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o n- 

















M. Man. 

B. Bat. 

S. Sheep. 

C. Cat 

11 nter national Science Xibran? 


Evolution of Man 



Principal Points of Human Ontogeny and 






VOL. I. 


TTbe Werner Company 

36oofc Manufacturers 

Bftron, ©bio 

Authorized Edition 







List of Plates ... ••• ••• •«• ••• ••• xi^ 

Liat of Woodcuts ... ... ... ... ••• xv 

List of Genetic Tables ... ... ... ... ... xriii 

Preface to the First Edition ... ... ... xix 

Preface to the Third Edition ... ... ... ... xxyii 

Prometheus ... ... ••• ... **. iutu 

Faust ». ••• ••• ••• ••• ••• xxxni 



General Significance of the History of the Evolution of Man. — Ignor- 
ance of it among the so-called Educated Classes. — The Two 
Branches of the History of Evolution. — Ontogeny, or the History 
of Germs (Embryos), and Phylogeny, or the History of Descent (or 
of the Tribes). — Causal Connection between the Two Series of 
Evolution. — The Evolution of the Tribe determines the Evolution 
of the Germ. — Ontogeny as an Epitome or Recapitulation of Phy- 
logeny. The Incompleteness of this Epitome. — The Fundamentaj 
Law of Biogeny. — Heredity and Adaptation are the two Formativ* 
Functions, or the two Mechanical Causes, of Evolution. — Absence 
of Purposive Causes. — Validity of Mechanical Causes only. — Sub- 
stitution of the Monistic or Unitary for the Dualistic or Binary 
Cosmology. — Radical Importance of the Facts of Embryology to 
Monistio Philosophy. — Palingenesis, or Derived History, and Keno 
genesis, or Vitiated History. — History of the Evolution of Forme 
and Functions. — Neoesaary Connection between PhyeiogeHv am 


I • 

Morphogeny. — The History of Evolution as yet almost entirely 
the Product of Morphology, and not of Physiology. — The History 
of the Evolution of the Central Nervous System (Brain and Spinal 
Harrow) is involved in that of the Psychic Activities, or the ALnd 1 



Caspar Friedeich Wolff . 

The Evolution of Animals as known to Aristotle. — His Knowledge of 
the Ontogeny of the Lower Animals. — Stationary Condition of the 
cientific Study of Nature during the Christian Middle Ages — 
First Awakening of Ontogeny in the Beginning of the Seventeenth 
Century. — Fabricius ab Aqnapendente. — Harvey. — Marcello Mal- 
ighi. — Importance of the Incubated Chick. — The Theories of Pre- 
formation and Encasement (Evolution and Pre-delineation). — 
Theories of Male and Female Encasement. — Either the Sperm, 
animal or the Egg as the Pre-formed Individual. — Animalculists : 
Leeuwenhoek, Hartsoeker, Spallanzani.— Ovulists : Haller, Leib- 
nitz, Bonnet. — Victory of the Theory of Evolution owing to the 
uthority of Haller and Leibnitz. — Caspar Friedrich Wolff. — His 
Fate and Works. — Tae Theoria Oener'ationis. — Re-formation, or 
Epigenesis. — The History of the Evolution of the Intestinal Canal. 
— The Foundations of the Theory of . Germ-layers (Four Layers, 
Leaves). — The Metamorphosis of Plants. — The Germs of the 
Cellular Theory. — Wolff's Monistic Philosophy ... ... ... 26 



Karl Ernst Baer. 

Karl Ernst Baer, the Principal Disciple of Wolff. — The Wurzhnrg School 
of Embryologists : Dollinger, Pander, Baer. — Pander's Theory of 
Germ. layers. — Its Full Development by Baer. — The Disc-shaped 
first parts into Two Germ-layers, each of which again divides into 
Two Strata. The Skin or Flesh-stratum arises from the Outer or 
Animal Germ- layer. The Vascular or Mucous Stratum arises from 
the Inner or Vegetative Germ-layer. The Significance of the 
Germ-layers. — The Modification of the Layers into Tubes. — Baer's 
Discovery of the Human Egg, the Germ-vesicle, and Chorda Dor* 



galig. — The Four Types of Evolution in the Four Main Groups of 
the Animal Kingdom. — Baer's Law of the Type of Evolution and 
the Decree of Perfection. — Explanation of this Law by the Theory 
of Selection. — Baer's Successors : Hathke, Johannes Muller, Bis- 
clioff, Kolliker. — The Cell Theory : Schleiden, Schwann. — Its Appli- 
cation to Ontogeny : Robert Keniak.— Retrogressions in Ontogeny: 
Reichert and His. — Extension of the Domain of Ontogeny : Darwin 48 



Jean Lamarck. 

Phylogeny before Darwin. — Origin of Species. — Karl Linnaeus' Idea of 
Species, and Assent to Moses' Biblical History of Creation. — The 
Delnge. — Palaeontology. — George Cuvier's Theory of Catastrophes. 
— Repeated Terrestrial Revolutions, and New Creations. — Ly ell's 
Theory of Continuity. — The Natural Causes of the Constant Modi- 
fication of the Earth. — Supernatural Origin of Organisms. — 
Immanuel Kant's Dualistic Philosophy of Nature. — Jean Lamarck. 
— Monistic Philosophy of Nature. — The Story of his Life. — His 
Philo.<ophie Zoolngique. — First Scientific Statement of the Doctrine 
of Descent. — Modification of Organs by Practice and ITabit, in 
Conjunction with Heredity. — Application of the Theory to Man. — 
Descent of Man from the Ape.— Wolfgang Goethe. — His Studies 
in Natural Science. — His Morphology. — His Studies of the 
11 Formation and Transformation of Organisms." — Goethe's Theory 
of the Tendency to Specific Differences (Heredity) and of Meta- 
morphosis (Adaptation) ... ... ... ... ... 7C 


Charles Dakwin. 

Relation of Modern to Earlier Phylogeny. — Charles Darwin's Work on 
the Origin of Species. — Causes of its Remarkable Success. — The 
Theory of Selection : the Interrelation of Hereditary Transmission 
and Adaptation in the Struggle for Existence. — Darwin's Life and 
Voyage Round the World — His Grandfather, Erasmus Darwin. — 
Charles Darwin's Study of Domestic Animals and Plants. — Com- 



pari son of Artificial with Natural Conditions of Breeding. — The 
Struggle for Existence. — Necessary Application of the Theory of 
Descent to Man. — Descent of Man from the Ape. — Thomas Hux- 
ley. — Karl Vogt. — Friedrich Rolle. — The Pedigrees in the Qenerelle 
Morphologxe and the " History of Creation." — The Genealogical 
Alternative. — The Descent of Man from Apes deduced from the 
Theory of Descent. — The Theory of Descent as the Greatest Induc- 
tive Law of Biology. — Foundation of this Induction. — Palaeon- 
tology. — Comparative Anatomy. — The Theory of Rudimentary 
Organs. — Purposelessness, or Dysteleology. — Genealogy of the 
Natural System. — Chorology. — GSkology. — Ontogeny. — Refutation 
of the Dogma of Species. — The " Monograph on the Chalk 
Sponge*;" Analytio Evidence for the Theory of Descent ... 9b 



The Egg of Man and of other Animals is a Simple Cell. — Import and 
Essential Principles of the Cell Theory. — Protoplasm (Cell-sub- 
stance), and the Nucleus (Cell-kernel), as the Two Essential Con- 
stituent Parts of every Genuine Cell. — The Undifferentiated Egg- 
oell. compared with a highly Differentiated Mind-cell or Nerve-oell 
of the Brain. — The Cell as an Elementary Organism, or an Indi- 
vidual of the First Order. — The Phenomena of its Life. — The 
Special Constitution of the Egg-cell. — Yelk. — The Germ -vesicle. — 
The Germ-spot. — The Egg- membrane, or Chorion. — Application of 
the Fundamental Principle of Biogeny to the Egg-cell. — One-oelled 
Organisms. — The Amoebae. — Organization and Vit&) Phenomena. — 
Their Movements. — Amoeboid Cells in Many-celled Organisms. — 
Movements of suoh Cells, and Absorption of Solid Matter. — Absor- 
bent Blood Corpuscles. — Comparison of Amoeba with Egg-cell. — 
Amoeboid Egg-cells of Sponges. — The Amoeba as the Common 
Ancestral Form of Many-celled Organisms ... .„ ... 120 



Development of the Many-celled from the One-celled Organism. — The 
Cell-hermit and the Cell-state. — The Principles of the Formation 
of the State. — The Differentiation of the Individuals as the 



Standard of Measurement for the Grade of the State. — Parallel 
between the Processes of Individual and of Race Development. — 
The Functions of Evolution. — Growth. — Inorganic and Organic 
Growth. — Simple and Complex Growth. — Nourishment and Change 
of Substance. — Adaptation and Modification. — Reproduction. — 
Asexual and Sexual Reproduction. — Heredity. — Division of Labour, 
or Differentiation. — Atavism, or Reversion. — Coalescence. — The 
Functions of Evolution as yet very little studied by Phyniology, 
and hence the Evolutionary Process has often been misjudged. — 
The Evolution of Consciousness, and the Limits to the Knowledge 
of Nature. — Fitful and Gradual Evolution. — Fertilization. — Sexual 
Generation. — The Egg-cell and the Sperm-cell. — Theory of the 
Sperm-animals. — Sperm-cells a form of Whip-cell. — Union of the 
Male Sperm-cell with the Female Fgg-cell. — The Product of this is 
the Parent-cell, or Cytula. — Nature of the Process of Fertilization. 
— Relation of the Kernel (Nucleus) to this Process. — Disappear- 
ance of the Germ-vesicle. — Monerula. — Reversion to the Monera- 
form. — The Cytula ... ... ... ... ... ... 148 



First Processes after the Fertilization of the Egg-cell is complete. — 
Original or Palingenetic Form of Egg-cleavage. — Significance of 
the Cleavage-process. — Mulberry-germ, or Morula. — Germ-vesicle, 
or Blastula Germ-membrane, or Blastoderm. — Inversion (In- 
vagination) of the Germ-vesicle. — Formation of the Gastrula. — 
Primitive Intestine and Primitive Mouth. — The Two Primary 
Germ -layers ; Exoderm and Entoderm. — Kenogenetic Form of Egg- 
cleavage. — Unequal Cleavage (segmentatio inequalis) and Hood- 
gaetrula (Amphigastrula) of Amphibia and Mammalia. — Total and 
Partial Cleavage. — Holoblastic and Meroblastic Eggs. — Discoidal 
Cleavage (segmentatio discoidalis) and Disc-gastrula (Discogastrula) 
of Fishes, Reptiles, Birds. — Superficial Cleavage (segmentatio super- 
ficialis) and Vesicular Gastrula (Peri-Gastrula) of Articulates 
(Arthropoda). — Permanent Two-layered Body-form of Lower 
Animals. — The Two-layered Primeeval Parent-form ; Gastraea. — 
Homology of the Two Primary Germ-layers in all Intestinal 
Animals (Metaz'u). — Significance of the Two Primary Germ- 
layers. — Origin and Significance of the Four Secondary Gei 
layers. — The Exoderm or Skin-layer gives rise to the Skin-sensor; 



Layer and the Skin-fibrous Layer.— The Entoderm or Intestinal 
Layer gives rise to the Intestinal-fibrous Layer and the Intestinal- 
glandular Layer ... ... ... ... .. ... ... ... 184 



K elation of Comparative Anatomy to Classification. — The Family-rela- 
tionship of the Types of the Animal Kirgdcni. — Different Signi- 
ficance and Unequal Value of the Seven Animal Types. — The 
Qastra-u Theory, and the Phylogenetic Classification of the Animal 
Kingdom. — Descent of the Gastrasa from the Protozoa. — Descent 
of Plant-animals and Worms from the Gastraea. — Descent of 
the Four Higher Classes of Animals from Worms. — The Verte- 
brate Nature of Man. — Essential and Unessential Parts of the 
Vertebral Organism. — The Amphioxus, or Lancelet, and the Ideal 
Primitive Vertebrate in Longitudinal and Transverse Sections. — 
The Notochord.— The Dorsal Half and the Ventral Half.— The 
Spinal Canal. — The Fleshy Covering of the Body. — The Leather- 
skin (corium). — The Outer-skin (epidermis) . — Body -cavity (caeloma). 
— The Intestinal Tube.— The Gill-openings. — The Lymph-vessels. 
— The Blood-vessels. — The Primitive Kidneys and Organs of Re- 
production. — The Products of the Four Secondary Germ-layers ... 244 



The Original (Palingenetic) Development of the Vertebrate Body from 
the Gastrula.— Relation of this Process to the Later (Kenogenetic) 
Germination, as it occurs in Mammals. — The most important act in 
the Formation of the Vertebrate. — The Primary Germ-layers, and 
also the Secondary Germ-layers, which arise by Fission of the Prima- 
ries, originally form Closed Tubes. — Contemporaneously with the 
Completion of the Yelk-sac, the Germ-layers flatten, and only later 
again assume a Tabular Form. — Origin of the Disc-shaped Mamma- 
lian Gerui-area. — Light Germ-area (area pellucida) and Dark Germ- 
area (area opaca). — The Oval Germ-shield, which afterwards 
assumes the Shape of the Sole of a Shoe, appears in the Centre of 
the Light Germ-area (a. pellucida). — The Primitive Streak 



separates the Germ-shield into a Right and Left Half. — Below the 
Dorsal Furrow the Central Germ -layer parts into the Notochord 
and the Two Side-layers. — The Side-layers split horizontally into 
Two Layers : The Skin-fibrous Laver and the Intestinal -fibrous 
Layer. — The Primary Vertebral Cords separate from the Side- 
layers. — The Skin-sensory Layer separates into Three Parts : the 
Horny Layer, Spinal Canal, and Primitive Kiduey. — Formation of 
the Caelora and the First Arteries. — The Intestinal Canal proceeds 
from the Intestinal Furrow. — The Embryo separates from the Germ- 
vesicle. — Ai'ound it is formed the Amnion -fold, which coalesces 
over the back of the Embryo, so as to form a Closed Sac. — The 
Amnion. — The Amnion-water. — The Yelk-sac, or Navel-vesicle. — 
The Closing of the Intestinal and Ventral Walls occasions the 
Formation of the Navel. — The Dorsal and Ventral Walls ... 274 




Essential Agreement between the Chief Palingenetic Germ Processes 
in the case of Man and in that of other Vertebrates. — The Human 
Body, like that of all Higher Animals, develops from Two Primary and 
Four Secondary Germ-layers. — The Skin-sensory Layer forms the 
Horn-plate, the Medullary Tube, and the Primitive Kidneys. — The 
Middle Layer (Mesoderm) breaks up into the Central Notochord, 
the Two Primitive Vertebral Cords, and the Two Side-layers. — 
The latter split up into the Skin-fibrous Layer and the Intestinal- 
fibrous Layer. — The Intestinal-glandular Layer forms the Epi- 
thelium of the Intestinal Canal, and of all its Appendages. — Onto- 
genetic and Phylogenetic Fission of the Germ-layers. — Formation 
of the Intestinal Canal. — The Two-layered Globular Intestinal 
Germ-vesicle of Mammals represents the Primitive Intestine. — 
Head Intestinal Cavity, and Pelvic Intestinal Cavity. — Mouth 
Groove and Anal Groove. — Secondary Formation of Mouth and 
Anus. — Intestinal" Navel and Skin-navel. — Movement of the 
Primitive Kidneys from the Outside to the Inside.— Separation of 
the Brain and Spinal Marrow. — Rudiments of the Brain- bladders. 
The Articulation or Metameric Structure of the Body. — The 
Primitive Vertebrae (Trunk- Segments, or Metamera). — The Con- 
struction and Origin of 4he Vertebral Column. — Vertebral Bodies 
and Vertebral Arches. — Skeleton-plate and Muscle-plate. — Forma- 


tion of the Skull from the Head-plates. — Gill-openings and Gill- 
arches. — Sense-organs. — Limbs. — The Two Front Limbs and the 
Two Hind Limbs ... ... ... ... ... ... ... 828 




ie Mammalian Organization of Man. — Man has the same Bodily 
Structure as all other Mammals, and his Embryo develops in 
exactly the same way. — In its Later Stages the Human Embryo is 
not essentially different from those of the Hig-her Mammals, and in 
its Earlier Stages not even from those of all Higher Vertebrates. — 
The Law of the Ontogenetic Connection of Systematically Related 
Forms. — Application of this Law to Man. — Form and Size of the 
Human Embryo in the First Four Weeks. — The Human Embryo in 
the First Month of its Development is formed exactly like that of 
any other Mammal. — In the Second Month the First Noticeable 
Differences appear. — At first, the Human Embryo resembles those 
of all other Mammals ; later, it resembles only those of the Higher 
Mammals. — The Appendages and Membranes of the Human 
Embryo. — The Yelk-sac. — The Allantois and the Placenta. — The 
Amnion. — The Heart, the First Blood-vessels, and the First Blood, 
arise from the Intestinal-fibrous Layer. — The Heart separates 
itself from the Wall of the Anterior Intestine. — The First 
Circulation of the Blood in the Germ-area (a. germinativa) : Yelk- 
arteries and Yelk-veins. — Second Embryonic Circulation of the 
Blood, in the Allantois: Navel-arteries and Navel-veins. — Divisions 
of Human Germ-history ... ... ... ... ... ••* 868 




Causal Significance of the Fundamental Law of Biogeny. — Infiuenoe 
of Shortened and Vitiated Heredity. — Kenogenetic Modification of 
Palingenesis. — The Method of Phylogeny based on the Method of 
Geology. — Hypothetic Completion of the Connected Evolutionary 
Series by Apposition of the Actual Fragments. — Phylogenetio 
Hypotheses are Reliable and Justified. — Importance of the Axnpsi* 



oxus and the Ascidian.— Natural History and Anatomy of the 
Amphioxus. — External Structure of the Body. — Skin-covering. — 
Outer-skin (Epidermis) and Leather-skin (Corium). — Notochord.— 
Medullary Tube. — Organs of Sense. — Intestine with an Anterior 
Respiratory Portion (Gill-intestine) and a Posterior Digestive 
Portion (Stomach-intestine). — Liver. — Pulsating Blood-vessels. — 
Dorsal Vessel over the Intestine (Gill-vein and Aorta). — Ventral 
Vessel under the Intestine (Intestinal Vein and Gill -artery). — 
Movement of the Blood. — Lymph-vessels. — Ventral Canals and 
Side Canals — Body-cavity and Gill-cavity. — Gill-covering. — 
Kidneys. — Sexual Organs. — Testes and Ovaries. — Vertebrate 
Nature of Amphioxus. — Comparison of Amphioxus and Young 
Lamprey (Petromyzori). — Comparison of Amphioxus and Ascidian, 
— Cellulose Tunic. — Gill-sao. — Intestine. — Nerve-centres.— ^HearVi. 
— Sexual Organs ... ... ... ••• . M ... 406 




Relationship of the Vertebrates and Invertebrates. — Fertilization of the 
Amphioxus. — The Egg undergoes Total Cleavage, and changes into 
a Spherical Germ-membrane Vesicle (Blastula). — From this the 
Intestinal Larva, or Gastrula, originates by Inversion. — The 
Gastrula of the Amphioxus forms a Medullary Tube from a Dorsal 
Furrow, and between this and the Intestinal Tube, a Notochord : 
on both Sides the latter is a Series of Muscle-plates ; the Matemera. 
— Fate of the Four Secondary Germ-layers. — The Intestinal Canal 
divides into an Anterior Gill-intestine, and a Posterior Stomach- 
intestine.— Blood-vessels and an Intestinal-muscle Wall originate, 
from the InteBtinal-fibrous Layer. — A Pair of Skin-folds (Gill- 
roofs) grow out from the Side-wall of the Body, and, by Coales- 
cence, form the Ventral Side of the Large Gill-cavity. — The 
Ontogeny of the Ascidian is, at first, identical with that of the 
Amphioxus. — The same Gastrula is Developed, which forma 
a Notochord between the Medullary and Intestinal Tubes. — 
Retrogressive Development of the same. — The Tail with the 
Notochord is shed. — The Ascidian attaches itself firmly, and 
envelops 'tself in its Cellulose Tunic — Appendicularia, a Tunicate 
which remains throughout Life in the Stage of the Larval Ascidian 
and retains the Tail-fin with the Chorda (Chordoma). — General 
Com pari eok. and Significance of the Amphioxus and the Ascidian 439 



Plate T. (Frontispiece). Development of the face in Mammals 
(Man, Bat, Cat, Sheep) in three different stages 

Explanation vol. ii. 346 
I^ate II. (between p. 240 and p. 241). Total egg-cleavage. Gas- 
trulation of holoblastic eggs (primordial and unequal cleavage) 

Explanation 240 

Plate III. (between p. 240 and p. 241). Partial egg-cleavage. 
Gastrulation of meroblastic eggs (discoidal and superficial 
cleavage) ... ... ... ... Explanation 240 

Plate IV. (between p. 320 and p. 321). Diagrammatic transverse 
section through various ontogenetic and phylogenetic stages 
in the development of the human body, showing the formation 
of this from the four secondary germ-layers ... Explanation 321 

Plates V*. (between p. 320 and p. 321). Diagrammatic longitu- 
dinal sections through various germ and tribal forms of Man 
showing their formation from the four secondary germ-layers 

Explanation 323 

Plate VI. (between p. 362 and p. 363). Comparison of the 
embryos of a Fish, an Amphibian, a Reptile, and a Bird, in 
three different stages of evolution ... ... Explanation 362 

Plate VII. (between p. 362 and p. 363). Comparison of the 
embryos of four different Mammals (Pig, Ox, Rabbit, and 
Man) in three different stages of evolution ... Explanation 363 

Plate VIII. (between p. 404 and p. 405). Representation of two 
human embryos, the one of nine, the other of twelve weeks : 
the latter within the egg-membranes ... Explanation 406 

Plate IX. (between p. 404 and p. 405). Representation of a 
human embryo of five months, natural size, within the egg- 
membranes ... ... ... ... Explanation 405 

Plate X. (between p. 438 and p. 439). Germ-history of Ascidian 

and Amphioxus ... ... ... ... Explanation 436 

Plate XI. (between p. 438 and p. 439). Structure of the body 
of Ascidian, Amphioxus, and larva of Petromyzon 

Explanation 437 



1. Human egg-coll • . 122 

2. Hainan liver-cell • • 124 

3. Epithelium cell from tongue 124 

4. Thorny cells of epidermis 125 


Human bone-cells . 



Enamel cells of tooth • 



A mind-cell 



Blood -cells in process ol 


division . 



Active lymph-cells . 



Primitive eggs of various 


animals . 

, 134 


Mammalian egg-cell . 

. 136 


Egg-cell of Hen 

, 139 


An Amoeba 

. 142 


Egg-cell of a Chalk-sponge 

» 144 


Blood-cells absorbing mat 

tor ... 

. 145 


Blood-cells dividing . 

. 159 


Sperm-cellB (seed-cells) 

. 173 


Fertilization of mammaliai 


egg ... 

. 175 


Monerula of Mammal 

. 179 


Moneron dividing . 

. 180 


Cy tula of Mammal • 

. 181 


22. Germination of a Coral . 190 

23. Gastrula of Gastrophysema 193 

24. Gastrula of Sagitta . . 193 

25. Gastrula of Uraster . . 193 

26. Gastrula of Nauplius . 193 

27. Gastrula of Limnaeus . 193 

28. Gastrula of Amphioxus . 193 

29. Gastrula of Olynthus . 195 

30. Cells of primary germ- 

layers . . . .198 

31. Cleavage of Frog's egg . 203 
32-35. Gastrulationof the Toad 206 

36. Monerula of Rabbit . . 210 

37. Cytula of Rabbit . . 210 

38. Rabbit-egg with two cells 210 

39. Rabbit-egg with four cells 212 

40. Rabbit-egg with eight cells 212 

41. Gastrula of Rabbit . . 213 

42. Egg of an Osseous Fish . 217 

43. G astrnla of an Osseous Fish 219 

44. Egg-cell of Hen . . 223 

45. Egg-cleavage of Bird . 225 

46. Mulberry-germ of Chick . 228 

47. Bladder-germ of Chick . 228 

48. Iuvaginated germ of Chick 228 



49. Gastrula of Chiok . 

50, 61. Four secondary germ- 

layers . 

62-56. Diagrammatic longitu- 
dinal and transverse sec- 
tions through the ideal 
Primitive Vertebrate 

57, 68. m m 

59. » * 

60. » m 

61. ft M 

62-69. Diagrammatic trans- 
verse sections through 
the most important germ- 
forms of the ideal Primi- 
tive Vertebrate 

70. Diagrammatio transverse 

sections through various 
mammalian germs (ex- 
plaining the separation 
of the intestine from the 
yelk-sac). . 

71. Gastrula of Mammal . 

72. Intestinal germ-vesiole of 


73. Transverse section through 

the intestinal germ- 
vesicle of Mammal 

74. Bxoderm-cells of the above 

75. Entoderm-oells of the above 

76. Transverse section through 

germ-area . . 
77-81. Intestinal germ-vwiicle 

of Babbit 
82, 83. Germ-area of Rabbit . 

M. » » 

IK. •  














86. Sole -shaped germ-shield 

of Dog. 

87. Sole-shaped germ-shield 

of Chick 

88. Transverse section 

through germ-shield 

89. „ ' m 

9& *» t$ 





n » 

94. Development of egg -mem- 

branes . 

95. Transverse section 

through germ of Chick . 
96,97. „ „ 

98. ,, w 

99. „ „ 

100. Separation of the intes- 

tine from the yelk- sac 

101. Longitudinal section 

through embryo Chick . 

102. Longitudinal section 

through head of an 
embryo Chick 

103-105. Lyre-shaped Chick 

106, 107. Germ-diso or germ- 
area of Babbit 

108,109. „ „ 

110, 111. Skeleton of Man 

112. Transverse section 

through germ of Chick . 

113. Human neck-vertebra 

114. Human ohest- vertebra . 
116. Human lumbar-vertebra . 


















116, 117. Head of embryo 


Development of egg- mem- 

Chick . 


branes .... 


118. Head of embryo Dog 



Development of amnion . 


119. Budiments of the limbs . 





120. „ „ 



>» »» 


121. Lyre-shaped germ of Dog 



144. Development of heart 


122. Human germs from the 


146. „ „ 


second to the fifteenth 


» n 


tt ©t?jC • • • • 



First circulation of the 

128, 124. Anatomy of human 

blood . • . • 


germs (four and five 


» » 


weeks) .... 



n n 


125. Head of Nose-ape . 



Amphioxus lanceolatus . 


126. Head of Julia Pastrana . 



Transverse section 

127-131. Human eggs and 

through Amphioxus . 


germs from second to 


An Asoidian . • • 


sixteenth weeks . 



Another Asoidian . . 


132, lo3. ,, jf 



Ga8trula of Amphioxus . 


134. „ „ 



Gastrula of Sponge . 


135. Chick germ with allantois 



Transverse section 

136. Dog germ with allantois . 


through Amphioxus larva 


137. » * n 





188. Pregnant human uterus 


Transverse section 

with egg - membranes 

through Vertebrate 


and narel-ODrd • • 



Appendicularia • • 




I. Systematic Survey of the main branches of Biogeny ... 24 
II. Systematic Survey of the constituent parts of the one- 
celled germ-form before and after fertilization ... 18U 

III. Systematic Survey of the most important ditl'erences in 

the egg-cleavage and gastrulation of animals ... 241 

IV. Systematic Survey of the five first germinal stages of 

animals, with reference to the four main forms of 
egg-cleavage ... ... .... ... ... 242 

V. Systematic Survey of some of the most important obser- 
vations in the rhythm of egg-cleavage ... ... 243 

VL Systematic Survey of some of the most important organs 
of the ideal Primitive Vertebrates, and their de- 
velopment from the germ-layers ... ... ... 273 

VIL Systematic Survey of the development of the human 

organ-systems from the germ-layers ... ... 327 

VIII. Systematic Survey of the most important section of 

human germ-history ... ... ... ... 402 

IX. Systematic Survey of the most important homologies 
between the embryo of Man, and the embryo of 
the Ascidian and the Amphioxus in a corresponding 
'stage of development, on the one hand, and the 
developed Man on the other ... ... ... 465 

X, Systematic Survey of the relationship in form of the 
Ascidian and Amphioxus on the one hand, of the Fish 
and Man on the other, in a fully developed condition 466 

XI Ontogenetic cell pedigree of the Aniphioxua ••« ••• 467 



These chapters on Anthropogeny are the first attempt 
to render the facts of human germ-history accessible to a 
wider circle of educated people, and to explain these facts 
by human tribal history. I have not overlooked the great 
difficulty and danger involved in thus entering for the 
first time on ground which is so especially full of risks. 
No other branch of natural science yet remains so ex- 
clusively confined to its own technical students ; no other 
branch has been so wilfully obscured and mystified, by 
priestly influence, as has the germ-history of Man. If, 
even now, we say that each human individual develops 
from an egg, the only answer, even of most so-called edu- 
cated men, will be an incredulous smile ; if we show them 
the series of embryonic forms developed from this human 
egg, their doubt will, as a rule, change into disgust. Few 
educated men have any suspicion of the fact, that these 
human embryos conceal a greater wealth of important 
truths, and form a more abundant source of knowledge than 
is afforded by the whole mass of most other sciences and 
of all so-called "revelations." 


Nor is this surprising, when we see what a little way 
the knowledge of human evolution has spread even among 
the very students of Nature. Even in most works devoted 
to the Natural History, Anatomy, Physiology, Ethnology, 
and Psychology of Man, it is evident at a glance that their 
authors, if not ignorant, have at least a very superficial 
knowledge of human germ-history, and that tribal history 
lies far beyond them. The name of Darwin is, indeed, in 
every mouth. But few persons have really assimilated 
the theory of descent, as reformed by him ; few have made 
it part of themselves. To show how far even biologists of 
repute are from thoroughly understanding the history of 
evolution, no more remarkable recent instance can be 
found than the well-known address, on " The Limits of 
Natural Knowledge." delivered by the celebrated physio- 
logist, Du Bois Reymond, in 1873, before the naturalists 
assembled at Leipzig. This eloquent address, the source 
of such triumph to the opponents of the theory of evolu- 
tion, the cause of such pain to all friends of intellectual 
advance, is essentially a great denial of the history oj 
evolution. No thoughtful naturalist will disagree with the 
Berlin physiologist when, in the first half of his address, 
he explains the limits of natural knowledge, as they are at 
present set to man by his vertebrate nature. But it is 
equally certain that every monistic naturalist will protest 
against the second half of the address, in which, not only 
is another limit, assumed to be different (but in reality 
identical), indicated for human knowledge, but the con- 
clusion is finally drawn, that man will never pass over 
these limits : " We shall never know that ! Ignorabimus ! " 

As the unanimous thanks of the Ecclesia militans have 


been gained by the author of this "Ignorabimus," the most 
deserving student of the electricity of nerves and muscles, 
we must here most emphatically protest in the name of 
advancing natural knowledge and of all science capable 
of development. Had our one-celled Amoeba-ancestors of 
the Laurentian Period been told that their descendants 
would afterwards, in the Cambrian Period, produce a many- 
celled Worm-like organism possessed of skin and intestine, 
muscles and nerves, kidneys and blood-vessels, they would 
certainly not have believed ; nor, again, would these Worms 
have believed, had they been told that their descendants 
would develop into skull-less Vertebrates, such as the 
Amphioxus ; nor would these Skull-less Animals have 
credited that their posterity would ever become Skulled 
Animals (Craniota). Our Silurian Primitive-fish ancestors 
would have^ been equally hard to convince that their off- 
spring of the Devonian Period would acquire amphibian 
form, and yet later, in the Triassic Period, would appear 
as Mammals ; the latter, again, would have deemed it im- 
possible that, in Tertiary times, a very late descendant 
of theirs would acquire human form, and would gather the 
splendid fruits of the tree of knowledge. All these would 
have answered : " We shall never change, nor shall we 
ever understand the history of our evolution ! Nunquam 
mutabimur ! Semper ignoxabimus ! " 

With this Ignorabimus the Berlin school of Biology 
tries to stop science in its advance along the paths oi 
evolution. This seemingly humble but really audacious 
" Ignorabimus" is the " Ignoratw" of the infallible 
Vatican and of the " black international " which it leads ; 
that mischievous host, against which the modern civilized 


state has now at last begun in earnest the " struggle 
for culture." In this spiritual warfare, which now moves 
all thinking humanity, and which prepares the way for a 
future existence more worthy of man, spiritual freedom 
and truth, reason and culture, evolution and progress 
stand on the one side, marshalled under the bright banner 
of science ; on the other side, marshalled under the black 
flag of hierarchy, stand spiritual servitude and falsehood, 
want of reason and barbarism, superstition and retrogres- 
sion. The trumpet of this gigantic spiritual warfare 
marks the dawn of a new day and the end of the long 
darkness of the Middle Ages. For modern civilization, in 
spite of the progress of culture, lies bound in the fetters 
of the hierarchy of the Middle Ages ; and social and civil 
life is ruled, not by the science of truth, but by the faith 
of the church. We need but mention the mighty influence 
which irrational dogmas still exercise on the elementary 
education of our youth; we need but mention that the 
state yet permits the existence of cloisters and of celibacy, 
the most immoral and baneful ordinances of the " only- 
saving " church ; we need but mention that the civilized 
state yet divides the most important parts of the civil 
year in accordance with church festivals ; that in many 
countries it allows public order to be disturbed by church 
processions, and so on. We do indeed now enjoy the 
unusual pleasure of seeing " most Christian bishops " and 
Jesuits exiled and imprisoned for their disobedience to the 
laws of the state. But this same state, till very recently, 
harboured and cherished these most dangerous enemies oi 

In this mighty " war of culture," affecting as it does 


the whole history of the World, and in which we may well 
deem it an honour to take part, no better ally than Anthro- 
pogeny can, it seems to me, be brought to the assistance 
of struggling truth. The history of evolution is the heavy 
artillery in the struggle for truth. Whole ranks of dualistic 
sophisms fall before the monistic philosophy, as before the 
chain shot of artillery, and the proud structure of the 
Roman hierarchy, that mighty stronghold of infallible 
dogmatism, falls like a house of cards. Whole libraries 
of church wisdom and false philosophy melt away as soon 
as they are seen in the light afforded by the history of 
evolution. The church militant itself furnishes tne most 
striking evidences of this, for it never ceases to give the 
lie to the plain facts of human germ-history, condemning 
them as "diabolical inventions of materialism." In so 
doing it gives the most brilliant witness that it recognizes 
as unavoidable the conclusions which we have drawn from 
these facts as to tribal history, as to the true causes of 
these facts. 

In order to render these little known facts of germ- 
history and their causal explanation by tribal history 
accessible to as wide a circle of educated readers as pos- 
sible, I have followed the same course as that which 
I adopted, six years ago, in my " Natural History of 
Creation," of which the " Anthropogeny " forms a second, 
more detailed part. In the summer of 1873 I had the 
academical lectures, on the outlines of the history of 
human evolution, which I have delivered during the last 
twelve years in Jena before a mixed audience of students 
of all faculties, taken down in shorthand by two of that 
audience, Messrs. Kiessling and Schlawe. The task I 

xxIt preface to the first edition. 

undertook in publishing these was indeed much harder than 
that incurred in the " Natural History of Creation ; " for 
tthile the latter passed lightly through the widest circle 
of biological phenomena, and touched only on the most 
interesting points, I was obliged, in the " History of the 
Evolution of Man," to exhibit a much more limited series 
of phenomena in their proper connection, of which, indeed, 
each individual one is interesting in its proper place, 
although they are of very various degrees of interest. 
Moreover, the comprehension of form-phenomena, with 
which human germ- history deals, is among the most 
difficult of morphological tasks ; the academical lectures 
on the history of human evolution are rightly considered 
even by medical men, who are previously acquainted with 
the anatomical features of the human body, as the most 
difficult to understand. I saw, therefore, that, if I desired 
to make the road into this dark region, entirely closed as 
yet to most men, really accessible to the educated laity, 
I must, on the one hand, limit myself as far as possible in 
my selection from the abundance of empiric matter, and 
yet, on the other hand, that I must be careful not to pass 
entirely over any essential part of this matter. 

Although, therefore, I have throughout taken pains to 
present the scientific problem of Anthropogeny in as 
popular a form as possible, I do not imagine that I have 
completely accomplished this very difficult task. I shall, 
however, have gained my object if I succeed in affording 
educated persons an approximate conception of the most 
essential outlines of human germ-history, and in con- 
vincing them that the sole explanation and comprehension 
of the matter is afforded by the corresponding tribal 


history. Perhaps, at the same time, I may hope to con- 
vince some of those specialists, who deal indeed daily 
with the facts of germ-history, but who neither know nor 
wish to know anything about the true causes of these, 
which lie hid in tribal history. As this is quite the first 
attempt to present the Ontogeny and Phylogeny of man in 
their whole causal connection, I fear that, at best, the 
point at which I aim lies far beyond the point gained. 
But by this each thinking man will, it is to be hoped, be 
convinced that only by recognizing this connection does 
the history of human evolution become a science. On- 
togeny can only be really understood through Phylogeny. 
The history of the tribe lays bare the true causes of the 
history of the germ. 

Eknst Heinkich Haeckel. 

Jena, July 13, 1874. 



When, two years ago, I published the first edition of the 
" History of the Evolution of Man," and this was followed, 
a few months later, by an unaltered second edition, I was 
fully conscious of the hazard involved in so doing, and 
was prepared to meet with numerous attacks. These were 
not slow to come ; and if I were now obliged to answer all 
my opponents, this third edition might easily be doubled in 
size. I think, however, that I may satisfy myself with but 
a few remarks. 

The great majority of my opponents are determined 
enemies of the Doctrine of Descent, who altogether deny 
a natural evolution of organic nature, and who can 
only explain both the origin of man and that of animal 
and plant species with the help ot miracles, by super- 
natural creative acts. These adherents of the Creation 
Theory I need not answer; for Anthropogeny, as the 
special application of the Theory of Descent to Man, 
naturally starts from the recognition of this latter theory : 
ten years ago, in my Generelle Morphologie, and again in 
the " Natural History of Creation," I explained my own 
conception of this in sufficient detail. 


I cannot, however, refrain from defending my stand- 
point against those naturalists, who, taking their position 
indeed on the Theory of Descent and on Darwinism, yet 
combat my individual conception of this, and, especially, 
regard my application of the theory to Anthropogeny as 
erroneous. Many of these naturalists, who were formerly 
determined opponents of the Theory of Descent, have 
recently passed over to Darwin's camp, merely in order 
not to stand entirely inactive at the barren standpoint 
offered by negation. Against two of these false Darwinists, 
Wilhelm His and Alexander Goette, I have defended 
myself in a special work on " The Aims and Methods of the 
Modern History of Evolution" (" Ziele und Wege der Heuti- 
gen Entwickelungsgeschichte." Jena, 1875). To that work 
I now refer. On the other hand, I have been forcibly 
attacked by naturalists who are really esteemed as well- 
known and convinced adherents of the Theory of Evolu- 
tion. Of these, Karl Vogt and Albert Kolliker require a few 
words of answer. 

Vogt, whose many services in furthering Zoology I have 
always most readily acknowledged, ranked second to Huxley 
among those naturalists who, but a few years after the 
appearance of Darwin's " Origin of Species," attempted to 
apply the theory contained in that work to Man and 
represented this as necessary. He afterwards, however, 
made no further progress in the same direction. While, as 
I am convinced, the mass of facts already accumulated in 
Comparative Anatomy, Ontogeny, Palaeontology, and Sys- 
tematic Zoology, is amply sufficient to afford the most 
general points on which to base the hypothetic human 
pedigree, Karl Vogt now holds opposed views, and entirely 


rejects the ancestral series as I have arranged it. He 
says : " We have been able to prove the assertion that Men 
and Apes must have originated from a common line ; — more 
than this we have never asserted, and further back than 
this it is absolutely impossible to prove anything or even 
to show with any degree of probability more than that. 
at farthest, the higher Mammals may perhaps have de- 
veloped from Pouched Animals (Marsupialia)." Against 
this view of Vogt's, I assert, that with the same logica^ 
" certainty or probability " the common descent of all 
Mammals from lower Vertebrates, primarily from Am- 
phibia, less immediately from Fishes, may be " proved." 
With the same " certainty or probability " — I assert again 
— the descent of all these Skulled Animals (Craniota) from 
Skull-less forms (Acrania, allies of Amphi ^xus), the descent 
of these latter from Chorda Animals (Chordoma, forms 
allied to Ascidia), and the descent of these Chorda -Animals 
from low Worms, " may be proved." With the same 
"certainty or probability" — I say finally — "we have been 
able to prove the assertion," that these Worms must, 
in their turn, have originated from a Gastraea (resembling 
the gastrula), and these Gastrseads from a one-celled 
organism (resembling the undifferentiated Amoeba). Proofs, 
as I believe, of these assertions are given in Chapters 
XIII.-XXY. of this edition. 

The whole of this hypothetic pedigree Karl Vogt entirely 
rejects, without, however, substituting another He espe- 
cially denies our relationship with the Selachii and the 
Amphioxus, with the Ascidia and the Gastraea, although the 
especially great phylogenetic significance of these instruc- 
tive animal-forms is almost unanimously recognized by the 


first authorities in our science. Whilst Vogt completely 
opposes himself to these important views, which from 
day to day become more firmly established, he refers to 
Karl Semper, a "gifted" naturalist, who shares these 
views of Vogt's, and who derives Vertebrates from Kinged 
Worms (Annelida). I regret that I can make no use of 
this reference; nor do I find reason to answer Semper's 
polemic on "Haeckelism in Zoology" (" Haeckelismus in 
der Zoologie." Hamburg, 1876) ; for, apart from his de- 
fective education and his insufficient acquaintance with the 
whole subject of Zoology, this " gifted " zoologist is so 
much at variance with logic, as also with truth, that 
refutation seems superfluous. (Cf. vol. i. p. 91 and p. 426.) 
An example is sufficient to show this : In order to indicate 
the scientific value of "Haeckelism," and in order "to 
show that this tendency must continually diverge more 
and more widely from the really scientific study ol 
nature," Semper brings forward the fact that, " according 
to Haeckel's own statement, Darwinism should be the 
religion of every naturalist." This last statement, which 
I consider absurd, is not mine, but that of my determined 
opponent, Professor Riitimeyer, and I quoted the sentence 
in the preface to the third edition of the " Natural History 
of Creation " merely to show the singular ground occupied 
by its author. 

The wide cleft which separates my standpoint of the 
history of evolution and of natural science, as a whole, 
from that of Yogt and Semper cannot be better indicated 
than by our mutual position towards philosophy. Karl 
Vogt, like his friend Karl Semper, was a sworn contemner 
of all philosophy. The former seizes every opportunity to 


mock at philosophic tendencies and researches ; and the 
latter knows no more severe charge to bring against me 
than that I seek to unite empiricism and philosophy, 
experience and idea, "observation and reflection." I am 
certainly firmly convinced that a really scientific study of 
nature can no more dispense with philosophic reflection, 
than can healthy philosophy ignore the results of natural 
scientific experience. "An exact empiricism," without 
those philosophic thoughts which combine and explain the 
raw material of facts, merely results in the accumulation 
of a lifeless store of knowledge ; on the other hand, 
" speculative philosophy " which knows nothing of the firm 
basis afforded by natural scientific observation, can only 
produce transient cloud-pictures. The most intimate com- 
bination and blending of empiricism and philosophy can 
alone enable us to construct a permanent and sure scientific 
structure. I still hold as decidedly as ever the much- 
abused views which I expressed, ten years ago, about this 
matter in my Generelle Morphologic, and the fundamental 
ideas which I have here reproduced. 

Moreover, he must be very one-sided or short-sighted 
who does not recognize the natural approximation, which 
is now becoming more close in all branches of human 
knowledge, between experimental and reflective study. The 
enormous enlargement of the field of empiric knowledge 
which has been brought about by the progress of the last 
half-century, has resulted in a corresponding specialization 
of separate researches, and consequently in an isolation of 
diverging aims which cannot possibly continue to satisfy. 
All thoughtful observers feel, more acutely in consequence 
of this, that they must raise themselves from the wearisome 


task of accumulating dry details to wider views, and thus to 
gain sympathy with allied aims. On the other side, the 
sterility of such pure speculative philosophy as ignores all 
those enormous advances in empiric knowledge, has so 
forced its way into the consciousness of all sound thinkers, 
that they earnestly desire to fall back on the firm basis 
afforded by experimental science. 

The ever-increasing flood of writings on natural philo- 
sophy, and essays on the relation of philosophy to natural 
science, plainly indicates the happy growth of this scientific 
unitary tendency. Nothing is more favourable to this, 
nothing better advances the combination of the various 
scientific lines, than the new theory of evolution. The 
extraordinary importance ascribed to this theory, rests 
especially on the fact that it supplies a philosophic central 
point, and just for this very reason it has in so short a 
time gained the active interest of all thoughtful minds. 
It raises us from a knowledge of facts to a knowledge of 
causes, and thus affords a deeper satisfaction to the 
demand for causality innate in human reason than a mere 
experimental science could ever supply. When, therefore, 
Karl Vogt and many other naturalists entirely reject philo- 
sophy, and will not allow that it has any point of union 
with what is called " exact " natural science — they volun- 
tarily renounce all the higher aims of investigation. 
(Cf. vol. ii. p. 887.) 

Albert Kolliker occupies a similarly one-sided stand- 
point. This author, in the second edition of his " History 
of the Evolution of Man and the Higher Animals " (" Ent- 
wickelungsgeschichte des Menschen und der Hoheren 
Thiere," 1876), in especially attacking the fundamental law 


of Biof/eny, has impugned the very foundation on which 
Anthropogeny rests. Most of his objections are, it appears 
to me, refuted by the explanations which I have given 
in this third edition as to the very important relations 
of Palingenesis and Kenogenesis. (Compare especially 
Chapters L, VIII., and X.) Kolliker will not recognize the 
Gastraea Theory because he has been unable to discover a 
gastrula in Mammals and Birds. But his experiences are 
opposed to the most recent researches of Van Beneden and 
Kauber, of whom the former in the case of the Babbit, the 
latter in the case of the Chick, describes a kenogenetic 
gastrula-form, which, in accordance with the Gastraea 
theory, may easily be referred to the palingenetic gastrula 
of the Amphioxus. Kolliker says finally : " As the lasi and 
most important argument, I bring forward the fact that 
Phylogeny as read by Darwin and Haeckel does not, it 
appears to me, represent the truth." This "most im- 
portant argument " is a simple petitu, principii. The sen- 
tence might as well be, " phylogeny is not true because it 
does not represent the truth." 

How very different in other respects Kolliker's concep- 
tion of the history of evolution is from mine is most clearly 
indicated in the " General Observations " (§ 29) at the end 
of his book. The learned Wiirzburg anatomist there 
explains with reference to germ-history, his " essential 
agreement in fundamental conceptions " with the un- 
learned Leipzig anatomist Wilhelm His. I have explained 
the nature of these " mechanical fundamental conceptions " 
in Chapter XXIV. of this book (vol. ii. p. 352 , and in 
greater detail in my work on " The Aims and Methods of 
the Modern History of Evolution " (" Ziele und Wege der 



Heutigen Entwickelungsgeschichte "). The celebrated 
theories of His, of which I have spoken as the " envelope 
theory," "gum-pouch theory," "waste-rag theory," etc., 
are the brilliant results of that " gifted " author's efforts 
and mathematical calculations. And yet many have 
allowed themselves to be dazzled by the " exact " appear- 
ance of his mathematical formulae. The history of the 
evolution of organisms, equally with the history of human 
civilization, can never be the subject of " exact " investi- 
gation. The history of evolution is in its very nature an 
historic natural science, as is geology. To regard and 
treat these and other historic natural sciences as "exact " 
leads' to the greatest errors. This is as true of germ- 
history (Ontogeny) as of tribal history (Phylogeny) ; foi 
between the two there is the most intimate causal 

Many naturalists have especially blamed the diagram- 
matic figures given in the Anthropogeny. Certain tech- 
nical embryologists have brought most severe accusations 
against me on this account, and have advised me to substi- 
tute a larger number of elaborated figures, as accurate as 
possible. I, however, consider that diagrams are much 
more instructive than such figures, especially in popular 
scientific works. For each simple diagrammatic figure 
gives only those essential form-features which it is intended 
to explain, and omits all those unessential details which in 
finished, exact figures, generally rather disturb and confuse 
than instruct and explain. The more complex are the 
form -features, the more do simple diagrams help to make 
them intelligible. For this reason, the few diagrammatic 
figures, simple and rough as they were, with which Baer 


half a century ago accompanied his well-known " History 
of the Evolution of Animals, " have been more serviceable 
in rendering the matter intelligible than all the numerous 
and very careful figures, elaborated with the aid of 
camera lucida, which now adorn the splendid and costly 
atlases of His, Goette, and others. If it is said that my 
diagrammatic figures are "inaccurate," and a charge ol 
"falsifying science" is brought against me, this is equally 
true of all the very numerous diagrams which are daily used 
in teaching. All diagrammatic figures are "inaccurate."*' 

The important advances in many different directions 
made during the ]vs$ two years, both by germ-history and 
tribal history, especially the reconstruction of the germ- 
layer theory and the development of the Gastraea theory, 
have compelled me essentially to modify the second and 
third sections of the Anthropogeny. Chapters VIII., IX., 
XVI., and XIX. especially appear in a new form ; but even 
in Sections I. and IX. I have been compelled to modify 
much and to improve many parts. At the same time I 
have exerted myself to the utmost, by improving the formal 
exposition, to render the extremely dry and unacceptable 
matter more interesting. This is, of course, an unusally 
hard task, and I am well aware how far even this third 
edition, in spite of all my efforts, is from affording a really 
popularly intelligible explanation of the Ontogeny and 
Phylogeny of Man. Because the defective natural scien- 
tific instruction in our schools, even in the present day, 
Waves educated men quite or nearly ignorant of the struc- 
ture and arrangement of their bodies, the anatomical and 
physiological foundation is usually wanting, on which alone 
a true knowledge of human germ-history, and consequently 


of human tribal history, can be based. And yet, as Baer 
says, "no investigation is more worthy of a free and 
thoughtful man than the study of himself." (Cf. vol. i. 
p. 244.) Hoping, as I do, that I may have aided to some 
extent to bring about this true self-knowledge, I shall have 
gained my purpose if my labours arouse an active interest 
in wider circles in the historic evolution of our animal 
organism, and if they advance the knowledge of this most 
significant process. 


Jena, October 6, 1876. 


Enviil thine heaven, Zeus, with vaporous cloud. 

And practise, like a boy beheading thistle*, 

On oaks and mountain summits ; 

Yet must thou let my earth alone to stand, 

And these my dwellings, which thou didst not baiki ; 

And these my flocks, for whose bright glow 

Thou enviest me. 

I know not au^ht more wretched 

Beneath the sun than you, ye Gods ! 

Who nourish piteously, 

With tax of sacrifice and reek of prayer; your glory 

Would starve, if children were not yet, and suppliant*, 

So full of hope — and fools. 

When I was young, and knew not whence nor whither, 

I used to turn my dazzled eyes to the gun, 

As if above me were 

An ear to listen to my crying, 

A heart, like mine, to pity those oppress'd. 

Who aided me against the Titans' arrogance ? 

Who rescued me from death, from slavery ? 

'Tis thou alone hast wrought it all, thou holy, glowing* heart. 

Thou didst glow young and fresh, though cheated ; thanks tot 

That slumbering one above. 

Why ifeould I honour thee ? 

Hast thou e'er lighten'd the woes of the laden ones ? 

Hast thou e'er dried the tears of the sorrowful? 

It was not thou who welded me to manhood, 

But Time the almighty, Fate the everlasting, 

liy Lords and thine. 



Dost fondly fancy I shall hate my life, 
And hie me to the waste, because not all 
My blossom-dreams bear fruit ? 

Here sit I forming manhood in my image, 

A race resembling me, 

To sorrow, and to weep, 

To taste, to hold, to enjoy, 

And not take heed of thee, 

As II 



Earth's narrow circle is well known to me ; 
What is above the eye can never see. 
Fool, who peers thither with his vision dim, 
And feigns a crowd of beings like to him ! 

Let him look round him, standing without fear, 
This world speaks plain for who has to hear, 
He need not stray within the vast to be 
But clasp what he can feel and see. 

So let him wander all his earthly days, 

Though ghosts should walk, still let him go his way, 

In every progress woe and joy betide, 

Though every- moment be unsatisfied. 

Yes, in this thought, I fix unswerving; 

Wisdom gives thus her judgment form ; 
Those are of Freedom, Life deserving, 

Who daily take them both by storm. 






fleneral Significance of the History of the Evolution of Man. — Ignorance of 
it among the so-called Educated Classes. — The Two Branches of the 
History of Evolution. — Ontogeny, or the History of Germs (Embryos), 
and Phylogeny, or the History of Descent (or of the Tribes). — Causal 
Connection between the Two Serie^ of Evolution. — The Evolution of 
the Tribe determines the Evolution of the Germ. — Ontogeny as an 
Epitome or Recapitulation of Phylogeny. The Incompleteness of this 
Epitome. — The Fundamental Law of Biogeny. — Heredity and Adapta- 
tion are the two Formative Functions, or the two Mechanical Causes, 
of Evolution.— Absence of Purposive Causes. — Validity of Mechanical 
Causes only. — Substitution of the Monistic or Unitary for the Dualistic, 
or Binary Cosmology. — Radical Importance of the Facts of Embryology 
to Monistic Philosophy. — Palingenesis, or Derived History, and Keno- 
genesis, or Vitiated History. — History of the Evolution of Forms and 
Functions. — Necessary Connection between Physiogeny and Morpho- 
geny. — The History of Evolution as yet almost entirely the Product of 
Morphology, and not of Physiology. — The History of the Evolution of 
the Central Nervous System (Brain and Spinal Marrow) is involved 
in that of the Psychic Activities, or the Mind. 

u The History of the Evolution of Organisms consists of two kindred and 
olosely connected parts : Ontogeny, which is the history of the evolution of 
individual organisms, and Phylogeny, which is the history of the evolution 
of organio tribes. Ontogeny is a brief and rapid recapitulation of 
Phylogeny, dependent on the physiological /"actions of Heredity (reproduo- 


ticm) and Adaptation (nutrition). The individual organism reproduce fa 
the rapid and short course of its own evolution the most important of the 
ohanges in form through which its ancestors, according to laws of Heredity 
and Adaptation, have passed in the slow and long course of their palaeonto- 
logical evolution." — Hajeckel's Oenerelle Morphologie (1866). 

The natural phenomena of the evolutionary history of man 
claim an entirely peculiar place in the wide range of 
the scientific study of nature. There is surely no subject 
of scientific investigation touching man more closely, or in 
the knowledge of which he is more deeply concerned, than 
the human organism itself; and of all the various branches 
of the science of man, or anthropology, the history of 
his natural evolution should excite his highest interest. 
For it affords a key for the solution of the greatest of those 
problems at which human science is striving. The greatest 
problems with which human science is occupied — the inquiry 
into the true nature of man, or, as it is called, the question 
of " Man's Place in Nature," which deals with the past 
and primitive history, the present condition, and future 
of Man — are all most directly and intimately linked to this 
branch of scientific research, which is called The History 
of the Evolution of Man, or briefly, " Anthropogeny." l 
It is, however a most astonishing but incontestable fact, 
that the history of the evolution of man as yet constitutes 
no part of general education. Indeed, our so-called "edu- 
cated classes" are to this day in total ignorance of the 
most important circumstances and the most remarkable 
phenomena which Anthropogeny has brought to light. 

In corroboration of this most astounding fact, I will 
only mention that most " educated people " do not even 
know that each human individual is developed from an 
egg, and that this egg is a simple cell, like that of any 


animal or plant. They are also ignorant of the fact that, 
in the development of this egg, an organism is first formed 
which is entirely different from the fully developed human 
body, to which it bears no trace of resemblance. The 
majority of "educated people n have never seen such a 
human germ, or embryo, in the early stages of development, 2 
nor are they aware that it is not at ail dilferent from those 
of other animals. They do not know that, at a certain 
period, this embryo has essentially the anatomical structure 
of a Lancelet, later of a Fish, and in subsequent stages 
those of Amphibian and Mammal forms ; and that in the 
further evolution of these mammal forms those first appear 
which stand lowest in the series, namely, forms allied to 
the Beaked Animals (Ornithorhynchus) ; then those allied 
to Pouched Animals (Alarsupialia), which are followed by 
forms most resembling Apes ; till at last the peculiar human 
form is produced as the final result. These significant facts 
are so little known that, when incidentally mentioned, they 
are commonly doubted, or are even regarded as unfounded 
inventions. Every one knows that the butterfly proceeds 
from a pupa, the pupa from a caterpillar, to which it bears 
no resemblance, and again the caterpillar from the egg of the 
butterfly. But few, except those of the medical profession, 
are aware that man, in the course of his individual evolution, 
passes through a series of transformations no less astonishing 
and remarkable than the well-known metamorphoses of the 
butterfly. The mere tracing of this wonderful series of forms, 
through which the human embryo passes in the course of its 
development, is, of course, of great general interest. But our 
understanding will be satisfied in a far higher degree, if we 
refer these remarkable facts to their final causes, and recognize 


that these natural phenomena are of the utmost importance 
to the entire range of human knowledge. They are of 
special importance to the " History of Creation," and, in 
connection with this, to philosophy in general, — as we shall 
presently see. Further, as the general results of all human 
striving after knowledge are summed up in philosophy, it 
follows that every branch of scientific research comes more 
or less in contact with, and is influenced by, the History of 
the Evolution of Man. 

In undertaking to describe the most important character- 
istics of these significant phenomena, and to trace them 
back to their final causes, I shall assign a much greater 
scope and aim to the History of the Evolution of Man than 
is usual. The lectures given on this subject in German 
universities during the past fifty years have been exclusively 
designed for medical students. It is true that the physician 
is most deeply interested in becoming acquainted with the 
development of the bodily organization of man, with which 
he deals, practically, from day to day, in his profession. I 
shall not here attempt to give a special account of the course 
of the evolution of the individual, such as has usually been 
given in embryological lectures, because few of my readers 
have studied human anatomy, or are acquainted with 
the physical structure of the developed man. Hence, I 
shall have to confine nryself in many points to general 
outlines, neglecting many of the remarkable details, which 
would have to be discussed in treating of the evolution of 
special human organs, but which from their complicated 
nature, and because they are not easy to describe, can only 
be completely understood by the aid of an intimate ac- 
quaintance with human anatomy. I shall strive, however 


to present this branch of the science in as popular a form as 
possible. A satisfactory general idea of the course of the 
evolution of the human embryo can, indeed, be given without 
going very deeply into anatomical details. As numerous 
successful attempts have recently been made to awaken 
the interest of larger classes of educated persons in other 
branches of Science, I also may hope to succeed in this 
department, though it is in many respects especially beset 
with difficulties. 

The History of the Evolution of Man, as it has been 
usually treated in lectures for medical students at the 
universities, has only concerned itself with Embryology, 8 
so-called, or more correctly with Ontogeny, 4 in other words, 
with the history of the evolution of individual human 
organisms. This, however, is only the first part of the task 
before us, only the first half of the History of the Evolution 
of Man in the wider sense which will here be attributed 
to the term. The second part, equal in importance and 
interest, is Phylogeny, 5 which is the history of the evolution 
of the descent of man, that is, of the evolution of the 
various animal forms through which, in the course of count- 
less ages, mankind has gradually passed into its present 
form. All my readers know of the very important scientific 
movement which Charles Darwin caused fifteen years ago, 
by his book on the Origin of Species. The most important 
direct consequence of this work, which marks a fresh epoch, 
has been to cause new inquiries to be made into the 
origin of the human race, which have proved the natural 
evolution of man through lower animal forms. The. Science 
, which treats of the development of the human race from 
the animal kingdom is called Phylogeny, or the tribal 


history of man. The most important source from which 
the science derives its material, is Ontogeny, or the history 
of germs, in other words, of the evolution of the individual 
Palaeontology, or the science of petrifactions, and, in a yet 
greater degree, Comparative Anatomy, also afiord most im- 
portant aid to Phylogeny. 

These two divisions of our science, Ontogeny, or the 
history of the germ, Phylogeny, or the history of the 
tribe, are most intimately connected, and the one cannot 
be understood without the other. The close intertwining 
of both branches, the increased proportions which germ- 
history and tribal history lend to each other, alone raise 
Biogeny 6 (or the history of organic evolution, in the widest 
sense) to the rank of a philosophic natural science. The 
connection between the two is not external and superficial, 
but deeply internal and causal. Our knowledge of this 
connection has been but very recently obtained ; it is most 
clearly and accurately expressed in the comprehensive state- 
ment which I call " the fundamental law of organic 
evolution'' or more briefly, " tlte first j/rinciple of Biogeny" T 

This fundamental law, to which we shall recur again 
and again, and on the recognition of which depends the 
thorough understanding of the history of evolution, is briefly 
expressed in the proposition : that the History of the Germ 
is an epitome of the History of the Descent ; or, in other 
words : that Ontogeny is a recapitulation of Phylogeny ; or, 
Bomewhat more explicitly : that the series of forms through 
which the Individual Organism passes during its progress from 
the egg cell to its fully developed state, is a brief, compressed 
reproduction of the long series of forms through which the 
animal ancestors of that organism (or the ancestral form* 


of its species) have passed from the earliest periods of so- 
called organic creation down to the present time. 

The causal nature of the relation which connects the 
History of the Germ (Embryology, or Ontogeny) with that 
of the tribe (Phylogeny) is dependent on the phenomena 
of Heredity and Adaptation. When these are properly 
understood, and their fundamental importance in deter- 
mining the forms of organisms recognized, we may go 
a step further, and say: Phylogenesis is the mechanical 
cause of Ontogenesis. The Evolution of the Tribe, which 
is dependent on the laws of Heredity and Adaptation, effects 
all the events which take place in the course of the Evolution 
of the Germ or Embryo. 

The chain of different animal forms which, according to 
the Theory of Descent, constitutes the series of ancestors, or 
chain of forefathers of every higher organism, and hence 
also of man, always forms a connected whole. This un- 
broken succession of forms may be represented by the letters 
of the Alphabet A, B, C, D, E, etc, down to Z, in their 
alphabetical order. In apparent contradiction to this, the 
history of the individual evolution, or the Ontogeny of most . 
organisms show us only a fragment of this series of forms, so 
that the interrupted chain of embryonic forms would be 
represented by something like : A, B, F, H, I, K, L, etc, ; or, 
in other cases, thus : B, D, H, L, M, N, etc. Several evolu- 
tionary forms have, therefore, usually dropped out of the 
originally unbroken chain of forms. In many cases also 
(retaining the figure of the repeated alphabet) one or more 
letters, representing ancestral forms, are replaced in the 
corresponding places among the embryonic forms by equi- 
valent letters of another alphabet. Thus, for example, in 


place of the Latin B or D, a Greek B or A is often found 
Here, therefore, the text of the biogenetic first principle is 
vitiated, while in the former case it was epitomized. This 
gives more importance to the fact that, notwithstanding 
this, the sequence remains the same, so that we are enabled 
to recognize its original order. 

Indeed, there is always a complete parallelism between the 
t wo series of evolution. This is, however, vitiated by the 
fact that in most cases many forms which formerly existed 
and actually lived in the phylogenetic series are now wanting, 
and have been lost from the ontogenetic series of evolution. 
If the parallelism between the two series were perfect, and 
if this great fundamental law of the causal connection between 
Ontogeny and Phylogeny, in the strict sense of the word^ 
had full and unconditional sway, we should only have to 
ascertain, with the aid of microscope and scalpel, the series of 
forms through which the fertilized human egg passes before 
it attains its complete development. Such an examination 
would at once give us a complete picture of the remarkable 
series of forms through which the animal ancestors of the 
human race have passed, from the beginning of organic 
2reation to the first appearance of man But this repro- 
duction of the Phylogeny in the Ontogeny is complete only 
in rare instances, and seldom corresponds to the entire series 
of the letters of the alphabet. In fact, in most cases the 
epitome is very incomplete, and greatly altered and per- 
verted by causes which we shall investigate hereafter. Hence 
we are seldom able to determine directly, 'by means of its 
Ontogeny, the different forms through which the ancestry of 
each organism has passed ; on the contrary > we commonly 
finc^ — and not less so in the Phylogeny of man, — a number 


of gaps. We are, however, able to bridge over the greater 
part of these gaps satisfactorily by the help of Compa- 
rative Anatomy, though not to fill them up directly by 
ontogenetic research. It is therefore all the more im- 
portant that we are acquainted with a considerable number 
of lower animal forms which still find place in the history of 
the individual evolution of man. In such cases, from the 
nature of the transient individual form, we may quite safely 
infer the nature of the ancestral animal form. 

For example, from the fact that the human egg is a 
simple cell, we may at once infer that there has been at a 
very remote time a unicellular ancestor of the human race 
resembling an Amoeba. Again, from the fact that the 
human embryo originally consists merely of two simple 
germ-layers, we may at once safely infer that a very ancient 
ancestral form is represented by the two-layered Gastrsea. A 
later embryonic form of the human being points with equal 
certainty to a primitive worm-like ancestral form which is 
related to the sea-squirts or Ascidians of the present day. 
But the low animal forms which constitute the ancestral 
line between the unicellular amoeba and the gastraea, and 
further between the gastraea and the ascidian form, can only 
be approximately conjectured with the aid of Comparative 
Anatomy and Ontogeny. On account of a shortened process 
of Heredity, various ontogenetic intermediate forms, which 
must> have existed phylogenetically, or in the ancestral 
lineage, have in the course of historic evolution gradually 
dropped out from these gaps. But notwithstanding these 
numerous and sometimes very considerable gaps, there is, on 
the whole, complete agreement between the two series of 
evolution. Indeed, it will be one of my principal objects to 


prove the deep harmony, and original parallelism, be- 
tween the two series. By adducing numerous facts, I hope 
to convince my readers that from the actually existing 
series of embryonic forms which can be shown at any time, 
we are able to draw the most important conclusions as to 
the genealogical tree of the human species. We shall thus 
be able to form a general picture of the series of animal 
forms which succeeded each other as the direct ancestors of 
man, in the long course of the history of the organic world. 

In this phylogenetic significance of ontogenetic phe- 
nomena, it is of course most important to distinguish clearly 
and exactly between the original, palingenetic processes of 
evolution, and the later kenogenetic processes of the same. 
The term Palingenttic process 8 (or reproduction of the history 
of the germ) is applied to all such phenomena in the history 
of evolution as are exactly reproduced, in consequence of 
conservative heredity, in each succeeding generation, and 
which, therefore, enables us directly to infer the corre- 
sponding processes in the tribal history of the developed 
ancestors. The term Kenogenetic process 9 (or vitiation o/ 
the history of the germ) is applied to all such processes in 
the germ-history as are not to be explained by heredity 
from primaeval parent-forms, but which have been acquired 
at a later time in consequence of the adaptation of the 
germ, or embryo form, to special conditions of evolution. 
These kenogenetic processes are recent additions, which do 
not allow of direct inference as to the corresponding pro- 
cesses in the tribal history of the ancestral line, but which 
rather falsify and conceal the latter. 

This critical distinction between the primary palinge- 
netic and the secondary kenogenetic processes is of course 


of the greatest importance to scientific Phytogeny, which, 
from the available empiric material supplied by Ontogeny, 
by Comparative Anatomy, and by Palaeontology, seeks to 
infer the long extinct historical processes of tribal evolution. 
It is of the same importance to the student of evolution 
as is the critical distinction between corrupt and genuine 
passages in the text of an old writer to the philologist ; the 
separation of the original text from interpolations and corrupt 
readings. This distinction between Palingenesis or inherited 
evolution, and Kenogenesis or vitiated evolution, has not, 
however, yet been sufficiently appreciated by naturalists. 
But I believe that it is the first condition requisite, if the 
history of evolution is to be really understood, and I think 
that two separate main divisions, based on this distinction, 
must be made in germ-history ; Palingenesis or inherited 
history, and Kenogenesis or vitiated history. 

Let us illustrate this highly important distinction by a 
few examples taken from the evolution of man. In Man, as in 
all other higher Vertebrates, the following incidents of germ 
history must be regarded as palingenetic processes : the 
formation of the two primary germ-layers, the appearance 
of a simple notochord (Chorda) between the spinal tube and 
the intestinal tube, the transitory formation of gill-arches 
and gill-openings, of primitive kidneys, of the primitive brain 
bladder, the hermaphrodite rudiment of the sexual organs.. 
etc All these, and many other important phenomena have 
evidently been accurately handed down, by constant heredity, 
from the primaeval ancestors of Mammals, and must, there- 
fore, be referred directly to corresponding palaeontological 
evolutionary incidents in the history of the tribe. On the 

other hand, this is not the case with the following germinal 


incidents, which must be explained as kenogenetic pro- 
cesses ; the formation of the yelk-sac, of the allantois and 
placenta, of the amnion and chorion, and, generally, of the 
different egg-membranes and the corresponding systems of 
blood-vessels; also the transitory separation of the primitive 
vertebrate plates and the side-plates, the secondary closing 
of the stomach wall and the intestinal wall, the formation 
of the navel, etc. All these, and many other phenomena 
are evidently not referable to corresponding conditions of 
an earlier, independent, and fully developed parent form, 
but must be explained as solely due to adaptation to the 
peculiar conditions of egg-life or embryo-life (within the 
egg-membranes). With reference to this fact we may now 
define our "first principle of Biogeny" more exactly as 
follows : " The evolution of the germ (Ontogeny) is a com- 
pressed and shortened reproduction of the evolution of the 
tribe (Phylogeny) ; and, moreover, this reproduction is more 
complete, in proportion as, in consequence of constant 
heredity, the original inherited evolution (Palingenesis) is 
more closely retained ; on the other hand, the repetition 
is more incomplete, in proportion as the later vitiated 
evolution (Kenogenesisj is introduced by changing adapta- 
tion." 10 

The kenogenetic vitiations of the original, palingenetic 
incidents of evolution depend in great measure on a gradually 
occurring displacement of the phenomena, which is effected 
in the course of many thousands of years by adaption to the 
changed conditions of embryonic existence. This displace- 
ment may effect either the place or the time of the 
phenomena. If the former, it is called Heterotopy ; if the 
latter, Heterochrony. 


" Displacement in position, " or " Heterotopy," especially 
affects the cells or elementary parts which compose the 
organs; but it also affects the organs themselves. For 
example, the sexual organs of the human embryo, as well as 
those of many higher animals, appear to originate from 
the middle germ-layer. But the comparative Ontogeny of 
the lower animals shows, on the other hand, that these 
organs did not originally arise from this layer, but from one 
of the primary germ-layers ; the male sexual organs from 
the outer germ-layer, the female from the inner. Gradually, 
however, the germ-cells have altered their original site, and 
have made their way, at an early period, from their original 
position into the middle germ-layer, so that they now 
appear actually to originate in the latter. An analogous 
heterotopism affects the primitive kidneys in the higher 
Vertebrates. Even the appearance of the mesoderm itself 
is very greatly affected by a displacement in position, which 
is connected with the transition of embryo cells from one 
germ-layer into another. 

The kenogenetic " displacements in time," or * Hetero- 
chronisms," are equally significant. They are seen in the 
fact that in the gerni-history (Ontogeny) the sequence in 
which the organs appears differs from that which, judging 
from the tribal history (Phylogeny), would be expected. By 
heterotopy the sequence in position is vitiated ; similarly, 
by heterochrony the sequence in time is vitiated. This 
vitiation may effect either an acceleration or a retardation 
in the appearance of the organs. We must regard the 
following incidents in the germ-history of man as examples 
of ontogenetic acceleration : the early appearance of the 
heart, the gill-openings, the brain, the eyes, the chorda, 


etc It is evident that these organs appear earlier in 
relation to others than was originally the case in the 
history of the tribe. The reverse is true of the retarded 
completion of the intestinal canal, the body-cavity, and tht 
sexual organs. It is evident that in these cases there is an 
ontogenetic postponement or retardation. 

It is only by critically appreciating these kenogenetic 
incidents in relation to the palingenetic, and by constantly 
allowing for the changes in inherited evolution effected 
by vitiated evolution, that it is possible to recognize the 
fundamental significance of the first principle of Biogeny, 
which in this way attains its true value as the most im- 
portant explanatory principle of the history of evolution 
When it is thus critically appreciated, this first principle 
also proves to be the " red thread " on which we can string 
every one of the phenomena in this wonderful domain ; 
this is the thread of Ariadne, with the aid of which alone 
we are able to find an intelligible course through this com- 
plicated labyrinth of forms. Even at an earlier period, when 
the history of the evolution of the human and the animal 
individual first became somewhat more accurately known — 
which is hardly half a century ago ! — people were greatly 
surprised at the wonderful similarity existing in the onto- 
genetic forms, or the stages of the individual evolution, of 
very different animals. They noticed also the remarkable 
resemblance between these and certain developed animal 
forms of allied lower groups. Even the older natural philo- 
sophers recognized the fact that in a certain way these 
lower animais permanently represent in the system of the 
animal kingdom forms which appear transiently in the 
evolution of individuals of higher groups. But formerly 


it was impossible to understand and interpret aright this 
remarkable resemblance. Darwin's greatest merit is that 
he has now enabled us to understand this circumstance. 
This gifted naturalist was the first to place the pheno- 
mena of Heredity on the one hand, and of Adaptation on 
the other, in their true light, and to show the fundamental 
significance of their constant interaction in the production 
of organic forms. He was the first to point out the im- 
portant part played by the continual Struggle for Existence 
in which all organisms take part, and how under its in- 
fluence, through Natural Selection, new species of organisms 
have arisen, and still arise, entirely by the interaction of 
Heredity and Adaptation Darwin thus enabled us properly 
to understand the immensely important relation existing 
between the two divisions of the History of Evolution : 
Ontogeny, and Phylogeny. 

If the phenomena of Heredity and Adaptation are left 
unnoticed, if these two formative physiological functions of 
the organism are not taken into account, then it is entirely 
impossible thoroughly to understand the History of Evolution; 
so that before the time of Darwin we had no clear idea of 
the real nature and causes of the development of germs. 
It was utterly impossible to explain the strange series of 
forms through which a human being passes in its embryonic 
evolution; it was impossible to com prebend the reason of 
the curious series of various animal-like forms which appeal 
in the Ontogeny of man. Previously it was even generally 
believed that the whole human being, with all its parts 
foreshadowed, existed even in the egg, and that his evolution 
was only an unfolding of the form, a simple process of 
growth. But this is not at all the case. On the contrary, 

l6 the evolution of man. 

the entire process of the evolution of the individual presents 
to the eye a connected series of diverse animal forms ; and 
these various animal forms exhibit very diverse conditions 
of external and internal structure. The reason why every 
human individual must pass through this series of forms in 
the course of his embryonic evolution, was first explained 
to us by the Theory of Descent of Lamarck and Darwin. 
From this theory we first leam the efficient causes (caused 
pjjicientes) of individual evolution ; by the aid of this theory 
we first perceive that such mechanical causes alone suffice 
to effect the evolution of the individual organism, and 
that the co-operation of designing, or teleological causes 
(causae finales), which were formerly universally assumed, 
is unnecessary. Of course, these final causes still play an 
important part in the prevailing school-philosophy ; but in 
our new natural philosophy we are enabled to replace them 
entirely by the efficient causes. 

I allude to this matter at this early stage, in order to 
call attention to one of the most important advances made in 
any branch of human knowledge during the past ten years. 
The history of philosophy shows that in the cosmology of 
our day, a& in that of antiquity, final causes are almost 
universally deemed to be the real ultimate causes of the 
phenomena of organic life, and especially those of the life 
of man. The prevailing Doctrine of Design, or Teleology, 
assumes that the phenomena of organic life, and in particular 
those of evolution, are explicable only by purposive causes, 
and that, on the contrary, they in no way admit of a 
mechanical explanation, that is, one entirely based on 
natural science. The most difficult problems in this respect 
which have been before us, and which seemed capable o J 


solution only by means of Teleology, are, however, precisely 
those which have been mechanically solved in the Theory 
of Descent. The reconstruction of the history of the evolu- 
tion of man, which this theory has effected, has actually 
removed the greatest difficulties. We shall see in the 
course of our inquiries how, through Darwin's reform oi 
the Doctrine of Evolution, the most wonderful problems, 
hitherto deemed unapproachable, of the organization ol 
man and animals have admitted of a natural solution, of a 
mechanical explanation, by non-purposive causes. It lias 
enabled us to substitute everywhere unconscious causes 
acting from necessity, for conscious purposive. causes. 11 

If the recent progress in the Doctrine of Evolution had 
accomplished only r this, every thoughtful person must have 
admitted that even in this an immense advance had been 
made in knowledge. In consequence of it, the tendency 
called unitary or monistic, in contradistinction to the dual- 
istic, or binary, which has heretofore prevailed in speculative 
philosophy, must ultimately prevail throughout philosophy. 12 
This is the point at which the history of the evolution of 
man at once penetrates deeply into the very foundations 
of philosophy. For this reason alone it is very much to be 
desired, in fact is indispensable, that any one who aspires to 
philosophic culture should learn the most important facts in 
this field of research. 

The significance of the facts of Ontogeny is so great and 
so evident that the dualistic teleological philosophy, finding 
them extremely inconvenient, has of late endeavoured to 
meet them by simple denial. Such, for instance, has been 
the case w^th the fact that every human being develops 
from an egg, and that this egg is a simple cell, like the egg- 


cell of all other animals. When in my "History of Creation" 
I had discussed this fundamental fact, and had directed 
attention to its immense significance, several theological 
periodicals pronounced it a malicious invention of my own. 
The evident fact that at a certain stage of their evolution 
the embryos of Man and of the Dog are entirely in- 
distinguishable from one another was also denied. 

The fact is that an examination of the human embryo in 
the third or fourth week of its evolution shows it to be 
altogether different from the fully developed Man, and that 
it exactly corresponds to the undeveloped embryo-form 
presented by the Ape, the Dog, the Rabbit, and other 
Mammals, at the same stage of their Ontogeny. At this 
stage it is a bean-shaped body of very simple structure, 
with a tail behind, and two pairs of paddles, resembling the 
fins of a fish, and totally dissimilar to the limbs of man and 
other mammals, at the sides. Nearly the whole of the front 
half of the body consists of a shapeless head without a face, 
on the sides of which are seen gill-fissures and gill arches 
as in Fishes. (Cf. Plate VII. at the end of Chapter XI.) 
In this stage of evolution the human embryo differs in no 
essential way from the embryo of an Ape, Dog, Horse, Ox, 
etc., at a corresponding age. Even such facts as these, 
which can be easily and promptly demonstrated at any time 
by placing side by side the corresponding embryos of Man, 
a Dog, a Horse, etc., have been spoken of by theologians 
and teleological philosophers as inventions of materialism ; 
and even naturalists, who were presumably acquainted with 
them, have tried to deny them. No stronger proof, surely, 
of the immense radical importance of these embryological 
facta in favour of the monistic philosophy can be given than 


these efforts on the part of the dualistic school to meet them 
by simple denial or utter silence. They are indeed 
extremely distasteful to that school, and are totally 
irreconcilable with their teleological cosmology. We must 
therefore take especial care to place them in their true light. 
We are entirely of the opinion of Huxley, who, in his able 
" Evidence as to Man's Place in Nature," says that these 
fa^ts, " though ignored by many of the professed instructors 
of the public mind, are easy of demonstration, and are 
universally agreed to by men of science ; while their 
significance is so great, that whoso has deeply pondered 
over them will, 1 think, find little to startle him in the 
other revelations of Biology." 

Although our chief inquiry is primarily directed to the 
history of the evolution of the bodily form of Man and of 
his organs, and to their external and internal structural 
relations, I must here at once observe that the history of 
the evolution of the functions is inseparably connected with 
this. Everywhere in Anthropology, just as in Zoology, of 
which the former is but a part, and throughout the whole 
field of Biology, these two branches of research are thus 
inseparably connected. The peculiar form of the organism 
and its organs, both internal and external, is always closely 
related to the peculiar manifestations of life, of the organism 
and its organs, or, in other words, to the physiological func- 
tions performed by these. This intimate relation between 
form and function is also shown in the evolution of the organ- 
ism and its various parts. The history of the evolution oi 
forms, which primarily occupies us, is at the same time the 
history of the evolution of functions ; and this is equally 
true of the human and of all other organisms. 


But I must here add at once, that our knowledge of tha 
solution of functions is as vet far from beinof so advanced 
as our knowledge of the evolution of forms. Indeed, properly 
speaking, the entire history of evolution, or Biogeny, includ- 
ing both Ontogeny and Phylogeny, has as yet been almost 
exclusively a history of the evolution of forms, while the 
Biogeny of functions hardly exists even in name. The fault 
lies solely with Physiology, which has as yet scarcely given 
a thought to the history of evolution, which it has left 
entirely to the care of Morphology. 

The two chief divisions of biological research — Mor- 
phology and Physiology — have long travelled apart, taking 
different paths. This is perfectly natural, for the aims, as 
well as the methods, of the two divisions are different. 
Morphology, the science of forms, aims at a scientific under- 
standing of organic structures, of their internal and external 
proportions of form. Physiology, the science of functions, 
on the other hand, aims at a knowledge of the functions 
of organs, or, in other words, of the manifestations of life. 13 
Physiology, however, has, especially during the last twenty 
years, been far more one-sided in its progress than Mor- 
phology. Not only has it entirely neglected to apply the 
comparative method, by which Morphology has gained its 
greatest results, but it has altogether disregarded the History 
of Evolution. Hence it has come to pass that, within the 
past few decades, Morphology has advanced far beyond 
Physiology, although the latter is pleased to look haughtily 
down upon the former. It is Morphology which has gained 
the greatest results in the fields of Comparative Anatomy 
and Biogeny, and almost everything stated in these pages 
as to the History of the Evolution of Man, is due to the 


exertions of morphologists, and not of physiologists. Indeed 
the direction at present taken by Physiology is so one- 
sided that it has even neglected the recognition of the most 
important functions of Evolution, namely, Heredity and 
Adaptation, and has left this entirely physiological task to 
morphologists. We owe to morphologists, and not to physi- 
ologists, nearly all that we yet know of Heredity and 
Adaptation. The latter still works as little at the functions 
of evolution as at the evolution of the functions. 

It will, therefore, be the task of a future Physiogeny to 

grasp the history of the evolution of the functions with the 

same earnestness, and with the same success, with which 

Morphogeny has long ago undertaken the study of the history 

of the evolution of forms. A few instances will show how 

closely the two are connected. The heart of the human 

embryo has at first a very simple structure, such as appears 

permanently only in Ascidians and other inferior Worms, 

and connected with it is a circulation of the blood of 

the most simple kind. When, on the other hand, we see 

that with the fully developed form of the human heart there 

is connected a function of the circulation of the blood totally 

different from the former one, and far more complicated, the 

study of the evolution of the heart necessarily enlarges 

from a task which was originally morphological to one 

which is physiological also. It is the same in the case of 

all other organs and their activities. 

Thus, for instance, a careful comparative study of the 
history of the evolution of the form of the intestinal canal, 
the lungs, and the organs of generation, affords us also most 
important information as to the evolution of the respective 
functions of these organs. 


This important relation is most clearly seen in the 
history of the evolution of the nervous system. In the 
economy of the human body, this system performs the func- 
tions of sensation, of voluntary movement, volition, and 
finally the highest psychical functions, namely, those of 
thought ; in a word, every one of the various activities which 
constitute the special subject of Psychology, or the science 
of the mind. Modern Anatomy and Physiology have demon- 
strated that these functions of the mind, or psychic activities, 
are immediately dependent upon the more delicate structure 
of the central nervous system, upon the internal conditions 
of the form of the brain and the spinal marrow. Here 
are placed the extremely complex mechanism of cells, whose 
physiological function constitutes the mind-life of Man. 
It is so complex that to most people its function appears 
to be something supernatural, and incapable of mechanical 
explanation. But the history of the evolution of the in- 
dividual furnishes us with the most surprising and signi- 
ficant information as to the gradual origin and progressive 
formation of this most important system of organs. For the 
first rudiment of the central nervous system in the human 
embryo makes its appearance in the same most simple lorra 
in which Ascidians and other inferior Worms retain it 
throughout life. A perfectly simple spinal marrow, without 
brain, such as throughout its existence represents the organ 
of the mind of the Amphioxus, the lowest of Vertebrates, 
first develops from this rudiment. It is only at a later 
period that a brain develops from the anterior extremity 
of this spinal cord, and this brain is of the simplest form, 
3imilar to the permanent form of this organ in the lower 
Fishea Step by step this simple orain develops still 
further, passing through forms corresponding to those of 


the Amphibia, Beaked Animals (Omithostoma), Pouched 
Animals, or Marsupials, and Semi-apes (Prosimice), until the 
highly organized form is reached which distinguishes the 
Apes from all other Vertebrates, and which finally attains 
its highest development in the human brain. But step by 
step with this progressive evolution of the form of the 
brain, the evolution of its peculiar function, the psychical 
activities, moves on hand in hand, and it is therefore the 
history of the evolution of the central nervous system which 
for the first time enables us to understand the origin of life 
of the human mind from natural causes, and the gradual 
historic development of the psychic activities of man It i<* 
impossible without the aid of Ontogeny to perceive how 
these highest and most brilliant functions of the animal 
organism have been historically developed. In a word, the 
history of the evolution of the spinal marrow and the brain 
of the human embryo at the same time directly leads us 
to understand the Phylogeny of the human mind, that most 
sublime activity of life which in the developed human being 
we are accustomed to regard as something wonderful and 

There is no doubt that this special result of the study 
of the history of evolution is among the greatest and most 
important. Happily, our knowledge of the Ontogeny of the 
central nervous system of Man is so satisfactory, and agrees 
so perfectly with the supplementary results of Comparative 
Anatomy and Physiology, that it affords us a perfectly 
clear insight into one of the highest problems of philosophy, 
namely, the Phylogeny of the psyche, the mind, or the 
history of the ancestral Lineage of Man's psychic activities, 
and leads us into the only path by which we shall ever be 
&bl« to solve this the highest of all problems. 




