upon the rate of growth of the living embryo,
for by the aid of such a study one may hope to penetrate further into
the fundamental nature of the process. That chemical influences can
also exert a great effect on the growth of the embryo is obvious, but
they will be dealt with in succeeding chapters, such as the sections
on vitamines, and general metabolism. At present the discussion will
be strictly confined to the effects of radiant energy (heat and light)
on the rate of growth, and all teratological considerations will be
kept in the background.
2' 14. The Effect of Heat on Embryonic Growth
The accelerating influence of rise of temperature on embryonic
growth was known to William Harvey, though in an unformulated
kind of way, for referring to differences in gestation time, or what he
calls "the diversity of going with Child", he says: "For the same
thing befalls them as happeneth to Plants, whose fruits and seeds,
do more slowly and seldom arrive to maturity in cold Countries,
than do other Plants of the same kind which are in a fat and warme
soile. So Orenges in England adhere to the trees almost two whole
years together, before they come to maturity: and Figgs also scarce
ever arrive at any perfection here, which are ripe in Italy twice or
thrice a year. And the like befalleth the fruits of the Womb". But
perhaps the earliest quantitative observation of the effect of heat on
SECT. 2] AND WEIGHT 499
development was that of Gaspard, who in 1822 constructed the fol-
lowing table :
Developmental
Temperature time of snail's
(°C.) eggs in days
6-8 45
12 38
20 21
Gaspard also worked on frog's eggs. Davy and Coste, both in 1856,
published some figures showing the acceleration of development of
the eggs of the salmon in warmed water. Most of the early work
was, of course, fragmentary, and for various reasons unsatisfactory.
Thus Philipeaux in 1871 observed that the hatching time of axolotl
eggs was shortened from 25 to 8 days as warmth increased but he
did not take the temperatures, and Vernon stated in 1895 that the
optimum temperature for the development of the embryos of
echinoidea was from 7° to 22°, outside which limits they rapidly
became abnormal. Semper found that nauplii of Branchus and Apus
hatch out at a temperature of 30° in less than 24 hours, whereas at
16-20° they require some weeks. Again, lobster larvae reared at
23-27° passed the fourth moult in about 10 days, i.e. 3 days earlier
than larvae reared at 19° C., according to Herrick. Cuenot gave the
following figures for the hatching time of the locust :
°c. ...
... 25
20
15
10
Days
... 50
55
60
65
Table 68, partly from Davenport, shows the time taken in em-
bryonic development at various temperatures for certain fishes.
As we shall see further in the section on resistance and susceptibility,
there are to be found among embryos very obvious adaptations to
the environment in which they are to grow ; thus Rauber has shown
that the eggs of minnows and salmon which develop during the winter
will not grow at all at temperatures much above 12-15°, but will
do so at 0°. The critical points below and above which normal
development will not go on have been determined for amphibians
and birds by various observers, and are tabulated in Table 68. The
optimum temperature may be regarded as that at which the smallest
number of abnormal embryos are produced ; on each side of it the
amount of teratological effect will more or less rapidly increase, while
at the same time the rapidity of development will on the one hand
be increased and on the other hand be retarded. When a certain
32-2
500
ON INCREASE IN SIZE
[PT. Ill
Tab
•le 68.
Days
taken
in development.
Temperature
water (°C.)
of
Cod
(Earll)
Herring
(Meyer and Kupffer)
Shad
(Rice)
— 2- O-O
30-0
—
—
o- 1-9
2- 39
4- 5-9
6- 7-9
8- 9-9
IO-II-9
325
22-0
160
130
40
II
—
13-5
20-23-0
14-20-0
E
7
II
3-5
Early quantitative observations.
Time taken to reach definite stage, 38° taken as unity [Rana temporaria)
Temperature °C. 20 21 22
23
24
25
26
27 28 29
30 31
Edwards' figures o-oi o-oi 0-02
0-02
0-03
003
0-04
0-07 0-I2 0-15
0-35 0-55
Temperature °C. 34 35 36
37
38
39
40
41 42
Fere's figures 0-65 o-8o 0-72
10
I -00
1-25
1-51 0-0
Lillie & Knowl ton's figures:
Increase in length in mm. from 24
to 48
hours after hatching.
°C.
Rana virescens
Bufo lentiginosus
9-1 0-9
4-5
3-0
11-12-9
5-3
5-3
13-14-9
4-3
15-5
15-16-9
—
16-3
17-18-9
9-5
■ —
19-20-9
19-8
21-2
21-22-9
—
—
'
23-24-9
■ —
41-3
25-26-9
31-5
39-0
27-28-9
40-0
—
29-30-9
47-5
56-8
31-32-9
40-2
55-3
33-34-9
43-5
—
Higginbottom's figures. Frog [Ranafusca):
Temperature °F.
f
■^
Date
60
56
53
51
March 11
Egg
Egg
Egg
Egg
20
Hatch
23
External gills
—
—
—
25
—
Hatch
—
—
27
Internal gills
—
—
—
28
—
External gills
—
—
31
—
■ —
Hatch
Hatch
April 4
—
—
External gills
—
6
—
Internal gills
■ —
External gills
10
Big tadpole
—
^
—
May 22
Metamorphosis
Big tadpole
Internal gills
Internal gills
Aug. 18
—
Metamorphosis
—
—
28
—
—
Metamorphosis
—
Oct. 31
—
—
—
Metamorphosis
SECT, 2]
AND WEIGHT
501
Table 68 [cont.).
Marcus' figures. Sea-urchin {Strongylocentrotus lividus).
Days from
fertilisation
o
7
22
29-5
47-5
555
70-5
80
95-5
118-5
Temperature °C.
9
17-19
22
Egg
Egg
Egg
Blastula
Blastula
Gastrula
Bias tula
Mesenchyme
Gastrula
Blastula
Gastrula
Pluteus
Mesenchyme
Pluteus
—
Beginning of gastrulation
Old pluteus
—
Gastrulation
—
Gastrulation
End of gastrulation
Prismatic gastrulation
—
—
Environmental temperatures.
Temperattire
A
°C.
Species
Minnow
Salmon
Sea-urchin [Echinus esculentus)
Frog (Rana palustr is)
,, (Rana vires cens)
Toad (Bufo lentiginosus)
Hen {Callus domesticus)
Salamander {Amblystoma
tigrinus)
Frog {Rana fusca) ...
Hen [Callus domesticus)
Green bug [Toxoptera gram-
nium)
Texan cattle-fever tick [Mar-
garopus annulatus)
Brown-tailed moth [Euproctis
anysonhea)
Bombycid moth [Samia cecro-
pia)
Mealworm [Tenebrio molitor)
Colorado potato-beetle [Lep-
tinotarsa decemlineata)
Pine bug [Dendrolimus pini)
Hen [Callus domesticus)
Rhea [Rhea americana)
Locust [Melanoplus differen-
tialis)
Locust [Melanoplus femur-ru-
brum)
* Atkinson considers that
heat is conveyed to the eggs
thin sheet of rubber.
Mini-
mum Optimum
o —
2-5
3
6
25
o
17-22
30
32
38
Maximum
12-15
12-15
42
o —
28 35-39
29 —
20-5 —
- 37-6 (ist week)
38-3 (2nd „ )
39-1 (3rd „ )
1-65 —
7-5 —
II —
11 —
9 —
12 —
14 —
- 38-3*
- 38-5
18 36
18 36
43
32
32-5
28
Investigator
Rauber
>j
Vernon
Morgan
Lillie & Knowlton
Rauber
Lillie & Knowlton
Schultze
Kaestner
Prevost & Dumas
Edwards
Eccleshymer
Hunter & Glenn
Hunter & Hooker
Sanderson
Regener
Phillips & Brooks
Rozanov
Bodine
better results in artificial incubation are obtained when the
by conduction, not radiation, e.g. by covering them with a
502
ON INCREASE IN SIZE
Table 68 [cont.).
Environmental temperatures (cont.).
[PT. Ill
Temperature ° C.
A
f
Mini-
"^
Species
mum
Optimum
Maximum
Investigator
Japanese cuttlefish {Omma-
12
15
18
Sasaki
strephes sloani)
Frog {Rana fused) ...
—
20 rising to 2^
—
Hertwig *
Toad (Bufo lentiginosus)
—
28 „ 31
—
j>
Frog {Ranafusca) ...
—
—
23 rising to
30
King*
„ {Rana esculenta)
—
—
33
34
J)
Toad {Bufo lentiginosus)
—
—
33
38
35
Roundworms :
{Toxascaris limbata)
6-7
—
38-40
Zavadovski & Sic
{Ascaris megaloeephald) . . .
6-7
—
38-40
9J J>
{Ascaris suilla) ...
7-8
—
36-38
{Ankylostoma duodenale)
lO
20-30
32
Looss
5> 5J
—
21-25
Svensson
Trematode of Japanese Bil-
—
27-5
—
Miyakawa
harziosis {Schistosomum ja-
ponicum)
Japanese teleost {Hypomesus
6-0
—
17-5
Nakai
olidus pallas)
Goosefish {Lophius amerieanus)
4-0
lO-O
i8-o
BerrUl
Plaice {Pseudopleuronectes
4-5
—
Scott
amerieanus)
Salmon {Salmo irideus)
3-0
6-0
I2-0
Kawajiri
* Hertwig's and King's figures are interesting because Rana fusea and temporaria lay
early in spring when the water is often freezing: Rana palustris and esculenta later, and
Bufo lentiginosus later still. Giglio-Tos has emphasised the importance of this in ecology.
degree of external heat is reached, the mechanism of development
will be irreversibly interfered with and the growth-rate will suddenly
drop. On the cold side the growth-rate will fall off steadily. It is
necessary to distinguish these two types of effect, for it is only the
latter that concerns us here. According to the kinetic theory, rise
in temperature leads inevitably to an increased vigour of molecular
movement, and, as is well known, the extent to which this happens
gives a measure of the extent to which the process under examination
is physical or chemical. For in the former case the amount of increase
in molecular motion will only exert a direct effect in speeding up the
process, but, if the possibility of a chemical combination is present
to complicate matters, the effect will be much greater. The older
workers believed on these grounds that it would be very simple to
determine whether the nature of any given "master reaction" in
living matter was physical or chemical, but maturer consideration
and extended experiments have shown that such determination is
SECT. 2] AND WEIGHT 503
attended with very great difficulty. Bayliss has drawn attention
forcibly to this. The position as regards the embryonic growth-
process is therefore doubtful, and, although its temperature coefficient
has been many times estimated, we cannot yet be certain what the
real significance of this is. Nevertheless, the recent researches of
Crozier and his school have brought us nearer to a sound judgment
upon the matter.
2-15. Temperature Coefficients
The older quantitative observations were few in number and not
very accurate; we owe them to Fere; Lillie & Knowlton; Higgin-
bottom; Driesch; Chambers; Edwards; Semper; and Bury. Some of
them are shown in Table 68. They were too few in number to lead
to any well-based conclusions, though they certainly demonstrated
the fact that, within certain limits, the higher the temperature, the
higher the growth-rate. But the classical paper on this subject is
that of O. Hertwig, who in 1898 subjected developing frog embryos
to various temperatures. He had been preceded by Baudrimont
& de St Ange, who, as early as 1846, had observed the accelerating
effect of temperature on the developing egg of the frog. His figure,
which has often been reproduced, is shown as Fig. 76, and from it
one can easily see that the time taken to reach the seventh stage, for
instance, is 16-7 days at 20° but 55-6 days at 10°. The time taken
at the lower temperature, therefore, is just 3-33 times as long as
that at the higher temperature, so x^° of the van't Hoflf equation
Vt
= X"
will be 3-33, where Vt is the reciprocal of the weight gained at a
certain temperature and V {t -\- n) is the reciprocal of the weight
gained at n degrees higher temperature. Therefore 10 log a: =
log 3-33 = 0-5224; therefore log a: = 0-05224 and a; =1-128. In
other words, if m days are taken to complete a certain stage of de-
velopment at 10°, it will take m x 1-128" days when the temperature
is n degrees less for the same stage to be arrived at, D'Arcy Thompson
calculated all the values of Hertwig's experiments from this simple
exponential formula, and obtained a series of curves convex to the
abscissa, which showed fair agreement with those plotted from the
experimental observations.
504
ON INCREASE IN SIZE
[PT. Ill
Here 1-128 is the temperature coefficient for 1° (in the common
terminology Q.i) but, as it is usual to employ the expression Q,io ^.s
24.0 23° 22° 21° 20° 19° 18° 17° \&°- 15° 14° 13° 12° 11° 10° 9° 8° 7° 6° 5° 4° 3° 2° 1°
Fig. 76.
the temperature coefficient, the value for the frog is 1-128^°, i.e.
3-34, or, in other words, the time taken to reach a definite stage at
20° multiplied by 3-34 will give the time taken to reach the same
SECT. 2]
AND WEIGHT
505
stage at 10°. Thus the velocity of the process is more than tripled
by a rise of 10° in the temperature, as would ordinarily be expected
in the case of a chemical reaction. Judging from Hertwig's curves
alone, one should theoretically be able to increase the speed of the
growth to infinity by heating the system up, but this of course is
not the case, and Fig. 77 taken from Faure-Fremiet's work on
Sabellaria eggs shows how, after a certain temperature is reached,
the growth-rate may get slower again, owing to the destructive effects
of excessive heat on protoplasm. The subject was thoroughly gone
into, apparently at Abegg's suggestion, by Peter in 1905, who made
many measurements of the development of echinoderm eggs and
calculated out the temperature coefficients of Hertwig's results more
correctly. His results were as follows:
Table 69.
Sphaere-
^
chinus
Echinus
Ranafusca
Q.1
2-137
First cell-division
2-040
2-352
2-277
Second cell-division
2-404
2-402
1-758
Third cell-division ...
2-264
2-437
1-956
Gastrula
2-490
2-277
2-509
Medullary plate
3-487
2-177
2-272
Closure of medullary folds ...
3-701
2-625
2-042
Appearance of tail-bud
4-102
1-575
1-830
Appearance of external gill
3-151
1-600
1-609
Appearance of tail-fin
3- 1 38
—
1-831
Beginning of operculum
3-245
—
1-707
—
1-546
2-cell stage ...
4-cell stage ...
8-cell stage ...
i6-cell stage ...
32-cell stage ...
Blastula
First mesoderm
First spicule ...
Full spicules ...
Prisma
Young pluteus
Old pluteus ...
From these figures Peter concluded that the average tempera-
ture coefficients of the three embryos were not very different,
i.e. Sphaerechinus 2-15, Echinus 2-13, and Rana 2-86. He regarded
the similarity between the values for the echinoderms as significant.
He observed also that Q,io ^^^ i^ot quite a constant : thus between
2'5 and 14-5° it was 3-28 (average) and between 14 and 24° it
was 2-26; or in other words that the development at low tem-
peratures was rather slower than it ought to have been if the
same temperature coefficient held for all temperatures. Next Peter
noted that Q_^q was not the same for each stage of development,
for, on the whole, the average coefficient for the cleavage pro-
cesses was higher than that for the later events. This rule, however,
5o6
ON INCREASE IN SIZE
[PT. Ill
only held for the two echinoderms studied and not for the frog,
thus:
Cleavage stages Later stages
Sphaerechinus
Echinus
Rana ...
2-29
2-30
2-23
2-03
2 -08
3-34
Such a comparison obviously suggested interesting conclusions in
view of the fact that the two alecithic eggs behaved in the same way
30'
29
28
27
26
25
24
23
22
21
20
19
]8
o 16
c14
11
10
9
7 -
6 -
5 -
4
3
2
1 h
20 40 60 80 100
120
2
150
200
Minutes
4 Hours
Fig. 77-
while the yolk-laden one did not. Peter's final procedure was to
set side by side a quotation from van't Hoff, in which chemical
reactions were said to be increased 2| times for each rise of tem-
perature by 10°, and the overall average of his own work, i.e. 2-499.
Nor did he fail to point out that in inorganic chemical reactions also
the QjyQ increased as the temperature was lowered.
Subsequent workers found many cases in which embryonic growth
followed the exponential rule. Erdmann & R. Hertwig gave figures
SECT. 2]
AND WEIGHT
507
for Strongylocentrotus lividus, from which Kanitz calculated temperature
coefficients agreeing with Peter's. Bialascewicz confirmed the work
of Hertwig on the amphibian embryo, but obtained a lower average
temperature coefficient, namely, 2-40. For the early development of
Strongylocentrotus lividus Loeb found in 1908 an average Qj^q of 2-86,
which varied thus with temperature:
°c.
aio
3-13
391
4-14
3-88
5-15
3-52
7-17
3-27
9-19
2-04
10-20
1-90
12-22
1-74
For the early development of Ascaris megalocephalus, Faure-Fremiet
found Ohio's as follows :
0-16 6-25
16-23 393
23-32 I 82
and for that o^ Sabellaria alveolata:
°c.
Q.10
7-13
2-66
2-l6
16-19
1-28
19-22
1-27
Ephrussi (in 1926) studied the different phases of mitosis in sea-
urchin and nematode eggs, allotting to each a characteristic tempera-
ture coefficient. Thus he obtained the following figures, working
always between 18 and 25° C. and calculating for each period
separately, not in a cumulative fashion from zero hour.
Table 70. n
A (from zero hour to disappearance of nuclear membrane, zero being
copulation of pronuclei in S. lividus and laying in A. megalocephala
B (from disappearance of nuclear membrane to the first appearance
of the equatorial plate)
A plus B, i.e. the whole of the prophase
C (duration of the equatorial plate stage)
Z) (duration of the whole of the anaphase)
E (reconstitution of nuclei after mitosis)
A plus B plus C plus D plus E, i.e. the whole mitotic process
B plus C plus D plus E, i.e. from disappearance of nuclear membrane
to reconstitution of nuclei after mitosis
Strongylo-
centroius
lividus
Ascaris
megalo-
cephala
I 2-31
2-20
?
1-66
2-33
i-oo
2-07
1-22
1-71
2-00
1-39
1-70
1-33
I -80
I •41
1-45
5o8
ON INCREASE IN SIZE
[PT. Ill
As Ephrussi says, the correspondence can hardly be a mere coin-
cidence, and in any case the fact that change of temperature has
almost no effect on process C must be significant.
We do not know whether the effect of increased temperature is
greater on the mitotic or inter-mitotic period. Bucciante main-
tained chick embryo explants at 31° and at 41° and counted no more
mitoses during a given amount
of growth in the latter than in ^
the former, so he concluded that '~
the effect was the same.
Loeb & Wasteneys found 2-3
for the embryonic growth of
Arbacia and Bohr 2-9 for that of
the snake Coluber natrix. Warburg
gave 2-5 for the respiratory in-
crease of Arbacia eggs. Herzog,
working on the figures of Dan-
nevig for the hatching time of the
plaice egg {Pleuronectes platessa),
got a Q^io of 2-5, on those of Earll
for the cod egg [Gadus morrhua)
3-4 and 2-3, and on those of
Ainsworth and Metzger for two
different kinds of trout egg 3-4
and 5-3. Later, Bachrach &
Cardot obtained a value of 3-2
for the embryonic growth of the
slug Agriolimax agrestis between
6 and 23°, and one of 3-06 for that of the water snail Limnaea stagnalis
between 1 1 and 32°. For Ascaris megalocephala Zavadovski got a Q^^q of
2-8, but it was markedly higher at low temperatures than at higher
ones. Then later, Zavadovski & Sidorov gave Q^io (18-28°) 3-12 for
Toxascaris limbata, 3-12 for Ascaris megalocephala and 3-23 for Ascaris
suilla, but 1-45, 1-46 and 0-73 respectively between 28 and 38°. Brown
has also published a Q^^q for Ascaris, Berrill for Lophius americanus,
Harukawa for the oriental peach-moth, Kawajiri for Salmo irideus,
Nakai for Plecoglossus altivelis, Kojiyama for Pagrosoma major.
But the older phase of the work on the effect of temperature on
embryonic growth could not lead to any definite conclusions about
SECT. 2]
AND WEIGHT
509
the nature of the Hmiting factor or master process, in view of the
fact that Q^^Q itself was not a satisfactory standard. When a physical
process of diffusion (Nernst) could have a coefficient practically
identical with a chemical process (saponification of ethyl butyrate
by baryta, Trautz & Volkmann), both being about 1-3, it was difficult
to say what might be producing a given Q,io • But more serious was
the fact which soon became apparent, namely, that many cases of
embryonic development could equally well be said to be simply
linear functions of the temperature. Krogh was the first to find this.
In considering the figures which had been obtained by Krogh &
Johansen and by Dannevig on the rate of development of fish eggs
and the hatching times at diflferent temperatures, he found that the
relation with velocity was a straight line and not an exponential
curve, and that the relation with time was a curve which was
not exponential but was a hyperbola. This is seen in Figs. 78 and
5IO ON INCREASE IN SIZE [pt. iii
79, where the Hnear relationship clearly appears, and the velocity
increment is directly proportional to the temperature increment. In
1913-he continued this line of thought by finding that the eftect of
temperature on (a) the rate of segmentation in the frog's egg, {b) the
rate at which definite later developmental stages in the frog embryo
are reached, and {c) the hatching time of a water-beetle, Acilius
sulcatus, could in all these cases also be expressed by a straight line.
In the case of the frog, as Krogh pointed out, it would have been
absurd to express this linear relation in terms of Q^^q , for Q^^q would
thus become:
3-5
5-3
5-10
4-1
10-15
30
15-20
2-0
and could hardly be said to be a constant if it was constantly
varying. Krogh did not compare his results with those of Hertwig,
for the latter used rather different technique, e.g. transferring
his embryos and eggs very slowly and by stages to the constant
temperature basins. This procedure Krogh did not find necessary,
though he observed that the limits of normal development are
narrower during the earlier stages than they are later. The linear
relationship thus found deviated, however, to some extent below 7°.
Exactly the same straight line was found when the velocities at
which later developmental stages were attained were plotted against
the temperature, though here divergence began as high as 12°. The
hatching time of the water-beetle eggs behaved in the same way.
Next Krogh took the figures of Loeb and Loeb & Wasteneys for
echinoderm eggs already mentioned, and found that they also were
best expressed by a straight line when —. ; was plotted
■^ ^ in mmutes
against temperature. These results are all summed up in Table 71,
which gives the geometrical characters of the straight lines, i.e. their
slopes, together with other data about the embryos in question. In a
further paper Krogh showed that the relation between the tempera-
ture and the carbon dioxide production of the chrysalides of Tenebrio
molitor, the mealworm, could also be best expressed by a linear relation,
and was not susceptible of description by the van't Hoff formula.
At this point the question became a matter of dispute concerning
the proper way of calculating the results obtained in such experi-
SECT. 2]
AND WEIGHT
511
ments, Kanitz contested the truth of Krogh's view that the van't
HofF equation was inapphcable, on the grounds that the straight
Hnes might be the result of two or three flat exponential curves.
(Kanitz seems not to have avoided confusion between the time taken
for development and the reciprocal of the time taken.) Kanitz
plotted the log. of the velocity (log 1000/^) against the temperature,
and obtained two straight lines in each case, just as Crozier did
afterwards, with kinks between 10 and 15°. He assumed therefore
that the Arrhenius formtila could be used and calculated Q^^q from it.
Table 7 1 . Effect of temperature on embryonic development.
Krogh's figures;
Embryo
Cod (Gadus morrhua
and aeglifinus)
Plaice {Pleuronectes
platessa)
Cod {Gadus merlangus)
Plaice {Pleuronectes flesus)
„ {Pleuronectes
platessa)
Sea-urchin {Strongylo-
centrotus purpuratus)
Frog {Rana butyrhina)
>J 5!
Water- beetle {Acilius
sulcatus)
Sea-urchin {Arbacia)
2^ a
1 J3
« 0
«
feicia
a!:c3
temp
een w
Itionsl
d
temp
een w
ent w
ill
J3 M°
^5 S
C4
„ a! "H M
•M U.S2
■(J
.t;.Q u „
.S-^.2c
%%-
•2^
i-i?
esi>°
Investi-
Process
£;§-§
>-3
J3:S.g
•r! 3 01 0
iJ «T3 Ml
gator
Fertilisation to
-3-3
12-5
3-5-14
-I-I4
Dannevig
hatching
Do.
-2-1
11-6
6-5-12
jj
Do.
O
i8-8
55-14
—
3)
Do. .
-1-4
30-3
6-5-12
•
5J
Fertilisation to
-0-4
13-8
3-5-13
0-14
Johansen &
4-9 mm. larva
Krogh
Cleavage i
i-o
0-99
4-20
3-5-22
Loeb
Cleavage 2
—
1-57
—
,,
Cleavage i
2-7
0-511
7-21
3-5-22
Krogh
Later devel.
6- 1
—
12-21
7-5-25k
?j
Fertilisation to
9-4
0-65
15-25
15-28
55
hatching
Cleavage i
9-2
I 03
15-28
7-31
Loeb &
Wasteneys
Kraf ka in 19 1 9 found that in Drosophila the effect of temperature
on rate of development could be expressed best by a straight line,
but its effect on eye-facet number could be expressed best by an
exponential curve. The appearance of both forms in one and the
same living material made it unlikely that either relation needed
correction. It was more probable that the formulae themselves were
inadequate for proper analysis. Other lines of approach were made
by entomologists on the one hand and by marine biologists on the
other. Thus Reibisch worked over the original results of Dannevig,
and concluded that a certain amount of heat was necessary for
development and had to be supplied from outside, forgetting that
[uj LIBRARY
512
ON INCREASE IN SIZE
[PT, III
no matter how hot the environment, the egg gets all its energy
for development from the inside. Apstein, who noted with Krogh
that the time/temperature relation was linear, i.e. the product was
a constant, formulated another "day-degree theory", which, how-
ever, was no improvement. On the Reibisch-Apstein view, a certain
sum of heat, a certain number of temperature units or day-degrees,
is necessary to complete embryonic growth. Thus if an embryo
hatches after lo days at 12° 120 day-degrees would be said to have
been required, so that at 6° 20 days would be required. Krogh &
Johansen were easily able to show the superficiality of such a treat-
ment.