List of the principal branches of Btogeny, or the Htstory of Osgaivu 
Evolution, with reference to the four chief stages of Organic In- 
dividuality — Cell, Organ, Person, and Race. 14 

F!rtt branch of Biogeny, 
•or of the history of the 
evolution of organisms: 

Germ-History, or On- 
togeny (history of the 
development of the 
embryo of the in- 
dividual organism). 


Germ -hi story of 


2. Germ-hiVtory of 
(1'hysiugtny ) 

r l. Germ-history of the cells (and cytods) 
and of the tissues composed of the cells 

2. Germ-history of the organs, and of the 
systems and apparatus composed of the 
organs. Oryanuyeny. 

3. Germ-history of the persons (called 
" the history of the evolution of bodily 
form "). Blustogeny. 

4. Germ-his ory of races (or of social 
aggregates composed of persons: fa- 
milies, communities, states, etc. Cor- 

The germ-history of the functions, or the 
history of the development of vital 
activities in the individual, has not yet 
been accurately and scientifically in- 


Becond branch of Biogeny, 
or of the history of the 
evolution of orga isms: 
Teibal History, or 
Phylog ny (history of 
the pulajoutologicakevo- 
lution of organic 

(\. Tribal history of the cells (hardly at- 
tempted as yet). Histopliyly. 

2. Tribal hi>tory of organs (an unrecog- 
nized main object of comparative ana 
tomy). Oryanupliyly. 

•^ 3. Tribal history of persons (an unrecog- 
nized main object of the natural system 
of classification). Bla&topkyly. 

. Tribal history of races (or of social 
aggregates composed of persons: fa- 
milies, communities, states, etc. Cor- 

3. Tribal history 
of Forms. 

4. Tribal history 

of Functions. 


/"The tribal history of the functions, or the 
history of the pala-ontological develop, 
ment of viial activities, has, in the case 
of most organisms, not yet been ex- 
amined. In the case of man, a large 
part of the lretorj of culture falls oudei 
this head. 



Caspar Friedrich Wolff. 

The Evolution of Anim'als as known to Aristotle. — His Knowledge of the 
Ontogeny of the Luwer Animals. — Stationary Condition of the Scien- 
tific Study of Nature during the Christian Middle Ages. — First Awaken- 
ing of Ontogeny in the Beginning of the Seventeenth Century. — Fa- 
bricius ab Aquapendente. — Harvey. — Marcello Malpighi. — Importance 
of the Incubated Chick. — The Theories of Pre-formation and Encase- 
ment (Evolution and Pre-delineation). — Theories of Male and Female 
Encasement. — Either the Sperm-animal or the Egg as the Pre formed 
Individual. — Animalculists : Leeuwenhoek, Hartsoeker, Spallanz&ni.— 
Ovulists : Haller, Leibnitz, Bonnet. — Victory of the Theory of Evolution 
owing to the Authority of Haller and Leibnitz. — Caspar Friedrich Wolff. 
— His Fate and Works. — The Theoria Generationis. — Re-formation, or 
Epigenesis. — The History of the Evolution of the Intestinal Canal. — 
The Foundations of the Theory of Germ-layers (Four Layers, or Leaves). 
— The Metamorphosis of Plants. — The Germs of the Cellular Theory 
— Wolff's Monistic Philosophy. 

M He who wishes to explain Generation must take for his theme the 
organic b-xly and its constituent parts, and philosophize about them ; he 
must show how these parts originated, and how they came to be in that rela- 
tion in which they stand to each other. But he who learns to know a thing ., 
not only directly from its phenomena, but also its reasons and causes ; and 
who, therefore, not by the phenomena merely, but by these also, is compelled 
to say : ' The thing must be so, and it cannot be otherwise ; it is necessarily 
of such a character ; it must have such qualities ; and it is impossible for 
it to possess others ' — understands the thing not only historically but 
truly philosophically, and he has a philosophic knowledge of it. Our own 


Theory of Generation is to be such a philosophic comprehension of an onranie 
body, rery different from one merely historical." — Caspab FaiBDEiCH WoLf* 

In approaching each science it is, in several respects, pro- 
fitable to glance at the course of its evolution The well- 
known principle that " whatever has come into being can 
only be known from the process by which it came into 
being " is applicable to science. By tracing its gradual 
development, we shall most clearly perceive its tasks and 
aims. We shall also find that the present condition of the. 
History of the Evolution of Man, with all its peculiar cir- 
cumstances, can only be properly understood by taking into 
consideration the history of the evolution of the science 
itself. The examination will not detain us long ; for the 
History of the Evolution of Man is one of the very youngest 
of the Natural Sciences. This is equally true of its two 
divisions : the History of the Germ, or Ontogeny, and the 
History of the Tribe, or Phylogeny. 

Passing over such most ancient germs of the science as 
are found in classical antiquity, and which we shall have 
to discuss presently, the true History of the Evolution of 
Man, as a science, really begins in the year 1759, when 
Caspar Friedrich Wolff, one of the most eminent of German 
naturalists, published his Theoria Oenerationis. This was 
the first foundation-stone for a true history of animal 
germs. In 1809, exactly fifty years later, Jean Lamarck 
published the Phiiosophie Zoohgique, the first attempt at a 
History of Descent ; and in 1859, another half century later, 
appeared Darwin's work, which must be regarded as the 
first to give a scientific basis to that attempt But, before 
carefully examining this as the real foundation of the 


History of the Evolution of Man, we must rapidly £ lance at 
the great philosopher and naturalist of antiquity, who, in 
this as well as in all other branches of research in Natural 
Science, stands quite alone for a period of more than two 
thousand years. This was Aristotle, "the Father of Natural 

Among the extant writings of Aristotle on Natural 
History, treating of various aspects of biological research, 
and the most important of which is the History of Animals, 
there occurs also a smaller work, specially confined to the 
History of Evolution. It is entitled Peri Zoon Geneseos 
(" On the Generation and Development of Animals "). 16 
This work is of great interest, if merely because it is the 
most ancient, and the only one of its kind, which has 
reached us from classical antiquity in a fairly complete 
condition. It is important also because, like others of 
Aristotle's writings on subjects of Natural History, it 
entirely controlled the science for two thousand years. The 
philosopher was a careful observer and an ingenious 
thinker ; yet, while his importance as philosopher has never 
been doubted, his merits as an observant naturalist have 
only lately been duly appreciated. Those students of 
Nature who have lately more accurately examined his 
writings on Natural History, have been astonished at the 
mass of interesting statements, and the remarkable observa- 
tions which abound in them. With regard to the History 
of Evolution, it is specially noticeable that Aristotle traced 
it in the most diverse classes of animals, and that he was 
acquainted, especially in connection with the lower animals, 
with several of the most remarkable facts which we have 
re-discovered only towards the middle of the present 
century. K 


It is certain, for example, that he was thoroughly 
acquainted with the entirely peculiar method of propagation 
and development of the Cuttle-fishes, or Cephalapods, the 
embryo of which has a bag of yelk protruding from the 
mouth. He knew, also, that embryos of Bees can be 
developed from the egg even when it has not been fertilized. 
The so-called parthenogenesis, or virginal generation, of 
Bees has been proved in our days only lately by the 
meritorious zoologist, Siebold, of Munich, who also showed 
that male Bees develop from unimpregnated, and female 
bees only from impregnated eggs. 16 Aristotle further 
relates that some Fishes (of the species Serranus) are 
hermaphrodites, inasmuch as each individual has male 
and female organs, and impregnates itself. This fact, also, 
has only lately been established. He also knew that the 
embryos of several species of Sharks are connected with 
the mother's womb by a sort of placenta — an organ of 
nourishment, full of blood, which otherwise occurs only 
in Man and the higher Mammals. This placenta of the 
Shark was for a long time considered mythical, until, in 
1839, Johannes Miiller, of Berlin, proved it to be a fact. 
We might quote many other remarkable observations from 
Aristotle's account of Evolution, which would prove the 
accuracy of this great naturalist's acquaintance with onto- 
genetic investigations, and the great degree in which he 
was in advance of subsequent times in this respect. 

In most of his observations he was not satisfied with 
merely stating the facts, but he added reflections on their 
significance. Some of these theoretical thoughts are of 
special interest, because they indicate a right fundamental 
perception of the nature of the processes of evolution. He 


conceives the evolution of the individual to be a new 
formation, in which the several parts of the body develop 
one after t&e other. According to him, when the human 
or animal individual develops, either within the mother's 
body or out of it in the egg, the heart is formed first, and . 
is the beginning and the centre of the body. After the 
heart has been formed, the other organs appear ; of these 
the interior precede the exterior, and the upper, or those 
above the diaphragm, precede the lower, or those below it. 
The brain is formed at a very early stage, and out of it 
grow the eyes. This assertion is, indeed, quite accurate. On 
trying to obtain from these statements of Aristotle an idea 
of his conception of the processes of evolution, we find that 
they indicate a faint presentiment of that theory of evolution 
which is now called Epigenesis, and which Wolff, some two 
thousand years later, first proved It is especially remark- 
able that Aristotle altogether denied the eternity of the 
individual. He admitted that the kind or species, formed 
from individuals of the same kind, might possibly be 
eternal ; but asserted that the individual itself was tran- 
sient, that it came into being anew in the act of genera- 
tion, and perished at death. 

During the two thousand years after Aristotle no 
essential progress in Zoology in general, or in the History of 
Evolution in particular, is to be recorded. People were 
content to expound Aristotle's zoological writings, to copy 
them, to deface them greatly by additions, and to translate 
them into other languages. There was hardly any 
independent research during this long period. During the 
Middle Ages of Christianity, when insurmountable obstacles 
were laid in the way of independent researches in 


natural science by the development and diffusion of 
influential conceptions of faith, a re-commencement of 
biological researches was especially out of the -question. 
Even when, in the sixteenth century, human Anatomy 
again began to be studied, and independent investigations 
of the structure of the body of the developed human being 
were again first made, anatomists dared not extend their 
investigations into the condition of the yet undeveloped 
human body, into the formation and development of the 

The prevailing fear of such researches was due to 
several causes. This seems but natural when we remember 
that by the bull of Pope Boniface VIII. greater excom- 
munication was pronounced against all who dared to dis- 
member a human corpse. While anatomical investiga- 
tion of the developed human body was a crime which 
drew down the curse of the Church, it is evident that the 
examination of the body of the child, hidden in the 
mother's womb, and which the Creator himself seemed, 
by its concealed position, to have intentionally withdrawn 
from the curious gaze of naturalists, would have appeared 
much more criminal and impious The omnipotence of 
the Christian Church, which at that time caused many 
thousands to be executed and burned for heresy, and which 
even then with correct instinct foresaw danger threatened 
to itself from the deadly enemy which was then growing 
up in Natural Science, took care that the latter should 
not make too rapid strides. 

It was only when the Reformation broke the all- 
embracing power of the Only-Saving Church, and a new 
And fresh intellectual impulse began to release enslaved 


science from the iron chains of dogmatism, that human 
Anatomy and the History of the Evolution of Man could 
move again more freely, with the re-opening of research in 
other natural sciences. But Ontogeny remained far behind 
Anatomy, and it was only in the beginning of the seven- 
teenth century that the first ontogenetic publications 
appeared. The first to begin was the Italian anatomist, 
Fabricius ab Aquapendente, Professor at Padua, who pub- 
lished two works — De Formato Foetu (1G00), and De 
Formatione Foetus (1604), — which contain the oldest 
figures and descriptions of the embryo of Man and other 
Mammals, and also of the Chick. Similar imperfect 
representations were given soon after by Spigelius — De 
Formato, Foetu (1631) — by the Englishman, Needham 
(1667), and his celebrated countryman, Harvey (1652). The 
latter discovered the circulation of the blood in the animal 
body, and made the important assertion : Omne vivum ex 
ovo ("Everything living comes from an Qgg"). The Dutch 
naturalist, Swammerdam, in his " Bible of Nature," pub- 
lished the results of the first investigations into the 
embryology of the Frog, and the so-called segmentation of 
its yelk. The most important ontogenetic researches of the 
seventeenth century, however, were those of the Italian, 
Marcello Malpighi of Bologna, who gave a fresh impetus 
both to Zoology and to Botany. His two dissertations, De 
Formatione Fulli, and De Ovo Incubato (1687), contain the 
first connected description of the history of the development 
of the chick in the incubated egg. 

Here I must make some remarks on the great importance 
of the Chick in relation to our science. The history of the 
formation of a Chick, as well as of all birds, accurately 


corresponds in its essential characteristics with that of all 
other higher Vertebrates ; and, therefore, also of Man. 
The three higher classes of Vertebrates, Mammals, Birds, 
and Reptiles (Lizards, Snakes, Turtles, etc.), are from the 
beginning of their individual development so surprisingly 
similar in all essential features of their bodily structure, 
especially in the earlier stages, that for a long while it is 
impossible to distinguish them. (Cf. Plates VI. and VII.) 
It has long been known that the accurate study of the 
evolution of the embryo of the Bird, which is most readily 
obtained as the subject of research, is all that is necessary 
in order to learn the essentially similar mode of evolution 
of Mammals, therefore also of Man Even as early as the 
middle and the end of the seventeenth century, when 
human embryos, as well as those of all other Mammals, 
began to be examined in their earlier stages, this most 
important fact was soon recognized. It is of the greatest 
importance, both for theoretical and for practical purposes. 
Conclusions of the highest importance to the theory of 
evolution may be drawn from the similarity of structure 
of the embryos of widely differing animals. This simi- 
larity is invaluable in practical ontogenetic research, 
because the ontogeny of Birds, which is accurately known, 
most completely supplements and explains the embryology 
>f Mammals, which has been but imperfectly studied. 
Hen's eggs can be obtained at all times and in any quan- 
tity, and by hatching them artificially the evolution of 
the embryo may be traced step by step. On the other 
hand, it is much more difficult to study the evolution of 
Mammals, because the embryo of these does not develop 
in a large egg that has been laid, or, in other words, in an 


independent and isolated body, but in a small egg, which, 
until maturity, remains enclosed and concealed in the body 
of the mother. For this reason it is very difficult to pro- 
cure all the stages of development in any large number, 
for the purpose of making connected investigations, not 
to mention external reasons, such as the great cost, the 
technical difficulties, and the many other obstacles, which 
lie in the way of any extended series of researches into 
fecundated mammals. For this reason, from that time to 
the present day, the Chick during the process of incubation 
has been the subject oftenest and most closely investigated. 
The perfection of hatching-machines has made it yet easier 
to obtain embryo-chicks in any required stage of evolution 
and in any quantity, in order to examine the whole process 
of formation step by step. 

About the end of the seventeenth century the history of 
the evolution of the incubated Chick had already been 
advanced as far, and its more essential, external, and less 
delicate conditions were as well known, owing to the 
labours of Malpighi, as investigations with the imperfect 
microscopes of that time rendered possible. Of course, the 
perfection of the microscope and of technical methods of 
research was a necessary condition for more accurate em- 
bryological research. For vertebrate embryos in their 
earlier stages are so small and delicate, that it is impossible 
to examine them without a good microscope, and without 
applying, peculiar technical methods. But these means 
were not applied, and the microscope was not essentially 
perfected till the beginning of our century. 

Throughout the whole of the first half of the eighteenth 
century, during which time the systematic Natural History 


of animals and plants received so great an impulse from 
Linnaeus' famous Systema Naturae, the History of Evolution 
made scarcely any progress. It was in the year 1759 that 
Caspar Friedrich Wolff made his appearance, and his genius 
gave an entirely new direction to this science. Until then 
Embryology was almost exclusively occupied in unsuccessful 
attempts to construct various theories of evolution from the 
scanty material already acquired. 

The theory which at that time gained almost universal 
acceptance, and which continued to be generally received 
during the entire eighteenth century, is in Germany com- 
monly called the Theory of Unfolding (Auswickelung), or 
Evolution, but is better spoken of as the Theory of Pre- 
formation. 17 Its main idea is the following : no really new 
formation takes place during the evolution of each indi- 
vidual organism, animal or plant, including therefore Man ; 
there is only a growth and an unfolding of parts, all 
of which have, from eternity, been present, pre-formed, and 
complete, though only very minute, and wrapped together 
Every organic germ, therefore, contains all the parts and 
organs of the body pre-formed and represented in their 
subsequent form, position, and connection, and the entire 
course of the evolution of the individual, the entire onto- 
genetic process, is nothing but an evolution in the most 
exact meaning of the word; namely, an unwrapping of 
wrapped-up parts already formed. Hence, for example, in 
a hen's egg we do not find a simple cell which undergoes 
division, and the generation of cells of which form layers of 
germs, and by various changes, separations, and new for- 
mations, ultimately bring into being the body of the Bird ; 
but every hen's egg contains from the beginning a complete 


Chick, with all its parts pre-formed and wrapped together, 
and during the development of the incubated egg these 
parts are merely drawn out and grow. 

As soon as this theory was carried out logically, it 
necessarily led to the Theory of Encasement. According to 
this, every species of animal or plant was originally created 
only as a pair or as a single individual ; but this one indi- 
vidual already contained, encased within itself, the germs of 
all the other individuals of its species which have ever lived 
or will live. As at that time the age of the earth was 
calculated, according to the Biblical history of creation, at 
five or six thousand years, people thought they could 
approximately calculate the number of germs of every 
species of organism which had lived during that period, and 
consequently the number which had existed encased in the 
first " created " individual of the species. The theory was 
logically extended to mankind, and it was accordingly 
maintained that our first common mother Eve held in her 
ovary the germs of all the children of men, one encased in 
the other. 

This Theory of Encasement was then developed so that 
the female individuals were considered to be the created 
beings which were encased one in another. It was believed 
that only a single pair of each species was originally 
created ; but the ovary of the female individual contained, 
encased within it, all the germs of all the individuals of 
the kind, of both sexes, which were ever to develop. But 
the Theory of Pre-formation took quite another shape when, 
in 1690, Leeuwenhoek, the Dutch microscopist, discovered 
the human spermatozoids, or seminal threads, and proved 
that a large number of extremely delicate and actively 


moving threads exist in the sperm or seminal fluid of the 
male. (Cf. Fig. 17 in Chap. VII.) This astonishing discovery 
was at once interpreted to the effect that these minute 
living bodies, briskly swimming about in the seminal fluid, 
were genuine animals, the pre-formed germs of future 
generations. When at the time of fecundation the two 
generative substances, male and female, came in contact 
with each other, these thread-like seminal animalcules were 
to penetrate into the fruitful soil of the ovary and there to 
attain their development like vegetable seeds in the fruitful 
soil of the earth. According to this theory every single 
seminal animalcule of Man is a complete human being ; all 
the separate parts of the body would be entirely pre-formed 
in it, and subject only to a mere unwrapping and enlargement 
as soon as they reached the favourable matrix of the female 
egg. This theory also was logically carried out to the effect 
that in every single thread-like body were contained all the 
subsequent generations of its descendents, one encased in 
the other, each in the most extreme degree of fineness, and 
of the minutest size. The seminal gland of Adam, therefore, 
contained the germs of all the children of men who have 
ever peopled our planet, who inhabit it at present, or will 
occupy it in the future " until the end of the world." 

Of course, this Doctrine of Encasement in the Male was 
utterly opposed to the Doctrine of Encasement in the Female, 
which had previously prevailed. The only ground common 
to both was the false idea that the germs of innumerable 
generations, previously formed and encased one in another, 
existed in every organism ; a conception on which was also 
founded the curious Prolepsis Theory of Linnaeus. 

The two opposite theories of encasement soon began e 


vigorous contest, which resulted in the division of the 
physiologists of the eighteenth century into two large 
bodies of combatants, entirely opposed and contending 
vehemently. These were the Animalculists, and the Ovu* 
lists. The dispute between these two parties appears 
laughable to us now, for the theory of the one is just as 
unfounded as that of the other. As Alfred KirchhofF says, 
in an excellent biographical sketch of Wolff, " this dispute 
was as little capable of settlement, as the inquiiy whether 
the angels lived in the East or in the West of the heavenly 
regions. u 

The Animalculists, or the Believers in Sperm, looke'd 
upon the moving seminal threads as the real animal germs, 
ind they found support on the one hand in the lively 
movement, and on the other in the form of these seminal 
Animalcules. For in the case of man, as well as of a large 
majority of other animals, they appear to have a somewhat 
oblong, egg-like, or pear-like head, a thin intermediate 
segment, and a very thin tail, narrowing to a hair-like 
form (Fig. 17). In reality, the whole formation is but a 
simple whip-shaped cell. The head is the cellular nucleus, 
surrounded by cell-matter, which is protracted into the 
thinner portions in the middle, and to the hair-like, move- 
able tail ; the latter is the whip, or thread-like appendage of 
other whip-shaped cells. The Animalculists, however, con- 
sidered the head to be a real animal head, and the rest of 
the body to be a complete animal body. Leeuwenhoek, 
Baitsoeker, and Spallanzani were the chief defenders of 
this theory of Pre-delineation. 

The opposite party, the Ovulists (Ovists), or Believer* 
in Eggs, who adhered to the older Theory of Evolution, 


maintained that the egg was the real animal germ, and 
that the seminal animalcules, at the time of fecundation, 
only gave the impulse which caused the unfolding of the 
egg in which all generations were encased one in the other. 
This opinion prevailed with the majority of biologists 
during the whole of the last century, though Wolff, in 
1759, demonstrated its utter want of foundation. Its 
acceptance was specially due to the fact that the most 
celebrated biological and philosophical authorities of that 
time had pronounced in its favour, — among them princi- 
pally Haller, Bonnet, and Leibnitz. 

Albrecht Haller, Professor at Gottingen, who has often 
been called " the Father of Physiology," was a very learned 
and comprehensively educated man, but, as an interpreter 
of the more profound natural phenomena, occupied no 
very high position. He has best described himself in the 
celebrated and often-cited saying, that " Into the inner side 
of Nature no created mind ever penetrates ; happy he to 
whom she shows only her outer husk !" The best answer 
to this " husk " view of nature was given by Goethe, in his 
splendid poem which ends with the lines : 

" Nor husk nor kernel Nature brings — 
For all one only type of things ; 
Yet prove thyself, and seek to know 
If husk or kernel thou dost show." 

Attempts have, however, been recently made to justify 
Haller's " husk " view. Wilhelm His has made himself the 
special defender of this strange conception. Haller, in his 
well-known work, Elementa Physiologice, adopted the Theory 
of Evolution (Theory of Pre-formation) in a most decided 
manner, in these words : " There is no coming into being I 


{Nulla est epigenesis). No part of the animal body was made 
previous to another, and all were created simultaneously 
(Nulla in corpore animali pars ante alio.m facta est. 
et omnes simul creator existunt)." In reality, therefore, 
he denied any actual evolution in the natural sense, and 
in this went so far as to maintain even the existence of a 
beard in the new-born boy, and the existence of the horns 
in the hornless fawn ; all the parts were already present 
in a complete state, but hidden for a while from the human 
eye. Haller even calculated the number of human beings 
which God, on the sixth day of His work of creation, at 
once created and encased in the ovary of Eve, the Mother 
of all. He estimated them at two hundred thousand 
millions, by assuming the creation of the world to have 
been six thousand years ago, the average human life thirty 
years, and the number of human beings alive at the same 
time one thousand million. And the celebrated Haller 
advocated all this rampant nonsense, and the inferences 
drawn from it, most successfully, even after Wolff had dis- 
covered the true Epigenesis, and proved it by investigation. 
Leibnitz was the most important of the philosophers 
who adopted the Theory of Evolution (Pre-formation), and 
by his great authority, as well as by his talented exposition, 
gained numerous followers for it. Based upon his Theory 
of Monads, according to which soul and body are in an 
eternally inseparable union, and in their bi-unity constitute 
the individual (the Monad), Leibnitz quite logically applied 
the Theory of Encasement to the soul also, and denied all 
real development for it, equally with the body. In his 
TJieodiccB, for instance, he says : " I think that souls, which 
will some day be human souls, as in the case of those of 


3ther species, pre-existed in the semen ; that they existed 
in the ancestors as far back as Adam, therefore since the 
beginning of things, always in the form of organized bodies.* 

The Theory of Encasement seemed to receive its most 
important experimental support in the researches of Bonnet, 
ane of its most zealous adherents. He observed, for the 
first time, in Plant-lice, the so-called " virginal generation," 
or parthenogenesis, which is an interesting form of propaga- 
tion lately proved by Siebold and others, in many other 
articulated animals, such as various Crabs and Insects. 16 
The females of these and other lower animals of certain 
groups propagate for several generations without having 
been impregnated by a male. Such eggs, which for their 
evolution do not require to be impregnated, are called 
" false eggs," Pseudova, or Spores. Bonnet, in 1745, for the 
first time observed that a female Plant-louse, which he had 
completely shut off, as in a nunnery, and shielded from all 
contact with males, after shedding its skin four times, gave 
birth on the eleventh day to a living * female, and within 
the next twenty days produced as many as ninety-four 
other females ; and that soon all of these, without having 
come in contact with a male, multiplied again in the same 
virgin manner. Thereupon, of course, it seemed that a 
tangible proof of the truth of the Theory of Encasement, 
according to the interpretation of Ovulists, had been 
abundantly furnished, and it naturally became almost uni- 
versally accepted in this sense. 

The case stood thus, when suddenly, in the year 1759, 
Caspar Friedrich Wolff, then a young man, appeared, and 
with his new Theory of Epigenesis gave the death-blow to 
the entire Theory of Pre-formation. Wolff was born at 


Berlin, in 1733. He was the son of a tailor, and studied 
natural science and medicine at first in Berlin, at the 
Medico-surgical College, under the celebrated anatomist 
Meckel, and subsequently in Halle. Here, in the twenty - 
sixth year of his age, he passed his examination for his 
doctor's degree ; and on the 28th of November, 1759, in his 
dissertation as doctor, he defended the new doctrine of true 
evolution, the Theoria Generationis, founded on Epigenesis. 
This dissertation, in spite of its small limits and difficult 
language, ranks among the most important essays ever 
written in the whole range of biological literature. It is 
equally distinguished by its abundance of new and most 
careful researches, and by its far-reaching and very sug- 
gestive ideas given in connection with the observations, 
which latter he developed into a brilliant Theory of Evolu- 
tion entirely true to nature. Yet this remarkable publica- 
tion had at first no results whatever. Although the study 
of Natural Science was then flourishing in consequence of 
the impetus given by Linnaeus; although botanists and 
zoologists soon numbered, not dozens, but hundreds; yet 
hardly anybody took any interest in Wolffs Theory of 
Generation. And the few who had read it, foremost among 
whom was Haller, considered it totally false. 

Although Wolff proved the truth of Epigenesis by 
means of the most accurate research, and refuted the un- 
founded hypotheses of the Theory of Pre-formation, yet 
the " exact " physiologist Haller continued to be the most 
zealous adherent of the latter, and rejected the correct 
doctrine of Wolff with his dictatorial decree : Nulla est 
epigenesis ! It is not surprising that the entire body of 
physiological scholars of the second half of the eighteenth 


century submitted to the dictum of this physiological pope, 
and opposed Epigenesis as a dangerous innovation. More 
than half a century elapsed before Wolffs labours met with 
their deserved acknowledgment. Only after Meckel, in the 
year 1812, had translated into German another most im- 
portant publication of Wolffs, " On the Formation of the 
Intestinal Canal " (published 1764), and had drawn attenr 
tion to *its extraordinary significance, people began to re- 
occupy themselves with this almost forgotten author, who, 
of all the naturalists of the preceding century, had made the 
deepest progress into the knowledge of the living organism. 

Thus, as so often happens in the history of human know- 
ledge, new-born truth succumbed to all-powerful error, 
upheld by the weight of authority. The knowledge of Epi- 
genesis, clear as the sun, was not able to pierce through the 
thick fog of the Dogma of Pre-formation, and its ingenious 
discoverer was vanquished in the fight for the truth by the 
overwhelming power of the enemy. 

The result was that all progress in the History of Evo- 
lution was for a while arrested. This is all the more to be 
regretted because Wolff was finally compelled, by untoward 
circumstances, to quit his German Fatherland. From the 
first without means, he had only been able to finish his clas- 
sical work in the face of great difficulties, and was then com- 
pelled to earn his bread as a practising physician. During 
the Seven Years' War he was busy in the Silesian hospitals, 
and gave excellent lectures on Anatomy in the field hospital 
of Breslau, attracting the attention of Cothenius, the 
eminent Director of Hospitals. When peace had been con- 
cluded, this distinguished patron tried to procure a chair 
in Berlin for Wolff, but failed on account of the narrow- 


mindedness of the professors of the Berlin Medico-surgical 
College, who were averse to all scientific progress. This 
most learned faculty persecuted the Theory of Epigenesis 
as one of the most dangerous heresies ; just as is the case 
now with the Theory of Descent. Although Cothenius, 
and other patrons in Berlin, took a warm interest in Wolff, 
it was impossible even to procure permission for him to 
give public lectures on Physiology in Berlin. The conse- 
quence was, that Wolff was obliged to accept a summons 
with which the Empress Catherine of Russia honoured him 
in 17G6. He went to St. Petersburg, where he remained 
for twenty-seven years, devoting himself in undisturbed 
quiet to his deep researches, and enriching the publications 
of the St. Petersburg Academy with the productions of his 
brilliant talents. He died there in 1794. 19 

The progress which Wolff made in the entire science of 
Biology was so great that the naturalists of that time could 
not grasp it. The mass of important new researches, and 
of fruitful and great ideas accumulated in his publications, 
is so enormous that their full value has only been gradually 
appreciated, and their bearing properly understood during 
the present century. Wolff opened up the right path into 
the most various branches of biological investigations. 
Firstly, and above all, by the Theory of Epigenesis, he 
first made the real nature of organic evolution intelligible. 
He proved satisfactorily that the evolution of every organ- 
ism consists of a series of new formations, and that no 
trace of the form of the developed organism exists either in 
the egg or in the semen of the male. These are simple 
bodies of an entirely different significance. The germ, or 
embryo which develops from the egg, shows in the various 



phases of its evolution an internal structure and an externa] 
form totally different from those of the developed organism. 
In none of these phases do we find any pre-formed parts ; 
nowhere any encasement. In these days we can scarcely 
continue to call this Theory of Epigenesis a theory, for are 
have been thoroughly convinced of its correctness in fact, 
and we are able to demonstrate it in any moment under the 
microscope. Nor, during the last decade, has any doubt of 
the truth of Epigenesis been expressed. 

Wolff supplied detailed proof of his Theory of Epigenesis 
in his scholarly treatise " On the Formation of the Intestinal 
Canal (1768)." In its complete condition the intestinal 
canal of the Chick is a very complex, long tube, to which 
the lungs, the liver, the salivary, and many smaller glands 
are attached. Wolff showed that there is no trace of this 
complex tube, with all its various parts, in the embryo 
Chick during the first period of -incubation, but that m its 
place there is a flat, leaf-shaped body ; and that the whole 
embryo-body in the earliest period is also of a flat, oblong, 
leaf-like form. Considering the difficulty of accurately ex- 
amining conditions so extremely minute and delicate as the 
first leaf-shaped beginnings of the body of the bird with the 
indifferent microscopes of the last century, we cannot but 
admire the rare talent for observation possessed by Wolf£ 
who actually proved the most important facts known in 
this the darkest portion of Embryology. From this very 
difficult investigation he even drew the correct conclusion 
that the entire embryonic body of all higher animals, &i 
well as of birds, is for a while a flat, thin, T ieaf-shaped 
plate, which at first appears simple, but subsequently 
as if composed of several layers. The lowest of all these 


layers, 01 leaves, is the intestinal canal, the development of 
which Wolff examined thoroughly, from its beginning to its 
completion. He showed that the leaf-like rudiment first 
forms a groove, the edges of which curve towards each other, 
thus growing into a closed tube, and that, finally, at the 
ends of this tube the two openings, mouth and anus, arise. 

Nor did Wolff overlook the fact that the other organic 
systems of the body originate, in an entirely similar way 
from leaf-shaped rudiments, which afterwards assume the 
form of tubes. Like the intestinal canal, the nerve, muscle, 
and vascular systems, with all the various organs belonging 
to the last, develop from a simple layer-like or leaf-like 
rudiment. Thus in 1768 Wolff learned the very significant 
fact, which, half a century later, was first formulated by 
Pander, in the fundamental " germ-layer theory." The 
sentence in which Wolff expressed the main idea of this 
theory is so remarkable, that I quote it. " This very 
wonderful analogy between parts which in Nature are so 
widely separated, an analogy which is not imaginary, but 
is founded on the most reliable observations, is in the 
highest degree worthy of the attention of physiologists ; 
for it will be granted that it has a deep significance and 
that it is most intimately connected with the generation, 
and with the nature of animals. The different systems 
which compose the whole animal seem to be successively 
formed, at different times but on one plan; and these 
systems are therefore like one another, even though in their 
nature they are distinct. The system which is first pro- 
duced, which first assumes a peculiar definite form, is the 
nerve-system. When this is completed the flesh-mass, 
which properly speaking constitutes the embryo, is formed 


on the same plan. A third, the vascular system, now appears, 
which is certainly sufficiently similar to the two earlier 
structures to allow of its form being easily recognized as 
that which has been described as approximately common to 
all the systems. The fourth system, the intestinal canal, now 
follows ; this, again, is formed on the same plan, and, when 
completed and closed, resembles the three earlier systems." 
In this most important discovery Wolff laid the first 
foundations of the fundamental " germ-layer theory " which 
was not completely developed till long afterwards, by 
Pander (1817) and by Baer (1828). It is true that WolfFs 
propositions are verbally incorrect, but in them he reached 
the truth as nearly as was then possible, and as was to be 
expected. We shall presently see how nearly he approached 
to the real state of the case. 

Wolff owes much of his comprehensive conception of 
nature to the fact, that he was as good a botanist as a 
zoologist. He studied the history of the development of 
plants also, and in the field of botany first founded the 
theory which Goethe afterwards developed in his brilliant 
treatise on the " Metamorphosis of Plants." Wolff was the 
first to show that all the various parts of plants may be 
traced back to the leaf as their common rudiment, or 
' fundamental organ/' Flower and fruit, with all their 
parts, consist only of modified leaves. This discovery must 
have seemed all the more surprising to Wolff, from the fact 
that he had discovered a simple leaf-like rudiment to be 
the first form of the embryonic body of animals, as it is of 

We therefore find in Wolff distinct traces of those 
theories of which, at a much later period, other gifted 


naturalists were to construct the foundation of the know- 
ledge of the morphology of the animal and vegetable body. 
But our admiration for this eminent genius is still greater 
when we discover that he also first indicated the famous 
cellular theory. Indeed, Wolff had, as Huxley first pointed 
out, an evident presentiment of this fundamental theory, 
for he considered minute microscopical vesicles to be the 
real elementary parts constituting the germ-layers. 

Finally, particular attention must be directed to the 
monistic character of the profound philosophical reflections 
which Wolff published in connection with all his admirable 
investigations. Wolff' was a great monistic Natural Phi- 
losopher, in the best and most correct sense of the word. 
It is true that his philosophical researches, like his ex- 
perimental ones, were ignored for more than half a century, 
and have not even yet met with the recognition which 
they deserve ; but we therefore emphasize yet more 
strongly the fact that their tendency was strictly in that 
line of philosophy which we call monistic, and which alone 
can be considered correct 


Kabl Ernst Baeb. 

Karl Ernst Baer, the Principal Disciple of Wolff. — The Wurzburg School ol 
Embryologists : Dollinger, Pander, Baer. — Pander's Theory of Germ- 
layers. — Its Full Development by Baer. — The Disc-shaped first parts 
into two Germ-layers, each of which again divides into Two Strata. 
The Skin or Flesh-stratum arises from the Outer or Animal Germ-layer. 
The Vascular or Mucous Stratum arises from the Inner or Vegetative 
Germ-layer. The Significance of the Germ-layers. — The Modification 
of the Layers into Tubes. — Baer's Discovery of the Human Egg, the 
Germ-vesicle, and Chorda Dorsal is. — The Four Types of Evolution in 
the Four Main Groups of the Animal Kingdom. — Baer's Law of the Type 
of Evolution and the Degree of Perfection. — Explanation of this Law by 
the Theory of Selection. — Baer's Successors : Rathke, Johannes Muller, 
Bischoff, Kolliker. — The Cell Theory : Schleiden, Schwann. — Its Appli- 
cation to Ontogeny : Robert Remak. — Retrogressions in Ontogenj : 
Reiohert and His. — Extension of the Domain of Ontogeny : Darwin. 

" The History of Evolution is the real source of light in the investigation 
of organic bodies. It is applicable at every step, and all our ideas of the 
correlation of organic bodies will be swayed by our knowledge of the 
history of evolution. To carry the proof of it into all branches of research 
would be an almost endless task." — Karl Ernst Baer (1828). 

If we wish to separate our historic survey of the course 
of the development of the Science of Human Ontogeny 
into parts, it is most convenient to make three. The first 
of these occupied the last chapter, and includes the whole 
preparatory period of embryological researches ; it extends 


from Aristotle to Caspar Friedrich Wolff, to the year 1759, 
when the Theoria Generationis appeared and laid the 
foundation for future work. The second, to which we now 
turn our attention, comprises exactly a century ; that is, 
to the year 1859, in which appeared Darwin's work on 
"The Origin of Species," which reformed the whole basis of 
the science of Biology, and especially of Ontogeny. The 
beginning of the third division is as recent as the time of 

As Wolff's labours remained entirely unnoticed during 
half a century — till the year 1812 — we are not quite 
accurate in assigning the exact duration of a century to the 
second division. During fifty-three years not one book 
appeared which followed in the lines laid down by Wolff, 
and carried on his Theory of Evolution. His opinions, 
which were perfectly correct and founded directly on actual 
observations, were only occasionally mentioned, and then 
only to be rejected as erroneous. His opponents, followers 
of the prevalent and mistaken theory of Preformation, did 
not even deign to refute him. This was owing, as I have 
said before, to the extraordinary authority possessed by 
Albrecht Haller, Wolff's distinguished opponent, and the 
circumstance furnishes one of the most remarkable examples 
of the influence which a great authority may, as such, long 
exert against the clear recognition of facts. The neglect 
of Wolffs labours was so universal that in the beginning 
of this century two naturalists, Oken (1806) and Kieser 
(1810), undertook independent investigations into the 
development of the intestinal canal in the Chick, and 
obtained a correct insight into Ontogeny, without being 
aware of the existence of Wolff's important work in the 


same field, and trod in his very footsteps unconsciously. 
That they really did not know his works is proved by the 
fact that they did not advance as far as Wolff had done. 
In the year 1812 when Meckel translated Wolff's book on 
the Evolution of the Intestinal Canal into German, and 
called attention to its great importance, the eyes of anato- 
mists and physiologists were for the first time suddenly 
opened, and a great number of Biologists soon after under- 
took new embryological investigations, following out and 
corroborating Wolff's theory step by step. 

This revival of Ontogeny, and the first confirmation and 
further development of the only true theory of Epigenesis, 
started from the university of Wiirzburg. The distinguished 
biologist, Dollinger, was then lecturing there. He was the 
father of the famous theologian of Munich, who has done 
such good service in our day by his opposition to the new 
dogma of papal infallibility. Dollinger was both a thought- 
ful natural philosopher, and an accurate biological observer. 
He felt the greatest interest in the History of Evolution, 
and was much occupied with it. Yet he himself was unable 
to produce any very important work in this department, 
from want of means. But in the year 1816, a young doctor 
of medicine, who had just graduated, and whom we shall 
soon learn to know as the most important follower of Wolff, 
came to Wiirzburg. This was Karl Ernst Baer. His con- 
versations with Dollinger on the History of Evolution 


resulted in a renewal of the investigations. Dollinger ex- 
pressed a wish that, under his direction, some young 
naturalist should undertake a series of independent re- 
searches into the evolution of the Chick during the hatching 
of the egg. But neither he nor Baer possessed the cod* 

dollinger, baer, and pander. 51 

siderable pecuniary means then necessary to provide a 
hatching-apparatus, such as would afford uninterrupted 
observations of the process, or to pay a skilled artist to 
depict in a reliable form the successive stages of develop- 
ment. They, therefore, confided the execution of the plan 
to Christian Pander, a wealthy, early "friend of Baer's, by 
whom he had been induced to come to Wurzburg. Dal ton, 
a skilful artist, was engaged to prepare the necessary copper- 

Thus was formed, as Baer says, " that combination, ever 
memorable in the history of science, in which a veteran, grown 
gray in physiological researches (Dollinger), a youth glowing 
with zeal for science (Pander), and an artist without a peer 
(Dalton), united their powers to lay a firm foundation for 
the History of the Evolution of the Animal Orgariism. ,, In 
a short time the history of the evolution of the Chick, in 
which Baer took, though indirectly, a most active part, 
was so far advanced that Pander, in his dissertation 20 for 
the degree of doctor, published in 1817, was able to give 
the first complete sketch of the history of the evolution of 
the Chick on the basis of Wolffs theory. He was able to 
define clearly Wolffs Theory of Germ-leaves, and to prove 
from observation the evolution of the complex system of 
organs from simple leaf-shaped primitive organs, as anti- 
cipated by Wolff. According to Pander, the leaf-shaped 
germinal appendage of the hen's egg separates before the 
twelfth hour of incubation into two distinct layers — an 
outer serous layer, and an inner mucous layer. Between 
the two, a third, vascular layer, subsequently develops. 

Baer, who was one of those most active in inducing 
Pander to make his investigations, and who retained the 


liveliest interest in them after his departure from Wiirzburg, 
began his own much more comprehensive researches in 
1819, and nine years later published, as the fruit of these 
lesearches, a work on! " The History of the Evolution of 
Animals," which even now is generally and rightly con- 
sidered the most important and valuable contribution to 
embryological literature. This book, a true model of careful, 
experimental investigation, combined with ingenious philo- 
sophical speculation, appeared in two parts ; the first in 
the year 1828, the second in 1837. 21 It is the firm founda- 
tion on which the whole history of the evolution of the 
individual rests to this day, and so far surpasses its pre- 
decessors, including Pander's outline, that, next to the 
labours of Wolff, it must be regarded as the most important 
basis of modern Ontogeny. As Baer, who died at Dorpat in 
November, 1876, was one of the greatest naturalists of our 
century, and has exerted a most important influence on 
other branches of Biology also, it may be of interest to give 
some account of the life of this extraordinary man. 

Karl Ernst Baer was born in 1792, in Esthonia, on the 
little estate of Piep, which his father owned. He studied 
at Dorpat from 1810 to 1814, and then went to Wiirzburg, 
where Dollinger not only initiated him into Comparative 
Anatomy and Ontogeny, but also exercised over him, by 
his own interest in philosophical studies, a highly stimu- 
lating influence. From Wiirzburg Baer went to Berlin, 
and then, accepting a call from the physiologist Burdach, 
to Konigsberg. There he delivered lectures on Zoology and 
Evolution, with some interruptions, until 1834;, and com- 
pleted his most important works. In 1834 he went to St 
Petersburg as a member of the Academy of that place 

baer's work. 53 

There, however, he forsook almost entirely his former field 
of labour, and occupied himself with researches of a totally 
different nature, in various branches of Natural Science, 
especially in Geography, Geology, Ethnography, and Anthro- 
pology. His works on the History of the Evolution of 
Animals are far the most important ; nearly all of these 
were completed while he was in Konigsberg, though some 
of them were not published until later. Their merits, like 
those of Wolffs writings, are many-sided, and extend over 
the whole domain of Ontogeny in very various directions. 

Baer especially perfected the fundamental Theory of 
Germ-layers, as a whole as well as in detail, so clearly and 
completely, that his idea of it yet forms the safest basis of 
our knowledge of Ontogeny. He showed that in Man and 
the other Mammals, as in the Chick — in short, as in all Ver- 
tebrates — first two, and then four germ-layers are formed, 
always in the same manner, and that the modification of 
these into tubes gives rise to the first fundamental organs 
of the body. According to Baer, the first rudiment of the 
body of a Vertebrate, as it appears on the globular yelk 
of the fertilized egg, is an oblong disc, which first separates 
into two leaves or layers. From the upper or animal layer 
evolve all the organs which produce the phenomena of 
animal life : the functions of sensation, of motion, and the 
covering of the body. From the lower or vegetative layer 
proceed all the organs which bring about the growth of tho 
body : the vital functions of nutrition, digestion, blood- 
making, breathing, secretion, reproduction, and the like 
Each of these two original germ -layers separates again into 
two thinner layers, or lamellae, one lying above the other 
First, the animal layer separates into two, which Baer calls 


the skin, or dermal layer, and the flesh, or muscular layei. 
From the uppermost of these two lamellae, the skin-layer, 
are formed the outer skin, the covering of the body, and the 
central nervous system, the spinal cord, the brain, and 
the organs of sensation. From the lower, or flesh-layer, 
the muscles, or fleshy parts, the internal or bony skeleton, — 
in short, the organs of motion, arise. Secondly, the lower, 
or vegetative germ -layer, parts in the same way into two 
lamellae, which Baer distinguishes as the vascular and the 
mucous layer. From the outer of the two, the vascular 
layer, proceed the heart and the blood-vessels, the spleen, 
and the other so-called blood-vessel glands, the kidneys, 
and the sexual glands. Finally, from the lowest, and fourth 
or mucous layer, arises the inner alimentary membrane of 
the intestinal canal, with all its appendages, liver, lungs, 
salivary glands. Baer traced the transformation of these 
four secondary germ-layers into tube-shaped fundamental 
organs as ingeniously as he had successfully determined 
their import and their formation in pairs by the segmen- 
tation of the two primary germ-layers. He was the first 
to solve the difficult problem as to the process by which 
the entirely different body of the vertebrate develops from 
this flat, leaf-shaped, four-layered original germ ; the procesa 
was the transformation of the layers into tubes. 

In accordance with certain laws of growth, the flat 
layers bend, and become arched ; the edges grow towards 
each other so that the distance between them is continually 
decreased ; finally they unite at the point of contact. By 
this process the flat intestinal layer changes into a hollow 
intestinal tube ; the flat spinal layer becomes a hollow 
spinal tube, the skin-layer becomes a skin-tube, etc 


Among the many and great services which Baer ren- 
dered in detail to Ontogeny, especially to that cf Vertebrates, 
his discovery of the human egg must be especially men- 
tioned here. Most, even of the earlier naturalists, had 
assumed that man proceeds, like other animals, from an 
egg. The Theory of Evolution (pre-formation) had, more- 
over, assumed that all past, present, and future generations 
of the human race existed encased in the ova of Eve, the 
common mother. Yet the ova of Man and other Mammals 
were not actually known till the year 1827. For the egg 
ifi exceedingly small, a spherical vesicle or bladder of only 
one-tenth of a line in diameter, which can be seen with the 
naked eye only under very favourable circumstances. This 
spherical vesicle, when in the ovary of the mother, is en- 
closed in a number of peculiar spherical vesicles of much 
larger size, called Graafian follicles, after their discoverer 
Graaf, and these were formerly universally regarded as the 
actual eggs. It was not until the year 1827 — not fifty years 
age — that Baer proved that these Graafian follicles are not 
the actual eggs, which are much smaller, and only imbedded 
in the Graafian follicles. (Cf. end of Chapter XXV.) 

Baer was also the first to observe the so-called germinal 
vesicle of Mammals, that is, the little spherical bladder 
which is first developed from the impregnated egg, and the 
thin wall of which consists of a single layer of uniform 
polygonal cells. (See Chapter VIII.) Another discovery of 
Baer's, of great importance in understanding the types of 
the lineage of the Vertebrates, and the chaiacteristio 
organization of this group of animals in which Man is 
included, was that of the Cttorda Dorsalis. This is a long, 
thin, cylindrical cartilaginous cord, which in all Vertebrates 


passes lengthwise through the whole body of the embryo 
It is developed at a very early stage, and is the first formjv- 
tion of the spine, the firm axis of Vertebrates. In the 
Lancelet {Amphioxus), the lowest of all Vertebrates, the 
entire inner skeleton is limited to this Chorda throughout 
life. But in Man and all the higher Vertebrates, first the 
spine, and later the skull, are developed round this cord. 

Important as these and many other discoveries of Baer's 
were in the Ontogeny of Vertebrates, yet the great im- 
portance of his researches rested especially on the fact that 
he was the first to apply the comparative method to the 
study of the evolution. It was, of course, the Ontogeny of 
Vertebrates, and principally of Birds and Fishes, that Baer 
first and especially investigated. Yet he by no means 
limited himself to these ; for he included various Inverte- 
brates in his investigations. The most general result ot 
these comparative embryological researches was that Baer 
assumed four totally different courses of evolution for the 
four principal groups of the animal kingdom. These four 
chief groups, or types, which at that time had just begun 
to be distinguished, in consequence of George Cuvier's 
researches in Comparative Anatomy, are : (1) Vertebrates 
(Vertebrata) ; (2) Articulated animals (Arthropoda) ; (3) 
Soft-bodied animals (Mollusca) ; and (4) the lower animals, 
which at that time were all erroneously grouped under the 
term Radiata. Cuvier, in the year 1816, demonstrated for 
the first time that these four groups of the animal kingdom 
show very essential and typical distinctions in their whole 
inner structure, and in the arrangement and position of the 
organic systems ; that, on the other hand, the internal 
structure of all animals of one type, for example, of all Ver- 


tebrates, is essentially similar, notwithstanding the great 
variety of outward forms. Baer, however, independently 
and almost simultaneously, furnished proof that the foui 
groups develop from the egg by entirely different processes 
and further, that the order of the series of embryonic form* 
in the course of evolution is from the very beginning 
identical in all animals of the same type, but, on the other 
hand, different in those of different types. Up to that 
time, in making a classification of the animal kingdom, an 
endeavour had always been made to arrange all animals, from 
the lowest to the highest, from the infusoria to man, in 
a single connected series of forms ; and the false idea had 
always been maintained, that there was a single unbroken 
gradation of development from the lowest animal to the 
highest. Cuvier and Baer proved that this coneeption is 
totally erroneous, — and that, on the contrary, there are foui 
wholly distinct types of animals, which must be distin- 
guished not only as to their anatomical structure, but also 
as to their embryonic evolution. 

As a result of this discovery, Baer succeeded in estab- 
lishing a very important law, which we shall name in his 
honour Baer's Law, and which he expresses as follows : 
* The evolution of an individual of a certain animal form 
is determined by two conditions : firstly, by a continuous 
perfection of the animal body by means of an increasing 
histological and morphological differentiation, or an increas- 
ing number and diversity of tissues and organic forms ; 
secondly, and at the same time, by the continual transition 
from a more general form of the type to one more specific." 
The degree of perfection of the animal body depends on 
the greater or less amount of heterogeneity there is in its 


elementary parts, and in the segments of its compoeil 
organs, — in a word, in the degree of histological and mor- 
phological differentiation. The type, on the other hand, 
is the order of the arrangement of the organic elements and 
of the organs. The type is quite distinct from the degree of 
perfection; the same type may exist in several degrees 
of perfection ; and, conversely, the same grade of perfection 
may be reached in several types. This explains the phe- 
nomenon that the most perfect animals of any type, — for 
example, the highest Arthropods and Molluscs, — are much 
more perfectly organized, or more highly differentiated, 
than the most imperfect animals of other types, — for ex- 
ample, than the lowest Vertebrates and Star-animals. 

Baer's Law has been of the greatest importance in 
advancing our knowledge of animal organization ; though 
it was not until a later period that Darwin enabled us to 
perceive and value its real significance. Here we may 
at once remark that it can only be really understood by 
means of the Theory of Descent, by the recognition of the 
very important part played by Heredity and Adaptation 
in the construction of organic form. As I have shown in 
my Generelle Morphologie (vol. ii. p. 10), the type of 
evolution is the mechanical result of Heredity ; the degree 
of perfection is the mechanical result of Adaptation. 
Heredity and Adaptation are the mechanical factors in the 
production of organic forms, which were first brought to 
bear on Ontogeny by Darwin's Theory of Selection, and 
which have enabled us for the first time to understand 
Baer's Law. 

Baer's labours marked the beginning of a new epoch, 
and aroused an extraordinary interest in embryological 


research throughout a very wide circle. We find, therefore, 
that a large number of investigators occupied the newly 
found field of research, and, with praiseworthy industry, 
made a great number of distinct new facts in a short time. 
The majority of these new embryologists are industrious 
specialists, who have been very useful in collecting fresh 
materials, but who have, as *a rule, done but little to ad- 
vance the general problem of the History of Germs. I can, 
therefore, limit myself to the mention of a few names. 
Of special importance are the investigations of Heinrich 
Rathke, of Konigsberg (died 1861), who did much to advance 
the History of the Evolution of Invertebrates (Crabs, In- 
sects, Molluscs), as well as of Vertebrates (Fishes, Turtles, 
Snakes, Crocodiles). In the subject of the Embryology of 
Mammals, the widest conclusions are due to the careful 
experiments of Wilhelm Bischoff. of Munich. His History 
of the Evolution of the Rabbit (18-40), of the Dog (1842), of 
the Guinea-Pig (1852), and of the Roe-Deer .(1854), are as 
yet the most important basis of study in this department. 
Among the numerous works on the History of the Evolution 
of Invertebrates, those of the well-known zoologist, Johannes 
Muller, of Berlin, on Star-animals (Echinodemia), are espe- 
cially noteworthy ; also those of Albert Kolliker, of Wiirz- 
burg, on Cuttle-fishes (Cephalopoda); of Siebold and Huxley, 
on Worms and Plant-animals ; of Fritz Muller (Desterro), on 
the Crustacea ; of Weismann, on Insects, etc. The number oi 
labourers in this field has of late greatly increased, although 
not very much of special importance has been accomplished 
It is evident, from the majority of recent publications on 
Ontogeny, that their authors are not familiar enough with 
Comparative Anatomy. The most important of the latest 


ontogenetic works are those of Kowalevsky, E. Ray Lan- 
kester, and Eduard van Beneden, to which we shall presently 
again refer. 22 

A more decided advance in general knowledge than was 
effected by all these separate investigations, dates from the 
year 1838, when the proof of the Cellular Theory suddenly 
opened a new field of research in the History of Evolution. 
The distinguished botanist, Schleiden, of Jena, having 
proved by means of the microscope that every vegetable 
body is composed of innumerable elementary parts, the so- 
called cells, Theodor Schwann, of Berlin, a pupil of Johannes 
Muller, applied this discovery directly to the animal body. 28 
He showed, that not only in plants, but also in the bodies 
of the most dissimilar animals, these same cells are dis- 
tinguishable, under the microscope, in all the tissues, and 
that they form the actual building material of organisms. 
All the numerous tissues of the animal body, such as the 
entirely dissimilar tissues of the nerves, muscles, bones, 
outer skin, mucous skin, and of other similar parts, are 
originally composed of cells; and the same is true of 
all the various tissues of the vegetable body. These cells, 
which we shall hereafter consider more closely, are inde 
pendent living beings, the citizens of the state, which con- 
stitute the entire multi-cellular organism. The knowledge 
of this most important fact was, of course, of direct service 
to the History of Evolution also, in that it raised many 
new questions, chieiiy these : What relation have the cells 
to the germ-layers \ Are the germ -layers already com- 
posed of cells, and how are they related to the cells of the 
tissues which afterwards appear ? What place does the 
Qgg hold in the Cell Theory ? Is it itself* a cell, or is it 


composed of cells ? These were the important questions 
which the Cell Theory at once raised in the study of Em- 

Several naturalists attempted in different ways to 
furnish the right answers, but the excellent " Investigations 
into the Evolution of Vertebrates," by Robert Remak, of 
Berlin (1851), became conclusive. By somewhat remoulding 
the Cellular Theory of Schleiden and Schwann, this gifted 
naturalist was able to overcome the great obstacles which 
this theory, in its first form, had placed in the way of 
Embryology. It is true that the anatomist, Karl Boguslaus 
Reichert, of Berlin, had previously attempted to explain the 
origin of the tissues. But this attempt was necessarily a 
total failure, owing to the fact that the extraordinarily 
confused mind of the author was equally destitute of every 
correct idea of the History of Evolution, of the Cellular 
Theory as a whole, and of a sound view of the structure 
fcnd development of tissues in particular. The inaccuracy 
of Reichert's observations, and the falsity of the conclusions 
drawn from them, is shown by every accurate test applied 
to his so-called discoveries. By way of illustration, it may 
be said that he declared the whole of the upper germ-layer, 
from which the most important parts of the body — brain, 
spinal cord, outer skin, and the like — proceed, to be merely 
a transient " en veloping-skin " of the embryo, and that it 
had nothing to do with the formation of the body ; that 
many of the first formations of the separate organs did not 
proceed from the primary germ-layers, but came one by 
one from the yelk of the egg } and joined the layers after- 
ward. Reichert's preposterous embryological labours suc- 
ceeded in gaining a passing attention, only because they 


were put forward with unusual presumption, and professed 
to disprove Baer's Theory of Germ-layers. They are 
written in so clumsy and confused a style, that no one 
co\ild quite understand them ; but for this very reason they 
won the admiration of many readers, who supposed that 
a nucleus of profound wisdom was hidden somewhere be- 
hind these obscure oracular and mysterious sayings. 

Remak was the first to throw full light on the great 
confusion which Reichert had caused, by explaining, in the 
simplest possible manner, the evolution of the tissues. Ac- 
cording to him, the egg of animals is always a simple 
cell, and the germ-layers, which proceed from the egg, are 
also composed only of cells, and those cells, which alone 
constitute the egg, are produced in a very simple manner 
by the continuous and repeated segmentation or dividing 
up of the original simple egg-cell. This cell divides, or 
parts, first into two, and then into four; from these four 
arise eight, then sixteen, and then thirty-two, and so on. 
Hence, in the individual evolution of every animal, as well 
as of every plant, from the one simple cell, constituting the 
egg, is formed, by repeated segmentation, an aggregate of 
cells, as Kolliker had already maintained in 1844. The 
cells of such a mass spread themselves out flatly, and so 
form into layers, so that every one of these layers is 
originally composed of but one kind of cell. The cells oi 
the layers differentiate themselves, or assume various forms ;• 
and then there is a further differentiation, or, in other 
words, a division of labour of the cells within the layers 
themselves, and this latter differentiation produces all the 
various tissues of the body. 

These are the very simple principles of Histogeny, 01 

REMAK. 63 

the Science of the Evolution of Tissues, as first elaborated 
by Remak and by Kolliker in this comprehensive sense. 
By thus proving more definitely the part which the germ- 
layers take in the formation of the various tissues and 
systems of organs, and applying the Theory of Epigenesis 
to the cells and the tissues formed from them, Remak raised 
the Germ-layer Theory, at least as regards Vertebrates, to 
that degree of perfection in which we shall find it hereafter 
when we examine it in detail. According to him, the two 
germ-layers, of which the so-called germinal disc, the first 
simple leaf-shaped formation of the body of a Vertebrate, 
is composed, are soon increased by another layer, produced 
by the lower layer separating into two. These three have 
entirely distinct relations to the various tissues. First, 
from the upper layer proceed those cells which compose the 
outer skin (epidermis) of the body, together with the parts 
belonging and necessary to it (hair, nails, and the like) — 
that is, the external covering which envelops the whole 
body ; and, remarkable as it is, it produces also the cells 
which constitute the central nervous system, — the brain and 
spinal marrow. Secondly, from the lower germ-layer spring 
the cells which form the intestinal epithelium, — that is the 
whole inner coating of the intestinal canal and its append- 
ages (liver, lungs, salivary glands, and the like); in other 
words, the tissues which take up the food of the animal body 
and attend to its digestion. Finally, from the middle layer, 
lying between these two, arise all the other tissues of the 
body of the Vertebrate ; flesh and blood, bones and liga- 
ments, and the like. Remak also proved that the middle 
layer, which he calls the " motor-germinative " layer, again 
separates, secondarily, into two layers. In this way we get 


the same four layers which Baer had previously assumed. 
The upper part of the middle layer after its cleavage 
(Baer's " Flesh-stratum "), Remak calls the skin-lamella 
(Hautplatte, or better, Hautfaserplatte); it forms the outer 
wall of the body (the true skin, cutis vena, the muscles, 
hones, and the like). The lower part (Baers " Vascular 
stratum "), he calls the intestinal-fibrous lamella (Darm* 
faserplatte) ; it forms the outer covering of the intestinal 
canal, and of the heart, the blood-vessels, and so on. 

Based on the firm foundation which Remak thus supplied 
to the History of the Evolution of the Tissues, or the science 
called Histogeny, numerous investigations of special points 
which have considerably extended our information have 
been made. Of course many attempts have been made 
to give much narrower limitations to Remak's doctrines, or 
to remodel them altogether. Reichert, of Berlin, and Wil- 
hclm His, of Leipsic, have specially busied themselves to 
establish, in comprehensive works, an entirely new view of 
the evolution of the body of Vertebrates, according to which 
the rudiments of the body of the Vertebrate does not consist 
solely of the two primary germ-layers. But these works, 
owing to their total lack of the necessary knowledge of 
Comparative Anatomy, and clear knowledge of Ontogeny, 
and to the fact that they do not even glance at Phylogeny, 
could exert but a very transient influence. Only the total 
want of critical ability and comprehension of the real 
problems of the History of Evolution can explain the fact 
that many people for a time regarded the strange fancies of 
Reichert and His as a great gain. 

His, in 1868, in a large book, on "The Eaily Evolution 
of the Chick in the Egg," detailed* his entirely erroneous 


news in a very learned form, and under the banner of a 
new and very exact mathematical and physical method, he 
has recently expressed the same views in a general form in 
his book on u Our Body and the Physiological Problem of 
its Origin" (Leipsic, 1875). As His, in order to increase 
the circulation of the latter book, has allowed it to be 
publicly advertised as " important to readers of Haeckel's 
Anthropogenic," I shall only remark that my treatise on 
"The Aims and Methods of the History of Evolution'' 
(Jena, 1875) frees me from the necessity of further answer. 
To the most important points in his false theories I shall 
refer again (See Chapter XXIV.) 

Quite recently, however, His and Reichert's books on 
Ontogeny, which had previously ranked as the most per- 
verted and unfortunate of the larger works on this science, 
have been far eclipsed, in that respect, by a ponderous work 
by Alexander Goette, of Strasburg, on the " History of the 
Evolution of Bombinator igneus, as the Basis of a Com- 
parative Morphology of Vertebrates " (Leipsic, 1875). Thin 
monograph is the biggest existing contribution to the 
literature of Ontogeny — a thick volume of 964 pages, ac 
companied by a very beautiful folio atlas of 22 plates 
These splendid plates, containing as many as 382 accurate 
and very carefully executed drawings, representing the 
history of the development of the Bombinator, are the 
result of years of incessant labour, and excite a most 
favourable interest in the huge work. Unfortunately, 
however, the reader who is induced by this splendid 
picture-book to expect a corresponding degree of excellence 
in the voluminous text, will be sadly disappointed. Not 
only is the whole account most obscure, confused, and 


contradictory, but, further, the entire treatment shows that, 
by his whole scientific education, the author is incapable 
of the heavy task. I should not pronounce this harsh 
judgment, but that Goette flatters himself that, as the 
reformer of the science, he is about to place it on an 
entirely new basis ; and but that, consequently, he treats 
the great leaders of the science — Baer, Remak, Gegenbaur, 
and others — in the most insolent manner, as narrow-minded 
labourers who, * by reason of their lack of knowledge of the 
history of evolution, have missed their aim." The following 
samples seem to show the mode in which the new science 
is constituted by Goette : " Perfect life renders evolution 
impossible. The capacity of evolution in the mature egg 
excludes real life. Egg-cleavage is not a living process of 
evolution. The egg neither as a whole nor as to its parts, 
neither in its origin nor in its complete state, is a cell. The 
cells of the various tissues are not organisms, are not organic 
individuals. The individuality of an organism is only a 
peculiar expression of the end of its evolution," and so on. 

In these and many other statements Goette abruptly 
upsets the whole science, as at present constituted. The 
Cell Theory and the Protoplasmic Theory are rejected as 
worthless ; even Comparative Anatomy is, according to this 
writer, of no scientific value ; Phylogeny is no science, and 
so on. I have explained the most incredible of Goette's 
assertions and his most unexampled errors in my work on 
"The Aims and Methods of the History of Evolution M 
(Leipsic, 1875) ; in which book I have also criticized the 
views held by His and Agassiz. Errors of this sort are no 
longer possible in other sciences. Their occurrence in the 
History of Evolution is explained partly by the great 


difficulty of the very complex task which lies before this 
science, and partly by the insufficient general preparation 
possessed by most of the more recent students. 

All valuable modern investigations into Animal Onto- 
genesis have only tended to confirm and add to the Theory 
of Germ-layers as established by Baer and Remak. As the 
most important advance made in this direction, it is deserv- 
ing of mention, that the same two primary germ-layers, 
from which the body of Vertebrates, including Man, 
develops, have recently been shown to exist in all inver- 
tebrate animals also, with the single exception of the lowest 
group, that of the Primaeval animals {Protozoa.) The dis- 
tinguished English naturalist, Huxley, in the year 1849, 
had already shown that this is also true of Plant-animals 
(Medusce). He drew attention to the fact that the two 
cell-layers, from which the body of this Plant-animal 
develops, correspond, morphologically as well as physio- 
logically, to the two primary germ-layers of Vertebrates. 
The upper germ-layer, from which the outer skin and the 
tlesh proceed, he named Ectoderm, or Outer layer ; the 
lower, which forms the organs of digestion and reproduc- 
tion, he called the Entoderm, or Inner layer. But during 
the past ten years, the two germ-layers have been found to 
exist among many other Invertebrates, The indefatigable 
Russian zoologist, Kowalevsky, has found them among 
widely differing groups of Invertebrates, in Worms, 
Star-animals (Echinoderma), Soft-bodied animals (Mollusca), 
Articulates (Arthropoda), and the like. 

In my Monograph on the Calcareous Sponges, which 
appeared in 1872, 1 have shown that this same pair of primary 
germ-layers forms the basis of the body of the Sponges, and 


that they are to be regarded as occupying the same relative 
place, or as being homologous, throughout all the various 
classes of animals, from the Sponges to Man. This homology 
of the two primary germ-layers, which is of extraordinary 
significance, extends throughout the animal kingdom, with 
only a few . exceptions in the lowest class, the Primaeval 
animals (Protozoa). These animals are of an exceedingly 
low organization, and do not advance to the stage of form- 
ing germ-layers, and consequently never form real tissues. 
The whole body merely consists, either of a single cell, as 
in Amoebae and Infusoria, or of a loose mass qf but slightly 
differentiated cells, or, as in Monera, it does not even attain 
a form as high as that of a cell. But from the egg-cell of 
all other animals two primitive germ-layers first proceed, 
the outer, animal layer (Ectoderm or Exoderm), and the 
inner, or vegetative layer (Entoderm), and from these the 
various tissues and organs arise. This is equally true of 
Sponges, of the other Plant-animals, and of Worms ; it is as 
true of Soft-bodied animals (Mollusca), Star-animals (Echin- 
oderma), and Articulates (Arthrojioda), as of Vertebrates. 
All these animals may be comprised under the head of 
Intestinal Animals (Metazoa), in distinction from the 
Primaeval Animals (Protozoa), which have no intestine. 

It is perhaps more correct not to place the Protozoa 
among the true animals at all, but to class them in the 
neutral kingdom of the Protista, those humblest primaeval 
beings which are neither true animals nor true plants. 
According to this view the Metazoa can alone be considered 
as true animals, and the origin from two primary germ- 
layers may be held to form the primary character of thv 
animal kingdom. 


In the lowest forms of Metazoa, the body consists 
throughout life of these two primary germ- layers. But in 
all higher Intestinal Animals, each of these forms by 
cleavage two other layers, so that the body is thenceforward 
composed of four secondary germ-layers. In my "Gastrsea 
Theory" (1873), I have tried to show the general homology 
of these four layers in all Metazoa, and I have pointed out 
the important bearing of this fact on the natural system of 
the animal kingdom.- 4 

But though the most important facts in the individual 
evolution of the human and animal body had been suffi- 
ciently established by these advances in Animal Ontogeny, 
yet the most difficult task remained, — namely, the discovery 
of the causes by which the evolution of organisms and the 
production of their forms is effected. The real mechanical 
causes of individual evolution were first explained in 1859, 
in Darwin's work, in which the facts of Heredity and 
Adaptation were for the first time scientifically discussed, 
and their bearing on Ontogeny correctly interpreted. Only 
by the Theory of Descent, and by the aid of the laws of 
Heredity and Adaptation, are we really able to understand 
the facts of individual evolution, and to explain them by 
efficient causes. This is the point in which the Darwinian 
Theory is so important to the History of the Evolution of 
Man and to the immediate connection of the first part of 
our science, Germ-history, or Ontogeny with the second 
part, Tribal-history, or Phylogeny. 


Jean Lamarck. 

'hylogeny before Darwin. — Origin of Species. — Karl Linnaeus' Idea of 
Species, and Assent to Moses' Biblical History of Creation. — The 
Deluge. — Palaeontology. — George Cuvier's Theory of Catastrophes. — 
Repeated Terrestrial Revolutions, and New Creations. — Lyell's Theory 
of Continuity. — The Natural Causes of the Constant Modification 
of the Earth. — Supernatural Origin of Organisms. — Immanuel Kant's 
Dualistic Philosophy of Nature. — Jean Lamarck. — Monistic Philosophy 
of Nature. — The Story of his Life. — His Philosophie Zoologique. — First 
Scientific Statement of the Doctrine of Descent. — Modification of 
Organs by Practice and Habit, in Conjunction with Heredity. — Applica- 
tion of the Theory to Man. — Descent of Man from the Ape. — Wolfgang 
Goethe. — His Studies in Natural Science. — His Morphology. — His 
Studies of the " Formation and Transformation of Organisms." — 
Goethe's Theory of the Tendency to Specific Differences (Heredity 
&nd of Metamorphosis (Adaptation). 

•* It would be an eaBy task to show that tho characteristics in the organi- 
sation of man, on account of which the human species and races are 
grouped as a distinct family, are all results of former changes of occu- 
pation, and of acquired habits, which have come to be distinctive of indi- 
viduals of his kind. When, compelled by circumstances, the most highly 
developed apes accustomed themselves to walking erect, they gained 
the fcsoendant over the other animals. The absolute advantage they 


enjoyed, and the new requirements imposed on them, made them change 
their mode of life, which resulted in the gradual modification of their 
organization, and in their acquiring many new qualities, and among them 
the wonderful power of speech." — Jean Lamarck (1809). 

Those researches into the history of the individual evolution 
of man and animals, the history of which we surveyed in 
the last two chapters, had until recently hardly any other 
object than that of practically determining the changes of 
form undergone by the organism in the course of its growth. 
But until within the past fifteen years, no one dared to 
seek for the causes of these phenomena. During the entire 
century, from the year 1759, the date of the publication of 
Wolffs Theoria Generationis, until the year 1859, when 
Darwin published his " Origin of Species," the causes of 
the evolution of the germ remained entirely hidden. 
During the whole century nobody thought of seriously ex- 
amining the real causes of the changes of form which take 
place in the evolution of the animal organism. Indeed, 
the task was looked upon as so difficult that it entirely 
surpassed the powers of human comprehension. It was 
reserved for Charles Darwin to declare all these causes. 
We may therefore point to this gifted naturalist, who, 
in other respects, has effected a complete revolution 
throughout the whole range of Biology, as the founder of 
a new era in the field of Ontogeny also. It is true that 
Darwin himself has not really entered very deeply into 
embryological investigations, and even in his well-known 
chief work on the phenomena of individual evolution has 
but casually touched upon these, yet, by his reform of the 
Theory of Descent, and by constructing what he has named 
the Theory of Selection, he has placed in our hands the 


means of tracing the causes of the Evolution of Forms. It 
seems to me that it is in this respect that this great naturalist 
has had such an extraordinary effect on the entire subject of 
the History of Evolution. 

In glancing, as we must now do, at the last period, but 
just begun, of ontogenetic research, we enter at the same 
time into the second division of the History of Evolu- 
tion, namely, the History of the Descent, or Tribe 
(Phylogeny). In the first chapter I drew attention ti> 
the exceedingly important and intimate causal connec 
tion which exists between these two main branches of the 
History of Evolution, — that of the individual, and that ot 
his ancestors. We stated this connection in the funda- 
mental Law of Biogeny : the brief Ontogeny, or the 
Evolution of the Individual, is a swift and contracted 
reproduction, a compressed recapitulation, of the Phylogeny, 
or the Evolution of the Species. This proposition in reality 
comprises everything essentially relating to the causes of 
evolution, and we shall try everywhere, in the course of 
these chapters, to establish it, and to uphold its truth, 
by adducing actual facts in proof. The meaning of this 
fundamental Law of Biogeny, in relation to this causal 
significance, is perhaps yet better expressed as follows : 
" The evolution of the species, or tribes (phyla), contains, 
in the functions of heredity and adaptation, the determin- 
ing cause of the evolution of individual organisms;" or, 
quite briefly : " Phylogeny is the mechanical cause of 

It is owing to Darwin, that we are now able to trace 
the causes of individual evolution, which were previously 
deemed quite unapproachable, and to understand their real 


nature ; we therefore give his name to the new era of the 
History of Evolution. But before considering the grand 
discovery by means of which Darwin enabled us to under- 
stand the causes of evolution, we must glance rapidly at the 
efforts made by earlier naturalists in the same direction. 
The historical survey of these endeavours will be much 
shorter even than that of the labours in the field of On- 
togeny. There are really but few names to be mentioned. 
At the head stands the great French , naturalist, Jean 
Lamarck, who, in 1809, for the first time gave a scientific 
value to the so-called Theory of Descent. But even before 
this, the most important German philosopher, Kant, and 
the greatest German poet, Goethe, had both entertained 
the idea. During the previous half-century, however, their 
statements on this matter remained almost unnoticed. It 
was only in the commencement of our century that " Natural 
Philosophy " took up the question. Previously no one even 
dared to inquire seriously into the Origin of Species, which, 
properly speaking, is the culminating point of the History 
of Descent, or "Phylogeny. 

The entire Phylogeny of Man, and also of other animals, 
is most intimately connected with the question as to the 
nature of species, and » with the problem, how the distinct 
kinds of animals, which in systems are called species, really 
originated. The idea of species occupies the foreground. 
This idea was first presented by Linnaeus, who, in 1735, 
in his 8y sterna Naturae, attempted the first accurate dis- 
crimination and nomenclature of animal and vegetable 
species, and made a systematic list of the species then 
known. Since that time species has retained its place 
in descriptive Natural History, in systematic Zoology and 


Botany, as the most important collective term, although 
incessant strife has been waged as to the particular meaning 
of the term. Linnneus himself gave no clear, scientific defi- 
nition of the real nature of organic kind, or species. On the 
contrary, he took as a basis the mythological views of this 
subject, which the prevailing religious " faith," grounded on 
the Mosaic History of Creation, had introduced, and which 
are even now very generally maintained. He even adhered 
directly to the Mosaic History of Creation, and assumed 
that, as it is written in Genesis " male and female created 
he them," there had originally been but one pair of each 
animal and vegetable kind, or species. He supposed that 
all the individuals of a kind were descendants of the 
original pair created on the sixth day of Creation. Lin- 
naeus held that only a single individual was created of 
those organisms which are hermaphrodite, that is, which 
unite in their bodies both sexual organs, for these already 
possessed in themselves the qualifications for propagating 
their own species. In further developing these mytho- 
logical ideas, Linnaeus adhered to the Mosaic account 
and utilized the so-called " Deluge/' and the myth of the 
ark of Noah connected with it, to explain the choiology 
of organisms, the doctrine, that is, of the geographical and 
topographical distribution of animal and vegetable species. 
In harmony with Moses he assumed that all plants, animals, 
and human beings had been destroyed by the Deluge, with 
the exception of a single pair, which was saved in the ark 
to perpetuate the species, and which was put on land on 
Mount Ararat after the waters had subsided. Mount 
Ararat seemed to him specially adapted for this disembark- 
ation, because it is in a warm climate and rises to a height 

cuvier's system. 75 

of more than sixteen thousand feet, so that in its several 
zones of elevation it possessed all the climates necessary 
for the preservation of the various species of animals. The 
animals used to a cold climate could climb to the highest 
parts of the mountain ; those accustomed to a warm climate 
could descend to the foot ; and those from temperate zones 
could occupy the intermediate portions. From this moun- 
tain the animal and vegetable species could spread anew 
over the face of the earth. 25 

A scientific development of the History of Creation was 
impossible in the time of Linnaeus, because, among other 
reasons, the science of petrifactions, or Palaeontology, one 
of its principal bases, did not as yet exist. This science 
of petrifactions, or of the remains of extinct animals and 
plants, is most intimately connected with the whole 
History of Creation. Without reference to it, it is impos- 
sible to answer the question as to the manner in which the 
animals and plants now in existence came into being. But 
the knowledge of these petrifactions arose in much later 
times, and the real founder of Palaeontology, as a science, 
was the eminent zoologist, George Cuvier, who followed 
Linnaeus in constructing a System of Animals, and who, 
in the beginning of this century, brought about a com- 
plete reform of Systematic Zoology. The influence of this 
celebrated naturalist, who displayed an especially great 
power with extraordinary results during the first thirty 
years of this century, was so great that he opened new 
paths in almost every branch of Zoology, but especially in 
Classification, Comparative Anatomy, and Palaeontology. 
It is, therefore, important to glance at his views of the 
nature of species. In this respect he followed Linnaeus and 




the Mosaic account of Creation, though it was very difficult 
for him to do so, on account of the knowledge which he had 
of fossil animal forms. He was the first to show clearly 
that a number of totally different series of inhabitants had 
lived on our globe. He also showed that we must dis- 
tinguish at least ten or fifteen different main periods in the 
history of the earth, each of which exhibits a series of 
animals and plants of its own, peculiar to itself. 

Of course, Cuvier was at once confronted with the ques- 
tion, whence these various series of inhabitants had come, 
and whether they had any connection with each other 
He answered this question negatively, and maintained that 
these several " creations ' were totally independent of each 
other ; hence, that the supernatural act of creation by which, 
according to the received account of creation, the animal 
and vegetable species came into being, was repeated several 
times. Consequently, a series of quite distinct periods of 
creation must have followed one another, and in connection 
with them there must have occurred several vast alterations 
of the whole surface of the earth, — revolutions and cataclysms 
similar to the mythical Flood. These catastrophes and 
upheavals were favourite subjects with Cuvier ; especially 
as at that time the science of geology was also beginning 
to move greatly, and made rapid progress towards a know- 
ledge of the structure and origin of the earth. Others, 
especially the geologist Werner and his school, were occupied 
in carefully examining the various layers of the crust of the 
earth, and systematically investigating the fossils found 
in these. The result of their researches also was the recog- 
nition of several periods of creation. The inorganic crust 
.( the earth, the stratified surface, bore evidence of having 


been just as different at every period as were the animals 
and plants then inhabiting it. Combining this view with the 
results of his own pakeontological and zoological researches, 
and striving to understand clearly the whole course of the 
evolution of Creation, Cuvier arrived at the hypothesis 
usually called the Theory of Cataclysms or Catastrophes, or 
the Doctrine of Violent Upheavals. According to it several 
revolutions occurred on our earth at certain times, suddenly 
destroying every living inhabitant ; and at the end of each 
of these catastrophes an entirely new creation of organisms 
took place. But as the latter cannot be conceived as 
having been effected wholly by natural means, we must 
suppose, in explanation, that the Creator supernaturally 
interfered in the natural course of things. This Doctrine of 
Revolutions, treated by Cuvier in a separate work, which 
has been translated into several modern languages, was 
soon generally accepted, and for half a century continued 
to prevail among biologists ; there are even yet a few 
prominent naturalists who advocate it. 

It is true that more than forty years ago Cuvier's ' 
Doctrine of Catastrophes was altogether renounced by 
geologists ; and first of all by the English geologist, Charles 
Lyell, the most important authority in this branch of 
natural science. As early as the year 1830, in his famous 
"Principles of Geology," he proved that that doctrine is 
utterly false so far as the crust of the earth itself is con- 
cerned ; and he showed that in order to explain the structure 
and evolution of mountains, there is no need of having re- 
course to supernatural causes or universal catastrophes. On 
the contrary, the ordinary causes which even now unceasingly 
effect the transformation and reconstruction of the earth, are 


amply sufficient to explain these phenomena. These causes 
are : atmospheric influences ; water in its various forms — 
such as snow and ice, fog and rain, the running stream 
and the surging sea ; and finally, the volcanic phenomena 
contributed by the hot liquid mass in the interior of the 
earth. The most convincing proof was furnished by Lyell, 
that these natural causes are quite sufficient to explain all the 
phenomena of the structure and development of the crust 
of the earth. The geological teaching of Cuvier as to the 
revolutions and new creations was, therefore, soon totally 
abandoned, but in Biology the doctrine prevailed unopposed 
for thirty years longer. Zoologists and botanists, as far as 
they at all permitted themselves to think on the origin of 
organisms, adhered to Cuvier's false doctrine of repeated 
new creations and re-formations of the earth. This is cer- 
tainly one of the most curious examples of two closely 
related sciences long pursuing utterly divergent courses. 
One — Biology — remains far behind in the dualistic path, 
and even denies the possibility of solving "questions of 
creation " by the study of natural phenomena. The other — 
Geology — moves far ahead in the monistic path, and solves 
those very questions by the discovery of the actual causes. 

As an instance how utterly biologists refrained from in- 
quiries into the origin of organisms, and the creation of the 
animal and vegetable species, during this period from 1830 
to 1859, I mention, from my own experience, the fact that 
during all the whole course of my studies at the university, 
I never heard a single word on these most important and 
fundamental questions of biology. During this time, from 
1852 to 1857, I had the good fortune to listen to the most 
distinguished teachers in all branches of the science o/ 


organic nature; but not one of them ever spoke of this 
fundamental point, or even once alluded to the question of 
the origin of species. Not a word was ever spoken in 
reference to the earlier attempts toward understanding the 
origin of the animal and vegetable species; it was never 
thought worth while to allude to Lamarck's valuable 
Philosopkie Zoologique, in which that attempt had been 
made in the year 1809. The enormous opposition which 
Darwin met with when he first took up this question 
again may, therefore, be understood. His attempt seemed 
at first to be unsubstantial and unsupported by previous 
labours. Even in 1859 the entire problem of creation, the 
whole question of the origin of organisms, was considered 
by biologists as supernatural and transcendental. Even in 
speculative philosophy, in which this question should 
necessarily be approached from various sides, no one dared 
to take it seriously in hand. 

The dualistic position taken by Immanuel Kant, and the 
extraordinary importance attached, during the whole of this 
century, to this most influential of modern philosophers, 
probably offer the best explanation of the last-mentioned 
fact. For while this great genius, equally excellent as p 
naturalist and a philosopher, in the field of inorganic nature 
aided essentially in constructing a "Natural History ol 
Creation," he for the most part adopted the supernatural 
view of the origin of organisms. On the one hand, Kant 
in his " Universal History of Nature and Theory oi 
the Heavens," made a most successful and important "at- 
tempt to treat the constitution and the mechanical origin 
of the entire universe according to Newtonian principles," 
or, in other words, to treat it mechanically, to conceive 


it inonistically : and this attempt of his to explain the 
origin of the entire world by means of naturally working 
causes (causae efficientes), forms to this day the basis oi 
all our natural cosmogony. But, on the other hand, Kant 
maintained that the " principle of the mechanism of nature 
here applied, without which, after all, there could be no 
science of nature," was wholly inadequate to explain the 
phenomena of organic nature, and especially the origin of 
organisms ; that it was necessary to assume supernatural 
causes effecting a design {causae finales) for the origin of 
these natural bodies constructed with design. Indeed, he 
even went so far as to assert that "it is quite certain 
we cannot become adequately acquainted with organized 
beings, and their inner possibilities, by purely mechanical 
principles of nature, much less are we able to explain 
them ; and that this is so much the case that we may boldly 
assert that it is not rational for man even to enter upon 
such speculations, or to expect that a Newton will ever 
arise who, by natural laws not ordered by design, can 
render the production of a blade of grass intelligible ; in 
(act, we are compelled utterly to deny that it is possible 
for man ever to reach such knowledge." In these words 
Kant most definitely declared the dualistic and teleological 
standpoint which he adopted in the science of organic 

Kant sometimes, however, departed from this stand- 
point, especially in some very remarkable passages which 
I have discussed at some length in the fifth chapter of my 
'"History of Creation," in which he has expressed himself 
in quite the opposite, or monistic sense. With reference tc 
these passages, as I there showed, he might even be declared 

KANT. 8l 

an adherent of the Theory of Descent. Several very sig~ 
\rificant expressions, to which Fritz Schultze, in his interest- 
ing work on " Kant and Darwin, 86 has lately again called 
attention, actually enable us to recognize Kant 27 as the 
earliest prophet of Darwinism. He expresses with perfect 
clearness the great idea of an all-embracing, uniform evolu- 
tion ; he assumes " a variation from the primitive type of 
the tribe as the result of natural wandering." He even 
declares that man originally moved on four feet, and that 
it was only gradually that the human race raised their 
heads proudly over those of their old comrades, the beasts. 
But all these evidently monistic utterances are but stray 
rays of light ; as a rule Kant adhered in Biology to 
those obscure dualistic notions according to which the 
powers which operate in organic nature are entirely 
different from those which prevail in the inorganic world. 
This dualistic, or two-sided conception of nature is still 
dominant in school-philosophy ; most philosophers still 
consider these two domains of natural phenomena as 
entirely different. On one side is the field of inorganic 
nature, the so-called " inanimate ' world, where only 
mechanical laws (causae efficientes) are supposed to operate, 
of necessity and without purpose. On the other side is 
the field of " animated ' organic nature, all the phenomena 
of which in their profoundest essence and first origin can 
be made intelligible only by assuming pre-ordained pur- 
poses, or so-called (causce filiates) causes fulfilling a design. 
Although the question of the origin of animal and 
vegetable species, and the allied question as to the creation 
of man, remained until the year 1859 under the sway of 
these false dualistic prejudices, and were very generally 


declared to be a subject beyond the reach of scientific 
knowledge, yet even in the beginning of our century there* 
were independent eminent minds, who, undeterred by the 
prevailing doctrines, took these questions quite seriously in 
hand. The so-called earlier school of Natural Philosophy, 
which has so often been abused, deserves the highest praise 
in this respect. It was represented in France by Jean 
Lamarck, Buffon, GeofFroy St. Hilaire, and Ducrotay Blain- 
ville; in Germany, by Wolfgang Goethe, Reinhold Trevi- 
ranus, Schelling, and Lorenz Oken. 

The gifted naturalist and philosopher who must here 
be mentioned first, is Jean Lamarck. He was born at 
Bazentin, in Picardy, August 1, 1744, and was the son of 
a clergyman who destined him for the Church. He, how- 
ever, first joined the army, and as a boy of sixteen dis- 
tinguished himself by his braver}^ in the battle of Lippstadt 
in Westphalia, which resulted unfavourably for the French. 
He was then stationed for several years in a garrison in 
the south of France. Here he became acquainted with 
the interesting flora on the Mediterranean coast, which 
soon won him over to the study of botany. He resigned 
his commission, and published, as early as the year 1778, 
his valuable Flore Frangaise. For years he could gain no 
scientific position. It was only in his fiftieth year, in 1794, 
that he obtained a poor professorship of zoology at the 
museum of the Jardin de Plantes in Paris. His position 
caused him to enter more deeply into the study of zoology, 
towards the classification of which his labours were as 
valuable and important as those which he had dedicated 
to systematic botany. In 1802 he published his Considera- 
tions 8wr lea corps vivants, which contains the first germs of 


his Theory of Descent. In 1809 appeared the important 
Philosophie Zoologique, the principal work in which he 
elaborated this theory. In 1815 he gave to the world his 
comprehensive treatise on the Natural History of Inver- 
tebrates (Ilistoire naturelle des animaux sans vertebres), 
in the Introduction to which the same theory is again 
developed. About this time Lamarck entirely lost his eye- 
sight. Grudging fate never favoured him. While his 
principal opponent, Cuvier, was lucky enough to gain an 
influential position and the highest rank of scientific fame 
in Paris, Lamarck, who far surpassed Cuvier in clear and 
high-minded conception of nature, was obliged to struggle 
in lonely seclusion for the very necessaries of life, and could 
obtain no recognition. In 1829 his laborious life closed in 
the midst of the most needy circumstances. 28 

Lamarck's Philosophie Zoologique was the first scientific 
outline of a real history of the evolution of Species, a 
natural history of the creation of plants, animals, and 
men. The effect produced by this remarkable and im- 
portant book was, like that of Wolff's, none : neither was 
understood. No naturalist felt called upon to interest him- 
self seriously in this book, and to forward the development 
of the rudiments of the most valuable progress in Biology 
which it laid down. The most eminent botanists and 
zoologists threw the book entirely aside, and did not coii- 
sider it worth refuting. Cuvier,, who taught and laboured 
in Paris as a contemporary of Lamarck, in his account of 
the progress made in Natural Science, in which the most 
insignificant observations were mentioned, did not deem it 
worth whi]e to devote a syllable to this the greatest advance. 
In short, Lamarck's Zoological Philosophy shared the fat* 


of Wolffs Theory of Evolution, and was ignored for half a 
century. Even Oken and Goethe, the German natural 
philosophers, who were simultaneously employed in similar 
speculations, do not appear to have been aware of Lamarck's 
work. Had they known it, it would have been a great 
help to them, and they would have worked out the Theory 
of Evolution to a point beyond that which was otherwise 
possible to them. 

To enable my readers to judge of the great value of the 
Philosophie Zoologique, I shall here briefly mention some oi 
the most important of Lamarck's ideas. According to him 
there is no essential difference between animate and inani- 
mate nature ; all nature is a single world of connected 
phenomena, and the same causes which form and trans- 
form inanimate natural bodies are alone those which are at 
work in animate nature. Hence, we must apply the same 
methods of investigation and explanation to both. Life is 
only a physical phenomenon. The conditions of internal 
and external form of all organisms — plants and animals, 
with man at their head — are to be explained, like those of 
minerals and other inanimate natural bodies, only by 
natural causes (causce ejftcientes), without the addition of 
purposive causes (causce finales). The same is true of the 
origin of the various species. Without contradicting nature, 
we can neither assume for them one original act of crea- 
tion, nor repeated new creations as implied in Cuvier's 
Doctrine of Catastrophes, — but only a natural, uninterrupted, 
and necessary evolution. The entire course of the evolu- 
tion of the earth and its inhabitants is continuous and 
connected. All the various species of animals and plants 
which we now see around us, or which ever existed, have 


developed in a natural manner from previously existing, 
different species ; all are descendants of a single ancestral 
form, or at least of a few common forms. The most ancient 
ancestral forms must have been very simple organisms of 
the lowest grade, and must have originated from inorganic 
matter by means of spontaneous generation. Adaptation 
through practice and habit, to the changing external condi- 
tions of life, has ever been the cause of changes in the nature 
of organic species, and Heredity caused the transmission of 
these modifications to their descendants. 

These are the principal outlines of the theory of 
Lamarck, now called the Theory of Descent or Transmuta- 
tion, and to which, fifty years later, attention was again 
called by Darwin, who firmly supported it with new proofs. 
Lamarck, therefore, is the real founder of this Theory of 
Descent or Transmutation, and it is a mistake to attribute 
its origin to Darwin. Lamarck was the first to formulate 
the scientific theory of the natural origin of all organisms, 
including man, and at the same time to draw the two ulti- 
mate inferences from this theory : firstly, the doctrine of 
the origin of the most ancient organisms through spon- 
taneous generation ; and secondly, the descent of Man 
from the Mammal most closely resembling Man — the Ape. 

Lamarck attempted to explain the latter process, a most 
important one, and of special interest to us here, by the 
game efficient causes to which he had also referred the 
natural origin of animal and vegetable species. He con 
sidered that, on the one hand, practice and habit (Adapta- 
tion), and, on the other, Heredity, are the most important 
of these causes. The chief modifications of the organs 
cf animals and plants result, according to him, from the 


functions or actions of the organs themselves, from the 
exercise or absence of exercise, the use or disuse of these 
organs. To mention examples, the Woodpecker and the 
Humming-bird owe their peculiarly long tongue to their 
habit of using these organs to take their food out of 
narrow and deep crevices ; the Frog acquired a web between 
its toes from the motions of swimming ; the Giraffe gained 
its long neck by stretching it up to the branches of trees* 
Habits, the use and disuse of organs, are certainly of the 
greatest importance as efficient causes of organic form ; but 
they are insufficient to explain the modification of species. 
As a second and equally important cause, Lamarck fully 
perceived that Heredity must necessarily co-operate with 
Adaptation He maintained that the variations of organs 
arising from habit or use are in themselves at first but 
insignificant in each separate individual ; but that by the 
accumulation of the effects produced in each individual, 
transmitted from generation to generation in an ever increas- 
ing number, they become very significant. This was quite 
a correct fundamental idea ; but Lamarck did not reach the 
principle which Darwin subsequently introduced as the 
most important factor in the Theory of Transmutation, 
namely, the principle of Natural Selection in the Struggle 
for Existence. Lamarck failed to discover this most im- 
portant causal relation, and this, together with the low 
condition of all biological sciences at that time, prevented 
him from more firmly establishing his theory of the common 
descent of animals and man 

Lamarck also attempted to explain the evolution of Man 
from the Ape, as principally due to the progress made by 
the- Ape in its habits of life, the further development and 


Ucreased use of its organs, and to the fact that it trans- 
mitted the improvements thus acquired to its descend- 
ants. Lamarck considered the most important of these ad- 
vantageous variations to be the erect gait of Man, the differ- 
ing form of the hands and feet, the growth of language, 
and the correlative higher development of the brain. He 
assumed that the Apes most closely akin to Man, those 
which became the ancestors of .mankind, made the first 
step toward becoming human when they gave up the habit 
of climbing and living on trees, and accustomed themselves 
to an upright gait. This resulted in the carriage peculiar 
to Man and in the reconstruction of the spinal column and 
pelvis, as well as in the specialization of the two pairs of limbs 
— the fore pair developing into hands for the purpose of 
grasping and touching, while the hind pair were used only 
for walking, and thus developed into true feet. In con- 
sequence of the totally changed mode of life and of the 
correlation and interrelation of the various parts of the 
bociy and their functions, important changes occurred also 
in other organs and their functions. The change of food, 
for example, caused a change in the jaws and teeth, and, 
consequently, in the entire formation of the face. The tail, 
no longer used, gradually disappeared. As these Apes lived 
together in societies and acquired regulated family relations, 
such as are still found among the higher classes of Apes, the 
social habits, or so-called " social instincts," were especially 
developed. The Apes language of mere sounds grew into 
the word-language of Man, and abstract ideas were accxi- 
mulated from concrete impressions. The brain gradually 
developed in correlation with the larynx ; the organ of the 
mind in interrelation with that of speech. These important 


ideas of Lamarck contain the first and oldest genus of a 
real history of the human tribe. 

Toward the end of the preceding and the beginning 
of this century, the great poet Goethe, whose genius 
W8,s of the highest order, busied himself, independently of 
Lamarck, with the problem of creation, and his thoughts 
on this subject are of special interest. It is well known 
that Goethe's ready recognition of all the beauties of 
Nature, and his deep insight into her workings, early 
attracted him to natural scientific studies of the most 
various kinds. Throughout his life these formed the 
favourite occupation of his leisure hours. The theory of 
colours especially resulted in 1 his well-known and compre- 
hensive work on this subject ; but the most valuable and 
important of Goethe's natural scientific studies are those in 
connection with organic bodies, with " Life, that splendid, 
priceless thing." In Morphology, the doctrine of forms, 
he made most unusually deep researches. Aided by Com- 
parative Anatomy, he obtained most brilliant results in 
this, and went far in advance of his time. His cranial 
theory, his discovery of the temporal jawbone in man, and 
his doctrine of the metamorphosis of plants, must be espe- 
cially mentioned here. 29 These morphological studies led 
Goethe to make those researches into the formation and 
transformation of organisms which we must rank, after those 
of Lamarck, among the oldest and profoundest rudiments 
of phylogenetical science. He came so near the Theory of 
Descent that he must be classed with Lamarck among the 
founders of it. It is true that Goethe has nowhere given 
a connected scientific exposition of his theory of evolution ; 
but his brilliant miscellaneous writings, "Zur Morphologie* 


abound in most excellent ideas. Some of them may indeed 
be called the rudiments of the Theory of Descent. In 
proof of this it is sufficient to adduce some of his most 
remarkable propositions. He says : " This, then, is what wc 
have gained, fearlessly to assert that the more perfect natural 
organisms, such as Fishes, Amphibia, Birds, Mammals, and 
Man at the head of the last, have been formed after one 
primordial type, the very permanent parts of which only 
vary a little one way or another, and which in the course 
of reproduction is still being remoulded and perfected" 
(1706). This " primordial type " of Vertebrates, after which 
Man also has been shaped, answers to what we call " the 
common ancestral form of the vertebrate tribe," and from 
which all the various species of Vertebrates have arisen by 
constant " development, variation, and reproduction." In 
another passage Goethe says (1807) : "Plants and animals, 
regarded in their most imperfect condition, are hardly dis- 
tinguishable. This much, however, we may say, that from 
a condition in which plant is hardly to be distinguished 
from animal, creatures have appeared, gradually perfecting 
-themselves in two opposite directions, — the plant is finally 
glorified into the tree, enduring and motionless, the animal 
into the human being, of the highest mobility and free- 

That Goethe, in these and other utterances, did not 
apeak merely figuratively, that he grasped the internal 
relation and connection of organic forms in a genealogical 
sense, is yet more evident in remarkable separate passages in 
w hich he declares himself as to the causes of the external 
multiplicity of species, on the one hand, and of the internal 
unity of their structure on the other. He assumed that 


every organism is the product of the co-operation of two 
contrary constructive forces, or formative tendencies. One, 
the internal formative tendency, "the centripetal force," is 
that of the type, or " the tendency toward specification," 
which constantly aims at maintaining uniform the Organic 
forms of the species in the series of generations. This is 
Heredity. The other, the external formative tendency, 
" the centrifugal force," is variation, or " the tendency 
toward metamorphosis," which acts, through the continual 
changes made in the external conditions of their existence, 
so as continually to vary the species. This is Adapta- 

In this significant conception, Goethe very nearly con- 
ceived the two great mechanical factors, Heredity and Adap- 
tation, which are, we assert, the most important efficient 
causes of the formation of species. For example, he says, 
that " at the foundation of all organization there is an 
original intrinsic kinship " (which is Heredity) ; " the variety 
of forms, however, is due to the conditions of relation 
necessarily held to the external world, on account of which 
we may properly assume, for the purpose of explaining the 
present forms, which are both varied and unvaried, that 
there was diversity, originally and simultaneously, and that 
a progressive transformation is continually going on' 1 
(which is Adaptation). 

In order rightly to appreciate Goethe's morphological 
views it is, however, necessary to grasp the connection 
between the whole peculiar course of his monistic study of 
nature and his pantheistic conception of the world. Most 
significant in this respect is the lively and warm interest 
with which he followed the efforts which the French 


natural philosophers were making in the same direction, 
and especially the contest between Cuvier and Geoffroy St. 
Hilaire. (See Chapter IV. in "History of Creation.") It 
is also necessary to be in some degree master of Goethe's 
language and his process of thought, before it is possible 
rightly to understand the many expressions, often incidental, 
which refer to the doctrine of descent. He who does not 
know the great poet and thinker as a whole, may possibly 
even construe these very expressions in a contrary sense. 

In proof of this I adduce the strange fact that two 
second-rate German zoologists have recently discovered 
that Goethe was an extremely narrow-minded naturalist 
and a "willing adherent of the doctrine of constancy of 
species." Karl Semper, the gifted discoverer of " Haeckelism 
in Zoology," and Robby Kossman, the ingenious " Solver of 
the Rhizo-cephalic Problem," have extracted from Goethe's 
morphological writings that the latter needy Frankfort 
geniuses had neither a clear conception of the whole sig- 
nificance of organic forms, nor the faintest idea of the 
natural evolution of these forms, and of their connection 
by common descent. All who know the poor and narrow- 
minded literary productions of Semper and Kossman must 
smile at the sentence of annihilation thus pronounced on 
Goethe's conception of nature. 

Notwithstanding the condemnation by these great 

students of animal life, the rest of the world may continue 

to admire Goethe as a true prophet of the theory of descent. 

The numerous sentences which I have prefixed, as mottos 

to the chapters of the Generelle Morphologie, clearly 

show how far Goethe had advanced in his conception of 

the innate genealogical connection of the diverse organic 


forms. At the end of the last century he so nearly grasped 
the principles of natural tribal history, that we are justified 
in regarding him as one of the earliest forerunners of 
Darwin, although, unlike Lamarck, he did not formulate 
the Theory of Descent in a scientific system. 



Charles Darwin. 

Relation of Modern to Earlier Phylogeny. — Charles Darwin's Work on the 
Origin of Species. — Canses of its Remarkable Success. — The Theory ol 
Selection : the Interrelation of Hereditary Transmission and Adaptation 
in the Struggle for Existence. — Darwin's Life and Voyage Round the 
World. — His Grandfather, Erasmus Darwin.— Charles Darwin's Study 
of Domestic Animals and Plants. — Comparison of Artificial with 
Natural Conditions of Breeding. — The Struggle for Existence. — Neces- 
sary Application of the Theory of Descent to Man. — Descent of Man 
from the Ape. — Thomas Huxley. — Karl Vogt. — Friedrich Rolle. — 
The Pedigrees in the Generelle Morphologie and the " History of 
Creation." — The Genealogical Alternative. — The Descent of Man from 
Apes deduced from the Theory of Descent. — The Theory of Descent 
as the Greatest Inductive Law of Biology. — Foundation of this Induc- 
tion. — Palaeontology. — Comparative Anatomy. — The Theory of Rudi- 
mentary Organs. — Purposelessness, or Dysteleology. — Genealogy of the 
Natural System. — Chorology. — GEkology. — Ontogeny. — Refutation of 
the Dogma of Species. — The " Monograph on the Chalk Sponges ; " 
Analytic Evidence for the Theory of Descent. 

"By considering the embryological structure of man — the homologies 
which he presents with the lower animals — the rudiments which he retains — 
and the reversions to which he is liable, we can partly recall in imagination 
the former condition of our early progenitors ; and can approximately place 
fchem in their proper position in the zoological series. We thus learn that 
man is descended from a hairy quadruped, furnished with a tail and pointed 
probably arboreal in its habits, and an inhabitant of the Old World. 


This creature, if its whole structure had been examined by a naturalist, 
would have been classed among the Quadrumana, as surely as would the 
common and still more ancient progenitor of the Old and New World 
monkeys." — Charles Darwin (1871). 

In the short time that has passed since the appearance of 
Charles Darwin's book " On the Origin of Species in the 
Animal and Vegetable Kingdom," the History of Evolution 
has advanced so greatly that it is scarcely possible to point 
to an equally great advance throughout the whole record 
of the Natural Sciences. The literature of Darwinism is 
increasing day by day, not only in connection with Zoology 
and Botany — which are the special sciences most affected 
and reformed by the Darwinian Theory — but far beyond. 
It is applied in much wider circles with a zeal and interest 
which no other scientific theory has ever aroused. There 
are two distinct circumstances which principally explain 
this extraordinary success. In the first place, all the 
natural sciences, and especially Biology, made unusually 
rapid progress during the preceding half century, and from 
actual experience many new data for the theory of natural 
evolution were amassed. When compared with the failure 
of Lamarck, and the earlier naturalists to obtain recognition 
for their first attempts to explain the origin of organ- 
isms and of man, the success of the second attempt, made 
by Darwin, who had at his command such vast accumu- 
lations of well-attested facts, was all the more thorough. 
In availing himself of recent progress, the latter was able 
to employ quite other scientific evidence than Lamarck and 
Geofiioy, Goethe and Treviranus, could command. But, in 
the second place, we must give due weight to the fact that 
Darwin has the especial merit of having approached the 



question from an entirely new direction, and of having 
worked out that independent theory in explanation of the 
Doctrine of Descent which we properly call the Dai^winian 
Theory, or Darwinism. 

While Lamarck explained the variation of organisms 
descended from common ancestral forms, as especially the 
effect of habit and the use of the organs, but also by the 
aid of the phenomena of Heredity, Darwin independently, 
and on an entirely new basis, unfolded the actual causes 
which mechanically accomplish the modification of the 
various animal and vegetable forms by the aid of Adap- 
tation and Heredity. Darwin deduced his " Theory of 
Selection" from the following considerations. He com- 
pared the origin of the various breeds of animals and plants 
which man is able to produce artificially, — the conditions 
of " Selection " in horticulture, and in the breeding of 
domestic animals, — with the origin of wild species of 
plants and animals in a natural state. He thus found 
that causes similar to those which, in artificially breeding 
domestic animals, and raising cultivated plants, we apply 
to alter the forms, are also at work in Nature. He named 
the most effective of all the co-operating causes the 
Struggle for Existence. The gist of Darwin's theory, 
properly so called, is this simple idea : that the Struggle 
for Existence in Nature evolves new Species without design, 
just as the Will of Man produces new Varieties in Culti- 
vation with design. Just as the gardener and the farmer 
breed for their own advantage, and according to their 
own will, making judicious use of the productive effects 
of Heredity and Adaptation, so does the Struggle for 
Existence constantly modify the forms of vegetables and 


animals in an undomesticated state. This Struggle fo* 
Existence, or the universal efforts of organisms to secure the 
necessary means of existence, works without design, but 
yet in the same way modifies the organisms. But as under 
* its influence Heredity and Adaptation enter into most 
intimate reciprocal relations, there necessarily arise new 
forms, or variations, which are of advantage to the organ- 
ism, and which have, therefore, an object, although in 
reality not originating from a preconceived design. 

This simple fundamental idea is the real gist of Darwin- 
ism, or the "Theory of Selection." Its author conceived 
the idea long ago, but with admirable industry he employed 
twenty years in collecting data from actual experience for 
proving his theory before declaring it. In the "History 
of Creation" (Chapter VI.), I gave a full account of the 
method by which he reached his results, as well as of his 
most important writings, and his life. I shall, therefore, 
now only allude very briefly to some of the most important 
points. 30 

Charles Darwin was born on the 12th of February, 1809, 
at Shrewsbury, where his father, Robert Darwin, practised 
as a physician. His grandfather, Erasmus Darwin, was 
a thoughtful naturalist, who laboured in the line of the 
earlier natural philosophy, and who, toward the end of 
the eighteenth century, published several works on that 
subject. The most important of these is his "Zoonomy," 
which appeared in 1794, and in which he expressed views 
like those of Goethe and Lamarck, though he knew nothing 
of the similar efforts of these contemporaries. Erasmus 
Darwin transmitted to his grandson Charles, according to 
the law of latent transmission (Atavism), certain mole- 


cular movements of the cells in the ganglia of his powerful 
brain, which had not made their appearance in his son 
Robert. This fact is of great interest in relation to the 
remarkable law of Atavism which Charles Darwin himself 
has so well discussed. But in the writings of Erasmus 
Darwin, formative imagination too greatly outweighs 
critical judgment, while in his grandson, the two are evenly 
balanced. As, at present, many naturalists of limited 
genius regard imagination as superfluous in Biology, and 
their own lack of it as a great and "exact" advantage 
I take this opportunity of calling attention to a striking 
remark of an able naturalist, who was himself one of the 
leaders of the school called " exact," confining itself strictly 
to experience. Johannes Muller, the German Cuvier, whose 
works will always be regarded as models of exact investiga- 
tion, declared that continuous interaction and harmonious 
balance of imagination and reason, are the indispensable 
conditions of the most important discoveries. This passage 
is given in full as a motto at the beginning of the eighteenth 

After completing his university studies in his twenty - 
second year, Charles Darwin was fortunate enough to 
accompany an expedition which sailed round the world for 
scientific purposes. This lasted for five years, thus affording 
him an abundance of most instructive suggestions and of 
opportunities for the contemplation of Nature in its 
grandest forms. In the very beginning of the voyage, 
when he first landed in South America, he noticed a variety 
of phenomena, which suggested to him the great problem of 
his life-long work, the question of the "Origin of Species." 
On the one hand, the instructive phenomena of the geogra- 

Q8 the evolution of maw. 

phical distribution of species, and on the other, the relation 
between the living and extinct species of the same continent, 
suggested to him the idea that nearly allied species might 
have descended from a common ancestral form. On his 
return from his five years' voyage, he devoted himself for 
years most zealously to the systematic study of domestic 
animals and garden-plants, and he recognized the evident 
analogies between the formation and transmutations of these. 
and those of wild species in a state of nature. He had, 
however, no conception of natural selection through the 
struggle for existence, which is the most important feature 
in the construction of his theory, until after he had read 
the celebrated book of Malthus, the political economist, on 
the " Principles of Population." This gave him a clear 
conception of the analogy between the changing relations 
of population and over-population in civilized countries and 
the social relations of animals and plants in a wild state. 
He continued for many years to collect materials in order to 
accumulate a great mass of evidence for the support of this 
theory. At the same time, as a practical breeder, he insti- 
tuted many important experiments in breeding, and gave 
special attention to the instructive breeding of domestic 
pigeons. Ample leisure was afforded him by the quiet 
retirement in which, after his return from his journey 
round the world, he has lived on his property of Down, near 

It was not until the year 1858, that Darwin was induced, 
by the work of another naturalist, Alfred Russell Wallace, 
who had conceived the same Theory of Selection, to publish 
the outlines of his theory. In 1859 appeared his principal 
work, " On the Origin of Species," in which the theory ifl 


exhaustively discussed, and is established by the weightiest 
evidence. Having fully expressed my opinion of this book 
in my Generelle Morphologie, and in the " History of 
Creation," it will here be sufficient to recapitulate briefly 
the gist of the Darwinian theory, on the right under- 
standing of which everything depends. The whole is based 
on the simple fundamental idea that the Struggle for 
Existence in Nature modifies organisms, and produces 
new species by the aid of the same means by which man 
produces new domesticated varieties of animals and plants. 
These means consist in the constant preference or selection 
of the individuals most suitable for propagation, so that the 
interaction of Heredity and Adaptation acts as a modifying 
cause. 81 

The celebrated traveller Wallace had independently 
formed the same conclusions. He had, however, by no 
means determined the influence of Natural Selection in 
forming species as clearly and thoroughly as had been done 
by Darwin. But the writings of Wallace (especially those 
on Mimicry, etc.) contain many admirable, original con- 
tributions to the Theory of Selection. It is most unfor- 
tunate that the imagination of this gifted naturalist has 
since become diseased, and that he now only plays the part 
of a spiritualist in the spiritualistic society of London, 

The effect produced by Darwin's book on " The Origin of 
Species by Natural Selection " in the animal and vegetable 
kingdom, was extraordinarily great, though not at first in 
the special branch of science to which it most directly 
applied. Several years passed before botanists and zoolo- 
gists recovered from their surprise at the new views of 
nature advanced by this great reconstructive work. The 


effect of the book on the special sciences with which 
zoologists and botanists are concerned, has become really 
prominent only during the past few years, during which the 
Theory of Descent has been applied in Anatomy and On- 
togeny, and in zoological and botanical classification. In 
some ways it has already caused extraordinary progress and 
a great reform in the prevailing views. 

But in Darwin's first work of 1859, the point which 
most interests us here — the application of the Theory 
of Descent to Man — was not touched at all. For many 
years it was even asserted that Darwin had no intention of 
applying his theory to Man, but that he shared the preva- 
lent opinion, that an entirely peculiar place in creation must 
be assigned to Man. Not only men unversed in science, 
including very many theologians, but even educated natur- 
alists, asserted with the greatest ingenuousness, that the 
Darwinian Theory in itself was not to be combated, and 
was entirely correct, for it afforded an excellent means of 
explaining the origin of the various species of animals and 
plants; but that the theory was in no way applicable to 

In the mean time, however, many thoughtful people, 
naturalists as well as others, expressed the opposite opinion, 
that it necessarily follows as the logical conclusion from the 
Theory of Descent, as formulated by Darwin, that Man 
must have descended from other animal organisms, and, 
immediately, from Mammals resembling Apes. The truth of 
this conclusion was early recognized by many thoughtful 
opponents of the theory. Just because they regarded this 
as a necessary consequence, many felt that the whole theory 
must be rejected. The first scientific application of this 


theory to Man was made by Huxley, who now holds the 
first place among English zoologists. 82 This able and 
learned philosopher, to whom much progress in zoological 
science is due, published a little work entitled "Evidences 
of Man's Place in Nature," in the year 1863, contain- 
ing three essays : 1. On the Natural History of Man- 
like Apes ; 2. On the Relations of Man to the Lower 
Animals ; 3. On Some Fossil Remains of Man. In these 
three very important and interesting essays, it is clearly 
shown that the much-disputed descent of Man from the Ape 
is the necessary consequence of the Theory of Descent. If 
the Theory of Descent is correct as a whole, it is impos- 
sible not to regard the Apes most resembling Man as the 
animals from which the human race has been immediately 

Almost simultaneously Karl Vogt, a most acute zoologist; 
published a larger work on the same subject, entitled 
"Lectures on Man, his Place in Creation and in the History 
of the Earth." This author has since partly retracted his 
views, and has, indeed, quite recently adopted the strange 
assumption that the descent of Man can only be traced 
from the Apes, and not from the yet lower animals. This, 
however, only shows that Vogt has not followed the recent 
progress of Zoology, and that he has long ceased to sym- 
pathize with the most important parts of the History oi 

Gustav Jaeger ra and Friedrich Rolle M must be men- 
tioned among zoologists who, after the publication oJ 
Darwin's work, took up the Theory of Descent, advanced 
it, and drew the right logical conclusion, that Man is 
descended from the lower animala Friedrich Rolle, in 1866 



published a work on " Man, his Descent and Civilization, ia 
the light of the Darwinian Theory." 

At the same time, in the second volume of my Generelle 
Morphologic der Organismen, which appeared in 18C6, I 
made the first attempt to apply the Theory of Evolution to 
the entire classification of organisms, including Man. 85 I 
tried to sketch the hypothetical genealogies of the dif- 
ferent classes of the animal kingdom, of the kingdom of 
Protista, and of the vegetable kingdom, not only as they 
must be according to the principles of the Darwinian 
Theory, but also, as it is already really possible to do, with 
a certain degree of probability. For, if the Theory of 
Descent, as first definitely stated by Lamarck, and after- 
wards firmly established by Darwin, is correct in its general 
principles, then it must also be possible to interpret the 
natural system of plants and animals genealogically, and to 
place the smaller and larger divisions recognized in the 
system, as limbs and branches of a genealogical tree. The 
eight genealogical tables which I appended to the second 
volume of the Generelle Morphologie, are the first attempts 
to accomplish this. In the twenty-seventh chapter of the 
same work are given the most important stages in the 
ancestral line of the human race, as far as they can be 
traced in the descent of Vertebrates. I there attempted 
especially to determine the place in the mammalian class 
assigned to Man by the system, and, as far as seems possible 
at present, the genealogical significance of the latter. In 
the twenty-second and twenty-third chapters of my " His- 
tory of Creation," I materially improved on this attempt 
and explained it in a more popular form. 

A.t last, in 1871, Darwin himself published a very in 


teresting work, which contains the much -disputed applica- 
tion of his theory to Man, and which, therefore, completes 
his great doctrine. In this work, entitled " The Descent of 
Man, and Selection in Relation to Sex," 88 Darwin has 
openly and most logically drawn the inference, about which 
he had before purposely maintained silence, that Man also 
must have been evolved from lower animals. In a most 
masterly manner he discussed especially the very important 
part paid by Sexual Selection in the progressive exaltation 
of Man, and of all other higher animals. According to this 
theory, the careful selection which the two sexes exercise on 
each other, in relation to their sexual connection and re- 
production, and the aesthetic taste evinced by the higher 
animals in this matter, has a most important influence on 
the progressive evolution of forms and in the distinction of 
the sexes. The male animals seek out the most beautiful 
females, and, on the other hand, the females choose the 
finest males, so that the specific, and at the same time the 
sexual character is continuously ennobled. In this respect 
many of the higher animals exercise a better taste and a 
more impartial judgment than does man. But even among 
men sexual selection has given rise to a noble form of 
family life, which is the chief foundation on which civiliza- 
tion and social states have been built. The human race 
certainly owes its origin in great measure to the perfected 
Sexual Selection which our ancestors exercised in the choice 
of wives. (Cf Chapter XI. of the "History ui Creation," 
and pp. 244-247 in the second volume of the Generelle 

In all essential points Darwin approves of the general 
outline of the genealogical tree given in the Generelle Mor- 


phologie and the " History of Creation," and he expressly 
states that his experience points to the same conclusions 
It is impossible not to appreciate his great wisdom in not 
himself applying the Theory of Descent to Man, in his first 
work ; for the inference was of a sort to raise the strongest 
prejudices against the entire doctrine. It was at first only 
necessary to establish the theory in relation to the species 
of animals and plants. Its application to Man then inevit- 
ably followed sooner or later. 

It is most important to understand this connection 
rightly. If all organisms have sprung from a common root, 
Man is also included in this common descent. But if, on 
the contrary, each separate kind or species of organism has 
been separately created, then Man was also " created, not 
evolved." Between these two opposite views lies our 
choice ; and this decisive alternative cannot be often 
enough and prominently enough placed in the foreground. 
Either all the various species of the vegetable and animal 
kingdoms are of supernatural origin, created, not evolved — 
in which case Man is also the product of a supernatural act 
of creation, as is assumed in all the various systems of 
religious belief; or, the various species and classes of the 
animal and vegetable kingdoms have evolved from a few 
common and most simple ancestral forms ; and if this is the 
case, man himself is the latest product of the evolution of 
the genealogical tree of animals. 

The connection between the two maybe concisely stated 
as follows : the Descent of Man from lower animals is a 
special deductive law, necessarily following from the general 
inductive law of the entire Doctrine of Descent This 
sentence formulates the relation most clearly and simply. 


The Doctrine of Descent is really nothing but a great in- 
ductive law, to which we are led by grouping and compar- 
ing the most important empirical laws of Morphology and 
Physiology. We are obliged to draw our conclusions 
according to the laws of induction in every case in which 
we are unable to establish the truths of nature immediately 
by the infallible method of direct measurement, or mathe- 
matical calculation. In the study of animated nature, we 
are seldom able entirely to ascertain the significance of 
phenomena immediately, and by infallible mathematical 
means, as is possible in the much simpler study of inorganic 
bodies, in Chemistry, Physics, Mineralogy, and Astronomy. 
In the last especially, we can always employ the very 
simple and absolutely sure method of mathematical calcula- 
tion. But in Biology, this is for many reasons entirely 
impossible, and especially because the phenomena in it are 
far too complex to admit of immediate solution by mathe- 
matical analysis. We are therefore compelled to proceed 
inductively; in other words, from the mass of separate 
observation we must gradually draw general conclusions, 
which must be more and more approximately correct 
These inductive conclusions, it is true, cannot claim the 
absolute certainty of mathematical propositions ; but they 
are more and more approximately true in proportion with 
the increase in. extent of the experiences on which they are 
based. The importance of such inductive laws is in no way 
lessened by the circumstance that they must only be 
regarded as provisional scientific achievements, which may 
possibly be improved, or perfected, by the further progress 
of knowledge. This is equally true of the greater part of 
knowledge in other sciences ; for example, in Geology and 


Archaeology. However much particular items of such indue 
tive knowledge may in time be improved and modified, 
their general significance, as a whole, remains quite un- 

The Theory of Descent,, according to Lamarck and Dar- 
win, as a great inductive law, and indeed the greatest of 
all inductive biological laws, is in the first place based on the 
facts of Palaeontology, on the modification of species brought 
to light by the science of Petrifactions. From the condi- 
tions under which these fossils, or petrifactions, are found 
buried in the rock- layers of our earth, we draw the first 
sure conclusion, that the organic population of the earth, as 
well as the crust of the earth itself, has been slowly and 
gradually evolved, and that series of diverse populations 
have successively appeared at different periods of the 
earth's history. Modern geology shows us that the evolu- 
tion of the earth has been gradual, and without total and 
violent revolutions. Comparing the various plant and 
animal creations that have successively appeared during the 
course of the earth's history, we find, in the first place, that 
an increase in the number of species has been constant and 
gradual from the earliest to the most recent times ; and, in 
the second place, we perceive that the increase in the per- 
fection of the forms belonging to each of the larger groups 
of animals and plants is also constant. For example, the 
only Vertebrates existing in the earliest times are the lower 
Fishes ; then the higher kinds of Fishes ; later Amphibia 
appear ; still later, the three higher classes of Vertebrates, 
Reptiles first, then Birds, and Mammals ; of these only the 
most imperfect and lowest forms appear first ; it is only at 
y. very late period that the higher placental M^mmala 


appear, and among the latest and youngest forms of the 
latter is Man. Both the perfection of forms and their 
variety originate, therefore, only gradually, and in a period 
extending from the oldest time to the present day. This 
fact is of great importance, and can be explained only by 
the Doctrine of Descent, with which it perfectly agrees. If 
the various groups of plants and animals really descended 
one from another, then such an increase in number and 
degree of perfection, as the series of fossils actually exhibits, 
must necessarily have occurred. 

A second series of phenomena of great importance for 
the inductive law with which we are dealing, is contributed 
by Comparative Anatomy. This latter is that part of 
Morphology, or the Science of Forms, which compares the 
developed organic forms, and seeks, in their great variety, 
to find the one common law of their organization, or, as 
it was formerly called, the "general plan of structure." Since 
Cuvier first formed this science, at the beginning of 
this century, it has always been a favourite study of the 
most eminent naturalists. Goethe, even before him, had 
been greatly attracted by the charm of the mysteries which 
it solved, and had been drawn into the study of Morphology. 
It was especially Comparative Osteology, the philosophical 
observation and comparison of the bony skeletons of Verte- 
brates, which is really one of the most interesting branches 
of the science, that riveted his attention and led him to form 
his Theory of the Skull, which has already been mentioned. 
Comparative Anatomy teaches that in each line of descent 
in the animal kingdom, and in each class in the vegetable 
Kingdom, the'inner structures of all the animals belonging 

to the one, and of the plants belonging to the other, are in 



ail essential points in the highest degree similar, even 
though the outward forms are extremely unlike. Man, 
accordingly, in all essential features of internal organization 
so closely resembles other Mammals, that no comparative 
anatomist has ever doubted that he belongs to that class. 
Tho whole inner structure of the human body, — the disposi- 
tion of its various systems of organs, — the arrangement of 
the bones, muscles, blood-vessels, and the like, — the coarser 
and more minute structure of all these organs, corresponds so 
well with that of all other Mammals, — such as Apes, Gnawing 
animals (Rodentia), Hoofed animals (Ungulata), Whales, 
and Oppossums, — that the complete dissimilarity of the 
outward form is as nothing in the balance against it. We 
learn also from Comparative Anatomy that the fundamental 
characteristics of animal organization are so much alike, 
even within the various classes, numbering from thirty 
to forty in all, that they may fittingly be arranged in from 
six to eight principal groups. But even in these few groups, 
which represent the lineages or types of the animal kingdom, 
it can be shown that certain organs, especially the intestinal 
canal, were originally uniform. 

We can only explain this most , essential uniformity in 
all these various animals, notwithstanding their great ex- 
ternal dissimilarity, by the aid of the Theory of Descent. 
Only by considering the internal correspondence as the 
result of Heredity from common ancestral forms, and the 
external dissimilarity as the result of Adaptation to varied 
conditions of life, can this wonderful fact be thoroughly 

The recognition of this truth raised Comparative 
Anatomy itself to a higher rank, so that Gegenbaur, 87 the 


ablest living representative of this science, could say with 
perfect justice, that the Theory of Descent opened a new 
period in Comparative Anatomy, and that the former is 
the touchstone of the latter. " So far, no experience in 
Comparative Anatomy is contradictory to the theory of 
Descent; all rather lead to it. So that the theory will 
receive back from the science that which it has imparted 
to its methods; namely, clearness and certainty." Formerly, 
the remarkable internal similarity of structure in organisms 
had been a source of wonder, incapable of explanation. 
Now, however, we can understand the causes of these facts, 
and can prove that this wonderful uniformity is simply the 
necessary consequence of Heredity from common ancestral 
forms, and that the striking dissimilarity of the external 
form is the necessary consequence of Adaptation to the 
outward conditions of existence. 

There is a special branch of Comparative Anatomy 
which is peculiarly interesting in this respect, and at the 
same time of the most extended philosophical significance. 
This is the science of Rudimentary Organs, which we may 
call, in reference to their philosophical consequences, the 
Doctrine of Purposelessness, or Dysteleology.. Almost 
every organism, with the exception of the lowest and 
most imperfect, and especially every highly developed vege- 
table or animal body, man as well as others, possesses one or 
more structures which are useless to its organism, valueless 
for its life-purposes, worthless for its functions. Thus all of 
\i: have in our bodies various muscles which we never use ; 
for example, the muscles in the external ear and the parts 
immediately surrounding it. These outer and inner ear 
muscles are of great use to most Mammals, especially such 


as have the power of erecting the ears, because the form 
and position of the ear may thus be materially altered, in 
order to take in the various waves of sound in the best 
possible manner. In Man, however, and in other animals 
not possessing the power of pricking up the ears, the 
muscles, though present, are useless. As our ancestors long 
a£0 discontinued to make use of them, we> have lost the 
power of moving them. Again, there is in the inner corner 
of our eye a small crescent-shaped or semi-lunar fold of skin; 
the last remnant of a third inner eyelid, the so-called nicti- 
tating membrane. In our primitive relatives, the Sharks, 
and in many other Vertebrates, this membrane is highly 
developed, and of great use to the eye ; but with us it is 
abortive and entirely useless. On the intestinal canal we 
have an appendage, which is not only useless, but may 
become very injurious, the so-called vermiform appendage 
of the caecum. This little appendage of the intestine not 
infrequently causes fatal disease. If in the process of 
digestion, by an unlucky accident, a cherry-stone or some 
similar hard body is pressed into its narrow passage, a 
violent inflammation ensues, which usually causes death. 
This vermiform appendage is not of the slightest use in our 
organism ; it is the last and dangerous remnant of an organ, 
which was much larger in our vegetarian ancestors, and was 
of great use to them in digestion ; as it is still in many 
herbivorous animals, such as Apes and Rodents, in which 
it. is of considerable size, and of great physiological im- 

Other similar rudimentary organs exist in us, as in all 
higher animals, in different parts of the body. They are 
among the most interesting phenomena with which Com- 


parative Anatomy acquaints us ; firstly, because they afford 
the most obvious proof of the Theory of Descent, and 
secondly, because they most forcibly refute the custom- 
ary teleological philosophy of the schools. The Doctrine 
of Descent renders the explanation of these remarkable 
phenomena very simple. They must be regarded as parts 
which in the course of many generations have gradually 
been disused and withdrawn from active service. Owing 
to disuse and consequent loss of function, the organs 
gradually waste away, and finally entirely disappear. The 
existence of rudimentary organs admits of no other expla- 
nation. Hence, they are of the greatest philosophicaJ 
importance; they clearly prove that the mechanical, or 
monistic conception of the nature of organisms is alone 
correct, and that the prevailing teleological, or dualistic 
method of accounting for them, is entirely false. The very 
ancient fable of the all- wise plan according to which " the 
Creator's hand has ordained all things with wisdom and 
understanding," the empty phrase about the purposive 
" plan of structure " of organisms is in this way completely 
disproved. Stronger arguments can hardly be furnished 
against the customary teleology or Doctrine of Design, than 
the fact that all more highly developed organisms possess 
such rudimentary organs. 

The favourite phrase, "the moral ordering of the world," 
is ahc shown in its true light by these dysteleological 
facts Thus viewed, the " moral ordering of the world " i* 
evidently a beautiful poem which is proved to be false by 
the actual facts. None but the idealist scholar, who closes 
his eyes to the real truth, or the priest, who tries to 
keep his spiritual flock in ecclesiastical leading-strings, can 


any longer tell the fable of " the moral ordering of tk^ 
world." It exists neither in nature nor in human life, 
neither in natural history, nor in the history of civilization 
The terrible and ceaseless " Struggle for Existence " gives 
the real impulse to the blind course of the world. A 
" moral ordering," and " a purposive plan ' of the world 
can only be visible, if the prevalence of an immoral rule 
of the strongest and undesigned organization is entirely 

The Natural System of Organisms, which classifies all 
the various forms in larger and smaller groups, according to 
the degree of similarity or dissimilarity of these forms, is 
the widest inductive basis of the Theory of Descent. 
These groups or categories of the system, the varieties 
species, genera, families, orders, classes, and so' on, always 
show such relative co-ordination and subordination that 
they can be explained only genealogically, and the whole 
system can but be represented figuratively under the form of 
a tree with many branches. This tree is the genealogical 
tree of the groups related in form, and their relation in 
form really is their relation in blood. As no other explana- 
tion can be given of the fact that the system naturally 
assumes a tree-like form, we may regard this as an imme- 
diate and powerful proof of the truth of the Doctrine of 

Among the most important of the phenomena, testify- 
ing to the inductive law of the Theory of Descent, is the 
geographical distribution of animal and vegetable species 
over the surface of the earth, and their topographical distri- 
bution on the heights of mountains and in the depths of 
oceans, Alexander Humboldt gave a fresh impulse to the 


scientific investigation of these conditions, to the Science of 
Distribution, or Chorology ; but until Darwin, people were 
satisfied to observe the phenomena of Chorology, and tried 
principally to establish the demarcations of the distribu- 
tions of existing organic groups of greater or less extent 
But the causes of the remarkable phenomena of distribu- 
tion, the reasons why some groups exist only here, others 
only there, and why there are such numerous divisions ol 
the various species of plants and animals, it was impossible 
to explain. The Doctrine of Descent, for the first time, fur- 
nishes the key to the solution of this problem also ; it alone 
puts us in the right way to obtain an explanation, by ^how 
ing us that the various species and groups of species spring 
from common ancestral species, the widely diverging pos- 
terity of which gradually spread over the whole earth. 
Yet for every group of species there must be assumed a so- 
called " centre of creation " — that is, a common cradle, or 
original habitat, in which the common ancestral species of 
a group first evolved, and from which their immediate 
descendants dispersed in different directions. Individuals 
of these migrated species became in their turn the ances- 
tral species of new groups, which again, by active and 
passive migration, dispersed; and so on. As every form 
after its migration adapted itself to new conditions of 
existence in its new home, it underwent modification, and 
gave rise to new series of forms. 

Darwin, by the Theory of Descent, was the first to 
establish this highly important doctrine of active and 
passive migrations. At the same time he correctly pointed 
out the significance of the important chorological relations 
between the living population of each region and their fossil 


ancestors and allied forms. Moritz Wagner worked out this 
point most excellently under the name of " The Theory of 
Migration." ^ But, in our opinion, this famous traveller 
has over-estimated the importance of his " Theory of Mi- 
gration," in so far as he declares it to be a condition 
necessary to the rise of new species, and holds the " Theory 
of Selection " to be incorrect. The two theories are, how- 
ever, in no way opposed. On the contrary, migration, 
by which the ancestral species of a new kind becomes 
isolated, is only a special form of selection. The great and 
interesting series of chorological phenomena, since they can 
only be explained by the Theory of Descent, must also be 
considered as important inductive data of the latter. 

Exactly the same is true of all the remarkable pheno- 
mena which, in the " Household of Nature," we observe in 
the economy of the organisms. All the various relations of 
animals and plants, to one another and to the outer world, 
with which the (Ekology of organisms has to do, and espe- 
cially such interesting phenomena as those of parasitism, of 
family life, of the care of young, and of socialism, — all admit 
of simple and natural explanation only by the Doctrine of 
Adaptation and Heredity. While it was formerly usual to 
marvel at the beneficent plans of an omniscient and bene- 
volent Creator, exhibited especially in these phenomena, we 
now find in them excellent support for the Theory of 
Descent ; without which they are, in fact, incomprehensible. 

Finally, the whole of Ontogeny, the history of the indi- 
vidual evolution of all organisms, is an important inductive 
foundation of the Theory of Descent. But as this subject 
will be especially treated in later chapters, nothing further 
need be said of it here. Step by step, I shall endeavour 

" SPECIES." 115 

fco show that the whole of the phenomena of Ontogeny 
forms a connected chain of evidence in favour of the truth 
of the Theory of Descent, and that they can be explained 
only by Phylogeny. With the aid of this close causal 
connection between Ontogeny and Phylogeny, and by 
constantly appealing to our fundamental law of Biogeny, 
we shall be gradually able to prove from the facts of On- 
togeny that Man is descended from the lower animals. 

In conclusion, it must be mentioned that very recently 
the important theoretical question as to the nature and idea 
of " kind," or " species," which is the point on which really 
hang all the disputes about the Theory of Descent, has been 
definitely settled. For more than a century this question 
was discussed from the most varied points of view, without 
resulting in a satisfactory settlement. During that time 
thousands of zoologists and botanists have occupied them- 
selves in systematically distinguishing and describing 
species, without, however, any clear idea of the meaning 
of " species." Many hundred thousand vegetable and 
animal forms were set up and marked as good species, 
though even those who declared them such were unable to 
justify the proceeding, or to give logical reasons for thus 
distinguishing them. Endless disputes arose among the 
"pure systematizers," on the empty question, whether the 
form called a species was " a good or a bad species, a species 
or a variety, a sub-species or a group," without the question 
being even put as to what these terms really contained and 
comprised. If they had earnestly endeavoured to gain a 
clear conception of the terms, they would long ago have 
perceived that they have no absolute meaning, but are 
merely stages in the classification, or systematic categories, 
and of relative importance only. 


It is true that in the year 1857 a celebrated and abl^ 
but very untrustworthy and dogmatic naturalist, Louia 
Agassiz, attempted to give an absolute signification to these 
categories. He attempted this in an " Essay on Classification," 
in which the phenomena of organic nature were inverted, 
and in which, instead of explaining these by natural causes, 
he examined them through the seven-sided prism of theo- 
logical dreams. Every " good species, or bona species," is, 
according to him, " an embodiment of a creative thought of 
God." But this fine phrase is as little able to hold its 
ground against the criticism of natural science, as all other 
attempts to preserve an absolute conception of species. I 
chink I have demonstrated this sufficiently in my Generelle 
Morphologie (vol. ii. pp. 323-402), in the exhaustive critique 
there given of the morphological and physiological idea of 
species and of systematic categories. 

Moreover, Agassiz can himself hardly have believed his 
theosophic phrases. This great American, who, as Carus 
Sterne rightly said, laid the foundation of much natural 
science, 89 was, in reality, gifted with too much genius 
actually to believe in the truth of the mystic nonsense 
which he preached. Crafty calculation, and well-judged 
reliance on the want of understanding of his credulous 
followers, can alone have given him courage to pass the 
juggler's pieces of his anthropomorphic Creator as true coin. 
The divine Creator, as represented by Agassiz, is but an 
idealized man, a highly imaginative architect, who is always 
preparing new building plans and elaborating new species. 
(Cf. Chap. III. of the " History of Creation," and also " The 
Aims and Methods of the History of Evolution." Jena, 


When, in 1873, the grave closed over Louis Agassiz, the 
last great upholder of the constancy of species and of 
miraculous creation, the dogma of the constancy of species 
came to an end, and the contrary assumption — the assertion 
that all the various species descend from common ancestral 
forms — now no longer encounters serious difficulties. All 
the elaborate inquiries as to the real nature of species, and 
how it is possible that various species can proceed from 
a single ancestral species, have now been brought to a 
perfectly satisfactory close by the fact that the sharp de- 
marcations between species and variety on the one side, 
between species and genus on the other, have been entirely 
set aside. I have given the analytical evidence of this in 
my " Monograph on Chalk Sponges," 40 which appeared in 
1872. In it I closelv examined the variations of all the 
species of this small, but highly instructive group of animals, 
and demonstrated in every instance the impossibility of 
dogmatic distinctions of species. Just in proportion as the 
systematizer takes the ideas of Genus, Species, and Varieties 
in a wider or narrower sense, he distinguishes in the little 
group of Chalk Sponges, either only a single genus with 
3 species, or 3 genera with 21 species, or 21 genera witli 
111 species, or 39 genera with 289 species, or even 113 
genera with 591 species. But all these diverse forms are so 
intimately connected by numerous transitions and inter- 
mediate forms that the common descent of all the Chalk 
Sponges from a single ancestral form, the Olynthus, can be 
proved with certainty. 

I think I have thus given the analytical solution of the 
problem of the Origin of Species, and have thus satisfied the 
demands of those opponents of the Theory of Descent who 


wished to see the origin of allied species from a single 
ancestral form proved "in special instances." Those who 
are not satisfied with the synthetic proofs of the truth 
of the Doctrine of Descent, as afforded by Comparative 
Anatomy and Ontogeny, Palaeontology and Dysteleology, 
Chorology and Classification, may try to overthrow the 
analytic proofs in the "Monograph on Chalk Sponges," 
which was the product of five years of the closest observa- 
tion. I repeat : if any one still asserts, in opposition to the 
Theory of Descent, that the derivation of all the species 
of a group has hitherto never been convincingly shown 
in a special instance, the assertion is now completely with- 
out foundation. The "Monograph on the Chalk Sponges' 3 
furnishes this analytic proof in detail, entirely from facts, 
and, as I am convinced, also with incontrovertible certainty. 
Every naturalist who will examine the extensive material 
used in my investigations, and follow my statements, will 
find that in the Chalk Sponges, the various species can be 
traced step by step through the course of their evolution in 
statu nascenti. But, if this is really the case, if, in a single 
class or family, the derivation of all the species from a 
common ancestral form can be shown, then the problem of 
the Descent of Man has been definitely solved ; and we are 
able to demonstrate the derivation of man also from lower 

The demand which has been so often made, and which 
has recently been repeated even by well-known naturalists, 
that the derivation of Man from the lower animals, and 
immediately from Apes, yet requires " sure proof," has thus 
been satisfied. These " sure proofs " have been for some 
time available to all who would open their eyes to see them 


Quite vainly, many so-called " Anthropologists " demand as 
proof, that direct transitional forms between Men and 
Apes should be found, or even that a living Ape should 
be deliberately cultivated into a Man. Convincing and 
" sure " proofs are evident in the abundant material which 
has already been accumulated. The invaluable sources 
of Comparative Anatomy and Ontogeny afford the surest 
proof of Phylogeny. It is, therefore, unnecessary to search 
out fresh proofs of the descent of the human race, though 
it is necessary to recognize and to learn to understand the 
" sure proofs " which nave oeen uuc*uned. 


The Egg of Man and of other Animals is a Simple Cell. — Import and 
Essential Principles of the Cell Theory. — Protoplasm (Cell-substance), 
and the Nucleus (Cell-kernel), as the Two Essential Constituent Parte 
of every Genuine Cell. — The Undifferentiated Egg-cell compared with a 
highly Differentiated Mind-cell or Nerve-cell of the Brain. — The Cell as 
an Elementary Organism, or an Individual of the First Order. — The 
Phenomena of its Life. — The Special Constitution of the Egg-cell. — 
Yelk. — The Germ- vesicle. — The Germ-spot. — The Egg-membrane, or 
Chorion. — Application of the Fundamental Principle of Biogeny to 
the Egg-cell. — One-celled organisms. — The Amoebae. — Organization and 
Vital Pheuomena. — Their Movements. — Amoeboid Cells in Many-ceiled 
Organisms. — Movements of such Cells, and Absorption of Solid Matter. — 
Absorbent Blood Corpuscles. — Comparison of Amoeba with Egg-cell. — 
Amoeboid Egg-cells of Sponges. — The Amoeba as the Common Ancestral 
Form of Many-celled Organisms. 

"The ancestors of the higher animals most be regarded as one-oelled 
beings, similar to the Amoebae which at the present day occur in our rivers, 
pools, and lakes. The incontrovertible fact that each human individual 
develops from an egg, which, in common with those of all animals, is a 
simple cell, most clearly proves that the most remote ancestors of man 
were primordial animals of this sort, of a form equivalent to a simple celL 
When, therefore, the theory of the animal descent of man is condemned as 
a ' horriblo, shocking, and immoral ' doctrine, the unalterable fact, which 
can be proved at any moment under the microacope, that the human egg 


is n simple cell, which is in no way different to those of other mammals, 
must equally be pronounced * horrible, shockiag, and immoral.' " — Stamm 
baum dm Menschengkschlechts (1870.) 

In order clearly to understand Ontogeny, or the evolution 
of the individual Man, the most significant of the man) 
wonderful and varied facts which meet us must first 
be brought into y.>roininence, and then from the important 
points of view thus gained, the innumerable less weighty 
and important phenomena must be explained. The first 
and most important point of view, and, therefore, the 
starting-point of our ontogenetic studies, is the fact that 
every human individual is developed from an entirely 
simple cellular egg. The human egg-cell is, in its wholt 
form and constitution, not essentially different from those 
of other Mammals, though there is some difference between 
the ep-gr-cells of Mammals and those of other animals. 

This most important fact, the fundamental significance 
of which is hardly surpassed by any other, is of recent 
discovery. It was only in 1827 that Baer, by practical 
observation, discovered the human and mammalian egg. 
Before that, the larger vesicles, which in reality contain the 
true and much smaller egg, had been erroneously regarded 
as the eggs. Of course the important discovery that the 
mammalian egg is a simple cell like that of other animals 
could only be made after the establishment of the Cell 
Theory, which was first laid down, with respect to plants, 
by Schleiden, and extended to the animal kingdom by 
Schwann in 1838. The reader is already aware of the 
great importance of the Cell Theory in the complete ex- 
planation of the human organism and its evolution. It 
therefore seems desirable to say a few words as to the 



present position of the cell theory, and as to the views 
commonly held in connection with it. 

Fig. 1. — The human egg from the ovary of the female ; much enlarged 
The entire egg is a simple, globular cell. The greater part of the spherical 
egg-ceW is formed by the egg-yelk, or the granular cell-substance (proto- 
plasm), which is composed of innumerable, delicate yelk-granules, with a 
little intervening substance. The germ ; vesicle, answering to the cell- 
kernel (nucleus) lies in the upper part of the yelk. It contains a dark 
nucleolus or germ-spot. The globular mass of yelk is surrounded by a 
thick transparent egg-membrane (zona pellucida, or chorion). This is 
penetrated by the pore, canals, in the form of very numerous hair-like lines, 
which run radially towards the centre of the globe ; through these the 
thi'ead -shaped, moving sperm-cells pass, in the process of impregnation, into 
the egg-yelk. 

In order rightly to appreciate the Cell Theory, which 


has been regarded during the last thirty-five years as the 
true basis of all morphological and physiological know- 
ledge in Zoology and Botany, it is especially necessary to 
conceive the cell as an integral organism, or, in other words, 
an independent living being. When by dissection we have 
separated the developed body of a Man, or of any other 
animal or plant, into its organs, and when we then proceed 
further to examine by means of the microscope the more 
minute constituents of these larger organs, which give the 
form to the whole organism, we are surprised to find that all 
these various parts are made up of the same fundamental 
constituents or structural elements ; and these are cells. 
Whether we examine anatomically and by means of the 
microscope, a leaf, a flower, or a fruit ; or again, a bone, a 
muscle, a gland, or a piece of skin, etc., we everywhere find 
one and the same form-element, which has been called the 
Cell, since Schleiden gave it that name. Very different 
riews are held as to the real nature of this cell ; but what- 
ever we think of it, it must be regarded as an independent 
life-unit. The tiny cell is, as Briicke says, " an elementary 
organism," or, as Virchow expresses it, a " seat of life * 
(Lebensheerd). It is, perhaps, most accurately described as 
the organic unit of form of the lowest grade, as an indi- 
vidual of the first order (Generelle Movyhologie, vol. L 
p. 269) This unit is such both in anatomical form, and in 
physiological function. In the Protista, in the one-celled 
plants and primitive animals, the whole organism per- 
manently consists only of a single cell On the contrary, in 
most animals and plants, it is only in the earliest period 
of individual existence that the organism is a simple cell ; 
it afterwards forms a cell-society, or, more correctly, an 



organized cell-state. The human body is not in reality a 
simple life-unit, as is at first the universally current, simple 
belief of men. It is, rather, an extremely complex social 
community of innumerable microscopic organisms, a colony 
or a state, consisting of countless independent life- units, of 
different kinds of cells. 41 

The term cell is, in reality, not well chosen. Schleiden, 
who first introduced it as a scientific term in the sense in 
which it is used in the cell theory, named the little element- 
ary organisms " cells," because in a cross-section of most 
parts of plants, they look like chambers, which, like the cells 
of a honeycomb, are massed together, are separated by solid 
walls, and are filled with liquid or a soft pulpy substance. 
This conception of the cell, as held by Schwann, namely, 
that it was a small closed sac, or bladder, filled with a 
fluid, and surrounded by a solid envelope, or Avail, continued 
prevalent for a long time ; but in the case of most of the 
cells in the animal body, it is altogether inapplicable. The 
further the investigation of the cells of the animal body was 
carried, the more evident it became that the cell must be 

Fig. 2. — Ten cells from the liver ; one (b) has two 'cernete. 

Fig. 3. — Three epithelial cells from the mucous membrane of the tongue. 


entirely differently conceived. The cell is now usually 
defined as a small semi-solid or semi-fluid (i.e. neither solid 
nor fluid) dense body, the chemical nature of which is albu- 
minous, and in which another small roundish body, generally 
more solid, and also albuminous, is enclosed. An envelope 
or membrane may exist, as is the case with most plant- 
cells ; but it may be wanting, as in most animal-cells. 
Originally it is never present. The young cells are usually 
roundish in form, but they afterwards vary very greatly. 
The cells from different parts of the human body, repre- 
sented in Figures 2-6, may be compared as examples. 

Fig. 4.— Five thorny, or heckle-cells, the edges of which fit into each 
other, from the epidermis ; one (b) is separated from the rest. 

The most essential feature in the modern conception of 
the cells is, therefore, that the cell-body is composed of two 
distinct parts. The one constituent part is the inner, and 
is called the nucleus (cytvblasbus) ; this is generally of a 
round, oval, or spherical form, usually more solid, seldom 
softer than the protoplasm, and consists of a peculiar 
albuminous substance, the nuclein or kernel-substance ; the 
second essential constituent part of every cell is the cell- 
slime or cell-substance — the protoplasm, or primitive slime 
( Urschleim of the older natural philosophers). This proto- 
plasm, which surrounds the nucleus, also belongs, in point 
of chemical composition, to the class of albuminous sub- 
stances, and is a compound of carbon, containing some 



atoms of nitrogen. It remains throughout life in a soft 
condition of density, or aggregation, neither solid nor fluid. 
The albuminous composition of the protoplasm is similar 
to that of the nucleus, but is yet essentially and constantly 


Fig. 5. — Nine star-shaped bone- cells with branched processes. 

Fig. 6. — Eleven star-shaped enamel cells from a tooth ; they are con- 
nected by their branched processes. 


Nucleus and protoplasm, the inner cell-kernel and the 
outer cell-slime, are the only two essential constituents of 
every genuine cell. Everything else which occurs in and 
on the cell* is of secondary importance, as it develops after- 
wards ; the membrane, which may be variously constituted, 
and is often very thick ; the intermediate cell-mass, or inter- 
cellular substance, which is secreted between the contiguous 
cells ; and also the bodies of various kinds contained in the 
cell, such as fatty particles, crystals, grains of colouring 
matter, water-vesicles, etc. All these are subordinate and 
passive parts, which are formed by the activity of the 
protoplasm or are taken up from without, and are of no 
interest to us at present. The nucleus and the protoplasm 
are the only two active, essential, and always present parts 
of the cell-organism. 

In contrast to the simple cell (Fig. 1, p. 122), let us 
compare with it a large nerve-cell, or ganglion-cell of the 
brain. The egg-cell potentially represents the whole 
animal — that is, it possesses the capacity to develop from 
itself the entire multi-cellular animal body ; it is the 
common mother of all the generations of innumerable cells, 
which form the various tissues of the animal body: in a 
certain sense it unites in itself their various powers, but 
only potentially, only in design. In direct contrast to this, 
the nerve-cell of the brain (Fig. 7) is an extremely one- 
sided formation. It cannot, like the egg-cell, develop 
from itself numerous generations of cells, of which some 
transform themselves into skin-cells, some into flesh-cells, 
and others into bone-cells, etc. But instead, the nerve-cell 
which is formed for the highest activities of life, possesses 
the capacity to feel, to will, to think. It is a true mind- 




Pio. 7. — A large branched nerve-cell, or " mind-cell," from the brain of 

an Electric Fish (Torpedo) ; 600 times the natural size. The large, bright, 
globular kernel (nucleus) lies in the centre of the cell ; this nucleus contains 
a nucleolus, and in that, again, there is a nucleolinus. The protoplasm of 
the cell has separated into innumerable fine threads (or fibrillse), which are 
embedded in the inter-cellular substance, and which pass out into the 
branched processes of the cell. An un branched process (a) passes over 
into a nerve vessel. (After Max Schultze.) 

cell, an elementary organ of mental activity. Correspond- 
ingly, it has an extremely complex minute structure. Innu- 
merable filaments of exceeding fineness, which may be com- 
pared to the numerous electric wires of a great central 
telegraph station, traverse, crossing each other again and 
again, the finely granulated protoplasm of the nerve-cell 
and pass into branched processes, which proceed from this 
mind-cell, and connect it with other nerve-cells and nerve- 
fibres (a, b). It is scarcely possible to trace, even approxi- 
mately, the tangled paths of these filaments in the fine 
substance of the protoplasmic body. 

We thus have before us a highly complex apparatus, 
the more minute structure of which we have hardly begun 
to know, even with the help of our strongest microscope, 
and the significance of which we rather guess than know. 
Its complex mechanism is capable of the most intricate 
psychical functions. But even this elementary organ of 
mental activity, of which there are thousands in our brain, 
is only a single cell. Our whole intellectual life is but the 
sum of the results of the activity of all such nerve-cells or 
mind-cells. In the centre of each cell lies a laro;e trans- 
parent ball, which encloses a smaller dark body. This is 
the nucleus which contains the nucleolus. Here, as every- 
where, the nucleus determines the individuality of the 
cell, and shows that the entire formation, notwithstanding 


its minute and complex structure, is in form only a single- 

In contrast to this highly complex specialized mind- 
cell (Fig. 7) is the egg-cell (Fig. 1), which is in no way 
specialized. Yet here, also, we are obliged to infer from its 
active properties a highly complex chemical composition of 
its protoplasmic substance, and a minute molecular struc- 
ture, which are completely hidden from our eyes. 

The description of these cells as elementary organisms, 
or individuals of the first order, must be somewhat qualified. 
For cells by no means represent quite the lowest grade of 
organic individuality, as that is usually understood. There 
are yet more simple elementary organisms at which we 
will now give a passing glance, in order to return to 
them hereafter. These are cytods : living, independent 
existences which consist merely of an atom of plasson ; in 
other words, of an entirely homogeneous atom of an albu- 
minous substance, which is not yet differentiated into 
nucleus and protoplasm, but exercises the properties of both 
united. For example, the remarkable Monera are cytods 
of this kind. (Cf. Chapter XVI.) Strictly speaking, we 
should say : the elementary organism, or the individual of 
the first order, occurs in two different grades. The first and 
lowest is the cytod, wfyich consists merely of an atom of 
simple plasson. The second and higher grade is the cell, 
which has been differentiated into nucleus and protoplasm. 
Both grades, cytods and cells, are grouped together under 
the idea of sculptors or builders, because they alone in 
reality build the organism. 42 But in higher animals and 
plants, such cytods do not, as a rule, appear, so that only 
actual nucleated cells occur. Here, therefore, the elementary 



individual always consists of two different parts, the outer 
protoplasm and the inner nucleus. 

In order to be thoroughly convinced that every cell is 
an independent organism, it is only necessary to trace the 
active phenomena and the development of one of these tiny 
bodies. We then see that it performs all the essential life- 
function:; which the entire organism accomplishes. Every 
one of these little beings grows and feeds itself indepen- 
mtly. It assimilates juices from without, absorbing them 
from the surrounding fluid; the naked cells can even 
take up solid particles at any point of their surface, and 
therefore eat without using any mouth or stomach. 
(Cf. Fig. 15.) Each separate cell is also able to re- 
produce itself and to increase (Fig. 8). This increase 
generally takes place by simple division, the nucleus parting 
first, by a contraction round its circumference, into two 
parts; after which the protoplasm likewise separates into 
two divisions. The single cell is also able to move and 

Fig. 8. — Blood-cells, which increase by 
division, from the embryo of a young stag. 
Each blood-cell has originally a kernel, and is 
globular (a). When they are about to in. 
crease, the cell -kernel, or nucleus, first separ- 
ates into two kernels (b, c, d). The protoplas- 
mic body then becomes pinched in at a point 
between the two kernel^, which become more 
widely separated from each other (e) . Finally 
a complete separation between the two parts 
is effected at the point where the original cell 
was pinched in, so that there are now two 
cells (/). (After Frey.) 

creep about, if it has room for free motion, and is not pre- 
vented by a solid covering ; from its outer surface, it sends 



out and draws back again, finger-like processes, thereby 
modifying its form (Fig. 9). Finally, the young cell has 

Ftg. 9. — Active cells from the 
inflamed eye of a Frog (from the 
watery moisture of the eye, the 
humor aquevs). The naked cells 
move freely and creep about ; 
like Amoebae and Rhizopods they 
accomplish this by extending deli- 
cate processes from their naked 
protoplasmic bodies. These pro- 
cesses continually alter in number, 
form, and size. . The kernel of these 
amoeboid lymph-cells is not visible, 
being covered b\ the numerous deli- 
cate granules which are scattered 
in the protoplasm. (After Frey.) 

feeling, and is more or less sensitive. It performs certain 
movements on the application of chemical and mechanical 
irritants. Thus we can trace in every single cell all the 
essential functions, the sum of which constitute the idea of 
life : feeling, motion, nutrition, reproduction. All these 
properties which the multi-cellular, highly developed animal 
possesses, appear in each separate cell, at least in its youth. 
There is no longer any doubt about this fact, and we may 
therefore regard it as the basis of our physiological idea of 
the elementary organism. 

Without linp^erino; here over the extremelv interest- 
ing phenomena of cell-life, we will at once attempt to 
apply the Cell Theory to the egg. The comparison which 
we have made leads to the important result that we 
must regard every egg as originally a simple ceil. This 
is of the highest significance, because the whole Science of 


Ontogeny can be demonstrated in answer to the problem : 
" How does a many-celled organism arise from a one-celled 
organism ? ' Every individual organism is originally a 
simple cell, and as such, an elementary organism, or an 
individual of the first order. It is only at a later period 
that this cell gives rise, by division, to a multitude of cells, 
from which the many-celled organism, an individual of a 
higher order, is developed. 

If we next observe somewhat more closely the original 
composition of the egg-cell, we notice the very remark- 
able fact, that in its original condition it is so exactly 
the same in Man as in all other animals, that it is im- 
possible to discover any essential difference. At a later 
period, the eggs, even when they remain one -celled, are 
very different in size and form, have different coverings, etc. 
But, if they are sought in the place where they originate, 
in the ovary of the female animal, these primitive eggs, in 
the first stages of their life, are found to be always of the 
same formation — every primitive egg being originally an 
entirely simple, somewhat round, moving, naked cell,, pos- 
sessing no membrane, and consisting only of the nucleus 
and protoplasm (Fig. 10). These, two parts have long 
borne distinctive names ; the protoplasm being called the 
vitellus, or yelk, and the nucleus the germinal vesicle, 
(vesicula germinativa). As a rule, the nucleus of the egg 
is of a soft, often vesicular texture. Within this nucleus, 
as in many other cells, is enclosed a third body, which in 
ordinary cells is called the nucleolus. In the egg-cell it is 
called the germinal spot (macula germinativa). Lastly, in 
many, but not in all eggs, within this germinal spot, is found 
yet another little point, a nucleolinus, which may be called 




Fig. 10. — Primitive eggs of various animals, performing amoeboid move- 
ments (very much enlarged). All primitive eggs are naked cells, capable of 
change of form. Within the dark, finely granulated protoplasm (egg-yelk) 
lies a large vesicular kernel (the ^erm -vesicle), and in the latter is 
nucleolus (germ-spot) ; in the nucleolus a germ-point (nucleolinus) is oftei 
visible. Fig. A 1 — A 4. The primitive egg of a Chalk Sponge (Leuculmis 
echinus), in four consecutive conditions of motion. Fig. B 1 — B 8. The 
primitive egg of a Hermit-crab (Chnndrocanthvs cornutus), in eight con-e- 
cutive conditions of motion (after E. van Beneden). Fig. 1 — C 5. 
Primitive egg of a Cat, in four different conditions of motion (after Pfluger). 
Fig. D. Primitive egg of a Trout. Fig. E. Primitive egg of a Hen. Fig. 
£, Primitive human egg. 


the germinal point (punctwm germinativum). But these 
last two parts, the germinal spot and the germinal point, are 
only of subordinate importance ; only the first two parts are 
of primary importance, the protoplasm (Vitellus) and the 
nucleus (vesicula germinativa). 

In many lower animals, for example, in Sponges and 
Medusce, the egg-cells retain their entirely simple original 
nature until fertilization. But in most animals they 
undergo certain changes before that time ; they sometimes 
acquire certain additional Protoplasm, which serves for the 
nourishment of the egg (food-yelk), sometimes outer en- 
velopes or membranes, which serve for its protection 
(egg-membranes). An envelope of this sort appears on all 
mammalian eggs in the course of their further develop- 
ment. The little sphere is surrounded by a thick covering 
of a perfectly transparent, glass-like nature, which is dis 
tinguished as the zona pellucida, or chorion (Fig. 11). When 
this is very closely examined under the microscope, very 
fine radial lines may be distinguished, traversing the zona ; 
these are very fine canals. The human egg cannot be 
distinguished from that of most other Mammals either 
in its immature or in its more complete condition. Its 
form, its size, its composition, are approximately the same 
in all. In its fully developed condition, it has an average 
diameter of ^ of a line, or 02 millimetres. If the mam- 
malian egg is properly isolated and held on a glass plate 
toward the light, it appears to the naked eye as a very fine 
point. The eggs of most of the higher Mammals are of 
exactly the same size. The diameter of the spherical egg- 
cell almost always measures from -^ to ^ of a line (J — ^ 
of a millimetre). It has always the same spherical form, 



always the same characteristic thick covering ; always the 
same clear, round germinal vesicle with its dark germinal 
spot. Even under the highest magnifying power of the 

Fig. 11. — A human egg (much enlarged) from the ovary of a female. 
The whole egg is a simple spherical cell. The greater part of this cell is 
formed by the egy-yelk, by the granular cell-substance (protoplasm), con- 
sisting of innumerable yelk-gran ule.s with a little inter-granular substance. 
In the upper part of the yelk lies the bright, globular, germ -vesicle, corre- 
sponding with the cell-kernel (nucleus). This contains a darker germ-spot, 
answering to the nucleolus. The globular yelk mass is surrounded by 
a thick, light-coloured egg-membrane (zona pellucida or chorion). This is 
traversed by very numerous hair-like lines, radiating towards the central 
point of the mass; these are the porous canals, through which, in the course 
of fertilization, the thread-shaped, active sperm-cells penetrate into the 


best microscope, there appears to be no essential difference 
between the eggs of Man, of the Ape, of the Dog, etc. 
This does not mean that they are not really different 
in these different Mammals. On the contrary, we must 
assume that such differences, at least in point of chemical 
composition, exist universally. Even of human eggs, each 
differs from the other. In accordance with the law of 
individual variation, we must assume that "all individual 
organisms are, from the very beginning of their in- 
dividual existence, different, though often very similar." 
(Gen. Morph. vol. ii. p. 202). But with our rough and 
incomplete apparatus we are not in a position actually 
to perceive these delicate individual differences, which 
must often be sought only in the molecular structure. Yet 
in spite of this, the remarkable morphological similarity 
of human and mammalian eggs, which has the appearance 
of absolute similarity, remains a strong argument in favour 
of the common descent of Man and the other Mammals. 
The similar embryo-form bears witness to the common 
parent- form. On the other hand, there are striking pecu- 
liarities by which the ripe mammalian egg may be very 
easily distinguished from the ripe eggs of Birds and other 
Vertebrates. (Cf. the end of Chapter XXV.) 

The ripe egg of the Bird is especially different, although 
as a primitive egg (Fig. 10, E) it was entirely similar to 
that of Mammals. But the egg-cell of the Bird at a later 
period, though while still within the oviduct, absorbs a mass 
of food which it elaborates into the large and well-known 
yellow yelk. If a very young egg from the ovary of a hen 
is examined, it is found to be exactly like the young egg- 
cells of Mammals and other animals (Fig. 10). But it 


afterwards grows so considerably that it expands to the 
well-known yellow ball of yelk. The nucleus, or the germi- 
nal vesicle, of the egg-cell, is thus pressed on to the surface 
of the spherical cell, and is there embedded in a small mass 
of clear, so-called white yelk. This then forms a circular 
white spot, which is called the tread, or cicatricle (cica- 
tricula, Fig. 12, b). From the tread a thin cord of white 
yelk passes through the yellow to £he middle of the round 
cell, where it swells to a little central ball, the falsely-called 
yelk-cavity (latebra, Fig. 12, d). The yellow yelk, which 
surrounds this white yelk, appears in the hardened egg 
in concentric layers (c). The yellow yelk is encircled by 
a delicate structureless yelk-skin (membrana vitellina, a). 

Of late it has been widely believed that the large yellow 
egg-cell of the Bird, which in the case of the largest birds 
reaches a diameter of several inches, cannot be regarded as 
a simple cell. But, with Gegenbaur, we believe this view 
to be erroneous. The unimpregnated and unsegmented egg- 
cell of the Bird, with its simple nucleus, remains a simple 
cell, even though its yellow yelk-substance increases very 
greatly. Every animal which consists of a single cell, every 
Amoeba, every Gregarina, every Infusorial animal, is one- 
celled, and remains so, however much food of various kinds 
it absorbs. In the same way the egg-cell remains a simple 
cell, however much food-yelk it may afterwards collect 
within its protoplasm. Gegenbaur has proved this clearly 
in his excellent work on the embryos of Vertebrates. 43 

The Bird's egg, of course, assumes a different form as 
soon as it is fertilized. Its germinal vesicle, or nucleus, then 
separates by repeated division into many parts, and the 
protoplasm of the tread, which surrounds it, is corre« 



sponclingly divided. At this stage the egg consists of 
as many cells as there are nuclei in the tread. Hence, the 
yellow ball of yelk of the impregnated egg, as it is laid, 
and as we eat it every day, is already a many-celled body. 
Its tread is composed of many cells, and is now dis- 
tinguished as the germ-disc (discus blastodermicus). In 
the eighth chapter we shall refer to this again. 



Fig. 12. — A ripe egg-cell from the ovary 
of a hen. The yellow nutritive yelk (c) is 
composed of many concentric strata (d) and 
is surrounded by a thin yelk-membrane (a). 
The cell-kernel, or germ-vesicle, lies in the 
upper part, in the tread (b). From this the 
white yelk passes into the centre of the 
yelk-cavity (d r ). The two kinds of yelk 
are not, however, distinctly separated. 

After the ripe egg of the Bird (Fig. 12) has left the ovary 
and has been fertilized in the oviduct, it surrounds itself 
with various coverings which are secreted from the inner 
surface of the oviduct. The thick la}^er of transparent 
albumen first forms round the yell6w yelk ; this is followed 
by the formation of the outer calcareous shell, within which 
lies another envelope of skin. All these coverings and 
additions which are gradually formed around the egg, are of 
no importance to the development of the embryo ; they are 
parts that have nothing to do with the original simple egg- 
cell. Even in the case of other animals we often find veiy 
large eggs with thick coverings, — for example, in that of 
the Shark. In this case also the egg is originally exactly 
similar to those of Mammals, that is, it is a simple naked 

cell. But, as in the case of Birds, a considerable quantity 


of food-yelk is collected within the original yelk, as pro- 
vision for the growing embryo : various coverings are 
formed around the egg. The egg-cells of many other 
animals receive similar internal and external additions. 
But as these are always of subordinate importance in the 
formation of the embryo itself, serving either as food, or as 
a protecting covering for the embryo, we may disregard 
them entirely, and turn our attention to the most important 
point, — the essential similarity of the original egg-cells oj 
Man and other animals (Fig 10). 

Let us here make use for the first time of our funda- 
mental biogenetic law, and apply this causal law of develop- 
ment directly to the human egg-cell. This results in an 
extremely simple, but highly important conclusion. From 
the one-celled organization of the human egg and of the 
eggs of the other animals, the conclusion directly follows, 
according to this fundamental law of Biogeny, that all 
animals, including Man, descend originally from a one- 
celled organism. If that fundamental principle is really 
true, if germ-history or the development of the individual 
is an epitome or a brief reproduction of the tribal history or 
the development of the race (and it is impossible to doubt 
this), then, from the fact that all eggs are originally simple 
cells, we must necessarily conclude, that all many-celled 
organisms are descended from a one-celled organism. As, 
however, the original egg-cell has the same structure in the 
case of Man as in that of all other animals, we may reason- 
ably assume that this one-celled original form was prooably 
the common one-celled ancestral organism of the whole 
animal kingdom, including Man. But this last hypothesis is 
ny no means as certain as the former conclusion. 

AMCEBM. 141 

The inference from the one-celled germ-form tc the 
one-celled parent-form is so simple, and yet so full of sig- 
nificance, that it is impossible to lay too much stress upon 
it. Naturally the first question arising is, whether there 
exist at the present day any one-celled organisms, from the 
form of which we may draw an approximately correct 
conclusion as to the one-celled ancestors of many-celled 
organisms ? The answer to this question is undoubtedly 
affirmative. There are most certainly one-celled organ- 
isms now in existence, the whole organization of which is 
but that of a permanent egg-cell ; there are independent 
one-celled organisms, which never undergo any further 
development, which pass their whole lives as simple cells, 
and as such reproduce themselves without attaining to any 
higher development. We now know a great number of such 
one-celled organisms, — for example, the Gregarina, Flagellata, 
Acineta, Infusoria, etc. But one among them is especially 
interesting to us, because it affords the most complete 
answer to our question, and must be regarded as the one- 
celled primary organism which most nearly approaches the 
type-form of the race. This organism is the Amoeba. 

The name Amoebse has long been applied to a number of 
microscopic one-celled organisms, which are by no means 
rare, and are indeed widely distributed, principally in fresh 
water, but also in the ocean ; lately they have been found 
inhabiting moist earth. When one of these living Amcebse 
is placed in a drop of water under the microscope and 
greatly magnified, it appears to be a roundish body of very 
irregular and varying form (Fig. 13). Enclosed in the soft/ 
slimy, half-fluid body, which consists of protoplasm, we can 
>nly see a small solid or vesicular substance, which is the 



nucleus. This unicellular body now begins to move, and 
crawls about in various directions on the glass, on which we 
are observing it. The shapeless body accomplishes these 

Ftg. 13. — A creeping Amoeba (much en. 
larked). The fortn-value of the whole or- 
ganism is that of a simple leaked cell, which 
moves about by means of variable processes, 
sometimes extended from the protoplasm of 
its body, sometimes drawn in. In the centre 
is the round kernel, or nucleus, with its nu- 

movements by extending finger-like 
processes from various points of its 
surface, which are moved in slow but 
constant alternations, and draw the rest of the body after 
them. After a time something new is seen ; the Amoeba 
suddenly stands still, draws in its processes, and assumes 
a spherical form. But soon the little slimy ball begins to 
spread out again, and stretches its processes in different 
directions, and moves forward again. These variable pro- 
cesses are called false-feet (Pseudopodia), because they 
perform the office of feet, and yet are no special organs, in a 
morphological sense; for they vanish as quickly as they 
appear, and are only variable extensions of the semi-fluid, 
homogeneous, and structureless substance of the body. 

If one of these creeping Amoebse is touched with a 
needle, or if a drop of acid is added to the water, the whole 
body at once contracts in consequence of this mechanical or 
chemical irritation. Usually it reassumes its spherical 
form. Under certain circumstances, for example, if the 
impurity is retained in the water, the Amoeba begins to 
encase itself. It exudes a homogeneous envelope, or cap- 


sule, which immediately hardens, and in a state of repose 

assumes the form of a spherical cell surrounded by a pro- 
tecting membrane. The one-celled Amoeba obtains its 
food, either by absorbing dissolved substances directly from 
the water by imbibition, or by pressing into itself solid 
particles of foreign matter with which it comes into contact. 
The latter operation can be observed at any time if it is 
maple to eat. If finely pulverized colouring matter, such as 
carmine or indigo, is placed in the water in very small 
quantities, the soft body of the Amoeba can be seen to 
assimilate these particles of colouring matter, over which 
the soft substance of the cell flows together. The Amoeba 
can take food in this way at any point of the surface of its 
body, although it possesses no special organs for taking in 
and digesting nutritive matter, no true mouth or stomach. 
By means of this assimilation of nutriment and dissolving 
the particles in its protoplasm, the Amoeba grows; and, 
after it has reached a certain size by this process, it begins 
to reproduce. This occurs in the simplest way, by division. 
The enclosed nucleus first separates into two pieces. Then 
the protoplasm distributes itself between the two new 
nuclei, and the whole cell parts into two similar cells, in 
consequence of the growth of the protoplasm round the two 
nuclei. This is the usual method of propagation ; the 
nucleus first divides into two halves, which separate from 
each other, and act as centres of attraction to the surround- 
ing cell-substance or protoplasm (Fig. 8). 

Though the Amoeba is, therefore, only a simple cell, it 
shows itself capable of performing all the functions of a 
many-celled organism. It moves itself by creeping, it feels, 
it feeds, it reproduces its kind. Some species of Amoebse 



are quite visible to the naked eye ; but the greater number 
are microscopic. Our reasons for regarding the Amcebse as 
the particular one-celled organisms, the phylogenetic rela- 
tions of which to the egg-cell are of peculiar importance, 
will be evident from the following facts. In many lower 
animals, the egg-cell remains in its original, naked condition 
till it is fertilized; it acquires no covering, and is .often 
indistinguishable from an Amoeba. Like the latter, these 
naked egg- cells can extend processes and move about. In 
the Sponges, these active egg -cells creep freely about, as 
though they were independent Amoebae (Fig. 14), even 

Fig. 14. — Egg-cell of a Chalk Sponge (Olyn- 
thus). The egg-cell moves and creeps about within 
the Sponge, by means of variable processes which 
it extends. It is not distinguishable from the 
common Amoeba. 

within the parent organism. In this 
condition they were observed by earlier 
naturalists, and were mistaken for 
Amoebse, living as parasitical intruders in the body of 
the Sponge. It was only afterwards that it. was dis- 
covered that these supposed one-celled parasites were in 
reality the egg-cells of the Sponge itself. This remarkable 
phenomenon is also found in other lower animals, for ex- 
ample, in those pretty bell-shaped Plant-animals (Medusce) ; 
the eocrs of these also remain as naked, uncovered 
cells, which stretch out amoeboid processes, feed themselves, 
move, and from which, after fertilization, the many-celled 
Medusa-organism is indirectly or directly developed by 
repeated division. 

It is, therefore, certainly no wild hypothesis, but an 



entirely sober conclusion, which regards the Amoeba as the 
particular one-cellecl organism which gives us an approxi- 
mate representation of the ancient one-celled ancestral 
form common to ah manv-celled organisms. The naked, 
simple Amoeba possesses a less differentiated and more 
primary character than most other cells. To this may be 
added the circumstance, that similar amoeboid cells can be 
shown in the full-grown bodies of all many-celled animals. 
For example, they occur as the so-called white blood-cor- 
puscles among the red blood-cells (corpuscles) in human 
blood, and in that of all other Vertebrates. They also occur 
in many Invertebrate animals ; for instance, in the blood of 
the Snail ; and in 1859 T showed that these colourless blood- 
corpuscles, like independent Amoeba?, can assimilate solid 
particles, can, therefore, eat (Fig. 15). Lately, it has been 
found that very many different cells, if they have room, 

Fig. 15. — Devouring blood-cells of a Naked Sea-snail (Thetis) very 
much magnified. In connection with the blood-cells of this snail, I was 
the first to observe the important fact that " the blood. cells of invertebrate 
animals are uncovered lumps of protoplasm, and, like the Amoebae, by 
means of their peculiar movements can absorb matter," can, therefore, 
" eat.'' When at Naples {on the 10th of May, 1859) I had injected the 
blood-vessels of one of these Snails with pulverized indigo dissolved in 
water, I was much astonished to find after a few hours that the blood 
cells themselves were more or less filled with fine particles of indigo. By 
repeated experimental injections, I was able to watch the absorption of the 
colouring matter into the blood -cells, which was accomplished exactly as by 
Amoeba?. (See " Monograph of Radiolaria," 1862, pp. 104, 105.) 


are able to move, to eat, and to act entirely like Amoebee 
(Fig. 9). 