Williamson's experiments with herring, haddock and plaice eggs
also seem to show a linear
time/temperature relation, but
the variations are large. Elm-
hirst studied the hatching time
of Leander squilla and the de-
velopment of various decapod
Crustacea, and observed a con-
stant time/temperature factor.
He also went over the large
number of fragmentary papers
on the effect of temperature on
the development of fishes which
exist in the literature, mostly
in Fishery Board Reports, and
in many cases, though not in
all, succeeded in finding evidences of the Krogh straight-line relation-
ship between velocity and temperature. He thought that this emerged
more clearly if i -9° were added on to each temperature in order to have
the freezing-point of sea water as a basis. But, on the other hand,
in many cases, such as that of the cod (Fig. 80, taken from Fulton
from various official Reports) the time/temperature curve could cer-
tainly be said to have a temperature coefficient (2-3 for 10-20°
and 3-5 for 0-10°). Blunk working on Dytiscus embryos, and Blunk
& Janisch working on those of Blitophaga opaca, obtained straight
lines like Elmhirst for velocity/temperature graphs, i.e. hyperbolas
for time/temperature graphs.
Sanderson has reviewed the work that has been done on the
Fig. 80.
SECT. 2] AND WEIGHT 513
effect of temperature on the insect embryo, and has in many
cases recalculated the figures of earlier observers, such as those of
Abbe : hatching of the Rocky mountain locust [Melanoplus femur-
rubrum) .
Regener : hatching of the Pine bug {Dendrolimus pini) .
Hennings : hatching of the Typographic beetle ( Tomicus typographus) .
Quaintaince & Brues : hatching of the bollworm {Heliothis obsoleta) .
Girault : hatching of the bollworm {Heliothis obsoleta) .
Jenne; Hunter & Glenn: hatching of the codling moth [Carpocapsa
pomonella) .
Jenne ; Hunter & Glenn : hatching of a Braconid parasite [Lysi-
phlebus tritici) and of the green bug ( Toxoptera gramnium) .
Hunter & Hooker : hatching of the Texan cattle-fever tick [Mar-
garopus annulatus) .
All these data without exception, together with figures collected by
Sanderson himself on the incubation oiEuproctis, Samia, Tenebrio, Lepti-
wo^arj"a,Afa/(2coj'oma, etc., gave curves closely resembling those of Hertwig
for the development of the frog. Thus the Q^^q for Jenne's codling
moth curve would work out at approximately 2-8 between 14 and
24°, and that for Samia cecropia would be i-g or so between 20 and 30°.
In most cases the usual rise in temperature coefficient with decreasing
temperature was shown, and in Samia, for instance, Q^io"^ould be 2-5
between 10 and 20°. It is extraordinary that Sanderson made
no calculation of temperature coefficients, but he was particularly
interested in the practical application of his work and computed his
results according to an empirical day-degree system which, as Martini
showed, has no physico-chemical meaning, and even confuses in-
tensity with quantity of heat. It is curious that ichthyologists on
the one hand and entomologists on the other should have evolved
very similar treatments of time/temperature curves both equally
unsatisfactory. The temperature coefficients for these insect curves
would work out somewhat as shown in Table 72. Peairs later
tried to show that, for certain insects, the curve relating time taken
in embryonic growth with temperature was a hyperbola, and he did
indeed, over a short range, demonstrate the reciprocal to be a straight
line, as, if that were the case, it should be. Peairs paid no more
attention to the temperature coefficient question than did Sanderson.
Precisely analogous experiments and results were obtained by Blunk
working on Dytiscus marginalis and by Bodine on various grasshoppers.
NEI 33
514
ON INCREASE IN SIZE
[PT. Ill
Table 72.
Temperature coefficients of insect development.
Species
°C.
Q.IO
Investigator
Browntail moth {Euproctis chrysorrhea) . . .
16-26
2-0
Sanderson
Bombycid moth {Samia cecropia)
10-20
2-5
JJ
>> jj
20-30
1-9
,,
Mealworm {Tenebrio molitor) ...
10-20
3-8
JJ
,, ,,
20-30
2-1
JJ
Colorado potato-beetle {Leptinotarsa
io-20
2-5
JJ
decemlineata)
JJ JJ JJ
20-30
2-0
,,
JJ JJ JJ
20-30
2-2
Girault
JJ JJ JJ
20-30
2-0
Girault & Rosenfeld
Pine hug (Dendrolimus pini)
15-25
1-4
Regener
Typographic beetle ( Tomicus typographus)
15-25
2-9
Hennings
Bollworm (Heliothis obsoleta) ...
10-20
4-4
Quaintaince & Brues
JJ JJ
20-30
2-0
,, ,,
Codling moth {Carpocapsa pomonella) ...
15-25
2-8
Jenne
Braconid parasite {Lysiphlebus tritici) ...
O-IO
2-3
Hunter & Glenn
JJ JJ JJ
10-20
33
JJ JJ
Green bug {Toxoptera gramnium)
O-IO
2-5
)j J)
JJ JJ
10-20
33
JJ JJ
Texan cattle-fever tick {Margaropus annu-
10-20
3-1
Hunter & Hooker
latus)
JJ JJ JJ
20-30
21
JJ JJ
J) 5) 'J
30-40
1-8
JJ JJ
Codling moth {Carpocapsa pomonella) ...
15-25
2-7
Hammar
Gypsy moth {Porthetria dispar)
5-15
4-9
Peairs
JJ j>
15-25
1-9
JJ
Locust (Melanoplus dlfferentialis)
,, {Melanoplus femur-rubrum)
23-33
2-5
Bodine
23-33
2-4
,,
Green-striped locust {Chortophaga viridi-
23-33
2-8
JJ
fasciata)
Bachmetiev made some experiments like those of Sanderson
and Peairs, and Kanitz reviewed all the older literature in 191 5.
Kanitz also suggested that the variations found in the pregnancy
time of mammals, and especially man, might be related to the
differing body-temperature in individuals, for 2° at a Q^iq of 2-5
would in man make a difference of 10 days. This notion was sup-
ported by O. Wellmann, who went through the statistics of preg-
nancy time in mares and found that, while the average was 326 days
from July to September, it was 343 days from March to May. Since
the researches of Cobelli; Vos; Congdon; and Sumner have all shown
that the body-temperature is raised to a slight degree when the
environing atmosphere is warm, the gestation time might very
well be expected to show the seasonal rise and fall which it
actually does. Vicarelli's thermometric observations support Kanitz's
theory.
SECT. 2] AND WEIGHT 515
2- 1 6. Temperature Characteristics
The older phase of the subject ended, then, in a rather barren
doubt as to whether the van't Hoff equation was apphcable to hving
processes such as those of the growing embryo, or more correctly
whether the time/temperature relation was best expressed by a curve
of exponential form or by a hyperbola. In so far as it was applicable,
it gave definite information that the limiting factor of embryonic
growth was probably chemical rather than physical, but that was
all. Snyder's paper of 1908 introduced a new period, that of the
use of the Arrhenius equation. This expression is
K^ = K^eHk'i) or ^ = eHk-fX
where K^ and Kj^ are the velocity constants of the reaction in question
at the high and low temperatures, Tg and 7"i, chosen respectively,
e the base of Napierian logarithms, 2 the gas constant, and /a the
gram molecular energy of activation of the catalyst, i.e. the "critical
increment of the active substance" if the reaction is monomolecular,
or the sum of the gram molecular energies of the substances if the
reaction is bimolecular. The temperature is expressed in degrees
absolute. If the velocity constants cannot be calculated, the reci-
procals of the time taken to do a definite amount of embryo formation
or other work may be used instead, so that the relation becomes :
T2- Ti = 10, then -^ = d^^
where ^^ and ^2 ^re the times in question, e.g. 10 days from fertilisa-
tion to hatching at one temperature, 20 days at another. When
of the van't Hoflf equation. The relation may also be expressed:
r log Ko — log K-,
a = 4'6l —2 — 2 2 — i.
II
It was originally suggested by Arrhenius as an empirical description
of the facts, but it now has through the work of Rice; Rodebush;
33-2
5i6 ON INCREASE IN SIZE [pt. iii
and Thomson, a solid theoretical basis. The quantity fx is called the
temperature characteristic in distinction from the temperature co-
efficient, for it is the calculated "energy of activation" of the active
molecule in the reaction governing the slowest process in the complex
chain of processes under investigation. It is an index, therefore, of the
nature of the catalyst in operation, and should do much more than
merely distinguish between chemical and physical processes. Reactions
having the same catalyst have the same temperature characteristic.
For chemical reactions ju. varies between 4000 and 35,000, and can
be represented graphically by the slope of the straight line or lines, if
there is a break, relating the log. of the velocity in question to the
reciprocal of the absolute temperature. The break indicates that one
reaction of the catenary has ceased to be the slowest and another
has taken its place. The steeper the slope, the greater the increase
of velocity with unit rise, and the higher the value of fju. The log.
effect/reciprocal of absolute temperature relation does not always
give a straight line, but only if the process that is being measured is
irreversible. If an equilibrium effect is functioning, then the relation
will be a curve. Finally, fju has nothing to do with the Q^ of the
van't Hoff equation, for the former has reference to heat of activation
and the latter to heat of reaction.
The advantages of the Arrhenius equation over the van't Hoff
equation are considerable. The former has a much greater range of
values dealing with thousands instead of decimal units. The Arrhenius
equation also reveals abrupt breaks in the straight lines relating
log. effect to the reciprocal of absolute temperature at which one
limiting reaction is supposed to take the place of another ; these are
masked by the van't Hoff equation. Then Q,io is not a constant
while jLt is, at any rate between certain definite temperatures where
the breaks occur. But most important of all, Q^^q values cannot be
definitely associated with specific types of chemical reaction, such as
oxidation-reduction, hydrolysis, and synthesis, so that all one can
hope to find out by the van't Hoff equation is whether the process
is physical or chemical, while with the Arrhenius equation one may
discover, perhaps, of what nature the controlling change is at any
given moment.
Arrhenius himself in 191 5 made many applications of this equation
to biological processes, and it is interesting that he gives as an average
value for segmenting eggs fx 14,100, but without any reference or
SECT. 2] AND WEIGHT 517
even any intimation whether this was based on results obtained by
himself or by others. The matter is curious, for 14,000 is, as will
be seen, not now regarded as a zone occupied by growth processes.
But the equation was first applied in physiology by Snyder, who
calculated /x for various temperature/time effects in muscle action,
and compared them with others from various sources. Thus he found
a /x of 12,800 calories for the experiments of Peter with echinoderm
egg-cleavage and of 16,600 for those of Hertwig on amphibian egg-
cleavage. He did not, however, carry the question much further,
and it was left for Crozier and his collaborators, in a long series of
papers, to work out the temperature characteristic for a great number
of processes. Before discussing the findings in the case of embryonic
growth, we must outline the groups into which Crozier found he
could separate living processes as regards their temperature charac-
teristic.
The temperature characteristic of many respiration processes,
both in vivo, such as the oxygen consumption of Arbacia eggs, and
in vitro, such as the combination of oxygen with haemoglobin, was
about 16,600. In many cases an abrupt break in the straight lines
would occur; thus below 15° /^ for the oxygen consumption and
ciHary activity of mussel gills was 16,000 and above 15° it was
11,000, which latter value held at all temperatures tried for the
oxygen consumption of the guinea-pig uterus. On the other hand,
the rate of progression of all kinds of invertebrates usually had
a /A of between 12,000 and 12,500, and such phenomena as
the frequency of the firefly flash, the frequency of invertebrate
heart beats, and the frequency of noise production also came
in this group. A /x of 12,000 would seem to indicate the pre-
dominance of a nervous factor, and of 16,000 the predominance of
an oxidation. Other processes gave various values of /u,, e.g. 9240
for rate of movement of algae, from 4700 to 10,300 for protoplasmic
streaming in plant cells, and 7900 for rate of pulsation of infusorial
contractile vacuoles. These would possibly indicate the appearance
of a physical factor, probably diffusion. Rate of nerve conduction
gave 8080 and 10,700. It is probable that even a difference between
16,000 and 16,700 is significant, for the former may indicate that the
controlling catalyst in the oxidation is iron, while the latter may
indicate that a dehydrogenation mechanism is at work; again, the
value 11,500, which some oxidations show at some temperatures, is
5i8
ON INCREASE IN SIZE
[PT. Ill
closely associated with hydroxyl ion catalysis. Glaser suggested that
the value of 8000 which he found for a phase of Paramoecium loco-
motion was associated with hydrolysis. Crozier & Stier found that
the [x for various processes in the grasshopper was quite different after
decapitation to what it had been before, changing from 7900 to
16,500, and showing that, after central nervous system control had
been removed, an oxidation was the slowest reaction. "The velocities
of vital processes", said Crozier, "are determined by the velocities of
dynamically linked systems of chemical transformation differentially
affected by external conditions." Table 73 gives a resume of the
commonest [x values and their significances.
Table 73.
Significance
4,7001
8,000 I
9,000 f
1 0.000 j
11,300
1 2.000 1
I2,500[
16,000)
i6,6ooJ
18,000
20,000
24,000
27,000
Physico-chemical
Hydrolysis or diffusion
Biological
( Protoplasmic streaming
< Rate of nerve conduction
(Rate of moving algae
Hydroxyl ion catalysis of oxidations Oxygen consumption
Breaking of i sulphur linkage Nervous factor
Catalysis by iron dehydrogenation
Activation of iodine
Hydrogen ion catalysis
Certain hydrolyses
Oxidation, oxygen consumption
Growth
In 1926 Crozier marshalled all the values that had been obtained
for p, in biological processes into a frequency polygon, from which
it appeared that the commonest types of fx were roughly 8000,
11,000 and 16,000; above that the peaks or modes were not so
striking, for the observations were fewer. He also marshalled all the
values that had been obtained for breaks in temperature charac-
teristics at definite temperatures into another frequency polygon,
from which it appeared that 15° was by far the most important of
these, but that 8°, 20° and 30° were also associated to some extent
with abrupt breaks in the values of /x. Less common were breaks
at 4*5°, 25° and 27°. That both these polygons are definitely multi-
modal is a fact which makes the whole position more convincing
than it might have been; for, if chance had been operating alone,
there would have probably been but one peak.
SECT. 2]
AND WEIGHT
519
These preliminaries concluded, one can examine the results of
Crozier for growth in general and embryonic growth in particular.
He found on examining the figures for the growth at different
temperatures of Drosophila melanogaster embryos (and larvae) (Loeb
& Northrop and Krafka) and of the pupae of Tenebrio molitor (Krogh)
that there were two temperature characteristics, 27,000 below 15°
and about 10,000 above 15°. This was evidence that whatever the
00034
COOdD
00056
Fig. 81. Temperature characteristics of early development in the frog. A, Krogh's data
(first cleavage) ; B, Lillie & Knowlton's data (from first cleavage to disappearance of
yolk-plug); C, Lillie & Knowlton's data (from other cleavages to disappearance of
yolk-plug) ; D, Krogh's data (later development) .
limiting factor in growth was, it was not an oxidation process, a
conclusion that was strongly supported by the data of Loeb &
Wasteneys and Loeb & Chamberlain for the velocity of segmentation
(first cleavage) of echinoderm eggs. This worked out at 41,000 below
about 11°, 21,000 between 11° and 16°, and 12,400 above 16°.
Krogh's data, again, for the segmentation of frog's eggs, gave
22,600 below 13-5° and 10,200 above 13-5°. In no case, therefore,
was a typical oxidation temperature characteristic obtained for the
earliest stages of embryonic growth.
520
ON INCREASE IN SIZE
[PT. Ill
Crozier himself emphasised the view that these facts opposed
the theory of metabohc gradients (see Section 3 '8), and, in
so far as that theory holds that such tissues as have the highest
metabohc rate (respiration intensity) must also be growing the
0.0035
Vj° <abs.
0.00d6
Fig. 82. Temperature characteristics of early development in teleostean fishes.
A, By Dannevig's data {Gadus); C, Krogh & Johansen's data {Pleuronectes);
D, Higurashi & Tauti's data {Hypomesus) ; E, Higurashi & Nakai's data {Plecoglossus) .
fastest, Crozier's facts do oppose it. Other work on the temperature
characteristics of growth confirmed that of Crozier; Brown, for
instance, found jit's of 28,000, 17,210, 7410 and 19,800 for the adult
development of cladoceran arthropods, though in one instance it
must be admitted that he got a value of 16,950 {Simocephalus serru-
latus) .
In a later paper, Crozier considered fully the results of many other
SECT. 2] AND WEIGHT 521
investigations on the effect of temperature on embryonic growth.
Figs. 81 and 82 show some of the results. Fig. 81 includes the data
of Krogh; Lillie & Knowlton; and Hertwig on early development
in the frog, and Fig. 82 includes the data of Dannevig; Krogh &
Johansen; Higurashi & Tauti, and Higurashi & Nakai on develop-
ment in various teleost fishes. The regularity with which values are
obtained for /x of about 20,000 is remarkable; thus the first cleavage
o£Rana eggs has 21,900, the third cleavage to yolk-plug disappearance
in Rana has 22,000, the total incubation time oi Hypomesus olidus has
23,700 and of Plecoglossus altivelis 23,000. Blunk's figures for the rate
of development of Dytiscus marginalis give 19,300 and of Dytiscus
semisulcatus 20,000. Ziegelmayer's figures for the rate of develop-
ment of Cyclops give 15,700, which is on the low side. Bliss's figures
for Drosophila give a very similar result. The fact that the same
temperature characteristic holds good for various different periods
during one continuous developmental process obviously suggests that
the shape of the curve must be much the same at different tempera-
tures. The developmental process as a whole is therefore not deformed
by change of temperature, but simply lengthened or shortened, its
shape, as it were, remaining the same, or, in other words, either only
one velocity constant is involved, or else, if more than one, velocity
constants of the same temperature characteristic. To what extent
this is really the case will appear presently.
The only work on the temperature characteristic of the develop-
ment of the chick is that of Brody & Henderson, who studied the
effect of incubation at different temperatures upon the growth-rate
of the embryo. Fig. 83 a taken from their paper shows how at any
given day during development the weight attained is greater the
higher the temperature; thus at the 17th day the average weight
at 35-0° C. is 5-510 gm., at 37-3° 12-685 gm. and at 40-5° 20-427 gm.
The upper part of the same graph shows the log. weight plotted
against the age, as is usual in Brody's method, and the slopes of the
resulting series of straight lines give the instantaneous percentage
growth-rates, as follows :
Phase or cycle
I St
2nd
3rd
°c.
{k)
{k)
{k)
40-5
70
3^
20
37-3
56
36
24
35-0
62
34
20
34-4
46
22
—
522
ON INCREASE IN SIZE
[PT. Ill
The instantaneous percentage growth-rate is shown in Fig. 83 b
related to the temperature. The effect of temperature on the earliest
^
3ms.
30
20
10
>>f.,
o105°F=40-5°C
A 99°F=37-22°C
A 95°F = 35°C
X 94° F = 34'44°C
• Temperature un
^^
y-:%
~^.
\
known
/ (
"C
y\
fl
90
f-^
zy
y
>
>y
^
8
A
' ^
if
i,
y
y
/
/
y
i
/
/
/
y
5
/
/
/
J
/
o?>^
''.*.
/
fy V
^
X
3
. 2
/n-V
/a
/
Xy
y
r?
^
i
-y
^
i
1^
f
'\^y
/
y
V
• Gms.
/
) J
K
Y
/
^
/
X
y
y
r
35
30
'/
y
/
5 1
J^S>/-
;^^
1
I
1
f
/
4
i
ti^/z
•8
^
' J
1
7
t '
k
^
/
/
/
/
T/l
25
/^Li^jL^
/
/
y
/
•A
/
/
\(
yLif'7 <3'/_
'
/
y
A
W
• ^
i
y
y
20 -^
•3
vf
^A
/
y
r
J
r
/
f'^
/
1
\
/
-5?
15^
(
•2
,/ /I
y
I
(
w
b
/^
Y
y
V
k
>^
/
1
10
•1
>
^
l\
p
/
.'O
H
r
^■^
^
y
y
<
r-1
-^
^
i
H
*-<
^
--^
0
*D£
^s
5
i
3
1
1.
0
1
.ba
Fi
2
bio
1
a.
4
9^
1
6
1
8
2
0
phase of growth is obviously the greatest of the three ; in other
words, in the earlier stages rise in temperature has a proportionally-
greater effect on the instantaneous percentage growth-rate than in
the later stages. In the intermediate stage the rise is not so steep
SECT, 2]
AND WEIGHT
523
adalb
body-
temp.
100K
70
65
60
j= 55
50
45
40
35
i. 30
25
20
15
with rise of temperature, and in the last stage, i.e. from about the
14th day onwards, the effect is nil, for the line is horizontal; or, in
other words, the heat-regulating mechanism in the embryo is suffi-
ciently developed at this stage to enable it to keep its body-tempera-
ture constant within the limits of the experimental temperatures so
that they do not get a chance of affecting its growth-processes. It
could indeed be argued that the
three curves show a progressively
increasing command of body-
temperature, so that higher
temperature coefficients and
steeper growth-rate/age curves
are inevitably found the younger
the embryo is. This question of
temperature-regulation will be
taken up again later in the book
(see Section 4'2i).
From these data the /x con-
stants of Arrhenius' equation
was calculated. For phase i it
was 25,700 below 37° and 7100
above it, while for phase 2 it was
39,050 below 37° and 6500 above
it. These values agree well with
those obtained for other in-
stances of embryonic growth, a
fact which is especially important
in view of the restriction of the
other data to amphibia, fishes
and arthropods. For the third
phase there was obviously no
^
v^^
'by^
X,
K
"V
/
/
/
^^t
\J0.
&
"^2
/
/
/
V
14
-21
dax^s
X3
35^
37=
39°
Temperature (c)
Fig. 83 b.
41° 41-2'=
temperature characteristic. Thus, before the age of about 13 days and
between the temperatures of 35 and 37° increase of temperature by
ten degrees more than doubles but does not quite triple the rate of
growth, while after the 14th day the growth of the embryo is practically
unaffected by change of temperature owing to its acquired homoio-
thermicity.
Taking the values which have been obtained for /x for growth and
development, it is clear that they all cluster rather closely round
524
ON INCREASE IN SIZE
[PT. Ill
certain definite magnitudes. Fig. 84, constructed from the data of
all the relevant investigations, shows a frequency polygon for the
values of fx associated with growth. Crozier's polygon for all fj,
values (286 investigations) is described by a continuous line, and
demonstrates the well-known modes at 8000, 11,000 and 16,000, as
well as the smaller ones at 20,000 and above. These include all bio-
logical reactions which have been studied at different temperatures.
But when only such fj, values as have been obtained for growth are
o
CT»6
O 4
C
— Crosiers frequency
polygon of all jx's.
M Embryonic growth.
— Growth as a whole
I.e instances of
adult + emb.
l\
l\
I 1
/ 1
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34
JU XIOOO
Fig. 84.
plotted, the picture is quite a different one, for the modes do not
correspond at all with those of the wider series. For the growth-
process, they exist at about 10,000, from 17,000 to 21,000 and at
27,000. The first of them falls between two peaks of Crozier's polygon,
and the second, though represented there, equals, in this narrower
consideration, the great 16,000 oxidation mode. The growth-
frequency polygon obviously has little to do with oxidations, which
are here graphically seen not to be the limiting factors in the growth-
process. If now embryonic growth alone is considered, an even
narrower situation is disclosed, for embryonic growth has a peak
at 10,000, practically nothing at all between 13,000 and 19,000, and
SECT. 2] AND WEIGHT 525
after that a sustained high series until 25,000 is reached, followed by
a spurt at 27,000. In a word, the peaks of growth as a whole and
embryonic growth are very much alike, namely at 10,000, 20,000 and
27,000, but the middle one of these is specially associated with the
development of the embryo. Now there can be no doubt but that the
velocity of growth (reciprocal of the time required to attain a given
size or stage of development) can be expressed well in its relation
to temperature by the Arrhenius equation. Unfortunately, there is
difficulty in deciding what is the exact meaning of the frequency
with which the value of 20,000 occurs. There seems to be no reason
why this should be so usual a feature of the time/temperature
relation in embryos, and the modes at 10,000 and 27,000 are equally
mysterious. No doubt it may be expected that further work, both
in the direction of precision of data and in the direction of explana-
tion of the significance of these values, will clear the matter up. The
identification of the reactions responsible would certainly be a
desirable thing. At present there is only one suggestion as to the
meaning of a /i of 20,000, namely, its association by Glaser with
"mobilisation hydrolysis", i.e. the production of oxidisable sub-
stances by hydrolytic action such as the formation of glucose from
glycogen. The mode at 10,000 might be ascribed to synthetic processes,
but this would apply better to the growth-values of 7000 or thereabouts.
"I am quite unable to attach possible specific significance to tem-
perature characteristics 8000 and 27,000", says Crozier in a private
communication. "It will be realised that in the case of the earlier
statements one is dealing with somewhat uncertain suggestions of
which the chief value may be that they serve to suggest specific
experiments. I should personally tend to deplore theories of the
physico-chemical control of diflferentiation and development which
might very easily be erected on the basis of such speculation." In
these circumstances all that can be done is to note the values of fi
which have been so far obtained for embryonic growth without
attempting to give them any meaning.
Perhaps the most important result of this kind of work is that no
embryonic growth-process so far investigated turns out to have a
temperature characteristic of 16,000, which is the typical one for
oxidations. In this connection the paper of Loeb & Wasteneys in
191 1 is interesting; it was entitled: "Are oxidation processes the
independent variables in life phenomena?" It occurred to them to
526
ON INCREASE IN SIZE
[PT. Ul
Table 74, Results of investigations of temperature characteristics.