Trie capacity of the naked cell to make these character- 
istic amoeboid movements depends on the contractility (or 
automatic movableness) of the protoplasm. This seems to 
be the universal property of all young cells. Where they 
are not surrounded by a strong membrane, or shut up in a 
" cell prison," they are all capable of amoeboid movements. 
This is as true of the uncovered egg-cell as of other un- 
covered cells, of the moving cells of various kinds, lymph- 
cells, mucous cells, etc. 

Our examination of the egg-cell and comparison of it 
with the Amoeba, has afforded us the best and surest basis 
for the history of the germ as well as for the history of the 
tribe. From it we have drawn the conclusions that the 
human egg is a simple cell ; that this egg-cell is not essen- 
tially different from those of other Mammals, and that we 
must therefore infer the existence of a primeval one-celled 
ancestral form, which in all essential points was of amoeboid 

The assertion that the first ancestors of the human race 
were simple cells of this sort, which, like the Amoeba, led 
an independent one-celled life, has not only been ridiculed 
as an empty scientific chimera, but has also been indig 
nantly rejected in theological periodicals as " horrible, 
shocking, and immoral." But, as I have already remarked 
in my lectures "On the Origin and Genealogy of the Human 
Race," the same righteous indignation must fall with equal 
justice on the " horrible, shocking, and immoral " fact, that 
every human individual develops from a single cell, and 
that this human egg-cell cannot be distinguished from 


those of other Mammals. This fact can be demonstrated 
at any moment under the microscope, and it is useless to 
close our eyes to this " immoral " fact. It remains as 
incontrovertible as the important conclusions which we 
have linked with it. 

The very important bearing which the Cell Theory has 
on the whole conception of organic nature is thus very 
clearly seen. The " place of man in nature " is radically 
explained by it. Without this theory, Man is an unin- 
telligible puzzle. Philosophers, therefore, and certainly 
psychologists, ought especially to acquaint themselves 
thoroughly with the Cell Theory. The human mind can 
only be really understood by means of this theory, and its 
simplest form is illustrated in the Amoeba. 

The extant Amoebae and the kindred one-celled organ- 
isms, Arcellae, Gregarinse, etc., are therefore of great 
interest, because they show us the simple cell in a per- 
manently independent form. The human organism and 
that of other higher animals, on the contrary, is only one- 
celled in its earliest, immature condition. As soon as the 
egg-cell is fertilized, it multiplies by division and forms a 
community, or colony of many social cells. These dif- 
ferentiate themselves, and by their specialization, by various 
modifications of these cells, the various tissues which com- 
pose the various organs are developed. The developed 
many-celled organisms of Man and of all higher animals 
resembles, therefore, a social, civil community, the numerous 
single individuals of which are, indeed, developed in various 
ways, but were original ly only simple cells of one common 



Development of the Many-celled from the One-celled Organism. — The Cell- 
hermit and the Cell-state. — The Principles of the Formation of th« 
State. — The Differentiation of the Individuals as the Standard of Measure- 
ment for the Grade of the State. — Parallel between the Processes of 
Individual and of Race Development. — The Functions of Evolution. — 
Growth. — Inorganic and Organic Growth. — Simple and Complex Growth. 
— Nourishment and Change of Substance. — Adaptation and Modification. 
— Reproduction. — Asexual and Sexual Reproduction. — Heredity. — Divi- 
sion of Labour, or Differentiation. — Atavism, or Reversion. — Coalescence. 
— The Functions of Evolution as yet very little studied by Physiology, 
and henoe the Evolutionary Process has often been misjudged. — The 
Evolution of Consciousness, and the Limits to the Knowledge of Nature. 
— Fitful and Gi'adual Evolution.— Fertilization. — Sexual Generation. — 
The Egg-cell and the Sperm-cell. — Theory of the Sperm-animals. — 
Sperm -cells a form of Whip-cell. — Union of the Male Sperm-cell with 
The Female Egg-cell. — The Product of this is the Parent-cell, or 
Cytula. — Nature of the Process of Fertilization. — Relation of the Kernel 
(Nucleus) to this Process. — Disappearance of the Germ-vesicle. — Mone- 
rula. — Reversion to the Monera-f orm. — The Cytula. 

" If the man of science chose to follow the example of historians and 
pulpit-orators, and to obscure strange and peculiar phenomena by employing 
a hollow pomp of big and sounding words, this would be his opportunity ; 
for we have approached one of the greatest of the mysteries which surround 
the problem of animated nature and distinguish it above all other problems 
of science. To discover the relations of man and woman to the egg-cell 
would be almost equivalent to solving all those mysteries. The origin and 
development of the egg-cell in the body of the mother, the transfer to it 


by means of the seed, of the physical and mental characteristics of the 
father, affect all the questions which the human mind has erer raised in 
regard to existence." — Rudolph Vikchow (1848). 

The discovery that every human being at the beginning 
of his existence is a simple cell, that this egg-cell is essen- 
tially similar to those of other Mammals, and that the 
forms arising during the evolution of this cell in Man and 
in the other higher Mammals, are at first similar, — supplies 
a basis from which we may trace the further ' processes of 
evolution. In the first place we have convinced ourselves 
of a fact which is of great importance to the empiric side 
of the history of development, relating to those ontogenetic 
facts which can be directly traced by means of the micro- 
scope ; and this fact is that in Man as well as in other 
animals the developed many-celled organism with all its 
various organs proceeds from a simple cell. Secondly, as 
regards the phylogenetic side of the question, the specu- 
lative part of the History of Human Development, which 
is based on those facts, we have reached the conclusion 
that the original ancestral form of Man as of the other 
animals was a one-celled organism. The whole difficult 
problem of the History of Evolution is thus now reduced 
to the simple question : u How has the complex many-celled 
organism arisen from the simple one-celled form ? By what 
natural process has the simple cell been transformed into 
that complex life-apparatus with all its various organs, the 
apparently rational and purposive construction of which we 
admire in the developed body ? " 

Turning now to answer this question, we must bear in 
mind the view to which we have already alluded, that the 
many-celled organism is ordered and constituted on the 


same principles as a civilized state, in which the several 
citizens have devoted themselves to various services directed 
towards common ends. This comparison is of the greatest 
service in enabling us thoroughly to understand the con- 
struction of Man from many cells of various kinds, and to 
understand also the harmonious co-operation of these 
various cells for an apparently pre-conceived purpose. If 
we bear this comparison in mind, and apply this significant 
idea of the developed many-celled organism as a civil union 
of many individuals, to the history of the evolution of this 
organism, we shall obtain a correct view of the real nature 
of the first and most important processes of evolution. We 
can even, on deeper reflection, guess the first stages of 
development, and establish them a priori, before we call 
observation, a posteriori knowledge, to our aid. 

For once we will reverse the process, and will not, as 
will be the case hereafter, first observe the facts of Ontogeny 
and then attach their phylogenetical significance to them. 
Beginning at the other end, let us here try to guess the 
course which evolution must have taken, if the comparison 
is well founded. Then if, afterwards, the facts of Ontogeny 
confirm our preconception, we shall be yet more firmly 
convinced of the truth of our views on Phylogeny. This 
agreement will afford us a more striking justification of our 
views than can be gained in almost any other way. 

Let us therefore first answer this question : " Granting 
the correctness of the fundamental law of Biogeny, how 
would the original one-celled organism which founded the 
first cell-state, and thus became the ancestor of the higher, 
many celled animals, — how must that organism have acted 
at the beginning of organic life on the earth, or at the 


beginning of creation, as it is usually expressed ? " The 
answer is very simple. It must have acted just as a man 
who founds a state or a colony for a given purpose. Let 
us trace this process in its simplest form, as, for example, 
may have easily taken place when any of the remote 
islands in the Pacific Ocean were first peopled. Two South 
Sea Islanders, a man and a woman, have gone in a boat 
to fish; they are overtaken by a storm, carried far away, 
and at length driven on to a remote island, as yet unin- 
habited. This " first human pair," remaining isolated, play 
the parts of Adam and Eve, and produce a numerous pos- 
terity, thus becoming the parents of the future inhabitants 
of the island. As they are entirely devoid of all resources, 
without the many means of support possessed by the 
founders of states of advanced civilization, the posterity 
of this uncivilized and isolated pair have first developed 
as genuine savages. Their only purpose in life for cen- 
turies has remained as simple as that of the lower animals 
and plants ; the simple aim of self-preservation and of the 
production of descendants ; they have been contented with 
the simplest organic functions, nutrition and reproduction. 
Hunger and love are their only motives of action. 

For a very long period, these savages, scattered over the 
whole island, must have aimed at the one single object 
of self-preservation. Gradually, however, several families 
collected at certain places, larger communities arose, and 
now many reciprocal relations began to arise between 
individuals ; in consequence, a rude division of labour took 
place. Certain savages continued to fish and hunt, others 
began to cultivate the ground, others devoted themselves 
to religion and medicine, which now began to develop^ 


and so on. In short, the ever-increasing division of laboui 
specializes the people into various ranks or castes, which 
always tend to become more sharply denned in propor- 
tion as the state becomes more highly developed : all 
follow diverse occupations, and yet work for a common end. 
In this way, from the descendants of a single human pair, 
a simple community of individuals, originally alike, first 
gradually arises, and this is followed by a more or less 
well-organized confederation. In this community, we. may 
regard the more or less complete division of labour among 
individuals, or the so-called specialization, as the standard 
by which the grade of development of its culture may be 

A process similar to this, and the details of which each 
can easily fill up for himself, took place millions of years 
ago, when, at the beginning of orgauic life on the earth, 
one-celled organisms at first developed, and were afterwards 
followed by many-celled forms. 

The single cells which arose by reproduction from the 
oldest parent-cells must at first have lived in an isolated 
condition ; each one performed the same simple offices as 
all the others; they were satisfied with self-preservation, 
nutrition, and reproduction. At a later period isolated 
cells gathered into communities. Groups of simple cells, 
which had arisen by the continued division of a single 
cell, remained together, and now began gradually to perform 
different offices in life. The first traces of specialization, or 
division of labour, soon occurred, as one cell assumed one 
office, another another. One set of cells may have devoted 
themselves especially to the absorption of food, or nutrition; 
other cells may have busied themselves only with repro- 


duction ; and others, again, have formed themselves into 
protecting organs for the little community, and so on. In 
short, various classes or caster must have arisen in the 
cell-state, following diverse occupations and yet working 
together for the common end. In proportion as this 
division of labour progressed, the many-celled organism* 
or the specialized cell-community, became more perfect or 

We may follow the comparison further. It may be 
asserted a priori, that in consequence of the reciprocity of 
relations which was occasioned by the struggle for existence 
and the gathering of many organic individuals in a common 
dwelling-place, when organic life first began on the earth, 
a community of many similar individuals arose from a one- 
celled organism ; that a division of labour afterwards took 
place among these similar cells, and that finally, in conse- 
quence of continuous specialization, a developed many- 
celled organism with many different organs, all working 
for a common end, arose. In order fully to realize the value 
of this significant comparison, it would be necessary to 
enter in detail into the theory of the division of labour, or 
specialization, which now plays a very important part in 
Biology, especially since ' Darwin's Theory of Selection, has 
enabled us to understand the true causes of these phe- 
nomena. At present I must refer for the more detailed 
elaboration, which would carry us too far to be entered 
into here, to Darwin's Doctrine of the Divergence of Cha- 
racter, and to my lecture on the Division of Labour. We 
shall hereafter return to this subject. 44 

At present we will rather examine whether the & priori 
views on Phylogeny which we preconceived, are in accord- 


ance with the facts which Ontogeny places before us ; 
whether in the evolution of the individual organism 
from the egg-cell, the same phenomena appear, which we 
have presupposed as necessary in this comparison. The 
ontogenetic structural process proves to be in very close 
harmony with our conclusions, and we find that the facts ol 
the evolution of the individual which can be seen under the 
microscope, do in fact correspond perfectly with the picture 
of the process of phylogenetic evolution which we have 
sketched & priori. The first processes which occur in the 
evolution of the individual from the egg-cell, and also the 
succeeding simple processes which first come under observa- 
tion, really correspond to the events which we have just 
traced in the development of a colony of savages, and have 
assumed as the first phylogenetic processes in the origin oi 
a many-celled organism. 

In the first stage of the evolution of the individual, 
many homogeneous cells first arise, from the simple egg-cell, 
by continuous division. These are exactly comparable to 
a community of human beings as yet uncivilized. These 
homogeneous cells increase still more, so that the accu- 
mulation of cells ever increases. As in making our 
comparison we found that an entire colony of savages pro- 
ceeded from the descendants of a single isolated human 
pair, so likewise all the homogeneous cells of this multitude 
(which we shall hereafter learn to know better under the 
name of cleavage-globules), are inter-related as the de- 
scendants of a single pair of cells. Their common father 
is the male sperm-cell, and their common mother the female 
*gg-celL J 

At first, all these numerous cells which arise by the con 


fcinuous division of the fertilized egg-cell, are exactly alike, 
and cannot be distinguished from each other. But gra- 
dually a division of labour occurs among them by their 
assuming different offices. Some accomplish nutrition, others 
reproduction, others protection, others locomotion, and so 
on. We may translate this into the language of the theory 
of the tissues and say : some of these cells become intestinal 
cells, others muscle-cells, others, again, bone-cells, nerve-cells, 
cells of the sense-organs, of the reproductive organs, etc. 
Thus we see that the whole course of the evolution of the 
individual corresponds in its essential features to that pre- 
supposed course of phylogenetic development, and thus 
affords a striking confirmation of our fundamental law of 

This observation naturally leads to a brief examination 
of the physiological functions, or vital activities, which are 
concerned in the evolution of the individual as in that of 
the race. At first sight a great number of complex pro- 
cesses seem to blend and co-operate here ; all of these can, 
however, in reality be reduced to a few simple organic 
functions. These vital activities are: (1) Growth; (2) 
Nutrition ; (3) Adaptation ; (4) Reproduction ; (5) Heredity ; 
(6) Division of Labour, or Specialization; (7) Atavism; 
(8) Coalescence. Heredity, Adaptation, and Growth are of 
especial importance in the evolution of the organic body ; 
these must, therefore, be regarded as especially formative 

Of all \ ital phenomena, growth may be regarded as the 
one which plays the chief part in the evolution of the 
individual organism, and as the really fundamental function 
of evolution. The bearing of this function on the evolu- 


tion of the germ is so great, that Baer expressed the most 
general result of his researches in the following proposition •. 
" The history of the evolution of an individual is the his- 
tory of the growth of individuality in every relation." 
Whenever a unit, an individual, develops in nature, growth 
is the first condition. This is equally true of inorganic 
(inanimate) and of organic (animate) natural bodies. ' In 
the former, in minerals, growth is often the only function 
of evolution. Growth is, therefore, especially interesting, 
because both in the inorganic individual, the crystal, and 
in the simplest organic individual, it is the necessary pre- 
liminary to all further evolution. Growth, the addition 
of homogeneous body-substance, is absolutely universal 
The inorganic crystal grows by absorbing homogeneous 
matter from the surrounding fluid medium, which then 
passes from a fluid into a solid condition. Similarly, the 
cell, the simplest organic individual, grows by attracting to 
itself particles from the surrounding medium, which is 
usually fluid, and by then transforming these particles into 
a semi-fluid, and more or less homogeneous condition 
(assimilation). The only difference between the growth of 
the crystal, and that of the simplest organic individual, the 
cell, is that the former adds the new substance externally, 


while the latter absorbs it internally. This essential differ- 
ence depends on the different conditions of density, or of 
aggregation, of the two different groups of bodies. The 
inorganic bodies may be either in a solid, fluid, or gaseous 
condition. They grow by apposition. Organic bodies, on 
the contrary, are in the fourth, the soft or semi-fluid con- 
dition of aggregation. They grow by intussusception. , 
Each individual or trophic growth is, however, only the 

GROWTH. 157 

simple or direct form of growth common to crystal and to 
simple organic individuals of the first order. This simple 
form of growth is secondarily opposed to compound or 
numerical growth, which is seen in the course of the evolu- 
tion of all many-celled organisms, in all individuals of the 
second, or higher order. In this case, the simple cell does 
not continually increase, as might be supposed, until the 
whole large organic individual, with all its parts, is formed ; 
but after the cell has attained a certain, very limited size, 
it does not increase further, but parts by self- division into 
two cells. Owing to the frequent repetition of this pro- 
cess of compound growth, a many-celled organism, which 
is far larger than the largest cell, at last arises. In this 
case, the growth of the ever-increasing organism is no 
longer the mere addition of homogeneous parts, but depends 
really on generation, i.e., the multiplication of the origin- 
ally simple individual. 

A further distinction between organic and inorganic 
growth depends on the fact that the former, unlike the 
latter, is connected with nutrition. Nutrition is necessary 
to the existence of every living organism, for loss of sub- 
stance of body-material is implied in all life-energies ; and 
this loss of substance must be replaced by the addition of 
new substance or food. This continual change of sub- 
stance, the absorption and assimilation of new matter, 
the expulsion of used-up particles, and briefly, all the 
processes included by the term nutrition, are conditions 
as necessary to the accomplishment of evolution as for all 
the other activities of life : they are as indispensable to the 
evolution of the single cell as to that of the entire many- 
celled organism. The usual method of nutrition in the 


case of the single cells is by the absorption by their soft 
semi-fluid cell-substance of food-material from the sur- 
rounding fluid; less frequently solid particles are pressed 
into the cell -substance. Similarly, the worn-out material 
is discharged, usually in a fluid, seldom in a solid form. 

Adaptation, the most important vital function, is 
directly connected with nutrition, and plays the most im- 
portant part in the progressive development of the organism. 
It is, in reality, the most influential cause of every advance 
and of all perfection of the organism. Adaptation effects 
all the modifications or variations which organic forms 
undergo under the influence of the external conditions of 
existence; it is the true cause of every modification. As 
I have very fully discussed the importance of modification 
and the various laws of Adaptation in my Generelle 
Morphologie, and in the " History of Creation," I may here 
dispense with any further reference to it. I shall only call 
attention to the fact, that all these various laws of Adapta- 
tion can appropriately be brought into the two classes that 
I have there distinguished; on the one side indirect, or 
Potential Adaptation, on the other direct, or Actual Adap- 
tation. I have shown in my Generelle Morphologie (vol. 
ii. pp. 193-226), that all these varied and important phe- 
nomena, if regarded from a physiological point of view, can 
be reduced to the mechanical function of nutrition, and, 
indeed, to the elementary conditions of cell-nutrition. 

Just as progressive Adaptation is linked with nutrition, 
so is conservative Heredity linked with reproduction. This 
latter activity of the organism may also be referred to the 
former functions. For radically " reproduction is a form of 
nutrition and a growth of the organism to a size beyond 


that belonging to it as an individual, so that a part is thus 
elevated into a (new) whole ' : (Generelle Morphologie, vol. 
ii. p. 16). The functions of growth and reproduction are 
therefore very intimately connected. Reproduction is only 
a* continuation of the growth of the individual. But the 
latter, again, depends in its compound form, on generation, 
that is, on the multiplication of the simple constituent indi- 
viduals. While, on the one hand, reproduction appears to 
be only a growth of the individual to a si»ze exceeding that 
of the individual, — compound growth, on the other hand, 
is the result of the reproduction of simple individuals of 
the first order. This view enables us clearly to understand 
reproduction and, consequently, Heredity, which otherwise 
appears to be an obscure and mysterious process. 

To prove the correctness of this view, we must start 
from the simplest form of reproduction, that is, division, as 
it occurs in the case of almost every cell. When the cell, 

Fig. 16. — Blood-cells (corpuscles), increas- 
ing by self-division, from the blood of the young 
embryo of a stag. Each has originally a kernel 
(nucleus), and is globular (a). When the cells 
are about to multiply, the kernel first separates 
into two (6, c, d). The protoplasmic body then 
becomes pinched in between the two kernels, 
which separate more and more from each 
other (e). Finally the cell parts into two, at 
the point where it was pinched in (/). (After 

having, by the absorption of nutrition, already reached its 
usual size, exceeds that measure, it divides into two cells 
(Fig. 16). Just in the same way in many-celled animals (for 
example, Corals), when the individual grows beyond the 


definite size proper to it, a separation into two new 
individuals necessarily takes place. Starting from this 
simplest form of reproduction, we can learn to understand 
the many complex forms with which we meet, especially in 
the lower animals and plants. Division is first followed by 
propagation by buds, then that by the formation of germ- 
buds, and propagation by germ-cells, or spores. All these 
forms of multiplication are classed under the name of 
asexual reproduction, or Monogeny ; in these cases it does 
not require the union of different individuals to effect the 
production of new, independent individuals. 45 

The conditions of sexual reproduction, or Amphigony, 
are quite different. Its nature consists in this ; that two 
distinct cells must unite in a particular way and blend in 
order to cause the production of a new individual. As we 
shall soon return to the subject of sexual reproduction, 
when we consider the fertilization of the egg, we need not 
here linger over it. We must only emphasize the fact, that 
this process of sexual reproduction, in spite of its peculiarity, 
is yet nearly related to the higher forms of asexual repro- 
duction, and especially to that by the formation of germ- 
cells. But while in the latter case a single cell separates 
from the confederacy of the many-celled organism and 
forms the foundation of a new individual, — in the former, 
two different elementary individuals, a female egg-cell and 
a male sperm -cell, must unite and blend into a single body 
to effect that purpose. The double cell formed in this way 
is alone capable of forming by division an aggregate of cells, 
from which a new many-celled organism then develops. 48 
(Cf. Chap. XXV.) 

Immediately connected with reproduction is a fifth 


highly important evolutionary function, Heredity. Just 
as we were able to trace Adaptation back to nutrition, we 
can also show that Heredity is a necessary phenomenon 
of reproduction ; and this is equally true of both kinds 
of Heredity — of conservative, as well as of progressive 
Heredity. As I have also fully explained these highly 
important Laws of Heredity, which maintain constant 
reciprocal relations with the Laws of Adaptation, in my 
"History of Creation," vol. i. Chapter VIII. p. 175, we 
will not stop to examine them here. (See also Generelle 
Morphologie, vol. ii. pp. 170-191.) 

Division of labour, or differentiation, which has but 
recently begun to be correctly valued, forms a sixth 
evolutionary function of especial importance. We have 
already seen that division of labour is the strongest impulse 
towards progressive evolution, not only in civic and social 
life, but also in the social cell-confederacy of every many- 
celled organism. A glance at any community or state 
organization shows that the first condition of all hiofher 
development and civilization, is, on the one hand, the divi- 
sion of the various duties among the various classes of the 
citizens ; and, on the other hand, the co-operation of these 
single individuals for the common purposes of the state. 
This is exactly the case also in every many-celled organism. 
Every multicellular individual in the plant or animal 
kingdom is more perfectly developed, and ranks higher in 
proportion as the division of labour among its constituent 
cells, the differentiation of Its cell-individuals, is more 
perfect. Therefore in the various classes of organisms we 
find this differentiation, sometimes in a more, sometimes in 
a less perfect condition. The simplest form of division oi 


labour occurs in those lower animals in the bodies of which 
only two kinds of cells have become differentiated. This 
is the case, for example, in the lowest Plant-animals, in 
Sponges, and the simplest Polyps, as well as in their 
common parent-form, the Gastrsea. Throughout the entire 
many-celled bodies of these, there are only two different 
kinds of cells ; the one kind effect the nutrition and repro- 
duction of the animal, the other kind are its organs of 
feeling and motion. These two kinds of cells are identical 
with those which first come to perfection in the first process 
of differentiation of the germ-layers in the human embryo. 
But in most higher animals the differentiation of the cells 
proceeds much further. Some take merely the office of 
nutrition ; others that of reproduction ; a third group con- 
stitute the outward covering of the body and form the 
skin ; a fourth group, the muscle-cells, form the flesh ; a 
fifth group, the nerve-cells, develop into the organs of 
sensation, of will, of thought, etc. All these different kinds 
of cells originally proceeded by differentiation or specialisa- 
tion from the simple egg-cell, and from the homogeneous 
descendants of that egg-cell, owing to division of labour. 
This differentiation of the cells, or this division of labour, 
originally arose in tribal history, from causes similar to the 
division of labour in the civilized states of men. Afterwards 
it appears in the germ-history, and by that time it has been 
made over to Heredity, and is merely repeated in accord- 
ance with the fundamental law of Biogeny. Now, although 
Differentiation usually leads to the progress of the whole 
organism as well as of its various constituent individuals, 
the single cells, yet it is also in many cases the occasion of 
retrogression, or atavism. Not only progressive, but also 


retrograde modifications take place in consequence of 
division of labour. 

Atavism, or reversion, must be regarded as a seventh 
function of evolution, and, as such, plays no unimportant 
part. In the evolution of almost every higher organism we 
observe that the progressive completion of most organs is 
accompanied by retrograde processes of evolution in single 
parts. In the cell this retrograde metamorphosis usually 
first occurs in consequence of the formation of fat-particles 
in the protoplasm. The cell is destroyed by the fatty 
degeneration of the protoplasm. During the course of 
phylogenetic, as of ontogenetic evolution, whole organs may 
thus retrograde by the dissolution of the cells which form 
them. Thus, for example, during the evolution of the germ 
of Man and of other Mammals, cartilages, muscles, etc., dis- 
appear which were of great importance in our primitive 
ancestors, the Fishes. This ontogenetic reversion reproduces, 
owing to Heredity, a corresponding phylogenetic process. 
The very interesting " rudimentary organs " are arrested — 
bodily growths of this kind, traces of which still remain in 
various stages of development (see p. 110). They are found 
in nearly every higher many-celled organism attaining to 
any considerable stago of evolution ; in this case the general 
progress of the whole is scarcely ever conditional on the 
equally progressive development of the cells ; on the con- 
trary, certain cells perish during Ontogeny, while others 
go on growing at their expense. This same phenomenon 
is met with in human society. In this it is always the 
case that many individuals perish without effecting any- 
thing; while the majority constantly develop more or 
less steadily. The comparison is perfectly apt. For the 


conditions of aggregation are the same in states as in 
many-celled organisms. 

Finally, we must mention an eighth and last function 
of organic development, viz. coalescence, or concrescence. 
As yet, this has been but little noticed, nor is it very 
striking; yet it is of real importance in certain processes. 
Coalescence consists in this, that two or more individuals 
which were originally separate afterwards combine and 
blend into one individual. We may regard the process 
of sexual generation as a coalescence of two cells. We also 
often find a similar coalescence of cells in other processes of 
evolution. Those tissues of the animal body which dis- 
charge the highest functions, viz. the muscular tissue, or 
flesh, which is concerned in locomotion, and the nervous 
tissue which performs the functions of sensation, will, and 
thought, consist in great part of coalescent cells. But not 
only cells, or individuals of the first order, but also 
organs, or individuals of the second order, coalesce very 
freely in the process of Ontogeny into a compound 
formation. Even independent organisms may coalesce, as 
is very often the case, e.g. in the Sponges. The process 
of coalescence (often also called conjugation or copulation), 
is in a certain sense the opposite process to that of propaga- 
tion. In the latter two or more new individuals arise 
from one, while in the former one individual results from 
several. As a general rule, this individual possesses a higher 
function than that of the two units from the coalescence of 
which it sprang. 

In reviewing for a moment the different vital activities 
of the organism which we have here enumerated as the 
e&sential functions of evolution— as the true formative forces 


of the nascent organism — it will easily be seen that they 
all admit of purely physiological investigation. And yet 
till very recently many of them were never closely studied, 
and consequently the processes of evolution have very often 
been regarded as something altogether enigmatical and 
peculiar, and even in some respects miraculous and super- 
natural. So that even yet many distinguished naturalists 
hold that the phenomena of evolution are beyond the limits 
of human knowledge, and are only explicable by the as- 
sumption of supernatural forces. 

This curious situation, reflecting as it does a somewhat 
unpleasant light upon the present status of our science, 
must be laid to the charge of modern Physiology. As I 
have already had occasion to remark, the Physiology of our 
day pays no attention either to the functions of evolution 
or to the evolution of the functions. With praiseworthy 
energy it has, it is true, exerted itself to perfect as far as 
possible the knowledge of certain groups of functions, to 
which an exact mathematical and physical treatment is 
directly applicable {e.g. the Physiology of the sense-organs, 
of muscular movement, of the circulation of the blood, etc.). 
But, on the other hand, it has paid but little attention to 
many important groups of functions, to which this exact 
method is not applicable. Among the latter are the choro- 
logical and cecological functions, many psychological pheno- 
mena and correlations of growth, and especially the most 
important of those functions of evolution which we have just 
enumerated — that of Heredity and Adaptation. Our present 
knowledge of these two most influential physiological 
functions of evolution has been almost entirely acquired 
by means of morphological, not physiological research, 


though Physiology had in the pursuit of its own objects 
occasion enough to devote itself* earnestly to the study of 
these functions. In the same way the important functions 
of growth and coalescence, as also those of differentiatioD 
and atavism, have as yet been very little studied from a 
physiological point of view. 

This neglect of the history of evolution explains the 
little interest and the lack of insight exhibited by the 
physiologists of our time with regard to the theory of 
descent. When Darwin, in his Theory of Natural Selection, 
gave a new basis to the theory of evolution, and so pointed 
out the way to a physiological explanation of the formation 
of species, a new and most interesting field of research was 
thrown open to Physiology. But Physiology has hardly yet 
entered this; and it has done as little to advance our 
knowledge of the processes of evolution in their ontogenetic 
as in their phylogenetic aspect. In fact, with a few 
illustrious exceptions, most physiologists have paid very 
little attention to the theory of descent, and to this day 
some of their most renowned leaders look on this most 
important biological theory as "an unproved and baselesn 

This want of comprehension of the history and signifi- 
cance of evolution can alone explain, for instance, the fact 
that the famous Berlin physiologist, Du Bois-Reymond, in 
his well-known address " On the limits of Natural Science," 
delivered at Leipsic in 1872, before the meeting of German 
naturalists, declared human consciousness to be a phenome- 
non absolutely and unconditionally transcending the bounds 
of human comprehension. It never occurred to him that 
consciousness, in common with every other cerebral activity, 


is in actual process of evolution. He overlooked the obvious 
consideration that even the consciousness of the human race 
must have arisen gradually by evolution through many 
phylogenetic stages precisely in the same way that even yet 
the individual consciousness of every child is gradually 
completed in the course of many ontogenetic stages. 

Again, this same want cf insight into the functions and 
the physiological process of evolution accounts for the fact 
that even at the present day esteemed and learned natural- 
ists are earnestly discussing the question whether the 
creation of species, or, in other words, the phyletic evolution 
of forms, took place suddenly or gradually. This dispute 
73 as irrational as would be a dispute as to whether the 
mouse is a great or a small animal. The elephant will of 
eourse declare the mouse to be a tiny creature, while the 
2-ouse, living on the skin of the mouse, must regard the 
latter as an animal of gigantic size. Just as in the one case 
the estimate of extension in space is purely relative, and only 
to be taken in a relative sense, so in the other case is the 
estimate of extension in tima 

Every process of evolution as such is always continuous, 
and real leaps or interruptions never occur. Natura non 
facit saltus — nature never leaps. This is true both of on- 
togenetic and of phylogenetic processes : of the evolution 
of the individual as well as of that of the species. It is 
true that in Ontogeny leaps sometimes appear to occur, 
e.g. when the butterfly is developed from the pupa into 
which the caterpillar has been transformed, or when a 
Medusa is developed from an entirely dissimilar hydra-form 
Polyp. But the morphologist who step by step studies the 
exact course of these processes of evolution, finds that* 


though certain stages seem omitted, the continuity is really 
unbroken, and that each new form arises directly from that 
which preceded it. Throughout there is a causal and un- 
broken connection ; nowhere a sudden leap. 47 But when the 
rapidity of the process of evolution is at one time retarded 
and again suddenly accelerated, or when heredity is cur- 
tailed, the result of the process appears to be a sudden leap. 

This unbroken causal connection of. the processes of 
evolution exists equally in germ-history, and in tribal 
history. For as Ontogeny is but a brief reproduction of 
Phylogeny, conditional on Heredity and modified by 
Adaptation, in the latter, therefore, as in the former, no leap 
or open gap can ever really exist between two consecutive 
evolutionary forms. As in the evolution of the individua 1 
so in that of the species, each new form arises directly fron 
that which preceded it ; and here also the physiologica 
process of development always preserves its continuity . 
Even in those extreme cases where a new form does indeeA 
seem to come into existence quite suddenly, as in what is 
called " sudden or monstrous adaptation," there is always 
under the surface, an unbroken physiological evolutionary 
process which has the appearance of being a " sudden leap r 
only because of its comparative rapidity, or of the magnitude 
of its result. 

As a striking instance, let us consider a frequently ob- 
served case of such "sudden variation." A common two- 
horned he-goat, the consort of which is also a common two- 
horned goat, begets a kid, from the skull of which grow four 
horns, in place of the two horns previously hereditary in tlis 
family of goats. In this case a new" variety of goats bear- 
ing four horns has " suddenly " arisen, and under favourable 


conditions this young he-goat may become the founder of . 
an entirely new four-horned race, or (by correlative adapta- 
tion and coustant heredity) of a new fixed species. 

But if we now search for the physiological functions of 
evolution which have " suddenly " formed this new race or 
species, we find that a change in the hereditary nutrition at 
two points in the frontal bone and in the skin covering the 
same is the prime cause. Owing to the excessive local 
nutrition of the osseous tissue, and the consequent propor- 
tionate multiplication of cells, a bony protuberance gradually 
appears at each of these points ; and in consequence o e 
correlative adaptation, the hairy skin covering both these 
protuberances, changes into a hard, bare horny sheath, 
analogous to the other two horns which have long been 
hereditary. As these bony protuberances grow, and their 
horny sheaths become correspondingly larger, a new, second 
pair of horns appears behind the old ones. All these func- 
tions of evolution which " suddenly and by a leap " produce 
this four-horned form of goat are in reality perfectly "gradual 
and continuous " changes in the evolution of those masses of 
cells of which we have spoken : they depend on a change 
in the nutrition of the tissue at these two points in the 
' frontal bone and skin. In this instance, therefore, an accu- 
rate examination of the physiological function of evolution 
alfords a perfectly natural explanation of an apparently 
miraculous process. This is equally true of individual and 
of phyletic evolution. 

This is also the explanation of a process of evolution 
which above all others is usually put under mystical veil 
as though it were a supernatural wonder; this is the 
process of fertilization, or sexual generation. In all the 


higher plants and animals this constitutes the first act in 
which the evolution of the new individual begins. But 
it must be noted here that this important process is by no 
means as universally distributed throughout the animal and 
vegetable world as is commonly supposed. On the contrary 
there are very many low organisms which always multiply 
asexually, e.g. the Amoebae, Gregarinae, Flagellata, Forami- 
niferse, Eadiolaria, Myxomycetae, etc. In these cases 
there is no form of impregnation : the multiplication of 
individuals, and the preservation of the species depend here 
simply on asexuai generation, under the forms of fission, 
propagation by buds or by germ-cells. On the other hand, 
in the case of all higher plant and animal organisms, sexual 
propagation is the general law, and asexual generation 
never or but seldom occurs. Among Vertebrates in par- 
ticular " virginal generation * (PartheTwgenesis) never 
occurs. This we must explicitly affirm in the face of the 
celebrated dogma of the "immaculate conception." "Im- 
maculate conception " has never been observed either in 
Man, or in any other Vertebrate. 48 

Sexual propagation in the various classes of animals 
and plants exhibits an especially large number of interest- 
ing correlations, especially those relating to fertilization 
and the transmission of the male sperm to the female egg. 
These correlations are of the utmost significance not only in 
regard to propagation, but also in the production of organic 
bodily forms, and especially of sexual differences. Very 
remarkable instances of interaction take place between 
plants and animals. The recent admirable researches of 
Darwin and Hermann Muller on the fertilization of flowers 
by insect agency, are especially interesting from this point 


of view. 49 As a result of this interaction we find a sexual 
apparatus of very complex anatomy. But in spite of the 
great interest of these phenomena, we cannot discuss them 
now, as they are only of subordinate importance in study- 
ing the essential nature of the process of fertilization. On 
the other hand, the nature of this process itself — the mean- 
ing of sexual generation, must be closely studied. 

In every process of fertilization, as has already been 
said, two different kinds of cell, male and female, are con- 
cerned. In animals generally the female cell is called the 
egg, or egg-cell (ovulum), and the male is called the sperm- 
cell, or seed-cell (zoospermium, spermatozoon). The female 
egg-cell, the form and structure of which we have already 
considered, is in all animals originally of the same simple 
structure. At first it is simply a globular, naked cell, 
consisting of protoplasm and cell-nucleus (Fig. 10, p. 134). 
When this cell lies free, and is capable of motion, it 
performs a number of slow, amoeboid movements, as we 
have seen in the case of the egg of the Sponges (Fig. 14, 
p. 144). But commonly at a later period it is enclosed in 
peculiar envelopes and coatings of a very heterogeneous and 
frequently very complex structure. On the whole, the egg- 
cell is one of the largest of cells. In nearly all animals it 
is larger than any of the other cells. 

On the other hand, the other cell which plays a part in 

impregnation, the male sperm-cell, is one of the smallest 

cells of the animal body. As a rule, fertilization results 

from a mucous fluid, secreted by the male, coming into 

contact with the egg-cell, either within or without the body 

of the female. This fluid is called the sperm, or male seed. 

The sperm, like the saliva and the blood, is not a simple 


clear fluid, but a dense mass of exceedingly numerous cells, 
floating about in a comparatively small quantity of fluid. 
It is not this fluid, but the cells suspended in it, which 
produce fertilization. In most animals, these sperm-cell* 
are possessed of two special properties. In the first place, 
they are extraordinarily small, usually the smallest cells in 
the organism ; and secondly, they are possessed of a very 
peculiar quick motion called the spermatozoid movements. 
The form of the cells is in correlation with this movement. 
In most animals, as also in many of the lower plants (but 
not in the higher), each of these cells consists of a very 
small naked cellular body, enclosing an oblong nucleus, 
and of a long vibrating filament attached to the body of 
the cell (Fig. 17). It was a very long time before it was 
discovered that these structures are simple cells. In former 
times they were universally regarded as actual animals, 
and were called sperm-animals (Spermatozoa). It is only 
through the searching investigations of the past few years 
that we have acquired positive evidence of the fact that 
each of these so-called spermatozoa is really a simple cell. 
It is, therefore, best to call them simply seed-cells or sperm- 
cells. In Man these possess the same form as in many 
other Vertebrates, and in the majority of Invertebrates. 
In many of the lower animals, however, the form of the 
seed-cells is very different. Thus, for example, in the Cray- 
fish, they are fixed, round cells, motionless, and furnished 
with peculiar stiff, bristly processes. So, too, in certain 
Worms, e.g. the Thread-worms, the sperm-cells possess a 
very anomalous form. Some of these are amoeboid, re- 
sembling very small egg-cells. Yet even in most of the 
lower animals, e.g. the Sponges and the Polyps, they possess 



the "pin-shaped form" which occurs m Man and other 
Mammals (Fig. 17). 

1 - I 1 Jr 

Fig. 17.— Seed-cells or sperm-cells from the semen of various Mammals. 
Tne broad side of the flattened, pear-shaped nucleus portion of the sperm- 
cell (the so-called "head of the sperm-animalcule") is represented in the 
drawings marked I; the narrow side in those marked II : A-, kernel of the 
sperm-cell; m, central portion (protoplasm); s, active tail-like process 
(whip) ; M, four human sperm-cells; A, two sperm-cells of the ape; K, of 
the rabbit; H, of the common mouse ; C, of the dog ; S, of the pig. 

In 1677, when the Dutch naturalist, Leeuwenhoek, first 
discovered these filamentous and very active tiny bodies 
in the human semen, they were generally supposed to be 
distinct, independent animalcules, resembling Infusoria, and 
they were at once named " seminal animalcules." As we 
have already observed, they played an important part 
in the erroneous theory of preformation which was then 
prevalent, according to which the whole of the developed 
organism with all its parts exists preformed, though very 
small and as yet unexpanded, in each seminal animalcule. 
(See p. 36.) These animalcules had only to penetrate 


into the fruitful soil of the female egg-cell in order that the 
preformed human body might unfold and grow in all its 
parts. This radically erroneous view is now completely 
refuted, and the most accurate researches have shown that 
these active small seminal bodies are genuine cells, of the 
Form called flagellate. In the earlier expositions of the 
subject a head, trunk, and tail were distinguished in each 
of these " seminal animalcules." The so-called " head ' 
'Fig. 17 k) is only the longish round or oval cell-nucleus , 
the body, the central portion (m), is only an aggregation 
)f cell material, a prolongation of which forms the tail (s). 
We now also know that the form of these seminal animal- 
cules is not even peculiar and unrepresented in other cells , 
for entirely similar vibratory cells occur in various othei 
parts of the animal body. When these cells are possessed 
of many processes they are called ciliate cells ; but if they 
have only one process, they are said to be flagellate. The 
ciliated sponge particles afford instances of flagellate cells 
resembling those of the sperm-cells. 

Thus the process of fertilization in sexual generation 
depends essentially on the fact that two dissimilar cells 
meet and blend. In former times the strangest views pre- 
vailed with regard to this act. Men have always been 
disposed to regard it as thoroughly mystical, and the most 
widely different hypotheses have been framed to account 
for it. It is only within the last few years that closer 
»tudy has shown that the whole process of fertilization is 
extremely simple, and entirely without any special mystery. 
Essentially it consists merely in the fact that the male 
Bperm-cell coalesces with the female egg-cell. Owing to itr 
sinuous movements, the very mobile sperm-cell finds its waj 



to the female egg-cell, penetrates the membrane of the latter 
by a perforating motion and coalesces with its cell-material. 

Fig. 18. — Fertilization of the egg- 
cell by the sperm-cells. The thread- 
shaped, lively sperm-cells penetrate 
through the porous canals of the egg- 
membrane into the granular mass of 
yelk, with which they amalgamate. 
The kernel (nucleus) of the egg-cell 
has disappeared. 

A poet might find in this 
circumstance a capital oppor- 
tunity for painting in glowing 
colours the wonderful mystery of the process of fertiliza- 
tion ; he might describe the struggles of the living " seed- 
animalcules ' eagerly dancing round the egg-cell shut up 
in its many coverings, disputing the passage through the 
minute pore-canals of the chorion, and then " of purpose ' 
burying themselves in the protoplasm of the yelk-mass, 
where, in a spirit of self-sacrifice, they completely efface 
themselves in the better " ego." Or a teleologist might 
here find occasion to admire the peculiar wisdom of the 
Creator, who made many fine pore-canals in the egg- 
membrane in order that the seed-animalcules might pass 
through them. But the critical naturalist very prosaically 
conceives this poetical incident, this " crown of love," as the 
mere coalescence of two cells. The result of this is that, in 
the first place, the egg-cell is rendered capable of further 
evolution ; and. secondly, that the hereditary qualities of 
both parents are transmitted to the child. 

The fertilized egg-cell is, therefore, of a nature entirely 
different from that of the unfertilized egg-cell. For since 


we regard the sperm-cell as well as the egg-cell as true cells 
and since fertilization essentially consists in the amalgama- 
tion of the former with the latter, therefore the cell which 
results from this amalgamation must be regarded as an en- 
tirely new independent organism. It contains, in the proto- 
plasm of the sperm-cell, a portion of the paternal, male body, 
and on the other hand, in the protoplasm of the original 
egg-cell, a portion of the maternal, female body. This is 
equally shown by the fact that the child inherits many 
qualities from both parents. Heredity from the father is 
transmitted through the sperm-cells, Heredity from the 
mother through the egg-cell. The new cell, which is the 
rudiment of the child, the newly generated organism, 
originates in an actual amalgamation or coalescence of the 
two cells. 

In order to gain a correct and clear knowledge ol 
fertilization, I think it is absolutely necessary to emphasize 
as quite fundamental this simple but most important 
process, which as yet is not sufficiently appreciated. I there- 
fore assign a peculiar name to the new cell, from which 
the child really proceeds, and which is usually inaptly 
called " the fertilized egg-cell " or " the first cleavage 
globule;" I shall call it the parent-cell (cytula), and its 
kernel (nucleus) the parent-kernel (cytococcus). The name 
" parent-cell " seems to me the simplest and most apt 
because all the other cells of the organism descend from it, 
and because it is in the most real sense both the male 
ancestor and the female ancestor of all the numerous 
generations of cells, which are afterwards employed in the 
formation of the many-celled organism. The very complex 
molecular movement of the protoplasm in this parent-cell, 



summed up in the word " life," is naturally entirely dif- 
ferent from that of the two distinct ancestral cells, the 
amalgamation of which gave rise to the parent-cell. The 
Life of the parent-cell (Cytula) is the product or resultant 
of the- paternal activities, transmitted through the sperm- 
sell, together with the maternal activities, transmitted 
through the egg-cell. 

All good recent observations agree in showing that 
the individual evolution of man and of other animals 
begins with the formation of such a parent-cell, and that 
in the course of further evolution this then separates 
by self-division, or cleavage, into a number of cells, the 
so-called cleavage-globules or cleavage-cells (segmentella). 
But the most active strife is still waged over the question 
of the mode in which the parent-cell (cytula) originates, 
and of the relative parts played by the sperm-cell and the 
egg- cell in the formation of the parent-cell and in the act 
of fertilization. Formerly it was usually assumed — and 
many well-known naturalists still adhere to this — that the 
original kernel (nucleus) of the egg-cell (p. 136, Fig. 11), 
the so-called germ- vesicle, is retained unaltered during 
fertilization, and that it directly transforms itself into the 
parent-kernel, "the kernel of the first cleavage-globule." 
But most more recent observers (with whom I agree) have 
become convinced that the germ-vesicle, the original egg- 
kernel, sooner or later disappears, and that the parent- 
kernel (cytococcus) forms itself anew. Here again, even 
the question as to the time and mode in which the new 
kernel of the parent-cell forms is at present still much 
debated. Some assume that the germ-vesicle disappears 
before fertilization, others say that this happens after ferti- 


lization. One party affirms that it is expelled from the 
egg-cell, the other that it dissolves in the yelk. Some are 
of opinion that it disappears entirely, others, that it only 
does so partially. 

We cannot here enter into the various views which have 
recently been formed as to this remarkable incident in fertili- 
zation, the examination of which presents great difficulties. 
Those who are particularly interested in it may be referred 
to valuable works on this subject by Auerbach, Biitschli, 
Hertwig, Strasburger, and others. 50 Here we can only 
briefly indicate the view which at present appears most 
probable. Most students of this point now assume as a 
universal incident in fertilization that the germ-vesicle, the 
original kernel of the egg-cell, disappears before fertilization, 
being either expelled from the egg or dissolved in the yelk. 
Either no part of the egg-cell, or only the germ-spot 
(nucleolus), remains as a defined part in the yelk. Accord- 
ing to Hertwig and others, this germ-spot amalgamates with 
the sperm-kernel, or the kernel of the intruding sperm-cell, 
and this amalgamation gives rise to the kernel of the 
parent-cell. On the contrary, according to other observers, 
the parent-kernel (cytococcus) is an entirely new formation 
in the protoplasm of the parent-cell (cytula, Fig. 21). 

At present, therefore, the majority of observers assume 
that between the original nucleated egg- cell and the 
known nucleated parent-cell there is a stage in which there 
is no real cell-kernel or nucleus, and in which, therefore, the 
form-value of the whole organic individual is no longer that 
of a true nucleated cell, but that of a non-nucleated cytod. 
i.e. a simple protoplasmic body in which no true cell-kernel 
(nucleus) is to be found. (Cf. p. 129.) Even if, with Hart- 



wig, we assume that the germ- vesicle does not completely 
disappear, but that the germ-spot (nucleolus) remains and 
amalgamates at the moment of fertilization with the 
nucleus (or nucleolus ?) of the sperm-cell, we may say that 
the kernel of the parent-cell arises anew in that act, and 
that, therefore, a non-nucleated germ-stage, in which the 
form-value of the germ is only that of a cytod, precedes the 
one-celled germ-stage (the parent-cell). For reasons which 
we shall presently recognize, we shall call this simplest 
(non-nucleated) stage, the Monerula. 51 (Fig. 19.) 

Fig. 19.— Monerula of a Mammal (Babbit) . The fertilized egg-cell, after 
the disappearance of the germ-vesicle, is a simple globe of protoplasm (d). 
The outer membrane is formed by the modified zona pellucida (z), together 
with a mucous layer (//) secreted on the outside of the zona. A lew single 
sperm-cells (.?) are still visible in the membrane. 

. We regard it as a fact of the greatest interest that the 
human child, like that of every other animal, is, in this 
first stage of its individual existence, a non-nucleated ball 
of protoplasm, a true cytod, a homogeneous, structureless 
body, without different constituent parts. For in this 
" Monerula-form " the structure of the animal, and thus of 


the human organism, is of the simplest conceivable nature. 
The simplest actually known organisms, and at the same 
time the simplest conceivable organisms, are the Monera, 
most of which are minute, microscopic, and formless bodies, 
consisting of a homogeneous substance, of an albuminous or 
mucous, soft mass, and which, though they are not com- 
posed of diverse organs, are yet endowed with all the vital 
qualities of an organism. They move, feed, and repro- 
duce themselves by division (Fig. 20). These Monera 

F/G. 20. — A Moneron (Protamceba) in the act of reproduction. A. The 
whole Moneron, which, like the Amoeba (Fig. 13), moves by means of change, 
able processes. B. The Moneron is pinched in at a central point, so that it 
is divided into two halves. C. The two halves have separated and each 
now forms an independent individual. (Much enlarged.) 

are of great importance, owing to the fact that they 
afford the surest starting-point for the theory of the origin 
of life on our earth. We shall presently have further oc- 
casion to point out their significance. (Cf. Chapter XVI.) 
Here we need only give due weight to the very remarkable 
fact that, both in germ -history and in tribal history, the 
animal organism begins its evolution as a structureless 
mucous ball. The human organism, like that of the higher 
animals, exists for a short time in this simplest conceivable 
form, and its individual evolution < ommences from this 
simplest form. The entire human child, with all its great 



future possibilities, is in this stage only a small, simple ball 
of primitive slime (protoplasm, Fig. 19). The membrane 
is still there, but seems to be an entirely passive part of the 
egg, and takes no real share in the active processes of the 
evolution of this egg. We may, therefore, for a time pass 
over this membrane, for we shall afterwards enter into the 
changes which it undergoes in a later stage ; as regards the 
actual process of evolution, it is entirely without significance. 
At present we need only concern ourselves with the contents 

Fig. 21. — Parent-cell or crtula of a Mammal (Rabbit) : k, parent- 
kernel ; n, nucleolus of the latter ; p, protoplasm of the parent-cell ; z 
modified zona pellucida ; s, sperm-cells; /;, external albaminous membrane. 

of the globular egg, the homogeneous yelk, which when 
in this condition we call the Monerula, in allusion to the 

Although morphologically we can see no denned con- 
stituent parts in the Monerula, yet chemically we must 
regard the latter as the complex product of at least four 
different constituents ; these are : (1) the protoplasm of the 
maternal egg-cell ; (2) the protonlasm of the naternal 


sperm-cell ; (3) the substance of the maternal germ-vesicle 
(kernel-substance or nuclein of the egg-cell) ; and (4) the 
substance of the paternal sperm-kernel (kernel-substance or 
nuclein of the sperm-cell). From the mixture of the two 
former substances (1, 2) the protoplasm of the parent-cell 
(Fig. 21, p) seems to originate ; from the mixture of the two 
forms (3, 4) the parent-kernel (cytococcus) seems to origin- 
ate (Fig. 21, h). 52 

The parent-cell (cytula, Fig. 21), which was formerly 
regarded as merely the "fertilized egg-cell," differs very 
essentially, therefore, from the original egg-cell, both in 
point of form (morphologically), and in point of composition 
(chemically), and lastly, also in point of vital qualities 
(physiologically). Its origin is partly paternal, partly 
maternal; we need not, therefore, be surprised, when we 
see that the child, which develops from this parent-cell, 
inherits individual qualities from both parents. 58 

The vital activities of each cell form a sum of mechani- 
cal processes, which depend radically on movements of the 
smallest "life particles," the molecules of the living sub- 
stance. If we call this active substance the Plasson, and 
the molecules the Plastidules, we may say that the indi- 
vidual physiological character of each cell depends on the 
molecular movements of its plastidules. The plastidule 
movements of the cytula are therefore the resultant of the 
united plastidule movements of the female egg-cell xnd oj 
the male sperm-cell. If we regard the two latter as the 
sides of the parallelogram of forces, then the plastidule 
movement of the cytula is the diagonal. In my work ou 
the " Perigenesis of Plastidules" (1876), I have explained 
the important bearing of this conception in explanation of 
the elementary processes of evolution. 

( ««3 ) 


Review of the Constituent Parts of the One-celled Gerni.organism, before 

and after fertilization. 

Cf. the works of Eduard Strasbnrger (" Ueber Zellbildung, Zelltheilnng 
ind Befruchtnng," 2nd Edition ; Jena, 1876) ; of Oscar Hertwig (" Beitrage 
zur Kentniss der Bildung, Befruchtnng, und Theilung des Thierischen Eies;" 
1875) ; of Leopold Anerbach (" Organologische Studien ; " 1874) ; and of Otto 
Piitschli (" Studien iiber die ersten Entwickelungs-Vorgange der Eizelle," 
Ma\; 1876) . M 

I. The Fertilizing Male, 
or Paternal Sexual 

The Sperm-cell. 
Syn. Thread - cell. 
Seed-animalcule. Sper- 
matozoa. Zoosperm. 
Fig. 17, p. 173. 

Constituent Parts. 
I. A. Protoplasm o^ the 


The central portion 
and the tail of the seed- 
thread, together with 
the outer sheath of the 
" head." 

I. E. Kernel (nucleus) 
of the Sperm-cell. 

Sperm -kernel (Hert- 
wig). " Head of the 
Bperm-animal " (with 
the exception of the 
thin outer gheath). 

II. The Fertilized Female, III. The New Cell, the 

or Maternal 


The Egg-cell. 

The Parent-cell, 



Syn. The unfertilized 

Syn. The fertilized 


egg. The first cleavage- 

Fig. 1, p. 122. 

globule. The oldest 

Fig. 10, p. 134. 

cleavaue-cell. Segmen- 

tella prima. 

Fig. 21, p. 181. 

Constituent Parts. 
II. A. Protoplasm of the 

Yelk, egg-yelk, Lecy- 
thus, vitellus. 

II. B. Kernel (nucleus) 
oi the Egg-cell. 

Germ-vesicle, or Pur- 
kinje's vesicle (Vesicula 
Qerminativa), contain- 
ing the germ-spot 
(Macula Qerminativa), 
or the nucleolus, which, 
aeoording to Hertwig, 
beoomes the egg-kemel. 

product of the Concre- 
scence of I. and II. 

Constituent Parts. 
III. A. Protoplasm of 
Parent-cell: Cleavage- 

Protoplasm of the 
first cleavage - globule 
(the product of the 
amalgamation of I. A 
and II. A. 

III. B. Kernel (nucleus) 
of the Parent-celL 


Cleavage-kernel ( Hert- 
wig). Germ - kernel 
(Strasburger). Kernel 
of the first cleavage- 
globule (product of the 
amalgamation of the 
■perm. kernel and the 
egg-kernel ?).** 




First Processes after the Fertilization of the E,2-g-cell is complete. — Original 
or Palingenetic Form of Egg-cleavage. — Significance of the Cleavage- 
process. — Mulberry-germ, or Morula. — Germ-vesicle, or Blastula. Germ- 
membrane, or Blastoderm. — Inversion (Invagination) of the Germ-vesicle. 
— Formation of the Gastrula. — Primitive Intestine and Primitive 
Mouth. — The Two Primary Germ -layers ; Exoderm and Entoderm.— 
Kenogenetic Form of Egg-cleavage. — Unequal Cleavage (segmentatio 
inequalis) and Hood-gastrula (Amphigastrida) of Amphibia and 
Mammalia. — Total and Partial Cleavage. — Holoblastic and Meroblastic 
Eggs. — Discoidal Cleavage {segmentatio discoidalis) and Disc-gastrula 
(Disrogastrula) of Fishes, Reptiles, Birds. — Superficial Cleavage (seg- 
mentatio stiperficialis) and Vesicnlar Gastrula {Peri-Gastrxda) of Ar- 
ticulates (Arthropoda). — Permanent Two-layered Body-form of Lower 
Animals. — The Two-layered Primaeval Parent-form ; Gastrsea. — 
Homology of the Two Primary Germ -layers in all Intestinal Animals 
(Metaz»a). — Significance of the Two Primary Germ-layers. — Origin 
and Significance of the Four Secondary Germ-layers. — The Exoderm 
or Skin-layer gives rise to the Skin-sensory Layer and the Skin, 
fibrous Layer.— The Entoderm or Intestinal Layer trives rise to the 
Intestinal-fibrous Layer and the Intestinal-glandular Layer. 

" The distinguishing of the strata, or layers, in the embryonic membrane 
was a turning-point in the study of the history of evolution, and placed 
later researches in their proper light. A division of the (disc-shaped) 
embryo into an animal and a plastic part first takes place. When this 
division is complete, each part has two layers. In the lower part (the 
plastic or vegetative layer) are a serous and a vascular layer, each of pecu- 



liar organization. In the upper part also (the animal or serous germ-layer) 
two layers are clearly distinguishable, a flesh-layer and a skin-layer." — Karl 

Ernst Baer (18-28). 

The first processes which occur in the evolution of the 
individual, after the impregnation of the egg-cell is com- 
plete, and after the formation of the parent-cell, are essen- 
tially similar throughout the whole animal kingdom, and 
always begin with the so-called yelk -cleavage, and the 
formation of the germ-layers. Only the lowest and simplest 
animals, the Primaeval Animals, or Protozoa, are peculiar in 
this respect. These latter include the Monera, Amoeba- , 
Gregarinae, Flagellata, Rhizopoda, Infusoria, and others. 
AH these Primaeval Animals reproduce themselves, as far as 
we yet know, only asexually, by division, the formation of 
buds, spores, germ-cells, and so on. On the other hand, they 
never have true eggs, i.e. germ-cells, to the evolution oi 
which fertilization is necessary. Nor do they ever form 
true germ-layers. All other animals, on the contrary, all 
true animals, or Metazoa (as we may call them, in contra- 
distinction from the Protozoa) have true eggs, and, from their 
impregnated eggs, form true germ-layers. This is as true 
of the low Plant-animals and Worms, as of the higher 
developed Soft-bodied animals (Mollusca,) Star-animals 
(Echinoderma), Articulated animals (Arthropoda), and Ver- 
tebrates. 55 

The most important processes of germination are essen- 
tially similar in all these true Animals (the Primaeval animals 
being excluded). In all, the parent-cell, which arose from 
the fertilized egg-cell, separates, by repeated cleavage, into 
a large number of simple cells. All these cells are direct 
followers or descendants of the parent-cell, and, for reasons 


vvliich will be explained later, are called Cleavage-cells or 
Oleavage-globules (segmentella). The repeated process of 
division of the parent-cell, which gives rise to the cleavage- 
cells, has long been known as egg-cleavage, or, inaccurately, 
as cleavage (segmentation). At an earlier or later stage, the 
entire mass of cleavage-cells divides into two essentially 
different groups, which range themselves in two separated 
cell-strata ; the two primary germ-layers. This formation of 
the germ-layers is a process of the greatest significance, and 
the real beginning of the formation of the true animal body. 

It is only quite recently that the fundamental germinai 
processes of egg-cleavage and the formation of the germ- 
layers have been thoroughly understood, and their real 
significance rightly estimated. In the various animal groups 
these processes exhibit various striking differences, and it 
was no easy task to show their essential similarity or 
identity throughout the whole animal kingdom (always 
excepting, of course, the Primaeval Animals, or Protozoa). 
It was only after I had established the Gastraea Theory,* 
in 1872, and afterwards, in 1875, had traced back indi- 
vidual forms of egg- cleavage and of the formation of the 
gastrula to one and the same type-form, that this important 
identity could be regarded as really proved. This furnished 
a single law which conditions the earliest germinal processes 
of all animals. 66 

The relation of Man to these earliest and most import- 
ant processes is entirely similar to that of other higher 
Mammals, and especially to that of Apes. As the human 
germ or embryo, even in a much later stage of its formation, 
when the brain-bladders, the eyes, the organs of hearing, 
the gill-arches, etc. are also present, does not essentially 


differ from the correspondingly developed embryos of other 
higher Mammals (Plate VII., 1st row), we may quite safely 
assume that the earliest germinal-processes, the cleavage of 
the egg and the formation of the germ-layers, also corre- 
spond. As yet, however, these processes have not been 
actually observed ; for there has never been an opportunity 
of dissecting a female of the human species immediately 
after fertilization is completed, and of seeking the parent- 
cell, or the cleavage-cells, in the oviduct. As, however, the 
youngest human embryo (in the form of germ-vesicles), 
which have yet been really observed, as well as the subse- 
quently developed germ-forms, correspond in all essential 
points with those of the Rabbit, the Dog, and other higher 
Mammals, no reasonable man can doubt that egg-cleavage 
and the formation of the germ-layers proceeds, in the one 
case as in the other, in the way represented in Plate II. 
Fig. 12-17. 57 

The particular form which egg-cleavage and the forma- 
tion of the germ-layers assume in the case of Mammals, is, 
however, by no means the original, simple, and palingenetic 
form of germination. On the contrary, it has been very 
much changed, vitiated, and kenogenetically modified in 
consequence of numerous embryonic adaptations. (Cf. p. 12.) 
It is, therefore, impossible from a mere study of it to learn 
its nature. On the contrary, in order to obtain this know- 
ledge, it is necessary to study and compare the various 
forms of egg-cleavage, and of the formation of the germ- 
layers, which occur in the animal kingdom ; and it is 
especially necessary to search for the original, palingenetic 
form, from which the modified, kenogenetic form of germi- 
nation of Mammals gradually arose at a much later time. 


This original, palingenetic form of egg-cleavage, and 
of the formation of the germ-layers is altogether unrepre- 
sented in the present day in the Vertebrate tribe, to which 
Man belongs, except in the lowest and oldest member of 
this tribe, the remarkable Lancelet or Amphioxus (Cf. 
Chapters XIII. and XIV., and Plates X. and XL). But it 
is still found in exactly this form in many low inverte- 
brate animals — for example, in the remarkable Sea-squirts 
(Ascidia), in the Pond-snail (Limnceus), in the Arrow- worm 
(Sagitta) ; also in many Star-animals (Echinoderma) and 
Plant-animals, — for example, in the common Star-fish and 
Sea-urchin, in many Medusae and Corals, and in the 
simplest Chalk Sponges (Olynthus). As an example, let us 
examine the palingenetic egg-cleavage and formation of 
the germ-layers of an eight-rayed single Coral, which I 
found in the Red Sea, and described in my Arabischen 
Korallen under the name of Monoxenia Darwinii. 5 * 

After the Monerula (Fig. 22, A) has changed into the 
parent-cell, or cytula (B), the latter divides into two similar 
cells (0). The kernel of the parent-cell first parts into 
two similar halves ; these part asunder, shrink from each 
other, and then act as centres of attraction to the surround- 
ing protoplasm ; after this the protoplasm becomes con- 
tracted by a circular groove running round its circumference, 
and then separates into two similar halves. Each of the 
two cleavage-cells, which are thus produced, again separates 
in the same way into two similar cells, the plane of division 
between these two latter lying at right angles to that 
between the two former (Fig. 22, D). The four similar 
cleavage-cells, the descendants in the second generation of 
the parent-cell, He in one plane. Each of these now again 


divides into two similar halves, the division of the cell- 
kernel again preceding that of the surrounding proto- 
plasm. The eight cleavage-cells thus produced bisect in 
the same way into sixteen. Thirty-two cleavage cells are 
formed from these by further division. As each of these 
again bisects, sixty-four of these cells are produced ; after- 
wards one hundred and twenty-eight, and so on. 59 These 
repeated and similar bisections finally result in the produc- 
tion of a globular rJass of similar cleavage-cells; we call 
this mass the mulberry -germ (morula). The cells lie as 
close together as the drupes of a mulberry or blackberry ; 
so that the entire surface of the round mass appears rugged 
(Fig. 22, E). (Cf. Plate II. Fig. 3. 60 ) 

After this egg-cleavage is completed, the solid mulberry- 
germ changes into a hollow globular vesicle. A watery 
liquid or jelly collects in the centre of the solid ball ; the 
cleavage-cells part asunder, and all seek the surface of the 
ball. Here by mutual pressure they become multilaterally 
flattened, assume the form of truncated pyramids, and range 
themselves in order, side by side, in a single stratum 
(Fig. 22, F, G) This cell-stratum is called the germ-mem- 
brane (blastoclerma) ; the cells (all of one kind), a simple 
stratum of which forms the germ-membrane, are called the 
germ-membrane-cells (cellulce blastodermicce) ; and the entire 
hollow ball, the walls of which are composed of these cells, 
\s called the germ-membrane-vesicle, or, briefly, the germ- 
vesicle, or vesicular-germ (blast ula ; formerly called the 
vesicula blastodeiwiica). 61 The inner cavity of the ball, 
which is filled with clear liquid or jelly, is called the 
cleavage-cavity (cavwm segmentarium), or the germ-cavito 


Fis. 22. — Germination of a Coral (Monoxenia Darwinix) : A> Monerula; 
B, Parent-cell (Cytula) ; C, two cleavage-cells ; D, fonr cleavage-cells; 
E, Mulberry-germ (Morula) ; F, the Germinal vesicle (Blastula) ; G, Ger- 
minal vesicle in section ; H, Germinal vesicle (inverted) in section ; I, 
Gastrula in longitudinal section; K, Gaatrula, or Gerui-oup, seen from 

In this Coral, as in many other low animals, the young 
animal-germ begins to move even in this stage, and 
swims about independently in the water. A long, thin, 
thread-like process, a whip or thong, grows out from each 
of the cells of the germ-membrane ; and these inde- 
pendently exert slow vibrations, which afterwards be- 
come quicker (Fig. 22, F). Each cell of the germ-membrane 
is thus transformed into a vibrating whip-cell. The whole 
globular germ-vesicle revolves or turns, and is driven about 
in the water by the united force of all these vibrating whip- 
processes. In many other animals, especially in those in 
which the germ is developed within closed egg-membranes, 
the vibrating whip-threads on the cells of the germ-mem- 
brane are not developed till a later period, or, even, are not 
formed at all. The germ- vesicle is capable of growing and 
extending, for the cells of the germ-membrane increase by 
repeated division, which occurs within the surface of the 
ball, and more liquid is secreted in the centre cavity. 

A most important and remarkable process now occurs ; 
this is the inversion of the germ- vesicle (invaginatio blas- 
tulce, Fig. 22, H). The ball, the wall of which is cellular, 
consisting of a single layer, changes into a cup with a two- 
layered cellular wall. (Cf. Fig. 22, G, H, I.) The outer sur- 
face of the ball becomes flattened at a particular point ; and 
this flattening deepens into a groove. The groove becomes 
deeper and deeper, growing at the expense of the central 


gerni-cavity, or cleavage-cavity. The latter decreases in 
proportion as the former extends. At last the central germ- 
cavity entirely disappears, while the inner, inverted portion 
of the germ-membrane, the wall of the groove, attaches its 
inner surface to the inner surface of the outer, uninverted 
portion of the germ-membrane. At the same time, the cells 
of the two parts assume a different form and size ; the innei 
cells become rounder ; the outer become longer (Fig. 22, I). 
The germ thus acquires the form of a cup or goblet- 
shaped body, the wall of which consists 'of two different 
cell-layers, while the cavity in its centre grows outward at 
one end, at the place where the inversion originated. This 
highly important and interesting germ-form is called the 
germ-cup or the intestinal larva (Gastrula, Fig. 22, 2, in 
longitudinal section; K, surface view). 62 

The Gastrula seems to me the most important and 
significant germ-form of the animal kingdom. For in all 
true animals, the Protozoa excepted, the egg-cleavage 
results either in a genuine, original, palingenetic gastrula 
(Fig. 22, I, K), or in an equivalent kenogenetic germ- 
form, which has arisen secondarily out of the earlier form, 
and which may be referred directly back to that form. 
P is certainly a most highly interesting and significant fact, 
that animals of the most diverse tribes, Vertebrates, Soft- 
bodied Animals (Mollusca), Articulated animals (Arihro- 
poda), Star-animals (Echinoderma), Worms, and Plant- 
animals {Zooyhyta) develop from one common germ-form. 
In most striking illustration of this, I place side by side 
several genuine Gastrula forms, taken from tribes of animals 
(Fig. 23-28, with the description). 

This extraordinary importance of the Gastrula makes 


x 93 

it necessary that we- should most carefully examine the 
structure of its body. Ordinarily it is invisible to the 

Fig. 24 

Fin. 25. 

Fig. 26. 

Fig. 27. 

Fig. 23. 

Fig. 28. 

Fig. 23. — (.4) Gastrula of a Zoophyte (Gastropliysema). (Haeckel.) 
Fig. 24.— (B) Gastrula of a Worm (Sagitta). (After Kowalevsky.) 
Fig. 25.— (C) Gastrula of au Echinoderm (Starfish, Uraster). (After 

Alexander Agassiz.) 

Fig. 26. — (D) Gastrula of an Arthropod (NavpUus). 

Fig. 27. — (E) Gastrula of a Mollusc (Pond-snail, Limnceus). (After Karl 


Fig. 28.— (F) Gastrula of a Vertebrate (Lancelet, Awphioxus). (After 

In all, d indicates the primitive intestinal cavity; o, the primitive mouth; 
8, the cleavage-cavity; t, the entoderm, or intestinal layer; e, the exoderm, 
or skin-layer. 


naked eye, or, at most, under favourable circumstances, it 
is seen as a tiny speck, usually ^ — -fe t or at most J — J 
millimetre in diameter ; it is hardly ever more. In form 
the body of the Gastrula is usually cup-like ; sometimes it 
is rather egg-shaped, sometimes rather ellipsoid or fusiform ; 
in other cases it is more hemispherical, or almost spherical ; 
and again in others, longer or almost cylindrical. The 
geometric outline of the body is highly characteristic; it 
is marked by a single axis with two differing poles. This 
axis is the main, or longitudinal axis of the future animal 
body ; one pole is the mouth, or oral pole ; the opposite is 
the aboral pole. This outline with one axis distinguishes 
the Gastrula very essentially from the globular Blastula 
and Morula, in which all the axes of the body are similar. 68 

I shall call the central cavity of the Gastrula-body the 
primitive intestine (protogaster'), and its opening the pri- 
mitive mouth (pr.otostoma). For this cavity is the original 
nutritive, or intestinal cavity of the body, and this opening 
originally served to admit food into the body. It is true 
that at a later period the primitive intestine and the 
primitive mouth appear very different in the different 
tribes of animals. This is especially true of Vertebrates, 
in which only the middle portion of the later-formed in- 
testinal canal proceeds from the primitive intestine ; and 
in which the later mouth-opening is a formation entirely 
independent of the primitive mouth, which closes. It is, 
therefore, necessary to distinguish clearly between the 
primitive mouth and intestine of the Gastrula on the one 
hand, and the later-formed intestine and mouth of the 
developed Vertebrate on the other. 64 

The two cellular layers which surround the cavity of 



the primitive intestine, and alone constitute the wall of 
the latter, are of very great significance. For these two 
which alone constitute the whole body, are, in fact, the 
two primary germ-layers, or primitive germ -layers (blas- 
tophylla). Their fundamental significance has already been 
pointed out in the historical introduction (Chapter III.). 
The outer cell-layer is the skin-layer, or exoderm (Fig. 29, e); 
the inner cell-layer is the intestinal layer, or entoderm 
(Fig. 29, e). The whole body of all true animals proceeds 
solely from these two primary germ-layers. The skin- 
layer furnishes the outer body-wall, the intestinal layer 
forms the inner wall of the intestine, and directly surrounds 
the intestinal cavity. At a later period a cavity forms 

Fig. 29. — The Gastrula of a Chalk Sponge (Olynthus) : A, external view. 
B, in longitudinal section through the axis ; g, primitive intestine (primitive 
intestinal cavity) ; o, primitive mouth (primitive month-onening) ; i, the 
inner cell-layer of the body-wall (the inner germ-layer, entoderm or intes- 
tinal layer) ; e, the outer cell-layer (the outer germ-layer, exoderm or skin- 
layer) . 


between the two germ-layers ; this cavity, filled with blood 
or lymph, is the body-cavity (c<xloma). m 

The two primary germ-layers, the outer or serous, and 
the inner or mucous layer, were first clearly distinguished, 
in 1817, by Pander, in the incubated Chick (p. 51). But 
their full significance was first thoroughly recognized by 
Baer, who, in his " History of Evolution " (1828), gave the 
name of animal layer to the outer layer, that of vegetative 
layer to the inner. These names are very apt, because it is 
the outer layer which especially (if not exclusively) gives rise 
to the animal organs of sensation and movement, the skin, 
the nerves, and the muscles ; while, on the other hand, it is 
especially from the inner layer that the vegetative organs 
of nourishment and reproduction, the intestine and blood- 
vessel system in particular, arise. Twenty years after- 
wards (in 1849) Huxley pointed out that in many low 
Plant-animals (Zoophyta), such as the Medusae, the whole 
body permanently consists only of these two primary 
germ-layers. The outer of these he called the ectoderm, or 
exoderm ; the inner he named the endoderm, or entoderm. 
Recently Kowalevsky and Ray Lankester especially have 
tried to show that other Invertebrate animals of the 
most diverse classes, in Worms, Soft-bodied Animals (Mol- 
lusca), Star-animals (Echinoderma), and Articulated animals 
(Arthroj)oda), form from the same two primary germ- 
la^ ers. Lastly, I have myself shown that this is the case 
also in the lowest Plant-animals, in Sponges ; and at the 
same time I tried to prove in my Gastrsea Theory that these 
two primary germ-layers must be considered as of the same 
significance, or as homologous, in all cases, from Sponges 
and Corals to Insects and Vertebrates, including Man. 


Ordinarily the cells of the Gastrula-germ, which com- 
pose the two primary germ-layers, already present recog- 
nizable differences. In most cases, if not in all, the cells 
of the skin-layer, or exoderm (Fig. 29, e), are smaller, more 
numerous, and brighter coloured ; on the other hand, the 
cells of the intestinal layer, or entoderm (Fig. 29, i), are 
larger, less numerous, and darker. The protoplasm of the 
exoderm cells is clearer and firmer than the darker and 
softer cell-substance of the entoderm cells; the latter are 
generally much richer than the former in fatty particles. 
The cells of the intestinal layer usually also have a much 
greater affinity for colouring matter, and take up carmine, 
aniline, and so on, from solution much more quickly and 
vigorously than do the cells of the skin-layer. 

These physical, chemical, and morphological differences 
in the two germ-layers correspond to their physiological dif- 
ferences, and are of great interest, because in them we see the 
first and earliest process of division or differentiation of the 
animal body. The germ-membrane (blasto derma), which 
forms the wall of the globular germ-vesicle, or Blastula 
(Fig. 22, F, G), consisted solely of a single layer of similai 
cells. These cells of the germ-membrane, or blastoderm, are 
usually formed in a very regular and even way, and are of 
entirely similar size, form, and qualities. Generally they 
are flattened by pressing against each other, and are often 
uniformly six-sided This uniformity of the cells disap- 
pears, at an earlier or later period, during the inversion 
(invaginatio) of the germ-vesicle. The cells, composing 
the inverted, inner part of the germ-vesicle (which after- 
wards form the entoderm) usually assume, even during the 
process of inversion (Fig. 22, H), a nature differing from 



that of the cells which constitute the outer, uninverted part 
(the future exoderm). When the process is completed, the 
histological differences in the cells of the two primary 
germ-layers are usually very strongly marked (Fig. 30). 
The small, bright-coloured cells of the exoderm (e) are 
clearly distinguishable from the larger, darker cells of the 
entoderm (i). 

Fig. 30. — Cells from the two primary germ- 
layers of a Mammal (from the two strata of 
the germ -membrane) : i, the larger, darker 
cells of the inner stratum, the vegetative 
germ-layer, or entoderm; e,the small, brighter- 
coloured cells of the outer stratum, the animal 
germ-layer, or exoderm. 

At present we have only con- 
sidered that form of egg-cleavage, of 
germ-layer and gastrulation, which 
on many and important grounds we are justified in regard- 
ing as the original, primary, and palingenetic form. We call 
this the primordial, or original, form of egg-cleavage ; and 
the Gastrula, resulting from this, we call the Bell-gastrula 
(Arckigcbstrula). In a form exactly similar to that of our 
Coral (Monoxenia, Fig. 22), we meet with this Bell-gastrula 
in the lowest Plant-animals, in the Gastrophysema (Fig. 23), 
also in the simplest Chalk Sponges (Olynthus, Fig. 29), 
in many Medusas and Hydra-polyps ; in low Worms of dif- 
ferent classes (Sagitta, Fig. 24; Ascidia, Plate X. Fig. 1-4); 
again, in many Star-animals (Echinoderma, Fig. 25); in 
low Articulated-animals (Arthropoda, Fig. 26), and Soft^ 
bodied Animals (Mollusca, Fig. 27) ; lastly, in the lowest 
Vertebrate (Amphioxus, Fig. 28; Plate X. Fig. 7-10). 


Although the animals which we have named belong to 
ihe most diverse classes, they all have this in common 
with each other and with many other animals, that, owing 
to constant heredity, they have retained the palingenetic 
form of egg-cleavage and Gastrula-formation, which they 
received from their oldest common ancestors, up to the pre- 
sent day. This is, however, not true of the large majority 
of animals. On the contrary, in them the original process 
of germination has, in the course of many million years, 
gradually changed in a greater or less degree, and has 
become vitiated owing to adaptation to new conditions of 
evolution. Both the egg-cleavage, or segmentation, and 
the formation of the Gastrula, or gastrulation, which 
succeeds the segmentation, have in consequence of this 
acquired an aspect which is in many ways different. In 
the course of time the differences have even become so 
marked that the cleavage process of most animals was 
wrongly interpreted, and the Gastrula of these animals was 
altogether unknown. It is only owing to the extensive 
comparative researches which I instituted in late years 
among animals of the most diverse classes, that I have been 
enabled to indicate the one common process which under- 
lies those processes of germination, apparently so different, 
and have traced back all the diverse forms of germination 
to the one original form, the form which has already been 
described. To distinguish them from this primary palin- 
genetic form of germination, I shall call all the secondary 
forms, varying from the primary, vitiated, or kenogenetic 
processes. The more or less varying Gastrula-form, which 
results from this kenogenetic egg-cleavage, may be called, 
generally, the secondary, modified Gastrula, or Metagaetrula. 


Among the many and various kenogenetic or vitiated 
forms of eg^-cleavage and gastrulation, I again distinguish 
three different chief forms : 1. Unequal cleavage (segmen- 
tatio inceqaalis, Plate II. Fig. 7-17) ; 2. Discoidal cleavage 
(segmentatio discoidalis, Plate III. Fig. 18-24) ; and 3. 
Surface cleavage (segmentatio superjlcialis, Plate III. Fig. 
25-30). Unequal cleavage results in a Hood-gastrula 
(Amphigastrula, Plate II. Fig. 11 and 17); discoidal cleavage 
results in a Disc-gastrula (Discogastrula, Plate III. Fig. 24); 
surface cleavage results in a Bladder-gastrula (Perigastrula, 
Plate III. Fig. 29). The last form does not occur among 
Vertebrates, with which we are now specially concerned ; 
it is, on the contrary," the usual form among Articulated 
Animals (Spiders, Crabs, Insects, etc.). In Mammals and 
Amphibia the cleavage is unequal, and the Gastrula is a 
Hood-gastrula ; this is equally true of the Ganoid fish and 
the Round-mouths (Lampreys and Hagfishes). On the other 
hand, in most Fishes, and in all Reptiles and Birds, we find 
the discoid form of cleavage, and a Disc-gastrula. (Cf. 
Table III.) - " 1 

As Man is a true Mammal, and as human germination 
is entirely similar to that of other Mammals, the cleavage 
in his case also is unequal, and results in the formation of a 
Hood-gastrula (Amphigastrula, Plate II. Fig. 12-17). But 
it is peculiarly difficult to investigate the first incidents in 
the egg-cleavage and gastrulation of Mammals. It is true 
that more than thirty years ago the anatomist Bischoff, of 
Munich, laid a foundation for this work in two books, which 
he published, on the germ-history of the Rabbit (1842), and 
on that of the Dog (1845) ; and that these were afterwards 
followed by two equally careful studies of the germination 


of the Guinea-pig (1852), and of the Roe-deer (1854). But it 
was only quite recently that Eduard van Beneden, an emi- 
nent Belgian zoologist, was able,- owing to the elaborated 
methods of preparation of the present day, to throw full 
light on the obscurity which surrounded the germination of 
Vertebrates, and to give a right explanation of its details. 
It still, however, remains so difficult to understand these 
details, that it is desirable to glance first at the germination 
of Amphibia. In common with Mammals, these animals 
exhibit unequal cleavage, and form a Hood-gas trula. But 
the details of germination are simpler and more evident in 
Amphibia than in Mammals, and they are more nearly akin 
to the original, palingenetic form of germination. 

The eggs of the common, tailless Amphibia, of the Frog 
and the Toad, afford the best and most convenient objects 
for this examination. Masses of them are easily obtainable 
in the spring from all ponds and pools ; and a careful 
examination of the eggs with a magnifying glass is suffi- 
cient to show at least the external features of the egg- 
cleavage. In order, how r ever, to obtain a correct idea of the 
intricate details of the whole process, and to understand the 
formation of the germ-layers and of the gastrula, the egg of 
the Frog must be carefully hardened, and, the thinnest 
possible sections having been cut with a razor from the 
hardened egg, these must be most minutely examined under 
a powerful microscope. 66 

The eggs of the Frog and of the Toad are globular in 
form, and have a diameter of about two millimetres ; they 
are laid in great numbers in masses of jelly, which, in 
the case of the Frog, form thick lumps, while those of the 
Toad form long strings. When the opaque, brown, grey, 


or black-coloured egg is minutely examined, the upper 
half appears darker than the lower. -In some kinds, tbe 
centre of the upper half is blacker, while the corresponding 
centre of the lower half is of a whiter colour. 67 This marks 
a distinct axis of the egg with two different poles. In order 
to give a clear conception of the cleavage of this egg, it is 
best to compare it to a globe, on the surface of which 
different meridian and parallel circles are marked. For 
the superficial boundary lines between the different cells, 
which result from repeated division of the egg-cell, have 
the appearance of deep furrows on the surface, for which 
reason the whole process has received the name of "the 
furrowing " (i.e. cleavage). 59 But this so-called cleavage, 
which was formerly regarded with astonishment as a very 
wonderful process, is, in reality, only an ordinary and often- 
repeated division of the cells. Therefore the " cleavage- 
globules," which result from it, are really true cells. 

Unequal cleavage, as we see it in the amphibian egg, is 
especially marked by the fact that it begins at the 
upper, darker pole — the north pole of the globe, according to 
our simile — and proceeds slowly downwards towards the 
lower, lighter pole, the south pole. During the egg-cleav- 
age the upper, darker hemisphere is in advance, and its 
cells divide more vigorously and quickly ; the cells of the 
lower hemisphere, therefore, appear larger and less numer- 
ous. 67 The cleavage of the parent-cell (Fig. 31, A) begins 
with the formation of an entire meridian-furrow, which 
starts at the north pole and ends at the south pole (B). 
An hour later, a second meridian-furrow arises in the same 
way, and cuts the first at right angles (Fig. 31, C). The 
sphere of the egg is thus divided into four similar segment*. 



Each of these four first cleavage-cells consists of an upper, 
darker, and of a lower, brighter half. A few hours after- 
wards a third furrow appears, perpendicularly to the two 

Fig. 31.— The cleavage of a Frog's egg (10 times enlarged) : .4. die 
parent-cell ; B, the two first cleavage-cells ; C, 4 cells ; D, 8 cells (4 
animal and 4 vegetative) ; E, 12 cells (8 animal and 4 vegetative) ; F, 
16 cells (8 animal and 8 vegetative) ; G, 24 cells (16 animal and 8 vege- 
tative) ; H, 32 cells; I, 48 cells; K, 64 cells; L, 96 cleavage-cells; M, 
160 cleavage-cells (128 animal and 32 vegetative). 

former (Fig. 31, D). This ring-furrow is generally, but 

wrongly, called the " equatorial furrow ; " it lies north from 

the equator, and should, therefore, rather be compared to the 


northern tropical line. The spherical egg now consists of 
8 cells, 4 smaller, upper, or northern, and 4 larger, lower, 
or southern. A meridian-furrow, starting from the norther n 
pole, now appears in each of the first four cells, each of 
which falls into two similar halves, so that 8 upper cells 
lie on 4 lower cells (Fig. 31, E). It is only later that the 
four new meridian cells place themselves slowly on the 
lower cells, so that the number mounts from 12 to 16 (F). 
Parallel to the first, horizontal ring-furrow, a new ring- 
furrow now appears, nearer the northern pole ; this, there- 
fore, we may compare to the arctic circle. The result of 
this is that we find 24 cleavage-cells : 16 upper, smaller 
and darker, and 8 lower, larger and brighter (G). The 
latter, however, soon separate into 16, for a third parallel 
circle appears in the southern hemisphere ; there are, there- 
fore, 32 cells in all (Fig. 31, H). Eight new meridian- 
furrows now arise at the northern pole, and, first cutting 
the upper, darker, cellular circle, afterwards intersect the 
lower, southern circle, and finally reach the southern 
pole. We thus find stages in which there are successively 
40, 48, 56, and finally, 64 cells (/, K). The inequality 
between the two hemispheres constantly becomes greater. 
While the inert southern hemisphere, for a long time, does 
not add to its 32 cells, the vigorous northern half of the 
globe furrows itself twice successively, and thus parts into 
64 ; and then into 128 cells (Fig. 31, L, M). In the stage 
in which we now see the egg, there are, therefore, 128 
small cells on the surface of the upper, darker half of the 
egg-sphere, and only 32 cells in the lower, brighter half: 
LtiO cleavage-cells in all. The inequality between the two 
hemispheres increases yet further ; and while the northern 


hemisphere parts into a very large number of small cells, 
the southern hemisphere consists of a much smaller number 
of larger cells. Finally, they almost entirely overgrow 
the surface of the spherical egg ; and it ie only at a small 
circular point in the middle of the lower hemisphere, at the 
south pole, that the inner, larger, and brighter cells are 
visible. This white space at the southern pole corresponds, 
as we shall presently see, to the primitive mouth of the 
Gastrula. The whole mass of inner, larger, and brighter 
cells (together with this white space at the pole) belongs 
to the entoderm, or intestinal layer. The outer envelope of 
dark, smaller cells forms the exoderm, or skin-layer. 