Species
Mealworm {Tenebrio molitor) (pupa)
5J 5) 55
Fruit-fly {Drosophila) (egg + larva + pupa)
55 55 55
55 55 (imago)
(egg+larva)
5J 55 55
(pupa)
55 55 55
Sea-urchin {Arbacia) (segmentation)
55 55 _ •••
Frog (Rana) (segmentation)
Frog (Rana) (ist segmentation to dis-
appearance of yolk-plug)
Frog (Rana) (3rd segmentation to dis-
appearance of yolk-plug)
Frog (Rana) (later)
,, (till hatching)
(early)
„ (later)
Cod {Gadus morrhua and aeglifinus)
,, {Gadus merlangus)
Vlaice {Pleuronectes platessa)
Japanese teleost {Hypomesus olidus)
„ ,, {Plecoglossus altivelis)
Water-beetle {Dytiscus marginalis)
„ {Dytiscus semisulcatus)
Water-flea {Cyclops)
55 55
Fruit-fly {Drosophila melamgaster)
Water-flea {Pseudosida bidentata)
,, {Moina macrocopa)
,, {Simocephalus semisulcatus)
55 55
Hen {Gallus dormsticus) ...
55 55
55 55 ••• •••
55 55 ••• ••• •"
Fruit-fly {Drosophila melamgaster)
95 55
55 _ 55
Trout {Salmo/ario)
„ J,
Unless otherwise stated the lower /x is above 15°.
compare the temperature coefficient of embryonic development in
the sea-urchin's egg with the temperature coefficient of respiratory
rate in the same material. Their figures for developmental rate gave
Embryonic
Growth
growth
Investigator
27,000
—
Krogh
10,000
—
,,
27,000
27,000
Krafka
10,000
10,000
55
I9>920
—
Loeb & Northrop
27,000
27,000
55 55
10,000
10,000
55 55
27,000
—
55 55
10,000
—
,, ,,
—
41,000
Loeb & Wasteneys
—
2 1 ,000
Loeb & Chamberlain
—
12,400
Krogh
—
21,900
95
—
10,800
95
—
19,100
Lillie & Knowlton
22,000
59 99
—
20,300
Krogh
—
20,000
Barthelemy & Bonnet
—
17,000
Hertwig
—
24,000
Lillie & Knowlton
—
11,800
20,000
Dannevig
—
20,000
Johansen & Krogh
—
23,700
Higurashi & Tauti
—
23,000
Higurashi & Nakai
—
19,300
Blunk
—
20,000
_99
—
10,400
Ziegelmayer
—
155700
99
335210
Bliss
16,850
—
99
7,100
—
99
19,800
—
Brown
28,500
—
,,
17,210
—
99
7,410
—
99
16,950
—
,,
4,780
—
99
25,700
Brody & Henderson
—
7,100
99 99
—
395O5O
55 99
—
6,500
99 99
27,800
—
Plunkett
17,100
—
99
9,000
—
99
—
24,500
Gray
—
20,000
Maitland
SECT. 2] AND WEIGHT 527
very considerable slowing at the low temperatures, so that although
the coefficient between 17 and 27° was 2-6, between 8 and 18° it
was 6-0. The temperature coefficient for the respiratory rate, how-
ever, did not show this, and, while it was 1-5 approximately round
30°, never rose higher than 2-45 even including the data for between
3 and 13°. There was thus no correspondence between the two sets
of coefficients below 15°. This circumstance, they thought, harmonised
with the fact that potassium cyanide in concentrations which stopped
development altogether still permitted 25 per cent, of the oxidations
to proceed, and further, with the fact that when no development was
going on normally, i.e. before fertilisation, there was still a certain
respiration. Of the two processes the morphological development
rate seemed to be the more easily abolished by external agents.
But above 32° when cleavage became impossible, respiration was
also impossible, and the oxygen uptake rapidly fell off. "These
facts", said Loeb & Wasteneys, "do not contradict the view that
oxidations are the independent variables in developmental processes.
But on the other hand they do not furnish any convincing evidence
in favour of such a view." The whole incident, as a matter of fact,
may now be regarded as an instance of the incapacity of the ordinary
van't Hoff equation to differentiate between two alternatives in a
complicated biological system, and the question which Loeb &
Wasteneys were trying to answer may be said to be solved by the
frequency polygon of temperature characteristics.
The results of all this work may be summed up by saying that by
the use of the heat effect on embryonic growth and development a
kind of dissection has been made of the process, for at one tempera-
ture one controlling reaction or pacemaker is laid bare, as it were,
and, at another temperature, another controlling reaction. This
exposing method has led to the unveiling of four or five reactions
of quite clear-cut temperature characteristics, which according to
the temperature and the circumstances may be in control of em-
bryonic growth. What these reactions are we do not yet know, but
admittedly a good step forward has been taken by the identification
of the heats of activation of their active molecules.
Crozier has discussed in some detail the relation between the
temperature characteristic of growth and the autocatakinetic growth
curve. Difficulties arose out of the experimental fact that temporary
alteration of the temperature during a period of development
528 ON INCREASE IN SIZE [pt. m
modified the temperature characteristic for the remainder of develop-
ment. This it could not be expected to do on the Robertsonian
view, for where there is only one velocity constant, as in his
presentation, its temperature characteristic must always be the same,
and changing the temperature would only multiply the time co-
ordinate by some constant, and drag out or compress the time
taken to complete the autocatalytic curve. Within one cycle the
temperature characteristics should be the same for all partial
developmental periods as well as for the whole. This, however,
was found not to be the case by Bliss for Drosophila, by Brown
for the cladoceran Pseudosida bidentata, and by Titschak for the
clothes-moth Tineola biselliela. If an animal is allowed to develop
for 50 per cent, of the total normal time at 15° C, and is then
transferred to a temperature of 25° C, it may take less or more time
to finish its development than would be predicted on the basis of
the fact that it has still 50 per cent, of its normal time to go
at 15°. Brown expressed the differences as per cent, gain or loss in
time under such treatment, and found that they were, though small,
quite significant statistically. This means that the growth equation
of Robertson, ,
= Kx (a — x)
dt
(where a is the initial amount of growth-forming substance, x the
amount formed after time t and K a constant), cannot hold, and
must give place to an equation with two velocity constants. Crozier
has suggested one, the differential form of which is
~={K, + K,x) [A - x),
where Kj^ is the velocity constant proper to the reaction A^x, but
in the absence of the catalytic effect of x, while K^ is the velocity
constant associated with the process when x is functioning as a
catalyst. The point of inflection will then be
The integral form of the differential equation is
/ =
, A {K^x + K^)
K^ + K^A = K^{A- x)
SECT. 2] AND WEIGHT 529
and though the form of its curve is sigmoid the point of inflection
depends on the ratio of the two velocity constants, so that it may be
very asymmetrical.
Before embarking on the exposition of the applications of the
Arrhenius formula to growth in general and embryonic growth in
particular, it was said that there was much difference of opinion as
to whether the time/temperature relation could be expressed best
by a hyperbola or an exponential curve. It was often pointed out
by Loeb and others that even a straight line might be produced by
the action of various factors on a true exponential curve causing it
to flatten out. Snyder suggested that an important factor which
might be expected to have such an effect was the protoplasmic
viscosity. This idea has since had a considerable popularity, although
Crozier has vigorously combated it. "The attempt to introduce con-
siderations of protoplasmic fluidity, presumably as influencing dif-
fusion, requires a theory of the general control of organic activities
by the whole body of the cell rather than at surfaces. This is un-
necessary", says Crozier, "and at present inadmissible." But it is by
no means easy to see why, and the possibilities contained in cor-
rections for viscosity should certainly be explored. The difficulty is
that viscosity varies in different ways with temperature according to
the animal or plant used. Some authors, moreover, seem to wish to
obtain a constant Q_io, although even in inorganic in vitro reactions
Q^io is never constant, but increases as the temperature is lowered.
Pantin, for instance, corrected various temperature coefficients, on
the basis of the viscosity changes in Nereis eggs, and found a much
greater approach to constancy, though even after correction, the Q^^q
rose a good deal at the lower temperatures. Krafka's work, already
referred to, is rather an obstacle to this point of view.
The same line of thought was carried further still by Belehradek,
who criticised the use of the Arrhenius equation on the ground that,
like the van't Hoff" formula, it also did not give a constant for all
temperatures at which normal development will proceed, but shows
up critical points which may or may not be real. This criticism was
also made by Heilbrunn. Belehradek used a modification of the
Esson-Harcourt equation, which he found would represent the time/
temperature relation fairly accurately:
y = ^ or logy = log a — b log x,
NEI 34
530
ON INCREASE IN SIZE
[PT. Ill
where jy is the time necessary to accompHsh a given amount of e.g.
embryonic development at the temperature x and a and b are
constants. Thus log. time/log. temperature gives a straight line. The
constant a varies according to the unit of time used, but the constant b
is independent of the time-unit and the actual velocity for it repre-
sents acceleration. If ^ = i then y = ajx, which is the equation for
a rectangular hyperbola, and covers the work of Peairs on insect
development as a special case of the general law. Again, the straight-
line relation of Krogh and others is another special case, for, ac-
cording to it, V ^ kx where v is the velocity and x the temperature
and, as jv = ijv and k = i/a, we get y = a/x, which is the original
equation once more. As b is usually greater than i, Q^^o usually
declines with rise of temperature. Belehradek regards the constant b
as a true temperature coefficient, for it does not change with tem-
perature, and he has calculated it for various systems which are
relevant here. Thus he gets for the embryonic development of
Copepod Cyclops fuscus (Ziegelmayer) ...
Fruit-fly Drosophila melanogaster (Loeb & Northrop) ...
Frog Rana virescens (Lillie & Knowlton)
Hen Callus domesticus (Fere)
and for the early segmentation of
Sea-urchin Strongylocentrotus lividus (Loeb)
Roundworm Ascaris megalocephalus (Faure-Fremiet)
It would appear that b increases with age, for
Fruit-fly Drosophila melanogaster (Loeb & Northrop) :
Larva
Pupa
b
i-i6
2-IO
2-36
4-10
I St
0-99
2-50
2nd
I-I3
and
and
and
Water-beetle Dytiscus semisulcatus (Blunk) :
Embryo
Larva (ist instar) ...
Larva (2nd instar)
Larva (3rd instar)
Prepupa
Pupa
Frog Ranafusca (Krogh) :
FertiHsation to ist cleavage
End of 1st cleavage to closure of neural fold
Closure of neural fold to appearance of external gills
Appearance of external gills to 3-branched gills
3-branched gills to 7 mm. embryo ... • ...
7 mm. embryo to 7-8 mm. embryo
Typographic beetle Tomicus typographus (Hennings) :
Embryo
Larva
210
2-28
i-io
1-14
1-26
1-38
1-48
I -60
I -20
1-64
1-76
1-92
1-69
2-52
2 -02
3-52
SECT. 2]
AND WEIGHT
531
B^lehradek interprets b as being a measure of protoplasmic viscosity,
so tliat in his opinion all temperature effects on growth can be
regarded as primarily effects upon viscosity, and only acting in-
directly upon the rapidity of the growth-process. Whether this point
of view will prove to be either more fruitful or more correct than
that of Crozier and his collaborators cannot at present be decided.
Crozier & Stier have, however, shown that the fit of B^lehradek's
formula in one case, at any rate, is not at all good, while Belehradek
has strengthened his case by studying the time factor in cooling and
heating protoplasm. But it is interesting that the constant b should
increase with age, for protoplasmic viscosity almost certainly does,
and this has an obvious importance in view of the gradual loading up
Exponential curve
Catenary
Fig. 85.
Hyperbola
of the cells of the body with paraplasmatic substances. Belehradek
has answered Crozier's criticisms and the papers must be consulted
for the details of the argument.
Janisch represents the time/temperature relation not by a simple
exponential curve, nor by a hyperbola, but by a " Kettenlinie " or
catenary curve. The reciprocal of this is not a straight line or a
sigmoid curve rising most rapidly at the point of inflection; it is a
sigmoid curve rising most slowly at the point of inflection. Such a
complex exponential curve should therefore be produced when the
velocity/temperature graph is drawn, and Janisch actually showed
that Krogh's straight lines turn into curves of this character when
the points which Krogh neglected at the upper and lower ends are
taken into consideration. Just the same can be shown for the figures
of Sanderson and Peairs. Fig. 85, which is taken from Janisch, shows
the relations between these curves. Fig. 86, also taken from Janisch,
shows a replotting of the data of Sanderson for Margaropus annulatus.
The time of development is shortest at 28°, and, just as Faure-
34-2
532
ON INCREASE IN SIZE
[PT. Ill
Fremiet showed for the segmentation of Ascaris, it lengthens a Httle
on the high-temperature side of that. The points are well described
by one of Janisch's catenary curves. The velocity of development,
i.e. the reciprocal of the development time, correspondingly rises to
a maximum at 28°, and has a sigmoid form on each side of this
maximum. This peaked curve, part of which, it should be remembered,
corresponds to the Krogh "straight-line", is identified by Janisch
with the peaked curve which Duclaux in 1899 pointed out would
be produced by the operation of two separate factors giving complex
exponential curves, i.e. differentially affected by temperature change.
This peaked curve could be either symmetrical or asymmetrical.
1
_
• /
, »
/'»
-
^,
/•
X
\
•
-
^''""^-.ui^
.
iu^
.^
^^^^
1 1 1 1 1
1
1
1 1
1 1 1
-^
160
140
120
100 I
80 Cl
60
40
20
38° 36 34 32 30 28 28 24 22 20 13 16 14 12 10 8°
Temperature
Fig. 86.
Perhaps further work along these lines will lead to conclusions about
critical temperatures and controlling reactions which would support
those of Crozier.
Janisch gave a full mathematical treatment of these questions, and
also reported an investigation of the effect of heat on the embryonic
development of the "Mediterranean flour-moth" Ephestia kuhniella
(see also Hase). The time/temperature relation here also can be
expressed by an equation of the form
y
2
%
where m and a are constants and y is the time taken to complete
a given amount of development at time x. Such presentations
SECT. 2] AND WEIGHT 533
of the data are very interesting, and possess advantages as well
as disadvantages in comparison with the equations of definite
physical meaning, such as that of Arrhenius. For, although
they do not seem to give immediately so great an insight into
the fundamental processes of the living cell, they are in value
independent of advances in physics which may destroy the basis
on which the other kind of treatment rests. In other words, their
status as short descriptions of the phenomena is safer if not so exalted.
I have now completed what it was necessary to say about
the effects of temperature on embryonic growth and develop-
ment. This knowledge is very valuable, but the unveiling of the
limiting factor by temperature control, and the identification of
the "master reaction" by fitting growth-curves, are perfect ex-
amples of what may be called the "short-cut" method. To a
certain type of mind the attractions of the short-cut are too great
to be resisted, but, as we have seen, the results are liable to be
enigmatic.
2-17. The Effect of Light on Embryonic Growth
The influence of light upon the course of embryonic growth has
been little studied, though the results of such work cannot now be
looked on with the scepticism of twenty years ago. Schnetzler's
paper of 1874 was the first serious contribution to the study, although
Edwards fifty years before had stated that frog eggs would not develop
at all in the dark, a groundless assertion which had been disproved
by Higginbottom and McDonnell. Schnetzler; and Baudrimont &
M. de St Ange also found no difference in hatching time, though the
former thought that the growth of the tadpoles was more rapid in
the light than in the dark. This was substantiated quantitatively by
Auerbach and by Yung, who placed freshly laid frog's eggs in vessels
containing 60 eggs in 4 litres of water, some in diffuse light, others
in the dark. Yung's figures were as follows:
Days
Size of light tadpoles compared
with dark (100)
development Length Breadth
25 125 124
30 117 118
60 106 107
It is interesting that the light effects diminish proportionally with
534 ON INCREASE IN SIZE [pt. iii
increasing age. Lessona and Camerano brought forward field evidence
which agreed with the controlled results of Yung, who was later
confirmed by Perna and by Chiarugi & Livini. Yung also found
that the eggs of Salmo trutta hatched i day earlier when kept in the
light than when kept in the dark, and that the eggs of Limnaea
stagnalis, the pond snail, hatched in 27 days in the light but 33 days
in the dark. Davenport, following earlier speculations of Millet and
Blanc, has commented upon the regularity with which developing
embryos are hidden away from the light either in the egg or in the
uterus, but as this is not the case with animals such as the echino-
dermata there seems no great significance in the idea. Bodine and
Carothers have observed a marked inhibitory effect on embryonic
development of certain orthoptera [Melanoplus differentialis, Chorto-
phaga viridifasciata, Circotettix verniculatus) of direct sunlight (not the
heat accompanying it) and Miyakawa affirms the same for Schistoso-
mumjaponicum*; but, on the other hand. Page, taking like precautions,
reported that shad eggs could be made to hatch 12-15 hours before
the controls by allowing direct sunlight to fall upon them. Ruffini's
curious observation that the eggs oiBufo vulgaris in their jellies always
orient themselves with their animal poles pointing toward the light has
never been confirmed, but may have significance. According to Good-
rich & Scott light has no effect on chick embryo cells growing in vitro.
Other researches have been made with the object of identifying
the most active wave-length. Beclard very early placed the eggs of
Musca carnaria behind screens of different colours, and found that
the embryos in the green light were the least developed after a
definite period, the rest following in the order, yellow, red, blue
and violet, in which last light they developed most rapidly. The
retarding effect of the green was also observed by Schnetzler.
Davidson's results are untrustworthy.
Very striking effects were observed by Yung, whose papers are the
most often quoted :
Length of tadpoles
Colour of light
r
I month
2 months
White
100
100
Violet
117
134
Blue
105
107
Yellow
99
102
Red
83
86
Green
77
All dead
* See also the work of Frederich & Steiner.
SECT. 2]
AND WEIGHT
535
Plus
Minus
2-5
—
—
1-3
—
45
—
4-8
—
6-9
—
8-9
As regards echinoderm embryos, we have the following table of
Vernon :
Length °/^ change from white length
Semi-darkness
Complete darkness
Blue
Green
Red
Yellow
Driesch stated that these colours had no effect on Echinus embryos.
Here the favourable action of violet or blue and the unfavourable
action of green are not apparent. Nor in the case of trout fry could
Schondorff find any differences between the colours. The hatching
time of various eggs, however, according to Yung, does bring it out
again, as Table 75 shows, and the qualitative experiments of Schenk
on Rana and Bufo eggs and Fatigati on other material (infusoria)
demonstrate the same relation.
Yung's figures:
Table 75.
Hatching time in days
Violet Blue
Loligo vulgaris
Salmo trutta ...
Limnaea stagnalis
50
32
17
53
35
19
Yellow
58
34
25
Red
58
36
Green
Dead
36
Dead
White
Dead
35
27
In 1 91 3 Grein subjected the eggs of Gadus virens to light which
had passed through the following screens :
Wave-length allowed to pass
A Red ... 610-710 /x/Li
B Red ... 600-720
C Red ... 600-700
D Green ... 440-460
E Blue ... All save light green
and dark yellow
Hatching times were quicker under D than under A, B or C, and
quicker under A, B or C than under E.
Supino's results, expressed in percentage of embryos hatched after
a fixed period, were as follows :
Blue
... 65
Yellow ...
... 43
White ...
- 53
Red
-. 39
Darkness
- 45
It may, on the whole, be concluded that the wave-lengths con-
tained in diffuse daylight which affect embryonic development are
536 ON INCREASE IN SIZE [pt. iii
the short ones at the violet end of the spectrum. It would be very-
desirable to go into the whole subject anew in the light of modern
technique and modern conceptions.
2- 1 8. The Effect of X-rays and Electricity on Embryonic
Growth
Other forms of radiant energy have also been investigated, but the
position is very complicated and unsatisfactory. Oilman & Baetjer
in 1904, judging from morphological and cytological evidence, con-
cluded that in Amblystoma, X-rays at first accelerated development
and then retarded it so that eventually the exposed embryos were
smaller than the controls.
For the silkworm Hastings, Beckton & Wedd have asserted that
X-rays accelerate the developmental time, and (Hastings) that the
secondary radiation excited by irradiating copper retards the de-
velopment of the silkworm. Ancel & Vintemberger in an elaborate
and long research, decided that in the case of the chick and the
frog, X-radiation had neither an accelerating nor a retarding in-
fluence. Nevertheless Colwell, Gladstone & Wakeley believed that
they had evidence of retardation by X-rays in the chick, as the
following table shows:
Table 76.
Length of embryo
Thickness of
on the eighth
Daily dose
(P.D.)
filter (mm.)
day (mm.)
0 (control)
—
22
I
4
4
I
I
4
I
2
4
f
i
8
t
I
10
1
2
12
i
I
13
i
2
13
i
i
15
i
I
15
i
2
15
Their second paper, though mainly morphological, confirmed the
earlier one. Richards obtained acceleration working with the egg
of the mollusc Planorbis, and the opisthobranch Haminea virescens.
Packard working with Arbacia, Lazarus-Barlow & Beckton with
Ascaris, Bohn with Strongylocentrotus eggs, found that short exposures
to radium accelerated development and long ones retarded it.
Lazarus-Barlow & Beckton found that /S and 7 radiations alone had
SECT. 2] AND WEIGHT 537
the same effect as all three together though the a radiation was
100 times as much as the other two. Forsterling obtained retardation
of growth in rabbit embryos by irradiating the mother, and other work
on mammals was done by Cohn, by Lengfellner, and by Bagg, but the
conditions are there so complicated that it is not worth discussing.
The effect of electricity upon embryonic growth has been fairly
often tried, but usually in an unintelligent way. Eggs have simply
been placed between the electrodes or between the poles of a magnet,
and conclusions have been drawn which would probably not bear
statistical examination for a moment. Thus Rusconi in 1840 stated
that frog's eggs hatched more quickly under the influence of electric
currents than otherwise. Lombardini remarked the same effect in
amphibian development but noted a large number of abnormalities,
as did Fasola. Windle, working with eggs of the trout, found no
acceleration of development either under the influence of electric
currents or of a large magnet, but he noticed that the hatched trout
died very quickly. Rossi found many anomalies in the eggs of urodele
amphibia subjected to electric currents, but no other effect. Slater
did not find any result at all when he subjected silkworm eggs to a
strong magnetic field, nor did Maggiorani when he subjected hen's
eggs to one. Finally, Benedicenti found no effect of constant weak
currents on the development of echinoderm eggs. The more recent
work includes that of Scheminzki on the trout — he could observe no
acceleration of development when the eggs were subjected to con-
stant sub-lethal currents, but the membrane at the end of the
development was weaker than usual so that the embryos tended
to hatch early. Gianferrari & Pugno-Vanoni passed currents of
9000 volts at 450,000 periods/second through suspensions of trout
{Salmo lacustris) and echinoderm {Echinus esculentus) eggs, but the
effects observed were merely teratogenic, and the abnormalities pro-
duced did not differ, apparently, from those produced in other ways.
The subject still awaits an investigator who will sweep up all this
debris into some coherent theory of the action of electrical currents
and magnetic fields on embryonic development.
The other factors, such as osmotic pressure, which have been
shown to affect growth so greatly in the post-natal stages and in
plants, do not exercise so much influence on the foetus, guarded as
it usually is from the world around it by the egg-shell or the uterus.
Their effects have therefore not been much studied in the case of
538 ON INCREASE IN SIZE [pt. iii
embryonic growth, and the few data that do exist on the subject
will be presented in the section on comparative susceptibility of the
embryo at the different stages in its development. The section on
biophysical phenomena also contains information on cognate points.
The process of cell-division, as such, is of course influenced by a
great variety of factors and substances, as the following table shows :
Table 77.
Substances which accelerate
cell-division Authority
Heat ... ... ... ... ... Laughlin
X-rays ... ... ... ... ... Gilman & Baetjer
Radium emanation ... ... ... Packard; Bohn; Shumway; and
Buddington & Harvey
Th>Toiodin ... ... ... ... Richards
Adrenalin ... ... ... ... Chambers
Alcohol Calkins; Woodruff
Potassium hydrogen phosphate ... Woodruff
Potassium sulphate ... ... ... „
Potassium bromide ... ... ... ,,
Oxygen ... ... ... ... Godlevski
Sodium and potassium hydroxide . . . Richards ; Loeb
Pilocarpine ... ... ... ... Richards
Ammonium hydroxide ... ... ,,
In most cases, however, the effects produced by these agents are
complicated, and the original papers should be referred to. Naturally,
all these substances and factors exercise a depressant action on cell-
division if they are applied in too brutal a manner. Moreover, though
they tell us a certain amount about the nature of mitosis, they do
not much assist in the understanding of the metazoal growth-process,
organising mitoses as it does on the large scale. As Richards says, " The
advantage gained in the segmentation-stages may later manifest itself
in more vigorous larvae than in more rapidly developing ones ", so that
agents which accelerate cleavage may not accelerate hatching-time.
The study of these agents has not led so far to any great advance in
our knowledge of the essential nature of growth and development.
2-19. The Effect of Hormones on Embryonic Growth
As regards the effects of endocrine organs upon embryonic growth,
practically nothing is known. Willier, who made chorio-allantoic
grafts of thyroid in chick eggs, observed that the host embryo was
always smaller, and in some cases as much as one-third smaller, than
the control. Hanan later, after a tliorough study of the difficult
question of appropriate dosage, injected 1/600 of a mgm. of thyroxin
SECT. 2]
AND WEIGHT
539
into the air-space of chick eggs on the 5th day of development. The
effect was absolutely nil, as is seen from Figs, 87 & 88 where his points
are plotted both for wet and dry weight. There is thus a certain con-
tradiction between the results of Willier and those of Hanan ; Okada
afterwards repeated Hanan's experiment, and observed no effect on
the general appearance, but an 18 % decrease in length and weight
of the chicks from the injected eggs. There were characteristic histo-
logical changes, mostly in the direction of precocity. Okada's results
would fit in with Butler's finding of a decrease in cell-division rate
under the influence of thyroxin in Arbacia eggs.