The often repeated division of the cells, which as 
cleavage or segmentation is plainly traceable on the surface 
of the egg-sphere, is not confined to this surface, but ex- 
tends to the whole interior of the ball of the egg. The 
cells also segment in strata, which approximately corre- 
spond to concentric strata of the sphere ; this process ad- 
vances more quickly in the upper than in the lower half. 
A large cavity, filled with liquid forms, has in the mean 
time arisen, in the interior of the egg-sphere ; this is the 
cleavage-cavity (s, dra wings of sections in Plate II. Fig. 
8-11). The first trace of this cavity makes its appearance 
in the middle of the upper hemisphere, at the point at 
which the three first cleavage-planes, which are at right 
angles to one another, intersect (Plate II. Fig. 8 s). During 
the progress of cleavage, this hollow extends significantly, 
and afterwards assumes an almost hemispherical form (Fig. 
32 F; Plate II. Fig. 9 s, 10 s). The arched roof of this 
hemispherical cleavage -cavity is formed by the smaller, 
darker-coloured cells of the skin-layer, or exoderm (Fig. 



32, D) \ on the other hand, the flat floor of the cavity is 
composed of the larger, whiter-coloured cells of the intes- 
tinal layer, or entoderm (Fig. 32 z). 



Fig. 32-35. — Four longitudinal sections of the segmented egg of a Toad, 
in four successive stages of evolution. In all, the letters indicate the same 
parts : F, cleavage-cavity ; D, the roof of this cavity ; B, dorsal half of 
the germ; B, intestinal half; P, the yelk-plug (white circular space at the 
lower pole) ; z, yelk-cells of the entoderm (the gland-germ of Remak) ; 
N, primitive intestinal cavity (prfltoga-ster, or Rnsconi's nutritive cavity). 
The primitive mouth is filled up by the yelk-plug (P) ; s, boundary between 
the primitive intestinal cavity (N) and the cleavage-cavity (F) ; h, V, section 
through the swollen circular lip or edge of the primitive mouth (the so- 
called anus of Rusconi). The dotted line between A- and A' indicates the 
former connection between the yelk-plug (P) and the central mass of yelk- 
cells (2) In Fig. 35 the egg has turned round 90°, so that the dorsal half 
of the germ (R) is seen above j the intestinal half (B) is now turned down- 
ward. (After Strieker.) 


A second cavity, narrower but larger, now arises, owing 
to an inversion of the lower pole, and to a separation in 
the white entoderm-cells next to the cleavage-cavity (Fig 
32-35, N). This is tne primitive intestinal cavity or 
stomach-cavity of the Gastrula, the Protoga&ter. It was 
first observed by Rusconi in the eggs of Amphibia, and 
is accordingly called Rusconi's "nutritive cavity." In the 
longitudinal section (Fig. 33) it appears bent and sickle- 
shaped, and extends from the south pole nearly to the 
north, for it folds a portion of the inner intestinal cells 
inward and upward — between the cleavage-cavity (F) and 
the dorsal skin (R). The primitive intestinal cavity is so 
narrow at first because the greater part of it is filled up 
with the yelk-cells of the entoderm. The latter also plug 
up the entire wide opening of the primitive mouth, and 
there form the so-called yelk -plug, which appears from the 
outside as the white, circular spot at the south pole (P). 
Round this yelk-plug the skin-layer thickens, swells, and 
forms the lip of the primitive mouth (the properistoma, 
Fig. 35 k, k'). Presently the primitive intestinal cavity (i\ r ) 
extends gradually at the cost of the cleavage-cavity (F) ; 
and, finally, the latter entirely disappears. A thin partition 
(Fig. 34, s) alone separates the two cavities. That portion 
of the germ in which the primitive intestinal cavity de- 
velops, afterwards becomes the dorsal surface (R). The 
cleavage-cavity lies in the anterior, the yelk-plug in the 
posterior part of the body. 68 

When the primitive intestine is complete, the Fiog- 
embryo has reached the Gastrula stage (Plate II. Fig. 11). 
But it is evident that this kenogenetic amphibian Gastrula 
differs greatly from the genuine palingenetic Gastrula, which 


we saw before (Fig. 23-29). In the latter, the Bell-gastrula 
(Archigastrula) , the body has but one axis. The primitive 
intestine is empty, and the opening of the primitive mouth 
is wide. The skin-layer and the intestinal layer consist 
each of a single cell stratum. The two lie close together, 
for the cleavage-cavity has entirely disappeared during the 
process of unfolding. The amphibian Hood-gastrula (Am- 
phigastrula) is entirely different (Fig. 32-35 ; Plate II. 
Fig. 11). In this the cleavage-cavity (F) continues for a 
considerable time side by side with the primitive intestinal 
cavity (JS T ). Yelk-cells fill the greater part of the latter ; 
and they also fill the primitive mouth (yelk-plug, P). Both 
the intestinal layer (z) and the skin-layer (a) consist of 
several strata of cells. Finally, the general outline of the 
entire Gastrula, instead of having only one axis, has three ; 
for the three axes which characterize the bilateral body 
of the higher animals, are indicated by the eccentric evo- 
lution of the primitive intestinal cavity. 

In the evolution of the Hood-gastrula (Amphigastrula) 
we are unable to distinguish sharply between the different 
epochs, which, marked by the mulberry -germ and the germ- 
vesicle, we saw followed each other in the case of the Bell- 
gastrula (Archigastrula). The Morula-stage (Plate II. Fig. 
9) is as indistinctly separated from the Blastu la-stage 
(Fig. 10), as the latter is from the Gastrula (Fig. 11). But 
in spite of this, we shall not have much difficulty in retra- 
cing the whole kenogenetic or vitiated course of evolution 
of this amphibian Amphigastrula to the genuine, palin- 
genetic origin of the Archigastrula of the Amphioxus. 

It Ls far harder to do this in the case of Mammals, 
although the course of egg-cleavage and gastrulation in 


these is, on the whole, very similar to that of Amphibia 
Until recently the growth of the mammalian embryo was 
entirely wrongly explained ; and it is onl} 7 lately (1875) 
that Van Beneden, whose views we adopt here, pointed out 
its real significance. 69 His studies were directed towards 
,he embryo of the Rabbit, an animal in connection with 
which Bischoff first discovered the history of the mamma- 
lian germ. As the Rabbit in common with Man belongs to 
the group of disco-placental Mammals, as this Rodent 
develops entirely in the same way as does Man, and as even 
at a later stage of evolution the embryos of Man and of the 
Rabbit are hardly distinguishable (cf. Plate VII. Fig. 
K y M), there is not the slightest reason to doubt that the 
egg -cleavage and gastrulation of the two are similar. 

When the fertilization of the egg of the Rabbit is com- 
plete, and the elaboration of the parent-kernel has trans- 
formed the Monerula (Fig. 36) into the parent-cell, or cytula 
(Fig. 37), the latter (the cytula) separates into the two first 
cleavage-cells (Fig. 38). In this process the parent-kernel 
first becomes fusiform and divides into two kernels (the 
two first cleavage-kernels). These repel each other and the 
two move apart. After this the protoplasm of the parent- 
ceD, attracted by the two kernels, parts into two halves, 
each of which assumes a globular form. They afterwards 
change from this globular to an ellipsoid form Tig. 38), 
These two cleavage-cells are not, as was formerly believed,, 
of the same size and significance. The one is larger, 
brighter, and more transparent than the other. Again, the 
smaller cleavage-cell takes a much deeper colour from car- 
mine, osmium, etc., than does the larger. The two cells 
thus already betray theii relations to the two primitive 



Ftg. 36. — Monerula of a Mammal (Rabbit). The fertilized egg-cell after 
loss of the germ-vesicle is a simple ball of protoplasm (<£). The outer 
envelope of this is formed by the modified zona pellucida (z) and by a mucous 
layer (7i), which is deposited on the outside of the zona. A few sperm-cells 
are still visible in this mucous laver (s). 

Fig. 37. — Parent-cell, or cytula, of a Mammal (Rabbit) : fe, parent-kernel, 
or nucleus ; n, nucleolus; p, protoplasm of the parent-cell; z, modified 
zona pellucida ; h, external albuminous envelope ; s, sperm-cells. 

Fig. 38. — Commencement of cleavage in the mammalian egg (Eabbit). 

The parent-cell has separated into two differing cells ; the brighter, 
mother-cell of the skin-layer (e), and the darker, mother-cell of the in- 
testinal layer (i) : z, zona pellucida; h, external albuminous envelope; 
s, dead sperm-cells. 


germ-layers. The brighter and harder cleavage-cell (Fig. 
38, e) is the mother-cell of the exoderm ; the darker and 
softer cleavage-cell (Fig. 38, i) is the mother-cell of the 
entoderm. All the cells of the outer germ-layer, the skin- 
layer, are produced from the exoderm mother-cell (Fig. 
38, e ; Plate II. Fig. 13, e). In the same way the whole of 
the cells of the inner germ-layer, the intestinal layer, 
descend from the entoderm mother-cell (Fig. 38, i; Plate 
II. Fig. 13, i). This interesting relation, which we thus see 
in the mammalian germ, is yet more pronounced in the 
germs of many lower animals. In many Worms, for 
example, at the beginning of cleavage, the parent-cell 
parts into two cleavage-cells of very dissimilar size and 
chemical qualities. In such cases the mother-cell of the 
exoderm is often very many times smaller than the ento- 
derm mother-cell, which contains a large store of nutritive 


The two first cleavage-cells of the Mammal, which are 
to be regarded as the mother-cells of the two primary germ- 
layers, now contemporaneously separate into two cells (Fig. 
39 ; Plate II. Fig. 14). These four cleavage-cells usually lie 
in two different planes, perpendicular to each other ; more 
rarely in one plane. The two larger and brighter cells 
(Fig. 39, e), the descendants in the first generation of the 
exoderm mother-cell, if placed in carmine, colour much 
more deeply than do the two smaller and darker cells, the 
descendants of the entoderm mother-cells (Fig. 39, i). The 
line which connects the central points of the two latter 
uleavage-globules is usually perpendicular to that which 
connects the central points of the two latter. Presently 
each of these four cells again divides into two similar cells; 



we therefore find that there are now eight cleavage-cells, 
the descendants in the third generation of the parent-cell 
(Fig. 40). Four larger, brighter, and firmer cells lie in one 
plane ; the descendants in the second generation of the 
exoderm mother-cell. Four smaller, darker, and softer cells 
lie in a second plane, perpendicular to the former ; the 
descendants in the second generation of the entoderm 
mother-cell. If we connect the central points of the oppo- 
site cleavage-cells of one plane, two and two, by straight 
lines, these lines meet each other at right angles. But the 
four connecting lines of the two parallel planes together 
intersect at an angle of forty-five degrees (Fig. 40). 

Fig. 39. — The four first cleavage -cells of a Mammal (Babbit) : e, the 
two exoderm-cells (larger and brighter) ; i, the two entoderrn-cells (smaller 
and darker) ; z, zona pellucida ; h, outer albuminous envelope. 

Fig. 40. — Egg of Mammal (Rabbit), with eight cleavage-cells: e, four 
exoderm-cells (larger and brighter) ; i, four entoderm -cells (smaller and 
darker) ; z, zona pellucida ; h, outer albuminous covering. 

Now, however, the eight cleavage-cells alter their 
original position, and lose their globular form. One of the 



four exoderm-cells makes its way into the middle of the 
cell-mass, and, together with its three fellows, forms a pyra- 
mid (or tetrahedron). The four exoderm-cells arrange 
themselves in the form of a cap over the poin£ of this 
pyramid (Plate II. Fig. 15). This is the beginning of a 
germinal process which must be regarded as a shortened 
and vitiated repetition of the inversion of the germ-mem- 
brane vesicle, and which results in the formation of a Gas- 
trula. From this time the further cleavage of the mam- 
malian egg adheres to a rhythm which is most essentially 
similar to that of the Frog's egff. While in the original 

© c>c> o 

(or primordial; egg- cleavage, the rhythm advances in regular 
geometrical progression (2, 4, 8, 16, 32, 64?, 128, and so on) ; 
in the modified progression of the mammalian egg, the 
sequence of numbers is the same as that of the amphibian 
egg : 2, 4, 8, 12, 16, 24, 32, 48, 64, 96, 160, etc' (Cf. Table V.) 

Fig. 41. — Gastrula of a 
Mammal (Amphi gastrula of a 
Rabbit), in longitudinal section 
through the axis : e, exoderm- 
cells (64 brighter and smaller) ; 
i, entoderm-cells (32 darker and 
larger) ; d, central entoderm- 
cells, filling up the primitive in- 
testinal cavity ; o, external ento- 
derm-cells, plugging the primi- 
tive mouth-opening (yelk-plug 
in the " anus of Rusconi "). 

This depends on the fact that from this time the more 
vigorous exoderm-cells increase at a quicker rate than the 
more inert entoderm-cells. The latter always remain 
behind the former, and are overgrown by them. This pro- 


cess in which the inner intestinal layer cells are overgrown, 
is really nothing but the inversion of the vegetative hemi- 
sphere into the animal hemisphere of the germ-vesicle ; i.e. 
the formation of a Gastrula (Fig. 41). 69 

Next, therefore, follows a stage in which the mamma- 
lian germ consists of 12 cleavage-cells ; 4 darker entoderm- 
cells form a three-sided pyramid which is covered by a cap 
of 12 lighter exoderm-cells (Plate II. Fig. 15 in section). 
The next stage, in which there are 16 cleavage -cells, is seen 
to consist of 4 entoderm-cells in the interior, 4 other outer 
and lower entoderm-cells ; while the 8 exoderm-cells, in the 
form of a hemispherical cap, cover the upper half of the 
germ. This cap of exoderm-cells, which increase in number 
from 8 to 16, continues to overgrow the inner cell mass; of 
the 8 entoderm-cells, 3, 4, or 5 lie in the centre of the germ, 
and the rest at the base of the globular germ (Plate II. 
Fig. 16). This 24-celled stage is followed by one in which 
there are 32, for the 8 entoderm-cells also double their 
number. This is afterwards succeeded by germ-forms in 
which there are 48 cleavage-cells (32 exoderm and 16 ento- 
derm) ; 64 cleavage-cells (32 skin-layer and 32 intestinal 
layer) ; 96 cleavage-cells (64 exoderm and 32 entoderm), 
and so on. 

When the mammalian embryo has acquired 96 cleavage- 
cells, a stage which, in the case of the Rabbit, is reached 
in about the 70th hour after fertilization, the charac- 
teristic form of the Hood-gastrula (Amjyhigastrula) becomes 
plainly visible (Fig. 41 ; cf. Plate II. Fig. 17 in section). 
The globular embryo consists of a central mass of 32 soft.. 
roundish, dark granular entoderm-cells, which, by mutual 
pressure, are flattened multilaterally, and which assume 


a dark brown colour when treated with osmic acid (Fig. 
41, i). This dark central cellular mass is surrounded by 
a brighter globular membrane, composed of 64 smaller cube- 
shaped and finely granulated exoderm-cells, which lie side 
by side in a single layer, and take up very little colour from 
osmic acid (Fig. 41, e). The exoderm-membrane is broken 
only at one single point, when 1, 2, or 3 entoderm-cells 
pierce to the surface. The latter form the yelk-plug 
which entirely occupies the primitive mouth of the Gastrula 
(o). The central primitive intestinal cavity is filled by 
entoderm-cells (Plate II. Fig. 17). The single axis of the 
outline of the mammalian Gastrula ia thus clearly indi- 
cated. 69 

Although the unequal egg-cleavage and gastrulation of 
Mammals and Amphibia present various peculiarities, it 
is comparatively easy to trace these processes back to the 
egg-cleavage and gastrulation of the lowest Vertebrate, the 
Amphioxus, which is entirely similar to the form of cleav- 
age carefully examined by us in the case of the Coral. (Cf. 
Fig. 22 and 28.) All these and many other classes of 
animals agree in that, in their egg-cleavage, the whole egg 
parts, by repeated division, into a large number of cells. 
All such animal eggs have long been called holoblastic, a 
name given them by Remak, because in them the cleavage 
into cells extends to the whole mass ; or, in other words, is 
total (Plate II.). 

In very many other classes of animals this is, however, 
not the case ; for instance, among Vertebrates, in Birds, Rep- 
tiles, and most Fishes ; among Articulated animals (Arthro- 
poda), in Insects, most Spiders and Crabs; among Soft- 
bodied animals {Mollusca), in Cephalopoda or Cuttle-nahea 


In all these animals, both the ripe egg-cell, and the parent- 
cell, into which fertilization transforms this egg-cell, consist 
of two quite distinct and separate parts, which are distin- 
guished respectively as the formative yelk and the nutritive 
yelk, The formative yelk (yitellus formativus, or morpho- 
leciihus) is the nucleated egg-cell, capable of evolution, which 
divides in the prpcess of cleavage, and produces the nu- 
merous cells which constitute the embryo. The nutritive 
yelk (yitellus nutritivus, or tropholecithus), on the other 
hand, is a mere appendage of the true egg-cell, and contains 
hoarded food-substance (albumen, fat, etc.); so that it forms 
a sort of storehouse for the embryo in the course of its 
evolution. The embryo absorbs a quantity of nutritive 
matter from this storehouse, and finally entirely consumes it. 
Indirectly, therefore, the nutritive yelk is of great import- 
ance in germination. Directly, however, it takes no share 
in the process, for it is not concerned in the cleavage, and 
is not cellular. Sometimes the nutritive yelk is smaller, 
sometimes larger; generally many times larger than the 
formative yelk ; for which reason, greater importance was 
formerly attached to the nutritive than to the formative 
yelk. All eggs which have this independent nutritive yelk, 
and of which, therefore, only a portion undergoes cleavage, 
are called meroblastic, the name given them by Remak; 
their cleavage is incomplete or partial (Plate III.). 

It is not easy correctly to apprehend this partial egg- 
cleavage, and the peculiar form of Gastrula which results 
from it; and it was only quite recently that comparative 
research enabled me to remove this difficulty, and to retrace 
this kenogenetic form of cleavage and gastrulation to the 
original, palingenetic form. The sea eggs of one of the 



Osseous Fishes (Teleostei), the evolution of which I studied 
at Ajaccio, in Corsica, in 1875, were of the greatest service 
to me in this respect (Plate III. Fig. 18-24). I found these, 
massed together in lumps of jelly, floating on the surface of 
the sea ; and as the tiny eggs were quite transparent, I was 
easily able to watch each stage in the evolution of the 
germ. 70 These eggs, probably those of a cod-fish of the 
Gaddoid family, but perhaps of a Cottoid, are colourless 
globules, as transparent as glass, and of rather more than half 
a millimetre in diameter (0'64 — 066 mm.). Within a thin, 
structureless but firm egg-membrane (chorion, Fig. 42, c) lies 

Fig. 42. — Egg of an oceanic Osseous 
Fish : p, protoplasm of the parent. cell ; Tc, 
kernel of parent-cell ; n, clear albumin- 
ous ball of nutritive yelk ; /, fat-globule 
of the latter ; c, external egg-membrane, 
or chorion. 

a large albuminous ball, which 
is quite transparent and as clear 
as water (n). At both poles of 
the axis of this ball there is a 

groove-like indentation. In the groove at the upper pole, 
which, in the floating egg, is turned downwards, lies a 
simple, lentil-shaped cell, containing a kernel (Fig. 42, p). 
In the unfertilized egg, this is the original egg-cell ; after 
fertilization it is the parent-cell. In the interval between 
these two nucleated stages there is probably a non- 
nucleated condition, representing the Monerula. At the 
opposite pole of the egg, in the lower groove, lies a simple, 
clear fat-globule (/). This small fat-globule and the large 
albuminous globule together form the nutritive yelk. The 


small cell alone is the formative yelk, and is the only pan 
concerned in the cleavage process, which does not extend 
to the nutritive yelk. 70 

The cleavage of the parent-cell, or the formative yelk, 
proceeds entirely independently of the nutritive yelk, and 
in quiet, regular, geometric progression. (Cf. Plate III. Fig. 
18-24.) Only the formative yelk, with the contiguous 
portion of the nutritive yelk (ri) t is represented in the 
perpendicular section (through a meridian-plane); the 
greater part of the nutritive yelk and the egg-membrane 
is therefore omitted. The parent-cell (Fig. 18), first sepa- 
rates into two similar cleavage-cells (Fig. 19). By repeated 
division, this gives rise to 4, then 8, then 16 cells (Fig. 20). 
By continued contemporaneous division, 32, and then 64 
cells originate from these ; and so the process goes on. All 
these cleavage-cells are alike in size and character. At last 
they form a lentil-shaped mass of closely layered cells (Plate 
III. Fig. 21). This entirely corresponds to the globular 
mulberry-germ of the primordial cleavage-process (Morula, 
Plate II. Fig. 3). The cells of this lentil-shaped mulberry- 
germ now move off in a peculiar centrifugal direction, 
so that the mulberry-germ changes into a vesicular germ 
(Blastula, Plate III. Fig. 22). The ordinary lentil be- 
comes a disc, in the shape of a watch-glass, with thickened 
edges. Just as a watch-glass lies upon a watch, this con- 
vex cellular disc lies on the upper, more slightly arched, 
'pole surface of the nutritive yelk. Meanwhile, liquid has 
collected between the disc and the surface of the nutritive 
yelk, so that a low circular cavity has been formed (Fig. 22, a). 
This is the cleavage-cavity, and corresponds to the cleavage- 
cavity in the centre of the palingentic Blastula (Plate II 



Fig. 4). The slightly arched floor of this low cleavage- 
cavity is formed of nutritive yelk (n) ; the more arched roof 
is of Blastula-cells. In fact, the embryonic Fish is now a 
vesicle with an eccentric cavity, as was the Blastula of the 
Frog (Plate II Fig. 10). 

The important process of inversion, resulting in gastru- 
lation, now takes place. In consequence of a further re- 
moval, or wandering, of the blastula-cells, and of a further 
increase in their number, the thickened edges of the cellular 
disc, which lie on the nutritive yelk, grow toward each 
other in a centripetal direction, and toward the centre of 
the cleavage-cavity (Fig. 23), at which point they finally 
unite. The whole cell-mass now forms a small flat sac lying 
on the top of the nutritive yelk. The cavity of this sac, 
the cleavage-cavity, soon, however, disappears, because the 
whole upper surface of the lower wall of the sac attaches 
itself closely to the whole lower surface of the upper wall 
(Fig. 24). This completes the gastrulation of this Fish. 

Fig. 43. — Disc-gastrula {Disco-gas- 
trula) of an Osseous Fish : e, exoderm ; 
i, entoderm ; iv, swollen edge, or primi- 
tive mouth-edge ; n, albuminous ball 
of nutritive yelk ; /, fat-globule with- 
in the latter ; c, outer egg-membrane 
(chorion) ; d, boundary between ento- 
derm and exoderm (former site of the 
cleavage - cavity) . 

In order to distinguish this third important form of 

Gastrula from the two previously mentioned, we will call it 

the Disc-gastrula (Disco-gastvula, Fig. 43). The cell-mass of 

this Gastrula forms a thin, circular disc. The lower concave 


surface of this disc lies immediately on the upper, convex 
surface of the nutritive yelk (n). On the other hand, the 
outer surface of. the disc is convex as in a Shark. If we 
make a perpendicular section through a meridian-plane of 
the globe-shaped egg, we shall find that it is composed of 
several layers of cells (in this particular case there are four) 
(Plate III. Fig. 24). Immediately above the nutritive yelk 
lies a single layer of larger cells (Fig. 24, i), which are 
characterized by a softer, less transparent, and more coarsely 
granulated protoplasm, and which take up a dark red colour 
from carmine. These form the intestinal layer, or entoderm, 
which arises by the ingrowth of the edges of the disc 
(infolded germ-layer). The three outer layers, lying on top 
of this lower layer, form the skin-layer, or exoderm (Fig. 24, e). 
They consist of smaller cells which take only a slight colour 
from carmine ; their protoplasm is firmer, more transparent, 
and more finely granulated. At the thickened edges of the 
gastrula, the primitive mouth-edge (properistoma), the 
entoderm, and the exoderm pass into each other without 
clear limits (Fig 43, w). 

It is evident that the most important peculiarities which 
distinguish the Disc-gastrula from the two typical Gastrula- 
forms which we before examined, are due to the large nutri- 
tive yelk. This takes no part in the cleavage, and from the 
first occupies the whole primitive intestinal cavity, while at 
the same time it extends far beyond the mouth-opening of 
the latter. If we imagine the original Bell-gastrula (Archi- 
gastrula, Fig. 23-29) attempting to swallow a globe of 
nutritive matter far larger than itself, in the attempt the 
Gastrula will be spread out in the form of a disc on the 
nutritive matter, much in the same way as in the Disc- 


gastrula (Disco-gastrula, Fig. 43). We may therefore infer 
that the latter is directly, or through the intermediate stage 
of the Hood-gastrala, descended from the original Eell- 
gastrula. It arose phylogenetically owing to the fact that 
a store of nutritive matter collected at one pole of the egg, 
and thus formed a nutritive yelk distinct from the forma- 
tive yelk. Yet, notwithstanding this, the Gastrula in this, 
as in the former cases, was originated by an inversion or 
invagination of the Blastula. We may, therefore, also refer 
this kenogenetic form of discoidal cleavage (segmentatio 
discoidalis) to the original and palingenetic form. 

Although it is thus tolerably easy and safe to trace back 
the descent of the small egg of this oceanic Osseous Fish, yet, 
on the other hand, it seems hard to do this with certainty 
in the case of larger eggs, such as occur in the case of most 
other Fishes, and in the case of all Reptiles and Birds. In 
the first place, the nutritive yelk of these is quite dispro- 
portionately large ; so large, indeed, that it almost causes 
the formative yelk to disappear. And, in the second place, 
the nutritive yelk contains a number of variously formed 
constituent parts, which are known as the yelk-granules, 
yelk -globules, yelk- vesicles, and so on. These definite yelk- 
elements have often even been explained as true cells, 
and it has been quite wrongly assumed that a portion 
of the body of the embryo is found in them. 71 This 
is by no means the case. The nutritive yelk, what- 
ever its size, always remains a lifeless store of nutritive 
matter, which, in the process of germination, is taken into 
the intestine during its development, and is consumed by 
the embryo. The latter develops solely from the living 
formative yelk, from the parent-cell. This is equally true 


of the small Osseous-fish which we have been examining, 
and of the huge eggs of the Primitive Fishes (Selachti), of 
Reptiles, and of Birds. 

The egg of the Bird is specially important to us, for 
most of the important researches into the evolution of 
Vertebrates have been founded on study of incubated hen's 
eggs. It is much harder to procure and to examine mam- 
malian eggs ; for which very practical and incidental reason 
the latter has been more rarely accurately studied. On 
the other hand, hen's eggs can always be obtained in 
any quantity, and artificial hatching enables us accurately 
to follow every stage in the changes undergone by the 
embryo in the course of its evolution. As we have seen, 
the chief difference which distinguishes the egg of the 
Bird from the minute egg of the Mammal is the very con- 
siderable size of the former, which is due to the accumula- 
tion of a very large mass of fatty nutritive yelk. This is 
the yellow mass which, daily consumed under the name of 
yelk of egg, is collected within the original yelk or proto- 
plasm of the egg-cell. In order to obtain a correct con- 
ception of the Bird's egg, the nature of which has very 
frequently been misrepresented, we must search for it in 
its earliest condition, and follow its evolution from its 
beginning in the ovary. In this stage, we find that the 
original egg is a very small, naked, and simple cell with 
a nucleus, and that it differs neither in size or shape 
from the original egg-cell of Mammalia and other animals. 
(Cf. Fig. 10 E, p. 134.) As in all SkuUed-animals (Crcmiota) 
the original egg-cell or primitive egg (protovum) is com- 
pletely covered by a continuous layer of smaller cells, M 
though by an epithelium. This skin-coat, or epithelium, is 


the so-called Graafian follicle ; immediately under this the 
structureless yelk-membrane is secreted by the egg-yelk. 

At a very early period the small protovum of the Bird 
begins to imbibe a mass of food-substance through the 
yelk-membrane, and to elaborate this matter into the so- 
called " yellow yelk." The protovum is thus transformed 
into the metovum (after-egg), which is many times larger 
than the protovum, but which, nevertheless, is only a single, 
enormously enlarged cell. 72 The accumulation of the large 
yellow-yelk mass within the ball of protoplasm forces the 
kernel (vesicula germinativa), which is contained in the 
latter, quite to the upper surface of the yelk-mass. Here 
the kernel (^vesicula germinativa) is surrounded by a small 
quantity of protoplasm ; and these two together form the 
lentil-shaped " formative yelk " (Fig. 44, b). This- appears 
ou the outside of the yellow yelk-mass, at a particular 
point of the upper surface, in the form of a small, white, 
circular point; the so-called "tread," or cicatricvla. A 

Fig. 44. — A mature egg-cell from the 
ovary of a Hen (in section). The yellow 
nutritive yelk is composed of concentric 
layers (c), and is surrounded by a thin yelk- 
membrane (a). The cell-kernel {vesicula germi- 
nativa), together with the protoplasm of the a < : 
egg-cell, forms the formative yelk (b), or the 
tread. The white yelk (here represented as 
black) passes from the tread to the yelk- 
cavity (a"). The two kinds of yelk are. 
however, not sharply distinguished. 

thread-like cord of white nutritive yelk (d), which contains 
no particles of yellow yelk, and is softer than the yellow 
nutritive yelk, passes from the tread directly to the 


centre of the yellow yelk-mass, and there forms a small 
central ball of white yelk (Fig. 44, d). The whole mass of 
this white yelk is, however, not sharply divided from the 
yellow yelk, which in hardened eggs shows a slight trace 
of concentric stratification (Fig. 44, c). Just as in this 
globular egg in the ovary, so also in the hen's egg after 
it has been laid; when the egg-shell is opened and the 
yelk taken out, a small, circular, white disc is seen on 
the upper surface of the latter. This disc represents the 
cicatricula, or tread. This small white germ-disc is, how- 
ever, far advanced in development, and is, in fact, the 
Gastrula of the hen. The body of the latter proceeds 
entirely from this Gastrula. The whole mass of white and 
yellow yelk is entirely without share in the formation of the 
Chick, for it is only used up as nutritive matter and con- 
sumed as food by the embryo in the course of its evolution. 
The transparent, tough, and voluminous mass of albumen, 
surrounding the yellow yelk of the Bird's egg, and the hard 
chalky shell of the egg, are formed round the egg t in the 
oviduct, after it is already fertilized. 

After the fertilization of the egg within the body of the 
parent Bird is complete, the germ-vesicle (vesicula ger- 
minativa) probably, as in other cases, first disappears ; and 
the reconstruction of a kernel results in a parent-cell 
(cytula). This lentil-shaped parent-cell now undergoes a 
discoidal cleavage (segmentatio discoidalis, Fig. 45) entirely 
similar to that of the egg of the Fish (Plate III. Fig. 18-24). 
Two similar cleavage-cells (-4) first arise from the parent- 
cell. These part into 4 (B), into 8, 16 (0), 32, 64 cells, and 
so on. As before, the division of the kernel always precedes 
the division of the cells. The planes of division between 



the cleavage-cells appear at the free surface of the " tread ' 
as " furrows." The two first furrows are at right angles 
to each other, in the form of a cross (B). Two new furrows 
then originate, which cut the former two at an angle of 45^, 
The tread, which is changing into the germ-disc, now forms 

Fig. 45. — Discoidal cleavage of a Bird's egg (diagrammatic, about ten 
times enlarged). Only the formative yelk (the tread, or cicatricula, is repre- 
sented in these 6 figures (A-F), because it alone is affected by cleavage. 
The much larger nutritive yelk, which does not share in the cleavage, is 
omitted, and only indicated by the dark, outer ring. A. The first furrow 
separates the parent-cell into two parts. B. These two first cleavage -eel Is 
are parted by a second furrow (perpendicular to the first) into four cells. 
C. 16 cells have originated from the 4 cleavage-cells, owing to the fact that 
between the first two bisecting furrows, two other, radial furrows have 
appeared, and that the central portions of these 8 radial segments by 
a furrow running round the centre. D. A stage with 16 radial furrows and 
about 4 concentric ring-furr6ws. E. A stage with 64 radial furrows and 
about 6 ring-furrows. F. The whole tread has been broken up into a heap 
of small cells by the further formation of radial and ring furrows ; the whole 
now forms the lentil-shaped mulberry-germ (Morula). The separation of 
the kernel always precedes the formation of the furrows. 


an eight-rayed star. A circular furrow now forms round 
the centre, so that the 8 three-cornered cleavage-cells 
become 16, of which 8 lie in the middle, surrounded by 
8 others ((7). After this, new furrows, some circular and 
others radiating from the central point, succeed each other 
more or less irregularly (D, E). Finally this cleavage- 
process, like the others, results in the formation of small 
cells of like character. 78 In this case also, the cleavage- 
cells form a circular lentil-shaped disc, which represents the 
mulberry-germ, and lies embedded in a slight deepening in 
the white yelk (Fig. 46, in perpendicular section). The 
Morula in the case of the Hen's egg is, however, thinner and 
flatter than that of the egg of the Osseous Fish (Plate III. 
Fig. 21). 

In the Hen's egg, just as in that of the Osseous Fish, a 
kenogenetic germ-vesicle, or Blastula, now arises (Fig. 47). 
The cleavage-cells of the Morula increase in number and 
move away from the nutritive-yelk, so that a disc, in 
the form of a watch-glass, with thickened edges (w), is 
again formed ; and a cleavage-cavity (s) is formed between 
this germ-membrane (Blastoderma, Fig. 47, b) and the 
nutritive yelk. After this the thickened, swollen edge 
turns inward, and a simple layer of larger, darker-coloured 
cells grows from the edge, centripetally towards the middle 
of the cleavage-cavity (Fig. 48). The meeting of these two 
edges at a central point gives rise to the intestinal layer, or 
entoderm (Fig. 48, i). This attaches itself immediately to 
the roof of the cleavage-cavity, the cells of which form the 
skin-layer, or exoderm (Fig. 49, i). This completes the 
Gastrula of the Chick, a flatly extended, disc-shaped Gas- 
trula (Dwcogastrula), resembling that of the Osseous Fish 


(Plate III. Fig. 24). While, however, in the latter case the 
nutritive yelk is attached directly to the lower surface of 
the entoderm, filling the whole primitive intestinal cavity, 
a low germ-cavity remains between the entoderm and the 
nutritive yelk in the Disc-gastrula of the Chick ; this is a 
part of the primitive intestinal cavity (Fig. 49, d), and must 
not be confused with the cleavage-cavity (Fig. 47, 8, 48, s). 
The latter lies between the nutritive yelk and the blasto- 
derm, the former between the nutritive yelk and the ento- 
derm. The inversion (invagination) of the Gastrula is 
complete when the primitive intestinal cavity has taken 
the place of the cleavage-cavity, the entoderm at the same 
time attaching its inner surface to the inner surface of the 

The germ-disc (Blastodiscus), which in an unincubated, 
freshly-laid Hen's egg lies at the tread, or cicatricula, is 
thus already a complete Disc-gastrula (Discogastrula, Fig. 
4i9). It is plainly visible to the naked eye, and appears 
like a small, circular, white spot, 4-5 mm in diameter, in 
the middle of the upper surface of the yellow yelk-mass. 
It is separated from the latter by the primitive intestinal 
cavity, and its thickened edges alone touch the latter. It 
is possible to lift up the entire Gastrula. The two primary 
germ-layers are plainly visible in the perpendicular section ; 
an upper or outer layer of smaller, brighter cells forming 
the skin-layer (exoderm, Fig. 49, e) ; and a lower or inner 
layer of larger, darker cells forming the intestinal layer 
(entoderm, Fig. 49 f). 74 

In order to complete our survey of the important pro- 
cesses of egg-cleavage and gastrulation, we will now finally 
glance quickly at the fourth type-form of these processes 



superficial cleavage (segmentatio superficialis, Plate III. 
Fig. 25-30). This form is entirely unrepresented among 
Vertebrates. It, however, plays the most important part 

Fig. 46-49. — Gastrulation of a Hen's egg. All four figures represent 
perpendicular, half-diagrammatic sections through the middle of the thin, 
circular tread, or germ-disc. Of the nutritive yelk (n) only the contiguous 
part (perpendicularly shaded) is represented. 

Fig. 46. — (A) Mulberry-germ (Morula) ; b, cleavage-cells. 

Fig. 47. — (J5) Germ-vesicle (Blastula) ; s, cleavage-cavity; b, blasto- 
derm-cells ; w, thickened or swollen edge of the germ-disc. 

Fig. 48. — (C) Germ-vesicle in the process of inversion (Blastula in- 
vaginata) ; e, exoderm ; i, entoderm ; n, nutritive yelk ; w, thickened edge 
s, cleavage -cells. 

Fig. 49. — (P) Gastrula (Discogastrula) of Chick : d, primitive intestinal 


in the very extensive articulated tribe (Arthropoda), in 
Insects, Spiders, Centipedes, and Crabs. The Gastrula 
which results from this form of cleavage is the Bladder- 
gastrula (Peri-gastrula, Plate III. Fig. 29). 

In eggs which undergo this superficial cleavage, just as 
in the eggs which have been mentioned, those of Birds, 
Reptiles, Fishes, and other animals, the formative yelk is 
quite distinct from the nutritive ; and the former is alone 
concerned in the cleavage, which does not touch the latter. 
But while in those eggs, the cleavage of which is discoidal, 
the formative yelk is eccentric, and lies at one pole of the 
single axis "of the egg, while the nutritive-yelk is massed 
together at the other pole ; in those eggs, on the contrary, 
which undergo a superficial cleavage, we find that the 
formative yelk is spread over the whole surface of the egg, 
surrounding the nutritive yelk in the form of a bladder, 
which is central, and situated in the middle of the egg. 
The cleavage, as it affects only the former, not the latter, 
is naturally entirely superficial ; the provision, which is 
massed in the centre, is entirely untouched by it. Other- 
wise, this superficial cleavage proceeds quite regularly, like 
the original cleavage, in geometrical progression (Plate 
III. Fig. 25-30 represents several stages of this process in 
perpendicular meridian section through the ellipsoid egg of 
a Crab, Peneus.) The parent-cell, or cytula (Plate III. 
Fig. 25), first parts into two similar cells ; from these, by 
repeated simultaneous division, arise first 4 (Fig. 26), then 
8, then 16 (Fig. 27), 64, 128, and so on. Finally, the whole 
formative yelk parts into numerous, small, similar cells, 
which lie side by side in a single layer over the whole 
surface of the egg, forming a superficial germ- membrane 


(Bladoderma, Fig. 28, 0). This germ-membrane is a simply 
completely closed vesicle, the space within being wholly 
filled with nutritive yelk. The chemical quality of the 
contents of this true germ-vesicle, or Blastula (Fig. 28) 
alone distinguishes it from the Blastula of the primordial 
cleavage-process (Plate II. Fig. 4). The latter contains 
water, or jelly as transparent as water; the former con- 
tains a dense mixture of albuminous and fatty substances, 
in which there is much nutritive matter. As this extensive 
nutritive yelk occupies the centre of the egg from the very 
beginning of the cleavage, there is naturally no difference in 
this case between the mulberry-germ and the vesicular 

When the germ-vesicle (Fig. 28) is quite complete, the 
important process of inversion (invaginatio), which produces 
the Gastrula, follows (Fig. 29). A circular, groove-like 
deepening first arises at a point on the surface, and this 
enlarges into a cavity, the primitive intestinal cavity of 
the Gastrula (Fig. 29, d) ; the point at which the inversion 
takes place forming the primitive mouth of this cavity (0). 
The inverted portion of the germ-membrane, the cells of 
which enlarge and assume a slender cylindrical form, consti- 
tutes the intestinal layer and surrounds the cavity of the 
primitive intestine. The superficial, uninverted portion ot 
the germ-membrane forms the skin-layer ; the cells of this, 
owing to continual self-division, become smaller and more 
flattened. The space between the skin-layer and the intes- 
tinal layer (the remnant of the cleavage-cavity) continues 
full of nutritive yelk, which is now gradually consumed. 
This is the only essential point in which the Bladder- 
gastrula (Peri-gastrula, Fig. 29) differs from the original 


form, that of the Bell-gastrula (Archi-gastrula, Fig. 6). It 
is evident that the former has gradually originated from the 
latter, in the course of a long period of time, by the accu- 
mulation of nutritive-yelk in the centre of the egg. 16 

■/ The fact that we have been thus enabled to retrace all 
the numerous and multiform phenomena in the germination 
of different animals to these four type-forms of egg-cleavage 
and gastrulation, may be regarded as an advance of the 
widest significance. Of these four type-forms we have been 
able to declare that one is the original, palingenetic form, 
and that the other three are kenogenetic forms descended 
from the first. The unequal, discoidal, and the superficial 
forms of cleavage have evidently all originated, in conse- 
quence of secondary adaptation, from the primary, original 
cleavage ; and we must consider that the most important 
cause of their origin was the gradual formation of a nutri- 
tive-yelk, and the distinction, which is always appearing in 
an earlier stage, between the animal and the vegetative 
parts of the egg } between the skin-layer and the intestinal 
layer. The inter-relation of the four cleavage-forms, with 
regard to the ordinary distinction between total and partial 
egg-cleavage is as follows : — 

I. Palingenetio (I. Original cleavage (Bell-^ 

cleavage, j gastrula). A. Total cleavage (with- 

I L out any independent 

/2. Unequal cleavage (Hood- 1 nutritive yelk). 

J gastrula). J 

II. Kenogenetic 
(modified by J 3. Discoidal cleavage (Diso-' 




4. Superficial cleavage (Blad- 
der, gastrula) . 

B, Partial cleavage (with 
an independent nu- 
tritive yelk). 

The lowest known intestinal animals (Metazoa), that is it 


Bay, the lew Plant-animals (Sponges, simplest Polyps, etc), 
remain throughout their life stationary in a structural stage 
which differs very little from the Gastrula; their whole body 
being composed of only two cell-strata or layers. This fact is 
of the very greatest significance. For we see that Man, and 
indeed all Vertebrates, pass quickly through a transitory 
two-layered structural stage, which is persistently retained 
throughout life by these lowest Plant-animals. By now 
again applying our first principle of Biogeny, we im- 
mediately obtain the following very important conclusion : 
Man and all those other animals, which in the first stages of 
their individual evolution pass through a two-layered struc- 
tural stage or a Gastrula-form, must have descended from a 
'primaeval, simple parent-form, the whole body of which 
consisted throughout life, as now in the case of the lowest 
Plant-animals, only of two different cell-strata or germ- 
layers. To this most important primaeval parent-form, to 
which we shall presently refer in detail, we will now pro- 
visionally give the name of the Gastraea (i.e. primitive intes- 
tinal animal). 24 

According to the Gastrsea theory, there is in all animals 
one organ which is originally of the same morphological 
and physiological significance ; this is the primitive intes- 
tine ; the two primary germ-layers, which form the wall of 
this intestine, must therefore in all cases be regarded as also 
of the same significance, or as " homologous." This import- 
ant " homology of the two primary germ-layers " is, on the 
one hand, demonstrated by the fact that the Gastrula in all 
cases originates in one way, that is, by the inversion (in- 
vagination) of the Blastula ; and, on the other hand, by the 
fact that in all cases the same fundamental organs arise 


from the two germ-layers. The outer or animal germ-layer, 
the skin-layer, or exoderm, always forms the outer body- 
wall with the most important organs of animal life; the 
skin-covering, nerve-system, organs of the senses, etc. On 
the other hand, the inner or vegetative germ-layer, the in- 
testinal layer, or entoderm, gives rise to the inner intestinal 
wall with the most important organs of vegetative life ; the 
organs of nutrition, of digestion, those which form the blood, 

In these low Plant-animals, especially in Sponges, 
the whole body of which remains permanently stationary 
in the same structural stage, these two functional groups 
(the animal and the vegetative acts) also continue strictly 
distributed between the two simple, primary germ-layers. 
Throughout life the outer or animal germ-layer retains the 
simple significance of a covering (an outer skin), and, at 
the same time, accomplishes the movements and sensations 
of the body. On the other hand, the inner cell-stratum, 
or the vegetative germ-layer, always retains the simple 
significance of an intestinal epithelium, a nutritive in- 
testinal cell-stratum, and in addition to this appears only 
to produce the reproductive cells. 40 

In all other animals, and especially in all Vertebrates, 
the Gastrula appears only as a very transitory germ-stage. 
The two-layered stage of their germ-rudiment changes 
quickly, first into a three-layered, and then into a four- 
layered stage. On the completion of the germ-layers, which 
Jie one over the other, we have again provisionally attained 
a fixed and definite point of view ; and one from which we 
may trace and explain the incidents in the construction, 
which are much more obscure and intricate. Trustworthy 


researches by many observers, embracing the Ontogeny 
of the most diverse higher animals, have now established 
the important fact that the germ in a certain stage is 
composed of four secondary germ-layers. It is most im- 
portant to notice that this is quite as true of Man as of 
other Mammals. 

In many cases there is a three-layered stage interme- 
diate between the two and the four-layered condition. 76 
But in proportion to the certainty of this conclusion, 
that there are at first two, and afterwards four layers, it is 
difficult to understand the way in which these four 
secondary layers arose from the two primary layers. In 
this respect the opinions of the many observers who have 
studied the question are so contradictory that comparison 
of them fails to enable us to reach the truth. There is, 
however, no doubt of the one fact, that these four layers 
result solely from the two original germ-layers, and that 
they are not partly independent of the latter, as E-eichert, 
His, and other confused observers have asserted. 71 But the 
question yet remains undecided whether the two middle 
layers both originate from one of the two primary layers 
(from the outer or the inner), or whether one of the two 
middle layers must be referred to the upper, the other to 
the lower of the primary germ-layers. 

In order to show the importance of this question to 
the whole history of evolution, I will now briefly indi- 
cate the significance of the two middle layers. We must 
call these two middle layers the second and the third, 
numbering the four secondary germ-layers in order from 
the outer tc the inner. The outer skin, the muscular mass 
or flesh of the trunk, the muscles, which move the body 


and limbs, as well as the inner skeleton, or bony frame- 
work of the body, arise from the second germ-layer, or the 
outer middle layer, which is called the skin-muscular 
layer, or the skin-fibrous layer. The muscles and vascular 
membranes, which first surround the inner cellular canal 
of the intestine and its glands, and which accomplish the 
digestive movements of the throat (pharynx), oesophagus, 
the stomach, and the various other parts of the intestinal 
canal, are all produced from the third germ-layer, the 
inner middle layer, which is called the intestinal-muscular 
layer, or the intestinal-fibrous layer; the heart and the 
most important blood-vessels also originate in this. The 
two middle layers, therefore, especially provide those cell- 
strata which are employed in the formation of the fibrous 
coverings, and of the flesh or muscles. The cells of the 
second layer change into the flesh and the bony framework 
of the trunk; the cells of the third layer change into the 
muscles and the fibrous coverings of the intestinal canal. 
Both middle or fibrous layers are therefore called muscular, 
or flesh-layers ; the outer is called the skin-muscular layer, 
because it lies on the first secondary layer, the skin-sensory 
layer ; the inner is called the intestinal-muscular layer, as 
it lies next to the fourth secondary layer, the intestinal- 
glandular layer (Fig. 50). 

Baer was the first naturalist who recognized and clearly 
distinguished the four secondary germ-layers of the higher 
animals. He did not, however, fully understand their 
origin and their wider significance, nor was he quite 
right in his explanation of the details of their respective 
purposes. But in the main, their significance did not 

escape him, and he even expressed that view of the origin 




of the two middle layers, which I, in opposition to most 
other authors, still hold to be correct. That is to say, he 
derived each middle layer separately from a primary germ- 
layer (by fission), and said, that the outer or animal germ- 

Fig. 50. — Transverse section through the embryo of an Earth-worm : hs, 
skin-sensory layer; hm, skin-fibrous layer ; df, intestinal-fibrous layer; dd, 
intestinal-glandular layer ; a, intestinal cavity ; c, body-cavity, or Cceloma ; 
n, nerve-centres ; u, primitive kidneys. 

Fig. 51. — Corresponding section of the larva of Amphioxus (after 
Kowalevsky). The letters indicate the same parts as in Fig. 50. 

layer separates into two strata, a skin-stratum and a flesh- 
stratum ; similarly the inner or vegetative germ-layer 
separates into two strata; the vascular stratum and the 
mucous stratum. In the following table this view of Baer, 
which I believe to be right in regard to the phylogenetic 
origin of the middle layers, is compared with the newer 
nomenclature, which is now in vogue : — 

A. The two primary germ-layers. 

I. The outer or animal germ- / 

layer (the skin -layer, or ! 
exoderm). ( 

II. The inner or vegetative 

germ-layer (the intesti- 
nal layer, or entoderm). 

B. The four secondary germ-layers. 

1. Skin-sensory layer (skin-stratum, Baer). 

2. Skin-fibrous layer (flesh-stratum, Baer)." 

3. Intestinal-fibrous layer (vascular stra 

turn, Baer). 

4. Intestinal -glandular layer (mucous stra- 

tum, Baer). 


Much recent research by Kowalevsky, Ray-Lankester, 
Van Beneden, and others has justified this "Four-layer 
Theory " of Baer. For instance, it can be plainly shown 
that in the Earth-worm (Fig. 50), in the Amphioxus (Fig. 
51), and in some other animals each of the two primary 
germ-layers parts into two secondary germ-layers; the 
skin, or outer-layer parts into the skin-sensory layer (hs), 
and the skin-fibrous layer (hm) : similarly the intestinal 
or inner layer separates into the intestinal-fibrous layer 
(df), and the intestinal-glandular layer (dd). The body- 
cavity, or cceloma (c), forms between the two fibrous layers. 

Contrary to this view, most recent observers assume 
that the two middle layers proceed from plane-division of 
a single, middle germ-layer (mesoderma). According to 
this, a third originates between the two primary layers, 
and by a secondary process of fission splits into two layers 
along the plane of its surface. Some observers, however, 
as certainly derive this third layer from the lower primary 
layer, as do the others from the upper primary layer. It 
is exactly this suspicious circumstance, together with many 
other grounds (based especially on Comparative Anatomy) 
that lead us to the conjecture, which I believe to be correct, 
Shat neither party is right, but that the outer middle 
layer rather proceeds from the animal, the inner middle 
layer from the vegetative germ-layer. It is true, as we 
shall presently find, that only a single middle layer 
(Remak's " motor-germinative germ-layer") usually arises 
between the two primary germ-layers of mammals, and 
that by the fission of this, the two different middle layers, 
the skin-fibrous layer and the intestinal-fibrous layer, 
originate only secondarily. There are, however, strong 


grounds for the assumption that this process is the effect 
of vitiated Heredity. The simple middle germ-layer of 
Vertebrates has most probably originated only secondarily 
by the coalescence of two distinct primary middle layers, 
and, therefore, the fission of the former into the two latter 
must be regarded as a tertiary process. 

However this may be, we have now reached the im- 
portant, definite point in the History of Evolution, in 
which the whole Vertebrate body, in common with that 
of most higher animals, forms a tube, the wall of which is 
composed of four layers, lying one over the other. This 
is not a figurative comparison; these constituent parts 
of the tube-wall are actually layers, or thin plates, which 
lie fixed one over the other. They can even be mechanically 
parted or split off from each other. This separability is 
connected with the fact that the cells in each one of the 
four layers are alike, while those of each are already in 
some degree distinct or differentiated from those of the 
other three layers. The first, the skin-sensory layer, con- 
sists of cells differing from those of the second, or skin- 
fibrous layer ; the cells of the latter are again different from 
those of the third, the intestinal-fibrous layer ; and these 
latter are of a somewhat different nature from the cells of 
the fourth, the intestinal-glandular layer. We find the 
same four germ-layers as in Man and other Vertebrates 
(Fig. 51), also in Soft-bodied Animals (Mollusca), Articulates 
(Arthropoda), Star-animals (EchinoderTna), and again in 
the higher Worms (Fig. 50). This fact in Comparative 
Ontogeny is of the greatest phylogenetic significance. In 
all cases, fchpse four secondary germ -layers develop from 
the two primary germ-layers ; it is only in the lower Plant- 

Wolff's knowledge of the gekm-layers. 239 

animals (Zoophytes), especially in Sponges, that the latter 
retain their original simplicity. 

Finally, as a special proof of the prophetic genius of 
Caspar Friedrich Wolff, due emphasis must be given to the 
remarkable fact that that naturalist assumed the existence 
of these four secondary germ-layers under the name of 
" four systems formed on one type," the proof of which 
was not furnished till half a century later by Baer. 77 
(Cf. p. 46.) 

Remak's three germ-layers. 

Outer, or 

Inner, or 

I. Outer, or upper 
germ-layer (sen- 
sory layer) 

II. Middle germ- 
layer (motor-ger- 
minative layer) 

III. Inner, or under 
germ-layer (tro- 
phic layer) 

The four secondary 

1. Skin-sensory 

1 2. Skin -fibrous layer 
'3. Intestinal-fibrous 

dular layer 

The two primary 


Animal layer, 
Exoderm, or skin 

"Vegetative layer, 
Entoderm, or intes 
tinal layer. 

Modified ontogenetic process. 

Original phyiogenetic process. 



Egg-cleavage and GA8TBULATION. 7 • 

These two plates are intended to illustrate, by means of diagrammatic 
sections, the most important differences in animal egg-cleavage and gas- 
trnlation. Plate II. represents holoblastio eggs (with total cleavage) ; 
Plate III. meroblastic eggs (with partial cleavage). The animal halves of 
the eggs (exoderm) are coloured gray ; the vegetative halves (entoderm with 
nutritive yelk) red. The nutritive yelk is perpendicularly shaded. All the 
sections are perpendicular meridian sections through the axis of the primi. 
tive intestine. In all, the letters indicate the same parts : c, parent-eell 
(Cytula) ; /, cleavage -cells (Segmentella) ; w, mulberry-germ (Morula) ; b, 
germ-vesicle (Blastula) ; g, cup-germ (Gastrula) ; s, cleavage-cavity; d, 
primitive intestinal cavity j o, primitive mouth ; n, nutritive yelk ; *, intes- 
tinal layer (Entoderm) ; e, skin-layer (Exoderm). 

Fig. 1-6. — Original or primordial egg-cleavage of the lowest Vertebrate 
(Amphioxus). Fig. 1, parent-cell (Cytula) ; Fig. 2, cleavage-stage with 
4 cleavage-cells ; Fig. 3, mulberry -germ (Morula) ; Fig. 4, germ-vesicle 
(Blastula) ; Fig. 5, the same, in process of inversion (Invaginatio) j Fig. 6, 
Hell-gastrula (Archigastrula). 

Fig. 7-11. — Unequal egg-cleavage of an amphibian (Frog). Fig. 7, 
parent-cell (Cytula) ; Fig. 8, cleavage-stage with 4 cleavage-cells ; Fig. 9, 
mulberry-germ (Morula) ; Fig. 10, germ-vesicle (Blastula) ; Fig. 11, Hood- 
gastrula ( Amphigastrula) . 

Fig. 12-17. — Unequal egg-cleavage of a Mammal (Man). Fig. 12, 
parent-cell (Cytula) ; Fig. 13, cleavage-stage with 2 cleavage-cells (e } 
mother-cell of the exoderm ; t, mother-cell of the entoderm) ; Fig. 14, 
cleavage stage with 4 cleavage-cells ; Fig. 15, beginning of the inver- 
sion of the germ-vesicle ; Fig. 16, further advanced inversion ; Fig. 17, Hood- 
gastrula (Amphigastrula). 

Fig. 18-24. — Discoidal egg-cleavage of an Osseous fish (Motella? Cottus 7). 
The greater part of the nutritive yelk (n) is omitted. (Cf. Fig. 42, 43, pp. 
217, 219.) Fig. 18, parent-cell (Cytula) ; Fig. 19, cleavage stage with 
2 cells; Fig. 20, cleavage-stage with 32 cells; Jfig. 21, mulberry-germ 
(Morula) ; Fig. 22, germ-vesicle (Blastula) ; Fig. 23, toe same, in process of 
inversion; Fig. 24, Disc-gastrula (Discogastrula). 

Fig. 25-30. — Superficial egg-cleavage of a Grab (Peneus). Fig. 25, 
parent-cell (Cytula) ; Fig. 26, cleavage-stage with 4 oelia ; Fig. 27, oleavage- 
stage with 32 cells ; Fig. 28, mulberry-germ (Morula), and at the same 
time the germ-vesicle (Blastula) ; Fig. 29, Bladder-gastrula (Perigastrula) ; 
Fijf. 30, Nauplius-germ ; the pharynx-cavity has formed in front of the 
primitive mouth (d), owing to an inversion from without. 







kaxckhl's svolutiom of man. 







List of the most important differences in Animal Egg-cleavage and 

Gastrulation. 7 ' 

The letters a-/ indicate the 6 animal tribes (the primitive animals being 
excluded) : a, Plant-animals ; b, Worms ; c, Soft-bodied animals (MolVussa) ; 
i, Star-animals {Echinoderma) ; e, Articulates (Arthropoda) ; /, Vertebrates. 


Total Cleavage. 

Segmmtatio totalis. 
Hoioblastic Eggs. 

Oafrtrula with- 
nutritive yelk. 


I. Original Cleavage. 
(Segmentatio primordialis, 

Arcbiblastic eggs. 

Bell-gas trula. 

v Archigastrula .) 

(Plate II. Fig. 1-6.) 


a. Most low Plant-animals (low 
Sponges, Hydrapolyps, Me- 
dusae, Corals). 

6. Many low Worms (Sagitta, 
Phoronis, Ascidia, many 
Nematodes, etc.). 

c. A few low Soft-bodied animals 

(Mbllusca) — Terebratula, Ar- 
giope, Pisidium. 

d. Most Star - animals (Echino- 


e. A few low Articulates (some 

Brancbiopods, Copepods, Tar- 
/. The Skull-less Vertebrate (Am- 

n. Unequal Cleavage. 

(Segmentatio inaqualis.) 
Ampbiblastic eggs. 



(Plate II. Fig. t-17.) 

a. Numerous Plant-animals (many 

Sponges, Medusae, Corals, 
Siphonophores, Ctenophorae). 

b. Most Worms. 

c. Most Soft-bodied Animals (MoU 


d. Individual Star-animals (vivi- 

parous species and a few 

c. A few low Articulates (Arthro- 
poda) -both Crustaceans and 

/. Cyclostoma, Ganoids, Amphibia, 
Mammals (all ?). 


Partial Cleavage. 

Segmentatio partialis. 

Meroblastic eggs. 

Gastrula with 

nutritive yelk. 


m. Discoidal Cleavage. 

(Segmentatio discoidalis.) 

Discoblastic eggs. 



(Plate III. Fig. 18-24.) 

e. Cuttle-fish, or Cephalopods. 

e. Some Articulates (Arthropoda}, 
Millepedes, Scorpions, ard 

/. Primitive Fishes (Selachii), 
Osseous Fishes, Reptiles, 
Birds (and Monotremes ?). 

IV. Superficial Cleavage. / a. A few Sponges (?). 
(Segmentatio super [facialis .) 
Periblasts eggs. 

Alcyonium (?). 
6. Individual Worms (?) 

Bladder-gastru 1 a. 

(Plate III. Fig. 26-30.) 

«. The great majority of Articu- 
lates (Arthropoda)— Crus- 
taceans, Myriopocto, Spiders, 


8ystematio Surrey of the five earliest germinal stages of Animals witfc 
reference to the four different type-forms of Egg-cleavage. 

A. Total Cleavage. 

(Segmentatio totalis.') 

a. Original or primor- 
dial cleavage. 

I. «. Archi- 

(Fig. 22^1, p. 191.) 
A cytod in which 
the formative and 
nutritive yelk are not 

n. «. Arohi- 


(Plate n. Fig. 1.) 
Parent-cell which 
has arisen out of the 
archi-monerula by the 
formation of the pa- 

HI. a. Archi- 

(Plate H. Fig. 3.) 
A solid (generally 

globular) neap of 

similar cells. 

TV. •. Archl- 

(Plate II. Fig. 4.) 
A hollow (usually 
globular) vesicle, the 
wall of which consists 
of a single layer of 
similar cells. 

V. «. Archi- 

(Plate II. Fig. 6.) 
Fig. 23-28, p. 193. 
Primitive intestine 
empty, without nu- 
tritive yelk. Pri- 
mary germ - layers 

6. Unequal cleavage. 

I. 6. Amphi- 

A cytod which 
includes formative 
yelk at the animal 
pole, nutritive yelk 
at the vegetative 
pole : the two are 
not very distinct. 

n. b. Amphi- 


(Plate II. Fig. 7, 12.) 

Parent-cell which 
has arisen out of the 
ainpbi-monerula by 
tbe formation of the 

III. b. Amphi- 

(Plate II. Fig. 9.) 
A roundish heap 
formed of two kinds 
of cells, the animal 
cells at one, the vege- 
tative cells at the 
other pole. 

IV. 6. Amphi- 

(Plate II. Fig. 10.) 
A roundish vesicle, 
tbe wall of wbich at 
the animal pole con- 
sists of smaller cells, 
at the vegetative 
pole of larger cells. 

V. 6. Amphi- 


(Plate II. Fig. 11,17.) 

Fig. 32-35, p. 206, 

Fig. 41. 

Primitive intestine 
partly filled with 
segmented nutritive 
yelk. Germ-layers 
often many-layered. 

B. Partial Cleavage. 
(Scgmcntatio partialis.) 

e. Discoidal cleavage. 

I. e. Disco- 

A cytod which 
includes formative 
yelk at the animal 
pole, nutritive yelk 
at the vegetative 
pole: the two are 
quite distinct. 

II. c. Disco- 


(Plate III. Fig. 18.) 

Parent-cell which 
has arisen out of the 
disoo-monerula by 
the formation of the 

HI. c. Disco- 
(Plate III. Fig. 21.) 
A flat disc, com- 
posed of similar cells 
on the animal pole 
of nutritive yelk. 

IV. c. Disco- 


(Plate III. Fig. 22.) 
A roundish vesicle, 
the small hemisphere 
of which consists of 
cleavage - cells, the 
larger of nutritive 

V. c. Disco- 


Disc-gas trula. 

(Hate III. Fig. 24.) 

Fig. 43, p. 219, 

Fig. 49, p. 228. 

Primitive intestine 
filled with unseg- 
mented nutritive 
yelk. Flat germ-disc. 

d. Superficial cleavage. 

I. d. Peri- 
A cytod which in- 
cludes formative yelk 
in the outer wall, 
nutritive yelk in the 

n. d. Perl- 

(Plate III. Fig. 25.) 

Parent-cell which 
has arisen out of the 
peri-monerula by the 
formation of the pa- 

III. d. Peri- 


(Plate III. Fig. 27.) 

A closed vesicle : a 
cellular stratum sur- 
rounds the whole cen- 
tral nutritive yelk. 

IV. d. Peri- 

(Plate III. Fig. 28.) 

A closed vesicle: a 
cell layer surrounds 
the whole nutritive 
yelk (= Peri-morula). 

V. d. Peri- 


Bladder-gas trula . 

(Plate III. Fig. 29.) 

Cleavage-cavity fill- 
ed with unsegmented 
nutritive yelk. Pri- 
mitive intestine dif- 


Systematic Survey of some of the most important rhythmical variations in 

Egg-cleavage. 80 

Only the first column (Amphioxus) presents the original, palingenetio 
cleavage- hythm, in regular geometrical progression. All the other columns 
show the descended kenogenetio modifications. 

c = parent-cells. s = cleavage-cells. « = exoderm-cells. 

i = entoderm-cells. 























(le + 10 



(le + It) 


(le + 10 



(2e + 2i) 



(2e + 10 


(2e + 10 



(4e 4- 4t) 


(4e -f 4i) 

(4e + 4i) 


(4e + li) 

(3«+ 10 


(8e -f 4d) 


(8e + 4t) 


(8e 4- 4n) 


4e + 2t) 


(4e+ 10 



(8e + 8») 


(8e + 8i) 

(16e + 4d) 


(8e + 20 


(5e + 10 


(16e + 8i) 


(16e 4- 8i) 

(16e + 8t) 


(8e + 30 


(6e + 10 



(16e + 16i) 


(16e + 160 


(32e + 80 


(16e 4- 30 

(7* + 10 

(32e + 16i) 

(32e + 160 

(32e+ 12*) 


(16e + 50 


(8e + 10 


(32e + 32i) 


(32e + 320 

(64e + 120 

(32e + 50 

(9« + 10 

(64e + 32i) 

(64e + 32i) 

(64e + 20i) 

(32e + 60 



(128e + 32i) 


<128e + 20t) 

(64e + 60 



Relation of Comparative Anatomy to Classification. — The Family-relation- 
skip of the Types of the Animal Kingdom. — Different Significance 
and Unequal Value of the Seven Animal Types. — The Qastrcea Theory, 
and the Phylogenetic Classification of the Animal Kingdom. — De- 
scent of the Gastraea from the Protozoa. — Descent of Plant-animals 
and Worms from the Gastraea. — Descent of the Four Higher Classes of 
Animals from Worms. — The Vertebrate Nature of Man. — Essential and 
Unessential Parts of the Vertebral Organism. — The Amphioxus, or 
Lancelot, and the Ideal Primitive Vertebrate in Longitudinal and 
Transverse Sections. — The Notochord. — The Dorsal Half and the Ven- 
tral Half.— The Spinal Canal. — The Fleshy Covering of the Body. — 
The Leather-skin (corium). — The Outer-skin (epidermis).— Body- 
cavity (cceloma). — The Intestinal Tube. — The Gill-openings.- — The 
Lymph-vessels. — The Blood-vessels. — The Primitive Kidneys and 
Organs of Keproduction. — The Products of the Four Secondary Germ, 

" Know thyself ! This is the source of all wisdom, said the great thinkers 
of the past, and the sentence was written in golden letters on the temple of 
the gods. To know himself, Linnaeus declared to be the essential indis- 
putable distinction of man above all other creatures. I know, indeed, in 
study nothing more worthy of free and thoughtful man than the study of 
himself. For if we look for the purpose of our existence, we cannot possibly 
find it outside ourselves. We are here for our own sake." — Karl Ernst 
Baer (1824). 

A. difficult task now lies immediately before us in this 
history of our individual development ; we must trace the 
complex human body with all its various parts, — organs, 


limbs, etc. from the simple Gastrula. The two primary 
germ-layers which form the entire body of the Gastrula fall 
by fission into the four secondary germ-layers, which have 
already been named ; and of these four the whole complex 
form of the perfected human and animal body constructs 
itself. It is so difficult to understand this process of con- 
struction, that we will first look around us for an ally 
capable of helping us over many obstacles. 

This powerful ally is the science of Comparative 
Anatomy. Its object is, by comparison of the perfected 
bodily forms of the various groups of animals, to discover 
the universal structural laws, in accordance with which the 
animal body develops ; and at the same time, by critically 
determining the degrees of difference between the various 
classes, and the larger groups of animals, to establish their 
relations to each other and to the whole system. There was 
a time when this task was attempted from a teleological 
point of view, and in the actually existing apt organization 
of animals proof was sought of a pre-arranged "plan of con- 
struction " by the Creator ; but, recently, the establishment 
of the Theory of Descent has enabled Comparative 
Anatomy to go deeper, and its philosophical task has de- 
veloped into the explanation of the variety of organic forms 
by Adaptation, and their similarity by Heredity; it has 
also to seek to discover the various degrees of blood - 
relationship in the graduated and various form-relationships, 
and to prove as nearly as possible the genealogy of the 
animal kingdom. In this way Comparative Anatomy is 
most closely allied to the classification of organic bodies, 
which, starting from the opposite direction, aims at the 
same result 


In asking ourselves what place the most recent dis- 
coveries of Comparative Anatomy and the Science of Classi- 
fication, among other organisms, assign to Man, what light 
is thrown by a comparison of developed bodily forms on the 
position of Man in the whole animal system, we receive 
a very simple and significant answer ; and this answer 
affords conclusions of extreme importance in explanation of 
the evolution of the embryo, and as to the phylogenetic 
interpretation of this evolution. 

Since the time of Cuvier and Baer, since the great 
progress originated by these two great zoologists in the first 
decades of this century, the whole animal kingdom has 
been universally held to be divisible into a small number of 
main divisions, or Types. They are called types, because 
a certain typical or characteristic structure of body is 
invariably maintained within each one of these main 
divisions. 81 Of late, since the Development Theory has been 
applied to this celebrated Doctrine of Types, it has been 
discovered that all animals of the same type stand in direct 
blood-relationship to each other, and can be traced from 
a common parent-form. Cuvier and Baer assumed four 
of these types ; more recent research has raised the number 
to seven. These seven types, or tribes (Phyla), 82 of the 
animal kingdom, are: (1) the Protozoa ; (2) the Plant-animals 
(Zoophytes) ; (3) the Worms (VerTnes) ; (4) the Soft-bodied 
animals (Mollusca) ; (5) the Star-animals (EchinoderTna) ; 
(6) the Articulated-aaimals (Arthropoda); (7) the Vertebrata. 

I may at once introduce the reader to the genealogi- 
cal inter-relations of these seven types as I am fully con- 
vinced they are phylogenetically constituted. For this 
purpose I will give as briefly as possible the outlines 0/ 


my Gastrsea Theory, 84 on which I base the monophyletic 
genealogy of the animal kingdom, and which I am con- 
vinced must supersede the Theory of Types which now 
prevails. According to this Gastrsea Theory, which I 
enunciated in the " Monograph on the Chalk Sponges " 
(vol. ii. pp. 4G 5-467), the seven types or tribes of the animal 
kingdom possess an entirely different significance and an 
entirely unequal value. Only the four higher tribes 
— Vertebrates, Arthropods, Molluscs, and Echinoderms — 
are types in the sense of Cuvier and Baer, and even these 
only in a limited sense, not as originally meant by the 
authors of the theory. On the other hand, the lowest 
type, that of the Primitive-animals, is not really a " type," 
but the sum of all the lowest animals ; it was from a 
branch of the Primitive-animals that the Gastrsea developed. 
The two remaining types, the Plant-animals and the Worms, 
stand between the Primitive-animals and the four higher 
types. They are more specialized and typical than the 
Primitive-animals, and less typically organized and charac- 
terized than the four higher tribes. 

The Gastrsea Theory is founded on the fact that we 
have proved the two primary germ-layers to be the rudi- 
mentary bodily-structure common to the six higher groups 
of animals. But it is also proved that a single original 
organ is of the same use, or homologous, in all these 
animals; this is the intestine (jorotogaster), the primitive 
intestinal or stomach cavity, in its most simple form. In 
the Gastrsea itself, and in the extant Gastreads (Haliphy- 
sema, Gastrophysema), the entire, simple, spherical or oval 
body consists only of this simple primitive cavity, open at 
one pole of the axis (the primitive intestine and primitive 


mouth), and of the two primary germ-layers which sur- 
round it in their simplest original form (Entoderm and 
Exoderm). But in none of the Protozoa are there germ- 
layers, and therefore no primitive intestine. The entire 
protozoan body is formed either of a very simple cytod, a 
little shapeless mass of protoplasm, as in the Monera, or a 
very simple cell, as in Amoebae and Gregarinae, or a colony 
of simple cytods or cells (as in most Protozoa). But in the 
last case the cells of this cell-community are either entirely 
homogeneous, or but slightly differentiated, and never 
separated into true germ-layers. A real intestine never 
appears in the Protozoa. The Infusoria, which reach the 
highest degree of physiological perfection among Protozoa, 
do indeed appear to have an intestine with a mouth and 
vent. But as the entire body, notwithstanding the con- 
siderable differentiation of its individual parts, retains only 
the form-value of a simple cell, we cannot compare this 
physiological food-canal with its openings, with the true 
many-celled intestine, which in other animals are morpho- 
logically characterized by their covering of germ-layers. 83 

We must therefore primarily divide the whole animal 
kingdom into two main divisions ; on the one side the 
Protozoa, without a primitive intestine or germ-layers, 
without yelk-cleavage or differentiated many-celled tissues ; 
on the other side, the Intestinal animals (Metazoa) with 
intestines, with two primary germ-layers, with yelk-cleav- 
age, with differentiated many-celled tissues. The Intestinal 
animals, or Metazoa, in which we include the six higher 
groups of animals, have all descended from the Gastraea, 
the previous existence of which may be, even at this day, 
proved with certainty by means of the Gastrula. Thii 


Gastrula, or intestinal larva, which recurs in a remarkably 
similar form in the history of the individual development 
of the several groups of animals, is of the greatest 
significance. From this Gastrula the lowest Vertebrate 
develops, just as the lower forms of Worms, Soft-bodied 
Animals, Star-animals, Plant-animals, etc (Cf. Plates II., 
Ill, and Fig. 22-28, pp. 191, 193.) The Gastrula at the 
present day presents a correct picture of the primitive 
Gastraea, which must have developed from the Protozoa in 
the Laurentian period. 

Comparative Anatomy and Ontogeny teach us, further, 
that from this Gastrsea the animal kingdom at first de- 
veloped in two diverging directions or lines. In the one 
direction proceeded the low group of the Plant-animals 
(Zoophytes), to which the Sponges, Polyps, Corals, Medusae, 
and many other marine animals belong ; and among fresh- 
water animals the well-known Hydra, or fresh-water Polyp, 
and the Spongilla, or fresh-water Sponge. In the other 
direction, the very important group of the Worms, in the 
narrower sense in which the present zoological classification 
limits this group, developed from the Gastraea. In the 
Linnaean system, and generally in earlier times, all the 
lower animals, Infusoria, Worms, Soft-bodied Animals, 
Plant-animals, Star-animals, etc., were included under the 
name of Worms ; the name is now, however, much more 
narrowly restricted to the true Worms. Under it are in- 
cluded Earth-worms, Leeches, Ascidians, and also the 
various parasitic Worms, Tape-worms, Round-worms, 
Trichinae, etc. Different as all these worms appear, in their 
perfect state, they can all be traced back to the Gastraea. 
(Cf. Table XVIII. in Chap. XVII.) 


We must look for the original parent-form of the four 
higher tribes of animals among the numerous branch-forms 
of the Worm Tribe. The comparative Anatomy and 
Ontogeny of these four tribes certainly teach that all origi- 
nated from four different branches of Worms. This tribe is 
the common ancestral group of the four higher animal tribes. 
These last are : (1) the Star-animals (Echinoderma — Star- 
fishes, Sea-urchins, Sea-lilies, Sea-cucumbers) ; (2) the im- 
portant class of the Articulated-animals (Arthropoda — 
Crabs, Spiders, Centipedes, Insects); (3) the Soft-bodied- 
animals (Mollusca — Lamp-shells, Mussels, Snails, etc.) ; and 
finally (4) the Vertebrata, the most highly developed tribe 
of animals, to which Man belongs. 

These are the principles of the unified or monophyletic 
genealogy of the animal kingdom, as they present them- 
selves, provisionally, according to the Gastrsea Theory, at 
the present stage of zoological classification and of embryo- 
logical knowledge. If I am right in asserting the original 
similarity or homology of the primitive intestine and the 
two primary germ-layers enclosing it in all intestinal 
animals, this phylogenetic classification of the animal 
kingdom may supersede the systems hitherto based on the 
Type Theory. According to this, therefore, the seven 
types of that theory acquire a wholly different significance. 
Of these seven tribes (Phyla), (1) that of the Protozoa 
remains at the foot of the scale; from it springs (2) the 
Gastrsea, which branches into the two lines of the Plant- 
animals and Worms ; and from the Worms develop (3) the 
four higher groups of animals ; these last are four diverging 
lines, which are only connected together at the base, among 
the lowest Worms, but are not otherwise comparable. 


In specially observing the position of Man in the animal 
lystem, it cannot be doubted for a moment that the entire 
bodily structure of Man is that of a Vertebrate, and that 
Man possesses in the characteristic position and combination 
of his organs all those peculiarities which appear only in 
the Vertebrate class, and are totally wanting in all other 
animals. The Vertebrates are either in no way related to 
the three other higher groups of animals, or they are so 
only in their common descent from the Worms and from 
the Gastnea ; on the contrary, a relationship really exists, 
and may be clearly proved between Vertebrates and some 
forms of Worms. I may now enunciate the proposition, 
which we shall hereafter prove, that the entire Vertebrate 
tribe has developed from the Worm tribe. On the other 
hand, the Vertebrates have certainly not descended from 
the Articulated-animals (Arthropoda), the Soft-bodied 
Animals (Mollusca), or Star-animals (Echinoderma). There- 
fore by far the greater part of the animal kingdom may be 
entirely overlooked in our future investigations, whether 
Ontogenetic or Phylogenetic. We have nothing further to 
do with these. The three groups which alone interest us, 
are the Primitive Animals (Protozoa), the Worms, and the 
Vertebrates. " 

Those people who regard the descent of Man from the 

animal kingdom as a more or less degrading stigma, and 

are ashamed of it, may take such consolation as they can 

from the fact that the greater part of the animal kingdom 

is not akin to them. The Vertebrates have no connection 

with the great group of Articulated-animals (Arthropoda) ; 

but to the latter belong not only the Crabs, but also the 

Spiders and Insects, which last form a single class, com- 


prising probably as many, if not more, distinct species than 
all the other classes of animals together. Unfortunately, 
we lose by this the relationship which might otherwise 
connect us with Termites, Ants, Bees, and other virtuous 
members of the Articulate class. Among these insects are 
many well-known patterns of virtue, which the fable 
writers of old classic times held up as examples for men. 
In the civil and social arrangements of the Ants especially, 
we meet with highly developed institutions which we may 
even yet regard as instructive examples. But unfortu- 
nately these highly civilized animals are not related to us. 

Our next task must now be, to enter in greater detail 
into the vertebrate nature of Man, and to determine the 
special position which he holds in the system of Verte- 
brates. Here it is necessary to point out the most essen- 
tial facts in the particular structure of the vertebrate 
body ; for, otherwise, we shall be quite unable to enter 
rightly into the difficult question of Ontogeny. The evolu- 
tion of even the simplest and lowest Vertebrate from the 
simple Gastrula is so complex a process, and is so difficult 
to trace, that it is necessary to understand the principles 
of the organization of the perfect "Vertebrate, in order to 
comprehend the principles of its evolution. But it is equally 
important that in this brief anatomical description of the 
vertebrate organism, we should stop only at the essential 
facts, and leave all others untouched. Therefore, in giving 
an ideal anatomical sketch of the main form of the Verte- 
brate and its inner organization, I leave out all secondary 
and non-essential circumstances, and confine myself to 
those most essential 

Many particulars, which will probably appear highly 


important and essential to the reader, are shown by the 
History of Evolution and Comparative Anatomy to be of 
secondary and subordinate importance, or even entirely non- 
essential. For example, from this point of view the head 
with the skull and the brain are non-essential, as are also the 
extremities, or limbs. It is true that these parts of the body 
possess a very high — even the very highest physiological 
importance; but for a morphological conception of the 
Vertebrate, they are non-essential, because they appear 
only in the higher Vertebrata, and are wanting in the lower. 
The lowest Vertebrates possess neither a clearly marked 
head with a brain and skull, nor extremities, nor limbs. 
The human embryo also passes through a stage in which it 
possesses no head, no brain, no skull, in which the trunk 
is still entirely simple and undivided into head, neck, 
breast, and abdomen, in which there is no trace of limbs, 
arms, or legs. In this stage of evolution, Man, as well as 
every other higher Vertebrate, essentially resembles that 
simplest Vertebrate form, which is represented only by a 
single existing Vertebrate, retaining the form throughout 
life. This single lowest Vertebrate, which deserves the 
closest consideration, and, next to Man, must undoubtedly 
be called the most interesting of all Vertebrates, is the well- 
known Lancelot, or Amphioxus (Plates X. and XL). As we 
shall afterwards examine this animal minutely (in Chapters 
XIII. and XIV.), I shall say but little about it now. 

The Amphioxus lives buried in sea-sand ; it attains a 
length of 5-7 centimetres, and in its adult condition is 
shaped exactly like a long, lanceloate leaf. It it>, therefore, 
called the Lancelet. The narrow body is compressed on 
both sides, is similarly pointed in front and at the back, 


without any trace of external appendages, without any 
division of the body into head, neck, breast, abdomen, etc 
Its whole form is so simple, that its first discoverer declared 
it to be a naked Snail. Not until much later (about forty 
years ago) was the remarkable little being more closely 
examined, and it then became evident that it is a true 
Vertebrate. Later investigations have shown that its bearing 
on Comparative Anatomy and human Embryology and 
Phylogeny is of the highest importance. For the Lancelet 
enables us to solve the weighty question as to the descent 
of Vertebrates from Worms, with certain lower forms 
(Asddia) of which it is immediately connected in its de- 
velopment and bodily structure. N , 

Now, if we make several sections through the body of 
the Amphioxus, — first, perpendicular longitudinal sections 
through the whole body from front to back, and secondly, a 
perpendicular cross-section through it from right to left, we 
shall obtain two instructive anatomical pictures. (Cf. Plates 
X. and XI.) In all essential points they correspond to the 
abstract ideal, which, aided by Comparative Anatomy and 
Ontogeny, we are able to conceive as the primitive type, 
as the picture of the Primitive Vertebrate; of that long 
extinct parent-form, to which the whole Vertebrate tribe 
owes its origin. We need only make very slight and im- 
material alterations in the actual sections of the Amphioxus, 
in order to obtain such an ideal anatomical picture or 
diagram of the primitive form of the Vertebrate, as it is 
represented in Fig. 52-56. The Amphioxus differs so 
little from this primitive form that it may be accurately 
described as a Primitive Vertebrate. (Cf. Plates X. and XI 
with Fig. 52-56.) * 


In the longitudinal section of the type of the Vertebrate, 
a thin but firm rod, of cylindrical form, and pointed at the 
posterior and anterior ends (Fig. 52, x), is seen in the middle 
of the body. This passes through the whole length of the 
centre of the body, and represents the original rudiments 
of the spine or vertebral column. This is the notochord, 
the chorda dorsalis, or chorda vertebralis, which is also called 
the vertebral chord or spinal axis, or, briefly, the chorda. 
This firm, but flexible and elastic chord, consists of a cartila- 
ginous mass of cells, and forms the central inner axis of the 
skeleton or main support of the body ; it occurs exclusively 
in Vertebrates, and is entirely wanting in all other animals. 
As the first rudiment of the spine, it possesses the same sig- 
nificance in all Vertebrates, from the Amphioxus to Man 
But in the Amphioxus alone the notochord is retained, 
throughout life, in its simplest form. In Man and all the 
other higher Vertebrates, on the contrary, it is found in this 
form only in the earliest embryonic stages, and afterwards 
develops into the articulated vertebral column. 

The spinal axis, or notochord, is the fixed main axis of 
the Vertebrate body, corresponding with the ideal axis of 
length, and at the same time serving as a sure guide by 
which we learn the true bearing of the typical relative posi- 
tions of the most important organs of the Vertebrate body. 
By means of it we can picture the body of the Vertebrate in 
its original natural arrangement, in which the axis of length 
lies horizontally ; the dorsal side lies above, and the ventral 
side below (Fig. 52). If we make a vertical section through 
the whole length of this axis, the whole body separates into 
two similar and symmetrical parts, the right and left halves. 
In both halves exactly the same organs originally lie, in the 







i; h% -ma C 

Fig. 53. 

471 S I 

Fig. 54. 

Fig. 55. 

Fig. 52. — The ideal Primitive Vertebrate type, seen from the left side : 
rnr, medullary tube ; oc, chorda ; na, nose ; au, eyes ; g, ear-vesicle ; md, 
mouth; fc, gill -body ; ks, gill-openings; kg, gill-arches; ma, stomach; I, 
liver ; d, small intestine ; af, anus ; v, intestinal vein ; hz, heart ; a, body, 
artery ; n, primitive kidney canal ; e, ovary ; h, testicles ; c, body-cavity 
(visceral cavity); ms, muscles; Ih, leather-skin (corium); oh, outer-skin 
(epidermis) ; /, skin-fold, acting as fin. 

Fig. 53. — Same as above, viewed from the ventral side. 


Fig. 54. — Transverse section of the same in the anterior part (throng* 
the gill-body, at kg, Fig. 53). 

Fig. 55. — Transverse seotion of the same in the central part (in the 
neighbourhood of the heart, at hz, Fig. 53) . 

Fig. 56. — Transverse section of the same in the posterior part (through 
the ovary, at e, Fig. 53). The letters indicate the same parts in all the 

same relative position and connection ; but their positions 
in relation to the central plane of t section are exactly re- 
versed ; the left half resembles the right, as though reflected 
in a mirror. The two halves are called counterparts, or 
antimera. The perpendicular line of section which divides 
the two halves, passes from the back to the abdomen, and is 
called the sagittal or dorso- ventral axis. If, on the other 
hand, we make a horizontal section lengthwise through the 
chord, the whole body falls into a dorsal and a ventral half. 
The line of section which passes through the body from the 
right to the left side is called the cross or lateral axis. (Cf. 
f'lates IV. and V. 84 ) 

The two halves of the Vertebrate body which are 
separated by this horizontal, transverse axis, have an 
entirely different significance. The dorsal half is especially 
the animal part of the body, and contains the greater part 
of the so-called animal organs, of the nerve-system, muscle- 
system, bone-system, etc. The ventral half, on the other 
hand, is essentially the vegetative part of the body, and 
contains the greater part of the vegetative organs of the 
vertebrate, the digestive system, the reproductive system, 
etc. The two outer secondary germ-layers are, therefore, 
specially employed in the formation of the dorsal half, and 
the two inner in the formation of the ventral half. Each 
of the two halves develops in the form of a tube, and 
surrounds a cavity in which another tube is enclosed. 


The dorsal half encloses the spinal cavity, which lies above 
the notochord, and contains the tube-shaped central nerve 
system, the spinal marrow or spinal tube. The ventral 
half, on the other hand, encloses the much larger intestinal 
or ventral cavity, which lies below the notochord, and con- 
tains the intestinal canal with all its appendages. 

The spinal, or medullary tube, as the central nerve 
system or mental organ of Vertebrates is called in its primi- 
tive condition, consists in Man, as in all higher Vertebrates, 
of two very different parts : the large brain lying within the 
skull, and the long spinal cord which extends from the brain 
Along the whole back (Plate V. Fig. 16, m). But no part of 
this structure is seen in our primitive vertebral type. In this 
the highly important mental organ, which occasions the feel- 
ing, willing, and thinking of the Vertebrate, appears in an 
extremely simple form. It is composed of a long cylindrical 
tube which passes lengthwise through the body immediately 
above the notochord, and encloses a narrow central canal filled 
with fluid (Fig. 52-57, mr). We find that the Amphioxus 
at the present day retains throughout life this simplest 
form of the spinal canal, just as it existed in all the older 
and lower Vertebrates (Plate XI. Fig. 15, m). It is enclosed 
in a tube of skin which proceeds from the immediate 
surrounding of the notochord, the so-called notochord 
sheath, and in which, at a later period, the bony vertebrae 
of the higher Vertebrates are developed. 

Of organs of sense, the parent-form of Vertebrates 
probably possessed an olfactory groove, as the simplest 
rudiment of a nose (Fig. 52, 53, na), a pair of eyes (em), 
and a pair of auditory vesicles (cf) of the most simple cha- 
racter. 86 Some of these organs of sense are not represented 



in the Amphioxus, probably in consequence of secondary 
reversion. (Cf. Chap. XIII.) 

Fig. 57. — Transverse section through the 
anterior part of the primitive vertebrate type : 
mr, spinal tube; x, chorda (notochord) ; msi, 
dorsal muscles; kb, gill-vent ; k, gill-intestine. 

On both sides of the spinal tube 
of all Vertebrates, and the notochord 
which underlies it, great masses of 
flesh are seen, which form the muscular 
parts of the trunk and accomplish its movements. Although 
in developed Vertebrates these masses are differentiated and 
combined in various ways (corresponding to the variously 
differentiated parts of the bony skeleton) yet in our ideal 
primitive Vertebrate we can distinguish only two pairs of 
main muscles which traverse the whole length of the body 
parallel to the notochord. These are the upper, or dorsal, 
and the lower, or ventral, side-muscles of the trunk. The 
upper (dorsal) side-muscles of the trunk, the primitive 
back-muscles (Fig. 58, msi) form the thick mass of the 
flesh of the back. The lower (ventral) side-muscles, the 
primitive abdominal muscles, on the other hand, form 
the fleshy wall of the abdomen (Fig. 58, ms2). 


171 ■s 

Fig. 58. — Transverse section through the 
central portion of the ideal Primitive Verte- 
brate : /, skin-fold, forming fin ; mr, spinal tube ; 
a?, chorda; msi, dorsal muscles; msi, ventral 
muscles ; a, aorta (in the mesentery) ; ma, 
stomach-cavity ; c, body-cavity (visceral cavity) ; 
hz, heart. 

Outside this wall we find the outer firm covering 
of the whole body, called the leather-skin {corium, or 


cutis, Ih). The lower layers of this tough and tlick 
covering consist principally of fat and loose connective 
tissue ; the outer layers of skin-muscles and firmer connec- 
tive tissues. It covers the whole surface of the fleshy body, 
with which it is connected, and it lies immediately below the 
thin outer skin (epidermis, oh). In the case of the higher 
Vertebrates, hairs, nails, feathers, claws, scales etc., arise 
from this outer skin. With all its appendages and pro- 
ducts, it consists entirely of simple cells, and contains no 
blood-vessels. Its cells are connected with the ends of the 
sensory nerves. Originally the outer skin (epidermis) is an 
entirely simple covering for the outer surface of the body, 
and consists of but one kind of cell. In higher Vertebrates, 
it afterwards separates into two strata, an outer, firmer 
horn-stratum, and an inner, softer mucous stratum ; many 
external and internal appendages arise from it at a later 
period ; the hair, nails, etc., externally, and the sweat and 
sebaceous glands internally. 

In the primitive Vertebrate the skin probably arose 
along the middle line of the body in the form of an erect, 
perpendicular seam used for floating purposes (/). The 
Amphioxus and the Cyclostomi yet retain a similar seam, 
which passes almost entirely round their bodies ; one is also 
found on the tail of the larval Frog, or Tadpole (Fig. 194). 

From these external parts of the vertebrate body we 
will now turn to the inner organs, which we find beneath 
the notochord, in the large body, or intestinal cavity. To 
avoid confusion, we will in future call this cavity the 
codoma. In Anatomy it is usually called the pleuro-peri- 
toneal cavity (Fig. 58, c). In Man and all other Mammals, 
but in no other animals, this ccelom, when developed, if 


separated into two distinct cavities, which are completely 
divided by a transverse partition, the muscular midriff, or 
diaphragm. The first, or chest-cavity, contains the oesopha- 
gus, the heart, and the lungs ; the other, the ventral cavity, 
contains the stomach, small intestine, large intestine, liver, 
spleen, kidneys, etc. But in mammalian embryos, these 
two form a single connected cavity, a simple coelom, before 
the diaphragm is developed, and this we find to be the 
case in all lower Vertebrates throughout life. This coelom is 
covered by a delicate layer of cells, the intestinal epithelium. 
The most important of the viscera within the body- 
cavity (cceloma), is the nutritive intestinal tube, the organ 
which forms the whole body of the Gastrula. This is a 
long tube, more or less differentiated, enclosed in the coelom, 
and having two openings; a mouth-opening for taking in 
food (Fig. 59, 60, md), and an anal opening for discharg- 
ing waste-matter or excrement (a/). Numerous glands, all 
of which proceed from the intestine, are attached to the 
intestinal canal, which are of great importance in the verte- 
brate body. These are the salivary glands, lungs, liver, 
and numerous smaller glands. A pair of simple liver- 
pouches (Fig. 59, 60, I) were probably present even in the 
parent-form of Vertebrates. The walls of the intestinal 
canal and of all these appendages, consist of two very 
different parts or layers ; the inner cellular covering is the 
intestinal-glandular layer, or the fourth germ-layer ; the 
outer fibrous envelope, on the other hand, proceeds from 
the third germ-layer, the intestinal-fibrous layer; it is 
mainly composed of muscle-fibres, which effect the digestive 
movements of the intestine, and of a tissue of connective 
fibres forming a firm covering. The mesentery, a thin, 


ribbon-like layer, by which the intestinal canal is attached 
to the ventral side of the notochord, is a continuation of 
this. In addition to this, the most important parts of the 
blood-vessel system, especially the heart, and the greater 
arteries, also develop from this intestinal-fibrous covering. 
In Vertebrates the intestinal canal, as a whole as well as 
in its separate parts, is modified in various ways, although 
its original very simple form is always the same. As 
a rule, the intestinal canal is longer, often many times 
longer, than the body, and therefore lies, in many convolu- 
tions, enclosed in the cceloma, especially in the back part. 
In higher Vertebrates it is also often divided by valves 
into various separate parts; the parts being distinguished 
as the mouth, throat, oesophagus, stomach, small intestine, 
large intestine, and rectum. All these parts arise from a very 
simple formation, which originally (and, in the Amphioxus, 
permanently) is a straight, cylindrical canal running from 
front to rear below the notochord. 

As the intestinal canal, in a morphological sense, may be 
regarded as the most important organ of the animal body, 
it is interesting to get a clear conception of its essential 
nature in Vertebrates, setting aside all non-essential parts. 
In this respect, it is especially necessary to give due 
weight to the fact that the intestinal canal in all Verte- 
brates shows a very characteristic division into two parts, 
a front half (Fig. 59, k) which serves especially for respira- 
tion, and a hind half which serves entirely for digestion 
(d). In all Vertebrates peculiar clefts appear, at a very 
early period, on the right and left sides of the front divi- 
sion of the intestinal canal ; these, the so-called gill-open- 
ings (ks), are most closely connected to the primitive 


respiration of Vertebrates. All lower Vertebrates, the 
Amphioxus, Lampreys, and Fishes, continually take in 
water through the mouth, and let it pass out through 

g kji x nvr T?„ z. z_ ms I v i „ ' ' , „/f 

Fig. 59.— The ideal Primitive Vertebrate, seen from the left side : na, 
nose ; an, eye ; g, ear ; md, mouth ; ks, gill-openings ; x, chorda ; mr, 
spinal tube ; kg, gill-vessels ; k, gill-intestine ; hz, heart ; vis, muscles ; 
ma, stomach ; v, intestinal vein ; c, body-cavity ; a, aorta ; I, liver ; d, small 
intestine ; e, ovaiy ; h, testes ; n, kidney canal ; af, anus ; Hi, leather skin ; 
oh, outer skin (epidermis) ; /, skin-fold, acting as fin. 

the lateral openings of the neck. The water that passes 
through the mouth serves for breathing. The oxygen 
contained in it is inhaled by the blood-channels which 
extend along the " gill-arches ' (kg), situated between 
the gill-openings.' These very characteristic gill-openings 
and gill-arches are found in the human embryo, and in 
the embryos of all higher Vertebrates, at an early period 
of their development, in that form in which they are 
retained throughout life by the lower Vertebrates. In 
Mammals, Birds, and Reptiles they never act as true organs 
of respiration, but gradually develop into very different 
organs. The fact that they originally actually exist in the 
same form as in Fishes, is, however, one of the most interest- 
ing proofs of the descent of these three higher classes of 
Vertebrates from the Fishes. 



Not less interesting and significant is the circumstance 
that the later respiratory organs of Mammals, Birds, and 
Reptiles develop from the front, or respiratory portion of the 
intestinal canal. A bladder-like fold develops at an early 
period from the throat of the embryo, and soon takes the 
form of two large sacs, which are afterwards filled with 
air. These sacs are the two air-breathing lun^s which take 
the place of the water-breathing gills. But this bladder- 
like fold, from which the lungs arise, is simply the well- 
known air-filled bladder which is called the swimming- 
bladder in Fishes, and serves throughout life as a hydro- 
static organ, a swimming-apparatus lightening the specific 
gravity of the Fish. Human lungs are a modification of 
the swimming-bladder of Fishes. 

The vascular system of Vertebrates stands in the closest 
morphological and physiological relation to the intestinal 
canal, its most important parts being developed from the 
intestinal-fibrous layer. It consists of two distinct parts, 
which are, however, immediately dependent on each other, 

Fig. 60. — Ideal Primitive Vertebrate, ventral view : na, nose ; au, 
eyes ; g, ear ; md, mouth ; k, gill-body ; ks, gill-openings ; kg, vascular 
gill-arches ; hz, heart; v, intestinal vein; ma, stomach; I, liver; d, small 
intestine; af, anus; n, primitive kidneys; e, ovary; h, testicles; c, body- 
cavity ; ms, muscles ; /, skin-fold, acting as float. 


the system of blood-vessels and the system of lymphatic 
vessels. The cavities of the former contain the red blood ; 
ihose of the latter, the colourless lymph. To the lymphatic 
system belongs the ccelom (the so-called pleuro-peritoneal 
cavity) ; and also numerous lymphatic ducts which extend 
through all the organs, absorbing the juices which have 
been consumed from the tissues, and conveying them into 
the venous blood. Finally, the chyle- vessels, which absorb 
the white chyle or milky nutritive juice prepared by the 
mtestines, carry it into the blood. 

The blood-vessel system of Vertebrates is developed in 
various ways, but seems originally to have existed, in the 
Primitive Vertebrate, in the simple form in which it now 
permanently exists in the Ringed-worms (Annelida) — for 
example, the common Earth-worm — and in the Amphioxus. 
Two large unequal blood-channels, which are originally 
situated in the fibrous wall of the intestine, and which run 
along the intestinal canal in the central plane of the body 
(one underneath the intestinal canal, and the other above), 
must especially be regarded as essentially and originally the 
most important part of the blood-vessel system. These two 
principal channels give rise to many branches which traverse 
all parts of the body, and pass into each other in curves at 
the anterior and posterior ends of the body ; we will call 
them the primitive artery and primitive vein. The former 
represents the dorsal vessels, the latter the ventral vessels 
of the Worms. The primitive artery or primordial aorta 
(Fig. 59, a) lies on the top of the intestine, along the central 
line of the dorsal side, and conveys oxygenated or arterial 
blood from the gills into the body. The primitive or 
primordial principal vein (Fig. 60, v) lies below the intes- 


tine, along the central line on the side toward the abdomen, 
and conveys carbonated, or venous blood, from the body 
back to the gills. In the front part of the gill-division of 
the intestine, these two main channels are connected by 
several connecting branches, which rise in the form of 
arches between the gill-openings. These "vascular gill- 
arches " (kg) pass along the gill-openings, and directly 
accomplish respiration. Immediately behind their base the 
front end of the primitive vein enlarges into a spindle-shaped 
bladder (hz). This is the simplest rudiment of the heart, 
which, in higher Vertebrates and in Man, afterwards as- 
sumes the form of a four-chambered, pulsating organ. 

In the lowest part of the body-cavity of Vertebrates, 
on the under side of the dorsal wall, near and on both sides 
of the notochord and the mesentery, lie the sexual glands, 
which form the reproductive cells ; in the female the ovary, 
in the male the testis. Recent study of the development 
of these parts seems to show that the original formation 
of the sexual glands in mankind and in all other Verte- 
brates, is hermaphroditic, or sexless. The embryonic glands 
of the Vertebrate contain the rudiments of both kinds of 
reproductive organs — the ovary of the female, which forms 
the ovule; and the testis of the male, which forms the 
sperm. These two kinds of sexual glands, each of which at 
a later stage of development is distributed to one only of 
the two sexes, are originally united in the embryo. This 
fact leads us to the conviction, which appears probable on 
other grounds also, that Vertebrates, in common with lower 
animals, were originally hermaphrodite, that each indi- 
vidual was capable of reproducing itself independently, and 
that the separation of the sexual organs took place at a 


later period. We may, therefore, assume that the primitive 
Vertebrate possessed both ovaries (Fig. 60, 61, e) and 

testes (h). 

Eig. 61. — Transverse section through the 
posterior part of the ideal Primitive Vertebrate : 
/, float ; mr, spinal tube ; x, notochord ; vis, 
muscles ; e, ovaries ; n, primitive kidney ducts ; 
a, body-arteries ; d, intestine ; r, intestinal vein. 

The sexual organs of Vertebrates 
are most intimately connected with the 
primitive kidneys, two glands running 
along near the notochord, which, in the embryo, secrete the 
urine, and in Fishes and Amphibia, remain permanently as 
urinary organs. 87 In higher Vertebrates, their place is taken 
at a later period by the permanent kidneys, which arise 
from the posterior portion of the primitive kidney ducts. 
In their earliest and simplest form, the primitive kidneys 
appear to be a pair of simple ducts, running along either 
side of the notochord within the body-cavity, and having 
openings at their posterior ends (Fig. 60, n). In this form 
they yet appear transiently in the embryo of higher Verte- 
brates, and permanently in the Worms. 

The organs which we have thus enumerated in a 
general survey of the primitive Vertebrate, and have ex- 
amined in relation to their characteristic positions, are 
those parts of the organism which are repeated in all 
Vertebrates without exception, in the same mutual rela- 
tions, though they are modified in very various ways. We 
have turned our attention principally to the transverse 
section of the body (Fig. 54-56), because it shows most 

distinctly the peculiar relative positions of these organs. 


But, in order to perfect our picture, we must turn for a 
moment to pay special attention to their articulation or 
metameric structure, which is best seen in the longitudinal 
section (Fig. 52, 53). The body of Man, as of all developed 
Vertebrates, appears to be composed of a string or chain of 
like members lying one behind the other along the longi- 
tudinal axis of the body. In Man the number of these 
like segments or metamera is about forty ; in many Ver- 
tebrates, for example, in Snakes and Eels, it is several 
hundred. As this inner articulation corresponds essentially 
with the vertebral column and the muscles surrounding it, 
these members, segments or metamera, are called primitive 
vertebrae. Now, this structure of these primitive ver- 
tebrae, or internal metamera, is correctly regarded as a 
prominent characteristic of Vertebrates, and the various 
forms into which it is differentiated bear greatly on the 
different groups of Vertebrates. But in our present task, 
that of tracing the development of the simple body of the 
primitive Vertebrate from the Gastrula, the segments or 
metamera are of subordinate significance, and we need not 
deal with them till later. 

Putting these metamera temporarily aside, I think that> 
in the above brief description of the essential parts, I have 
said everything necessary as to the fundamental structure 
of Vertebrates. The chief organs which have been men- 
tioned are the original and most important parts, nearly all 
of which are to be found, in a similar form, in the adult 
Amphioxus, and which re-occur in the original rudimentary 
germ of all members of this tribe. Many very important 
parts, which appear to be entirely essential, will, it is true, 
be missed in this review. As I have already remarked, the 


(Specialized head of the Vertebrate with skull and brain is 
a non-essential, secondary formation ; and the same may be 
said of the limbs or extremities. Important as these parts 
of Man and the higher vertebrates are physiologically y they 
are morphologically unimportant, for originally they were 
absent, and they develop only at a later period. The older 
Vertebrates of the Silurian Period had neither skull nor 
brain, and were entirely without limbs. 

If we pay no attention to those parts which are second 
arily formed, and are therefore unimportant, and if we 
provisionally examine only the essential, primary parts, we 
shall greatly simplify our task. This task is essentially 
to trace the described organism of the " primitive Verte- 
brate" from the simple germ-form of the Gastrula. That 
simplest Vertebrate body is, as is usually said, composed of 
two symmetrical, double tubes ; of a lower tube, the body- 
wall, which surrounds the intestinal tube, and of an upper 
tube, spinal canal, which surrounds the spinal marrow. 
Between the spinal tube and the intestinal tube, lies the 
notuchord, the most essential part of the inner axis of 
the skeleton which characterizes the Vertebrate. This 
characteristic arrangement of the most important organs 
re-occurs in all Vertebrates from the Amphioxus to Man. 
(C£ Plate IV., with explanation.) We must, therefore, 
now examine the way in which these organs develop from 
the two primary germ-layers of the Gastrula, and from the 
four secondary germ-layers which arise by fission of the two 

In order to solve this difficult problem it seems desirable 
to begin with a statement of the most important conclusions 
of ontogenetic study. The distant goal will be more easily 



reached if we see it clearly before us. I will now, there- 
fore, mention as briefly as possible the relations which 
these particular organs of the vertebrate organism bear to 
the four different germ-layers. 

The first of the secondary germ-layers, the skin-sensory- 
layer, produces, — firstly, the outer covering of the whole 
body; the outer skin, or epidermis, and, in higher Ver- 
tebrates, the hair, nails, sweat and sebaceous glands, and 
all other parts developing secondarily from the originally 
simple outer skin (epidermis). In the second place, from 
this layer arises also the central nerve-system, the medullary 
or spinal canal. It is remarkable that this mental organ 
develops from the outer surface of the epidermis, and, only 
afterwards, during the course of the development of the 
individual, gradually moves inward, so that, at a later 
period, it is situated internally, surrounded by muscles, 
bones, and other parts. Thirdly, the primitive kidney of 
the Vertebrate which secretes the urine, probably develops 
from the outer germ-layer. It may be presumed that this 
primitive kidney was originally a secretory gland of the 
skin, like the sweat-glands, and, like them, developed from 
the outer skin (epidermis) ; at a later period it lies deep 
within the body. 

From the second of the secondary germ-layers, the skin- 
fibrous layer, arises the principal mass of the vertebrate 
body, namely, all those parts lying between the epidermis 
and the inner ccelom, and forming the firm body-wall. To 
these belong, firstly, the leather-skin (corium), which lies 
at the surface directly under the epidermis, — the firm, 
fibrous covering which contains the nerves and blood-vessels 
of the skin; secondly, the great masses of muscle of th« 


whole trunk, or the flesh, surrounding the vertebral column, 
and consisting of two main groups of muscles ; the dorsal, 
or upper side-muscles of the trunk, and the ventral, or lower 
side-muscles of the trunk. To these must be added, in the 
third place, the inner skeleton, which is especially character- 
istic of Vertebrates, the central formation of which is the 
spinal axis or notochord, developing at a later period 
into the articulated vertebral column; also all the bones, 
cartilages, ligaments, etc., which form the vertebral skeleton 
in all more highly developed Vertebrates, and are connected 
by the sinews and muscles belonging to it. Fourthly and 
finally, from the innermost layer of cells of this secondary 
germ-layer develops the exocoelar, that is, the outer, or 
parietal ccelom-epithelium, the cell-layer which forms the 
inner covering of the body- wall, and which is also probably 
the original site of the male sexual cells. 

The third secondary germ-layer is the intestinal-fibrous 
layer. From this is developed, firstly, the endoccelar, that 
is, the inner, or visceral ccelom-epithelium, the layer of 
cells, covering the outer surface of the whole intestine, pro- 
bably also the site of the female sexual cells. Secondly, from 
this layer originates the heart, and the great blood-vessels 
of the body, as well as the blood itself, so that it has been 
called, in a peculiar sense, the vascular layer. The great 
blood-channels, or arteries, going from the heart and the 
great veins passing to the heart, as well as the chyle-ves- 
sels, which open into the latter, are formed, like the heart, 
the lymph, and the blood itself, from this third germ- 
layer. Thirdly, arises the muscular tube of the intes- 
tines, or the mesenteric tube, that is, the whole of those 
fibrous and fleshy parts which form the outer wall of the 


intestinal canal, as well as the mesentery, the thin, fibrous 
membrane by which the intestinal canal is connected with 
the ventral side of the vertebral column. 

The history of the fourth secondary germ-layer, or the 
intestinal-glandular layer, is very simple and clear. It* 
only product is the intestinal cellular covering, or the Epi- 
thelium of the intestinal canal with all its appendages, the 
large and small intestinal glands, among which are the 
lungs, liver, salivary glands. (Cf. Plates IV., V.) 


Syitematio Surrey of the principal organs of the ideal Primitive Vertebrate, 
the hypothetical parent-form of Vertebrates, and of their development 
from the germ-layers. 

Primary Germ- 

Secondary Germ- 

Most important Organs of 
the Primitive Vertebrato, 


(Animal germ-layer, / 

Lamina dermalis, H. 



Skin-sensory layer 

(Skin-stratum, Baer), 

Sen?ory layer. 
Lamina neurodermali$ t 3 


Skin -fibrous layer 

(Flesh-stratum, Baer), 


Flesh -layer. 
Lamina inodermalU, E. 



Outer skin (Epidermis). A 
simple cell-covering of the 
outer surface. 

Spinal tube (Tubu$ nedul~ 
laris) (with the organs of 
sense : the nose, eyes, organs 
of hearing). 

Primitive Kidneys (Protone- 
phra) (a pair of simple ducts, 
primitive kidney ducts). 

True skin (Corium) (CWi* and 

Muscles of the trunk (dorsal 
and ventral muscles). 

Note-chord {Chorda dorsalis). 

Exoccelar, or Parietal Ccelom- 
epitbelium ("the inner cell- 
covering of trie body-wallX 

8. Male sexual glands (Teste 

Coeloma, or Body-cavity. A space between the body-wall and the 
intestinal wall, between the exoderm and the entoderm, filled with lymph 
(colourless blood). 


Intestinal laver 
(Vegetative germ 

layer, Baer). 
Lamina gastralis 




Intestinal-fibrous layer 

(Vascular stratum, Baer), 


Vascular laver. 
Lamina inogastralit, H. 


Intestinal-glandular layer 

(Mucous stratum, Baer), 


Mucous layer. 
lamina mycogattrali*, M. 

I. Female sexual glands (Ovary). 

). Endoccelar, or Visceral Cos- 
lom-epithelium (the outer 
cell-covering of the Intestinal 
11. Principal blood-vessels (primi- 
tive artery or dorsal vessel, 
and the primitive vein or 
ventral vessel). 

I. Mesentery. 

>. Muscular intestine wall (fi- 
brous intestinal wall). 

14. Intestinal epithelium (innel 

cell-covering of the intestinar 

15. Intestinal glandular epithe- 

lium (liver-cells and other 
Intestinal glandular cells\ 



he Original (Palingenetio) Development of the Vertebrate Body from 
the Gastrula. — Eelation of this Process to the Later (Kenogenetio) 
Germination, as it occurs in Mammals. — The most important act in the 
Formation of the Vertebrate. — The Primary Germ-layers, and also the 
Secondary Germ-layers, which arise by Fission of the Primaries, 
originally form Closed Tnbes. — Contemporaneonsly with the Completion 
of the Yelk-sac, the Germ-layers flatten, and only later again assume 
a Tnbnlar Form. — Origin of the Disc-shaped Mammalian Germ-area. 
— Light Germ-area (area pellucida) and Dark Germ-area (area 
opaca). — The Oral Germ-shield, which afterwards assumes the Shape 
of the Sole of a Shoe, appears in the Centre of the Light Germ-area 
(a. pellucida). — The Primitive Streak separates the Germ-shield into 
a Eight and Left Half. — Below the Dorsal Furrow the Central Germ- 
layer parts into the Notochord and the Two Side-layers. — The Side- 
layers split horizontally into Two Layers : the Skin-fibrous layer and 
the Intestinal-fibrous layer. — The Primary Vertebral Cords separate from 
the Side-layers. — The Skin-sensory Layer separates into Three Parts : 
the Horny Layer, Spinal Canal, and Primitive Kidney. — Formation of 
the Coelom and the First Arteries. — The Intestinal Canal proceeds from 
the Intestinal Furrow. — The Embryo separates from the Germ-vesiole. 
— Around it is formed the Amnion-fold, which coalesces over the back 
of the Embryo, so as to form a Closed Sac. — The Amnion. — The 
Amnion-water. — The Yelk-sac, or Navel- vesicle. — The Closing of the 
Intestinal and Ventral Walls occasions the Formation of the NaveL — 
The Dorsal and Ventral Walls. 

" The development of the Vertebrate proceeds from an axis upward, in 
two layers, which coalesce at the edges, and also downward, in two layers, 


winch likewise coalesce at the edges. Thus two main tnbes are formed, one 
above the other. During the formation of these, the embryo separates into 
strata, so that the two main tubes are composed of subordinate tubes which 
enclose each other as fundamental organs, and are capable of developing 
into all the organs." — Kael Eenst Baee (1828). 


The mammalian egg, in the stage of development in which 
we left it, presented an extremely important and remark- 
able germ-form, the Gastrula (Fig. 41, p. 213, and Plate II. 
Fig 17). The whole body of this globular Gastrula con- 
sists solely of the two kinds of cells which compose the 
two primary germ-layers. A single stratum of lighter- 
coloured and firmer cells forms the outer germ-layer, and con- 
stitutes an outer covering over the whole surface of the body 
of the Gastrula. The whole interior of the latter is filled 
by the darker and softer cells of the inner germ-layer : it 
is only at a single point that these latter cells appear at 
the outer surface of the spherical body; this point is the 
mouth of the Gastrula, the primitive mouth (protostoma, 
Fig. 41, o). 

It is no easy task to explain how the complex mamma- 
lian organism originates from this simple Gastrula. In 
order to lighten the task, we have, as a preliminary, made 
ourselves acquainted with the typical structure of the 
simple primitive Vertebrate (Fig. 52-56, p. 256). We chiefly 
based our study of that directly on the real conditions 
which may yet be actually seen in the structure of the 
body of the lowest extant Vertebrate, the Amphioxus. In 
most important points of internal organization we may 
regard the Amphioxus as a correct, palingenetic picture of 
the long-extinct parent-form of all Vertebrates, the form to 
which the origin of Man must also be referred. It is only 
in a few unimportant points that the Amphioxus appears to 



Fig. 62-69. — Diagrammatic transverse sections through the most im 
portant germ-forms of the ideal Primitive Vertebrate (Fig. 52-61). 89 

Fig. 62. — A. Transverse section through the Gastrnla ; two-layered germ 

Fig. 63. — jB. Three-layered germ. 

Fig. 64. — C. Four-layered germ (four secondary germ-layers). 


p^. 65. — j). The body-cavity appears between the skin-layer and the 
intestinal layer. 

Fig. 66. — E. The notochord appears between the spinal furrow and the 



Fig. 67. — F. The primitive kidneys and primitive vertebrae appear ; the 
spinal tube is olosed. 

Fig. 68. — O. The rudiments of the sexual organs appear near the primi- 
wve kidneys. The primitive vertebras surround the notochord and the 
jpinal tube. 

Fig. 69. — H. The main blood-vessels appear above and below the intestine. 

The letters indicate the same parts in all : d, the intestinal cavity ; dd, 
the intestinal-glandular layer ; df, the intestinal-fibrous layer ; g, mesen- 
tery ; y, female germ-glands (rudimentary ovary) ; », male germ-glands 
(rudimentary testes) ; a, aorta (primitive artery) j vd, intestinal vein 
(primitive vein) j vc, cardinal vein ; eh, notochord ; uw, primitive ver- 
tebrae ; w, vertebrae ; m, dorsal muscles ; bm, ventral muscles ; u, primi- 
tive kidneys ; mf, spinal furrow ; mr, spinal tube ; hs, horn -plate. In all, 
the four secondary germ-layers are indicated by shading : the intestinal 
glandular layer (dd) is dotted. The intestinal-fibrous layer (df) is per- 
pendicularly shaded. The skin-fibrous layer (hf) is horizontally shaded. 
The skin-sensory layer (hs) is black. 

be kenogenetically altered, and we must suppose that 
the conditions were originally different. This is equally 
true of the very important germ-history of this lowest Ver- 
tebrate. In a later chapter (XIV.) we shall enter into the 
details of this. Here, however, we may base our argument 
on this germ-history so far as we are able, from a compara- 
tive study of the germination of the various Vertebrates, to 
form an approximate conception of the course of individual 
evolution, as it originally occurred in the oldest and simplest 
Vertebrates. Only after we have gained a general view of 
this, can we turn to the far harder task of tracing the 
construction of the mammalian organism, and especially 
that of Man, from the Gastrula. 

The palingenetic Bell-gastrula of the Amphioxus (Fig 
28, p. 193) affords a safe starting-point. A series of dia* 


grammatic transverse sections through those germ-forma 
which first develop from the Gastrula, 'will best and most 
easily afford us the desired view. (Cf. Fig. 62-69, and 
Plates IV., V.) In the first place, a third layer, the middle 
layer, or fibrous layer (mesoderrna, Fig. 63 m&), arises be- 
tween the two primary germ-layers of the Gastrula (Fig 
62). Then, this three-layered stage is followed by one in 
which there are four layers (Fig. 64). As we have already 
stated, each of the two primary germ-layers probably 
originally contributed to the formation of the middle layer 
(mesoderrna), although it is usually asserted that the latter 
originates from one only of the former. It is probable that 
the exoderm, or skin-layer (e), separated into the skin- 
sensory layer (hs) and the skin-fibrous layer (hf) y and 
correspondingly, the entoderm, or intestinal layer, into the 
intestinal-fibrous layer (df) and the intestinal-glandular 
layer (dd). 

When the four germ-layers are completed, the form of 
the Gastrula, which had but one axis, has become symme- 
trically bilateral (cf. p. 257). In consequence of the body 
becoming flat, a distinction is formed between the dorsal 
and ventral sides, between the right and the left. Parallel 
with the axis of length, a delicate streak, the indication of 
a furrow, appears in the centre of the dorsal side. The side 
walls of this furrow, which is called the " spinal furrow ' 
(m/), rise in the form of two parallel ledges (Fig. 65 m/) ; 
these are the spinal swellings (medullary or dorsal swell- 
ings). Their two parallel edges bend toward each othe* 
(Fig. 66 mf) and finally coalesce, so that the trench 
becomes a tube ; this is the spinal tube (Fig. 67 mr): 
Along the longitudinal axis of the body, a solid cylindrical 


cord arises between the spinal tube and the intestinal tube ; 
this is the notochord, or chorda (ch). It originates from 
the central portion of the skin-fibrous layer, while the side 
portions of the latter supply the true skin and the great 
part of the flesh. This flesh-mass separates into the dorsal 
muscles (Fig. 68, 69 rra) and the ventral muscles (6m). 

The separation of the four secondary germ-layers is 
followed by a separation between the skin-fibrous layer (hf) 
and the intestinal-fibrous layer (df). Between the two, 
a chink-like cavity, filled with fluid, arises ; this is the true 
body-cavity (cceloma, Fig. 65-69 c). The intestinal tube lies 
freely in this, being only supported along the length of the 
notochord by a band of the intestinal-fibrous laver, which 
afterwards extends into the mesentery (Fig. 68 g). Two 
narrow canals, filled with blood, form within the intestinal- 
fibrous layer, and traverse the whole length of the intestine, 
one passing underneath, and the other above; these are the 
first blood-vessels. The upper of the two is the dorsal 
vessel (Fig. 69 a), the latter is the ventral vessel (vd) ; the 
one afterwards gives rise to the aorta, the other to the 
intestinal vein and the heart. 

Finally, the first rudiments of the urinary and sexual 
glands make their appearance on either side of the in- 
testinal tube and the notochord attached to the dorsal 
wall of the body-cavity. The primitive kidneys (u) re- 
semble two narrow canals, traversing the body, parallel 
to the notochord, opening at the anterior end into the 
body-cavity, and at the posterior end through the outer 
ftVJTi (or in the last chamber of the intestine). They 
probably originally arose as skin-glands, formed by an 
inversion of the skin-sensory layer (Fig. 66-68 u). In 


fcheir immediate neighbourhood are the sexual organs, in the 
form of simple heaps of cells, which are attached to the 
body- wall, near the mesentery. Presumably, they originated 
as hermaphrodite glands, the female gland (y) from the 
inner, the male gland (x) from the outer germ-layer. The 
former becomes the ovary, the latter the testes. Simul 
taneously with these changes, the spinal tube has completely 
detached itself from its original site, the skin-sensory layer, 
and has made its way far into the body. A sheath has 
formed round the notochord, and processes from this " nota 
chord sheath " grow round the spinal tube, imbedding it in 
a vertebral canal (Fig. 68, 69 w). 

If, for a moment, we leave the transverse sections, and 
trace the evolution of the primitive Vertebrate in longi- 
tudinal sections, we see that at a very early period the 
intestinal tube is divided into a gill-intestine and a stomach - 
intestine, in consequence of the appearance of gills in the 
anterior portion. The primitive mouth of the Gastrula 
closes ; the two permanent openings of the future intestine 
arise as new formations from the exterior; the mouth in 
front, the anus behind. Moreover, the outer body-wall 
becomes articulated, owing to the fact that the fleshy mass 
of the trunk-muscles assumes the form of a number of 
similar, consecutive portions, segments, or metamera. In 
correspondence with these, each of the respective portions 
of the nerve and vascular systems becomes distinct 

The following processes must, therefore, be emphasized 
as the chief acts by which the simple Gastrula changes into 
the typical vertebrate organism : 1. The two primary germ- 
layers part by fission into four secondary germ-layers. 
2. The Gastrula becomes flattened, so that, instead oi a form 


with a single axis, it assumes the bilateral vertebrate form. 
X The body-cavity (coelorna) arises, in consequence of the 
disconnection of the skin-fibrous layer and the intestinal- 
fibrous layer. 4. Along the central line of the dorsal 
surface the nerve-centre appears in the form of a trench- 
shaped furrow; it then changes into the spinal-tube and 
completely detaches itself from the skin-sensory layer. 
5. Immediately below the spinal tube, the notochord origi- 
nates from the central part of the skin-fibrous layer, while 
the side parts of the same layer form the true skin and the 
trunk-muscles ; the latter articulate themselves into meta- 
mera. 6. In the outer stratum of the intestinal wall, in 
the intestinal-fibrous layer, originate the main blood-vessels, 
a dorsal vessel (aorta) above the intestinal tube, and a ven- 
tral vessel (primitive vein) below the latter. 7. The intes- 
tinal tube separates into two main parts ; the gill-intestine 
in front, the stomach-intestine behind. Several gill-open- 
ings form on either side of the gill-intestine. 8. The 
intestinal tube acquires two new openings, a mouth in front, 
an anus behind ; the original primitive mouth of the Gas- 
trula closes. 9. Close by the intestine and notochord, and 
on either side of them, arises a tube which separates urine, 
and which opens into the body-cavity in front, outside the 
body in the rear ; this is the primitive kidney canal. 
10. Close by this canal, between it and the notochord, develop 
the rudiments of the sexual glands (the testes and ovary), 
in the form of roundish cellular masses, which penetrate 
from the wall of the body-cavity to this position (the un- 
defined boundary of the skin-fibrous layer and the intestinal- 
horous layer). 90 

These chief, fundamental, and palingenetic acts in the 


evolution of the individual, the assumption of which ia 
justified by the comparative germ-history of Vertebrates, 
re-occur essentially in all branches of this tribe, though in 
single cases they are more or less changed, or kenogenetically 
modified. In their simplest and earliest form, which is 
certainly mainly palingenetic, we yet find them in the 
Amphioxus ; in the Round-mouths (Cyclostomi), Fishes, and 
Amphibia they have already become much changed and 
vitiated, kenogenetically transformed ; and this is true 
in a much greater degree of the three higher vertebrate 
classes, Reptiles, Birds, and Mammals. In these the gradual 
formation of a very large nutritive yelk and of peculiar 
egg-membranes has introduced so many changes, or 
secondary kenogenetic modifications, that at first sight it 
is hardly possible to recognize the primary palingenetic 
incidents of evolution. 

In these, the kenogenetic relation of the germ to the 
nutritive yelk is especially prominent, and till quite recently 
caused an entirely false conception of the first and most 
important conditions of the germ of the higher Vertebrates, 
introducing many false views as to the Ontogeny of these. 
Previously, the germ-history of the higher Vertebrates was 
universally based on the view that the first rudiment of the 
germ is a flat layer-shaped disc ; and for this reason the 
cell strata which compose the germ-disc (also called the 
germ-area) were called " germ-layers." This flat germ-disc 
which is at first circular, afterwards oval, and which in the 
hen's egg we have learned to call the tread (cicatricula), 
lies at a particular point on the outer surface of the large 
globular nutritive yelk. When germination begins, the flat 
germ-disc arches outwards and detaches its outer surface 



from the large yelk-ball which lies beneath it. The flat 
layers become tubes, in consequence of their edges inclining 

Fig. 70. — Separation of the disc-shaped mammalian germ from the 
yelk-sac; in transverse section (diagrammatic). A. The germ-disc {h,hf) 
lies flat on one side of the intestinal germ-vesicle (kb). B. The spinal tube 
(rar) appears in the centre of the germ-disc, and underneath t'his the 
notochord (ch). C. The intestinal-fibrous layer (df) has grown round 
the intestinal-glandular layer (dd). D. The skin-fibrous layer (hf) and the 
intestinal-fibrous layer (df) separate in the outer wall ; the intestine (d ) 
begins to separate itself from the navel- vesicle (nb). E. The intestinal tube 
(rar) is closed ; the body-cavity (c) begins to form. F. The primitive verte- 
brae (w) separate ; the intestine {d) is almost entirely closed. G. The 
primitive vertebrae (w) begin to grow round the spinal tube (mr) and the 
notochord (ch) ; the intestine (d) is separate from the navel-vesicle (nb). 
H. The vertebras (w) have grown round the spinal tube (rar) and the noto- 
chord ; the body -cavity (c) is closed, the navel-vesicle has disappeared. The 
amnion and serous membrane are omitted. 

In all, t e le.ters indicate the same parts : h, horn-plate ; mr, spinal 
tube; hf, skin-fibrous layer; w, primitive vertebrae; ch, notochord ; c, body- 
cavity (cceloma) ; df, intestinal-hbrous layer; dd, intestinal-glandular layer j 
d, intestinal cavity ; nb, navel-vesicle. 


towards each other and coalescing (Fig. 70). The genu 
growing at the expense of the nutritive yelk, the lattei 
continually becomes smaller; it is completely surrounded 
by the growth of the germ-layers. At a later period the 
remnant of the nutritive yelk forms only a small globular 
sac, the yelk-sac, or navel-sac (saccus vittelinus, or vesicula 
umbilicalis, Fig. 70 nb). This is surrounded by the intes- 
tinal layer, and connected with the central portion of the 
intestinal tube by a thin stalk, the yelk-duct (ductus 
vitellinus), and, in most Vertebrates, is at last completely 
absorbed by the intestinal tube (Fig. 70 H). The point at 
which this happens, and at which the intestine finally 
closes, is the intestinal navel. In Mammals, in which the 
remnant of the yelk-sac remains outside and gradually 
dwindles, the yelk-duct pierces the outer ventral wall to the 
last. The navel cord parts. at birth at this point, which per* 
manently remains as the navel (umbilicus) in the outer skin. 
As in the germ-history of the higher Vertebrates, based 
chiefly on that of the Chick, the distinction between the 
germ (or formative yelk) and the nutritive yelk (or yelk- 
sac) has up to the present time been regarded as original, 
the flat, leaf-shaped rudiment of the germ-disc has also 
necessarily been regarded as the original germ-form, and 
the greatest weight has been laid on the fact that these 
Hat germ-layers curve, and thus become hollow trenches. 
and that, by the concrescence of their edges, they become 
closed tubes. 

This view, which has governed all past expositions of 
the germ-history of the higher Vertebrates, is, I am con- 
vinced, entirely false. For the Gastrasa Theory, the full 
i'-nificance of which now becomes evident, teaches us that 



the real state of the case is originally just the opposite. 
The Gastrula, in the body- wall of which the two primary 
germ-layers appear from the first as closed tubes, is the 
original germ- form of ail Vertebrates, as of all Invertebrate 
animals ; and the fiat germ-disc, with its flatly extended 
germ-layers, is a later, secondary germ-form, which arose 
in consequence of the kenogenetic formation of the large 
nutritive yelk, and the consequent extension of the germ- 
layers over the surface of the latter. 91 The curving of these 
germ-layers, which actually occurs, and their coalescence 
into tubes is, therefore, not original and primary, but a much 
later, tertiarv incident of evolution. Accordingly the three 
following stages of germ-formation must be distinguished in 
the Phylogeny of Vertebrates : 

A. First Stage : 

B. Second Stage : • 

C. Third Stage : 







Process of Germ- 

Process of Germ- 

Process of Germ- 




From the first, the 

The germ. layers 

The germ -layers form 

germ-layers form closed 

spread themselves out 

a flat germ-disc, the 


in the form of a leaf, 

edges of which incline 

No nutritive yelk. 

in consequence of the 

toward each other, and 

formation of a large 

coalesce into u closed 

yelk-sac from the cen- 


tre of the intestinal 


If this view is correct, and, as the logical conclusion 
from the Gastraea Theory, I am obliged to believe it is so, 
then the explanation of the process as at present accepted 
must be exactly reversed. The yelk-sac must no longer 
be treated as though it were originally distinct from the 


germ or embryo, but as essentially a part of the latter, a 
part of its intestinal tube. According to this view, the 
primitive intestine (protogaster) of the Gastrula of the 
higher animals has separated, in consequence of the keno- 
genetic formation of the nutritive yelk, into two different 
parts : the after-intestine (metagaster) or the permanent 
intestinal canal, and the yelk-sac or navel-vesicle. 

If the germ-histories of the Amphioxus, the Frog, the 
Chick, and the Rabbit are comparatively studied (Plates II., 
III.), I am convinced that there can be no doubt as to the 
correctness of this view, which I have thus explained. In 
the light afforded by the Gastraea Theory we shall regard 
the structural proportions of the Amphioxus alone of all 
Vertebrates, as original and but slightly varied from the 
palingenetic germ-forms. In the Frog these proportions are 
on the whole but slightly kenogenetically altered. In the 
Chick, on the contrary, they are very much altered, and 
most of all in the Rabbit. Both in the Bell-gastrula of the 
Amphioxus and in the Hood -gastrula Of the Frog, the germ- 
layers are visible from the first in the form of closed tubes 
(Plate II. Fig. 6, 11). But, on the other hand, the em- 
bryonic Chick (in the freshly-laid, unincubated egg) appears 
in the form of a flat, circular disc ; it was only quite 
recently that the true gastrula-character of this germ-disc 
was recognized by Rauber and Goette. 74 This Disc-gastrula 
grows and surrounds the huge globular yelk, and the after- 
intestine (metagaster) parts off from the external yelk-sac ; . 
in these two processes all that is diagrammatically repre- 
sented in Fig. 70 is accomplished ; and these are the pro- 
cesses which have been regarded as main acts, though they 
are in reality only secondary acts. 


In Mammals the corresponding germinal processes are 
fery complex and peculiar. Till quite recently they were 
entirely wrongly explained ; the recently published researches 
of Eduard van Beneden 69 which placed them, for the first 
time, in a true light, enabled us to bring them into agree- 
ment with the principles of the Gastnea Theory, and to trace 
their relation to the germination of the lower Vertebrates. 
Although there is no independent nutritive yelk, distinct 
from the formative yelk, in the mammalian egg, and 
although the cleavage is therefore total, a large yelk-sac 
arises from the embryo which is produced by this cleavage, 
and the true germ spreads itself in a layer-like form on the 
surface of this yelk-sac, as in the case of Reptiles and Birds, 
the eggs of which have a large nutritive yelk and undergo 
partial cleavage. As in the latter, the flat, leaf-shaped 
germ-disc of Mammals detaches itself from the yelk-sac, 
its walls incline towards each other and coalesce into tubes. 

This striking contradiction can only be explained as a 
consequence of very peculiar, strange, kenogenetic modi- 
fications of the germ, the causes of which are not yet fully 
explained. They are evidently connected with the fact 
that the ancestors of the viviparous Mammals were Amnion- 
animals, which laid eggs, and which only gradually became 
viviparous. When the Hood-gastrula (Amphigastrula) of 
the Mammal is complete (Fig. 71), it changes into a large 
globular vesicle, filled with fluid. According to Van 
Beneden, this happens in the following way : The Gastrula- 
mouth disappears in consequence of the entoderm-cell (0), 
which formed the yelk -plug, disappearing into the interior, 
to the other cells of the intestinal layer (oQ. The mam- 
malian germ now forms a solid ball, consisting of a quantity 



of dark, multilateral entuderm-cells (i), and covered by a 
light-coloured globular membrane, which is composed of a 
single stratum of exoderm-cells (e). A transparent bright- 
coloured liquid now collects at a point between the two 
germ-layers ; and this increases so greatly that it expands 
the exoderm cellular membrane into a large globular vesicle. 
The mass of entoderm-cells, forming a ball of smaller 
diameter, remains attached to one point of the exoderm ; 
(according to Van Beneden, this point is that of the yelk- 
plug, o). The entoderm mass now becomes flattened, first 
assuming a hemispherical, then a lentil-shaped, and finally 
a discoidal form: this is accomplished by a movement 
among the cells, which spread themselves out in a one- 
layered circular disc. 

Fig. 71. — Gastrula of a Mam- 
mal (Amphigastrula of a Rabbit) 
in longitudinal section through 
the axis : e, exoderm-cells (64, 
lighter-coloured and smaller) ; i, 
entoderm-cells (32, darker and 
larger) ; d, central entoderm-cells, 
occupying the primitive intes- 
tinal cavity ; o, peripheric ento- 
derm-cells, plugging the primi- 
tive mouth-opening (the yelk- 
plug in the "anus of Kusconi"),, 

This vesicular condition of the mammalian germ w T as 
detected two centuries ago (1677) by Regner de Graaf. He 
discovered small, globular, transparent vesicles, vith a 
double membrane, lying free in the uterus of a Rabbit four 
days after impregnation. But Graaf 's statement was not 
accepted. It was not till 1827 that these vesicles were 



re-discovered by Baer ; those of the Rabbit were afterwards 
more minutely described by Bischoff, in 1842. They may 
be found in the uterus (matrix) of the Rabbit, the Dog, and 
other small Mammals within a few days after impregnation. 
The ripe mammalian eggs, having left the ovary, are fer- 
tilized, either here or in the oviduct, by the active sperm- 
cells which make their way in. 9 ' 2 (On the" uterus and 
oviduct cf. Chapter XXV.) Cleavage and gastrulation 
take place within the oviduct. Either while the mam- 
malian Gastrula is still in the oviduct, or after it has entered 
the uterus, it changes into the globular vesicle which is 
represented in Fig. 7'2 (the surface) and in Fig. 73 (in 

Fig. 72.— Intestinal germ-vesicle (Gastrocijstis) of a Rabbit (the so-called 
"Germ-vesicle," or vesicula blastoderm ica, of other writers): a, external 
egg-membrane (chorion) ; b, skin-layer (exoderrna), forming the whole wall of 
the germ-yelk vesicle ; c, heap of dark cells, forming the intestinal layer 
(entoderma) . 

Fig. 73.— The same in section. The letters as in Fig. 72 : d, hollow 
space within the intestinal germ-vesicle 

section). The thick, external, structureless membrane which 
surrounds this is a modification of the original egg-mem- 
brane (zona pellucida, p. 135), with the addition of an 


albuminous stratum, which has been externally deposited. 
In future we shall call this membrane the outer egg-mem- 
brane, the primary chorion (prochorion, a). The real wall 
of the vesicle, surrounded by this outer egg-membrane, 
consists of a simple layer of exoderm-cells (6), which have 
been regularly flattened by mutual pressure, and 'most of 
which are hexagonal ; a light-coloured kernel is visible 
through their finely granulated protoplasm (Fig. 74). On a 

Fig. 74. — Four exoderm-cells from the intestinal germ-vesicle of a 

Fig. 75. — Two entoderm-cells from the same. 

point on the inside of this hollow sphere lies a circular disc, 
formed of darker, softer, and rounder cells, of the dark 
granulated entoderm-cells (Fig. 75). 

The characteristic germ-form in which the developing 
Mammal now is has usually been called the " germ-vesicle ' 
{, Bischoff) ; the "sac-germ" (Baer) ; the "vesi- 
cular embryo," or the " germ-membrane vesicle ' (vesicula 
blastodermica, or, briefly, blastosphcera). The wall of the 
hollow sphere, consisting of a single cell-stratum, was called 
the " germ-membrane," or blastoderm, and it was supposed 
to be equivalent to the cell-stratum, called by the same 
name, which forms the wall of the true germ-membrane 
vesicle (Blastula) of the Amphioxus (Plate II. Fig. 4), and 


of many Invertebrate Animals (e.g. of the Monoxenia, Fig. 
22, F, G). This true germ-membrane vesicle has, up to the 
present time, been universally regarded as homologous with 
the germ-vesicle of Mammals. It is not so, however. The 
so-called " germ-vesicle " of Mammals and the true germ- 
membrane vesicle of the Amphioxus and of many Inverte- 
brates are entirely different germ-forms. The latter (the 
Blastvla) precedes gastrulation. The former (yesicula 
blastodermica) follows gastrulation. The globular wall of 
the blastula is a true germ-membrane (Blastoderma), and 
consists entirely of cells of one sort (blastoderm-cells) ; it 
is not yet specialized into the two primary germ-layera 
On the other hand, the globular wall of the mammalian 
" germ- vesicle " is the specialized skin-layer (exodtrma), 
and a circular disc of entirely different cells lies at a point 
on the inside of this ; this disc is the intestinal layer 
(entoderma). The spherical cavity, filled with clear liquid, 
in the interior of the blastula, is the cleavage-cavity. On 
the other hand, the similar cavity in the interior of the 
mammalian germ-vesicle is the yelk-sac cavity, which is 
joined on to the developing intestinal cavity. 

On all these grounds, which have been very recently 
brought to light by the researches of Van Beneden, it is 
very necessary to recognize the secondary " intestinal germ- 
vesicle " of Mammals (Gastrocystis) as a peculiar germ-form, 
occurring only in this class of animals, and as quite distinct 
from the " germ-membrane vesicle " (Blastula) of the Am- 
phioxus and of the Invertebrates. The wall of this mam- 
malian " intestinal germ- vesicle " consists of two distinct 
parts. Far the larger part is one-layered, and is formed by 
the exoderm, For the smaller part, the circular disc, which 


is formed of both primary germ-layers, we will adopt Van 
Beneden's name, and call it the intestinal germ -disc (Gas- 

The small, circular, dull whitish spot which lies at a 
particular point on the outer surface of the bright-coloured, 
transparent, and spherical " intestinal germ- vesicle," and 
which is the " intestinal germ-disc " (Gastrodiscus), has long 
been known to naturalists, and was compared with the 
" germ-disc " of Reptiles and Birds. Sometimes, therefore, it 
was called the " germ-disc ' (discus blastodermicus), some- 
times the " embryonic spot " (tache embryonnaire), but 
more usually the germ-area (area germinativa). The 
further evolution of the germ proceeds especially from this 
germ-area. On the other hand, the greater part of the 
intestinal-germ-vesicle of Mammals is not directly employed 
in the formation of the future body, but in the formation oi 
the transitory " navel- vesicle." The embryo-body pinches 
itself off from the latter more and more, in proportion as 
it grows and develops at the expense of the latter ; the 
two become no longer connected except by the yelk-duct 
(the stalk of the yelk-sac) ; and the latter forms the indirect 
communication between the cavity of the navel-vesicle and 
the intestinal cavity in the course of development (Fig. 70). 

The germ-area, or the intestinal germ-disc of Mammals, 
originally consists, like the germ-disc of Birds, merely oj 
the two primary germ-layers, each of which is formed of a 
single cell-stratum. Soon, however, a third cell-stratum, the 
rudiment of the middle fibrous layer (mesoderma), appears 
in the middle of the circular disc, between the two earlier 
strata. According to most observers, the mesoderm arises 
trom the inner primary germ- layer ; according to others, op 



the contrary, it arises from the outer of the two ; 03 both are 
probably concerned in the process. The middle of the 
germ-area, or germ-disc, now consists of three germ-layers, 
while the circular rim consists of two ; the rest of the wall 
of the intestinal germ-vesicle consists only of a single germ- 
layer, the outer. But this wall also now becomes two-layered. 

Fig. 7<5.— Section through the germ-area of a Mammal, at right angles 
to the surface (diagrammatic) : e, exoderm (the simple cell-stratum of the 
skin- layer) ; m, mesoderm (the several cell-stratum of the middle laver) ; 
i, entoderm (the simple cell-stratum of the intestinal layer) ; k, hollow 
cavity in the intestinal germ -vesicle. 

While, in the centre of the germ-area, the fibrous layer 
becomes greatly thickened, in consequence of cell-growth, 
the inner germ-layer simultaneously extends and grows in 
all directions from the edge of the disc. Everywhere closely 
applied to the outer germ-layer, it completely overgrows the 
inner surface of the latter ; it covers first the upper, and 
then the lower hemisphere of the inner surface, and finally 
closes in the centre of the latter. (Cf. Fig.. 77-81.) The 
whole wall of the intestinal germ-vesicle now, therefore, 
consists of two cell-strata: the exoderm without, the entoderm 
within. In the centre only of the circular germ-disc, which, 
in consequence of the excessive growth of the middle 
layer, continually increases in thickness, this germ-disc 
consists of all three germ-layers. Simultaneously, small 
structureless knots, or warts, secrete themselves on the 
'surface of the outer egg-membrane {prod tor ton), which 



-tf'iG. 77. — Egg from the uterus of a Eabbit (4 mm. in diameter). The 
perm men brane vesicle (6) has slightly retreated from the smooth outer 
egg-membrane (prochorion, a). The circular <jerm-area (c) is visible in the 
centre of the germ-membrane, and at the edge of the former (at d) the inner 


stratum of the germ-vesicle is already beginning to extend. (Fig. 77-81, 
after Bischoff.) 

Fig. 78. — The same, seen from one side. The letters as in Fig. 77. 

Fig. 79. — Egg from the uterus of a Rabbit (6 mm. in diameter). The 
germ-membrane is already to a great extent double-layered (b). The outer 
egg-membrane (prochorion) becomes knotty, or warty (a). 

Fig. 80. — The same, seen from one side. The letters as in Fig. 79. 

Fig. 81. — Egg from the uterus of a Rabbit (8 mm. in diameter). Nearly 
the whole of the germ-membrane vesicle is already double-layered (b) ; only 
below (at d) there is still only one layer. 

has raised itself from the intestinal germ-vesicle (Fig. 
79-81 a). 

We need not at present pay any attention either to this 
outer egg-membrane (prochorion) or to the larger portion of 
the germ- vesicle, and may turn our full attention to the 
germ-area (or germ-disc). For it is in this part alone that 
the important modifications which result in the specializa- 
tion of the earliest organs first appear. In this respect it is 
quite immaterial whether we examine the germ-area of a 
Mammal (e.g. a Rabbit), the germ-disc of a Bird or a Reptile 
(e.g. a Lizard or a Tortoise). For in all members of the three 
higher vertebrate classes, all called Amnion-animals, the 
germinal processes which immediately follow are essentially 
alike. In this respect Man is like the Rabbit, the Dog, the 
Ox, etc. ; and in all these Mammals the germ-area undergoes 
essentially the same modifications as in Birds and Reptiles. 
It is in the Chick that these have been chiefly and most 
accurately traced, for any requisite number of incubated 
hen's eggs, in all stages, can always be obtained. Within 
a few hours from the beginning of incubation the cir- 
cular germ-disc of the Chick also passes from a two- 
layered to a three-layered stage, in consequence of the 
development of the mesoderm between the exoderm and 
the entoderm. 


The first modification of the discoidal, three-layered 
germ-area consists in the fact that -the cells round its edge 
increase more rapidly, and accumulate dark granules in their 
protoplasm. In this way a darker ring is formed, which is 

Fig. 82. — Circular germ-area of a Eabbit, distinguished into toe central, 
light-coloured germ-area (area pellucida), and the peripheric dark germ-area 
(a. opaca). As it makes itself visible through the dark part, the area 
pellucida appears the darker. 

Fig. S3. — Oval germ-area (a. germinativa) . The dull whitish area opaca 
appears on the outside. 

more or less distinctly marked off from the lighter centre of 
the germ-disc (Fig. 82). The latter we shall in future call 
the light germ-area (area pellucida) ; the darker ring we 
shall call the dark germ-area (area opac< (In reflected 
light, as in Fig. 82-84, it appears reversed ; the light germ- 
area appears dark, because the dark ground makes itself 
seen from below ; the dark germ-area appears lighter in 
comparison.) The circular form of the germ-area changes 
to an elliptic, and immediately afterwards to an oval form 
(Fig. 83). One end appears broader and more abruptly 



rounded off, the other is smaller and more pointed; the 
former represents the hinder portion of ■the future body. 
The characteristic bilateral form of the body, the distinc- 
tion between anterior and posterior, between right and 
left, is thus already indicated. 

In the centre of the light germ-area a dull-coloured, 
large, oval spot now appears ; at first it is very delicate and 
hardly noticeable, it soon however becomes more sharply 
distinguished, and presently appears as an oval shield, 
surrounded by two rings (Fig. 84). The inner, lighter ring 

Fig. 8-1. — Germ. area or 
gerrn-disc of a Rabbit (about 
ten times magnified). As 
the delicate, half-transparent 
germ- disc lies on black 

and, the light germ-area 
appears as a darker ring, the 
dark uerm-area (situated on 
the outside), on the contrary, 
as .-, white ring. The oval 
germ-shield, situated in the 
centre, also appears whitish ; 
along its axis the dark spinal 
furrow is already visible. 
(After Bischoff.) 

is the remnant of the 
light germ-area ; the 
outer, darker ring is 
the dark germ-area : but the dull-coloured shield-shaped 
spot itself is the first rudiment of the dorsal portion 
of the embryo. We will call it briefly the " germ-shield " 
(not<tspis). u Kemak called it the "double shield," because 
it arises from a shield-shaped thickening of the outer and the 
middle gerni-layers. In most books this gernvshield is 



spoken of as the "first germ-rudiment or embryonic rudi- 
ment," as the " primitive germ," or " the first trace of the 
embryo." But these designations, which are based on the 
authority of Baer and BischofF, are incorrect. For in reality 
the germ or embryo already exists in the parent-cell, in the 
Gastrula, and in all the subsequent germ-stages. The germ, 
shield is merely the earliest rudiment of that dorsal part 
which first becomes defined. 

Fig. 85.— Germ-area or germ-disc of a Rabbit, with a sole-shaped germ- 
shield (about ten times enlargeu). The light, circular tract (d) is the dark 
area (a. opaca). The light area (a. pellucida) (c) is lyre- shaped, as is the 
germ-shield itself (b). Along its axis the dorsal furrow or spinal furrow (a) 
is seen. (After BischofF.) 

Fig. 86. — Sole-shaped germ-shield of a Dog ("double shield" of Eemak, 
"embryo-rudiment" of other authors) In the centre is the dorsal furrow, 
on either side are the spinal swellings, or medullary swellings. 

Fig. 87.— Sole-shaped germ-shield of Chick. 

After the oval germ-shield has become distinctly defined, 
in the centre of the light germ-area, along its centred line 
a delicate, white streak appears, which soon becomes pro- 


miment ; this is the " primitive streak " of Baer, the " axial 
plate " of Remak. This phenomenon is due to the fact that 
the upper and middle germ-layers coalesce along their 
central lines, thus forming the axis-band at this point. 
(Cf. Fig. 88, 89.) In the centre of the primitive streak an 
even, dark line, the so-called primitive groove, becomes 
denned (Fig. 84, 85, a). This separates the germ-shield into 
two symmetrical halves, a right and a left half. While the 
primitive groove deepens, the oval germ-area (a. germina- 
tiva) resumes its earlier circular form. 

The germ-shield, on the other hand, leaves its oval 
form and assumes the so-called lyre-shape, or sole-shape. 
Its elliptical leaf-shaped body becomes somewhat pinched 
in the middle, while its anterior and posterior ends become 
somewhat enlarged (Fig. 85) This very characteristic 
shape, which is most aptly compared to the sole of a shoe, 
a violin, or a lyre, is retained for some time by the embryo 
of the Mammal (Fig. 86, 87), and also by that of the Bird 
and the Reptile. The human germ-shield assumes this sole- 
form as early as the second week of its development. 
Towards the end of that week its length is about two 

We will now leave the peripheric part of the germ- 
area, for its changes are only interesting to us at a much 
later period, and we will give our whole attention to 
the sole-shaped germ-shield, from which the further evolu- 
tion of the body directly proceeds. In order correctly to 
understand this, we must employ a method which was first 
turned to full account by Remak, viz., that of viewing 
sections made from right to left perpendicularly through the 

thin disc of the germ-shield. It is only by very carefully 




studying these sections, one by one, in every stage of the 
evolution, that it is possible fully to Understand the pro- 
cesses by which the exceedingly complex structure of the 
vertebrate body is developed from the simple leaf-shaped 

If we now make a perpendicular section through the 
sole-shaped germ-shield (Fig. 86, 87), the first thing we 
notice is the difference between the three germ-layers, as 
they lie one over the other (Fig. 88). The germ-shield con- 
sists, as it were, of three shoe-soles overlying each other. 
The undermost, or innermost, of these (the intestinal-gland- 
ular layer) is the thinnest stratum, and consists of a single 
layer of cells (Fig. 88 d). The middle of these shoe-shaped 
bodies (the mesoderm) is considerably thicker and more or 
less evidently appears to be composed of two closely con- 
nected layers. The third and uppermost, or outermost sole- 

ID m 1 1 EB 

TGSgS8^S83 rerj 

Fig. 88. — Transverse section through the germ-disc of a Chick (a few 
hours after the beginning of incubation) : h, skin-sensory layer; m, skin- 
fibrous layer ; /, intestinal-fibrous layer (the two latter are united into the 
middle-layer, or mesoderm) • d, intestinal-glandular layer. All the four 
secondary germ-layers have coalesced in the middle and from the thick 
axis-band (xy) ; n, first trace of the primitive groove ; u, region of the 
future primitive kidney rudiment. (After Waldeyer.^ 

shaped body (h), is the skin-sensory layer, and consists of 
smaller and lighter-coloured cells. In the middle of the 
transverse section, along a considerable part of the longi- 


tudinal axis of the sole, all three soles coalesce, and here form 
the thick axial band (Fig. 88, xy). This coalescence is very 
significant. It causes an exchange of cells between the 
primary germ-layers. These cells move, alter their position, 
and multiply, so that exoderm-cells penetrate among the 
entoderm-cells, and entoderm-cells among those of the exo- 
derm. The middle layer, or mesoderm, therefore, contains 
cells from both of the two primary germ-layers. Even 
if Remak's explanation, according to which the mesoderm 
is originally split off from the entoderm, is correct, in 
consequence of the coalescence at the central point, exo- 
derm cells may also afterwards make their way into the 
mesoderm. The fibrous layer indeed soon plainly shows 
that it is composed of two different strata ; the outer, 
which, phylogenetically, must be referred to the skin-layer, 
and the inner, which must be referred to the intestinal 
layer. The outer is the rudiment of the skin-fibrous layer 
(Fig. 88, m, 89, m) ; the inner becomes the intestinal-fibrous 
layer (Fig. 88, /, 89, /). Soon after the coalescence of the 
germ-layers in the axial portion of the germ-shield has 
taken place, and the cells have been exchanged, the small 
rectilineal primitive groove (Fig. 89, n) becomes visible in 

& & 

Fig. 89.— Transverse section through the germ-shield of a Chick (in a 
stage rather later than in Fig. 88). The letters indicate the same parts as 
m Fig. 88. In the middle of the axis-band (y) the chorda dorsalis, or noto- 
chord, becomes denned (x). (After Waldeyer.) 


the central line of the outer surface. On each side of this, 
the dorsal swellings rise in the form of low ridges. In the 
centre of the lower side of the primitive groove a cylin- 
drical band separates itself from the cell-mass of the thick 
axis-band ; this, which in transverse section appears 
roundish, is the first rudiment of the notochord (chorda 
dorsalis, x). The four secondary layers separate more and 
more distinctly. The intestinal-fibrous layer (f) appears 
as the product of the intestinal -glandular layer (d), and 
distinct from the skin-fibrous layer (m), which arises from 
the skin-sensory layer (ft). 

Ftg. 90. — Transverse section through the germ-shield of an incubated 
Chick (about the end of the first day) ; about 100 times the natural size. 
The skin-sensory layer (the outer germ-layer) separates into two different 
parts : (1) the thinner, peripheric horn-plate (ft), from which the outer skin 
with its appendages arises; (2) the thicker, axial spinal plate (m), which 
gives rise to the spinal tube (tubus medullaris) ; this originates from the 
dorsal furrow (Rf), the deepest part of which forms the primitive groove 
(Pv). The boundaries between the spinal plate (m) and the horn-plate (/<) 
form the prominent, parallel dorsal swellings. The middle germ-layer, the 
compound fibrous layer (the " motor-germinative "), is already distinguished 
into the notochord (ch) and the two side-layers (sp). The inner portion of 
these side-plates soon becomes defined as the primitive vertebral band 
(«wp). The tiny fissure in the side-plates is the first rudiment of the 
future body-cavity (uwh). The inner germ-layer (the intestinal-glandular 
layer) (d d) is not yet modified. (After Kolliker.) 

The primitive groove (Fig. 90 Pv) soon becomes con- 
siderably deeper and so fashioned as to constitute the bed 
of the broader spinal furrow (medullary or dorsal furrow) 



(Rf). On both sides of this rise the two parallel dorsal 
swellings, or spinal swellings (m). At the same time the 
central notochord, or chorda dorsalis (Fig. 90 ch), separates 
entirely and definitely from the two lateral portions of the 
mesoderm. These we will henceforth regard as side-layers 
(sp) in reference to the axial chord. They are usually 
called side-plates. In the middle of each of these side- 
layers a horizontal fissure appears, where the upper or outer 
skin-fibrous layer separates from the lower or inner intestinal- 
fibrous layer. This fissure (Fig. 90 uwh) is very significant, 
for it represents the first rudiment of the future body- 
cavity (cwloma). (Cf. Plate IV. Fig. 2, c and 3, c.) 

In sp a king of these side-layers, which are usually 
called " side-plates," I would say a word or two about 
those figurative expressions "layers' 5 and "plates," which 
have been universally employed since Baer's time. The 
" layers " (/amirae), as well as the " plates " (lamellw), are 
leaf-like or plate-shaped bodies originally consisting of a 
single homogeneous cellular stratum, or of several lying one 
above the other, and constituting the first basis of the 
organic systems and of the organs of the body.' But the 
language of Ontogeny distinguishes considerably between 
a layer, or leaf (lamina), and a plate (lamella). The first 
and )ldest cell-layers of the germ, which overspread the 
whole germ, and form the first basis of whole organ-systems, 
are layers, or leaves (lamintu). On the other hand, the 
term plates (lamella}) is applied to separate portions of the 
layers, or leaves, and to the cellular strata produced from 
the latter, which only belong to a part of the germ and 
serve to form single organs of variable size. 

Of course this distinction is by no means sharply 


drawn : thus, for instance, the two middle, secondary germ- 
layers are usually called the skin-fibre plate and the intes- 
tinal-fibre plate (instead of layers, or leaves). Conversely, 
the horn-plate (which is a portion of the skin-sensory 
layer) is usually called the horn-layer, or leaf. As far as 
possible we shall, however, maintain this important distinc- 
tion : we shall only use the term layers, or leaves, of the 
two primary, and the four secondary germ-layers ; naturally, 
however, we must speak of the side-plates as side-layers, or 
leaves, as they first originate by a coalescence of the two 
primary germ-layers. On the other hand, we shall speak 
of the so-called horn-layer and of all the layer-like rudi- 
mentary organs, which are split off or differentiated from 
the four layers, or leaves, as plates ; e.g. the muscle-plate, 

After the chorda has entirely separated from the two 
side-layers, a portion, in the shape of a long, thick cord, 
breaks off, in the posterior portion of the germ-shield, from 
the inner edge of each of the side-layers (Fig. 90, uwp, 91, u). 
We will call this the primitive vertebral plate, or better, 
the primitive vertebral cord, for it afterwards develops into 

w ..<v 

Fig. 91. — Transverse section through the germ-shield of a Chick (at the 
end of the first day), rather more developed than in Fig. 90; about twenty 
times the natural size. The two edges of the spinal plate (m), which, as 
spinal swellings (w), separate the latter plate from the horn-plate (h), incline 
towards each other. On both sides of the notochord (ch) the inner portion 
of the side-layers (u) has separated itself as a primitive vertebral band 
from the outer portion. The intestinal-glandular layer (d) is not yet 
modified. (After Remak.) 


the primitive vertebrae and the neighbouring parts. It forms 
the first rudiment ot the individual segments of the verte- 
bral column, the primitive vertebrae. At a later period these 
primitive vertebrae become very closely connected with the 
chorda dorsalis which they surround, and this whole axis- 
mass then develops into the vertebral column, which is 
afterwards articulated in so many complex ways. The 
peripheral parts of the two side-layers, which remain after 
the separation of the primitive vertebral cord, are hence- 
forth called the side-plates (lamellce), the term being thus 
used in its restricted sense. They develop into the two 
fibrous layers, which have already been mentioned. In the 
anterior half of the germ-shield, representing tb<s future 
head, there is no separation between the inner primitive 
vertebral mass and the outer side-layers. 

During these processes, this intestinal-glandular layer, 
the inner germ-layer, remains quite unaltered; no separations 
are to be seen in it (Figs. 90, dd, 91, d). The changes, there- 
fore, which take place at this period in the skin-sensory 
layer, the outer germ-layer, are all the more remarkable. The 
continuous elevation and growth of the dorsal swellings tends 
to make the upper, free margins of these prominent ridges 
incline towards each other, and as they continually ap- 
proach each other (Fig. 91, w), they finally coalesce. Thus 
the open dorsal furrow, the separation at the top of which 
grows narrower and narrower, is transformed into a closed 
cylindrical tube (Fig. 92, mr). This tube is of the greatest 
importance, for it is the first basis of the central nervous 
system — the brain and spinal cord. This rudiment is called 
the medullary tube (tubus medullaris). Formerly this fact 
was regarded with wonder as an inexplicable enigma, but 


the Theory of Descent explains it as but a perfectly natural 
process. It is quite natural that the central nervous system, 
the organ by which all intercourse with the outer world, 
all mental activities, and all sensory perception are accom- 
plished, should be developed by detachment from the outer 
skin (epidermis). At a later stage the medullary tube 
separates entirely from the outer germ-layer, is surrounded 
by the primitive vertebrse, and is forced inwards. From this 
time, the remaining portion of the skin-sensory layer (Fig. 
92 h), is called the horn-plate or " horn-layer," because the 

. j uuo ao ' /j * v r / 

Fig. 92. — Transverse section through the germ-shield of a Chick (second 
day of incubation) ; about 100 times the natural size. In the outer germ- 
layer, the axial dorsal furrow, having completely closed, forms the spinal 
tube (rar), and has pinched itself off from the horn-plate (h). In the middle 
germ-layer, the axial notochord (ch) is entirely separated from the primitive 
vertebral bands (uiv), in the interior of which a transitory cavity (uwh) 
afterwards forms. The side-layers have split into the outer skin-fibrons- 
layer (hpl) and the inner intestinal -fibrous layer (df), the two being still 
connected by the middle plates (mp). The fissure (sp) between the two is 
the fh\st rudiment of the body -cavity (ccelnma). In the gap between the 
primitive vertebral bands and the side-layers, on either side, is the primitive 
kidney (uny), and on the inside the primitive artery (ao). (After Kolliker.) 

outer skin (epidermis), with its horny appendages — nails, 
hair, etc. — develops from it. (Cf. Plates TV. and V.) 

At a very early period, in addition to the central nervous 
system another or wholly different organ is seen to arise 
from the oute'e skin ; this is the primitive kidney, which 


accomplishes the excretory functions of the body, and se- 
cretes the urine of the embryo. The primitive kidney 
originally consists of an entirely simple, tubular, elongated 
passage, a straight duct situated on each side of the ventral 
aspect of the primitive vertebral cord, running from an 
anterior to a posterior direction (Fig. 92, wag). It 
apparently arises from the horn-plate, and at the side 
of the medullary tube (spinal tube), in the space between 
the primitive vertebral cord and the side-plates. Even 
while the medullary tube is separating from the horn-layer, 
the primitive kidney is visible in this gap. Some authors, 
however, hold that the first rudiment of the primitive 
kidney is not furnished by the skin-sensory layer, but by 
the skin-fibrous layer. 

While the skin-sensory layer is thus splitting up into 
the horn-plate, the spinal tube, and the primitive kidneys, 
the mesoderm, or fibrous layer, also separates into three por- 
tions, viz. : (1) the notochord in the central line of the germ- 
shield (Fig. 92, ch) ; (2) the primitive vertebral bands on 
each side of the notochord (uw) ; and (3) the side-layers which 
separate from the exterior of primitive vertebral bands. 
These side-layers still show the original separation of the 
middle germ-layer into the outer skin-muscle layer (or skin- 
fibrous layer, hpl), and the inner intestinal-muscle layer 
(or intestinal-fibrous layer, df). The point of union of the 
two fibrous layers is called the middle plate, or mesentery- 
plate (mp). The narrow fissure (sp), or empty space 
which arises between the two fibrous layers, is the first 
rudiment of the body-cavity (coeloma), the great visceral 
cavity, in which the heart, lungs, intestines, etc., are after- 
wards situated. In Mammals this is separated, at a later 


period, into two distinct cavities by the formation of the 
diaphragm ; these are the chest, or thoracic cavity, and the 
abdominal cavity. Immediately below the mesentery-plate, 
in the gap between the intestinal-glandular layer, the in- 
testinal-fibrous layer, and the primitive vertebral bands, 
another organ appears at an early stage, in the form of a 
tube with a thin wall (Fig. 92, ao). This is the first rudi- 
ment of a large blood-vessel, the primitive artery, or aorta. 
It arises by fission from the intestinal-fibrous layer. 

During these processes the inner germ-layer, the intes- 
tinal-glandular layer (Fig. 92, df), remains quite unaltered, 
and it is only somewhat later that it begins to show a very 
shallow, channel-like depression along the central line of the 
germ-shield, immediately below the notochord. This is the 
intestinal channel, or intestinal furrow, and it already indi- 
cates the future destination of this germ-layer. For as the 
intestinal channel gradually deepens, and its lower edges 
bend towards one another, it assumes the form of a closed 
tube, the intestinal tube, precisely as the dorsal furrow 
became the spinal or medullary tube (Fig. 92). The in- 
testinal-fibrous layer (/), which lies on the intestinal-glan- 
dular layer (d), naturally follows the curve of the latter. 
Thus f'om the time when it first begins to develop, the 
intestinal wall is composed of two strata, internally ol 
the intestinal-glandular layer, externally of the intestinal- 
fibrous layer. 

The formation of the intestinal tube is so far similar to 
that of the spinal tube, that in both cases a rectilineal trench, 
or furrow, first appears along the central line of a flat germ* 
layer. The edges of this furrow then incline towards each 
other, and by coalescence form a tube (Fig. 93). But the 



two processes are in reality quite different For the spinal 
tube closes along throughout its entire length into a' cylin- 

Fig. 93. — Three diagrammatic transverse sections through the germ- 
shield of a higher Vertebrate, showing the origin of the tubular rudimentary- 
organs from the bent germ-layers. In Fig. A the spinal tube (n) and the 
intestinal tube (a) are still open trenches ; the primitive kidneys (it) are still 
simple skin-glands. In Fig. B the spinal tube (n) and the dorsal wall have 
already closed, while the intestinal tube (a) and the ventral wall are still 
open ; the primitive kidneys are pinched off. In Fig. C both the spinal tube 

! with the dorsal wall above, and the intestinal tube with the ventral wall 
below, are closed. All the open trenches have become closed tubes ; the 
primitive kidneys have penetrated into the interior In all three figures the 

I letters indicate the same parts : h, skin-sensory layer ; n, spinal tube, or 
medullary tube ; u, primitive kidneys ; x, notochord ; s, vertebral rudiments ; 
r, dorsal wall; b, ventral wall ; c, body-cavity (ccelonw) ; /, intestinal-fibrous 
layer: t, primitive artery (aorta); v, primitive vein (intestinal vein); d, 

"intestinal-glandular layer ; a, intestinal tube. (Cf. Plates IV. and V.) 

drical tube, while the intestinal tube remains open in the 
middle, and, till a much later stage, this cavity remains 
connected with the cavity of the intestinal germ- vesicle. 
The connection between these two cavities is closed only at 
a very late period, by the formation of the navel. The 
closing of the medullary tube proceeds from both sides, the 
tight and left edges of the dorsal furrow coalescing. The 


closing of the intestinal tube, on the other hand, takes plac« 
not only from the right and left, but by a concrescence of the 
walls on all sides of the intestinal groove towards the navel, 
as a central point. Moreover, the whole process of the 
secondary formation of the intestine in the three higher 
classes of Vertebrates is most closely connected with the 
formation of the navel, with the "pinching in" of the 
embryo from the yelk-sac (navel-vesicle). (Cf. Fig. 70, p 
283, and Plate V. Figs. 14 and 15.) 

In order to be quite clear about these points, it is neces- 
sary to bear in mind the relation of the germ-shield to the 
germ-area and to the intestinal germ-vesicle. This is best 
accomplished by comparing the rive stages which are repre- 
sented in longitudinal section in Fig. 94. The germ-shield (e)\ 
which at first protruded only slightly from the surface of 
the germ-area, soon begins to raise itself from the latter, and 
to pinch itself off the intestinal germ-vesicle. During this 
the germ-shield, seen from the dorsal side, still retains its 
original simple sole-shape (Figs. 86, 87, p. 298). There is as 
yet no appearance of any distinction into head, neck, trunk, 
or limbs. But the germ-shield has grown much thicker, espe- 
cially in the anterior portion It now, therefore, protrudes 
from the surface of the germ-area like a thick, much arched, 
oval swelling, and begins to separate and free itself 
completely from the intestinal germ-vesicle, to which it is 
attached by its ventral surface. The progress of this separ- 
ation renders the back continually more curved; in proportion 
as the embryo grows and becomes larger, the germ-vesicle 
decreases and becomes smaller, till at last it hangs, in the 
form of a small bladder, from the abdomen of the embryo 
(Fig. 94, 6 ds). In consequence of the processes of growth 


which effect this separation, a furrow-like depression is first 
formed round the embryo-body on the upper surface of the 
germ- vesicle, surrounding it like a trench ; round the out- 
side of this trench a circular wall, or dike, is formed by 
the elevation of the adjoining parts of the germ- vesicle 
(Fig. 94, 2 ka). 

In order to get a clear and connected view of thib 
important process, we may compare the embryo to a 
fortress surrounded by a moat and a wall. This moat, or 
trench, consists of the outer part of the germ-area, and 
ceases where the germ-area passes into the intestinal germ- 
vesicle. The important process of fission in the middle 
germ-layer which occasions the formation of the large 
body-cavity, extends over the whole germ-area along the 
periphery of the embryo. At first the extent of this 
middle germ-layer is co-extensive with that of the germ- 
area; the whole remaining part of the intestinal germ- 
vesicle originally consisting only of the two original germ- 
layers, the outer and the inner. Thus, over the extent 
of the germ-area, the middle germ-layer splits into the two 
layers which we knew as the outer skin-fibrous layer, 
and the inner intestinal-fibrous layer. These two layers 
separate widely, a clear fluid collecting between them 
(Fig, 94, 3 am). The inner layer, the intestinal-fibrous 
layer, remains lying on the inner layer of the intestinal germ- 
vesicle (on the intestinal-glandular layer). The outer layer 
the skin-fibrous layer, on the contrary, attaches itself closely 
to the outer layer of the germ-area, to the skin-sensory layer, 
and the two together rise up from the intestinal germ- vesicle. 
From these two united outer layers, a connected membrane 
dow arises. This is the circular wall, which continues to 



Fig. 94. — Five diagrammatic longitudinal sections through the maturing 
mammalian germ and its egg-membranes. In Fig. 1-4, the longitudinal 
section passes through the sagittal plane, or the central plane of the body, 


*rhich separatee the right and left halves ; in Fig. 5, the germ Is seen from 
the left side. In Fig. 1, the tufted (d) chorion encloses the germ .vesicle, 
the wall of which consists of the two primary germ-layers. Between the 
outer (a) and inner (i) germ-layers, the middle germ-layer (to) has developed, 
co-extensively with the germ-area. In Fig. 2, the embryo (e) is beginning to 
separate from the germ- vesicle (ds) f while the wall of the amnion-fold is 
developing round it (in front as the head-sheath, fc«, in rear as tail-sheath, 
ss). In Fig. 3, the edges of the amnion-fold (am) meet above the back oi 
the embryo and thus form the amnion -cavity (ah) ; while the embryo (e^ 
separates still more from the germ-vesicle, the intestinal canal (dd) if 
developed, and from the posterior end of this, the allantoie (al) grows out 
In Fig. 4, the allantois (al) becomes larger ; the yelk-sac (ds) smaller. In 
Fig. 5, the embryo shows the gill-openings and the rudiments of the two 
pairs of limbs ; the chorion has formed branching tufts. In all the five 
figures e signifies embryo ; a, outer germ-layer ; m, middle germ-layer ; t, 
inner germ-layer ; am, amnion (fcs, head-sheath ; ss, tail-sheath) ; ah, 
amnion-cavity ; as, amnion-sheath of the umbilical cord; kh — ds, intestinal 
germ- vesicle ; ds, yelk-sac (navel-vesicle) ; dg, yelk-duct j df, intestinal, 
fibrous layer; dd, intestinal-glandular layer; al, allantois; vl=hh, region 
of the heart ; d, yelk-membrane (prochorion) ; d", tufts on prochorion ; sh, 
serous membrane ; sm, tufts of the foregoing ; ch, tufted membrane or 
chorion; r, the space between the amnion and chorion, filled with fluid. 
(A-Ooording to Kolliker.) Compare Table V. Fig. 14 and 15. 

raise itself higher and higher around the entire embryo, and 
at last coalesces above it (Fig. 94, 2 , 3 , 4 , 5 , am). To 
keep up the simile of a fortress imagine that the sur- 
rounding wall of the fortress becomes extraordinarily high, 
and towers far above the fortress. Its edges arch like 
the crests of a jutting cliff which is about to enclose the 
fortress ; they form a deep cavern, and at last grow together 
above. At last the fortress lies entirely within the cavern 
forme by the concrescence of the edges of this mighty 
wall. (Cf. Figs. 95-98, p. 319, and Plate V. Fig. 14.) 

These two outer strata of the germ-area, rising in this 
way in the form of folds around the embryo and coalescing 
above it, at last form a spacious sac-like envelope around 
it. This envelope bears the name of germ-membrane. 


water-membrane, or amnion (Fig. 94, am). The embryo 
swims in a watery fluid, which fills the space between it 
and the amnion, and is called the amnion-water, or germ- 
water (Fig. 94, 4 , 5 ah) We shall return hereafter to the 
significance of this remarkable formation. It is of no 
interest to us at present, because it bears no direct relation 
to the formation of the body. 