Pituitrin has also been injected into hen's eggs, by Cunningham
& Stanfield, who obtained the following results :
Average weights (gm.)
Injection 7th day, opened gth
Injection 7th day, allowed to hatch
Injection 6th day, opened 8th
/■
>
Control
Injected
embryos
embryos
1-578
i-goi
37-4
400
0950
I 203
540 ON INCREASE IN SIZE AND WEIGHT [pt. iii
This may mean that growth is accelerated, but a larger number
of experiments with more complete statistical treatment will be
required to settle the question.
A word should be said about the possible total reversibility of
growth. Regression to the embryonic state was first brought about
by Caullery, and recently Davydov has succeeded in reducing the
nemertine Lineus lacteus to a state which cannot be distinguished from
its blastula. Nothing is known of the chemistry of this process.
In conclusion it may be said that modern embryology has more
and more come to follow the lead of Leonardo da Vinci in subjecting
all the aspects of the growth of the embryo to exact measurement.
In doing so, it has, as we have already seen, put itself in a position
to fuse its studies with those of biochemistry and biophysics. The
fruit of this union can only be the understanding of the molecular
mechanisms underlying embryonic growth and development, from
the egg-cell into the newly hatched animal, whose shape and con-
stitution must in the last analysis be regarded as among the properties
of what we call matter.
SECTION 3
ON INCREASE IN COMPLEXITY AND
ORGANISATION
3-1. The Independence of Growth and Differentiation
The previous section has been concerned solely with the question of
increase in size and in mass as the embryo develops; other syn-
chronous processes were perforce neglected. These may be compre-
hended in one word, differentiation, or increase in complexity and
organisation both macroscopic and microscopic. "All generation",
said Sir Kenelm Digby, "is made of a fitting, but remote, homogeneall
compounded substance; upon which outward Agents, working in
the due course of nature, do change it into another substance, quite
different from the first, and do make it less homogeneall than the
first was. And other circumstances and agents do change this second
into a third, that third, into a fourth ; and so onwards, by successive
mutations that still make every new thing become lesse homogeneall
than the former was, according to the nature of heat, mingling more
and more different bodies together, untill that substance be pro-
duced, which we consider to be the period of all these mutations."
Or, as William Harvey puts it, "For though the Head of the Chicken,
and the rest of its Trunck, or corporature (being first of a similar
constitution) do resemble a Mucus, or a soft glewey substance: out
of which afterwards all the parts are framed in their order; yet by
the same operation, and the same Operatour, they are together made
and augmented: and as the substance resembUng glew doth grow, so
are the parts distinguished. They are Generated, Altered, and
Formed at once". Having discussed then in the preceding section
the "augmentation" of the embryo, we have now to discuss its
"making" or in Harvey's other term, its "framing".
Framing is undoubtedly not an increase in complexity alone.
Woodger has well said, "It is often stated that organisms are just
complicated physico-chemical reactions, and it is because they are
so complicated that biology has so far made so little progress. But
it is evidently not simply a question of complicatedness, because
there are plenty of compUcated goings on in the world which no one
542
ON INCREASE IN COMPLEXITY
[PT. Ill
would mistake for organisms. The 'something going on' which we
call a thunderstorm, for example, is very complicated. An organism
from some points of view is comparatively simple, otherwise biology
would not have got as far as it has, and this simplicity appears to
be the outcome of its organisation. What happens in development
is not merely an increase in complexity nor an increase in spatial
structure, but a gradual rise in the level of organisation of the
developing organism".
It has often been said that the interesting thing about any magni-
tude is not so much its absolute as its relative size, and not so much
its relative size as the rate at which its relative size is changing. The
first question to ask therefore in
the discussion of the physico-
chemical aspect of differentia-
tion is whether the rate of
differentiation is the same as
the rate of growth at each
embryonic stage. That the two
processes are independent in
the sense that one can be in-
duced without the other has
long been known. Panum, for
instance, as long ago as i860
observed chick embryos with
well-developed blastoderms but no primitive streak, but he did not
associate this with any definite causative factor. Then Broca found he
could get this growth without differentiation by keeping fertile eggs
for a month or more at room temperature before incubating them,
and Dareste observed that very high as well as very low temperatures
maintained during the first 24 hours of development would bring
about the same effect. Rabaud confirmed this : for further information
see the review of Tur. Finally, Edwards found that cell-division with-
out embryonic organisation always occurred in eggs incubated from
20 to 27°. Continued growth may take place also in the primitive
streak stage as well as in the simple blastoderm when there is not a
sufficient degree of temperature to permit of normal differentiation.
Fig. 89, taken from Edwards' paper, shows the effects he obtained.
The controlled action of temperature can also bring about nuclear
division without corresponding cell-division in Echinus^ according to
" 12° 23°
TemperaJture
Fig. 89.
SECT. 3] AND ORGANISATION 543
Driesch, and many other workers have observed these polynuclear
undifferentiated masses in other embryos with abnormal temperatures.
Even in bacteria growth in size can take place without cleavage,
according to Henrici, and the work on the endocrine control of
amphibian metamorphosis affords other instances of continuous
growth without differentiation.
The most remarkable case of this, perhaps, is contained in the
work of Hoadley, who transplanted embryonic organs of the chick
on to the chorio-allantoic membrane and allowed them to develop
there until they had reached a degree of differentiation equivalent
to that of the controls. Then by making and weighing wax models
he ascertained the relative weights, and always found that the con-
trols were much heavier. But there was a direct relation with age,
for the younger the transplant at the time of transplantation the
smaller the eventual (fully-differentiated) organ, thus:
At the
time
of transplantation
A
Control : Transplant =
r ■
■\
Embryo age
No. of
X
Organ
(hours)
somites
(after about 8 days)
Spinal cord
48
28
0-353
Eye
48
28
0-219
>>
35
14
0-098
}}
20
0
0-0136
„ 40 0-00075
Another instance of the independence of growth-rate and differentia-
tion-rate is afforded by the genetic races of rabbits which differ
considerably in size. At birth the large-sized race is twice as heavy
as the small-sized race, and at the adult stage, three times as heavy.
In 1928 Painter observed that equally differentiated 12-day embryos
showed the characteristic weight-differences and thought that the
effect might be of endocrine origin, but Castle & Gregory were later
able to trace it back as far as the morula stage. The large-sized race
showed consistently more rapid cell-multiplication and size increase
in the embryonic period with unaltered differentiation-rate. There
is no difference in egg-size: but the large-sized race grows more
^ ^ ' ' Embryo age Average no. of Diameter of
in hours blastomeres blastodermic vesicle
Large-sized race ... ... 48 21*75 —
Small-sized race ... ... 48 14-00 —
Large-sized race ... ... 100 — 31*4
Small-sized race ... ... 100 — 16-3
Another good instance of this dissociation of the fundamental
544 ON INCREASE IN COMPLEXITY [pt. iii
embryogenic processes is seen in the interesting work of Twitty, who
studied the development and the nature of the action of the ciha on
the skin of amphibian embryos, previously described by Assheton and
Woerdemann. He found that the polarity of the ciliary cells is deter-
mined during the closure of the neural folds, for cilia grafts rotated
1 80° before that stage beat in the same direction as the cilia of the
adjacent ectoderm, but cilia grafts transplanted later retained their
original direction of waving. Thus the determination here (in
Amblystoma punctatum) occurred much later than the main point of
chemodifferentiation (see p. 575). Now in embryos allowed to
develop at low temperatures, Twitty found that this ciliary polarity
appeared at a much earlier stage. Evidently the determinative
process had been thrown out of gear with the morphological ones
by the cold, and the two were proceeding more or less independently.
Again, certain treatments, as is well known, inhibit segmentation,
and cause the production of cilia, while histogenesis and organo-
genesis are easily separable as is seen in teratological experiments
(Ranzi) and in explantation work (Hoadley)*.
3*2. Differentiation-rate
Under certain conditions then, such fundamental processes as
growth and differentiation can be shown to be proceeding indepen-
dently of each other. That their normal velocities differ might or
might not be the case. All who have had any practical contact with
embryology know that there is much more difference in shape and
form between a chick embryo of the 2nd day and one of the 5th than
there is between one of the 12th day and one of the 15th. But the
difficulty has always been to evolve some method of measuring change
in shape — a more elusive entity than change in weight. Murray,
however, has made an admirable attempt in this direction, choosing as
the organ for investigation the heart, which, as we have already seen
in the work of Schmalhausen, appears to keep pace exactly as regards
growth with the embryo as a whole. He tested this by measurements of
the surface area of the organ, calculating the percentage growth-rate
of the heart from Cohn's work with a projectoscope and a planimeter.
The result was one of the usual descending curves which closely
resembled that for the embryo as a whole, and the log. surface area
of the heart/log. age of embryo relation was also a straight line.
* Thus Waddington, working with chick embryos cultivated in vitro, found that the
i2-somite stage was much smaller than the corresponding stage in the egg.
bo
35
546
ON INCREASE IN COMPLEXITY
[PT. Ill
"No satisfactory method", Murray said, "of measuring changes
in form quantitatively is known, so that it was necessary to resort to
the expedient of selecting forms spaced by a visual impression so as
to represent approximately equal degrees of gross change. In other
words, from a series of drawings made at frequent intervals, certain
ones were chosen which seemed by inspection to be equally spaced
from one another in respect to their relative complexity of form. The
test is thus necessarily arbitrary and open to criticism because of its
<6
O
O
o
100
o-|
50
k
o^
N
'^
-—
o -
Ds.ys z
6 8 10
Incubd^tion 5.ge
Fig- 91-
a
subjective nature. By taking the average result of many eggs it was
then determined what were the exact incubation ages of the embryos
with heart forms such as those selected." The illustration shows these
clearly (Fig. 90). The reciprocals of the time intervals between suc-
cessive drawings were used as rough criteria of the rate of form
development. The graph showing this done is reproduced in Fig. 91,
from which it is clear that the rate of evolution of external form falls
precipitously at first, and then ever more slowly, essentially resembling
in this way the instantaneous percentage growth-rate, according to
Schmalhausen, or the averages of the steps in the fall of the in-
stantaneous percentage growth-rate, according to Brody. Growth
SECT. 3] AND ORGANISATION 547
and form would therefore seem to be what Murray calls "aligned"
with one another, for the most marked form changes occur at the
initiation of embryonic development when the growth-rate is at its
highest. Murray's interim method affords a basis of quantitative
comparison between growth-rate and differentiation-rate. But if
these two processes change together with like velocity, others do not.
The shifts of chemical and physical change within the embryonic
body follow an entirely different course In 1925 I had drawn
attention to the fact that, making allowance for the increase in
size of the embryo and for its consequently increased daily turn-
over of matter and energy, chemical activity seemed to be more
intense at the end of development than at the beginning. "One
pictures", I had said, "the gradual elaboration of structure up to
the beginning of the third week, followed then by a burst of chemical
activity consequent upon the assumption of function by the organs
already formed. In this hypothesis is involved the view that in the
first fortnight chemical change is limited to those compounds which
are required mainly for structural purposes, while toward the end
of incubation the opening up of functional operations causes marked
and profound chemical changes of other kinds." This was proposed
in connection with the three periods which modern embryologists
have come to recognise in all embryonic development. The same
idea was elaborated further by Murray in relation to change in
growth-rate and differentiation-rate. Taking the rate of change in
chemical constitution, he concluded (largely from his own experi-
ments) that the most marked changes occurred after the loth day,
not before it, as is the case with growth-rate. "Internal integration",
he said, "may be regarded as a process characterised by the con-
centration of solid substances within the body, whereas chemical
differentiation is a change in the composition of the solid substances
thus integrated." Fig. 92, taken from Murray, demonstrates these
relationships, for it may be observed that the percentage of ash in
the embryo, the percentage of total solids and the percentage of fat,
together with the rate of production of carbon dioxide, all rise or
fall in the same way, i.e. more rapidly at the end of development
than at the beginning, and precisely opposite to the growth-rate and
the differentiation- rate. Thus, just as morphological growth and
differentiation take place more rapidly the younger the embryo, so
chemical growth and differentiation take place more rapidly the
35-2
548
ON INCREASE IN COMPLEXITY
[PT. Ill
older the embryo. A note of caution must be inserted here, how-
ever, in view of the fact that, until we know more about the behaviour
of individual organs in the light of these conceptions, we cannot
estimate the part played by ossification, for instance, in producing
these overall results. Murray also afterwards found that the curve
Per Pep
cent cent
9 11 13
Incubation age
Fig. 92.
relating age with oxygen absorbed by the embryo per gm, per day
did not decline quite like the carbon dioxide evolution curve, i.e.
slowly at first, and then quicker and quicker, but was rather S-shaped,
the slowest rate occurring at the loth day. His line for this, how-
ever, depended for the initial fall exclusively upon one very high
point for the 6th day, and this may quite possibly have been wrong,
in which case the curve for oxygen consumption would fall with the
curve for carbon dioxide output.
SECT. 3]
AND ORGANISATION
549
Murray next showed that the curves for growth-rate of heart
tissue in explanted culture, obtained by himself working with Cohn
& Rosenthal (see p. 461), fell in the same way as those for the intact
embryo in its egg, but that the curve for latent period (see p. 462)
behaved, on the contrary, exactly like those for integration and
Deo/35
11 13
Incubation age
Fig- 93-
chemical differentiation, rising very slowly at first, and thereafter
more rapidly. Thus chemical constitution, metabolic rate, and latent
period of explant growth are to be correlated and distinguished from
growth-rate both in the intact animal and in tissue culture. Fig. 93
shows the difference between growth-rate and metabolic rate, as
judged from the rate of carbon dioxide production.
As a result of this analysis of embryonic development, at least five
550 ON INCREASE IN COMPLEXITY [pt. iii
fundamental processes may be distinguished, falling into two groups.
The first group consists of those processes which have their maximum
rate of change early in development, and it includes both growth-
rate and differentiation-rate. The second group consists of those
processes which have their maximum rate of change late in develop-
ment, and it includes, firstly, the concentration of solids in the
embryo (called by Murray "chemical growth ") , secondly, the increase
in complexity or change of composition in the solid substance (called
by Murray "chemical differentiation") *, and thirdly, the metabolic
rate or respiratory intensity. These two groups of processes correspond
to two type curves which in some circumstances show skew symmetry
round a central point. The first group has been termed by Murray
the group of "primary integration" and the second one the group
of "secondary integration", and he has in various papers associated
the two groups with the primary and secondary redistributions of the
evolutionary process in Herbert Spencer's scheme, or, as they are
often called, simple and compound evolution.
Murray found that the surface volume ratio was also a member
of the first group, changing most rapidly in the earliest stages and
falling in a way not unlike the growth-rate. He calculated it from
the formula of Meeh : 2
S= KW\
and, although such an application to the chick embryo rests in-
evitably on several unproved assumptions, it is probable that his
curve is not wholly misleading (see Fig. 94) . More interesting still,
he found in a later paper that the absorption curve (the grams
absorbed per gram dry weight per day) was not a member of either
group, for it was sigmoid, falling rapidly for a time, then less rapidly,
and then more rapidly again. His absorption curve, however, was
derived from the oxygen measurements which have already been
criticised, a fact which might account for its S-shaped character.
About the same time, I also calculated the curve of absorption-rate,
not from respiration experiments but from direct measurements of
protein, fat and carbohydrate, and I obtained a curve belonging to
group I, i.e. changing most rapidly at the beginning of development.
It is very interesting that the curve of absorption-rate should be
found to follow the curve for surface/volume ratio (see Fig. 253).
* This process of "chemical differentiation" must not be confused with "chemo-
differentiation " (seep. 571).
SECT. 3]
AND ORGANISATION
551
Murray's presentation of the facts does bring an order into a realm
where order is much needed. But it is not ahogether easy to see why
increase in dry weight should be regarded as "chemical growth", for
water is just as much a chemical compound as anything else. Why should
the title of chemical compound be restricted to those bodies which
do not happen to be liquids at room temperature? And the wetness
Deyas
11 13
Incubation age
Fig. 94-
of the embryonic body must also exercise a profound influence on
the speed and nature of the chemical reactions going on within it,
as is emphasised in the work of Ruzicka and his school. There is
therefore some reason for objecting to the term "chemical growth",
though the simple fact that rate of increase in dry weight follows an
opposite course from rate of increase in wet weight (i.e. "growth")
is plainly of importance. There is less reason for objecting to the
term "chemical differentiation", for Murray uses it to refer to such
entities as percentage of ash and percentage of fat, and shows that,
although for by far the greater part of the embryonic period the
552 ON INCREASE IN COMPLEXITY [pt. iii
former is descending and the latter ascending, they are doing it in
the same phase, i.e. slowly at first and more quickly afterwards.
Probably the most significant result which emerges from Murray's
symmetrically diphasic plan is that growth-rate and differentiation-
rate go together, and in opposition to metabolic rate.
Enriquez's paper of 1909 contains a foreshadowing of the idea of
chemical differentiation (see also Scammon & Ness).
S'S. Chemical Processes and Organic Form
In considering the processes of differentiation in the embryo, there
has been much disinclination to admit their physico-chemical
nature. It is the great credit of His that he took the lead in this
direction, pointing out that processes such as the production of the
neural and amniotic folds were the inevitable results of unequal
growth controlled by physico-chemical factors taking place in what
was, to start with, an undifferentiated sheet of embryonic cells.
His's artificial blastoderms, again, made of pills of dough to which
varying amounts of yeast in various places are added, provide a
close model for the embryo. D'Arcy Thompson has described the
incredulity and opposition which the views of His met with, especially
after the publication of his classical letter in the Proceedings of the
Royal Society of Edinburgh in 1888. As Garbovski put it, "it is absurd
to treat the living being as if it were made up of vesicles, cylinders,
and plates, and not of vital units". Embryologists such as Hertwig
and Balfour held that, in the study of development, a sufficient causal
explanation of one stage had been given when the immediately
preceding stage had been adequately described. " My own attempts",
said His in a famous passage, "to introduce some elementary
mechanical or physiological conceptions into embryology have not
been generally agreed to by morphologists. To one it seemed ridiculous
to speak of the elasticity of the germinal layers; another thought
that by such considerations we put the cart before the horse; and
one more recent author states that we have better things to do in
embryology than to discuss tensions of germinal layers and similar
questions, since all embryological explanations must necessarily be of
a phylogenetic nature. This opposition to the application of the
fundamental principles of science to embryological questions would
hardly be intelligible if it had not a dogmatic background. No other
explanation of living forms is allowed than heredity and any which
SECT. 3] AND ORGANISATION 553
is founded on another must be rejected — yet to think that heredity-
will build organic beings without mechanical means is a piece of
unscientific mysticism."
Such opposition to mechanical explanations in embryology has
of course long been dead, and there are many authors who have put
forward such theories to account for the phenomena of differentiation
in the embryo. But these theories are for the most part quite physical,
depending on physical properties such as elasticity and torsion, so
that they cannot be considered in detail in this book. It is also
very unfortunate that, owing to the exceedingly small size of the
parts undergoing such changes, practically no direct work has been
done, and investigators have focused their energies on the prepara-
tion of models, made of a variety of materials, such as rubber and
plasticine, which can be made, like the yeast pills of His, to exhibit
the phenomena of gastrulation and the like. Roux in his valuable
review of the technique of " Entwicklungsmechanik " has described
such rubber models, and an even greater collection of them is to be
found in the paper of Rhumbler. Rhumbler gives in full the litera-
ture on this subject (e.g. Morgan; Schaper & Cohen; and Spek), and
has many illustrations of invagination models, etc. (see especially his
Section F onwards) . These investigations are interesting indeed, but
they do not contribute a great deal to our knowledge of what happens
in the early stages of embryonic development, although they cer-
tainly set forth a number of ways in which it might conceivably
happen. Robertson's theory of cell-division, again, in which lecithin
was supposed to be broken down at the two nuclear regions of the
dividing cell to provide phosphoric acid for new nuclein, and the
resulting free choline to diffuse away, reaching its greatest concentra-
tion in the equatorial plane at right angles to the line joining the
two nuclei, was never shown to hold in actual fact. Like so many
of the "Nachahmung" models, it rested upon an unwarranted
simplification of the material under discussion. Nevertheless, it was
an ingenious suggestion, and, in spite of McClendon's criticisms, it
may still be found to contain a modicum of truth, but the reason
why it and others like it will not be taken up in detail here is because
they are not sufficiently close to the facts. The purpose of this book
is to give all the facts that are known about the physico-chemical
aspects of embryonic development, and not the theories, which,
indeed, would demand a much larger treatise.
554 ON INCREASE IN COMPLEXITY [pt. m
That there is at present a certain gulf between physico-chemical
research and the "form" and "shape" of the morphologists is a fact
which must be faced. In present-day biology, there lives on, still
very hale and hearty, the essence of the distinction made by Aristotle
between vXt] and eloo<;, matter and form. It is probably at bottom
this which inspires the statements so often made by morphologists
that do what one will with chemical methods, the meaning of form
in animals will always elude one's grasp, for it belongs to another
order of existences, a range of concepts intrinsically remote from
physics. In so far as the form of living organisms is an expression
of a degree of organisation higher than anything with which the
sciences of the non-living world have to deal, it is true that we have
to deal with something very different from mere heaps of molecules,
but crystal form and the colloidal state, which exhibit an inter-
mediate degree of organisation, exist in the inorganic world and can
be dealt with by the quantitative methods of physics and chemistry.
Examples of the extreme morphological point of view are common;
thus Cunningham in 1928, discussing chemical embryology, re-
marked of two eggs in the same incubator, "Why are the bones
formed in one case the bones of a chick and in the other case the
bones of a duckling?" For him the fundamental problem is, not
how does the rabbit get out of the hat, but why a particular kind of
hat should produce a particular kind of rabbit. In one sense the
question is simply a special case of the general question, why is the
universe what it is and not something else? and thus reduces to a
query concerning the fundamentally alogical character of the universe.
With such conundrums the scientific investigator is not concerned
and Cunningham should have addressed his inquiry to meta-
physicians. Cunningham's question had already been raised by
C. D. Broad. "The ultimate question", said he, "is, how do these
particular material systems called organisms come to have their
particular structure or components. So long as we explain their
origin by laws, whether mechanical or otherwise, we are merely
referred back to earlier collocations of matter, and so on ad infinitum.
The explanation in terms of a designing mind on the analogy of
humanly constructed machines seems to involve a circle or to end
in a mind so different from any that we know that the analogy fails
and it is hardly worth while calling it a mind. The explanation by
entelechies rests on a confusion and avoids no difficulty which is
SECT. 3] AND ORGANISATION 555
raised by the notion of an external designer. The problem, as far
as I can see, is extra-scientific and quite insoluble."
There is a sense, however, in which Cunningham's question has
a meaning for science, and all that can be said, in answer to it is
that the exact biologist has a hope, a belief, that in the long run
the outward forms and shapes of living animals are as much de-
pendent on the properties of what we call matter, as anything else
about them. To object that this is to endow atoms and electrons with
occult properties and potentialities is not reasonable, for the only
alternative is to abandon all hope of bringing form and shape within
the coherent scheme of the scientific world-picture. We cannot resign
ourselves to leaving them out in this way, and there is the less cause
for despondency when we see what great progress has been made
along such lines of research as that pursued by d'Arcy Thompson
and summarised in his Growth and Form.
Nevertheless there are real difficulties in this subject, and J. H.
Woodger, who has acutely felt them, has even gone so far as to
maintain that biology may for ever consist of two irreconcilable
divisions, morphology and physiology. Experimental and causal
embryology, in his view, is only physiology disguised. He would
regard morphological description as an end in itself, and the ultimate
aim of the morphologist to come down to solid geometry instead of
to causal relationships. There may be in this view an element of
truth, but in so far as any morphologist holds such an opinion of
his goal he must admit himself to be an artist searching for the
aesthetic experience of significant form rather than a scientific in-
vestigator seeking for understanding of how the thing works — surely
the essence of scientific explanation.
Another thinker who has vigorously opposed the extension of
physico-chemical concepts to include morphology is E. Rignano.
In a discussion of the relations of biochemistry with embryology he
said, "Now what do the ultra-mechanists do in the presence of these
teleological manifestations of the generative and regenerative pro-
cesses? They direct all their efforts to an attempt to prove that
given chemical substances exercise a morphogenetic action on par-
ticular developments, hoping to conclude triumphantly that the
entire series of morphogenetic phenomena, constituting the onto-
genetic development, may be explained completely by physico-
chemical action. But in this attempt they have mistaken a mere
556 ON INCREASE IN COMPLEXITY [pt. iii
release of morphogenetic activities already potentially in the de-
veloping embryo for a genuine morphogenetic action". Now it is
easy to adduce in the light of biochemical researches, chains of causes
which can have the effects seen in the developing embryo. One could
start with the experiments of Huxley on the morphogenetic action
of thyroxin. We know from Ahlgren's work that thyroxin has definite
effects on oxidation-reduction mechanisms in vitro, just as other
hormones have, and anything that may locally affect oxidation-
reduction mechanisms has every chance of affecting the local fatty
acid concentration, as is indicated by the work of Hopkins, so that
in due course the lipocytic constant or some other such cellular
value (cf. Mayer & Schaeffer) will alter and change correspondingly
the surfaces of the intracellular phases in that region, with the final
outcome that, as in the models of Warburg, one chemical substance
may be formed instead of another. These two alternatives may be
thought of, for instance, as scleroprotein on the one hand or phos-
phoprotein on the other, and thence it requires little imagination
to picture the most profound morphological changes taking place.
These causal chains are being unravelled every day.
But Rignano was probably prepared to admit the cogency of these
mechanisms within their own sphere; what he wanted to know was,
why should one embryo pass through all these changes and come
out at the end a dogfish, and another pass through them and come
out a skate. The standard answer of exact biology has, of course,
been that the genetic constitution of the former animal governs
the chemical morphogenetic processes, catalysing this and inhibiting
that, so as to produce the results we find. Rignano, Haldane, and
their associates have often replied that such a preformationism implies
a complexity too great to be imagined when it is faced with the
facts of biology as a whole, but this depends on one's imagination.
The alternatives, it might be argued, are much worse.
The adversaries of genetics too often seem to suppose that every
one of the infinite number of characters which they see in a given
animal has to be represented in some way, within the nucleus. They
forget that large blocks, as it were, of the specific- characters may be
the result of single processes set in action by a gene, and do not
consider the possibility that a good deal of morphogenesis may be
associated with a "delegation of function", the gene activating
secondary key-factors, just as statesmen delegate many of their
SECT. 3] AND ORGANISATION 557
duties to subsidiary but competent officials. This possibility becomes
a probability when the apparently mysterious and arbitrary grouping
of characters is considered, colour x always going together with pollen-
shape jv and so on.
In any case there is no need to load all the responsibility for the
adult skate and dogfish on to their genetic equipment; any more
than we need suppose the constitution of their eggs to differ simply
by the presence of two different entelechies, as Driesch would say
(see p. 22). It is true that in many respects the chemical constitution
of eggs is alike in different animals, but only when very broadly con-
sidered. We know that the provision of amino-acids is by no means
the same, and serological differences, which have so far only been
touched on by a few workers (see Section 19), are likely to be of
much importance. Riddle has also emphasised the importance of
the environmental factors in contributing to the final result of onto-
genesis, so the three principal sets of morphogenetic causes which
exact biologists at present accept are thus:
(i) The genetic constitution of the egg-cell.
(ii) The physico-chemical constitution of the egg's raw materials.
(iii) The environment during ontogeny.
Woodger observes that modern genetics owes a debt to the pre-
formation theory of the eighteenth century. The old theory identified
the "immanent factors" in the egg with the whole of the newly
separated individual, and imagined nothing but an increase in
volume. The modern theory identifies the immanent factors with
certain small bits of the individual, such bits being thought of as
related to the qualities of the individual as cause to effect. Woodger
considers that we shall get on better by sharply distinguishing
between genetics and embryology instead of by attempting to fuse
them, as is now the general aim, for he regards the formation of
parts as fundamentally or causally separate from the determination
of characters. I must refer those who are interested, to the original
discussion, but if we are not to treat parts as characters, an entirely
new conception of evolution will be required.
The difficulty of fusing morphology and physico-chemical biology
is, in fact, very real, and Rignano's objections, though they are far
from insuperable, are not merely restatements of finite teleology.
They bring up the question of how any real epigenesis can take
558 • ON INCREASE IN COMPLEXITY [pt. iii
place at all, i.e. how from moment to moment the level of actual
organisation in the embryo can rise, a question of much theoretical
importance to which I shall return in the Epilegomena. But
chemical embryology will never allow itself to be restricted to the
description of relatively superficial events in the life of the embryo,
such as the appearance of enzymes in the digestive tract. Ii will
insist on expanding physics and chemistry, if necessary, to cover
the animal level of organisation. It will affirm that if we
knew all that there is to be known about the physico-chemical
constitution of the egg, we should be able to predict the results
of its development. This affirmation does not imply that we
should be able to make such predictions before any given case had
been observed ; for the fact of emergence is real and it is true that
knowing all the properties of simpler systems, including how they
could combine with one another, does not tell us how in point of
fact they actually do combine. The schemes of science are resultant,
not emergent, they cannot describe the complex systems before they
have been observed, but they can and do offer reasonable causal
explanations of them with reference to their simpler factors, after
they have been observed. In this way we are not sure that physico-
chemical embryology will ever be able to say what a hitherto un-
known egg will develop into, but we do expect it some day to be
in a position to offer a reasonable causal explanation of the origin
of all measurable properties of adult living beings from the measur-
able properties of their eggs. And form is evidently one of these,
just as is physical constitution.
There is a great deal of confusion at the present time about such
questions and very few workers stop to ask themselves what is the
true aim of their studies in causal or exact biology. Moreover, many
biologists are uncertain as to the meaning of the facts with which
this book is dealing. "I have not so far been able to discern", writes
a Belgian embryologist to me, " the truly explicative value of quantita-
tive measures in embryology. Far be it from me to minimise the
interest of the analysis of metabolism in development, but I have the
impression that it is more thrilling for the biochemist than for the
embryologist. For the latter the cardinal problems remain the
setting-in-action of development, differentiation, or heredity, and
their mystery lies wholly in the laws of cell-life. Now these laws
have not so far been elucidated, and we hardly know anything of
SECT. 3] AND ORGANISATION 559
how one part of the cell acts on another; thus there remains an
enormous qualitative task to accomplish before we can begin to
enter on a numerical phase. I have been thinking of all this in reading
the monograph of M. Rapkine on the energetics of development,
where the physico-chemical facts are presented in a plain sort of
manner lending itself well to meditation. But I confess that it all
remains sibylline, enigmatic, to me, and some acute friends have
admitted the same embarrassment. Where is the explicative value,
the light, the link between all these facts so interesting, yet so
isolated? I cannot hit on it." This excellent passage, which my
friend has kindly allowed me to quote, expresses admirably the
doubtful air with which many biologists regard the extension of
physico-chemical concepts into morphology. In answer I pointed out
that chemical embryology is a very young science and if more
attention had been devoted to it in the past, would not show the
blank spaces and the gaps of which my friend complained. In a
word, Roux's decision to concentrate solely on the revelation of the
''secondary components" in embryogenesis has had momentous
effects, but there were always those who could not resist the tempta-
tion of going deeper down to the "primary components" before all
the mass of facts had been dealt with on the more superficial level.
Among such miners at the deeper levels are the physico-chemical
embryologists. It is true that an enormous qualitative task remains
to be accomplished, but there is room enough for both kinds of
work and it will surely be through the close co-operation of both
kinds of worker that the facts will lose their sibylline and enigmatic
character.
3-4. The Types of Morphogenetic Action
In order to have some concrete idea as to how the processes of
morphogenesis may be supposed to go on, and to form a background
for chemical facts, one may construct a table in which are placed
all the different kinds of process that may go on during differentia-
tion. Such classifications have been made by KeUicott and by
Jenkinson, and tables constructed from them are given here. (Charts
IV and V.)
Jenkinson's table may be taken first. The movements of single cells
which he enumerates are of much importance. The lower-layer cells
of the blastoderm of elasmobranch fishes, for instance, are free and
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562 ON INCREASE IN COMPLEXITY [pt. iii
have amoeboid properties ; they form an example of the first class.
Linear aggregation would be found in the formation of blood-vessels
and superficial aggregation in the formation of the yolk-sac in certain
mammals. Aggregation of cells into spherical masses is common in
organogeny, e.g. the origin of the spleen. The attachment of isolated
cells to another body takes place when tendon is united to bone,
and penetration by isolated cells into another body when the vitreous
humour of the eye is secreted. Phagocytosis plays a large part mostly
in post-embryonic differentiation such as the absorption of the tad-
pole tail, and perhaps in insect metamorphosis, but change of shape
in cells is usual in all embryos, i.e. from flat to columnar or from
neuron to neuron plus axon.
The movements of linear masses are more important still; thus
growth in length is seen in the back growth of the segmental duct
during the formation of the somites. Splitting occurs at the end of
linear masses whenever they branch as nerves and vessels frequently
do, and throughout their length in, for instance, the elasmobranch
segmental duct. Anastomosis is very common within the circulatory
system, and fusion with other organs occurs in the case of the vasa
eflferentia and the mesonephros, as also in the case of the formation
of the pituitary body. The movements of superficial aggregates,
sheets or layers, involve various alterations, such as increase in area
in the growth of the chick blastoderm over the yolk or the formation
of the mammalian blastocyst. This increase in area may be uniform
all over the surface, in which case the structure at the end of the
process will have much the same form as it had at the beginning,
or it may not be, in which case invaginations and foldings will be
produced. The optic cup is produced in this way, as also is the
medullary groove. As will later be seen, this unequal expansion of
surface has important connections with the theory of metabolic rate.
The sheet of cells may also thicken as the mammalian trophoblast
does in the formation of the placenta, or it may become thinner as
the roof of the thalamencephalon and medulla does, or the outer
wall of the lens. If it goes still further in this direction, it may break
altogether and a perforation be formed. This is what happens to
the floor of the archenteron in the amniota, and in many other
instances. Again, two separate layers or sheets of cells may fuse into
one by a process of concrescence, and this is what happens when
the medullary folds close over the medullary groove uniting flatly
SECT. 3] AND ORGANISATION 563
together, or when the stomodaeum unites with the gut. Here the
concrescence is one of surface, but it may also take place between
the margins of the layers, as in the union of the embryonic plate
with the trophoblast in some mammals. Finally, a layer may spHt
into two by lamination, which is what happens in the inner wall
of the pineal vesicle in lizards.
Aggregates of cells in mass formation may also be subject to quick
and profound changes of shape. The outgrowth of the limb-buds
affords an example of simple increase of size and weight, but more
complicated processes may take place also, such as the elaboration
of internal cavities or special internal structures. The blastocoele
cavity offers itself as an obvious instance of the former alteration,
but another one would be the origin of blood-vessel cavities from
soUd ropes of cells. Internal re-arrangement is also seen in the
formation of the concentric corpuscles in the thymus and the spleen.
Division of masses occurs plainly in the production of somites, i.e.
the metameric segmentation of mesoderm and neural crest. Fusion
of masses occurs when two gangUa unite and attachment of
masses is seen when the sclerotome and the notochord come
together.
Another version of the same catalogue is seen in the table taken
from Kellicott. Both of the tables include practically all known
morphogenetic processes. But the point to which I wish to direct
attention specially here is the possibilities which such classifications
hold out of laying bare the physico-chemical processes which accom-
pany the differentiation phenomena. Up to the present time, no
chemical experiments consciously directed towards this end have
been done, but the body of knowledge which has accumulated about
the chemistry of the embryonic body as a whole, and which will be
reviewed in the succeeding sections, does make it possible to draw
certain conclusions in relation to morphogenesis. These tables of
KelHcott and Jenkinson are given here to serve rather as a back-
ground for what follows than as a summary of the facts. We have
in them an assembly of the various means by which differentiation
takes place. Each of those means must have, as everyone would
grant, a physico-chemical aspect. We may prefer to say that each
of them must have a physico-chemical cause and basis. The main
aim of the chemical embryology of the future will be to unravel
these causes and their inter-relations.
36-3
564 ON INCREASE IN COMPLEXITY [pt. iii
General increase of thickness and extent is among the simpler
problems, and seems to be but a sub-section of the general question
of growth. All the discussion of this difficult subject is complicated
by the fact that nearly everything that has been ascertained about
the physico-chemical constitution and activity of the embryo has
been ascertained on the intact organism. It is only in very recent
times that the first step has been taken towards the penetration of
the internal balance of the embryo, namely, the evaluation of the
growth-rates and relative sizes at different times of its constituent
organs. But surer knowledge about this is bound to come in the
not too distant future. Meanwhile it is simple to picture the processes
of perforation, for example, as being due to local controlled autolysis.
Here the importance of local variations of intracellular pB. must not
be forgotten, and a study of the intracellular p¥L in a great variety
of regions at different times during development would be a most
valuable work, especially if it were to be done with methods much
better than those which have up to now been employed in the
estimation of intracellular pH. Buytendijk & Woerdemann have
made a good start in this direction (see Section 6- 1 ) , and there are other
investigations which have a similar theoretical importance, but they
will be referred to in their place. The powerful influence of />H on
cell-division, as in the work of Reding & Slosse and of Vies, Dragoiu
& Rose may, liowever, be mentioned in this connection. The two
tables should be borne in mind throughout all succeeding chemical
discussions.
Just as perforations may be probably due to local controlled
autolyses, so general disappearances of masses of cells may be due
to the same factor operating on a slightly larger scale. This con-
ception is made all the more Ukely by the work which has been
done since 191 9 on insect metamorphosis, and which goes to show
that there also an autolysis plays a large part. Again, the changes
in the form and size of individual cells which may grow larger or
smaller may probably depend on the increasing or decreasing con-
centration of storage materials in them, and here there is an extra-
ordinarily wide field for histochemical methods. Unfortunately these
are at present so unsatisfactory that it may be a long time before this
aspect of morphogenesis can be even approached. The separation of
cells and their fusion into surfaces and layers may also be due to
local variations in the sterol and lipoid fractions (tissue constants),
SECT. 3] AND ORGANISATION 565
the molecules of which might, by changing either their constitution
or their position in space or both, be the underlying factors responsible
for the process. An immense field lies open here for the application
of micro-estimation methods on sections of tissue dissected out from
embryos in the early stages of development, a field which has not
so far been entered by one investigator.
Processes such as chemotropism, moreover, probably have much
importance in morphogenesis. This has been emphasised by Kappers
whose work on neurobiotaxis has thrown much light on the early
development of the nervous system.
Associated with this conception is that of the localised presence of
enzymes. Everything, indeed, depends here on the word "localised".
The localisation of substances in the embryo is a study which has
not yet begun, for nearly all we know about its physico-chemical-
life is concerned with it as a whole and with its immediate sur-
roundings. The study of embryonic hormones, for instance, to which
much attention will later be given, has not so far been carried on
from this point of view, although it does seem clear that the embryo
in the early stages is quite devoid of "chemical messengers", and only
acquires them later at definite time-points in its development. One
may say that the "Nachahmung" school have done much to suggest
possible morphogenetic processes without doing anything to find out
what they actually are in point of fact. The only means by which this
can be done are applications of exact physico-chemical methods, and
these will have to be made use of more and more in the future.
We may, however, before leaving this subject take one instance
where chemical work has thrown a light on the state of affairs in
the young embryo. In 1926 I studied the free and combined carbo-
hydrate contained in the embryonic body of the chick embryo from
the 5th day until hatching. Thus 100 gm. of embryo (water and soHd
together) contained on the 5th day of development 160 mgm. of total
carbohydrate and 8 mgm. of free carbohydrate. During develop-
ment the former falls to about 100, and subsequently rises to over
300; the latter rises all the time in an S-shaped curve to about
48 mgm. This would indicate that the carbohydrate present in the
embryo on the 4th and 5th days, at which time there is more in
proportion than at any subsequent period, is not in the form of free
glucose, and it is certainly not glycogen. The dry weight data gave
even more striking results. 100 gm. of dry embryo contain on the 5th
566 ON INCREASE IN COMPLEXITY [pt. iii
day 3000 mgm. of total carbohydrate and about 100 mgm. of free
carbohydrate. After that point the total carbohydrate continuously
falls, reaching a value of 1750 on the i6th day, while the free carbo-
hydrate continuously rises, its highest point being reached on the
nth day with a value of 360 mgm., after which it falls, but not
below 220 mgm. We have therefore a considerable proportion
of the sugar present throughout not free and not in the form
of glycogen. It is very probable that this fraction can be called
"mucoprotein glucose", and that it is present in the form of com-
bination with a protein molecule. Its quantitative relationships are
interesting, for it amounts to 2550 mgm. per cent, dry weight on
the 5th day, but by the nth day it has fallen to about 1000 mgm.
per cent., after which it remains about steady at that level. No
stress is laid on the absolute magnitude of the figures, for the best
methods we have for estimating total carbohydrate almost certainly
give high results, but there is no reason for supposing that the relative
values are not significant.
It seems that this behaviour can be correlated with facts that have
long been known to histologists. Von Szily was the first to describe
a cell-free connective-tissue or fibrous ground-substance filling up all
the cavities of the embryonic body in the early stages. This has some
affinities with the cardiac jelly of Davis, and Baitsell has recently
examined its properties with the aid of a micromanipulator. An
account of its comparative histology is given by Biedermann. It
appears to be secreted by the cells, and provides them with a homo-
geneous matrix, a kind of natural culture medium in which even
migration may take place if and when it may be necessary. As
development proceeds, the substance does not disappear, but be-
comes relatively less and less important in relation to the body as
a whole. The nearest equivalent to this ground-substance elsewhere
is the Wharton's jelly of the umbilical cord and the vitreous humour
of the eye. It is significant that both these tissues are known to be
very rich in substances of the mucoprotein type, and the importance
of mucoprotein in the beginning of embryonic life leads inevitably
to a correlation with the jelly of von Szily. It explains, or is explained
by, the high proportion of combined glucose other than glycogen
at that period. This example is intended to give an idea of the kind
of correlation between chemical and morphological characteristics
which is necessary for the embryology of the future.
SECT. 3] AND ORGANISATION 567
Such correlations, in the position in which we now are, are difficult.
It is also very difficult to bring together the work of the "Entwick-
lungsmechanik " school and the results of physico-chemical work,
but close attention must be given to it in order to orientate
the point of view adopted in this book. Any exhaustive discussion
of it is rendered unnecessary in view of the papers by Huxley
and Spemann, and the excellent book of de Beer. Other recent
reviews of experimental embryology are those of de Beer; Przibram;
Mangold; Brachet; Weiss; Gilchrist; Hogben and Schleip.
It will not be possible here to do more than indicate a few of the
outstanding problems to which modern experimental embryology
has supplied tentative but fairly certain answers.
3-5. Pluripotence and Totipotence
One may first ask, therefore, what answer has been given to the
question of organisation in the unfertilised egg-cell, or, in other
words, what modicum of preformationism are we still obliged to
admit into our conception of the undeveloped egg. In the very
earliest stage of development two important critical points occur,
or two stimuli are received, the first causing the egg to become
radially symmetrical about an axis, and the second causing it to
become bilaterally symmetrical in any one of an infinite number
of planes passing through that axis. What these processes actually
are is entirely unknown, although we can make a guess, as will
later be seen. But the fact that they have to occur before differentia-
tion and growth can begin makes it impossible to speak of the spatial
arrangement of already differentiated material in the egg, which was
the basis of the classical preformationism. It is now correct to say
that heredity does not account for the individual but for a number
of potentialities, some of which are brought into being in the indi-
vidual. The only predetermination which exists is the assurance that,
if the potentialities in question are brought into actuality, they will
produce an organism belonging to the same species as its parents.
The appHcation of chemical methods to the undeveloped egg will
probably thus in the future not be so histochemical as would have
been the case if an arrangement of material had existed there.
Nevertheless, what has been said apphes to the vertebrate egg only;
there are invertebrate eggs, such as those of Cerebratulus , which do
seem to have an intracellular determinate arrangement. The state-
568 ON INCREASE IN COMPLEXITY [pt. iii
ment here made may be differently expressed by saying that pre-
formationism is only true in the sense that the constitution of the
fused nucleus must have reference to, though it dots not actually
resemble, the adult form. Protoplasm, and yolk, on the other hand,
as Lillie on Chaetopterus, Morgan & Spooner on Arbacia, and Morgan
on Cumingia have shown, are homogeneous as far as this goes, since
normal development can take place after centrifugation and dis-
placement of visible constituents. (See p. 346.)
It is known that eggs vary a great deal as to their dependence on
the arrangement of the initial material in the egg-cell. The extent
to which they do so is more or less ascertainable by the extent to
which they exhibit totipotence and the rapidity with which they lose
it. The phenomenon of totipotence was originally discovered by
Driesch in 1900, who found that, if the blastomeres of the sea-
urchin's egg were separated at the 2-cell stage, each nevertheless
gave rise to a perfect larva of half the normal size. If the blastomere
was taken from the 4-cell stage it would produce a larva of a quarter
the normal size. Blastomeres from the 8-, 16- and 32-cell stages did
not gastrulate well, and not at all if they were derived from the
animal pole. No blastomere from the 64-cell stage will produce a
gastrula. However, although one blastomere from the i6-cell stage
animal pole will not gastrulate, four together will, and will go on
to produce a perfect larva, so that the inability cannot be due to lack
of specific enteron-forming substance, but simply to too small size.
The gradual loss of totipotence, therefore, is an index of the degree
to which the egg depends on the localisation of organ-forming
materials in the original egg-cell. In some animals the blastomeres
are able to regulate their development, if isolated, and produce a
perfect larva, in other animals they are not: the former are termed
"regulation eggs" and the latter "mosaic eggs". Examples of the
former group, besides the echinoderm eggs just mentioned, are the
coelenterates, which, according to Zoja, can be totipotent up to and
including the 4-cell stage, and the nemertines; but, in this latter case,
blastomeres from the 4-cell stage are not perfectly totipotent, but give
rise to larvae lacking a few of the usual parts (Zeleny) . The group
of regulation eggs passes over without break into the group of mosaic
eggs. Thus in Amphioxus, totipotence exists only at the 2-cell stage,
as was shown by Wilson's experiments, and the same applies to
amphibia, but only under certain conditions, i.e. if each blastomere
SECT. 3] AND ORGANISATION 569
contains some of the grey crescent region, which in time will develop
into the dorsal lip of the blastopore (Brachet) . With the ctenophora
we pass to a purely mosaic egg, for, as Fischel has shown, though
the normal ctenophore has eight combs, the result of a 2-cell stage
blastomere is one with four combs, that of a 4-cell stage blastomere
is one with two combs and that of an 8-cell stage blastomere is one
with only one. Obviously, then, something was distributed more or
less evenly among the blastomeres as the cleavages took place, and
cleavage of the cells involved equal cleavage of the comb-forming
substance. Crampton has discovered very similar phenomena in
molluscan eggs {Ilyanassa), Wilson in those of Dentalium, Driesch
in those oi Myzostoma, Stevens in nematodes, and Conklin in ascidians.
Loss of totipotence may even take place gradually before fertilisation.
The organ-forming materials may be actually visible in the un-
developed egg, as, for example, the brownish yellow pigment in
Styela eggs (an ascidian) which was found by Conklin to be necessary
for the formation of muscle. On the other hand, in some cases, as
Morgan has shown, pigments and formed elements are present
which, while normally distributed to certain cells, can be centrifuged
into a different position in the egg without in the least affecting
development. Other examples of this have already been given. In
1926 Duesberg found that some of the elements which seemed to be
necessary for development, and could not be centrifugally displaced
without damage, were mitochondria. But the most significant experi-
ments were those of Jenkinson, who found that normal morpho-
logical differentiation could go on in the frog embryo even after the
egg had been centrifuged and the fat globules had been made to
get into the "wrong" cells. In this case the brain was formed quite
normally, though it contained, at any rate as far as histological
examination could show, a much greater quantity of fat than usual.
Although this process could not be carried beyond a certain point,
it did show that fat alone was a structural material rather than
a specific organ-forming substance. These regulatory processes oc-
curring after centrifugation were shown by Konopacka for the frog's
egg to be remarkably independent of temperature. Totipotence phe-
nomena probably underlie the remarkable cases of Polyembryony
which occur sporadically throughout the animal kingdom (cyclo-
stomatous Bryozoa, earthworms, parasitic Hymenoptera, and Arma-
dillos) and which have been reviewed recently by Patterson. The
570 ON INCREASE IN COMPLEXITY [pt. iii
Armadillo produces four identical quadruplets from one egg, and a
single Ggg of the parasite Litomastix truncatellus gives rise to about
1500 individuals.
3*6. Self-differentiation and Organiser Phenomena
The passage of time between fertilisation and hatching can, it has
been found, be divided into three periods. These may be summarised
thus: (i) division and re-arrangement of pre-existing material and
structure (up to the end of the formation of the germinal layers),
(2) primary or non-functional differentiation in which the organ
rudiments are determined irrevocably and their early differentiation
proceeds, and (3) secondary or functional differentiation, in which
the inception of active function by the new organs brings about
important consequences. "Up to 1910", it has been said, "the prin-
cipal achievements of experimental embryology could be summarised
thus: first, that nuclear division in early development was not
differential, the nuclei of the embryo all being equivalent; second,
that most fragments of the germ could, before the onset of gastrula-
tion, if of sufficient size, regulate themselves to produce a whole
embryo or an approximation to it; third, that in a few cases
definite 'organ-forming materials' existed in the fertilised &gg —
visibly differentiated regions causally correlated with the develop-
ment of certain organs; and fourth, that Roux's doctrine of the
struggle of the parts was valid in the later stages." The intermediate
part of development was not, in fact, very well understood. But
Spemann's discovery in 1918 that the region of the dorsal lip of
the blastopore was a differentiator or organisator, as it was called,
threw a new light on the middle period. By grafting a piece of it
into another embryo (of different species so as to recognise the parts
by their colours) he found that it would cause the host tissues between
it and the animal pole to form the primary axial organs, i.e. neural
plate, notochord and somites. It would initiate, in fact, the self-
differentiation process, and would cause cells which previously would
have had the potentiality of becoming almost anything according to
their position to set out on their path of irreversible differentiation.
Then Brachet showed that the organiser in the dorsal lip region
exerted its formative effect behind as well as before. If a cut was
made so that continuity between the organiser fragment and the
posterior region was interrupted, there would be no formation of
SECT. 3] AND ORGANISATION 571
notochord and somites behind this obstacle, and, on the other hand,
a cut between the organiser and the anterior region would exert
the same effect there. The influence evidently demands continuity
for its operation.
It is important to distinguish the actual differentiation of this
middle period from the invisible determination which foreruns and
controls it. Spemann showed, for instance, that in the early neural
plate stage, before any sign was present of the optic vesicle, there
was a predetermination of the eye as a whole, and also the separate
areas for optic stalk, retina, and tapetal layer. Bulbar, ventricular,
atrial, and sinus substances are similarly determined before any
trace of the heart appears. Huxley proposed the term "chemo-
differentiation " for this invisible point of decision as to the fates of
cells *. After this point the organism is a chemical mosaic of qualita-
tively unUke regions, in which regeneration is impossible. "If a
gastrula be cut into two", he says, "each half forms only those
organs which it would have formed as part of the whole; again, if
a piece of the future brain region is cut out of the embryo in the
neural plate stage and then grafted back in the reversed position,
its different parts still produce the structures that would have been
produced in its normal position. Each chemically determined region
is separate and irreplaceable." And this state of affairs proceeds until
the onset of function, after which, with nervous control and the
activities of hormones in humoral channels, a third and quite dif-
ferent period comes into being. It is most important to note, mean-
while, that the point of chemodifferentiation is associated with
gastrulation. In the section on resistance and susceptibility a good
deal of evidence will be presented which tends to show that gastrula-
tion is one of the main critical points in development, points at
which a less degree of external interference is required than at any
other, in order to make development go wrong.