Among the various appendages, the significance of which 
we shall presently recognize, we will mention, in passing, the 
allantois and the yelk-sac. The allantois, or urinary sao 
(Fig. 94, 3 , 4 at), is a pear-shaped bladder, which grows out 
from the hindmost part of the intestinal canal: the inner- 
most portion of it afterwards changes into the urinary 
bladder ; the outer part, with its vessels, forms the founda- 
tion of the placenta. In front of the allantois, the yelk-sac, 
or navel vesicle (Fig. 94, 3 , 4 ds), the remnant of the 
original intestinal germ-vesicle (Fig. 94, 2 kh), protrudes from 
the open abdomen of the embryo (Fig. 94, 3 , 4 ds). In a 
later stage of development of the embryo, in which the intes- 
tinal and ventral walls are nearly closed, this hangs out 
from the navel-opening in the form of a little stalked 
bladder (Fig. 94, 4 , 5 ds). Its wall consists of two layers, 
the inner of which is the intestinal-glandular layer, the 
outer the intestinal-fibrous layer. It is, therefore, a direct 
continuation of the intestinal wall. In proportion as the 
embryo grows larger, this yelk-sac becomes smaller. At 
first the embryo looks merely like a small appendage on 
the large intestinal germ- vesicle. But, on the contrary, at 
a later period, the yelk-sac, or the remnant of the intestinal 
germ-vesicle, looks like a little purse-shaped appendage of 
the embryo (Fig. 70). Finally, it loses all importance. The 


Tery wide opening by which the intestinal cavity at first 
communicates with the navel bladder, afterwards grows 
continually narrower, and at last altogether disappears. The 
navel, the little pit-like depression which appears in the 
middle of the ventral wall of the developed Man, is the 
place at which the remains of the germinal vesicle, the navel 
bladder, once entered the intestinal cavity, and by which it 
was connected with the intestine in the course of its evolu- 
tion. (Cf. Figs. 14 and 15 on Plate V.) 

The formation of the navel takes place at the same time 
as the closing of the outer ventral wall. The ventral wall 
originates in exactly the same way as the dorsal wall ; 
both are formed essentially from the skin-fibrous layer, 
and are covered outwardly by the horn-plate, the peripheric 
part of the skin-sensory layer. Both are formed by the 
modification of the animal germ-layer into a double tube; 
above, at the back, the vertebral canal, which encloses the 
spinal tube, — below, at the abdomen, the wall of the body- 
cavity, which encloses the intestinal tube (Fig. 93, p. 309). 

We will first notice the formation of the dorsal wall, 
and then that of the ventral wall (Figs. 95-98). In the 
centre of the dorsal surface of the embryo the spinal tube 
(mr) lies, originally immediately below the horn-plate (h), 
from the central part of which it has separated. But, at 
a later period, the primitive vertebral plates (uw) grow 
from the right and the left so as to penetrate between 
these two originally connected parts (Figs. 97, 98). The 
upper inner edges of the two primitive vertebral plates 
wedge themselves in between the horn-plate and the spinal 
tube, press these two apart, and finally coalesce between 

them in a suture corresponding with the central line of 


trie back. The closing is effected in exactly the same way 
as that of the spinal tube, which is now entirely enclosed 
by the vertebral canal. In this way the dorsal wall ip 
formed, and the spinal tube lies quite in the interior (Fig. 
98). In the same way the primitive vertebral mass grows 
lower down round the notochord (chorda dorsalis), there 
forming the vertebral column. In this lower part the inner 
under edge of the primitive vertebral plates on each side 
splits into two laminae, the upper of which passes in 
between the notochord and the spinal tube, while the under, 
on the contrary, penetrates between the notochord and the 
intestinal tube. These two laminae, by meeting from eacli 
side above and below the notochord, completely enclose the 
latter, and thus form the tubular outer notochord-sheath, 
the skeleton-forming layer, from which the vertebral 
column arises (Figs. 97, 98). (Cf. Figs. 3-6 on Plate IV, 
and the following chapter.) 

Processes similar to these which take place above, on 
the back, during the formation of the dorsal wall, are 
observed below, on the abdomen, during the formation of 
the ventral wall (Fig. 98, bh). Here the side-plates grow 
together round the intestine in a similar way to that in 
which the intestine itself closed. The outer part of the 
side-plates forms the ventral wall, or the lower body-wall, 
while on the inner side of the amnion-fold, which has been 
mentioned, the two side-plates curve more and grow toward 
each other from right and left. While the intestinal canal 
is closing, the closing of the ventral wall is also taking place 
from all sides. Thus the ventral wall, which encloses the 
whole ventral cavity below, also originates from two halvee, 
from the two side- plates, which incline toward each other ; 


these grow toward each other from all sides, and at last 
unite in the navel at the centre. We must, therefore, dis- 
tinguish between two navels, an inner and an outer. The 
inner or intestinal navel is the point at which the in- 
testinal wall finally closes, at which the communication 
between the intestinal cavity and the cavity of the yelk- 
sac was cut off (Fig. 70). The outer or skin-navel is the 
point at which the ventral wall finally closes, and which 
even in adults is visible as a depression. In each concrescence 
two secondary germ-layers are concerned ; at that of the 
intestinal wall, the intestinal-glandular layer and the in- 
testinal-fibrous layer ; at that of the ventral wall, the skin- 
fibrous layer and the skin-sensory layer. The intestinal 
wall, as a whole, arises, therefore, from the entoderm, and 
the ventral wall (and, indeed, the entire body- wall) from 
the exoderm. 95 

The processes by which the double tubular rudiment of 
the body originates from the four-layered germ-disc are, 
therefore, really very simple. They are not, however, at 
once easily understood, nor is it easy to describe them. 
Very much, doubtless, yet remains obscure to the reader, 


Fig. 95. 




Fig. 96. 

m A 

Fig. 97. 



Fig. 98. 

Figs. 95-98. — Transverse sections through embryo Chick : Fig. 95, the 
second day of incubation ; Fig. 96, the third ; Fig. 97, the fourth , and Fig. 
98, the fifth. Figures 95-97 are after Kolliker (magnified about 100 times) ; 
Fig. 98, after Remak (magnified about 20 times). 

h, horn-plate ; mr, spinal tube ; ung, primitive kidney duct ; un, pri- 
mitive kidney vesicle ; kp, skin-fibrous layer ; m = mu = mp, muscle- 
plate ; uw, primitive vertebral plate (u-h, membranous formation of the 
vertebral body ; wb, of the vertebral arch ; wq, of the rib, or transverse 
apophysis) ; inch, primitive vertebral cavity ; ch, spinal axis, or uoto- 
chord ; sh, notochord-sheath ; bh, ventral wall; g, posterior? v, anterior 
nerve roots of the spinal marrow ; a = af = am, amnion-fold ; p, body- 
cavity, or ccelom ; df, intestinal-fibrous layer ; ao, primitive aortas ; sa, 
secondary aorta; vc, principal veins; d = dd, intestinal-glandular layer; 
dr, intestinal groove. In Fig-. 95, the greater part of the right half of the 
cross-section is omitted, and in Fig. 96 the greater part of the left half. 
Only a small part of the wall of the yelk-sac, the remnant of the germ- 
vesicle, which lies below, is shown. 

especially to those who are not at all familiar with the 


anatomical features. If, however, the subsequent stages of 
development, which throw light on their predecessors, 
are accurately noted, and especially, if the transverse 
sections in the preceding figures and in Plate IV., repre- 
genting the complete vertebrate body and its germ, are 
carefully compared, the reader will probably obtain a cleai 
conception of the main features of mammalian Ontogeny. 
A close and thoughtful comparison of the transverse sections 
is of the greatest importance in this respect. 

It is true, however, that a deeper, phylogenetic know- 
ledge of these complex processes can only be gained with 
the aid of Comparative Anatomy and Ontogeny. These 
teach us that the ontogenetic process which we have 
described as resulting in the formation of the Vertebrate 
must be explained as kenogenetic, and that, in consequence 
of continual embryonic adaptation, these processes have 
departed very widely from the original palingenetic form. 
The Amphioxus alone of all living Vertebrates has, in con- 
sequence of tenacious heredity, approximately retained the 
palingenetic form. 96 (Cf Chapters XIII. and XIV.) 

As yet we have paid no attention to the various sections 
which are distinguishable in the length of the body : the 
head, neck, breast, abdomen, tail, etc. The transverse 
sections do not help us in this respect, and we must, there- 
fore, closely observe the articulation in the longitudinal axis 
of the mammalian body. 

hakckkl's evolution of man. PLATE IV. 


hasckbl's evolution of man. PLATE V. 


jr t -/»■ 


The two Plates IV. and V. exhibit, partly ontogenetically and partly 
phylogenetically, the mode in which the human body arises from the germ- 
tayers. Plate IV. contains only diagrammatic transverse sections (through 
the sagittal and transverse axes) ; Plate V. contains only diagrammatic longi- 
tudinal sections (through the sagittal and longitudinal axes), seen from the 
left side. The four secondary germ-layers and their products are distin- 
guished throughout by the same four colours, namely : (1) the skin-sensory 
layer is orange ; (2) the skin-fibrous layer, blue ; (3) the intestinal-fibrous 
layer, red ; and (4) the intestinal-glandular layer, green. In all, the letters 
indicate the same parts. In Fig. 1 and 9 alone the two primary germ- 
layers are represented — the outer, or skin-layer, orange ; the inner, or 
intestinal layer, groen. In all the figures the dorsal surface of the body is 
uppermost, the ventral surface underneath. All organs proceeding from 
the skin-layer are marked with blue letters ; all those proceeding from the 
intestinal layer, with red letters.* 7 

Plate IV. — Diagrammatic Transverse Sections. 

Pig. 1. — Transverse section through the Gastrula. (Compare Fig. 9, 
longitudinal section, and Figs. 22-28, p. 193.) The whole body is formed by 
the intestinal tube (ci) ; the wall of this oonsists solely of the two primary 

Fig. 2. — Transverse section through the larva of the Amphioxus, in tha 
early stage in which the body consists merely of the four secondary germ- 
layers. The intestinal tube (d), formed of the intestinal layer, is separated 
from the body-wall by the coelom (c), which is formed of the skin-layer. 

Fig. 3. — Transverse section through the germ-disc of a higher Vertebrate, 
with the rudimeuts of the earliest organs. (Compare the transverse section 
of the embryo Chick at the second day of incubation, Fig. 92.) The spinal 
tube (to) and the primitive kidneys (w) are separated from the horn-plate (h). 
On both sides of the notochord (ch) the primitive vertebrae (w) and the 
side-layers are differentiated. Between the skin-fibrous layer and the intes- 


tinal-fibrons layer, the first rudiment of the body-cavity, or the ooelom (c), is 
visible ; under it are the two primitive aortas (t). 

Fig. 4. — Transverse section through the germ-disc of a higher Vertebrate, 
somewhat further developed than in Fig. 3. (Compare the 15 transverse 
section of the embryo Chick at the third day of incubation, Fig. 95 and 96, 
p. 317.) The spinal tube (m) and the notochord (ch) are already beginning 
to be enclosed by the primitive vertebrae (uw), in which the muscle-plates, 
bone-plates, and nerve-roots are becoming distinct. The primitive kidneys 
(u) are already completely separated from the horn-plate (h) by the leather- 
plate (I) ; c, the coelom ; t, the aortas. The skin-layer, rising around 
the embryo, forms the amnion-fold (am) ; this gives rise to a hollow space 
(y) between the amnion-fold and the wall of the yelk-sac (ds). 

Fig. 5. — Transverse section through the pelvic region and the post3rior 
limbs of the embryo of a higher Vertebrate. (Compare the transverse 
section through Chick at the fifth day of incubation, Fig. 120.) The spinal 
tube (m) is already entirely enclosed by the two curving halves of the 
vertebras (wb), and similarly the notochord and its sheath by the two halves 
of the vertebral body (wk). The leather-plate (I) has entirely separated 
from the muscle-plate (mp). The horn-plate (h) has thickened very much 
at the head of the posterior limbs (x). The primitive kidneys (u) are pro. 
minent in the coelom (c), and lie very near the germ-epithelium, or the 
rudimentary sexual glands (fc). The intestinal tube (d) is attached to the 
dorsal surface of the body by the mesentery (g), beneath the main artery (t), 
and the two principal veins (n). Below, in the centre of the ventral wall, 
the stalk of the allantois (al) is visible. 

Fig. 6. — Transverse section through a developed Primitive Fish, or some 
other Vertebrate of a low order. The parts, on the whole, bear the same 
relation to each other as in the preceding transverse section, Fig. 5, and are 
marked in the same way. But the sexual glands (k) have developed into 
ovaries, and the primitive kidneys are transferred into oviducts, which open 
into the coelom. The two side protuberances (lb) of the intestinal tube (d) 
indicate the intestinal glands, for example, the liver. Below the intestinal 
tube, in the intestinal wall, lies the intestinal vein (y) ; above the intestinal 
tube lies the aorta (t), and above this, again, the two principal veins (n). 

Fig. 7. — Transverse section through one of the higher Worms (through 
the head of an Annelid), showing its essential agreement with the Verte- 
brates in the construction of the body from the four secondary germ, 
layers. It should be carefully compared with the diagrammatic trans- 
verse section through the low Vertebrate, Fig. 6 : m, the " brain," or " upper 
throat ganglion." The leather-plate (I) and the muscle-plate, which lies below 
the former, have differentiated from the skin-fibrous layer. The muscle-layei 
has separated into an outer circular muscle-stratum and along inner stratum, 
and the muscle of the latter has distributed itself into dorsal muscles (r) and 
ventral muscles (b). The two are separated by the primitive kidneyi (u), 


irhich extend from the horn-plate (h) to the coelom (c). Here the primitive 
kidneys have a funnel-shaped opening, through which they carry out the 
ovules, which fall from the ovaries (k) into the ccelom. The intestinal tube 
(d) has glands on its surface (liver-vesicles, lb). Below it lies the ventral vessel 
(the intestinal vein, v), above it the dorsal vessel (the aorta, t). The position 
and origin of all these primitive organs is entirely the same in Manandevenr 
other Vertebrate, as in the Worms. The only essential difference is that in 
the Vertebrates a notochord is developed between the spinal tube and the 
intestinal tube. 

Pig. 8. — Transverse section through the human thorax. The spinal tube 
(m) is entirely enclosed by the developed circular vertebrae (w). A curveo 
rib proceeds right and left from the vertebra, supporting the wall of the 
breast (rp). Below, on the ventral surface, between tht right and left rib, 
lies the breast-bone, or sternum (bb). Without, above the ribs, and the 
muscles between the ribs, lies the outer skin, formed from the leather-plate 
(l) and the horn -plate (h). The greater part of the breast-cavity (or the 
anterior part of the ccelom, c) is occupied by the two lungs (lu), in which 
the branches of the trachea ramify like a tree. These all open together 
into the unequal branches of the trachea (Zr), which opens further up 
at the neck into the oesophagus (sr). Between the intestinal tube and the 
vertebral column, lies the aorta (t). Between the trachea and the sternun 
lies the heart divided by a partition wall into two halves. The left hear 1 
(hi) contains only arterial, the right (hr) only venous blood. Each halt' o 
the heart is divided by a valved opening into an auricle and a ventricle 
The heart is here represented diagrammatically in its (phylogenetic) origin ti 
symmetrical position (in the centre of the ventral side). In the developed 
human being, and in apes, the heart lies in an unsymmetrical and oblique 
position, inclined to the left. 

Plate V. — Diagrammatic Longitudinal Sections, 

Fig. 9. — Longitudinal section through a Gastrula. (Compare Fig. 1, 
transverse section.) The intestinal cavity (<i) opens in front through the 
mouth (0). The body consists merely of the two primary germ-layers. 

Fig. 10. — Longitudinal section through an hypothetical Primitive Worm 
(Prothelmis) , the entire body of which consists of the four secondary germ 
layerB. The intestinal tube (d) is still very simple; but the anterior and 
posterior intestines begin to grow distinct. The mouth (0) is still the anus 

Fig. 11. — Longitudinal section through a low Coelomate Worm. The primi- 
tive brain (m), or the first nerve-centre overlying the throat, has separated 
from the horn-plate (h). The intestinal tube has acquired a second posterior 
anal opening (a) in addition to the mouth-opening (a) in front. A skin- 
gland has developed into primitive kidneys (it) and opens into the body 


cavity (c), which has formed between the skin -fibrous layer and the intes 
tinal-fibrous layer. «> 

Fig. 12. — Longitudinal section through an hypothetical Worm (Chordo* 
nium), which was among the common parent-forms of Vertebrates and 
Ascidians. The primitive brain (m) has lengthened into an elongated spinal 
tube. Between this spinal tube and the intestinal tube (d), the notochord 
(ch) has developed. The intestinal tube has differentiated into two divisions, 
an anterior gill-intestine (with three pairs of gill-openings, ks) whioh 
nerves for breathing, and a posterior stomach-intestine (with a liver- 
appendage, lb) which serves for digestion. In front, at the head-exti emity, 
an organ of sense (q) has developed. The primitive kidney (u) open*' into 
the body .cavity (c). 

Fig. 13. — Longitudinal section through a Primitive Fish (Proselachius), 
closely related to the existing Sharks, and hypothetical ancestors of Man 
(the fins are omitted). The spinal tube has differentiated into the five 
primitive brain-bladders (m l — -m 8 ) and the spinal marrow (m 8 ). (Compare 
Figs. 15 and 16.) The brain is enclosed in the skull (s), the spinal marrowin 
the vertebral canal (above the spinal marrow, the vertebral arches (wb) ; 
under it the vertebral bodies (wk) ; under the latter the origin of the ribs is 
indicated). In front an organ of sense (q t nose or eye) has developed from 
the horn-layer, — at the back, the primitive kidney (u). The intestinal tube 
(d) has differentiated into the following parts, lying one behind another: 
the mouth-cavity (mh), the throat -cavity with six pairs of gill-openings 
(ks), the swimming-bladder ( = lungs, lu), the oesophagus (sr), the stomach 
(mg), the liver (lb) with the gall-bladder («*). the small intestine (dd), and 
the rectum with the anus (a). Below the throat-cav'ty lies the heart, with 
the auricle (hv) and the ventricle (hk). 

Fig. 14. — Longitudinal section through a human embryo of three weeks, 
showing toe relation of the intestinal tube to its anpendages. In the centre 
the long-stalked yelk-sac (or the navel-vesicle, ds) projects from the intes- 
tinal tube (ds) ; similarly the long-stalked allantois (al) projects from the 
intestine at the back. The heart (hz) is visible beneath the anterior intes- 
tine. Amnion-cavity (ah). 

Fig. 16. — Longitudinal section through a human embryo of five weeks, 
(Compare Fig. 14.) The amnion and the placenta, with the urachus, are 
omitted. The spinal tube has differentiated into the five primitive brain-blad- 
ders (^i-^^s). and the spinal marrow (m 8 ). (Compare Figs. 13 and 16.) The 
skull (s) is formed around the brain ; below the spinal marrow the series of 
vertebral bodies (wk). The intestinal tube has differentiated into the 
following divisions, lying one behind another : the throat-cavity with three 
pairs of gill-openings (ks), the lung (lu), the oesophagus (sr), the stomach 
(mg), the liver (lb), the coil of the small intestine (dd), into which the 
yelk-sac (ds) opens, the urinary bladder (hb), and the rectum. Heart (hz). 

Fig. 16 —Longitudinal section through developed human female. All 


the parts are perfectly developed, but diagrammatically reduced and sim- 
plified, in order to exhibit clearly their relative positions and their relations 
to the four secondary germ-layers. In the brain, the five original brain- 
bladders (Fig. 15, m 1 -m s ) have been differentiated and transformed in the 
manner peculiar to the higher mammals: m l , fore brain (cerebrum), out- 
weighing and covering all the other four brain bladders; m t , twixt brain 
(" the centre of sight ") ; m 3 , mid brain (" the four bulbs ") ; m 4 , hind 
brain (cerebellum) ; m 5 , after brain, or prolonged marrow (medulla 
oblongata), passing into the spinal marrow (m 6 ). The brain is enclosed in 
the skull (s), the spinal marrow by the vertebral canal: above the spinal 
marrow the vertebral arches and spinal processes, under it the vertebral 
bodies (wk). The intestinal tube has differentiated into the following partB 
lying one behind another : the mouth-cavity, the throat-cavity (in which at 
an earlier period the gill-openings, ks, were situated), the trachea (Ir) with 
the lungs (lu), the oesophagus (sr), the stomach (mg), the liver (lb), with 
the gall-bladder (i), the ventral salivary gland, or pancreae (p), the small 
intestine (dd), the large intestine (dc), and the rectum with the anus (a). 
The body-cavity, or coelom (c), is divided by the diaphragm (z) into two 
distinct cavities ; the breast-cavity (c), in which the heart (hz) lies in. front of 
the lungs, and the ventral cavity in which most of the intestines lie. In front 
of the rectum lies the sheath (vagina, vg), which leads into the uterus (/) ; 
in this the embi'yo, indicated here by a small germ-membrane vesicle (e), is 
developed. Between the uterus and the os pubis lies the vesica urines (hb), 
the remains of the stalk of the allantois. The horn-plate (h) as the outer 
skin, covers the whole body, and also forms the coating of the cavities of 
the mouth, the anus, the vagina, and the uterus. The milk glands, or mamma 
( md), are also originally formixl from the horn. plate. 




Of the Meaning of the Letters in J'lates IV. and V. 

N.B. — The skin-sensory layer is indicated by orange, the wkin-fibroGt 
layer by blue, the intestinal -fibrous layer by red, and the intestinal-glanduliij 
layer by green. 

a, anal opening 
ah, amnion-cavity 

al, allantois (urine sao) 

am, amnion 

b, ventral muscles 

bb, breast-bone (sternwn) 

c, body. cavity (coeloma) 

c ti breast-cavity (cavitas pleurce) 

c„, ventral cavity (cavitas peritonei) 

ch, notochord (chorda) 

d, intestinal tube (tractus) 

dc, large intestine (colon) 

dd, small intestine (ileum) 
ds } yelk-sac (navel-vesicle) 

e, embryo or germ 
/, matrix (uterus) 

g, mesentery (mesenterium) 

h, horn- plate (ceratina) 

hb, urinary vesicle (vesica urince) 

hk, ventricle of heart 

hi, left (arterial) heart 

hr, right (venous) heart 

hv, auricle (atrium) 

hz, heart (cor) 

i, gall-bladder (vesica fellea) 

k, germ-glands (sexual glands) 

ks, gill-openings (throat -openings) 

'., leather-plate (corium) 

lb, liver (hepar) 

'r, windpipe {trachea) 

■' . lung (pulmo) 

medullary tube (tubus medul- 

m, — m 


the five brain -bladders 

m tt 

m t , 







spinal cord (medulla spinalis) 
md, mi Ik -glands (mamma) 
mg, stomach 
mh, mouth-cavity 
mp, muscle-plate (muscularis) 
n, principal veins 
o, mouth-opening (osculum) 
p, ventral salivary gland (pancreas) 
q, organ of sense 
r, muscles of the back 
rp, ribs (costce) 
s, skull (cranium) 
sb, os pubis 

sh, throat-cavity (pharynm) 
»r, gullet (oesophagus) 
t, aorta (main artery) 
u, primitive kidney (protovephron) 
uw, embryonic vertebra (metameron) 
v, intestinal vein (primitive vein) 
vg, vagina 
w, vertebra 
wb, vertebral archet 
wk, vertebral bodies 
0, legs, or limbe 
y, space between the amnion and 

the yelk-sac 
*, midriff (dwvphragma) 

( 327 ) 


Systematic Survey of the Development of the Organic Systems of Man from 
the Germ-layers. (Cf. Plates IV. and V.) 


Outer Primary 



Germ- layer, 




dermaiu, II. 




germ- layer. 








Lamella ceratina. 


Marrow- pin te. 
Lamella medullaris. 


Primitive kidney 


Lamella renalis. 

1. Outer-skin {epidermis). 

2. Appendages of the epi- 

dermis (hair, nails, etc/). 

3. Glands of the epidermu 

(perspiratory, sebaceoua, 
lacteal glands). 

4. Spinal marrow i medullary 

5. Brain J tube. 

6. Organs of the sense* (es- 

sential part). 

7. Primitive kidneys (.r) and 

the outlets, which irise 
from them for the sexual 
products (perhaps from 
the skin-fibrous layer ??) 





Skm -fibrous 



inodtrmalis, H. 



Lamella coriaria. 


Flesh -plate. 
Lamella carnosa. 


8. True skin (corium) and 
skin-muscle stratum? 

9. Trunk - muscle stratum 
(side muscles of the 
trunk, etc.). 

10. Inner skeleton (chord, ver- 
tebral column, etc.). 

11. Exoccelar.'(paiu-taiccelom- 
ej helium). 

12. Male glim-epithelium (ru- 
dimentary testes) ? ? 

Body-Cavity (Ccelomd) : A space between the skin-layer and the intestinal layer, between 
the body-wall and the intestinal wall, filled with lymph (colourless blood). 









gtutralii, E. 



germ- layer. 

fibrous layer 

tum, Baer.) 

Lamina ino- 
gastralis, II. 


Vascular plate. 
Lamella vasculosa. 

13. Female germ-epithelium 
(rudimentary ovary) ? ? 

14. Endoccel ar r (visceral cce- 

15. Main blood-vessels (heart, 
primitive arteries, prim- 
itive veins). 

16. Blood-vessel glands (lym- 
phatic glands and 



Lamella mesenterica 

17. Mesentery (mefenterium). 


{17. Mesentery (mesenter 
18. Intestinal - muscle 
(and fibrous int 






tum, Baer.) 

Lamina myco- 
gattralis, H. 


Mucous plate. 
Lamella mucota. 

19. Intestinal epithelium. (In- 

ner cell-cuating of the 
internal tube.) 

20. Intestinal gland epithe- 
lium. (Inner cell -coat- 
ing of the intestinal 




Essential Agreement between the Chief Palingenetic Germ Processes in the 
case of Man and in that of other Vertebrates. — The Human Body, like 
that of all Higher Animals, develops from Two Primary and Four 
Secondary Germ-layers. — The Skin-sensory Layer forms the Horn-plate, 
the Medullary Tube, and the Primitive Kidneys.— The Middle Layer 
(Me oderm) breaks up into the Central Notochord, the Two Primitive 
Vertebral Cords, and the Two Side-layers. — The latter split up into the 
Skin-fibrous Layer and the Intestinal-fibrous Layer. — The Intestinal- 
glandular Layer forms the Epithelium of the Intestinal Canal, and of 
all its Appendages. — Ontogenetic and Phylogenetic Fission of the 
Germ- layers. — Formation of the Intestinal Canal. — The Two-layered 
Globular Intestinal Germ-vesicle of Mammals represents the Primitive 
Intestine. — Head Intestinal Cavity, and Pelvic Intestinal Cavity. — 
Mouth Groove and Anal Groove. — Secondary Formation of Mouth 
and Anus. — Intestinal Navel and Skin-navel. — Movement of the Primi- 
tive Kidneys from the Outside to the Inside. — Separation of the 
Brain and Spinal Marrow. — Rudiments of the Brain- bladders. — The 
Articulation or Metamerio Structure of the Body. — The Primitive 
Vertebrae (Trunk -Segments, or Metamera). — The Construction and 
Origin of the Vertebral Column. — Vertebral Bodies and Vertebral 
Arches. — Skeleton-plate and Muscle. plate. — Formation of the SkuU 
from the Head-plates. — Gill-openings and Gill-arches. — Sense-organs. 
—Limbs. — The Two Front Limbs and the Two Hind Limbs. 

"The occurrence of an internal skeleton in definite local relations to the 
other organ-systems, and the articulation of the body into homologous 
segments, are points in the general organization of Vertebrates to which 
especial weight must be given. This metamerio structure is more or less 
definitely expressed in most of the organs, and as it extends to the axial 
skeleton, the latter also gradually articulates into separate segments, the 
vertebra*. The latter, however, must be regarded only as the partial ex* 


pression of a general articulation of the body, which is all the more 
important in consequence of its appearing prior to the articulation of the 
originally inarticulate axial skeleton. Hence this general articulation may 
be considered as a primitive vertebral structure, to which the articulation 
of the axial skeleton is related as a secondary process of the same sort." — 
Karl Gkgembauk (1870). 

The most important processes, which we have just noticed 
in the construction of the body from the germ-layers, are 
essentially similar in all Vertebrates. In these points Man 
entirely resembles the other Mammals ; nor do the latter 
essentially differ from other Vertebrates. It is true that a 
more exact study of germ-history brings various differemes 
to light, some of which are very striking: among these 
may be mentioned the formation of a large yelk -sac 
in most Fishes, in all Reptiles, Birds, and Mammals ; also 
the formation of the amnion and allantois in the three 
hio*her vertebrate classes. But all these remarkable struc- 
tural conditions, which react on the diversified development 
of other parts, were only kenogenetically acquired at a later 
^tage, in consequence of Adaptation to the conditions of 
egg-life ; on the contrary, the most important conditions of 
the original body-structure, which must be regarded as 
palingenetie, as transmitted by Heredity from the common 
parent-form of all Vertebrates, are, on the whole and in the 
main, everywhere the same. 

As such essential main acts in the germ-history of all 
Vertebrates, the following must be especially noted : — 1. 
The formation of a Gastrula (in the most original form in 
the Amphioxus, in a form which is modified from the 
latter in all other Vertebrates). 2. The fission of the four 
primary germ-layers into four secondary germ-layers (often 
with a three-layered stage intermediate between the two 



and the four-layered stages). 3. The axial soldering, 01 
the coalescence of the germ-layers along the longitudinal 
axis (giving rise to the axis-band). 4. The early sepa- 
ration of the medullary tube from the skin-sensory layer 
(by the formation of the dorsal furrow and the spinal 
swellings). 5. The early origin of the primitive kidnev 
ducts (probably from the skin-sensory layer). 6. The early 
division of the skin-fibrous layer into the chorda, the primi- 
tive vertebral cords, and the trunk-muscle plates. 7. The 
separation of the skin-fibrous layer from the intestinal- 
fibrous layer (giving rise to the body-cavity, or cueloma). 
8. The rudimentary primitive vessels, or aorta? (from the 
intestinal-fibrous layer). These important germ-process- 
result in the formation of ten different parts of the body, 
which we may call " the primitive organs," and which, in 
the following list, are represented in their relation to the 
germ-layers. (Cf. Fig. 99, and Plate IV. Fig. 3.) 

Pkylogenetic fission of the germ-layers. 

Primitive Organs 
{Fig- 99). 

Ontogenetic fission 

of the 


Outer primary germ- 


(Dermal laypr, or 

Inner primary germ- 
layer : 

Intestinal layer 
(Gastral layer, or 

I. Secondary germ- 
layer : 


II. Secondary germ- 
layer : 


in. Secondary germ- 
layer : 

fibrous layer. 

IV. Secondary germ- 
layer : 

glandular layer. 

1. Horn-plate (h). 

2. Medullary plate 


3. Primitive kidney 


4. Chorda (ch). 

5. Primitive verte- 

bral plate (tiio). 

6. Skin-muscle plaie 


7. Body-cavity (*p). 

8. Intestinal muscle 

plate idf). 

9. Primitive aorta 

19. Intestinal gland- 
epithelium (dd). 

A. Upper or 

Sensory layer, 


B. Middle or 


tive layer, 


C. Lower or 
Trophic layer, 


In the important transverse section through the germ- 
shield of a Chick (Fig. 99), which represents these primitive 
organs in their original relative positions, they are seen to 
be flattened and spread out ; and they are found in this 
same condition in a corresponding transverse section through 
the germ-shield of a Mammal. In order rightly to appre- 
ciate these instructive sections (with which Figs. 3 and 4 
on Plate IY. should be compared), it must be remembered 
that the layer-like extension of the flat germ-layers over 
the surface of the large yelk-sac represents a derived, 
kenogenetic condition, which has arisen in consequence of 
the gradual acquisition of a large nutritive yelk. In those 
low Vertebrates in which there is no such yelk-sac, and in 
which the original, palingenetic condition is more or less 

Fig. 99. — Transverse section through the germ-shield of a Chick (on the 
.second day of incubation, about 100 times enlarged). In the outer germ- 
layer the axial dorsal furrow has completely closed and forms the medullary 
tube (nir), which has separated itself from the horn-plate (h). In the 
middle germ-layer the axial notochord (ch) has entirely separated itself 
from the two primitive vertebral cords (uw), in the interior of which a 
transitory cavity (uwh) afterwards forms. The side-layers have split into 
the outer skin-fibrous layer (hpl) and the inner intestinal-fibrous layer 
(df), which are still connected by the middle plates (mp). The fissure 
(sp) between the two is the rudiment of the body-cavity. In the gap 
between the primitive vertebral cords and the side-layers on either side are, 
attached on the outer side, the primitive kidney (ting), on the inside the 
primitive artery (ao). (After Kolliker.) 
24 * 


, -retained, the germ-layers, even in the earliest stage, form 
closed tubes, which may be immediately referred to the 
tubular shape of an elongated Gastrula. (Cf. Figs. 62-69.) 

When, therefore, it was generally thought that the 
main object of the germ-history of Vertebrates was to 
derive the later organization of these from a primitive, 
flat, discoid form, the two-layered germ-disc (or the three- 
layered germ-shield), a grave error was committed. 91 For 
this flat, circular germ-disc, and the flat, sole-shaped germ- 
shield which arose from the former, are phylogenetic form- 
ations, which arose only secondarily, in consequence of the 
accumulation of a large mass of nutritive yelk in the 
primitive intestine of the primary Gastrula ; and so when, 
at a later period, the dorsal side of the flat germ-shield 
arches, and its edges bend towards each other and coalesce 
into tubes on the ventral side, the process is neither primary 
nor secondary, but tertiary. 

A right conception of the formation of the intestine is 
evidently the real point on which a thorough knowledge of 
these important germinal processes depends. The greatest 
difficulties are solved when a clear and correct conception 
of the formation of the intestinal canal has been acquired. 
For the primitive intestine is, according to the Gastraea 
Theory, the earliest and the most important organ of the 
animal body. In order to gain this clear idea of the forma- 
tion of the intestinal tube and the parts attached to it, it is 
especially necessary to note accurately the important modi- 
fication undergone by the intestinal-glandular layer of the 
mammalian germ. This, as has been said, is at first a 
simple layer of cells (an epithelium), which lines the inner 
surface of the globular intestinal germ- vesicle. It is a 


Bimple globule, the wall of which consists of a simple layer 
of homogeneous cells (Fig. 100, A dd). The first change in 


Fig. 100. — The separation of the discoidal mammalian germ from the 
yelk-sac, seen in section (diagrammatic). A. The germ-disc {h, hf) lies ex- 
tended on one side of the intestinalgerm-vesicle (kb). B. In the centre of the 
germ-disc the medullary furrow (mr), and under that the notochord (ch) 
appear. C. The intestinal-fibrous layer (df) has grown round the intestinal- 
glandular layer (dd). D. Skin -fibrous layer (hf) and intestinal-fibrous layer 
(df) part round the circumference of the germ-disc ; the intestine (d) begins 
to separate itself from the yelk-sac or navel-vesicle (nb). E. The intestinal 
tube (mr) is closed ; the body-cavity (c) begins to form. F. The primitive 
vertebrae (w) appear ; the intestine (d) is almost completely closed. G. 
The primitive vertebrae (w) begin to grow round the medullary tube (mr) 
and the notochord (ch) ; the intestine (d) is separated from the navel- 
vesicle (nb). H. The vertebrae (iv) have enclosed the medullary tube (mr) 
and the notochord (ch) ; the body-cavity (c) is closed ; the navel-vesicle has 
disappeared. The amnion and serous membranes are omitted. 

In all, the letters indicate the same parts : h, horn-plate ; mr, medullary 
tube ; hf, skin-fibrous layer ; iv, primitive vertebrae ; ch, notochord ; c, body- 
cavity ; df, intestinal-fibrous layer ; dd, intestinal -glandular layer ; d, in- 
testinal cavity ; nb, navel-vesicle. 


this globular formation is that at one point in the germ 
disc, immediately below the notochord, and, therefore, below 
the axis of the developing body, a furrow-like depression 
arises. This is the primitive groove (Fig. 100, B). It gradually 
becomes deeper and broader, assumes the form of a canal, 
and completely separates from the germ-vesicle, of which 
it originally formed a part (Fig. 100, D — H). At first 
the whole intestinal germ- vesicle is, in a certain sense, the 
intestinal cavity. We may, therefore, compare the entire 
intestinal germ vesicle of the Mammal, the wall of which, 
closed on all sides, is formed by the intestinal layer, with the 
primitive intestine of a Gastrula, the primitive mouth of 
which has closed. This primitive intestine separates into 
two parts, the permanent after-intestine (d), and the tran- 
sient navel-vesicle (nb). 

This is also true of the formation of the intestine in 
Birds and Reptiles. For in these, the large yelk-sac, filled 
with nutritive yelk, represents the smaller mammalian 
navel-vesicle, filled with clear liquid. In Birds and Reptiles 
again, the later, permanent intestine also separates itself 
from the yelk-sac by the intestinal groove changing into a 
canal, into the intestinal tube. This tube is formed from 
the intestinal-furrow in the same way as the medullary 
tube originates from the dorsal furrow. The groove grows 
deeper and deeper ; its edges grow downwards towards each 
other, and coalesce at the point at which they meet. But 
the difference between the structure of the intestinal tube 
and that of the medullary tube consists, as we have shown 
in the fact that the medullary tube is closed equally along 
its whole length in a suture, while the intestinal tube 
grows together more concentrically, not only from the two 


edges, but the ends also come together with the edges which 
close, and form a navel. 

With this concentric closing of the intestinal tube is 
connected the formation of two cavities, which are called 
the head intestinal cavity and the pelvic intestinal cavity. 
When the embryo gradually becomes detached from the 
wall of the germ-vesicle, on which it at first lies flat, the 
anterior and posterior ends are the first to be released, 
while the central portion of the ventral surface continues 
attached to the yelk-sac by the yelk-duct, or navel-duct 
(Fig. 101, m). In the mean time the dorsal surface of the 
body becomes much arched; the head end, on the other 
hand, bends downward and against the breast, while the 
tail end, in the same way, presses against the abdomen ; 
the embryo tries to roll itself together, as a hedgehog 
makes itself into a ball to ward off its enemies. This arch- 
ing of the back is caused by the quicker growth of the 
dorsal surface, and is directly connected with the detach- 
ment of the embryo from the yelk-sac (Fig. 101). In the 
head there is no separation between the skin-fibrous layer 
and the intestinal-fibrous layer, as is the case in the trunk, 
but the two layers remain attached and form the so-called 
"head-plates." Now as these head -plates free themselves 
at a very early period from the surface of the germ-area, 
and grow, first downward toward the surface of the 
intestinal germ-vesicle, and then backwards toward the 
point, at which the latter passes into the intestinal groove ; 
a small cavity is thus formed within the head portion, which 
represents the foremost blind end of the intestine. This 
is the small head intestinal cavity (Fig. 102, to the left 
of d) ; its opening into the middle intestine is called the 



anterior " intestinal gate ' (Fig. 102, at d). Just in the 
same way the tail curves back against the ventral surface ; 
the intestinal wall then encloses posteriorly a similar small 

r 3 


t 7i n 

vo I 3 i 

Fig. 101. — Longitudinal section through the embryo of a Chick (fifteenth 
day of incubation). Embryo with arched dorsal surface (black) : d, intes- 
tine ; o, mouth ; a, anus ; I, lungs ; h, liver ; g, mesentery ; v, auricle of 
heart; k, ventricle of heart; h, arterial arches; t, aorta; c, yelk-sac; m, 
yelk-duct ; w, allantois ; r, stalk of allantois ; n, amnion ; w t amnion- 
cavity ; s, serous membrane. (After Baer.) 

cavity, the hind end of which is blind ; this is the pelvic 
intestinal cavity. Its opening into the middle intestine 
-"is the " hind intestinal gate." 

In consequence of these processes the embryo assumes a 
form resembling a canoe lying bottom upward. Imagine a 
canoe with rounded ends, and fitted with a little deck fore 
and aft ; then turn this canoe upside down, so that its 
arched bottom is uppermost: this affords an approximate 
representation of this canoe-shaped embryo (Fig. 101,. e). 



The reversed convex keel represents the middle line of the 
back ; the little chamber under the fore-deck represents 
the head intestinal cavity, and that under the after-deck 
the pelvic intestinal cavity. (Cf. Fig. 94, p. 312.) 

Fig. 102. — Longitudinal section through the front half of a chick 
(at the end of the first day of incubation), seen from the left side : k, head- 
plates ; ch, notochord ; above the latter, the blind front end of the 
medullary tube (mr) ; below it the head intestinal cavity, the blind front 
end of the intestinal tube ; d, intestinal-glandular layer ; df, intestinal- 
fibrous layer; h, horn-plate; h.h, heart-cavity; hk } heart-cap; ks, head- 
sheath ; kk, head-cap. (After Remak.) 

With its two free ends the embryo now presses 
somewhat into the external surface of the germ-vesicle, 
and at the same time lifts the middle portion away from the 
germ-vesicle. The consequence is that the germ-vesicle 
soon appears to be merely a pouch-shaped appendage pro- 
truding from the middle of the body. This appendage, which 
continues to decrease in size, is afterwards called the yelk- 
sac, or navel-vesicle. (Cf. Fig. 94, 4 , 5 , ds; Fig. 100, and Plate 
V. Fig. 14.) The cavity of this yelk-sac, or cavity of the 
germ- vesicle, communicates with the growing intestinal 
cavity through a wide connecting aperture, which after- 


wards extends into a long narrow canal, the yelk-duct 
Let us suppose we are within the cavity of the yelk-sac ; 
we may then pass from it, through the yelk-duct (Fig. 101, m), 
directly into the middle part of the intestinal canal, which 
is still wide open. If from there we pass on into the 
head portion of the embyro, we reach the head intes- 
tinal cavity, the anterior end of which is blind. If, on the 
other hand, we pass from the middle of the intestine back- 
wards into the tail portion, we reach the pelvic intestinal 
cavity, the hind end of which is blind (Fig. 94, 3 ). The 
first rudiment of the intestinal tube now consists, therefore, 
strictly speaking, of three distinct sections : (1) the head 
intestinal cavity, the hind end of which opens, through the 
front intestinal gate, into the middle intestine ; (2) the 
middle intestinal cavity which opens downwards, through 
the yelk-duct, into the yelk -sac ; and (3) the pelvic intes- 
tinal cavity, the front of which opens, through the posterior 
intestinal gate, into the middle intestine. 

At first the mouth and anal openings are wanting. 
The whole primitive intestinal cavity is entirely closed, and 
is only connected in the middle by the yelk-duct with the 
cavity of the intestinal germ-vesicle, which is also closed 
(Fig. 94, 3 ). The two future openings of the intestinal 
canal, the anal opening and the mouth-opening, form only 
secondarily, on the outside, and from the outer skin; 
that is to say, a groove-like depression arises in the horn- 
plate at the point where the mouth is afterwards situated, 
and this grows deeper and deeper, growing towards the 
blind front-end of the head intestinal cavity : this is the 
mouth-groove. A similar groove-like depression appears 
posteriorly on the outer skin, at the point where the anus 


is afterwards found, and this also grows continually deepei 
and towards the blind anterior end of the pelvic intestinal 
cavity ; this is the anal groove. At length the innermost 
and deepest parts of these grooves touth the two blind end* 
of the primitive intestinal canal, from which they are now 
only separated by a thin membranous partition wall 
Finally, this thin skin is broken through, and the intestinal 
tube now opens outward in front through the mouth- 
opening, and in the rear through the anal opening (Fig. 
94, 4 ; 101). At first, then, we really have before us, if we 
look into these grooves, a partition wall separating them 
from the cavity of the intestinal canal, and it is only later 
that these partitions disappear. The mouth and anal 
openings develop secondarily. 

The remnant of the intestinal germ-vesicle, which we 
have called the navel- vesicle, or yelk-sac, grows smaller and 
smaller as the intestine develops, and finally hangs' like a 
small pouch from the middle of the intestine by a slender 
stalk, by the yelk-duct (Fig. 94, 5 ds). This yelk-duct is 
of no permanent importance, and, like the yelk-sac itself, is 
completely degraded and absorbed. Its contents are absorbed 
by the intestine, and the yelk-duct itself closes. The place 
at which it attaches itself to the navel is the intestinal 
naveL The complete closing of the intestine finally takes 
place at this spot (C£ Chap. XII., and Plate V. Fig. 
14, 15.) 

Just as the intestinal tube arose from the vegetative 
germ-layer, so from the animal germ-layer arises the outer 
ventral wall, which surrounds the entire body-cavity 
(c&loma), and includes the intestine. It develops from the 
outer portions of the side-layers. As has been already 


observed, these side-layers, which for a time were separated 
from the primitive vertebral cords, afterwards again adhere 
to the latter. While the inner portion of the side-layers 
(belonging to the intestinal-fibrous layer) is thus forming 
the external wall of the intestine, the outer portion of the 
same layers (belonging to the skin-fibrous layer) grows in 
a circle round the intestine, thus closing the body-cavity 
(Fig. 100, p. 333). The edges of the ventral plates, as these 
portions of the side-layers are called, grow toward each 
other from all sides, continually narrowing the slit-like 
ventral opening, from which the yelk-sac depends. Finally, 
the latter is, in Mammals, completely separated from 
the intestine by the closing of the ventral plates, while in 
Birds and Reptiles it is taken into the intestine. This point 
at which the ventral wall finally closes — the last point of 
coalescence — is the ventral navel, the externally visible skin- 
navel, which is commonly briefly called the navel. This 
must be distinguished from the inner, or intestinal navel, 
which is the point at which the intestinal canal closes, and 
of which no trace can afterwards be found. With the 
closing of the intestinal tube and of the ventral wall, 
the double tubular form of the vertebrate body is com- 

A few words must still be said concerning the modifica- 
tions which, while these processes are going on, take place 
in the primitive kidneys and in the blood-vessels. The 
primitive kidneys, which at first lie quite superficially just 
below the outer skin (epidermis, Fig. 99, ung), soon penetrate 
far into the interior in consequence of peculiar conditions of 
growth (Figs. 95, 96, ung, pp. 317, 318) ; at last they lie very 
far within, underneath the chorda dorsalis (Fig. 97,1^, p. 318) 



Similarly the two primitive aortse penetrate within, 
below the notochord, and there eventually amalgamate 
and form a single secondary aorta, which is situated under 
the rudimentary vertebral column. (Cf. Figs. 95-98, ao.) 
So, too, the cardinal veins, the first rudiments of the 
venous blood-vessels, penetrate further inwards, and after- 
wards lie directly over the primitive kidneys (Fig. 97, vc). 
In the same locality, at the inner side of the primitive 
kidneys, the first rudiments of the sexual organs soon 
become visible. The chief portion of this apparatus, apart 
from all its appendages, is, in the female, the ovary — in the 
male, the testes. Originally both these appear in the form 
of a simple hermaphrodite gland, formed from a small por- 
tion of the coelom-epithelium, the cellular lining of the body- 
cavity, at the point of contact between the skin-fibrous layer 
and the intestinal-fibrous layer. It is only secondarily that 
this hermaphrodite gland seems to connect itself with the 
primitive kidney ducts, which lie very close to them, and 
which are very importantly related to the sexual glands. 
(Cf. Chap. XXV., and Plate IV. Figs. 5-7.) 

We will now leave the transverse sections of the verte- 
brate body, the comparison of which has been so ex- 
tremely instructive and important, and by means of which 
we have solved the hardest problem of germ-history, the 
problem as to the part taken by the germ-layers in the 
formation of the body. In place of those, we will now 
examine the longitudinal form of the rudimentary embryo 
of the mammalian body, partly superficially, and partly in 
various longitudinal sections. 

Let us now examine the surface, from the dorsal side, of 
that very simple embryonic form which we called th© «ole- 




Figs. 103-5. — Lyre-shaped germ-shield of a Chick, in three consecutive 
stages of development, seen from the dorsal surface (about 20 times the 
natural size). Fig. 103, with six pairs of primitive vertebrae. Brain is a 
simple bladder (hb). The spinal furrow is open behind x ; at the posterior 
end it is very wide open at z ; mp, medullary plates; sp, si de -plate s ; y, 
limit between throat-cavity (s?i) and head -intestine (vd). — Fig. 104, with 
10 pairs of primitive vertebras. The brain has parted into three bladders : 
v, fore-brain ; m, mid-brain ; h, hind-brain ; c, heart ; dv, yelk-veins. The 
medullary furrow continues wide open at the posterior end (z) ; mp, medul- 
lary plates. — Fig. 105, with 16 pairs of primitive vertebrae. The brain has 
parted into five bladders : v, fore-brain ; z, twixt-brain ; m, mid-brain ; h, 
hind. brain ; n, after-brain ; a, eye-bladders ; g, ear-vesicles ; c, heart ; dv, 
yelk-veins ; mp, medullary plates j uw, primitive vertebrae. 


shaped or lyre-shaped germ-shield (Figs. 86, 87, p. 298). In 
the middle line of its dorsal surface the primitive groove 
first made its appearance, enclosed by the two parallel 
dorsal, or medullary swellings. The coalescence of these 
formed the medullary tube. When we examine the further 
modifications of this, we very soon perceive a difference 
between the formation of the anterior and that of the 
posterior ends. At the anterior end in Man, as in all the 
higher Vertebrates, the brain very soon begins to separate 
or differentiate from the medullary tube. The first rudi- 
ment of the brain is merely a roundish, bladder-like pro- 
tuberance of the vertebral canal (Fig. 103, hb). Very soon, 
however, this bladder is divided by two circular contrac- 
tions of its circumference, into three consecutive vesicles, 
the so-called primitive brain- bladders (Fig. 104, v m h). 
Two other similar contractions then appear, so that we now 
find five brain-bladders in a row (Fig. 105). This is the 
mode of development of the brain in all Mammals, from the 
simplest Fishes to Man. In all, we find a simple vesicle as 
the first rudiment of the brain, which is afterwards parted, 
by contractions in its circumference, into five smaller 
bladders. Though the brain, as the organ of the soul and 
the mental activities, afterwards develops in various Verte- 
brates in such very various ways, yet the first rudiment 
is of this simple and homogeneous form. This is a fact of 
the highest importance. 

Directly below the medullary tube, in the lyre-shaped 
germ-shield, we found the notochord. Right and left of the 
notochord the two parallel primitive vertebral cords had 
split away from the side-layers. But while the five brain- 
bladders are becoming distinct at the anterior end of the 



medullary tube, the two primitive vertebral cords in the 
centre of the primitive germ break up into a number of 
pieces, lying one behind another, and resembling small 
cubes on each side of the medullary tube. Two pairs 
usually first make their appearance simultaneously. Then 

Fig. 106-109. —The Germ - 
disc of a Rabbit (the circular 
germ -area with the lyre- 
shaped germ -shield), seen 
from the dorsal surface, in 
four consecutive stages of 
evolution (about ten times 
the natural size). (After 

In Fig. 106 the embryo (b) 
is as yet without primitive 
vertebrae ; the open dorsal 
furrow (a) surrounded by 
a narrow light germ-area 
(a. pelhicida, a), in the 
middle of the dark germ, 
area (a. opaca, d). 

In Fig. 107 seven pri- 
mitive vertebrae (c) may 
already be seen; the dorsal 
furrow is closed ; the first 
rudiment of the brain (a), 
a brain - bladder, behind 
which a second (b) is form, 
ing, is arising ; the light 
germ -area is now only 
visible at the anterior end, 
in the form of a dark sickle- 
shaped body on a black 



appear three, four, and five pairs, and finally a larger number 
of these pieces, which are called the primitive vertebrae. 

In Fig. 108 the em- 
bryo has eight primi- 
tive vertebrae and 
three brain-bladders ; 
the first brain -bladder 
(b) shows two lateral 
protuberances, the 
first rudiments of the 
eye -bladders (c) ; the 
second (d) and the 
third (e) brain-blad- 
ders are much 
smaller ; a indicates 
the edge of the head- 
sheath of the amnion. 

In Fig. 109 the 
embryo has ten 
primitive verte- 
brae ; in the germ- 
area the first 
traces of the net 
of blood-vessels 
appear, bounded 
by the ve7t,a term i- 
rialis (a) : b, tail- 
sheath, 6b, head- 
sheath of the 
amnion; the folds 
in the latter indi- 
cate the serous 


In Fig. 107 there are seven, in Fig. 108 there arc 
eight, and in Fig. 109 ten pairs of primitive vertebreB. Their 
number afterwards increases considerably, amounting in 
Man to upwards of thirty. As we shall presently see, out 
of each pair of these primitive vertebral segments an indi- 
vidual section of the trunk, a metameron, develops. Each 
pair of primitive vertebrae is not, as the name seems to 
indicate, merely the rudiment of a future vertebra, but, in 
addition to the vertebra, the appropriate muscles also 
develop from it, as does a pair of nerve-roots, etc. It is 
only the innermost portion of the primitive vertebra, the 
part lying next to the notochord, that gives rise to the 
rudiment of the articulated vertebral column, extending 
from the cranium to the tail, and composed of a number of 
bony vertebral rings. 98 

The breaking up of the vertebral cord into a double 
chain of primitive vertebral segments,' or, briefly, the 
forming of the metamera, is of the greatest importance, 
because it is in this process that the body of the Vertebrate 
passes from its originally inarticulate to its permanent 
articulate conditions. The developed Vertebrate is composed 
of a chain of homogeneous parts, lying one behind another 
precisely as are the Articulated Animals (Arthropoda). 
In the latter class, in Crabs, Spiders, Millipedes, and insects. 
this articulation is very clearly marked externally, the 
skin between each two members (metamera) having a ring- 
shaped contraction or dent round the circumference of the , 
body. In Vertebrates the articulation of the body is equally 
complete, but it does not appear externally, though internally 
it is fundamental. Every Vertebrate, in its perfect state, is 
an articulated person. Its personality forms a chain of 


members, metamera, or trunk-segments. In the same way 
in which the articulate and the externally articulated 
Worms developed from an inarticulate condition, so the 
internally articulated Vertebrate proceeded from an 
originally inarticulate condition. We shall presently ex- 
amine more closely the living representative of this con- 
dition, the Ascidia, a remarkable class of inarticulate Worm 
forms. (Chapters XIII. and XIV.) 

This process of articulation or metameric formation is, I 
repeat, of the highest importance in enabling us to under- 
stand each higher animal form, not only in its morpho- 
logical, but also in its physiological relations. This articu- 
lation is one of the most important conditions necessary 
to perfection : it is one of the principal causes of the 
complex body-functions of higher animals. The inarticulate 
animal can never attain so high a degree of perfection in 
form or in function as the articulated. And the reason is 
plain. These members, or metamera, are, in a certain sense, 
independent individuals. By division of labour, these 
originally homogeneous individuals develop into the different 
parts of the composite body-person, just as the embryonic 
cells fashion themselves, in consequence of division of labour, 
into the various tissues. The body of articulated animals 
may be likened to a railway train, in which the individual 
carriages, held together by the couplings, represent the 
metamera. The engine is the head of this articulated 
organism. Then come tender, mail-van, luggage-vans, 
passenger-carriages, cattle-trucks, etc. Each separate 
waggon or carriage is morphologically an individual and 
physiologically, yet the entire chain presents only a single 

individual, the railway train. As in this instance the 


various functions are distributed among the various kinds ot 
carriages — functions which each separate carriage can- 
not discharge simultaneously — so in the articulated animal 
body the division of labour among the metamera of the 
trunk must be regarded as a material advance. 

The best explanation of the nature of metameric 
formation is afforded by the articulated Worms, especially 
the Tape-worms and the Ringed- worms (Annelida). In 
these the members, or metamera, composing the ringed body, 
are all of the same structure and of the same form-value. 
The first member, the head, alone seems to be differently 
formed and more or less differentiated. In many Tape- 
worms the various members are so independent, that many 
zoologists regard each separate metameron as an individual 
animal, and the whole chain of members as a colony o. 
animals. In a certain sense this is quite correct, in so far 
as each separate metameron is an individual of a lower 
order, while the chain, composed of many metamera, is an 
individual of a higher order. But in proportion as the 
separate members relinquish their independence ; in pro- 
portion as they become differentiated in consequence ol 
division of labour, and become dependent on each other 
and on the whole body, and in proportion as the latter 
becomes centralized, the more perfect does the entire 
unitary organism become. In most Articulated Animals 
(Arthropoda), and in all Vertebrates, centralization has so 
far progressed that the individual metamera are no longer 
of any importance in themselves alone, and are to be con- 
sidered merely as the necessary component parts of the 
entire chain. 

Whan we investigate the origin of the metameric chain 


in the Worms, we find that it results, in consequence of 
repeated asexual generative processes, in consequence of 
what is called terminal budding, from an originally inarticu- 
late Worm-body, which is equivalent to a single metameron. 
Thus the Tape- worm embryo is at first ill head ; and on 
this head, which is only equivalent to a single metameron, 
repeated budding gives rise to one metameron after another; 
all, however, remain connected. So, too, in the Ringed 
Worms (Annelida) the originally inarticulate body puts out 
numerous buds from its posterior extremity, thus giving 
rise to the long articulated chain. Such is the nature of 
this process, which, however, in the germ-history of Articu- 
lated Animals and Vertebrates appears much compressed 
and secondarily modified. Originally, however, every ver- 
tebrate animal is just such a metameric chain, which has 
arisen, in consequence of terminal budding, from an in- 
articulated germ." 

As the metamera arise in this way, it will readily be 
understood that the anterior primitive vertebras are earliest 
found. Such is indeed the case. The earliest primitive 
vertebras, which are situated about the centre of the germ, 
are the first and second neck- vertebras. Then come the 
third and fourth neck-vertebrae, and so on. Each primitive 
vertebral segment in its turn soon produces, by the process 
of budding, a new metameron at its posterior extremity, 
till the chain is complete. The entire jointed body grows, 
therefore, in a direction from front to rear. In this way 
the articulated vertebral column of Man is at length pro- 
duced (Figs. 110, 111). In the developed Man it is composed 
of the cranium, with a chain of thirty- three or thirty-four 
different vertebrae : viz., seven neck-vertebrae, twelve chest- 


vertebrae, to which the ribs are attached, five lumbar- 
vertebrae, five cross- vertebrae (inserted into the pelvis), and 
four or five tail-vertebrae. Each of these represents a 
corresponding section of the nervous, muscular, and vascular 
systems, etc 

A further consequence of the mode of development of 
the metamera is, that nearly the whole front half of 
tho lyre-shaped germ-shield (Figs. 103, 107) mustre present 
the future head. The seven primitive vertebrae which 
occupy the third quarter of the whole length, form the neck, 
so that all the rest of the body originates from only the 
fourth and last quarter. This proportion seems strange at 
first, but its phylogenetic explanation, as the result of the 
terminal budding, is simple. The head portion of the 
vertebrate animal must accordingly be regarded phyloge- 
netically and originally as the oldest portion of the body — 
as a group of a few (six to ten) closely coalescent metamera, 
which, by continued budding at the posterior extremity, 
have produced the remainder of the body. The tail, on 
the other hand, is the most recent part, the latest in order 
of development. 

As has been already observed, the articulation affects 
the entire body of the Vertebrate, although the skin shows 
no external signs of articulation. The primitive vertebral 
pieces are, therefore, not merely rudiments of future 
vertebrae ; they are real metamera, or trunk-segments 
Each metameron first appears as a nearly cube-shaped, 
solid, roundly-hexagonal body, entirely composed of cells. 

Fig. 110. — Human skeleton, from the front. 

Fig. 111. — Human skeleton, from the right side. The arms havo been 
removed. (Both figures after H, Meyer.) 


--> 'i 




These cells are all the products of the stin-fibrous layer. 
At a very early period, a small cavity appears in each of 
these solid primitive vertebrae, which cavity, however, soon 
again disappears. This " primitive vertebral cavity " (Figs. 
95, 96, uwh, pp. 317, 318) is worthy of note only in so far as it 

Fig. 112. — Transverse section through the embryo of a Chick on the 
fourth day of incubation (about 100 times the natural size). The primitive 
vertebrae have separated into the outer muscle -pi ate (mp) and the inner 
skeleton-plate. The latter below, as the vertebral bodies (v;h), begins to 
surround the notochord (c/i) ; above, as the vertebral arches (w~b), begins 
to surround the medullary tube (m), the cavity of which (mh), is already 
very narrow. At ivq the primitive vertebra passes into the skin-muscle 
plate of the ventral wall (lip) ; hpr, leather-plate of the dorsal wall ; h, 
horn-plate ; a, amnion ; ung, primitive kidney duct ; un, primitive urinary 
canal; ao, primitive artery; vc, cardinal vein ; df, intestinal-fibrous layer; 
dd, intestinal-glandular layer j dr, intestinal groove. 


indicates an internal separation of the primitive vertebra 
into two entirely distinct parts : an inner part, which forms 
ihe skeleton — the skeleton-plate (Fig. 95, v/w, Fig. 112, wb), 
and an outer part, which forms the muscle — the muscle- 
plate (Figs. 95, 96, m, Fig. 112, mp), 

The skeleton-plate is formed of the entire inner half of 
each primitive vertebra, immediately adjoining the medul- 
lary tube (Fig. 112, wh, wb). Its lower part, the inner 
lower corner of the cube-shaped primitive vertebra, splits 
up into two lamellae, which grow round the chord, thus 
forming the basis of the vertebral bodies (wh). The 
upper lamella forces its way between the chorda and the 
medullary tube, the lower lamella between the chorda and 
the intestinal tube (Figs. 68, 69, p. 276 ; Fig. 93). As the 
lamellae of two opposite primitive vertebral pieces come 
together from right and left and unite, a ring-like sheath 
is formed round that particular part of the notochord. 
From this afterwards arises a vertebral body, i.e., the 
massive, lower, or ventral portion of the bony ring, which, 
as a vertebra in the strict sense, surrounds the medullary 
tube (Figs. 113-115). The upper or dorsal half of this 
bony ring, the vertebral arch (Fig. 112, wb) arises in just 
^the same way from the upper portion of the skeleton-plate ; 
i.e., from the inner, upper corner of the cube-shaped primi- 
tive vertebra. The two inner, upper corners of two oppo- 
site primitive vertebrae coalesce, from right to left, over the 
medullary tube, resulting in the closing of the vertebral 
arch. Between each pair of vertebral arches appear, at a 
later period, the roots of the spinal nerves, which arise from 
the same portion of the skeleton-plate (Fig. 98, <?, v, p. 318). 

The whole secondary vertebra, which thus results from 


the coalescence of the skeleton-plates of a pair of primitive 
vertebrae, and which encloses within itself a part of the 
chorda, consists originally of a somewhat soft cell-mass, which 
afterwards passes into a second firmer, cartilaginous state, 
and finally into a third, permanent, bony state. These three 
different conditions are generally distinguishable in the 
greater part of the skeleton of the higher Vertebrates ; at 

Pig. 113 — Third human neck -vertebra. 
Fig. 114. — Sixth human chest -vertebra. 
Fig. 115. — Second human lumbar-vertebra. 

first, most parts of the skeleton are quite tender, soft, and 
membranous; then, in the course of development, they 
become cartilaginous, and finally they ossify. 

All the bony vertebrae which afterwards compose the 
backbone, or vertebral column, arise, as we have already 
observed, entirely from the inner portion of the primitive 
vertebrae, from the skeleton-plate. The outer portion, on 
the other hand, which we have called the " muscle-plate ' 
(Fig. 112, Tnp), produces the great mass of the dorsal 
muscles (the dorsal " side muscles of the trunk "), as well as 
the leather skin, which covers the flesh of the back. This 
muscle-plate is in direct communication with that portion 
of the side-plates which develops into the ventral skin and 
the ventral muscles. 


In front, at the head end of the embryo, the middle 
layer (mesoderma) does not split into primitive vertebrae and 
side-plates, and the original fibrous layer here remains un- 
divided, forming the so-called " head-plates " (Fig. 102, k, p. 
337). From these arise the skull — the bony covering of the 
brain — as well as the muscles and leather-skin of the head. 
The skull develops precisely in the same way as the mem- 
branous vertebral column. The right and left head-plates 
arch towards each other over the brain-bladder, enclose the 
anterior extremity of the chorda, and thus eventually form 
a simple soft, membranous capsule round the brain. This 
afterwards changes into a cartilaginous primitive skull, 
similar to that which is retained throughout life by many 
fishes. It is only much later that the permanent bony 
skull, with all its different parts, is formed from this cartila- 
ginous primitive skull. 

In the embryo of Man, as in that of all other Vertebrates, 
the very remarkable and important structures, which are 
called the gill-arches and gill-openings, appear, at a very 
early period, on each side of the head (Plate I. Fig. 1, and 
Figs. 116, 118, /). These are among the characteristic and 
never-failing organs of the Vertebrates, for which reason 
we mentioned them in considering the typical primitive- 
Vertebrate (Figs. 52, 53, p. 256). On the right and left walls 
of the intestinal head-cavity, in the anterior portion, first 
one, and then several pairs of sac-like protuberances are 
formed, which break through the entire thickness of the 
side wall of the head. They thus become slits through 
which there is a free passage from without into the throat 
t cavity : these are the gill-openings, or throat-openings 
Between each pair of gill-openings the wall of the throat 



cavity grows thicker, and is changed into a bow-shaped or 
sickle-shaped ridge : these are the gill-arches ; on their 
inner side a vascular arch afterwards arises (Fig. 101, p. 336). 


Figs. 116, 117. — Head of a Chick, on the third day of incubation : 116 is a 
front view ; 117 from the right side ; n, nose-rudiments ; I, eye-rudiments ; 
g, ear-rudiments ; v, front-brain ; gl, eye-fissures. The first of the three 
pairs of gill-openings is separated into an upper jaw process (c) and a lower 
jaw process (u). (After Kolliker.) 

Fig. 118. — Head of embryo of a dog, from the front : a, the two side halves 
of the front brain-bladder ; b, eye-rudiments; c, middle brain-bladder ; de, the 
first pair of gill-arches (c, upper jaw process ; d, lower jaw process) ; /, /', /", 
the second, third, and fourth pair of gill-arches ; g h i k, heart (g, right, h, 
left auricle ; i, left, fe, right ventricle) ; I, origin of the aorta, with three 
pairs of aorta-arches, which pass on to the gill-arches. (After Bischoff.) 

The number of the gill-arches, and of the gill-openings, 
which alternate with the former, amounts in the higher Ver- 
tebrates to four or five on each side (Fig. 118, e, d, f, f, /'). 
The lower Vertebrates have a yet larger number. Origin- 
ally these remarkable formations discharged the function of 
a breathing-apparatus — were gills. Even yet in the Fishes 
generally, water, serving for respiration, and which is taken 
in through the mouth, passes out through the gill slits on 



the side of the gullet. In the higher Vertebrates they after- 
wards close. The gill-arches are transformed partly into the 
jaws, partly into the tongue-bone and the bonelets of the 
ear (ossicula auditus). (Cf. Plates I., VI., and VII.) 

Almost simultaneously with the development of the gill- 
arches, and immediately behind these, the heart with its four 
compartments is formed (Fig. 118, g h i h), and above, on the 
sides of the head, the rudiments of the higher sense-organs 
appear ; nose, eye, and ear. These highly important organs 

Fig. 119. — Transverse section through the shoulder region and the front 
limbs (wing-rudiments) of a Chick, on the fourth day of incubation (about 
20 times the natural size). Near the intestinal tube three lighter cords are 
visible on each side in the dark dorsal wall, which pass into the rudiments of 
the fore limbs or wiugs (e). The upper of these is the muscle-plate, the 
middle is the hind, and the lower is the front root of a spinal nerve. In 
the middle, below the chorda, is the aorta, and on each side of this a cardinal 
vein ; below the latter are the primitive kidneys. The intestine is almost 
closed. The ventral wall extends into the amnion, which forms a closed 
covering round the embryo. (After Remak.) 


originate in the very simplest form. The organ of smell, 
or the nose, appears quite in the front of the head, in the 
shape of two little pits above the mouth-opening (Fig. 
117, ri). The organ of sight, or eye, also in the form of 
a pit (Fig. 117, I, 118, 6), comes next, behind the organ of 
smell, towards which a considerable vesicular outgrowth of 
the fore-brain grows on both sides of the head (Fig. 105, a). 
Further back appears a third pit on each side of the head, 
the first rudiment of the organ of hearing (Fig. 117, g). 
No trace of the very marvellous future structure of these 
organs, or of the characteristic form of the face, is yet to 
be seen. 

The human embryo, having reached fchis stage of develop- 
ment, is yet hardly distinguishable from the germ of any 
of the higher Vertebrates. (Cf. Plates I., VI., and VII.) All 
the essential portions of the body are now begun : the head 
with its primitive skull, the rudiments of the three higher 
sense-organs, and the five brain-bladders, and the gill-arches 
and gill- openings ; the trunk with the medulla, the rudi- 
ments of the vertebral column, the chain of metamera, the 
heart and principal blood-vessels, and, finally, the primitive 
kidneys. Man, in this germ-stage, is a higher Vertebrate, and 
yet there is no essential, morphological difference between 
the human embryo and that of Mammals, Birds, Reptiles, 
etc. (Plates VI. and VII., upper line ol sections). This is an 
ontogenetic fact of the highest significance; from it are 
drawn the most important phylogenetic conclusions. 

There is, however, as yet no trace of limbs. Though 
the head and the trunk are already separated, though all 
the important inner organs are begun, there is as yet no 
trace of the limbs, or extremities, in this stage. These do 



not appear till later. This also is a fact of the profoundest 
interest; for it tells us that the older Vertebrates were 
footless, as the lowest living Vertebrates (Amphioxus and 

Fig. 120.— Transverse section through the pelvic region and the hind 
limbs of a Chick, on the fourth day of incubation (about 40 times the natura 
size) : h, horn-plate ; w, medullary tube ; n, spinal canal ; u, primitive kid- 
neys ; x, chorda ; e, hind limbs ; b, allantois canal in the ventral wall ; t, 
aorta ; v, cardinal veins ; a, intestine ; d, intestinal-glandular layer ; /, in- 
testinal-fibrous layer ; g, germ-epithelium ; r, dorsal muscles ; c, body-cavity 
(cceloma). (After Waldeyer.) 

the Cyclostoma) are at the present time. The descendants 
of these primaeval, footless Vertebrates did not acquire limbs 
till a much later period in the course of their development, 
when they acquired four limbs — a front pair and a hind pair. 
These limbs are all originally formed after .one model, though 
they afterwards develop very differently : in Fishes they 


become fins (pectoral and ventral) ; in Birds, wings and legs ; 
in creeping animals, fore and hind legs ; in Apes and in Man, 
arms and legs. All these parts arise from a first rudiment of 
the same perfectly simple form, which grows secondarily from 
the skin-layer (Figs. 119, 120). They always make their 
appearance in the form of two small buds, which at first 
resemble mere round knobs or plates. Gradually each of these 
plates increases into a more considerable projection, in which 
there is a more slender part, nearer the body of the embryo 
and distinct from the outer, broader, thicker part. This later 
portion is the rudiment of foot or of hand, while the former 
is the rudiment of arm or of leg. Plates VI. and VII. show 
how similar are the rudimentary limbs in very different 

A careful study and thoughtful comparison of the 
embryos of Man and other Vertebrates in this stage of 
development is very instructive, and reveals to the thought- 
ful many profounder mysteries and weightier truths than 
are to be found in the so-called "revelations" of all the 
ecclesiastical religions of the earth. Compare, for instance, 
carefully and attentively the three consecutive stages of 
development, represented in Plate VI. of the Fish (F), Sala- 
mander (S), Tortoise (T), and Chick (C), and in Plate VII. the 
corresponding embryos of the Hog (H), Calf (C), Rabbit (JK), 
and of Man (M). In the first stage (upper row of Section 
I.), in which the head with the five brain-bladders, and 
the gill-arches are indeed begun, though the limbs are still 
entirely wanting, the embryos of all Vertebrates from Fish 
to Man differ not at all, or only in non-essential points. In 
the second stage (middle row of Section II.), in which the 
limbs are indicated, differences begin to appear between the 



embryos of the lower and the higher Vertebrates ; as yet, 
however, the embryo of Man is hardly distinguishable from 
that of the higher Mammals. Finally, in the third stage 
(lower row of Section III.), in which the gill-arches have 
already disappeared and the face is formed, the differences 
become more evident, and grow, henceforth, more and more 
striking. The significance of such facts as these cannot be 
over-estimated. 100 

If there exists an inner, causal connection between the 
incidents of germ-history and those of tribe-history, as in 
accordance with the law of heredity, we must assume then 
these ontogenetic facts immediately afford most important 
phylogenetic conclusions. For the wonderful and compre- 
hensive harmony between the individual evolution of Man 
and that of other Vertebrates is only explicable by assuming 
the descent of these from a common parent-form. Indeed 
this common descent is now granted by all able naturalists 
who in place of a supernatural creation assume a non- 
miraculous evolution of organisms. 



Plates VI. and VII. are meant to represent the more or less complete 
agreement, as regards the most important relations of form, between the 
embryo of Man and that of other Vertebrates in early stages of individual 
development. This agreement is the more complete, the earlier the period 
at which the human embryo is compared with those of other Vertebrates. 
It is retained longer, the more nearly related in descent the respective 
matured animals are — corresponding to the " law of the ontogenetic con- 
nection of systematically related forms." (Cf. Chapter XII., p. 366.) 

Plate VI. represents the embryos of two of the lower, and two of the 
higher Vertebrates in three different stages : of a Fish (Osseous-fish, F); of 
an Amphibian (Land-salamander, j8) ; of a Reptile (Tortoise, T) ; and of a 
Bird (Chick, C). 

Plate VIII. shows the embryos of four Mammals in the three correspond- 
ing stages : of a Hog (H), Calf (C), Rabbit (R), and a Man (M). The con- 
ditions of the three different stages of development, which the three cross- 
vows (I., II., III.) represent, are selected to correspond as exactly as possible. 

The first, or upper cross-row, I., represents a very early stage, with gill- 
openings, and without limbs. The second (middle) cross-row, II., shows a 
somewhat later stage, with the first rudiments of limbs, while the gill- 
openings are yet retained. The third (lowest) cross-row, III., shows a still 
later stage, with the limbs more developed and the gill-openings lost. The 
membranes and appendages of the embryonic body (the amnion, yelk-sac, 
allantois) are omitted. The whole twenty-four figures are slightly magnified, 
the upper ones more than the lower. To facilitate the comparison, they are all 
reduced to nearly the same size in the cuts. All the embryos are seen from 
the left side ; the head extremity is above, the tail extremity below ; the 
arched back turned to the right. The letters indicate the same parts, in 
all the twenty-four figures, namely : v, fore-brain ; z, twixt-brain ; m, mid- 
brain ; h, hind-brain; n, after-brain; r, spinal marrow; e, nose; o, eye; 
0, ear ; fc, gill-arches j y, heart j w, vertebral column ; /, fore-limbs ; fc, bind, 
limbs 1 «, taiL 1 ** 

hakckkl's EVOLUTION of M.VV 


hatckel's evolution of man. 





The Mammalian Organization of Man. — Man hag the same Bodily Structure 
as all other Mammals, and his Embryo develops in exactly the same 
wav# — i n its Later Stages the Human Embryo is not essentially 
different from those of the Higher Mammals, and in its Earlier Stages 
not even from those of all Higher Vertebrates. — The Law of the 
Ontogenetic Connection of Systematically Belated Forms. — Application 
of this Law to Man. — Form and Size of the Human Embryo in the 
First Four Weeks. — The Human Embryo in the First Month of its 
Development is formed exactly like that of any other Mammal. — In the 
Second Month the First Noticeable Differences appear. — At first, the 
Human Embryo resembles those of all other Mammals; later, it 
resembles only those of the Higher Mammals. — The Appendages and 
Membranes of the Human Embryo. — The Yelk-sac. — The Allantois and 
the Placenta.— The Amnion. — The Heart, the First Blood-vessels, 
and the First Blood, arise from the Intestinal-fibrous Layer. — The 
Heart separates itself from the Wall of the Anterior Intestine. — The 
First Circulation of the Blood in the Germ-area (a. germinativa) : 
Yelk-arteries and Yelk-veins. — Second Embryonic Circulation of the 
Blood, in the Allantois : Navel-arteries and Navel-veins. — Divisions of 
Human Germ-history. 

" Is man a peculiar organism ? Does he originate in a wholly different 
way from a dog, bird, frog, or fish ? and does he thereby justify those who 
assert that he has no place in nature, and no real relationship with the 
lower world of animal life P Or does he develop from a similar embryo, 
and undergo the same slow and gradual progressive modifications P The 
answer is not for an instant doubtful, and has not been doubtful for the last 

thirty years. The mode of man's origin and the earlier stages of his 



development are undoubtedly identical with those of the animals standing 
directly below him in the scale j without the slightest doubt, he stands in 
this respect nearer the ape than the ape does to the dog." — Thomas Huxlk* 


The most important phenomenon, having a general bearing, 
that we have so far met with in the process of human germ- 
history, is surely the fact that the development of the 
human body proceeds from the beginning in exactly the 
same way as that of other Mammals. All the special 
peculiarities of individual development which distinguish 
Mammals from all other animals are found also in Man. 
Long ago, from the physical structure of the perfect Man 
the conclusion was drawn that his natural position in the 
system of the animal world can only be in the mammalian 
class. In 1735 Linnaeus, in his By sterna Naturae, placed 
Man in one and the same class with the Apes. This position 
is fully corroborated by comparative germ-history. We 
have evidence that, no less in embryonic development than 
in anatomical structure, Man closely resembles the higher 
Mammals, and especially the Apes. If we now seek, by 
applying the fundamental biogenetic law, to understand this 
ontogenetic agreement, the perfectly simple and necessary 
conclusion is that Man is descended from other mammalian 
forms. Hence we can no longer doubt the common descent 
of Man and the other Mammals from a single primaeval 
parent-form, or hesitate to believe that the blood-relation- 
ship is closest between Men and Apes. 

This essential harmony between the embryo of Man 
and of the other Mammals, in their whole bodily form and 
internal structure, exists even in that latest age of develop- 
ment, in which the mammalian body, as such, Is already 


unmistakable. (Cf. Plates VI. and VII., the second row.) 
But in a somewhat earlier stage, in which the rudi- 
ments of the limbs, the gill-arches, the sense-organs, etc., 
are already present, we cannot yet recognize mammalian 
embryos as such, nor can we distinguish them from the 
embryos of Birds and Reptiles. If we go back to still earlier 
stages of development, we are unable even to discover any 
distinction between the embryos of these higher Vertebrates 
and those of the lower, such as the Amphibia and Fishes 
(Plates VI., VII., upper row). Finally, if we go still further 
back, to the construction of the body from the four 
secondary germ-layers, we make the surprising discovery 
that these same four germ-layers exist, not only in all 
Vertebrates, but also in all the higher Invertebrates, and 
that they are everywhere concerned in the same way in 
forming the fundamental organs of the body. And if then 
we inquire into the origin of these four secondary germ- 
layers, we find that they develop from the two primary 
germ-layers, which are identical in all animals, with the 
exception of the lowest division, the Protista. (C£ Figs. 
23-28, p. 93.) Finally, we see that the cells, which compose 
the two primary germ-layers, universally originate by 
fission, from a single simple cell, from the egg-cell 

It is impossible to lay tpo much stress on this remark- 
able parallelism of the most important germ-conditions of 
man and animals. We shall afterwards turn the fact to 
account in support of the hypothesis of monophyletic 
descent, i.e., the assumption of the common, single line of 
descent of man and the higher animal tribes. It declares 
itself in the very beginning of the individual development ; 
in the cleavage of the egg-cell, in the formation of the 


germ-layers, in the fission of these, in the construction of the 
most important fundamental organs from these germ-layers, 
etc. The first rudiments of the principal parts of the body, 
and, above all, of the oldest main organ, the intestinal canal, 
are everywhere originally identical ; they always appear in 
the same simplest form. But all the peculiarities by which 
the various larger and smaller groups of the animal kingdom 
are differentiated from one another only make their appear- 
ance gradually, and secondarily, in the course of the evolu- 
tion of the germ ; and those which distinguish the animals 
most closely allied in the system of the animal kingdom 
are the latest to appear. This latter phenomenon can be 
formulated as a definite law, which may be regarded as, in 
some sense, an addition or appendage to the fundamental 
law of Biogeny. It is the law of the ontogenetic connection 
between systematically allied animal forms. The meaning 
of this is that the nearer two full-grown perfect animals are 
to each other in point of general body-structure, and hence 
the more closely they are allied in the system of the animal 
kingdom, the longer do their embryonic forms remain the 
same, and the longer are their embryos, and their young 
forms in general, either altogether indistinguishable, or dis- 
tinguishable only by subordinate characters. This law 
holds good of all animals in which the original form of evo- 
lution has been correctly inherited palingenetically, or by 
" inherited evolution ". Where, on the other hand, this ori- 
ginal form has been altered kenogenetically, or by " vitiated 
evolution," the law is less true in proportion as a greater 
aumber of new evolutionary conditions have been intro- 
duced by adaptation (cf. pp. 10-14). 101 

If we apply this law of the ontogenetic connection 



between systematically (and hence also phylogenetically) 
allied forms to Man, and if, with reference to this law, we 
rapidly run through the earliest human conditions, the first 
striking thing noticeable in the early history of the germ 
is the morphological identity of the egg-cells of Man and 
of other Mammals (Fig. 1). All the properties that cha- 
racterize the mammalian egg, are also observable in the 
human egg; especially that characteristic structure of its 
coating (the zona pelludda) which clearly distinguishes 
the mammalian egg from that of all other animals. When 

Fig. 121. — Lyre-shaped germ-shield of a dog. 
" Double shield " of Remak, " embryonic rudiment ' 
of other authors.) The dorsal furrow is visible in the 
centre ; on either side are the medullary swellings. 

the human embryo is fourteen days old, 
it, like a]l other mammalian embryos, is 
in the form of an entirely simple, lyre- 
shaped germ-shield. Along the middle 
line of the dorsal side of this, there ap- 
pears the rectilineal, groove-shaped medul- 
lary furrow, bordered by the two parallel 
dorsal, or medullary swellings. The ventral side is attached 
to the wall of the globular intestinal germ- vesicle. In this 
stage the human embryo is one line, or two millimetres 
in length. It is not distinguishable from that of other 
Mammals, e.g^ of the Dog (Fig. 121). 102 

A week later, or at the end of the twenty-first day, the 
human embryo has already attained twice this length : it is 
now two lines or about five millimetres in length., and already 
shows, when seen from the side, the characteristic curvature 
of the back, the swelling of the head end, the earliest rudi- 



ments of the higher sense-organs, and the rudiments of the 
gill-openings, piercing the sides of the neck (Fig. 122, III.  
Plate VII. Fig. M I.). The allantois has growr out from the 

Fig. 122. — Human germs or embryos from the second to the fifteenth 
week (natural size), seen from the left side, the arched back turned towards 
the right. (Principally after Ecker.) II., human embryo of 14 days ; III., of 
3 weeks; IV., of 4 weeks; V., of 5 weeks ; VI., of 6 weeks; VII., of 7 weeks; 
VIIL, of 8 weeks ; XII., of 12 weeks ; XV., of 15 weeks. * 

hind end of the intestine. The embryo is already entirely 
enveloped by the amnion, and is now only connected with 
the germ-vesicle, which is changing into the yelk-saa by 
means of the yelk-duct, in the centre of the abdomen. 


In this stage of development, the extremities, or limbs, 
are still entirely wanting ; there is as yet no trace either of 
arms or legs. The head end, however, has already become 
markedly distinct or differentiated from the tail end ; more- 
over, the first rudiments of the brain-bladders appear in 
front, and the heart appears more or less distinctly on the 
anterior intestine. A real face is, however, not yet formed. 
We may also search in vain for any character distinguishing 
the human embryo, in this stage, from that of other Mammals. 
(Cf. Fig. M I, B I., a I., and H I. on Plate VII.) 108 

Another week later, at the end of the fourth week, 
between the twenty-eighth and the thirtieth day of develop- 
ment, the human embryo is four or five lines in length, or 
about one centimetre (Fig. 122, IV. , Plate VII. Fig. M II.). 
The head with its various parts is now plainly distinguish- 
able : within, the five primitive brain-bladders (fore-brain, 
mid-brain, twixt-brain, hind-brain, and after-brain) ; at the 
lower end of the head, the gill-arches, which divide the 
gill-openings ; on the sides of the head the rudiments of 
the eyes, two indentations of the outer skin, towards which 
grow two simple bladders from the side- wall of the fore- 
brain. Far behind the eyes, above the last gill-arch, the 
bladder-like rudiment of the organ of hearing is visible. 
The head, which is very large, is attached to the trunk at 
a very considerable angle, almost a right angle. The trunk 
itself is still attached at the centre of its ventral side to the 
intestinal germ- vesicle ; but the embryo is already still 
further separated from the latter, which, therefore, protrudes 
and forms the yelk-sac. Like the front part, the hind part 
of the body is very much curved, so that the pointed tail 
end is turned towards the head. The head rests, face down- 



Fig. 123. — Human embryo of four weeks old, opened on the ventral side. 
The walls of the chest and abdomen have been cut away, so that the contents 
of the chest and ventral cavities are visible. All the appendages (amnion, 
allantois, and yelk-sac) have been removed, and also the middle portion of 
the intestine : n, eye ; 3, nose ; 4, upper jaw ; 5, lower jaw ; 6, the second 
gill-arch, and 6" the third ; o v, heart (o, right, o', left auricle ; v, right, v', 
left ventricle) ; b, origin of the aorta ; /, liver (u, navel-vein) ; e, intestine 
(with the yelk-artery, cut away at a') ; j', yelk-vein; m, primitive kidney; 
t, rudiments of the sexual glands; r, terminal intestine (with mesentery, 2, cut 
away) ; w, navel-artery ; w, navel-vein ; 7, anus ; 8, tail ; 9, front limb ; 9', 
hind limb. (After Coste.) 