The opportunities afforded by the tissue culture technique have
also been utilised in the study of the middle period of self-differentia-
tion or irreversible morphogenesis. Obviously the cultivation of an
explant of a certain region of embryonic tissue at different stages will
(or may) reveal whether it has been determined or not. This deter-
* We do not know how far this process is really chemical. Decision would be easier
if we knew the rate at which, say, medullary plate ectoderm becomes specifically eye-
forming substance. Determination-rate is probably at least as important as Growth-rate
and Differentiation-rate.
572 ON INCREASE IN COMPLEXITY [pt. iii
mination may from one point of view be regarded as the point
where the curve of decreasing totipotence reaches the base-line. For
totipotence persisting into later stages, such as blastula and gastrula,
demonstrates its presence by the capacity of any piece of tissue to
develop into structures which it never would have done if it had
been left to itself The name usually given to this first-period in-
determinateness is "plasticity". Thus before gastrulation Spemann
found that a piece of epidermis normally destined to fold in to form
the ectodermal nerve-cord can be exchanged for a piece of ordinary
epidermis with the result that the presumptive nerve-cord will turn
into skin and the presumptive skin into nerve-cord. Such plasticity
occurs all over the embryo before gastrulation, and Spemann was
able, in some very beautiful experiments, to work with the dark-
coloured Triton taeniatus and the light-coloured Triton cristatus to-
gether so that dichromic grafted embryos were produced. But if one
attempts to make such transformations as these after gastrulation
has been completed, one meets with no success whatever — even
though no differentiation at all is visible, the hidden process has
determined the fate of the cells.
In the preliminary plastic cells even the germ layers are inter-
changeable. Pieces of presumptive ectoderm can be planted just
beneath the dorsal lip of the blastopore, and being carried in as
invagination proceeds can afterwards be recognised as taking part
in the constitution of mesoderm or endoderm. In other words they
behave "Ortsgemass", i.e. according to the arrangements of their
host, and not "Herkunftsgemass", i.e. according to the arrange-
ments of their origin. Such pieces can be earmarked by the use
of vital stains, as Goodale in his work against the concrescence
theory was the first to show, and Vogt has in more recent years done
many experiments of this nature. The converse, namely, that pre-
sumptive endoderm or mesoderm can give rise to ectoderm, has been
demonstrated by Mangold, and similar remarks hold with respect
to the limb buds.
The fact that the organiser of one kind of Triton can bring about
its effects in another kind of Triton naturally led to the question of
what taxonomic distance was necessary to prevent the action of the
organiser. Geinitz, working on this point, found that the organiser
of even very different species such as Amblystoma or Bombinator could
be grafted into Triton embryos with efficient results. The grafts might
SECT. 3] AND ORGANISATION 573
even come from different genera, families, or sub-classes. Obviously
the region of the dorsal lip has in different embryos something very
much in common, which can act in a most remarkable way on
neighbouring tissues. More extraordinary still, the influence is not
a peculiar property of the blastopore lip, for other tissue can be
grafted into it, and then can be used as an organiser after having
been, as it were, "infected".
We have already seen that the organiser requires contact for the
spread of its influence and will not function across a cut. The spread,
Spemann found, also takes a measurable time, for at an early stage
the ectoderm near the organiser is determined to turn into noto-
chord, while further away it is still indifferent. Ruud & Spemann
threw light on the finding of Wilson already referred to when they
observed that, if the newt gastrula is divided into halves, only that half
which contains the organiser will develop. Brandt has thrown further
light on the process of chemodifferentiation and has divided it up
into three periods or phases, reversible, critical, and irreversible.
Thus different amphibian species which look externally in the same
developmental stage may or may not be equally chemodifferentiated.
The influence that radiates from the dorsal lip is not apparently
under the control of any other internal factor; it is therefore called
an "organiser of the first grade". But some of the organs which
differentiate under its influence show in their turn an organising
action on the structures developing near them; thus, the axial struc-
tures exert an effect on the development of the thoracic and abdominal
viscera [situs inversus viscerum experiments). They are therefore
called, as Spemann suggested, "organisers of the second grade".
In the same way the eye-cup is usually, though not always, an
organiser of the second grade with respect to the lens, as Spemann;
Lewis, and many others have shown.
There is certainly no doubt that some organs at a certain stage
contain within themselves all the factors necessary for their complete
development as far as the beginning of the third period, while others
do not, and seem to depend upon adjacent organs. So far only a
few of these relationships have been worked out, but it is very
probable that the factor of time is an important one, and, while
at one stage an organ or part may be dependent upon some neigh-
bouring second-grade organiser, at another stage it may be quite
self-differentiating. Examples of these relationships can be found in
574 ON INCREASE IN COMPLEXITY [pt. iii
the work of Streeter on the amphibian ear, which showed that the
membranous labyrinth is self-differentiating, and of Luther, which
showed that, on the contrary, the cartilaginous auditory capsule is
dependent-differentiating, i.e. in relation to the auditory vesicle as
to a second-grade organiser. Again, in Bombinator the visceral rudi-
ments are self-differentiatory, according to Holtfreter.
Tissue culture methods have been already referred to as important
in the analysis of the differentiatory power of a given anlage. These
may be of various kinds. One of the most valuable is the transplanta-
tion of the tissues of bird embryos on to the chorio-allantoic membrane
of another c^gg, where it can easily be seen whether or not they
continue to develop in isolation, and, if they do, what they develop
into. In this way Danchakov found that the blastoderm of the chick
develops properly in isolation, but only after some hours. The degree
of differentiation here depends entirely on the age of the embryo
from which the piece is taken. Thus Hoadley found that pieces of
an embryo only 4 hours old will, when transplanted on to the chorio-
allantoic membrane, produce an eye with pigment cells only; if the
age of the embryo is 6 hours, an eye with pigment and retinal cells
will be produced, and, if the age of the embryo is 8 hours, an eye
with pigment and stratified retina will result. At 10 hours the
primitive groove is formed, and, if a piece be taken from an embryo
after the beginning of somite formation, a completely self-differen-
tiating eye will develop on the chorio-allantoic membrane. Other
workers have made similar experiments with different regions of the
chick embryo as follows: limbs and limb girdles — Murray & Huxley;
Selby & Murray; and Spurling; eye — Barfurth & Dragendorff;
head — Lillie, and Danchakov; spinal cord — Agassiz & Danchakov;
nose, ear and mesencephalon — Hoadley; metanephros — Atterbury; otocyst —
Fell; gonads — Minoura; mesonephros and primordial germ cells —
Humphrey; various tissues — Hiraiwa; heart — Danchakov & Gagarin.
The other main form of explantation work is the in vitro technique
of tissue culture. This has been used to study self-differentiation
by Reinhov, who found that the kidney of an embryonic
chick would develop glomeruli, tubules, and capillaries in vitro,
and by Strangeways, who got good differentiation in embryonic
cartilage. Working with the same technique Strangeways &
Fell showed that the limb buds of embryonic chicks have only
a small power of self-differentiation in vitro (or in vivo in trans-
SECT. 3] AND ORGANISATION 575
plants), but that, on the other hand, the eye showed remarkable
capacities, producing all the constituents of the normal eye,
but not growing much in size. And with 6-day femurs, perfect
self-differentiation was produced in vitro by Fell & Robison. Most
interesting of all, Danchakov found, while studying the degree of
self-differentiation in various chick tissues, that the mesonephros
degenerated after a time in the grafts, just as it does in the
intact embryo. A commentary on the relations between tissues
acting as second-grade organisers and the tissues they influence is
seen in the observations of Champy, who found that epithelium and
connective tissue (from post-embryonic stages) in pure cultures by
themselves underwent dedifferentiation,iand reverted to an embryonic
condition. But if they were both present together in one culture the
differentiation was maintained, yet only so long as the connective
tissue constituent of such a culture was living. If it died the epi-
thelium dedifferentiated. In just the same way the kidney tissue of
the mouse grows alone as a sheet of undifferentiated Itissue, but can
be made to take on its usual character by the addition of connective-
tissue cells to the cultures (Drew).
"It will be seen", says Huxley, "that the discovery of the organiser
and of the gradient coordinate system enables us for the first time
to give a coherent formal account (however imperfect in detail) of
the early stages in development. In so doing we sail clear of the
difficulty which has beset so many minds of understanding how
differentiation can be compatible with absence of qualitative nuclear
division. Loeb in his book The Organism as a Whole was driven to
assume that the Mendelian chromosomal genes were only responsible
for minor characters, the main course of development being deter-
mined by the ovum, which, owing to its assumed possession of organ-
forming materials, was to be regarded as the 'embryo-in-the-rough'.
It is now seen that the egg cannot be held to become the ' embryo-
in-the-rough ' until chemodifTerentiation has started. After this
moment specific organ-forming substances are all-important, but in
most unfertiHsed eggs they scarcely exist. The production of these
organ-forming substances depends upon the varying interaction of
organiser and genes in regions of various activity. The differential
which determines the variation of activity is the system of metabolic
gradients, which, although definitely organised, is very far from
constituting the egg an embryo, however much in the rough. One
576 ON INCREASE IN COMPLEXITY [pt. iii
of the two main gradients is determined at or after fertilisation by
agencies external to the ovum. The other is determined in the un-
fertilised egg, but from an analogy with other forms, it is to be
expected that this too will be found to have been determined earlier
by agencies external to the oocyte (position of the oocyte in the
germinal epithelium, blood-supply, etc.)." Such is the general
scheme to which the recent researches in experimental embryology
have led. It is to be remarked that it includes in intimate association
with the working of the organiser the theory of metabolic gradients.
It will be necessary to treat this in some detail since it is based on
biochemical conceptions, and in some cases biochemical experiments.
But, before doing so, some further space must be devoted to the
organiser.
What determines the direction of the differentiation induced by
an implanted organiser? It might determine for itself the direction
in which it would radiate its influence or this might be a function
of the host. Geinitz's experiments have shown that the latter alter-
native is the more probable one. At exactly what stage the cells
of the dorsal lip acquire their organising power is naturally very
difficult to ascertain, and has not yet been ascertained with accuracy.
Moskovski long ago found that, if the grey crescent in the frog's egg
was injured by pricking with a hot needle or other means, no em-
bryonic anlage or organ-rudiments would be developed, although
gastrulation was not inhibited, and Spemann obtained precisely
similar results by eliminating the dorsal (i.e. grey crescent-containing)
blastomere at the 2-cell stage. The organiser must therefore originate
very early. But Spemann has so far not been able to get a trans-
planted piece of grey crescent to function as an organiser, although
Bautzmann reports success with parts of blastulae.
Of second-grade organisers Spemann says, "The optic vesicle can
be regarded as the organiser of the lens. Yet the vesicle itself, as well
as any organising power which it may possess, is not evolved through
mere self-differentiation of an anlage of an early gastrula: on the
contrary, the differentiation of this particular anlage itself is deter-
mined by an external stimulus. We have seen that at an early stage
of gastrulation the presumptive eye-anlage can be replaced by pre-
sumptive epidermis and it would be possible to choose the implant
so that it should contain the presumptive lens area. In this case
the two ectodermal layers would exchange their roles — the stimulator
SECT. 3] AND ORGANISATION 577
becoming the stimulated. The optic vesicle may then be termed
from this point of view an organiser of the second grade". The same
state of affairs exists between the medullary plate and the roof of
the archenteron. Spemann showed that a piece of presumptive
ectoderm, e.g. a piece of presumptive medullary plate, could be
made to develop into archenteron when implanted into the dorsal
lip of the blastopore and allowed to pass inwards with the gastrular
invagination. But, on the other hand, the same experiment can
be reversed, and a piece from the roof of the archenteron taken,
grafted somewhere else and so allowed to induce a medullary plate
in any indifferent epidermis. "Geinitz", said Spemann, "recently
combined these two experiments into one. A piece of presumptive
epidermis was removed at the beginning of gastrulation from an
embryo of Triton taeniatus, heavily stained with intra-vital stains and
implanted into the dorsal lip of the blastopore of another unstained
embryo at the same stage of development, in such a manner that
it invaginated and formed a portion of the archenteron. It was then
removed again and this time transplanted into the cleavage cavity
of a third embryo at the onset of gastrulation. In the course of
gastrulation it came to lie under the ectoderm and induced in the
latter the formation of a secondary medullary plate. This transplant,
if left in its normal environment, would have differentiated into
epidermis ; under the influence of the archenteron — ^into medullary
plate, perhaps into eye-anlagen and would then have induced the
formation of a lens. In its third environment it became itself an
organiser and induced the formation of a secondary medullary plate."
What is the extent of the region occupied by the organiser? This
question has been investigated by Bautzmann, who definitely settled
it by transplanting small trial pieces from the whole surroundings
into the blastocoele cavity of other gastrulae. Gastrulation brings
these under the ectoderm and then their power of induction, if they
have any, will manifest itself, as Marx showed, by the appearance
of a medullary plate in the overlying ectoderm. The region of the
gastrula, then, which may be said to contain the organiser, is a semi-
circular area above and beside the upper lip of the blastopore.
Normally it is invaginated, and gives rise to notochord and mesoderm.
A good deal is known about the structure of the organiser region.
It must have an axial longitudinal structure which is not lost by
transplantation, for the second embryo may readily be obtained
NEI 37
578 ON INCREASE IN COMPLEXITY [pt. iii
at right angles to the host embryo if the organiser is trans-
planted into it at right angles. It must also have some sort of
"laterality", as has been proved by Goerttler *. Roux long ago con-
cluded from work with half-embryos that each lateral half could
develop more or less independently of the other, and Vogt, by
subjecting salamander eggs to a temperature gradient by holding
them in a silver plate on each side of which water at different
temperatures was circulating, found just the same thing. The left
half could be made to hypertrophy in relation to the right half or
vice versa. Huxley made very similar experiments with a less abrupt
temperature gradient. These experiments showed that the embryo
as a whole has a half-structure. But Goerttler went further in re-
moving the left side of a dorsal lip and grafting a right side, as it
were, from another embryo into it; whereupon two right halves of
a medullary plate and two right medullary folds were formed. Thus
the organiser itself must possess laterality. It has also what might
be called a regional structure, in that different parts of one organiser
tend to produce different embryonic structures. But the question is
an extremely complicated one, for a given part of the organising
region tends to induce more than it ought, and there are signs
that the term "harmonious equipotential system", which was first
applied by Driesch to totipotent blastomeres, may also be required
for certain parts of the organiser region at certain times.
Of the physico-chemical nature of the organiser very little is
known. The small size of the amphibian material on which most
of these experiments have been made, and the body of experimental
difficulties as a whole, have made this side of the subject very back-
ward. Heteroplastic transplantations have shown, as we have seen,
that the organiser is not species-specific. It cannot act across a gap,
but requires continuity of the cell-mass for its effect. The inductive
power of the cells of the dorsal lip is not abolished by drying them,
according to Spemann, but freezing and thawing does lead to its loss.
Mangold has begun some interesting experiments on the quantitative
aspects of the working of the organiser.
Whatever is the mechanism of the organiser, there is evidence that
it retains its activity unimpaired for a long time. Thus Mangold
* Yet Spemann has found the inductive power still present in squashed pieces
from the organiser region. Does this mean that the laterality of the organiser is stereo-
chemical rather than cytological?
SECT. 3] AND ORGANISATION 579
found that a piece of the brain of a free-swimming larva would still
induce a medullary plate in the early embryo, so that the organising
power was still present, although it had long been unnecessary.
Again, Bautzmann transplanted a piece of notochord from a neurula
into the blastocoele cavity of a much younger embryo, and found
that the notochord fragment induced a medullary plate in the
ectoderm above it. It must, then, have retained the power to do so
long after it had been necessary to exercise it in the process of normal
development. Precisely analogous are the experiments of Wachs,
who found that regeneration of the adult amphibian lens takes place
under the inducing influence of the retina, just as had happened
originally in ontogenesis. Such experiments demonstrate that the
organiser persists into post-embryonic life.
Spemann & Mangold also found that, when they transplanted
a piece of medullary plate into the blastocoele cavity of an un-
gastrulated embryo, it would induce in its turn another medullary
plate. This process, which they called " homoiogenetic induction",
is really a special case of the action of a second-grade organiser, in
which a tissue produces a replica of itself.
A process occurring in the first and second periods of embryonic
development, which has not so far been touched on, is that of
"double assurance". Thus the eyeball in amphibia may induce a
lens in foreign epidermis, but in some amphibia the lens may develop
on its own in the absence of any eyeball, i.e. is self-differentiating,
and not dependent on the action of the optic vesicle in its capacity of
second-grade organiser. In Rana esculenta both faculties have been con-
clusively shown by Spemann and Filatov to coexist. Probably further
analysis of development will show that this double assurance principle
plays a great part in morphogenesis, and that cells only become what
they do under the influence of as many as three or four contributing
causes *. The double assurance principle may correspond to the
factors of safety which appear in structural engineering, so that, if
one process goes wrong, the embryo can still manage to complete
its development with the aid of the others.
* Thus Bautzmann got differentiation of medullary plate from fragments of Triton
blastulae which lacked the entire organising region. Similarly Hoadley got self-
differentiation of parts of the chick blastoderm at 4-6 hours, whereas Waddington showed
such parts to be still plastic up to 18 hours or longer. It is clear that chemical
determination is often controlled by more than one agency, and that the times of activity
of these agencies may overlap.
37-2
58o ON INCREASE IN COMPLEXITY [pt. iii
It is to be noted that all the experiments which have been de-
scribed have been carried out on amphibian material, but the
evidence which leads us to see in them a validity over all types
of embryo, even mammalian ones, is rapidly accumulating. Seidel
has extended the concept to insect eggs, while von Ubisch ; Runnstrom ;
and Horstadius find evidences of it in those of echinoderms, Wilson
in those of annelids, and Graper; Hunt and Waddington in those of
birds. It is highly probable that organiser phenomena will in time be
found to exist in all varieties of embryo. The division of embryonic
development into three main periods, however, is more generally
certain, and may be taken to hold in all cases.
As we have already seen, the junction between the first and second
periods of development varies with different eggs. In some (regula-
tion eggs), the point of chemodifferentiation does not occur till
gastrulation — this is its latest point — but in others it occurs earlier,
at some time during cleavage and blastula formation, while in pure
mosaic eggs, of which few are known, it occurs before fertilisa-
tion. The duration of the second period, the period during which
irrevocable differentiation is going on, is rather variable. It ends
with the beginnings of function on the part of the foetal organs.
3-7. Functional Differentiation
In this third period further differentiation may be dependent
on functional activity for its proper progression. The classical
example of these mechanisms is the circulatory system, in which
Oppel & Roux found that the structure and constitution of blood-
vessels depended largely on how they were being utilised by the
circulation as a whole. Fischer & Schmieden, for instance, trans-
planted a section of vein into the course of an artery, where it acted
perfectly well, but took on the characteristics of an artery, i.e. its
connective tissue content increased and its muscular walls were more
than doubled in thickness. Exactly the same thing happens with
regard to the central nervous system, and to the bones. "The
formation of the normal structure of the bones ", concluded Landauer,
"is caused largely by the static conditions of muscle tonus during
embryogeny." Diirken's well-known experiments may be men-
tioned, in which, when the hind limb buds of a frog embryo are
removed, the hind brain does not develop normally. Again, Babak
observed that in tadpoles the area of the active intestinal absorptive
SECT. 3] AND ORGANISATION 581
surface is directly proportional to the amount of vegetable matter
in the diet; if it is large, the intestines are capacious and long, if it
is small, the intestines are small. In this connection the case of the
changes in the intestinal tract of the opossum studied by Heuser
is of interest, for it has a gestation time of only 1 3 days, and has to
live some time on milk in the pouch. Babak's work was criticised by
Klatt, but has been confirmed more fully by Elven.
The three periods in the life of the embryo are now known to
have different relations to regeneration. Przibram's "law of apo-
genesis" holds true in the main, namely, that the younger an animal
the greater are its powers of regeneration (cf. the work of Abeloos
on Planaria dorotocephala) . But this must be qualified by the statement
that, during the intermediate period, no regeneration is possible.
Each individual part and organ of the embryo has its work cut out
to differentiate into its destined form, and any replacement of lost
parts cannot be made. Thus Spurling found that the limb buds of
the chick cannot be reformed during this middle period. But in the
later period of functional differentiation, regeneration is possible,
as has been shown by Olmsted; Morgan & Davis; Davidov; and
Nussbaum & Oxner. Perhaps the best study of this is the work of
Mackay, Mackay & Addis on compensatory hypertrophy after uni-
lateral nephrectomy. Their figures were as follows :
Age in months % hypertrophy
I 52-6
3 36-7
6 32-8
12 32-2
It is in this later period of functional differentiation that Roux's
doctrine of the struggle of the parts has its significance. The signs
of this equilibrium between organs and tissues do not emerge except
when the organism is subjected to the stress of an unfavourable
environment. Then some parts will be found to have the preference
and to take a relatively greater share than the others in the available
food-supply. The reproductive glands have an important position
here, e.g. the testis of the starved rat in Siperstein's work, and there
are many instances where the ova in course of preparation draw to
a great extent upon the remainder of the body. A discussion of these
facts will be found in the appendix on the maturation of eggs ; here
it may suffice to mention the work of Greene on the salmon, who found
a remarkable constancy in the chemical composition of the ovaries,
582 ON INCREASE IN COMPLEXITY [pt. iii
although that of the rest of the body was varying considerably
according to the food eaten.
3 -8. Axial Gradients
It is now time to turn to another aspect of the analysis of embryonic
development, the theory of axial gradients. A convenient transition
is afforded by Bellamy's discovery that the dorsal lip of the blasto-
pore and the animal pole in the frog's egg are regions of "high
protoplasmic activity". According to the theory of axial gradients,
this would mean that the metabolic rate at those points — obviously
of great importance as being the seat of the organiser — was higher
than anywhere else in the embryo at that moment. It is plain that
this is a matter of much interest, and it is therefore necessary to
examine in some detail the theory of metabolic, axial or physio-
logical gradients as a whole. Much of the evidence on which it is
based is to be found in the four books of Child, and Abeloos and
Ranzi have written valuable reviews of the subject.
The fundamental conception lying at the base of these views is
that of axiate pattern. Child emphasised in all his work the idea
of polarity and of gradients of activity between poles. He proposed
that we should think of the embryo or the animal as existing in a
three-dimensional graph or co-ordinate system, and being con-
stituted in a kind of pattern of axes of symmetry. Each axis would
pass from one pole to another, but along its length the protoplasmic
activity would not be constant; on the contrary, it would be very
high at one pole and very low at the other, dwindling away at a
definite and measurable gradient. Such an axis may or may not
at its origin in time have a visible morphological outward sign of
its existence. It may or may not last as long as the differential
growth to which it gives rise. In fact, ontogenesis from this point
of view is the clothing of the original protoplasmic axiate pattern
with a corresponding morphological axiate pattern. The anatomical
gross differentiation of parts with reference to a given axis is pre-
ceded, as it were, by the appearance of a gradient of physiological
activity along this axis. By a gradient of physiological activity Child
meant a series of quantitative differences between the properties of
the cells, following a definite orientation with reference to the
eventual pattern of the animal. Child may be said to have trans-
ferred those co-ordinate diagrams which d'Arcy Thompson used to
SECT. 3] AND ORGANISATION 583
demonstrate animal form from appliances of convenience into de-
scriptions of actual fact, and to have substituted for morphological
axes, axes of physico-chemical difference. Difficulty was bound to
arise when some sort of identification of the physico-chemical dif-
ferences was attempted, and Child, content with a loose and general
association, chose metabolic rate as the physico-chemical variable.
"Axial gradients have often been called metabolic gradients", he
said, "because differences in metabolism, or, more specifically, of
oxidative metabolism, as indicated by various experimental methods,
appear to be characteristic and conspicuous features of them." It
will be necessary presently to examine the evidence on which this
statement is based, but first of all a few theoretical remarks require
attention.
That gradients of various kinds exist within the developing embryo
has long been known. The "law of developmental direction"
(Jackson and Scammon) or of "cephalocaudal differential growth"
(Calkins), which we have already discussed in relation to the growth
of parts in the embryonic body, is simply, after all, a statement of
the fact known in general to Aristotle, that the head end of an
embryo develops quicker than the tail end. Again, as Minot showed,
the cephalic somites develop before the more caudal ones. "The first
parts to become morphologically visible", as Child puts it, "are the
apical or anterior regions, and these are followed in sequence by
the successively more posterior or basal parts." Again, there are
dorso- ventral gradients. In those bilaterally symmetrical inverte-
brates which have a ventral nerve-cord (including most worms and
arthropods) the ventral and median regions of the embryo at any
given level of the body develop more or less in advance of the dorsal
and lateral regions. On the other hand, in vertebrates, where the
nerve-cord occupies a dorsal position, the dorso-ventral gradient runs
the opposite way, and differentiation and growth proceed more
rapidly in the median dorsal region than in the lateral and ventral
regions. The antero-posterior gradient, however, is the same as in
the invertebrates. For a recent discussion of the law of cephalocaudal
differential growth Kingsbury's papers should be consulted.
Again, the rule which has been named after F. M. Balfour, that
the rate of cleavage in an embryonic region is inversely proportional
to the amount of yolk which the cells in it contain, is associated with
the gradient system of the egg. For the apicobasal gradient in the
584 ON INCREASE IN COMPLEXITY [pt. iii
amphibian egg, for instance, leading from the protoplasm-rich
rapidly dividing animal pole to the yolk-rich slowly dividing vegetal
pole, has an enormous effect on the type of development which takes
place. It is plain, too, that much more might be said about the
relation of gradients to the various classes of eggs, alecithic, telo-
lecithic, and centrolecithic. In 1905 Morgan advanced the hypothesis
that the gradation of materials in the egg was a factor in establishing
physiological polarity, and Boveri came to very similar conclusions
about the Ascaris egg.
Child made a step forward from these simple facts when he
propounded the conception of primary protoplasmic gradients of
which the morphological gradients were the obvious result, for he
brought the subject to some extent nearer the point where physico-
chemical analysis could begin. Moreover, he was above criticism
when he did not specify what sort of physico-chemical activity it was
that was responsible for the gradient. But it is difficult to follow
him when he concludes that the gradient is one of "oxidative
metabolism", "oxidising power" or metabolic rate. Apart altogether
from the fact that the experimental evidence will not carry this
conclusion, there are theoretical difficulties involved in it. (It
seems to have its roots, indeed, in that now abandoned idea,
^ which was once common among the followers of Jacques Loeb,
that oxidation-processes and growth were very closely allied, even
that the master reaction of the growth-phenomenon was an oxida-
tion. If no physiologist now adopts this notion it is because so
many researches have shown it to be false. The work of Crozier
and his school on temperature characteristics might be mentioned,
in which the [x value for growth in general and embryonic
growth in particular practically never turns out to be 16,000.
Murray's demonstration of the diametrically opposite course
taken by growth-rate and metabolic rate, again, is an instance
of the same thing. All the tendencies of recent years have been
against any close identification of oxidative processes with growth.
A little reflection is enough, moreover, to convince one that such an
association is far from being a priori necessary. In the case of the
embryo of the chick in the egg, for example, its increase in size
could be represented by the curve a-b, in Fig. 95. At the beginning
and at the end of the period x-j> the size of the embryo is of course
given by the height of the curve above the abscissa, but to conclude
SECT. 3]
AND ORGANISATION
585
Absorption
that the amount of solid absorbed by the embryo during that period
was equal to the difference between the size at x and the size atj
would be to assume that the efficiency of the embryo was 100 per
cent., which is certainly not the case. As the diagram shows, what
has actually happened could
be represented in abstract form
by a peaked curve super-
imposed on the growth-curve
representing by its upward
sweep the total quantity of
soHd absorbed during the
period in question and by
its downward fall the total
quantity of solid cataboHsed,
or "oxidised", and excreted
as carbon dioxide, uric acid,
etc. The fact that the down-
ward slope does not go down
as far as the upward slope has
come up is what makes growth
possible. The process of growth, then, might be related to that of
cataboHsm as rival, not as offspring, and instead of resulting from it
might compete with it for the available solid substance. Or, on the
other hand, the steeper the curve the higher the peaks might be above
it, in which case metaboHc rate would be highest when growth-rate
is highest. Everything depends on the relative magnitudes. The
experimental fact that the metaboHc rate of the whole embryo is
highest when the growth-rate is also highest reveals no simple causal
nexus between them, and it is not theoretically correct to assume
such a relation.
Child has used various methods to ascertain the existence and
distribution of his gradients:
Fig. 95-
(I
(2
(3
(4
(5
(6
(7
Direct susceptibility.
Indirect susceptibiUty.
Reduction of potassium permanganate.
Formation of indophenols.
Observation of electrical potential.
Estimation of carbon dioxide produced.
Estimation of oxygen taken in.
586 ON INCREASE IN COMPLEXITY [pt. iii
It is clear that the only methods capable of informing us whether
gradients of metabolic rate are involved (cubic millimetres of
oxygen used up per gram per hour or cubic millimetres of carbon
dioxide given off per gram per hour) are direct estimations of these
gases. Yet out of the hundred odd papers which make up the core
of the literature on physiological gradients, not more than a dozen
at the very outside are concerned with these fundamental measure-
ments. In 1915, when Child's first two books were published, the
only evidence available was due to Tashiro, who had, at Child's
request, examined the behaviour of planarian worms in his micro-
respirometer, and had concluded that the pieces from the cephalic
end gave off more carbon dioxide relatively than those from the
caudal end, though the figures seem never to have been published.
In view of the criticisms which Adam; Bayliss; and later Parker
brought against Tashiro's apparatus, not much weight can be
attached to these results.
In 1 92 1 Robbins & Child obtained evidence from a study of
regeneration in planarian worms that the larger amount of carbon
dioxide was produced relatively at the head end, and in the following
year Hyman & Galigher reported the same relationship to hold
as regards oxygen for the oligochaete worms Lumbriculus inconstans
and Nereis virens. Preliminary results on Corymorpha palma, a large
tubularian hydroid, were given by Child & Hyman to the American
Zoological Society in 1922 and 1923, and published in extenso by
Child & Hyman in 1926. Hyman extended this to oxygen uptake
of planarians in 1923. Her results were paralleled by figures for
carbon dioxide production estimated by a colorimetric method in
which the time taken by pieces of the stem to reach a definite acidity
was measured. But no precautions were used to ensure that the
acidity measured was due to carbon dioxide and not to other acids,
for the method did not involve passing a stream of air through the
water containing the stem under investigation. The oxygen deter-
minations, on the other hand, were all done by the Winkler method,
and were never checked by any differential manometer technique.
That this is a serious deficiency is evident from the remarks of
Shearer, who in a private communication says, "With Haldane's
and Barcroft's apparatus I could not get any results which even
begin to support Hyman's tables. I think that the Winkler method
is useless where a lot of slime is discharged into the water (as is the
SECT. 3]
AND ORGANISATION
587
case with Planarians) ". This view finds many supporters among those
famiHar with the Winkler method.
Hyman and Child made an attempt to gauge the concentrations
of glutathione at the different levels of the stem of tubularians,
using a modification of the nitroprusside test, from which they
concluded that glutathione gradients were present. Such a statement,
15
en
X
O
10-
o
- Heado
\
V
Tail X--
— ^
>»
"^.
>»
—X
■^
-^f—
—
X-
Days 4 5 6 7 8 9 10
Fig. 96.
however, cannot be accepted in the absence of exact quantitative
data, especially considering the unspecific character of the nitro-
prusside test applied to an intact animal.
Apart from these rather unsatisfactory researches, nothing has
been done by Child and his collaborators on the direct verification
of their theory. In spite of this, they carried over the conception
of metabolic gradients to the developing embryo without modification,
and the question naturally arises, to what extent were they justified in
doing so ? The position is, in a word, that it cannot yet be considered
as proved that gradients of metabolic rate exist, much less that they
accompany gradients of other entities, such as susceptibility and
588 ON INCREASE IN COMPLEXITY [pt. iii
electric potential, even in the case of invertebrates — tubularians,
planarians, annelids, etc. Are we then justified in asserting, direct
evidence being absent, that a similar relation holds for vertebrates
and embryos in general? Hyman's few figures for respiration of
Fundulus eggs tell us nothing about the metabolic rate, for no weighings
of embryos were made. As regards the embryo, it is most unfortunate
that the only other piece of relevant evidence is contradictory.
Shearer in 1923 made an investigation of the oxygen consumption
of the head end and the tail end of chick embryos, using the Barcroft
differential manometer. Fig. 96, taken from his paper, shows the
results which he obtained. On the 4th day the head pieces took up
more than three times as much oxygen per gram per hour as the
tail pieces did, but by the loth day the two curves had almost come
to coincide. Both of them fell, showing that the metabolic rate of
the cells was declining with time in the usual way. Such a graph
fits in very well with the morphological picture, for by the loth
day the axiate pattern has long been established, and subsequent
development mainly concerns growth in size. Shearer went on to
investigate the action of acetone powders in order to see whether
the higher metaboHc rate of the head pieces was dependent on
structural conditions. The powders were made in the same way as
acetone yeast preparations by first dehydrating the tissue in acetone,
and subsequently desiccating completely. Such powders on being
made into a thin emulsion with distilled water respire. The results
were as follows:
c.c. oxygen taken up by embryos of 6-7 days' development per amount of
tissue containing 1-4 mgm. nitrogen.
Exp. Head Tail
1 0-62 0-23
2 0-52 0-29
3 0-47 0-27
•This then aflforded a clear demonstration in favour of the identifica-
tion of metabolic rate gradients with axial gradients, and gave
evident support to Child's views. Unfortunately, in a later state-
ment Shearer reported that he had not been able to repeat these
findings, and that further experiments had very much modified the
original conclusions. "I have since concluded", says Shearer in a
private communication, "that I was dealing with the rate of cytolysis
SECT. 3] AND ORGANISATION 589
undergone by the head and tail fragments. The younger the embryo
the more readily the tissues disintegrated and cytolysed during an
experiment. The heads containing the large watery brain vesicles
cytolysed very quickly and gave a wholly abnormal respiratory rate,
but the tail fragments, being composed of less deHcate material, did
not undergo cytolysis so quickly. Whenever you get any cytolysis
the oxygen consumption is of course greatly increased." Shearer
afterwards extended the work with Barcroft microrespirometers to
planarian head and tail fragments, and absolutely failed to get the
results which should have been found on the Child theory. Here the
position was compHcated by muscular movement which did not go
on to the same extent in the head and tail fragments. The situation
is therefore at present a deadlock, and we are at a standstill until
further accurate work on the lines already laid down by Shearer is
carried through.
Practically no importance can be attached to the experiments
which have been made with potassium permanganate and indo-
phenol blue. Child & Hyman in 19 19 placed embryos in very
dilute solutions (Af/ 10,000) of potassium permanganate, and de-
scribed in all cases a gradient along the cephalocaudal axis with
maximum activity, i.e. maximum reduction of the permanganate,
at the anterior end. Similar work was afterwards done by Child
(on Corella), by Galigher, and by Hyman. As a demonstration of
contributory interest the permanganate method has its value, but it
is far too uncertain biochemically to serve as the basis for the
identification of axial with metaboUc gradients in embryos. As for
the indophenol blue reaction, Child applied it to the development
of the starfish egg, and observed that the apical third or half of the
body of blastula and gastrula stages was always stained a deep blue
before the blastopore region had become stained at all. In his
1924 book he stated that exactly similar gradients had been observed
with methylene blue, reduction of this dye being faster at the cephalic
than at the caudal end of the embryo. These observations cannot
by any means permit of conclusions about gradients of metabolic
rate.
These criticisms of one of the main aspects of Child's theory are
essentially the same as those made by Parker and by Loeb in his book
on Regeneration. "The unit for the measurement of metabolism",
said Loeb, acidly, "is the calorie, and the calories produced by an
590 ON INCREASE IN COMPLEXITY [pt. iii
animal or by one of its segments are not measured by the time
required to dissolve the animal or one of its segments in a solution
of potassium cyanide." Nevertheless, the susceptibility method does
show up the existence of gradients of something. The very numerous
studies of Child; Hyman; and Bellamy on the susceptibiHty of dif-
ferent regions of embryos at different stages are quite sufficient to
prove that. They will therefore be called in the remainder of this
discussion axial or physiological gradients, and their nature will be
left undefined. An example not tied up with any theory of the nature
of the gradients is the staining gradient of the chick embryo found
by McArthur to hold for many acidic and basic vital dyes. It must
be admitted that throughout Child's treatment of the subject he
confuses growth-rate with organisation- or differentiation-rate though
the two are certainly not identical. In fact, it is often difficult to
tell from Child's arguments whether he means growth-rate, differ-
entiation-rate or metabolic rate, and it is not very helpful to lump
them all in one as "level of physiological activity". But it is time
to come to the facts obtained by the use of the susceptibiHty method.
Child used two variations of the susceptibility method, the direct
technique and the indirect technique. In the former case the
resistance or susceptibility is determined directly by concentrations
of toxic agent which kill the animals within a few hours. For a
particular species a concentration must be determined which kills
without acclimatisation, but which does not kill so rapidly that no
differences between parts of the body are discernible. Child has
used all kinds of toxic agents in his experiments, various cyanides,
alcohol, ether, chloroform, chloretone, acetone-chloroform mixtures
and other narcotics, X-rays, ultra-violet rays, ammonia, soda. He
has obtained the best results with those substances which have a
narcosis time and a killing time very close together, in other words,
with those substances whose effects are not complicated with nar-
cosis. The most favourable poisons, he found, were the cyanides.
As Hogben has pointed out, it would have been far more favourable
to the metabolic gradient theory if Child had confined his attention
to the cyanides, which are known to have an inhibitory action upon
some, though not upon all, tissue oxidations. For then it would not
have been known that substances such as ether and chloroform had
precisely the same effects, and the association between axial gradients
and oxidation-rate would have been more convincing.
SECT. 3]
AND ORGANISATION
591
Since the death and disintegration of the different parts of the
body usually follow a regular time sequence, Child found it possible
to determine the time not merely of disintegration of the whole
animal, but of various regions of the body. The method used as a
rule was to examine the lot of animals at intervals of half-an-hour
and then to record the condition of each individual. Arbitrary values
having then been attached to the various stages of disintegration,
curves can be constructed showing the rate at which the process
has gone on. Fig. 97, taken from
Child's 1915 book, shows such a
curve constructed for specimens
of a flatworm, Planaria dorotoce-
phala, the curve ab representing
the susceptibility of young and
the curve cd representing that
of old animals. The correlation
between age and susceptibility
should be noted.
The indirect method involves
the principle of acclimatisation.
In general, according to Child,
the ability of an animal to
acclimatise itself to cyanide or
other toxic agent varies with its
metabolic rate, or rather its level
of physiological gradient. The
ability of parts of animals to ^^' ^^'
become acclimatised also alters with the same variable, so that the
indirect method affords a way of estimating gradients in the embryo.
The relation between age and survival time in solutions of the
concentration necessary for the indirect method is exactly the converse
of what it was in the direct method, for here the younger animals with
the higher metabolic rate live much longer than the older ones with the
lower metabolic rate.
The susceptibility method is obviously a very complicated one,
and conclusions from experiments with it have to be drawn with
caution. In lethal doses which are not concentrated enough to pro-
duce death within a short time after the beginning of the exposure,
the regions of higher activity are always affected first, so that above
Hourso 1
592 ON INCREASE IN COMPLEXITY [pt. hi
a critical concentration susceptibility varies directly as the physio-
logical activity, while below this concentration the reverse of this
relation is seen, in that regions of higher activity recover and adjust
themselves to the reagent more successfully than regions of lower
activity. "In applying the susceptibility method to embryonic de-
velopment, lethal concentrations may be used but not allowed to
act long enough to produce death in the embryo, and in such cases
they will, according to Child's interpretation, inhibit regions of
higher activity to a more marked degree than regions of lower
activity ; while on the other hand, in very low concentrations of the
reagent such as to permit acclimatisation and recovery, the region of
higher activity will be inhibited, according to Child's interpretation,
less than regions of lower activity." A simple instance of the operation
of Child's conceptions is the case of the eggs of some polychaete
worms, Chaetopterus and Nereis, which, when placed unfertilised in
lethal solutions of cyanide, exhibit a progressive dissolution from the
anterior end. As development proceeds, the region of maximum
susceptibility shifts round to the posterior region (where growth is
most active) so that, when the larva is ready to metamorphose,
the posterior region is the region which succumbs most readily to
lethal and recovers most readily from sub-lethal concentrations of
cyanide.
Child's work on the eggs of the starfish, Asterias forbesii, brought
out the fact that the susceptibility gradients in the unfertilised egg
were connected in some way or other with the mode of attachment
of the individual egg to the parent body in the ovary, obviously a
very important point for the question of the origin of polarity and
axial symmetry in the developing embryo. Wilson & Matthews
showed that the region where the nucleus lay nearest the surface
became in the starfish's tgg the apical or animal pole, and Child
found that it was from that point on the surface of the egg that dis-
integration began. From the behaviour of nuclei which had been
extruded during cytolysis in cyanide. Child concluded that the
nuclear susceptibility gradient ran in the same direction as the
cytoplasmic susceptibility gradient. The direction of the axis, he
concluded, was determined by the eccentricity of the nucleus. Child
found that during the earlier cleavage stages the general gradient
was obscured to a great extent by individual gradients in the blasto-
meres and other incidental factors, and that "in the early spherical
SECT. 3] AND ORGANISATION 593
blastula before movement begins the disintegration gradient is dis-
tinct, but the difficuky in identifying the animal pole and embryonic
axis makes it impossible to demonstrate that the gradient coincides
with the axis". But "in later free swimming stages of the blastula
the direction of movement with the apical region, the animal pole,
in advance, and before gastrulation the elongation of the embryo
in the direction of the axis and the increasing thickness of the cellular
layer toward the vegetative pole render orientation possible at a
glance. In these stages the disintegration gradient is very distinct.
It begins at the apical end and proceeds with a definite course along
the embryonic axis, ending in the region of the vegetative pole
where the gastrular invagination will occur". The susceptibility
gradient in the gastrula of the starfish Child found to be very similar
to that in the blastula, being greatest in the region of the apex and
least in the region of the blastopore. Moreover, the gradient in the
archenteron wall was exactly the same as that in the body-wall,
the apical end of the endodermal invagination being the region of
greatest susceptibility. After this time the gradients become less and
less distinct until by the bipennaria larva stage they have faded away
almost entirely.
Hyman extended in 1 9 1 6 the observations of Child on polychaete
eggs to those of a microdrilus oligochaete, Tubifex tubifex. She found
that, in the stage when the embryo has begun to elongate, its posterior
region was the most susceptible to cyanide, and the susceptibility
decreased as one passed forwards. In later stages the head end
became more susceptible, and finally exceeded greatly the tail end
in susceptibility, so that at hatching it was much the most easily
disintegrated region. Thus the posterior region of high physiological
level which is characteristic of the adult annelid arises very early in
development. After hatching, a worm placed in cyanide disintegrates
first at the head end, later at the tail end, so that the two waves of
disintegration reach and fuse at a point posterior to the middle of
the worm's length. Figs. 98 and 99, taken from Hyman's paper, show
the degeneration of embryos of different stages in cyanide.
Child and his associates frequently correlated their susceptibility
gradients with gradients of electric potential. Hyman & Bellamy in
1922 gave a full account of the work on this subject with a critical
discussion, and at the same time reported their results for frog
embryos. Hyde had previously found the heads of recently hatched
N EI 38
594 ON INCREASE IN COMPLEXITY [pt. iii
toad tadpoles to be galvanometrically negative to the heads, and
this Hyman & Bellamy confirmed for the frog. "The idea is ad-
vanced", they said, "that differences of potential in organisms,
particularly the permanent differences which exist along the main
axes of animals, are due to differences in metabolic rate at different
regions, the region of highest metabolic rate being the most negative
in the external circuit, most positive in the internal circuit." In the
tadpole, therefore, the highest physiological level appeared to lie
towards the tail.
Fig. 98.
0 ^.
Fig- 99-
Hyman has published a series of papers on the susceptibility
gradients of vertebrate embryos. The first of these she devoted to
the teleost embryos {Fundulus heteroclitus (minnow), Ctenolabrus ad-
spersus (cunner) and Gadus morrhua (cod)). She showed that, in
addition to the primary gradient of the embryo which has its high
level pole at the anterior or cephalic end, there were also other
"secondary" gradients arising from regions of high susceptibility
other than the anterior end. Certain organs, also, may have their
own axiate pattern, notably the heart.
The cod and cunner embryos were studied with cyanide in the
usual manner, but the impermeable egg-membranes of the minnow
made this impossible, so that ammonium hydroxide had to be used
instead. In the early blastoderm stages of the cunner and the minnow,
the central cells were observed to be the most susceptible, and from
them disintegration proceeded to the periphery of the blastoderm.
But in the case of the cod exactly the reverse relationship held true ;
SECT. 3] AND ORGANISATION 595
the periphery of the blastoderm was more susceptible than the
central part. In the later blastoderm stages of the cunner the region
of high susceptibility is shifted posteriorly, and a certain area along
the margin of the blastoderm succumbs very readily indeed to the
toxic agent. This is exactly where the embryo is about to arise. The
eggs of the minnow could not be examined at this stage. In the
cod, the germ ring was always much more susceptible than the central
part of the blastoderm, and at its circumference one region is more
susceptible than the remainder. This is where the embryonic shield
originates, so that the conditions in the cunner and the cod are now
very similar. As the embryonic shield grows forward, its anterior
margin is most susceptible, and disintegration extends posteriorly
from this.
Slightly later stages, when the embryo is visible in the centre of
the embryonic shield and the germ ring has advanced more than
half-way over the yolk, were not observable with certainty in the
cunner and the minnow. But in the cod they were clear enough, and
here toxic action obviously began at the anterior end of the embryo,
spreading backwards towards the posterior margin of the shield.
Still later, at the time of closure of the germ ring, disintegration
gradients were observable with ease in all three species, and always
the susceptibility was highest anteriorly, diminishing and spreading
backwards. The eyes are not very susceptible, and do not degenerate
until the wave has passed half-way back along the neural tube.
After the germ ring has closed, a secondary region of high suscepti-
bility appears at the posterior end of the embryo. From this point
onwards, there is no change in the gradients; there is a powerful
spreading backwards from the cephalic zone of high susceptibility
and a slight spread forwards from the caudal zone, with no compli-
cating factors. The minnow differs from the cunner in possessing
the two zones from the very earUest stages, and at certain early
points in development, the posterior zone is the more important
of the two. But as development proceeds the posterior zone declines
in susceptibility and the anterior one increases, especially after the
arrival of the optic vesicles, which fall an exceedingly easy prey to
the toxic agent. In very late stages in the minnow, a region of high
susceptibility develops in the hind brain where the cerebellum is
forming. By this time the tip of the tail has become free from the yolk
and somewhat more susceptible again. The cod embryo behaves in
38-2
596 ON INCREASE IN COMPLEXITY [pt. iii
much the same way as the other two in the later stages, always
having two regions of high susceptibility, the anterior preceding
before the closure of the germ ring and the posterior preceding
afterwards. Minor variations in susceptibility of eyes, fore brain,
etc., were noted. The somites in all three teleosts disintegrate from
each end, but more from the anterior than from the posterior. Special
observations were made on the heart gradients, which agreed in
many particulars with subsequent work by the same author on the
gradients of the embryonic heart of the chick, e.g. the venous end
was the more susceptible, and the gradient decreased towards the
arterial end.
Hyman was able to draw several conclusions from this work
important for pure embryology, such as that, in different teleost
embryos, the amount of material contributed to embryo formation
by the germ ring is variable, being very little in the cunner and con-
siderable in the minnow. This reconciled the views of older workers,
such as Morgan; Sumner; and Kopsch. These points, however,
together with the fact that her results gave no support to the
concrescence theory, are not so important for the present purpose as
the delineation of the regions of high susceptibility for comparison
with other embryos. The double gradient (anterior and posterior
zones of high susceptibility) is also regarded by Child & Hyman as
important for a comparison which they make between segments in
segmental animals and separate individuals, suggesting that, whereas
in annelids the posterior zone is permanent and never comes under
the control of the anterior zone, in vertebrate embryos it eventually
dies away, so that further segmentation ceases. This need not,
however, detain us here.
In the same paper as has already been mentioned, Hyman
measured the rate of oxygen consumption of Fundulus eggs during
their development. Unfortunately, the figures do not give us any
information which would either support or weigh against Child's
theory of metabolic gradients. Owing to the small size of the embryo,
the determinations had to be expressed in relation to looo eggs
(i.e. embryos + yolks), and, as we have no idea how the wet and
dry weights of the eggs or the embryos were varying during this
period, we cannot calculate the metabolic rate. This work will be
discussed in detail in the section on the respiration of the embryo.
Hyman also made a summary of the teratological results which had
SECT. 3] AND ORGANISATION 597
been obtained by various workers on Fundulus, and concluded that
in all cases the malformations produced most easily were those of
the fore brain, the head in general, the sense organs, especially
the eyes, the heart, the circulatory system, and the tail. These results
obviously fitted in very well with those appearing from the use of
the direct susceptibility method of Child, and this outcome of the
physiological gradient conception is perhaps one of its most attractive
aspects, for no other point of view serves to account for so many of
the facts of teratology. It was long ago pointed out by Dareste that
no relation seemed to exist between the application of a certain
physiological or physical condition and the resulting teratological
modification. Thus Herbst's "lithium larvae" and Stockard's "mag-
nesium embryos" have been shown to be obtainable with a great
variety of agents. There is, we may say, practically no specificity
in teratological action. "Any type of abnormality", as Bellamy puts
it, "may be produced under the influence of any inhibiting agent
by controlling the concentration or intensity of action, the length
of exposure, and the stage, i.e. physiological condition, of the Ggg
or embryo or parts of the egg or embryo when exposed." Since the
differences then do not reside in the teratological agents employed,
they must do so in the embryo itself — an admirably Kantian con-
clusion, which can only be explained on some basis which maps
out the embryo into a logical system. The only basis we have is the
conception of physiological gradients. It is difficult also to imagine
any other view which could explain such instances as the production
of the usual terata by fertilising eggs with foreign or injured sperma-
tozoa or treatment of the eggs before fertilisation as in the experi-
ments of Gee. Full discussions of the teratological literature and
interpretations of it from this physico-chemical standpoint will be
found for the fish embryo in the paper of Newman, for the chick
in the paper of Hyman, and for the frog in the paper of Bellamy.
It is very noteworthy that new teratological modifications not before
obtained have been predicted on the basis of physiological gradients,
and have afterwards been verified.
Hyman next studied the gradients during the development of the
brook lamprey, Entosphenus appendix, an organism of considerable
interest, in view of the fact that, like the amphibia, the cyclostomes
are one of the three vertebrate groups which develop by holoblastic
unequal cleavage. Alcohol and acetic acid were used as the reagents
,.-Of|j^
■■^afet-
Fig. lOO. Physiological gradients in the egg of the lamprey (Hyman) . 1-4. Unfertilised
egg. 5-8. Four-cell stage. 9-12. Eight-cell stage. 13-18. About thirty-two-cell
stage, 16 hours. 19-22. Morula, 20 hours.
Fig. loi. Physiological gradients in the egg of the lamprey (Hyman). 23-26. Early
blastula, 24 hours. 27-31. Late blastula, 40 hours. 32-35. Beginning of gastrula-
tion, 48 hours. 36-39. Gastrula, 60 hours. 40-43. Late gastrula, 70 hours. 44-46.
Late gastrula, neural groove about to appear, 75 hours. From no. 36 onwards, the
anterior end is to the right.
6oo ON INCREASE IN COMPLEXITY [pt. m
for producing differential death. An interesting point was that the
embryos were first stained with neutral red, and the change in tint
of this indicator noted as soon as the killing solutions were poured
on them, obviously showing that the differences in killing time of
different parts were not entirely due to differences of permeability.
Cannon and Huxley were at one time inclined to attribute most of
the results obtained by the direct susceptibility method to such
differences. But in any case differences of cell-membrane permeability
would be included in the variables which might be changing along
the physiological gradient.
The disintegration gradient of the unfertilised lamprey egg was
found to be a perfect example of a simple primary gradient, the
degeneration beginning at the animal pole and spreading regularly
to the vegetative pole. At the 4-cell stage the disintegration begins
at the animal tips of the four blastomeres, and passes backwards
to meet a slight secondary zone of high susceptibility at the vegetative
ends. These changes are seen in Figs. 100 and loi, taken from
Hyman's paper. In the 8-, 12- and i6-cell stages, the degeneration
begins in the micromeres at the animal pole, then passes on through
the macromeres. In later stages the disintegration constantly begins
at the animal pole, but in addition isolated cells or groups of cells
are to be seen in either animal or vegetal hemisphere which dis-
integrate in advance of the region in which they are situated.
Hyman supposed that these were taken by the killing solution in
the act of cleavage, and were thus more susceptible than their
neighbours. The secondary region at the vegetal pole is now dis-
appearing for good. The early blastula stages show the usual single
gradient, but an important change occurs in the later blastulae,
namely, that the spread takes place more rapidly along one surface
of the egg than the others. This foreshadows the differentiation of
that surface as the dorsal surface. In very late blastulae, there is a
small zone of susceptibility near the vegetal pole, which foreshadows
the gastrular invagination. The gastrula stages, as shown in the
figures, are characterised by disintegration beginning at the anterior
end of the embryo and proceeding backwards, but first dorsally
and then ventrally to meet the spread from a secondary zone
originating around the blastopore. These conditions continue un-
changed during the formation of the neural groove and the neural
tube, though in the late stages of the latter there is a slight double
SECT. 3] AND ORGANISATION 601
gradient in it. No further changes occur during elongation and
hatching.
The gradients in the chick embryo were also studied by Hyman,
using potassium cyanide, and ammonium and sodium hydroxides,
sometimes preceded by staining with neutral red. The earliest stages
were very difficult to deal with, but some evidence was obtained of
an antero-posterior gradient in the central opaque area of the
germinal disc at 7 hours' incubation. This was quite certainly
demonstrable, however, at the typical primitive streak stage, and
the stage of the head process. The medullary plate stage marked
the beginning of the double gradient (see Fig. 102), two regions
of high susceptibility being present, one at the anterior end of the
primitive streak and the other at the anterior end of the medullary
plate. When the first somites appear the same zones are seen, the
former spreading backwards and forwards along the embryonic axis,
the latter backwards only. As the neural folds close, they present
a region of high susceptibiUty, but this soon disappears, and the
embryo reverts to the simple double antero-posterior gradient system.
This holds good up to the 8-somite stage; from the 9th onwards the
rapidly increasing susceptibility of the optic cups is noticeable. At
the i2-somite stage the optic zone has died away and there is a
new one of high susceptibility in the hind brain, foreshadowing
the turning of the head, but this also disappears by the 3rd day
of development. Summing up the results, one may say that the
general picture is one of an antero-posterior gradient complicated
from time to time by the appearance of zones of high susceptibility
at different points along the embryonic axis.
In vertebrate embryos in general, it would seem that the forma-
tion of these two regions of high susceptibility is the regular mode
of development. The two centres are always located in the same
position with respect to the future embryo, one at the anterior end
of the antero-posterior axis and one in the axis at a more or less
posterior point. This posterior centre is the dorsal lip of the blastopore
in cyclostome and amphibian embryos, the posterior end of the
embryonic axis in teleostean fishes, and the primitive knot, sub-
sequently the tail bud, in the chick. "This posterior centre", says
Hyman, "is Hke a growing point which, passing backwards, deposits
the trunk of the embryo anterior to it." The presence of two centres
of activity was long ago recognised in frog and rabbit embryos
2'^ 3
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20
26
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27
p;,r ,no Phvsioloeical grad ents in the chick embryo (Hyman)
tig. I02. rnysioiogic^i gi'i" A/f<.rq,.ll^rv n ate sta£
e 4-6. Head-process stage. ?■
romi.fs?ie;t6i7°FTv:rm!,r:.is.s'ho-;^g'h?rgh^^^^^^
fold stage. i4-i7- One-somite stage
somite stage. 26-; .
of fusion of neural folds.
i-Q. Primitive streak
" ly '
18-20. Three-somite stage. 21-25.
I the cnicK emuiyu i^ixyx^ciw;. . -J. ' PcrWrK-nral
..age. 4^. Hjad-process stage. 7-.0. MeduUary (.a.e staje. ^. ,-.3. Early neu„^^
PT.ra,sECT.3] ON INCREASE IN COMPLEXITY 603
by Assheton, who referred to them as primary and secondary centres
of cell-proHferation, and this point of view has been adopted by many
embryologists, e.g. Eccleshymer; Adelmann; and Kingsbury.
In addition to these researches on embryos, Child has studied
the gradients during the development of the sea-urchin egg {Arbacia
punctulata), those of polychates {Nereis, Chaetopterus and Arenicola) and
those of an ascidian [Corella willmeriana) . In these cases, the primary
simple apicobasal gradient of the fertilised egg-cell was succeeded
by a double gradient resulting from the appearance of a zone of
high susceptibility at the posterior end of the embryo. Mention of
the work of Bellamy on the amphibian egg brings us back to our
starting-point, namely, the recent investigations on the organiser and
the mechanics of amphibian development. There is no need to
give a detailed description of Bellamy's results on the frog embryo,
for they resembled in many ways those of Hyman on the brook
lamprey. But they may be briefly summarised for the purpose of
comparing them with the work of Spemann and his school.
Bellamy found that in the unfertilised amphibian zgg the beginnings
of polarity were to be found in the position in the ovary. He observed
by injections and by actual observation of blood-flow that the blood-
vessels to the eggs in the oogonia pass arterially over the pigmented
part of the egg and venously over the unpigmented part. It is more
than probable that the first polarity of the egg arises because the
animal pole is that point on the surface of the egg which happens
to be most well supplied with a capillary network, not that which
happens to be attached to the ovary by the pedicle. The initial
physiological gradient, therefore, would seem to be a matter of
position in the ovary. It may be mentioned here that very similar
conclusions were come to by Lillie for Chaetopterus and Sternapsis
eggs, by Child for Phialidium (hydromedusa) eggs, and by Boveri and
Jenkinson for Strongylocentrotus eggs.
The early stages of development in the frog's egg are very resistant
to toxic agents. But it was possible to show that the fertilised but
undivided egg began to disintegrate at the animal pole and the
degeneration passed downwards and outwards with a special bias
towards the grey crescent. Much the same state of affairs was
seen in the 4-cell stage, but in the morula stages there are two
zones of high susceptibility, the second one appearing just above
the grey crescent, and contributing to the general spread down-
6 04
ON INCREASE IN COMPLEXITY
[PT. Ill
wards from the animal pole. Eggs in an early gastrula stage always
disintegrated first at the dorsal Up region and shortly afterwards
in the same meridian about 120 to 130° above the blastopore.
From the upper point at the animal pole the wave spreads down-
wards, and meets the disintegrated area of the dorsal Up, which has
spread apically and now includes the lateral lips, after which all
the pigmented cells become
gradually involved, though the
cells at the vegetal pole retain
their structure with their yolk
long after the rest of the egg has
died. Lateron, when elongation
has begun, the dorsal lip now
takes up the posterior position,
and the double gradient still per-
sists, disintegration beginning
from both ends, from the apical
point at the anterior end and
from the dorsal lip at the pos-
terior end. This description
applies to most of the later
Fig. 103.
stages, including the time of opening and closing of the neural folds, the
appearance of ventral suckers, etc. Eventually, with the differentia-
tion of various organs, local susceptibility differences begin to appear,
and the tail bud, the optic vesicles, the nasal pits, and other rapidly
growing regions show much susceptibility. (See Fig. 103.)
On the basis of these fundamental results, Bellamy was able to
make and verify teratological predictions, and to control experi-
mentally or modify development, e.g. the gastrular angle and the
cleavage ratio (the ratio between the sizes of animal and vegetal
pole cells). He was criticised by Cannon, whose main objection was
that the effects of the toxic agents were not uniform at the different
stages, but that the individual differences between eggs were so
large as to invalidate Bellamy's conclusions. When Cannon did
succeed in getting a lot of eggs to behave in the same way at the
same time, he found results quite at variance with Bellamy's, e.g.
the ventral, not the dorsal, region of neural tube stages was the
more highly susceptible. These criticisms were replied to in detail
by Bellamy & Child, who successfully rebutted them, and whose
SECT. 3] AND ORGANISATION 605
paper should be consulted for further details. Other criticisms of
the general theory of physiological gradients have been made by
Wilson; Kingsbury; Lund; Allen; and others, but they do not
affect the main conclusions which have been described.
The significance of Huxley's remarks in 1924, which have already
been quoted, can now be better appreciated. At the time of chemo-
differentiation, the various irreversibly determined regions differ
from each other by the presence not only of qualitatively different
substances but also by the presence of varying concentrations of the
same substance, according to the conception of axial gradients. "If
it is asked", said Huxley, "how we can imagine the process as
originating, the answer must, I think, follow some such lines as these.
During gastrulation every portion of the embryo has a definite
relation to the system of axial gradients. The two main gradients
extend both on the surface and internally and together constitute
a three-dimensional system of gradient co-ordinates. Every portion
of the embryo, therefore, has its own rate of activity corresponding
to its position in the existing co-ordinate system, and its own charac-
teristic proportions of yolk, glycogen, cytoplasm, etc., depending on
the previous effects of the apicobasal gradient during the growth
of the egg. When the organiser in the dorsal lip exerts its admittedly
as yet unexplained though not unparalleled action of initiating
differentiation, every region of the embryo is in a different con-
dition from every other. The substrate is different from place to
place, the result, therefore, also differs." The position of a given
point in the embryo on the physiological three-dimensional graph
is thus of greater importance than the proportions of primary
materials which it contains, and, further, the relative velocities
of processes going on there are more important than the actual
amount of substances of different kinds that happen to be present
there. The substances which can be distinguished in the unsegmented
ovum of a vertebrate are thus merely raw materials, and the or-
ganising influences are to a large extent expressible in terms of gradients
of activity. The first of these is probably determined before the egg is
laid at all, the second arises from the action of agencies external to
the egg approximately at fertilisation. We are thus left with the
conception of parts of the embryo as pacemakers of growth and
differentiation relatively to the rest: though in what exactly their
influence consists we do not as yet know. "The first step in organisa-
6o6 ON INCREASE IN COMPLEXITY [pt. m
tion and in embryonic development", says Child, "results from the
establishment in one way or another of some region or portion of
this protoplasmic reaction-system as a region of higher rate of
dynamic activity. This region dominates development, becomes the
apical or head region and determines the axial gradient or gradients
which constitute the dynamic basis of polarity and of individuation."
3-9. Organised and Unorganised Growth
An idea which has much importance for the study of the inter-
relations between growth and differentiation was contained in a
paper by Faris on the pigmentation of Amblystoma embryos. The
details of his investigations into the ontogenesis of this pigment will
be referred to again in the section on pigments; here I am con-
cerned to refer to his distinction between "proliferation metabolism"
and "differentiation metabolism". He observed that, during the
development of the myotomes of Amblystoma embryos, the pigment
accumulated in the cells proportionally to differentiation and not to
growth. He therefore suggested that it could be regarded as an
index of the difference in nature between the type of metabolism
associated with growth and that associated with differentiation, ad-
mitting, of course, that the two processes were only completely
separable in the abstract. "Proliferation", according to Faris, "must
lack the wear-and-tear processes that are characteristic of differentia-
tion and for that reason it lacks the function of pigment production."
It must be admitted that these concepts are vague enough, and they
rest on an unsatisfactory, because unquantitative, basis. But they have
a real interest, in view of two lines of recent thought : firstly, the
distinction made by Murray between the groups of processes in
embryonic development according to velocity at different times, and
secondly, the numerous papers of Warburg and his school (see Section
4-20), which introduce into these problems the concepts of organised
and unorganised growth metabolism. These investigations will be
referred to in detail in the next section. If it should turn out that
Faris's conception of two types of metabolism found a quantitative
basis in the data of Warburg and his collaborators, an interesting
and quite important avenue would be open for further investigation.
The notion of a distinction between organised and unorganised
growth had not, however, its sole origin in Germany, for Byerly
had also come to it from very different ground. Setting out
SECT. 3] AND ORGANISATION 607
from the idea originally suggested by Jordan that haemopoiesis
was associated with lack of oxygen or accumulation of carbon
dioxide, he allowed chick embryos to develop for 24 hours or
so, and then, breaking off the shell covering the air-space, immersed
the whole egg in water-glass solution so that the respiratory
exchange was quite stopped, but incubation allowed to proceed
till 96 hours. He then examined the suffocation effects so pro-
duced. These were (i) normal body-form only at the anterior
end, (2) no allantois, (3) extraordinarily large blood-vessels and
anomalous sinuses, (4) constantly recurring fatty necrosis of tissues.
Since the circulation of blood had stopped in these embryos, the
end-products of metabolism were accumulating in their cells. The
enormous amounts of blood found obviously suggested an unusual
haemopoietic activity. But the heart, being unable to beat in the
toxic anoxaemic blood, allows it to accumulate in the vessels, and,
as more blood-cells are continually being formed, sinuses develop.
Cessation of the circulation having led to a struggle for existence
between the various parts of the body, some may find it possible
to live on the rest, and unorganised "anarchistic" or unregulated
growth may occur. This is what Byerly actually found in the suf-
focated embryos, as regards the formation of blood, btit he did not
bring forward any evidence that an unusual quantity of lactic acid
was produced in the suffocated embryos, as would have been the
case on Warburg's view if one tissue had taken on an anaerobic Hfe
and was growing at the expense of the others. Holmes was later
unable to repeat these observations of Byerly's on the chick embryo,
but the number of her experiments was insufficient to negative
definitely Byerly's conclusions.
In his second paper, he continued the study of "dead" embryos,
which he observed to show three types of behaviour. One class
remained on the surface of the yolk and showed anarchistic growth,
one absorbed liquid to form a bladder-like vesicle on the surface
of the yolk, and one sank beneath its surface. Beha\dour of the tissues
of embryos of the first class varied according to the age of the embryo
at the time when the heart was made to stop beating. If cardiac
failure occurred after 4 or 5 days' incubation, only the liver cells
and certain of the blood-cells were still proliferating at the end of a
week's anaerobiosis. But if the failure took place at 72 hours' in-
cubation or less, then, in addition to liver and blood growth, the
6o8 ON INCREASE IN COMPLEXITY [pt. iii
nervous tissue went on proliferating for nearly a week. In such
embryos the mesenchyme cells seemed almost unaffected by the
suffocation, and continued to divide, but took on the histological
characteristics of blood-cells. The important point about the picture
in anaerobic embryos was that each cell became a relatively free
unit, and all correlation of growth as well as all further differentiation
ceased with the circulation. Only unorganised cell-life was possible.
No consideration of organisation in growth could omit the subject
of mitogenetic rays, from which it appears that radiant energy of
definite wave-length is given off by cells in mitosis as an excitant
to adjoining cells not in mitosis. For detailed information on
this subject, the book of Gurwitsch & Gurwitsch and the memoir
of Borodin should be referred to. Genuine uncertainty still exists,
however, about these rays, and a whole literature is growing up,
partly consisting of reports of workers who confirm Gurwitsch's
results, and partly of the reports of those who do not.
Anikin has already attempted to relate the activity of these
" mitogenetische Strahlen" to organiser phenomena, and Sorin &
Kisljak-Statkewitch have examined all the parts of the hen's egg,
using onion roots as detectors for the rays. Negative results were
obtained with albumen from the 2nd day of development, the vegetal
pole of the yolk throughout incubation, the white and yolk of in-
fertile eggs, even if incubated, the "Brei" of germinal spots 36 hours
old, cerebro-spinal fluid and brain tissue from 5-day embryos, the
amniotic liquid and certain other parts. On the other hand, positive
results were always obtained with the substance immediately under
the germinal spot up to the 6th day of development and with the
blood. Karpass & Lanschina also find peptic and tryptic digests of
egg-yolk to be powerful sources of mitogenetic rays.
3-10. Chemical Embryology and Genetics
The relations between physiological and chemical embryology and
genetics, on the other hand, afford a more solid basis for discussion.
It has frequently been found that the behaviour of organisms of
known genetic constitution during their embryonic period affords a
means of marking out the points in ontogenesis at which genetic
factors come into play*. The simplest case of this kind is the question
* Some embryological factors seem to be determined entirely by the maternal organism
(Toyama for pigmentation of silkworm eggs, Diver, Boycott & Garstang for dextrality
and sinistrality of Limnaea eggs) .
SECT. 3] AND ORGANISATION 609
of embryonic mortality, which will be mentioned again in Section 18,
and which provides a striking temporal field for the display
of genetic characteristics. To take only one example, Dunn &
Landauer studied the relative embryonic mortality of a variety of
chick called the "creeper". Cutler had first reported that, in
"creeper" fowls, the leg and wing bones were shorter and thicker
than in normal fowls, and that creepers never bred true, but usually
produced {a) normal chicks, {b) creeper chicks and (c) chicks with
extreme leg defects. The case was thus analogous to that of the yellow
mouse, which is always heterozygous, because the homozygous yellow
embryos die early in development. The relative embryonic mortalities
were found to be as follows : _^
Percentage of total embryos incubated.
Dead in shell
Hatched
alive
27-3
66-2
Creeper male x creeper female
Creeper male x normal female
1-6 days
45-5
4-2
7-13 days
15-2
16-9
14-21 days
I2-I
12-7
These results lead naturally to the assumption that the high early
mortality of the creeper x creeper matings was due to the death
of homozygous creeper embryos early in development. The creeper
variation would thus seem to be due to a single dominant gene which
is lethal in the homozygous condition. But the important point for
this discussion is that, if the point of action of a gene can be found
to occur at a definite point in development, a new outlook in genetics
becomes possible, for what we know to be taking place in the physio-
logical and chemical activity of the embryo at that period may tell
us a good deal about the gene itself.
Much thought has been given to these questions in recent years.
Danforth, for instance, asked the question whether genes interact
with one another during embryonic development to produce struc-
tural and functional characters, or whether they each exert their
influence separately, thus
Experiments with mice led him to regard the latter view as
the more probable. The three known effects of the Y-gene occur
N EI 39
6io ON INCREASE IN COMPLEXITY [pt. in
at 6, 25 and 90 days after fertilisation of the egg, and a complex
which modifies one of these effects may have no effect on the other
two, e.g. if colour is replaced by albinism, yellow is entirely sup-
pressed but senile adiposity is not influenced.
Plunkett made a systematic analysis of the way in which genetic
factors and environment interacted to produce bristles during the
embryonic development of Drosophila. These bristles can easily be
counted, and so afford a quantitative variable. What actually
happens finally as regards bristles depends on (i) genetic factors,
e.g. missing-bristle genes, extra-bristle genes, etc., (2) the tempera-
ture during the developmental period, (3) other environmental
conditions such as nutrition, (4) differences of internal environment
and (5) random internal variations. The last two of these can be
avoided by suitable methods. Rise of temperature, Plunkett found,
tends to suppress bristle formation. It must act, then, either by
decreasing some bristle-forming reaction or by increasing some
bristle-inhibiting reaction. If the temperature characteristic, he
argued, of this reaction is less than development as a whole, the
former must hold, but if it is greater, then the latter must hold. As
the evidence came out, it was in favour of the former theory, for the
critical thermal increment was 36,400, a value often found for heat
destruction of enzymes, although for development as a whole it was
27,800 from 14 to 17°, 17,100 from 17 to 25° and 9000 from 25 to 30°.
By various calculations, Plunkett showed that the reaction in question
began very early in the larval stage, and took place in three steps:
[a) production of R (a destructive agent for the enzyme) from some
protoplasmic component, {b) destruction of B (enzyme from the
gene) by R and [c] the formation of the bristles catalysed by B.
A bristle-reducing gene might therefore throw its weight into the
catalysis of the formation of R. Plunkett suggested that all genes act
by differential acceleration of enzyme actions during embryonic de-
velopment. Similar work on temperature characteristics of gene
action has been done by Nadler; and Gowen, after 'K-ra.ying Drosophila
at various stages of development, reported that the abnormalities
produced, being strictly confined to certain groups of cells, showed
that the gene had been affected prior to any action, and while it
was still, as it were, lying latent in the nucleus.
Huxley & Ford and Ford & Huxley have undertaken interesting
studies on the eye-pigment of Gammarus chevreuxi. Allen & Sexton
SECT. 3]
AND ORGANISATION
611
had found that the red colour of the eyes in this organism darkened
with development almost to black, and, by working with an arbitrary
scale of colours, Huxley & Ford were enabled to make a quantitative
examination of the rates of action of the various genes which in-
fluence eye-colour. Fig. 104, taken from their paper, demonstrates
diagrammatically the relationships found. The blackening appears
to be due to the deposition of melanin; in some cases this occurs
very rapidly, in others more slowly. Thus the steepest curve in the
diagram represents the dominant black-eye type which is black at
hatching. Embryos of this type, however, which have not completed
half their incubation, have no colour in their eyes, but soon they
Dominant Black Eije.
Below thj3 all are
Recessive Red
Moat Rapid Djrieninq
Me^n Rapid Daitieninq
Mean Slow Daritenm^
Absence of DdHteniiu)
Fig. 104.
become pale pink and later scarlet. Just before the end of embryonic
life, the eyes darken, until at about the time of extrusion from the
brood-pouch they are quite black. It has taken 10 days from fertilisa-
tion at 20° to bring this about. Other genetic types, however, hatch
with red eyes, and only much later in life approach to blackness,
as the diagram illustrates for various varieties. Thus, a definite
relation was found to exist between Mendelian genes and rate of a
chemical process.
Morgan, in 1923, discussed such questions as these. "It is to be
hoped", he said, "that in time the combined attack on the problem
6i2 ON INCREASE IN COMPLEXITY [pt. iii
of development by genetics and experimental embryology and
especially by chemistry may lead to the discovery of the physiological
action of the genes, but for the present we may confess ignorance."
A great step forward in this direction was taken by Goldschmidtt
in his important book on physiological genetics. He assumes that
genes are primarily of the nature of enzymes or substances which
can excite the action of enzymes. Working on the caterpillars of
Lymantria dispar he found that by crossing the European and Japanese
races, intersexes could be obtained, and he was able to identify their
various grades between the two poles of maleness and femaleness
with the time-process of development, i.e. with critical points earlier
or later in ontogeny. His conclusions were, firstly that the velocity
of the sex-determining (and all morphogenetic) reactions was pro-
portional to the quantity of the genes present, secondly that all the
morphogenetic reactions go on side by side, the most rapid one
controlling development and, thirdly, that the morphogenetic re-
actions involve determination-hormones brought into being by gene
action. In other words, the genes evoke the organisers, which evoke
the morphogenesis. It is obvious that the co-operation of the chemist
and the geneticist in the investigation of embryonic development will
be very fruitful in the future. With the work of Onslow on the
chemistry of coat-colour and of Brink and his collaborators on the
waxy gene in maize and its chemical effects, such co-operation may
already be said to have begun. Brink & Abegg, in an important
passage, point out that, though at present there is little likelihood
of valuable results emerging from further chemicafstudy of nuclear
material, i.e. the genes themselves, there is every chance of success
in the investigation of the chemical field of action of the genes. The
manner in which these units function in ontogeny plainly offers
a prodigious field for the future work of chemical embryologists.
Interesting reviews of this subject are those of F. R. Lillie and of
T. H. Morgan.
This section may fitly be concluded with the words of Sir W. B.
Hardy: "Let us consider the egg as a physical system. Its poten-
tialities are prodigious and one's first impulse is to expect that such
vast potentialities would find expression in complexity of structure.
But what do we find? The substance is clouded with particles, but
these can be centrifuged away leaving it optically structureless but
still capable of development. . . . On the surface of the egg there is
SECT. 3] AND ORGANISATION 613
a fine membrane, below it fluid of high viscosity, next fluid of rela-
tively low viscosity, and within this the nucleus, which in the resting
stage is simply a bag of fluid enclosed in a delicate membrane. How
shall sources and sinks of energy be maintained in a fluid composed
of 80 per cent, of water? They are undoubtedly there, for the egg
is a going concern, taking in oxygen and maintaining itself by ex-
penditure of energy. . . . The egg's simplicity is not that of a machine
or a crystal, but that of a nebula. Gathered into it are units relatively
simple but capable by their combinations of forming a vast number
of dynamical systems into which they will fall as the distribution of
energy varies.'*
END OF VOLUME I
CAMBRIDGE: PRINTED BY W. LEWIS, M.A., AT THE UNIVERSITY PRESS
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