Fio. 124. — Human embryo of five weeks old, opened on the ventral aide 
(as in Fig. 123). The chest and ventral walls, with the live A , have been 
removed; 3, outer nasal process ; 4, upper jaw ; 5, lower jaw ; z, tongue ; v, 
right, v', left ventricle of heart ; 0', left auricle ; b, origin of the aorta ; 
b' b" b"' y first, second, third arterial arches ; c, c', c", hollow veins (vena 
cava) ; ae, lungs (y, arteries of lungs) ; e, stomach ; w, primitive kidneys ; 
(y, left yelk- veins ; s, vena porta ; a, right yelk-artery ; n, navel-artery ; 
m, navel-vein) ; x, yelk-duct ; i, large intestine ; 8, tail ; 9, front limb ; 
9', hind limb. (After Coste.) 

ward, on the yet open chest. The curvature presently 
becomes so great that the tail almost touches the forehead 
(Fig. 122, V. ; Fig. 137). Three or four distinct curves of the 
arched dorsal side are now distinguishable ; a skull-curve or 
"front head-curve" near the second brain-bladder, a neck- 
curve or " hind head-curve " at the beginning of the spinal 
marrow, and a tail-curve at the hind end of the body. This 
marked curvature is shared by Man with the three 
higher classes of Vertebrates (the Amnion-animals), while 
in the lower classes it is either much less pronounced, or 
altogether absent. In this stage, when four weeks old, Man 
has a true tail, double the length of the legs. The rudi- 
ments of the limbs are now plainly marked : four entirely 
simple buds in the form of roundish plates, two fore limbs 
and two hind limbs, the former being a little larger than 
the latter. 104 

On opening the human embryo of the age of one month 
(Fig. 123), we find the intestinal canal already formed in the 
body-cavity, and that it is nearly completely separated 
from the germ-vesicle. The mouth-opening and anus 
already exist. The cavity of the mouth is, however, not 
yet separated from that of the nose, nor is the face in 
general yet formed. The heart, on the other hand, already 
shows all the four compartments ; it is very large, filling 


almost the entire chest-cavity (Fig. 123, ov). Behind it the 
very small rudiments of the lungs lie concealed. The 
primitive kidneys are very large (Fig. 123, m), occupying 
the greater part of the ventral cavity, and extending from 
the liver (/) to the pelvic intestine. Thus at the end of 
the first month, all the essential parts of the body are 
already begun ; and yet, in this stage, we are still unable 
to discern any characters essentially distinguishing the 
human embryo from those of the Dog, the Rabbit, the 
Ox, the Horse, or, indeed, of any of the higher Mammals. 
All these embryos are still of the same form, and at best 
differ from the embryo of Man only in the general dimen- 
sions of the body, or in the size of the individual organs — 
differences of no moment. Thus, for example, the head, 
relatively to the trunk, is a little larger in Man than in the 
Sheep; in the Dog the tail is somewhat longer than in 
Man. But these are all, evidently, very trifling differences 
indeed, and of no importance. On the other hand, the 
whole internal and external organization, the form, the 
disposition, and the connection of the separate parts of the 
body of the germ are essentially the same in the human 
embryo of four weeks, and in the embryos of other 
Mammals in a corresponding stage of development. 

But the case is different even in the second month of 
human development. Fig. 122 represents a human germ, 
VX, of six weeks, VII., of seven weeks, VIII., of eight 
weeks, in the natural size. The differences which distin- 
guish the human embryo from those of the Dog and the 
lower Mammals, now gradually begin to become more 
prominent. Even after the sixth, and yet more after the 
eighth week, considerable differences are visible, especially 


in the structure of the head (Plate VII. Fig. M III., etc). 
The size of the various divisions of the brain in Man is 
now greater, while, on the contrary, the tail appears shorter. 
Other differences between Man and the lower Mammals are 
to be seen in the relative dimensions of the interior parts. 
Yet even now the human embryo is hardly distinguishable 
from that of the nearest allied Mammals, the Apes, and 
especially the anthropomorphic Apes. The characters which 
distinguish the human embryo from those of Apes make their 
appearance much later ; even in a very advanced stage of 
development, in which the human embryo is instantly 
distinguishable from that of hoofed animals ( Ungulata), the 
former is still very similar to the embryo of the higher 
Apes. At length, in the fourth or fifth month these charac- 
ters make their appearance, and during the four last months 
of the embryonic life of the human being, from the sixth 
to the ninth month of pregnancy, the human embryo is 
readily distinguishable from those of all other Vertebrates ; 
then the characters which distinguish the various races of 
mankind also make their appearance, especially those in 
the structure of the skull. 

The striking resemblance which exists for a long time 
between the embryos of Man and of the higher Apes dis- 
appears, moreover, at a much earlier period in the lower 
Apes. It is naturally retained longest in the large anthro- 
pomorphic Apes (the Gorilla, Chimpanzee, Orang-outang, and 
Gibbon; Plate XIV.). The facial resemblance, which strikes us 
in these man-like Apes, continually decreases with age. On 
the other hand, it is retained throughout life by the remark- 
able Nose-apes (Semnopithecus nasicus) of Borneo (Fig. 125), 
the well-shaped nose of which might well be coveted by men 


in whom this organ is too short. On comparing the face of 
this nosed monkey with that of specially ape-like human 
beings {e.g., the noted Julia Pastrana, Fig. 126), the 

Fig. 125. — Head of a nose-ape (Semnopithecus nasicus) from Borneo. 
(After Brehm.) 

Fig. 126. — Head of Julia Pastrana. (From a photograph by Hintze.) 

former will appear a higher form of development than the 
latter. There are very many persons who believe that the 
"image of God" is unmistakably reflected in their own 
features. If the Nosed-ape shared in this singular opinion, 
he would hold it with a better right than some snub-nosed 
people. 105 

This gradually progressive separation, this increasing 
divergence of the human from the animal form, which 
depends on the law of the ontogentic connection between 
systematically allied forms, is seen not only in the external 
structure of the body, but also in the formation of the 
internal organs. It is even expressed in the formation 
of the coverings and appendages that are found round the 
outside of the embryo, and which we are now about to 
consider somewhat more in detail. Two of these appen- 
dages, the amnion and the allantois, belong only to the 


three higher vertebrate classes, while the third, the yelk- 
sac, occurs in most Vertebrates. This circumstance is very 
significant, and we shall afterwards find that it affords 
material assistance towards the construction of the 
genealogical tree of Man. 

The nature of the outer egg-membrane, which surrounds 
the entire egg embedded in the uterus of the Mammal, is 
the same in Man as in the higher Mammals. At first the 
egg is surrounded, as we have already stated, by the trans- 
parent, structureless zona pellucida (Fig. 1, p. 122, and Fig. 
36-40, pp. 210-212). Very soon, however, even in the first 
week of development, its place is taken by the permanent 
tufted membrane (chorion). This originates from the outer 
fold of the amnion, from the so-called serous membrane, 
the formation of which we shall presently examine. It is 
;ormed of a single stratum of cells from the outer germ- 
layer, the skin-sensory layer At its first appearance the 
serous membrane is an entirely simple, flat, closed vesicle , 
like a wide sac, closed in all directions, it surrounds the 
embryo with its appendages; the intermediate space be- 
tween the two is filled with clear watery fluid. At an 
early period, however, the smooth outer surface of the sac 
becomes covered with numerous small tufts or knots, which 
are really hollow processes, resembling the fingers of a glove 
(Fig. 127 ; 139, 4 sz, 5 chz). These branch and grow into 
the corresponding depressions formed by the bag-like glands 
of the mucous membrane of the maternal uterus ; the egg 
thus acquires its permanent, fixed position (Figs. 130, 132, 

In the human egg, even between the thirteenth and 
fourteenth day, this outer egg-membrane, which we shall 



Fig. 127. 


Fig. 128. 

Fig. 130. 

Fig. 131. 

Fig. 127- — Human egg between the twelfth and thirteenth day. After 
Allen Thomson. 1. Not opened ; natural size. 2. Opened, and enlarged. 
Within the outer tufted membrane (chorion) the small curved germ lies upon 
the left of the upper side of the large intestinal germ. vesicle. 

Fig. 128. — Human egg on the fifteenth day. After Allen Thomson. 
Natural size, and opened. The small germ lies in the upper right-hand part 
of the right half. 

Fig. 129. — Human germ oo the fifteenth day, taken from the egg; 
enlarged : a, yelk-sac ; b, region ot the neck (where the medullary furrow is 
already closed) ; c, head part (with open medullary furrow) ; d, hind part 
(with open medullary furrow) ; e, a shred of the amnion. 

Fig. 130. — Human egg between the twentieth and twenty-second day. 
After Allen Thomson. Natural size ; opened. The outer tufted membrane 
(chorion) forms a capacious vesicle, to the inner wall of which the small 
germ (above, on the right) is attached by a short navel-cord. . 

Fig. 131. — Human germ between the twentieth and twenty-second day, 
taken out of the egg; enlarged : a, amnion ; ?>. yelk-sac ; c, lower jaw process 
of the urst gill-arch; d, upper jaw process ot' the same; e, second gill-arch 
(behind it are two other small arches). Three gill-openings are very plainly 
seen j /, rudiments of the fore-limbs ; g, ear-vesicle ; h, eye ; i, heart. 



briefly call the tufted membrane {chorion), is completely 
covered with small knots or tufts, and forms a globe or 
sphere of 6-8 millimetres in diameter (Figs. 127-129.) In 
consequence of the accumulation of a large mass of liquid 
in the inside, the tufted membrane {chorion) continually 
increases in size, so that the embryo occupies only a small 
part of the space within the egg-bladder. At the same time 
the tufts on the chorion increase in number and size, and 

Fig. 132. — Human embryo, with amnion and allantois, in the third week ; 
with a large globular yelk-sac (below) and a bladder-like allantois (right) ; 
there are as yet no limbs. The germ and its appendages are surrounded by 
the tufted membrane (chorion). 

Fig. 133. — Human embryo, with amnion and allantois, in the fourth 
week. (After Krause.) The amnion (w) lies pretty close to the body. The 
greater part of the yelk-sac (d) has been torn away. Behind this the 
allantois (I) is visible, as a pear-shaped vesicle of considerable size. Arms 
(/) and legs (b) are just beginning; v, fore-brain; z, twixt-brainj m, 
mid-brain ; h, hind-brain  n, after-brain ; a, eye ; fe, three gill-arches j c, 
heart ; s, tail. 



become more branched. Though these tufts at first covered 
the whole surface, they afterwards degenerate over a great 
part of this ; they develop in consequence all the more 
vigorously at a particular point, at the place where the 
allantois forms the placenta. 

On opening the chorion of a human embryo of three 

Fig. 134. — Human embryo with its membranes, six weeks old. The outer 
covering of the embryo forms the chorion, which is covered with numerous 
branching tufts, and is lined internally by the serous membrane. The embryo 
is surrounded by the delicate membrane of the amnion-sac. The yelk-sac is 
reduced to a little pear-shaped navel-vesicle ; its thin stalk, the long yelk- 
duct, is enclosed in the navel-cord. In this cord, behind the yelk-duct, lies 
the much shorter stalk of the allantois, the inner layer of which (intestinal- 
glandular layer) in Figs. 132 and 133 presented a bladder of considerable 
size ; while the outer layer attaches itself to the inner wall of the outer 
egg-membrane, and at this point forms the placenta. 


weeks old, we find a large, round sac, filled with liquid, 
on the ventral side of the germ. This is the yelk-sac, the 
so-called navel- vesicle, the origin of which we have already 
examined (Figs. 132, 133). In proportion as the embryo 
grows larger, the yelk-sac grows smaller. At a later period 
it hangs, as a small pear-shaped vesicle, at the end of a long 
stalk (the yelk-duct), from the abdomen of the embryo 
(Fig. 139, 5 ds), and is finally detached from the body by the 
closing of the navel. The wall of this navel- vesicle consists, 
as we have seen, of an inner layer, the intestinal-glandular 
layer, and an outer layer, the intestinal-fibrous layer. It is 
therefore composed of the same constituents as the intestinal 
wall itself, of which it forms, in fact, a direct continuation. 
In Birds and Reptiles the yelk-sac is much larger, and con- 
tains a considerable quantity of albuminous and fatty nutri- 
tive matter. This penetrates through the yelk-duct into the 
intestinal cavity and serves as food. In Mammals the yelk- 
sac plays a much smaller part in the nourishment of the 
germ, and degenerates at an early period. The relation of 
the intestine to the yelk sac has very often been entirely 
mistaken According to the Gastrsea Theory the two form 
one whole. We may say that the primitive intestine of 
those Vertebrates which are without a skull afterwards 
separated in their descendants (in consequence of the 
accumulation of nutritive yelk) into two parts, a transitory 
embryonic organ (the yelk-sac), and a permanent intestine 
(the after-intestine). 

Behind the yelk-sac, a second and much more significant 
appendage forms, at an early period, on the abdomen of the 
vertebrate embryo. This is the allantois, or primitive 

urinary sac, an important embryonic organ, which occur* 

3 So 


only in the three higher classes of Vertebrates. It grows 
from the hind end of the intestinal canal, from the pelvic 
intestinal cavity (Figs. 133, 1, 135, r, u, 136, p, 139, at). Its 

r g 


r 7i 7i t>o 7 £ i 

Fig. 135. — Longitudinal section through the embryo of a Chick (in the 
fifth day of incubation). The embryo with curved dorsal surface (black): 
d, intestine ; o, mouth ; a, anus ; ?, lungs ; h, liver ; g, mesentery ; v, auricle ; 
k, ventricle ; b, arterial arches ; t, aorta ; c, yelk-sac ; m, yelk-duct ; w, 
allantois ; r stalk of allantois ; n, amnion ; w, amnion-cavity ; s, serous 
membrane. (After Baer.) 

first rudiment appears as a small vesicle on the edge of the 
pelvic intestinal cavity, representing an extension of the 
intestine, and therefore (like the yelk-sac) has a two-layered 
wall. The cavity of the vesicle is coated by the intestinal- 
glandular layer, and the outer lamella of the wall is formed 
by the thickened intestinal-fibrous layer. The small vesicle 
grows larger and larger, and forms a sac of considerable size, 
filled with liquid, and in the wall of which large blood- 
vessels form. It soon reaches the inner wall of the egg- 



cavity, and spreads itself out on the inner surface of the 
e irorion. In many Mammals the allantois becomes so large 
that it finally surrounds the whole embryo with its other 
appendages, as a great covering, and extends over the whole 
inner surface of the egg-membrane. On cutting such an 
egg, the first thing met with is a large space filled with 

Fig. 136. — Embryo of Dog, twenty-five days old, opened on the ventral side 
(as in Figs. 134 and 135). Chest and ventral walls have been removed: a, 
nose-pits; b, eyes ; c, under-jaw (first gill-arch) ; d, second gill-arch ; efg h, 
heart (e, right, /, left anricle ; g, right, h, left ventricle) ; i, aorta (origin of ) ; 
kk, liver (in the middle between the two lobes is the cut yelk-vein); I, 
stomach ; m, intestine ; n, yelk-sac ; 0, primitive kidneys : p, allantois ; q, 
fore-limbs ; It, hind-limbs. The crooked embryo has been stretched straight. 



fluid ; this is the allantois cavity, and it is only after the 
removal of this membrane that the real embryonic body, 
which is enclosed in the amnion, is found. 

In Man, the allantois does not attain so great a size, 
but losing its vesicular form, changes into the placenta 
soon after it has reached the inner wall of the chorion. 

Fig. 137. — Embryo of a Dog, from the right side : a, the firat brain, 
bladder; b, second; c, third.; d, fourth; e, the eye; /, the ear-vesicle; gh, 
first gill-arch (g, lower jaw, h, upper jaw) ; i, second gill-arch ; klm, heart 
(k, right auricle ; I, right ventricle ; in, left ventricle) ; n, beginning of the 
aorta ; o, heart pouch ; p, liver ; q, intestine ; r, yelk-duct ; s, yelk-sac (torn 
away) ; t, allantois (torn away) : u, amnion ; v t fore-limb ; a?, hind-limb. 
(After Bischoff.) 

Yet even in Man the first rudiment of the allantois is a 
stalked pear-shaped bladder (Fig. 133, I), just as in other 
Mammals. J stated this in 1874, in the first and second 


editions of this book, and explained it in the drawing now 
given in Fig. 137. I based the statement on a very apt 
deduction. For as the general form and the finer structure 
of the placenta is entirely similar in Man and in Apes, 
the origin of the organ could not be different in the two 
cases. As, however, the bladder-like form of the allantois 
of the human being had never been directly observed, I was 
gravely accused by Wilhelm His of falsifying science. His 
stated that " it is known that the allantois in the human 
being is never seen in the bladder-like form " (!). Luckily 
for me, this " never visible " bladder form was actually seen 
by Professor Krause of Gottingen in the following year 
(1875), and a drawing of it, reproduced in Fig. 133, was 
given. 106 

When the bladder-shaped human allantois has reached 
the inner wall of the tufted membrane (chorion), spreading 
itself flatly over the latter, it forms the placenta, which is 
very important to the nourishment of the germ. The stalk 
of the allantois, which connects the embryo with the 
placenta, and carries the large blood-vessels of the navel 
from the former to the latter, is enveloped by the amnion, 
and, together with the amnion-sheath, forms the so-called 
navel-cord (Fig. 138, a s). The large network of blood- 
filled vessels of the embryonic allantois attaches itself 
closely to the mucous membrane of the maternal uterus, 
and the partition wall between the blood-vessels of the 
mother and those of the child grows very much thinner, 
thus giving rise to the remarkable apparatus for nourishing 
the embryonic bod^ which we call the placenta, and to 
which we shall refer hereafter. (Cf. Chapter XIX.) At 
present, I will speak of it only in connection with the fact 



that it appears exclusively in the higher Mammals, not in 
the lower. Of the three sub-classes or principal groups of 
the Mammals, the two lower groups, the Beaked Animals 

A z- 

Fig. 138. — Egg-membranes 
of the human embryo (diagram- 
matic) : m, the thick, fleshy wall 
of the uterus ; plu, placenta (of 
which the inner layer (plu') 
sends processes in between the 
tufts of the chorion (chz) ; clif, 
tufted chorion ; chl, smooth cho- 
rion ; a, amnion ; ah, amnion- 
cavity ; as, amnion-sheath of 
the navel-cord (which passes 
below into the navel of the em- 
bryo, not represented here) ; 
dg, yelk-duct ; ds, yelk-sac ; 
dv y dr, decidua (dv, true, dr, 
false decidua). The cavity of 
the uterus {uh) opens below 

into the sheath (vagina), above, on the right, into the oviduct (t). (After 


(Ornithostoma) and Pouched Animals (Marsupialia), have no 
placenta, the allantois remaining a simple bladder, filled 
with fluid, as* in Birds and Reptiles. Only in the third and 
most highly developed mammalian sub-class, the Placental 
Animals, is a true placenta developed from the allantois. 
To the placental sub-class belong the Hoofed Animals, 
Whales, Beasts of Prey, Insect-eating Animals, Rodents 
Bats, Apes, and Men. This circumstance is direct evidence 
that man has developed from this group of Mammals. 

In connection with the line of descent of the human 
race, the allantois is, therefore, of twofold interest : firstly, 
because this appendage is entirely wanting in the lower 
classes of Vertebrates, and is developed only in the three 



Fig. 139.— Five diagrammatic longitudinal sections through the develop- 
ing mammalian germ with its egg-coverings. In Fig. 1-4 the longitudinal 
section is through the sagittal plane or the middle plane of the body, which 


separates the right and left halves ; in Fig. 5 the germ is seen from the left 
side. In Fig. 1, the prochorion (d), studded with tufts (d'), surrounds the 
germ-vesicle, the wall of which is composed of the two primary germ- 
layers. Between the outer (a) and the inner (i) germ-layer within the 
limits of the germ-area (area germinativa) the middle germ-layer (mesodtrma, 
m) has developed. In Fig. 2, the embryo (e) is already beginning to separate 
from the germ-vesicle (ds), and the wall of amnion-fold is beginning to rise 
round the embryo (in front as the head-sheath, ks, behind as the tail-sheath, 
ss.) In Fig. 3, the edges of the amnion-fold (am) meet over the back of the 
embryo, thus forming the amnion-cavity (ah) ; in consequence of the further 
separation of the embryo (e) from the germ-vesicle (ds), the intestinal- 
canal (dd) originates, and from the hind end of this the allantois (al) grows 
out. In Fig. 4, the allantois (al) is bigger ; the yelk-sac (ds) is smaller. 
In Fig. 5, the embryo already shows the gill-openings and the rudiments of 
the two pairs of limbs ; the chorion has formed branched tufts. In all five 
figures, e indicates embryo ; a, outer germ-layer ; m, middle germ-layer ; 
i, inner germ-layer ; am, amnion ; (ks, head-sheath ; ss, tail-sheath) ; ah, 
amnion-cavity ; as, amnion-sheath of the navel-cord ; kh, intestinal germ- 
vesicle ; ds, yelk-sac ; dg, yelk-duct ; df, intestinal -fibrous layer ; did, in- 
testinal-glandular layer ; al, allantois; vl=hh, region of the heart; d, yelk- 
membrane or prochorion; d', tufts of the latter; sh, serous covering; s$, 
tufts of the latter; ch, tufted membrane or chorion; chM, tufts of the 
latter ; st, terminal vein ; r, cavity, filled with liquid, between the amnion 
and chorion. (After Kolliker.) (Cf. PI. V. Fig. 14 and 15.) 

higher classes, in Reptiles, Birds, and Mammals; and, secondly, 
because the placenta is developed from the allantois only in 
the higher Mammals, including Man, and not in the lower 
Mammals. The former are therefore called " Placental 
Animals " (Pldcentalia). 

Another characteristic common to the three higher classes 
of Vertebrates alone, is the formation of the third appendage 
of the embyro, the amnion, which has already been men- 
tioned. We have already learned something of the amnion 
in noticing the separation of the embryo from the intestinal 
germ-vesicle. We found that the walls of the latter rise in 
a ring-shaped fold round the embryonic body. In front, this 
fold appears in the form of the so-called head-cap, or heatf- 


sheath (Fig. 139, 2 Jcs) ; at the back, it also arches upward 
and forms the tail-cap, or tail-sheath (Fig. 139, 2 ss) ; on the 
right and left sides, the fold is at first lower, and is here 
called the side-caps, or side-sheaths (Fig. 140; Figs. 95, 96, af, 
p. 317). All these caps or sheaths are only parts of a con- 

Fig. 140. — Transverse section through an embryonic Chick (a little 
behind the anterior opening of the intestine), at the end of the first day of 
incubation. The medullary furrow above and the intestinal funxnv below 
are still wide open. At each side, the rudiment of the body-cavity (cceloma) 
can be seen between the skin-fibrous layer and the intestinal-fibrous layer. 
On the right and left of it, at the outside, the side-caps of the amnion are 
beginning to rise. (After Remak.) 

nected ring-like fold, which passes round the embryo. This 
grows higher and higher, rises like a great encircling wall, and 
finally arches over the body of the embryo, so as to form a 
cavern-like covering over the latter. The edges of the ring- 
like fold meet and coalesce (Figs. 141, 142). The embryo, 
thus, at last lies in a thin membranous sac, filled with the 
amnion-fluid (Fig. 139, 4 , 5 ah). 

When the sac is completely closed, the inner layer of the 
fold, which forms the real wall of the sac, withdraws com- 
pletely from the outer layer. The latter attaches itself to 
the inside of the outer egg-membrane (" prochorion "). It 
supplants this prochorion, and forms the permanent tufted 
membrane, the true " chorion." This arises solely from 
the horn-plate (Fig. 139, 4 ah). The thin wall of the 
amnion-sac, on the other hand, consists of two strata : of an 
inner stratum, the horn-plate, and of an outer stratum, the 



skin-fibrous layer (Figs. 141, 142). The latter is indeed here 
very thin and delicate, but yet can be distinctly shown to 
be a direct continuation of the leather-skin (corium), and 

Fig. 141. — Transverse section through an embryonic Chick in the navel 
region (at the fifth day of incubation). The amnion-folds (am) almost 
meet over the back of the embryo. The intestine (d), still open, passes 
below into the yelk-sac : df, intestinal-fibrous layer ; sh, notochord ; sa, 
aorta; vc, principal veins; bh, ventral cavity, not yet closed; v, anterior, 
g, posterior nerve-roots of the spinal marrow ; rait, muscle-plate ; hp, 
leather-plate ; h, horn-plate. (After Remak.) 

is, therefore, the outermost layer arising from the fission of 
the middle germ-layer (mesoderma). Thus the outer 
peripheric portion of the skin-fibrous layer clothes only the 
inner lamella of the amnion-fold (the head-sheath, tail- 
sheath, etc.), and extends only to the edge of the fold itself. 
The outer lamella is formed entirely by the horn-plate, and 



it produces the tufted chorion, the hollow, branched tufts of 
which grow into the depressions in the mucous membrane 
of the maternal uterus. 


Fig. 142. — Transverse 
section through an em- 
bryonic Chick in the 
shoulder region (at the 
fifth day of incubation). 
The section passes midway 
between the rudiments of 
the anterior limbs (or 
wings, E). The amnion- 
folds have grown com- 
pletely together over the 
back of the embryo. 
(After Remak.) (Com- 
pare, as regards other 
points, with Figs. 139, 140, 
and 141, and Plate V. 
Fig. 14.) 

Tn human Phylogeny the amnion is particularly in- 
teresting, because it is a peculiar characteristic of the three 
higher classes of Vertebrates. Mammals, Birds, and Reptiles 
alone possess it, and therefore these three classes are 
grouped together under the name of Amnion Animals, or 
Amniota. All Amnion Animals, including Man, are de- 
scended from a common parent-form. All the lower Verte- 
brates, on the contrary, entirely want this amniotic formation. 

Of the three bladder-like appendages of the embryo just 
mentioned, the amnion has no blood-vessels at any period 
of its existence. On the contrary, the two other bladders, 
the yelk-sac and the allantois, are provided with large blood- 
vessels, which accomplish the nutrition of the embryonic 


body. Here we may speak of the first circulation of blood 
in the embryo, and of its central organ> the heart. The first 
blood-vessels and the heart, as well as the first blood itself, 
develop from the intestinal-fibrous layer. On this account 
the latter was called the vascular layer by the earlier em- 
bryologists. In a certain sense this name is quite correct ; 
only it must not be understood to imply that ail the blood- 
vessels of the body proceed from this layer, or that the 
whole of the vascular layer is applied only to the formation 
of the blood-vessels. Neither is the case. The intestinal- 
fibrous layer also forms, as we saw, the whole fibrous and 
muscular wall of the intestinal tube, and also the mesentery. 
We shall presently find that blood-vessels can form in- 
dependently from other parts, especially in the various parts 
proceeding from the skin-fibrous layer. 

The heart and blood-vessels, and the whole vascular 
system, are by no means among the oldest parts of the 
animal organism. Aristotle assumed that the heart of 
the Chick was formed first ; and many later authors have 
shared this view. This is, however, by no means the case. 
On the contrary, the most important parts of the body, the 
four secondary germ-layers, the intestinal tube, and the 
notochord, are already formed before the first indication of 
the blood-vessel system appears. This fact, as we shall after- 
wards find, is in complete harmony with the Phylogeny of 
the animal kingdom Our older animal ancestors possessed 
neither blood nor heart. 

We have already examined the first blood-vessels of the 
mammalian embryo in transverse sections. They are, first, 
the two primary arteries, or " primitive aortae," which lie 
in the narrow longitudinal cleft between the primitive 


spinal cords, the side-plates, and the intestinal-glandular 
layer (Figs. 92, ao, Fig. 95, 96, ad) ; and, second, the two 
principal veins, or " cardinal veins," which appear somewhat 
later, outside the former, above the primitive kidney ducts 
(Fig. 96, vc, Fig. 141, vc). The primitive arteries seem to 
arise by fission from the inner parts of the intestinal-fibrous 
layer ; the primitive veins, on the contrary, from the outer 
parts of the same layer. 

In just the same way, and in connection with these first 
blood-vessels, the heart also arises from the intestinal- 
fibrous layer, in the lower wall of the anterior intestine, 
near the throat, at the place where the heart remains 
throughout life in Fishes. Perhaps it will not seem very 
poetic that the heart develops directly from the intestinal 
wall. But the fact cannot be altered, and is also easily 
comprehensible phylogenetically. The Vertebrates are, at 
any rate, in this respect more aesthetic than the Mussels, 
in these the heart remains permanently lying behind on the 
wall of the rectum near the anus, so that the heart seems to 
be penetrated by the rectum. 

Midway between the gill-arches of the two sides of the 
head, and rather further back, at the throat of the embryo, a 
wart-like thickening of the intestinal-fibrous layer develops 
on the lower wall of the intestinal head cavity (Fig. 143, df). 
This is the first rudiment of the heart. This swelling is 
spindle-shaped, at first quite solid, and is formed entirely of 
cells of the intestinal-fibrous layer. It afterwards, however, 
curves in the form of an S (Fig. 144, c), and a little hollow 
is formed in its centre, in consequence of the accumulation 
of a small quantity of fluid between the central cells. 
Some single cells of the wall separate from the rest and 




float about in this fluid. These cells are the first blood- 
cells., and the fluid is the first blood. In the same way 

Fig. 143. — Longitudinal section through the head of an embryonic Chick 
(at the end of the first day of incubation) : m, medullary tube; ch, noto- 
chord ; d, intestinal tube (with a blind anterior end) ; k, head-plates ; df, 
first rudiment of the heart (in the intestinal-fibrous layer of the ventral 
wall of the head intestine) ; hh, cavity for the heart ; hk, membrane of the 
heart ; kk, head-cap of the amnion ; ks, head-sheath ; h, horn-plate. (After 

Fig. 144. — Human embryo, of 14 to 18 days, opened at the ventral side. 
Under the forehead-process of the head (£) the heart (c) appears in the 
heart-cavity (p) with the base of the aorta (b). The greater part of the 
yelk-sac (o) has been removed (at x, the opening of the anterior intestine) ; 
g, the primitive aortse (lying under the primitive vertebrae) ; t, terminal 
intestine, or large intestine ; a, allantois (u, its stalk) ; v, amnion. (After 



blood arises in the first rudimentary blood-vessels con- 
nected with the heart. These also, at first, are solid, round 

Fig. 145. — Transverse section through the head of an embryonic Chick 
of 36 hours. Below the medullary tube, the two primitive aortss (pa) are 
visible in the head-plates (s) on both sides of the notochord. Below the 
throat (d) can be seen the aortal-end of the heart (ae) ; Jih, heart-cavity ; 
hk, heart membrane ; &s, head-sheath, amnion-fold ; h, horn-plate. (After 

Fig. 146. — Transverse section through the heart-region of the same 
Chick (further back than the former). In the heart-cavity (hh), the heart 
(7i) is still connected by a heart -mesentery (hg) with the intestinal-fibrous- 
layer (df) of the anterior intestine : d, intestinal-glandular layer; up, 
primitive vertebral plates; g~b, rudiment of the ear-vesicle in the horn- 
plate ; hp, first rising of the amnion-fold. (After Bemak.) 

cords of cells. They then become hollow, while a fluid 
separates and gathers in the centre, and single cells detach 
themselves from the rest and become blood-cells. This is 
equally true of the arteries, which carry the blood from the 


heart, and of the veins, which carry the blood back to the 

At first, the heart lies within the intestinal wall itself, 
from which it has" developed, as do the first main blood- 
vessels proceeding from it. The heart itself is in reality 
only a local extension of one of these main blood-vessels. 
Soon, however, the heart separates from its place of origin, 
and now lies freely in a cavity, called the heart-cavity 
(Figs. 145, hh, 146, hh). This heart-cavity is merely the 
anterior part of the body-cavity (coeloma), which, as a 
horseshoe-shaped arch, connects the right and left divisions 
of the coelom (Fig. 140). The wall of the heart-cavity is 
therefore formed, like that of the remainder of the body- 
cavity, partly by the intestinal-fibrous layer (Fig. 146, df), 
and partly by the skin-fibrous layer (hp). While the heart 
is separating from the anterior intestine, it remains for a 
short time attached to the latter by a thin plate, a heart- 
mesentery (Fig. 146, kg). It afterwards lies quite freely in 
the heart-cavity, and is directly connected with the intestinal 
wall only by the main blood-vessels which pass from it. 

The anterior extremity of this spindle-shaped heart- 
formation, which soon assumes a curved, S-shaped form, 
divides into a right and a left branch. These two tubes 
are arched and curved upward, and represent the two first 
aortse-arches. They mount up in the wall of the anterior 
intestine, which, in a measure, they encircle, and they there 
unite above at the upper wall of the intestinal head-cavity 
in one large single main artery, which passes backward 
immediately under the notochord, and which is called the 
main aorta (aorta principalis, Fig. 147, a). The first paii 
of aortae-arches passes up on the inner wall of the first pair 



of gill-arches, and lies, therefore, between the first gill-arch 
(k) on the outside, and the anterior intestine (d) on the 

Fig. 147. — Diagrammatic 
transverse section through 
the head of an embryonic 
Mammal : h, horn-plate ; m, 
medullary tube (brain-blad= 
der) ; rar, wall of the latter ; 
7, leather-plate; s, rudiment- 
ary skull; ch, notochord; 
~k, gill-arch ; mp, muscle, 
plate ; c, heart-cavity, an- 
terior part of the body- 
cavity (cceloma) ; d, in- 
testinal tube ; del, intes- 
tinal-glandular layer ; df, 
intestinal-muscle plate ; Jig, 
heart -mesentery ; Jiw, heart- 
wall; hk, ventricle; ah, 
aorta-arches ; a, transverse 
section through the aorta. 

inside,— just as these vascular arches are situated in adult 
fishes throughout life. The single main aorta, which results 
from the union above of these two first vascular arches, 
soon again divides into two parallel branches, which pass 
backward on both sides of the notochord. These are the 
primitive aortse, which have been already spoken of; they 
are also called posterior vertebral arteries (arterice verte- 
brates posterior es). These two main arteries send out on 
each side from four to five branches at right angles, which 
pass from the body of the embryo into the germ-area, and 
are called the omphalic-mesenteric arteries {arterice omphalo- 
mesentericaz), or the yelk-arteries (arterial vitellince). 
They represent the first rudiments of a circulation within 
the germ-area. The first blood-vessels, therefore, pass out 

from the body of the embryo and extend to the edge of 

28 " 



the germ-area. Numerous blood-vessels form in the intes- 
tinal-fibrous layer of the germ-area. They are at first 
confined to the dark germ-area, or the so-called " vascular 

Fig. 148. — Canoe-shaped 
germ ,qf a Dog, from the 
ventnjfc j side ; enlarged 
about?fB times. In front, 
below the forehead, the 
first pair of gill-arches are 
visible ; below these is the 
S-shaped bent heart, close 
by, and on either side of 
which lie the two ear-vesi- 
cles. Posteriorly, the heart 
divides into the two yelk- 
veins, which spread them- 
selves over the germ-area 
(the greater part of this has 
been torn away). At the 
bottom of the open ventral 
cavity the primitive aortse 
lie between the primitive 
vertebras, and from which 
five pairs of yelk-arteries 
proceed. (After Bischoff.) 

area " (area opaca, or area vasculosa) ; but they afterwards 
extend over the whole outer surface of the intestinal germ- 
vesicle. The whole yelk-sac, finally, seems to be enveloped 
in a network of blood-vessels. It is the function of these 
blood-vessels to collect food-material from the contents of 
the yelk-sac and carry it to the body of the embryo. This 
is done by veins, by blood-vessels leading back, which pass 
in at the posterior opening of the heart, first from the germ- 
area and later from the yelk-sac. These veins are called 
yelk -veins (venw vitelline) ; they are also often called 
omphalic-mesenteric veins {venae omphalo-mesentericai). 



Thus the first circulatory system of the blood in the 
embryo (Figs. 148-150) occurs in all the higher classes of 

Fig. 149. — Embryo and germ -area of a Rabbiv, in which, the earliest 
rudiments of the blood-vessels appear,- -seen from the ventral side (magni- 
fied about ten times). The posterior end of the simple heart (a) divides 
into two large yelk-veins, which form a network of blood-vessels on the 
dark germ-area (which appears light on the black background). At the 
head extremity the fore-brain with the two eye- vesicles (bb) may be seen. 
The dark centre of the germ is the wide-open intestinal cavity. Ten 
primitive vertebrae are visible on each side of the notochord. (After 

Vertebrates in the following simple order. The very simple 
pouch-shaped heart (Fig. 150, d) divides both in front and 
behind into two vessels. Those at the back are veins 
leading to the heart. They take food-material from the 
germ- vesicle, or yelk- sac, and carry it to the body of the 



embryo. The vessels passing from the heart in front are the 
gill-arch arteries, leading from the heart, and which, rising 
as aorta-arches, encircle the anterior end of the intestine, 
and unite in the main aorta {aorta principalis). The two 
branches, which result from the division of this main artery, 

Fig. 150. — Embryo and germ-area of a Rabbit, in which the first system 
of blood-vessels is complete, — seen from the ventral side (magnified about 
five times). The posterior end of the heart (cZ), which is curved in the form 
of an S, divides into two large yelk-veins, each of which sends out an 
anterior branch (b) and a posterior branch (c). The ends of these unite in 
the circular boundary vein, or terminal vein (v. terminalis) (a). In the germ- 
area may be seen the coarser venous network (lying below), and the finer 
arterial network (lying nearer the surface). The yelk-arteries (/) open 
into the two primitive aortse (e). The dark area which surrounds the head 
like a halo, represents the recess within the head-cap or membrane. 
(After Bischoff.) 


the primitive aortse, send out right and left the yelk-arteries, 
which leave the body of the embryo and pass into the germ- 
area. Here, and in the circumference of the navel- vesicle, 
two layers of vessels are distinguishable — the superficial 
arterial layer, and the lower venous layer. The two are 
connected together. At first this system of blood-vessels 
is extended only over the superficial front of the germ-area 
as far as the edge. Here, on the edge of the dark vascular 
area, all the branches unite in a large terminal vein (vena 
terminalis, Fig. 150, a). This vein disappears at a later 
period, as soon as ; in the course of development, the for- 
mation of blood-vessels progresses further, and then the 
yelk- vessels traverse the whole yelk-sac. When the navel- 
vesicle degenerates, these vessels, of course, also degenerate, 
being of importance only in the first period of embryonic 

This first circulation in the yelk-sac is replaced, at a 
later period, by the second circulation of blood in the 
embryo, that of the allantois. Large blood-vessels are 
developed on the wall of the primitive urinary sac, or 
allantois, from the intestinal-fibrous layer. These vessels 
giow larger and larger, and are most intimately connected 
with the vessels that develop in the body of the embryo 
itself. This secondary allantois circulation thus gradually 
takes the place of the original, primary, yelk-sac ciiculaticn. 
When the allantois has grown to the inner wall of the 
chorion, and has changed itself into the placenta, its blood- 
vessels alone accomplish the nourishment of the embryo. 
They are called navel-vessels (vasa umbilicalia), and are 
originally in pairs : one pair of navel arteries, and one pair 
of navel veins. The two navel-veins (vence umbilicale^ 


Figs. 123, u, 124, it), which carry blood from the placenta to 
the heart, open, at first, into the united yelk-veins. These 
last afterwards disappear, and the right navel-vein simul- 
taneously disappears entirely, so that a single great vein, 
the left navel-vein, alone remains, which carries all the 
nutritive blood from the placenta into the heart of the 
embryo. The two arteries of the allantois, or the navel- 
arteries (arterice umbilicales, Figs. 123, n, 124, ri), are merely 
the last, posterior extremities of the two primitive aortas, 
which are afterwards greatly developed. It is not until the 
end of the nine months of embryonic life, when the human 
embryo is born and enters the world as an independent 
physiological individual, that the navel circulation loses its 
significance. The navel cord (Fig. 138, as), in which these 
larger blood-vessels pass from the embryo to the placenta, 
is removed with the latter at the so-called "after-birth," 
and an entirely new circulation of the blood, limited to the 
body of the child, comes into operation simultaneously with 
pulmonary respiration. 107 

Now, if, in conclusion, we briefly review the germ- 
history of Man as far as we have traced it, and endeavour 
to comprehend the whole subject in one connected view, it 
seems desirable to divide it into several main sections, or 
periods, and these into subordinate stages, or steps. With 
reference to the phylogenetic significance of this history, 
which we shall next consider more closely, it seems to me 
most appropriate to make the four main divisions and ten 
sub-divisions as distinguished in the following pages, which 
correspond to the most important phylogenetic stages of 
development of our animal ancestors. (Cf. Table XXV. 
at the end of the nineteenth chapter.) This will again 


shew that the germ-history of Man (according to the law 
of abbreviated heredity) is very rapid and compressed in 
the first stages of its course, but grows slower and slower 
in each succeeding stage. All the remarkable phenomena 
which we observe in the transformation of the human 
embryo in the whole course of our Ontogeny, are intel- 
ligible only with the help of Phylogeny, and are explicable 
only by reference to the historical metamorphoses of our 
animal ancestry. 108 

It is true that if the ontogenetic, and the phylogenetic 
stages (in Tables VIII. and XXII.) are carefully compared, 
a complete agreement between the two is not observable ; 
on the contrary, there are many individual divergences. In 
germ-history many organs appear earlier, others later, 
than the probable course of tribal history leads us to 
expect. But an adequate explanation of these divergences 
is found in the various kenogenetic modifications which 
the germ-history of the higher Vertebrates has undergone 
in the long course of its evolution. This will become quite 
clear when we carefully compare the germ-history of Man 
with the Ontogeny of the lowest Vertebrate, the Amphioxus, 
an Ontogeny distinguished by tenacious inheritance of the 
original course of evolution. 


Systematic Survey of the Periods in Human Germ-histort, 

(Cf. Table XXII.) 


Man as a simple Plastid. 

The human embryo possesses the form-value of a simple individual of the 
Brat order of a single plastid. 

First Stage : Monerula Stage (Fig. 36, p. 210). 

The human germ is a simple cytod (the impregnated egg-cell after the 
loss of the germ- vesicle). 

Second Stage : Cytula Stage (Fig. 37, p. 210). 

The human germ is a simple cell (the impregnated ovule-ocll with the 
re-formed kernel, or the parent-cell). 


Man as a many-celled Primitive Animal. 

The human embryo consists of many oells, which, however, as yet form 
no organs; it therefore possesses the form- value of an individual of the 
second order. 

Third Stage: Morula Stage (Fig. 40, p. 212, and PI. II. Fig. 14). 

The human germ is a globular cell-mass, of which one hemisphere consists 
of animal cells, the other of vegetative cells. 

Fourth Stage : Blastula Stage (PI. II. Fig. 16). 

The human germ is a vesicle, the wall of whioh consists of animal cells, 
its contents of vegetative c*»Ua. 



Man m an invertebrate Intestinal Animal. 

The human embryo possesses the form- value of an individual of the third 
order, an unarticulated person (a single metameron). The primitive in- 
testinal cavity is enclosed by two primary germ-layers, from the fission of 
which four secondary germ-layers are presently formed. 

Fifth Stage : Oastrnla Stage (Fig. 41, p. 218, and Fl. II. Fig. 17). 

The human germ forms an Amphigastrula, consisting solely of the two 
primary germ-layers, the skin-layer, and the intestinal layer. The cavity of 
the primitive intestine is occupied by entoderm cells, which also plug the 
primitive mouth. 

Sixth Stage : Chordonium Stage (Fig. 90, p. 302). 

The human germ possesses, in all essential points, the organization of a 
worm, of which the nearest existing allied form seems to be the asoidian 
larva. Four secondary germ-layers have developed from the two primary 
germ-layers, and coalesce along the central line. 


Man as a true Vertebrate. 

The human embryo possesses the form-value of an articulated person, 
or a metameric chain. The articulation principally affects the bony 
system (primitive vertebrae) and the muscle-system. The skin-eensory 
layer is divided into the horn-plate, the medullary tube, and the primitive 
kidneys. The skin-fibrous layer has separated into the leather-plate, the 
primitive vertebrae (muscle-plate and bone-plate), and the notochord. From 
the intestinal-fibrous layer proceed the heart with the principal blood- 
vessels, and the fleshy intestinal wall. From the intestinal-glandular layer 
the epithelium of the intestinal tube is formed. 

Seventh Stage : Acrauial Stage (Figs. 103, 107, pp. 342, 344). 

The human germ possesses, in essential points, the organization of a skull- 
less vertebrate, similar to the developed Amphioxus. The body already 
forms a chain of metamera, as several primitive vertebrae have become 
distinct. The head is, however, not yet distinctly separated from the 
trunk. The medullary tube has not yet differentiated into the brain- 
bladders. The skull is still wanting, as are also the heart and limbs. 


Eighth Stage : Cydortoma Stage (Fig. 132, p. 377, PI. VII. Fig. M I.> 

The human germ possesses, in essential points, the organization of a gill* 
less oranial animal (like the developed Myxinoida and Petromyzonta) . The 
number of metamera is increasing. The head is more distinctly differenti- 
ated from the trunk. The anterior extremity of the medullary tube swellg 
in the form of a bladder, and forms the rudimentary brain, which soon 
divides into five brain-bladders, lying one behind the other. On the sides of 
these appear the rudiments of the three higher sense-organs : the nose-pit, 
and the eye and ear vesicles. With the first circulation of the blood the 
heart begins its activity. The jaws and limbs are still wanting. 

Ninth Stage : Iehthyod Stage (Fig. 134, p. 378, PI. VII. Fig. M IL). 

The human germ possesses, in essential points, the organization of a fish 
(or a fish-like Skulled-animal). The two pairs of limbs appear in the simplest 
form, as fin-like processes : a pair of anterior limbs (dorsal fins) and one 
pair of posterior limbs (ventral fins). The gill-openings are completely 
formed, and between these the gill-arches form ; the first pair of gill-arches 
differentiate into the rudiments of the upper and lower jaws. From the in. 
testinal canal proceed lungs (swimming-bladder), liver, and pancreas. 

Tenth Stage : Amniotio Stage (PI. VTI. Fig. M III. ; PI. VIII.). 

The human germ possesses, in essential points, the organization of an 
Amnion-animal (of a higher gill-less Vertebrate). The gill-openings disap- 
pear by concrescence. From the gill-arches develop the jaws, the tongue- 
bone, and the bonelets (ossicles) of the ear. The allantois perfects itself, 
and changes into the peripheric portion of the placenta. All the organs 
gradually acquire the forms peculiar to the mammals, and at last the 
specific human form. (Compare on these points the Phylogeuy in the 
following chapters. 109 ) 

HAECKEL's evolution of man. 


haeckel's evolution of mam 


( 405 ) 


(Both Plates are copied from Erdl, " Entwickehing des Menschen" ,w ) 

Plate VIII. Fig. 1. — A human embryo of nine weeks, taken ont from tho 
egg-membranes and magnified three times. (Erdl, Plate XII. Fig. 1-5.) 
The skull is still quite transparent, so that the different divisions of the 
brain show through : the large mid-brain (" four-bulbs ") is separated from 
the scarcely larger fore -brain (cerebrum) by a shallow groove, but from the 
smaller hind-brain (cerebellum) by a deep indentation. The forehead is 
much arched in front ; the nose is yet very undeveloped; the eye is still dis- 
proportionately large and wide open. The upper lip is still very short and 
thickly swollen ; the under lip is very thin ; the chin is short and very re- 
treating. The whole face is very small in proportion to the skull. The ear- 
shell is also very small, but the outer opening of the ear very large. The 
neck is still very short ; the trunk, only about a third longer than the head, 
is of uniform thickness, and, towards the tail, is produced into a blunt point. 
The two pairs of limbs are already completely articulated. The anterior 
limbs (arms) are somewhat shorter than the posterior limbs. The upper 
and lower parts of the arm (arm and fore-arm) are very short in proportion 
to the hand, and, similarly, the upper and lower parts of the leg (thigh-bone 
and leg-bone) are short in proportion to the foot. The fingers on the hand 
are almost complete ; while, on the contrary, the toes on the foot are 
completely bound, as far as the points, in a swimming membrane, so that 
they form fins. 

Plate VIII. Fig. 2. — A human embryo of twelve weeks, within the egg- 
membranes; natural size. (Erdl, Plate XI. Fig. 2.) The embryo is com- 
pletely enclosed in the amnion, which is filled with the amnion fluid, as in a 
water-bath. The navel cord, which passes from the navel of the embryo to 
the chorion, is sheathed in a continuation of the amnion, which forms folds 
at the point where it is fastened. Above, the closely-crowded and branched 
chorion-tufts form the placenta. The lower part of the chorion (cut open 
and laid in many small folds) is smooth and destitute of tufts. Below it, 
the "decidua," which is also cut and spread out, is still hanging in deepei 
folds. The head and limbs of the embryo are already considerably moi e 
developed than in Fig. 1. 

Plate IX. — A human embryo of five months ; natural size. (Erdl, Plate 
XIV.) The embryo is enclosed in the delicate transparent amnion, whicl- 
has been cut open in front, so that the face and limbs are plainly seen 
through the opening. The back is bent, the limbs drawn together, so that 
the embryo occupies the smallest possible space in the egg-cavity. The 
eyelids are closed. From the navel the thick navel-cord passes, in ser- 
pentine folds, over the right shoulder to the back, and from there to the 
spongy placenta (below, on the right). The outer, thin, much-folded cover- 
ing is the outer egg-membrane, the chorion. 1 " 




Causal Significance of the Fundamental Law of Biogeny. — Influence <>' 
Shortened and Vitiated Heredity. — Kenogenetic Modification of Pali  . 
genesis. — The Method of Phylogeny based on the Method of Geology. 
Hypothetic Completion of the Connected Evolutionary Series by Appo- 
sition of the Actual Fragments. — Phylogenetic Hypotheses are Reliable 
and Justified. — Importance of the Amphioxus and the Ascidian.— 
Natural History and Anatomy of the Amphioxus. — External Structure 
of the Body. — Skin-oovering. — Outer-skin (Epidermis) and Leather-skin 
(Coriivm). — Notochord. — Medullary Tube. — Organs of Sense. — Intestine 
with an Anterior Respiratory Portion (Gill-intestine) and a Posterior 
Digestive Portion (Stomach-intestine). — Liver. — Pulsating Blood-vessels. 
— Dorsal Vessel over the Intestine (Gill-vein and Aorta). — Ventral 
Vessel under the Intestine (Intestinal Vein and Gill-artery). — Move- 
ment of the Blood. — Lymph-vessels. — Ventral Canals and Side Canah 
— Body-cavity and Gill-cavity. — Gill-covering. — Kidneys. — Sexual 
Organs.'— Testes and Ovaries. — Vertebrate Nature of Amphioxus. — Com- 
parison of Amphioxus and Young Lamprey (Petromyzon) . — Comparison 
of Amphioxus and Ascidiatn. — Cellulose Tunic — Gill-sac. — Intestine. 
— Nerve-centres. — Heart. — Sexual Organs. 

* The primitive history of the speoies is all the more fully retained in 
its germ-history in proportion as the series of embryonio forms traversed is 
longer j and it is more accurately retained the less the mode of life of the 
recent forms differs from that of the earlier, and the less the peculiarities 
of the several embryonio states must be regarded as transferred from a later 
to an earlier period of life, or as acquired independently." — Fkitz Mullek 

In turning from the germ-history to the tribal history 
of Man, we must constantly bear in mind the causal conneo 


fcion which exists between these two main branches of the 
hi ' f ory of human evolution. We found that this most 
significant causal connection was most simply expressed in 
"the fundamental law of organic evolution," the meaning 
and significance of which was explained in detail in the 
first chapter. According to that first biogenetic principle, 
Ontogeny is a short and compressed recapitulation of 
Phylogeny. If this reproduction of tribal history were 
always complete in germ-history, it would be an easy task 
to re-arrange Phylogeny by using Ontogeny as a guide. 
When any one wanted to know from what ancestors each 
higher organism is descended, therefore also from what 
ancestors Man is descended, and from what forms the whole 
human race has developed, it would only be necessary to 
trace accurately the series of forms which occur in the 
evolution of the individual from the egg; each form occur- 
ring in this series might then, without further trouble, be 
regarded as the representative of an old and extinct 
ancestral form. But, as a matter of fact, this immediate 
translation of ontogenetic facts into phylogenetic concep- 
tions is only directly allowable in the case of a com- 
paratively small part of animals. There are, it is true, a 
number of low, Invertebrate Animals (e.g., Plant-animals, 
Worms, Crabs) still extant, each germ-form of which we are 
justified in explaining, without further trouble, as the 
reproduction, or the portrait, of an extinct parent-form. But 
in mast animals, and in Man, this is impossible, because 
the germ-forms themselves have again been modified, and 
have partly lost their original nature, in consequence of the 
infinite variety in the conditions of existence. 

During the immeasurable course of the organic history 


of the earth, during the many millions of years, in the 
course of which organic life has been developing on our 
planet, modifications in the mode of germination have 
occurred in most animals ; this fact was first clearly recog- 
nized by Fritz Muller-Desterro, and was thus expressed 
in his able work, " Fiir Darwin." " The historical record 
preserved in the history of the evolution (of an individual) 
is gradually obliterated, in consequence of the fact that 
evolution continually strikes out a straighter road from 
the egg to the perfect animal, and the record is much 
vitiated by the struggle for existence which the .freely- 
living larvae have to undergo." The former phenomenon, 
the obliteration of the ontogenetic epitome, is effected by 
the law of simplified or abridged heredity. The latter phe- 
nomenon, the vitiation of the ontogenetic epitome, is caused 
by the law of modified or vitiated heredity. In accordance 
with this latter law, the young forms of animals (not only 
freely-living larvse, but also embryos enclosed in the mother's 
body) may be modified by the influence of the immediate 
surroundings, just as fully formed animals are modified by 
adaptation to the external conditions of their existence; 
the very species are sometimes modified during germination. 
In accordance with the law of shortened heredity, it is 
advantageous to all higher organisms (and the more so the 
higher their development) that the original course of 
development should be shortened and simplified, and, 
consequently, that the ancestral traditions should be 
obliterated. The higher the individual organism stands in 
the animal kingdom, the less completely does it reproduce, 
in its Ontogeny, the entire series of its ancestors, for 
reasons which are partly known, partly yet undiscovered. 


The fact is simply shown by a comparison of the various 
histories of individual evolution of higher and lower animals 
of the same tribe. 111 

In order to give its due weight to this significant 
relation, we have classed the whole series of ontogenetic 
phenomena, of the phenomena occuring in the evolution of 
an individual, in two different groups, placing the palin- 
genetic phenomena in one group, the kenogenetic in the 
other. To Palingenesis, or inherited evolution, we referred 
those incidents in germ-history which may be regarded as 
accurately inherited from the history of the tribe. On the 
other hand, we applied the term Kenogenesis, or vitiated 
evolution, to such ontogenetic processes as were not 
directly referable to corresponding phylogenetic incidents, 
but were, on the contrary, to be explained as modifications, 
or vitiations, of the latter. In consequence of this critical 
separation of palingenetic from kenogenetic germinal 
phenomena, the fundamental law of Biogeny was more 
accurately defined as follows : The short and quick history 
of the germ (Ontogeny) is a compressed epitome of the 
long and slow history of the tribe; this epitome is the 
more correct and complete, in proportion as the inherited or 
epitomized evolution (Palingenesis) is retained by heredity, 
and the less vitiated evolution (Kenogenesis) is introduced 
by adaptation. 10 

In order correctly to distinguish the palingenetic from 
the kenogenetic phenomena of germ-history, and from these 
rightly to infer the tribal history, we must especially apply 
ourselves to a comparative study of Ontogeny. It is only 
by comparing the germ-history of allied forms that we are 
able to discover the traces of their tribal history. For this 


purpose we may most advantageously apply the method 
which geologists have long used in determining the order 
of the sedimentary rocks in the crust of the earth. Most 
people know that the solid crust of our globe, a thin shell 
which surrounds the glowing and fluid main mass in its 
interior, consists of two chief classes of rocks : firstly, the 
so-called Volcanic, or Plutonic rocks, produced directly by 
the solidification of the molten internal mass of the earth 
upon the surface ; and, secondly, the so-called Neptunian, or 
Sedimentary rocks, produced from the former by the trans- 
forming agency of water, and deposited, in stratified layers, 
under water. At first, each of these Neptunian layers 
formed a stratum of soft mud ; but in the course of 
thousands of years they solidified into firm, hard masses of 
rock (sandstone, marl, chalk, etc.), at the same time perma- 
nently enclosing in their own mass such hard and imperish- 
able bodies as had found their way into the soft mud. 
Among the bodies, which were in this way either actually 
fossilized, or left the characteristic imprints of their forms 
in the soft clay, the harder parts of the animals and plants 
which lived and died on the spot during the stratification of 
mud are especially frequent. 

Each Neptunian rock-stratum contains its own charac- 
teristic fossils — the remains of such animals and plants as 
lived during that particular epoch of the earth's history. 
By comparing these strata, it is possible to review the whole 
connected series of earth-periods. All geologists are now 
agreed that such a positive, historical series of rock forma- 
tions is demonstrable, and that the lowest of these strata 
were deposited in primaeval times, the upper in the most 
recent times. But in no one place on the surface of the 


globe is the entire series of the strata-system perfect, with 
layer on layer in due succession ; in no place is the series even 
approximately complete. In fact, the order of the different 
strata of the earth and of the corresponding periods of the 
earth's history, as commonly conceived by geologists, is only 
hypothetical, and does not actually exist ; it is the result of 
the comparison of a number of separate observations of the 
sequence of strata at various points on the earth's surface. 

We shall treat the Phylogeny of Man in a similar way. 
We will endeavour to form the various phylogenetic frag- 
ments, occurring in very different groups of the animal 
kingdom, into an approximately correct representation of 
the ancestral line of Man. We shall find, that it is really 
possible, by rightly grouping and comparing the germ-history 
of very diverse animals, to obtain an approximately perfect 
picture of the palseontological development of the ancestors 
of Man and of Mammals ; a picture, such as could never be 
formed from the Ontogeny of the Mammals. In consequence 
of the kenogenetic processes to which we have alluded, in 
consequence of vitiated and of abridged heredity, whole 
series of the lower stages of evolution, especially in the most 
ancient periods, have fallen out from the germ-history of 
Man and of other Mammals, or have been vitiated by modifi- 
cation. But in the lower Vertebrates and in their Inverte- 
brate ancestors we meet with these very low form-stages 
in all their original purity. Especially in the lowest of all 
Vertebrates, the Amphioxus, the most ancient ancestral forms 
have been perfectly retained in the evolution of the germ. 
So, too, we find strong evidence in the Fishes, which stand 
midway between the lower and higher Vertebrates, and 
which explain several other phylogenetic periods. Lastly 



come the highest Vertebrates, in which the middle and the 
older stages of ancestral evolution have been either falsified 
or abridged, but in which the later stages of the phylo- 
genetic process are still well retained in the Ontogeny. 
Thus it is possible, by collating and comparing the history 
of individual development in the different groups of Verte- 
brates, to obtain an approximately complete picture of the 
palseontological history of the development of the ancestors 
of Man, within the vertebrate tribe. If we descend below 
the lowest Vertebrates, and compare the germ-history of 
these with that of the phylogenetically allied Invertebrates, 
we can trace the genealogical line of our animal ancestors 
much further, as far back as the lowest Plant-animals 
(Zoophytes) and Primitive-animals (Protozoa). 

In now treading the obscure path of this phylogenetic 
labyrinth, holding fast the Ariadne's clew of the funda- 
mental law of Biogeny and guided by the light of Com- 
parative Anatomy, we must, in accordance with the method 
we have just indicated, search out from among the diverse 
germ-histories of very different animals, those fragments from 
which we may construct the tribal history of Man ; and we 
must arrange these fragments in their proper order. Here 
again I would call special attention to the fact that we 
employ this method with the same certainty and with the 
same right as do geologists. No geologist has seen the 
actual process in which the gigantic rock-masses, composing 
the Carboniferous formations, the Jurassic, the Cretaceous, 
etc., were actually deposited by the water. Nor lias any 
geologist actually seen that these various sedimentary rocky 
formations originated in a particular sequence ; and yet all 
a^ree as to this sequence. The reason of this is that only 


on the hypothesis of this sedimentary stratification and of 
this sequence, is the nature and origin of these rock-masses 
intelligible. Since they are only conceivable and explicable 
by these "geological hypotheses," these hypotheses are 
universally accepted as " geological theories." 

On similar grounds, our phylogenetic hypotheses can 
claim precisely the same force. In proposing them we 
follow the same inductive and deductive methods, and with 
the same approximate certaint}^, as are followed by geolo- 
gists; because only with the aid of these phylogenetic 
hypotheses is the nature and origin of Man and of other 
organisms conceivable ; and because these hypotheses only 
can satisfy our reason in its demand for causality, therefore 
we hold these to be just ; therefore we claim for them the 
rank of "biological theories." And, just as geological 
hypotheses, which even in the beginning of the present 
century were derided as speculative castles in the air, are 
now universally accepted; so, too, before the close of this 
century will our phylogenetic hypotheses be received as 
valid, although they are at present ridiculed by the narrow- 
minded majority of naturalists as "the dreams of the 
physio-philosophers." It is true, our task, as we shall find, 
is not so simple as that of the geologists. It surpasses the 
latter in difficulty and complexity in the same proportion as 
the organization of Man is higher than the structure of the 
rock. 112 

When we approach our task, we obtain very essential 
aid by first closely studying the comparative germ-history 
of two low animal forms. One of these is the Lancelet 
(Amphioxus), and the other is the Sea-squirt (Ascidia) 
(Plates X. and XL). Both animals are extremely significant 


Both stand on the borderland between the two chief divisions 
of the animal kingdom, which since the time of Lamarck 
(1801) have been distinguished as the Vertebrates and 
the Invertebrates. The Vertebrates embrace the already 
mentioned classes from the Lancelet up to Man (Acephala, 
Lampreys, Fishes, Double-breathers, or Dipneusta, Amphibia, 
Reptiles, Birds, Mammals). In contradistinction to these, all 
other animals have usually, in agreement with the example 
of Lamarck, been classed as " Invertebrates." But, as we 
have already had occasion to remark, the Invertebrates in 
turn consist of several quite distinct tribes. Of these, the 
Star-animals (Echinoderma), the Soft-bodied Animals (Mol- 
lusca), and the Articulated Animals (Arthropoda), do not 
interest us here, because they are independent main branches 
of the animal genealogical tree, which are quite distinct 
from the Vertebrates. The class of Worms is, on the 
other hand, extremely interesting to us. In this group 
a very remarkable class of animals exists which has only 
recently been carefully studied, and which bears most 
significantly on the genealogical tree of Vertebrates. This 
class is that of the Mantle-animals (Tunicata). One 
member of this class, the Sea-squirts (Ascidia), very closely 
resembles in its internal structure and in its germination 
the lowest Vertebrate, the Lancelet (Amphiacus). Till a 
few years ago no one suspected the close connection be- 
tween these two apparently quite different animal forms, 
and it was a very lucky accident that just now, while the 
question as to the descent of the Vertebrates from Inverte- 
brates is foremost, the germ-history of these two most 
closely allied animals was discovered. In order rightly tc 
understand the germ-history of the Lancelet and the Sea* 


squirt, we must first consider these two remarkable animals 
in their perfect state afld compare their anatomies. 

We begin with the Lancelet, or Amphioxus, which, after 
Man, is the most important and interesting of all Vertebrates 
(Cf. Fig. 151, and Plate XI. Fig. 15.) The Lancelet was first 
described in 1778 by a German naturalist, named Pallas. 
He received this little animal from the British North Sea, 
and, thinking that in this animal he recognized a form 
closely allied to the common Naked Snail (Limax), he gave 
it the name of Li/max lanceolatus. For more than half a 
century, no one troubled himself about this reputed Naked 
Snail. Not till 1834 was this insignificant creature observed 
alive in the sand at Naples, by a local zoologist named 
Costa. He asserted that it was no snail, but a diminutive 
fish, and gave it the name of Branchiostoma lubricum. Just 
about the same time the English naturalist, Yarrell, showed 
that it possessed an internal axial skeleton, and called the 
animal Amphioxus lanceolatus. Then, in 1839, it was 
studied most closely by Johannes Muller of Berlin, to 
whom we are indebted for a very profound and thorough 
dissertation upon its anatomy. 118 Recently our knowledge 
of the animal has been greatly extended, and its more 
delicate structure especially has become better known 114 

The Amphioxus lives in flat, sandy localities on the sea- 
coast, partly buried in the sand, and appears to be very 
widely distributed in various seas. It is found in the 
North Sea (on the British and Scandinavian coasts, and 
also in Heligoland), in various parts of the Mediterranean 
(e.g., at Nice, Naples, Messina). It also occurs on the coast 
of Brazil and on the distant shores of the Pacific Ocean 
(the coast of Peru, Borneo, China, etc.). Everywhere this 
remarkable little animal appears in the same simple form. 58 


Johannes Muller referred the Lancelet to the class ol 
Fishes, though he insisted that the differences between this 
lowest of the Vertebrates and the lowest Fishes are much 
more considerable than the difference between all Fishes and 
the Amphibia. But this is far from expressing the real 
significance of this important little animal. Indeed, we 
might confidently lay down the proposition that the dif- 
ference between the Amphioxus and the Fishes is far greater 
than between the Fishes and Man and all other Yertebrates. 
Nay, so widely does the Amphioxus differ in its whole 
organization from the rest of the Vertebrates, that, according 
to the laws of systematic logic, we are forced to distinguish 
two main divisions of the vertebrate tribe: (1) the Skull -less 
Animals, or Acrania (the Amphioxus and the extinct allied 
forms) ; and (2) the Skulled Animals, or Craniota (Man and 
all other Vertebrates.) 115 

The first and lower division consists of Vertebrates 
without head, brain, or skull, for which reason they are 
called Skull-less Animals, or Acrania. Of these, the only 
extant representative is the Amphioxus, though in the 
earlier periods of the earth's history very numerous and 
varied forms belonging to this division must have existed. 
We may here lay down a universal law, which must be 
accepted by every adherent of the theory of evolution : viz., 
Buch entirely peculiar and isolated animal forms, as the 
Amphioxus — which apparently stands alone in the whole 
system of animals — are always the last survivors of an 
extinct group, numerous and diversified forms of which 
existed at an earlier period. As the whole Amphioxus is 
soft, and has no firm organs, capable of being fossilized, we 
may suppose that all its numerous extinct kindred were 


equally soft, and were, therefore, equally incapable of being 
petrified and of leaving any fossil impressions. 

Contrasted with the Skull-less Animals stands the other 
main division of Vertebrates, embracing all the rest of that 
class from Fishes to Man. These all have a head, clearly 
marked from the trunk, with a skull and brain; they all 
have a centralized heart, developed kidneys, etc. They are 
called Skulled Animals, or Craniota. But in the earliest 
stages of their existence even these are skull-less. As we have 
seen in the Ontogeny of Man, every Mammal, in the early 
stages of individual development passes through a condition 
in which it has neither head, nor skull, nor brain, and 
possesses only the well-known, simple form of a lyre-shaped 
disc, or of a shoe-sole, without any limbs or extremities. 
Comparing these early embryonic forms with the developed 
Lancelet, we may say, that the Amphioxus is in a certain 
sense a persistent embryo, a permanent germ-form of 
Skulled-animals ; it never passes beyond a certain low, 
early youthful condition, out of which we have long since 

The perfectly formed Lancelet (Fig. 151) is 5 to 6 cm. in 
length (above two inches), is either colourless or slightly 
reddish, and is shaped like a narrow lanceolate leaf. The 
body is pointed at both ends, and much compressed later- 
ally. There is no trace of limbs. The outer skin-covering 
is very delicate and thin, naked, translucent, and consists of 
two distinct strata ; a simple external skin, the outer skin 
(epidermis ; Plate X. Fig. 13, h), and a fibrous leather-skin 
(corium), lying below the epidermis (Fig. 13, 1). The central 
line of the back is traversed by a narrow fin-like ridge 
which widens behind into an oval tail-fin. and is prolonged 


underneath into a short, anal fin. The fin-like ridge ib sup 
ported by a great number of small and delicate quadrangular 
plates (Plate XI. 15, /). The delicate parallel lines under 
the skin, which describe an acute angle forward along the 
central line of each side, are the boundary lines of the 
numerous dorsal muscles (Fig. 15, r and b). 

In the centre of the body is a thin cartilaginous 
cord, which traverses the longitudinal axis of the entire body 
from front to rear, and is symmetrically sharpened at both 
ends (Fig. 151, i). This is the notochord (chorda dor satis), 
which in this case takes the place of the backbone, or 
vertebral column. In the Amphioxus the notochord does not 
develop further, but remains permanently in this most simple 
original condition. It is enclosed in a firm membranous 
covering, the notochord-sheath. The nature of the latter, 
and of the formations which proceed from it, may be best 
seen in the transverse section of the Amphioxus (Fig. 152 ; 
Plate X. Fig. 13, cs). Immediately above the chorda the 
notochord-sheath forms a cylindrical tube, and in this tube 
the central nervous system lies enclosed, the spinal or me- 
dullary tube (Plate XI. Fig. 15, m). This important mental 
organ retains throughout life this most simple form, that of 
a cylindrical tube, the anterior and posterior ends of which 
are almost equally simple, and the thick wall of which 
encloses a narrow canal. The anterior end is, indeed, rather 
rounder, and contains a small, hardly noticeable, bladder- 
like swelling of the canal (Fig. 15, rrij). This may be re- 
garded as the first indication of a real brain-bladder ; as a 
rudimentary brain. On the foremost end there is also a 
little black pigment-spot, the rudiment of an eye. Near 
this eye-spot, on the left side, there is a little ciliated groove, 


the single organ of smelL The organ of hearing is entirely 
wanting. This defective evolution of the higher sense- 
organs is probably in great measure explicable as not 
original, but as a degeneration. 

Below the notochord runs a very simple intestinal canal, 
a tube, which, on the ventral side of the little animal, opens 
in front in a mouth, and at the back in an anus. The mouth 
is oval, and surrounded by a cartilaginous circle, on which 
are 20 to 30 filaments of cartilage (organs of taste) (Fig. 
151, a). By a contraction in the centre, the intestinal cana] 
divijies in the centre into two very different parts, of about 
equal length. The anterior division acts as a respiratory 
organ, the posterior end as a digestive organ. The anterior 
half forms a wide gill-body, the lattice-like wall of which 
is pierced by numerous gill-openings (Fig. 151, d, and Plate 
XI. Fig. 15, k). The delicate bars of the gill-body, between the 
openings, are supported by small, firm parallel staves, which 
are connected together in pairs by cross-staves. The water 
which the Amphioxus takes in through its mouth passes 
through these openings in the gill-body into the large gill- 
cavity which surround the gill-body, and then passes further 
back and out through the breath-hole, or gill-pore (porus 
branchialis ; Fig. 151, c). On the ventral side of the gill- 
body there is, along the central line, a ciliated groove 
(the hypobranchial groove), which also occurs in Ascidians 
and in the larvae of Cyclostomi ; it is of interest because 
from it in the higher Vertebrates is developed the thyroid 
cartilage on the throat (on the lower part of the so-called 
Adam's apple; Fig. 15, y). 

Behind the breathing, or respiratory part of the intestinal 
canal comes, secondly, the digestive part. The small bodies 



which the Amphioxus takes up in the water it breathes —  
Infusoria, Diatomacese, parts of decayed plants, and animal 



Fig. 151. — Lancelet (Amphioxus lanceolatus) , twice 
the natural size ; seen from the left side (the longitu- 
dinal axis stands upright; the mouth end is turned 
upwards, the tail end downwards, as in Plate XI. 
Fig. 15): a, month-opening, surrounded by hairs; 
b, anal opening; c, gill-pore (porus branchialis) ; 
d, gill-body ; e, stomach ; /, liver ; y, small intestine ; 
h, gill-cavity ; i, notochord (below this the aorta) ; 
k, aorta-arch ; I, main trunk of the gill-artery ; m, 
swellings on the branches of the latter; n, hollow 
vein (vena cava) ; o, intestinal vein. 

bodies, etc. — pass back from the gill-body 
into the digestive section of the intes- 
tinal canal, and are there taken up as 
food and assimilated. From a rather wider 
'- section, corresponding to the stomach 
(Fig. 151, e), proceeds an oblong, pouch- 
like blind-sac (J), which passes directly 
forward, and ends on the right side of the 
gill-body. This is the liver of the Amphi- 
oxus, the simplest form of liver that we 
know of in any Vertebrate. In Man also, 
as we shall see, the liver develops as a 
pouch-shaped blind-sac, which protrudes 
from the intestinal canal behind the 

The structure of the system of blood- 
vessels in our little animal is not less re- 
markable than that of the intestine. For 
while all ofcher Vertebrates have a compressed, thick, purse- 
shaped heart, which develops at the throat from the lower 


wall of the anterior intestine, and from which the blood- 
vessels proceed, there is in the Amphioxus no special central 
ized heart, propelling the blood by its pulsations. Instead, 
the movement of the blood in the Amphioxus, as in the 
Ringed Worms (Annelida), is effected by the thin tubular 
blood-vessels themselves, which perform the functions of the 
heart, contracting and pulsating through their entire length, 
and thus driving the colourless blood through the whole 
body. This circulation is so simple and yet so remarkable, 
that we will briefly consider it. Let us begin in front at 
the lower side of the gill-body. In the central line of this 
lies a large main vessel, which corresponds to the heart of 
other Vertebrates and to the main gill-artery proceeding 
from its heart, and which propels the blood into the gills 
(Fig. 151, 1). The anterior, portion of this is swollen like 
a heart and is extended (immediately in front of the first 
gill-opening). Numerous little arching vessels rise on each 
side from this gill-artery, form little heart-like swellings 
(bulbs, m) at their point of departure, traverse the gill- 
arches, between the gill-openings, round the anterior intes- 
tine, and unite as gill- veins above the gill-body in a great 
main vessel, which passes below the notochord. This vessel 
is the primitive aorta (Plate X. Fig. 13, t ; Plate XI. 
Fig. 15, t). The aorta passes between the intestine and the 
notochord precisely as m all the higher Vertebrates. The 
branch- vessels which this aorta sends to all parts of the 
entire body, again collect into a large venous vessel, which 
passes to the lower side of the intestine, and which may 
here be called the intestinal vein (Fig. 151, ; Plate X. 
Fig. 15, v ; Plate XI. Fig. 13, v). It passes on further over the 
pouch-like liver, there forms a kind of cystic vein, weaving 


a fine vascular network around the blind-sac of the lirei 
and then passes, as a liver vein, into a vessel, directed 
toward the front, which we may call the hollow vein 
(Fig. 151, n). This last passes again directly to the ventral 
side of the gill-body, and here directly re-enters the gill- 
artery, which we took as a starting-point. Like a circular 
closed aqueduct, this single main vascular tube passes along 
the intestinal tube through the whole body of the Amphi- 
oxus, pulsating throughout its entire length both above and 
below. Within about a minute the colourless blood is thus 
driven through the whole body of the little creature. When, 
in pulsating, the upper tube contracts, the lower fills with 
blood, and vice versa. Above, the current of blood is from 
front to rear ; below, on the contrary, it is from the rear to 
the front. The entire long vascular tube, which runs below 
along the ventral side of the intestinal tube, and which 
contains venous blood, probably represents the so-called 
ventral blood-vessel of Worms (Plate IV. Fig. 7, v). On the 
other hand, the long straight vascular tube, which runs 
above along the dorsal line of the intestinal tube, between 
it and the notochord, and which contains arterial blood, is, 
on the one hand, evidently homologous with the aorta of 
other Vertebrates, and, on the other hand, with the so-called 
dorsal blood-vessel of Worms (Plate IV. Fig. 7, t). 

Johannes Muller recognized this important similarity in 
the formation of the system of blood-vessels of the Lancelet 
and of Worms. He directed special attention to the analo- 
gies of the two, and their physiological resemblance, the 
blood in both being driven by the pulsating contractions of 
the great vascular tubes throughout their entire length, and 
not by a centralized heart, as in all other Vertebrates. But 


we conceive that this important resemblance is more than 
a mere analogy. It has the deeper significance of a true 
homology, and rests on a morphological resemblance of the 
organs compared. Thus, the Amphioxus shows us that the 
aorta, the single main artery of Vertebrates, running 
between the intestine and the notochord, represents the 
dorsal blood-vessel of Worms. On the other hand, the ven- 
tral blood-vessel of the latter is retained only in the single 
intestinal vein passing below the intestine of the Amphioxus 
(and its anterior continuation ; cystic vein, liver vein, 
hollow vein (y. cava), gill-artery). In the developed body ' 
of all other Vertebrates this intestinal vein (originally the 
main venous blood-vessel !) is far outstripped by other 

Together with the real blood-vessels, special absorbing 
lymph-vessels seem to exist in the Amphioxus. Several 
canals, extending under the skin, have recently been 
regarded in this light, especially the narrow "ventral canals" 
(Fig. 152, S-i), and wide "side canals" (S). Both pass along 
the whole length of the ventral side and contain colourless 
lymph. The side canals (S) must possibly be regarded as 
the last remnants of degenerated primitive kidney ducts. 
They lie in the two parallel side folds of the ventral skin 
(F), ending blindly both in front and behind, and do not 
open outwards, as was supposed till recently. 

The real body-cavity (coeloma) in the Amphioxus (Fig. 
152, Lh) is extraordinarily narrow and small. It surrounds 
the intestinal tube in its narrow cavity, and is probably con- 
nected with the lymph spaces. Formerly it was confused 
with the large respiratory cavity or gill-cavity (A), which is 
of entirely different morphological and physiological signifi- 



Fig. 152. — Transverse section through the anterior part of a Lancelet. 
(After Eolph.) The outer covering forms the single cell-stratum of the outer 
skin (epidermis, E). Below this lies the thin leather skin (corium), the 
inner tissue of which is thickened below (TJ) ; partition walls of connective 
tissue pass inward from it between the muscles (M x ) and to the chorda- 
sheath ; N, medullary tube ; ch, notochord ; Lh, body-cavity (cceloma) ; A, 
gill-cavity; L, upper wall of the latter; E x , inner wall of the same; E 2 , 
outer wall of the same ; Est, gill-rods ; M, ventral muscles ; R, Raphe, or 
seam formed by the coalescence of the ventral folds (gill-roofs) ; 0, sexual 


cance. The true body-cavity (Lh) is filled with lymph, 
its inner wall being clothed by the intestinal-fibrous layer, 
its outer wall by the skin-fibrous layer. The gill-cavity (A) 
is, on the contrary, filled with water, and its whole wall is 
clothed by the skin-sensory layer. The latter envelopes the 
outer surface of the two large, lateral gill-roofs, the lateral 
processes from the body- wall, which grow together below 
round the original ventral side, and unite in the central line 
(in the ventral seam or raphe, Fig. 152, R). 

On each side of this ventral seam, on the inner surface of 
the gill-roofs, directly in front of the gill-pore (jporus 
branchialis), and over the ventral muscles (M) and between 
the sexual glands (G), lie the kidneys of the Amphioxus. 
These urinary glands are present in the simplest form, as 
glandular epithelial swellings of the skin-sensory layer. 
The epithelial cells of these are distinguished by peculiar 
size and nature, and contain crystalline deposits. As we 
regard the primitive kidneys of other Vertebrates also as 
originally skin-glands, and as we derive them from the skin- 
sensory layer, it is very interesting to find these organs 
permanently retained in the Lancelet as skin-glands. 

The sexual organs also appear in a perfectly simple 
form. On both sides of the gill-intestine, in the central part 
of the gill-cavity, lie from twenty to thirty small elliptical or 
roundly four-cornered sacs, which can easily be seen by the 
naked eye from without, through the thin transparent wall 
of the body. In the female, these little sacs are the ovaries, 
and contain numbers of simple egg-cells (Plate X. Fig. 13, e). 
In the male, these are replaced by the testes, heaps of 
smaller cells, which change into movable whip-cells (sperm- 
cells). Both kinds of sacs lie within on the inner wall of 


the gill-cavity, and have no special channels of exit. When 
the eggs of the female and the seed masses of the male are 
matured, they fall into the body-cavity, and are expelled 
through the gill-pore (p. branchialis). 

Now on trying to comprehend in one connected view the 
results of our anatomic study of the Amphioxus, and com- 
paring this conception with the known organism of Man, 
the contrast between the two seems immense. In fact, the 
most perfect vertebrate organism, represented by Man, is in 
every respect so far above that lowest stage in which the 
Lancelet remains, that it seems at first almost impossible to 
place both organisms in the same main division of the 
animal kingdom. And yet this classification is based on 
unassailable grounds. For Man represents only a further 
advance of the same vertebrate type, which in all its rudi- 
mentary characters is unmistakably seen in the Amphioxus. 
It is only necessary to recall the representation which has 
been given of the ideal form of the Primitive Vertebrate 
(p. 256) and to compare witi it the various lower stages of 
development of the human embryo, in order to become 
convinced of our near relationship to the Lancelet. 

It is true that a few zoologists have recently maintained 
the paradoxical view that the Amphioxus is in no way 
allied to Vertebrates. This was asserted especially by Karl 
Semper and Robby Kossman, the same learned pair who 
discovered in Goethe a narrow-minded upholder of the 
constancy of species (see p. 91). But these gentlemen can 
only have uttered this assertion in order, in the absence of 
positive merits, to make their names known by negative 
instances. One who at the present time maintains that 
the Amphioxus is not allied to Vertebrates goes back a 


whole century, even beyond Pallas (1778), and only proves 
that his notions of Comparative Anatomy and of the history 
of evolution are extremely weak. 

The Amphioxus does, indeed, stand very far below all 
other extant Vertebrates. It is, indeed, without the head 
containing a developed brain and skull, which distinguishes 
all other Vertebrates. It is without an organ of hearing, 
and without a centralized heart, such as all others possess ; 
perfect kidneys are also lacking. Each organ appears in a 
simpler and more imperfect form than in any other Ver- 
tebrate. And yet, the rudimentary characters, the connec- 
tion and relative position of all the organs, are the same as 
in all other Vertebrates : moreover, they all, during their 
embryonic development, pass, at an early period, through a 
stage in which their whole organization is not superior to 
that of the Amphioxus, but rather, agrees with it in all 
essential particulars. (Cf. Table IX.) 

In order to be thoroughly convinced of this important 
fact, it is specially instructive to compare the Amphioxus 
with the early forms of development of those Vertebrates 
which are most nearly allied to it in the natural system 
of this tribe. This is the class of the Round-Mouths 
(Cyclostomi). This remarkable class, which formerly com- 
prehended many species, contains at the present day but 
very few species, which are separable into two different 
groups. One group is formed by the Hags (Myxinoidce), 
which have been made known to us by Johannes Muller's 
classic work, " Vergleichende Anatomie der Myxinoiden." 
The other group is formed by the well-known Lampreys, or 
Hock-Suckers (Petromyzonta), which are eaten as a delicacy. 
AH these Round-Mouths are usually included in the class of 



Fishes. They stand, however, far below the true Fishes, and 
form a very interesting connecting group between them 
and the Lancelet. How near they stand to the latter, is 
clearly seen if an immature Lamprey ( Petromyzon, Plate XI. 
Fig. 16) is compared with the Amphioxus (Fig. 15). In 
both, the notochord (ch) is in the same simple form, as is 
also the medullary tube (m), lying above the notochord, and 
' the intestinal tube (d), lying below the notochord. But in 
the Lamprey, the medullary tube soon swells in front into 
a simple pear-shaped brain-bladder (m^, and on each side 
of this appears a very simple eye (aw) and a simple ear- 
vesicle (g). The nose (n) is still a single pit, as in the 
Amphioxus. The two sections of the intestine also, the 
anterior gill-intestine (k) and the posterior stomach-intes- 
tine (d), are very simple in the Lamprey, and very like 
those of the Amphioxus. On the other hand, there is 
decided progress in the organization of the heart, which 
appears below the gills as a centralized muscular pouch, and 
separates into an auricle (hv) and a ventricle (file). At a 
later period, the Lamprey attains to a considerably higher 
state of development, acquires a skull, five brain-bladders, 
a series of independent gill-pouches, etc. But this makes 
the remarkable similarity of its young larva to the de- 
veloped Amphioxus all the more interesting. 116 

The Amphioxus, which is thus directly connected, on 
the one side, with the Fishes through the Round-Mouths 
(Cyclostomi), and thereby to the series of higher Vertebrates, 
is, on the other hand, very nearly allied to a lower inver- 
tebrate sea-animal, from which, at first sight, it seems very 
far removed. This remarkable animal is the Sea-squirt, or 
Ascidian, which until very recently was regarded as being 


nearly related to the Mussels, and was therefore classed 
with the Soft-bodied Animals {Mollusca). But since 1866, 
when the remarkable germ-history of these animals was 
first understood, there has been no doubt that they are 
unconnected with the Soft-bodied Animals. On the con- 
trary, greatly to the surprise of zoologists, the entire mode 
of their individual development indicates that they are the 
nearest allies of the Vertebrates. In their matured con- 
dition the Ascidians are shapeless lumps, which at first 
sight certainly do not look like animals. The oblong body, 
often rough, or covered with uneven knobs, in which no 
definite outward organs are distinguishable, adheres firmly 
by one end to sea-weeds, stones, or to the bottom of the 
ocean. Some species resemble potatoes, others dried plums. 
Many Sea-squirts form very insignificant incrustations on 
the surface of stones and plants. Some of the larger kinds 
are eaten like oysters. Fishermen, who know them well, 
regard them not as animals, but as sea- weeds. They are 
frequently offered for sale together with other low sea- 
animals, in the fish-markets of many Italian seaside towns, 
under the name of Sea-fruit (frutti di mare). There is 
indeed nothing outwardly indicating an animal. When 
they are drawn from the sea in a drag-net, all that is 
noticeable is that they feebly contract their bodies, thus 
producing a spirting of water from certain parts. Most of 
the Sea-squirts are very small, only a few lines, or at most 
a few inches long ; a few species attain the length of a foot 
or rather more. There are a great many species, which are 
to be found in all seas. We find no fossil remains of this 
class of animals, because they have no hard parts capable 
of petrifaction; but they are certainly of very great an 


tiquity, and must have existed during the primaeval 

The whole class to which the Ascidians belong bears the 
name of Mantle-animals (Tunicata), because the body is 
enclosed in a thick and firm membrane, as in a mantle or 
tunic. This tunic, which is sometimes soft and jelly-like, 
sometimes tough and leather-like, sometimes firm and 
cartilaginous, is distinguished by many remarkable charac- 
teristics. Probably the most remarkable of these is, that it 
consists of a woody mass or cellulose, the same plant-cell 
material which forms the firm exterior of the cells of 
plants, the substance of the wood. The Mantle-animals are 
the only class of animals which really possess a cellulose 
covering, a wood-like envelope. Sometimes the cellulose 
tunic is variegated, at other times it is colourless. Not 
uncommonly it is set with spines or hairs, like a cactus. 
Many foreign substances, such as stones, sand, fragments of 
mussel-shells, and so forth, are often embedded in the tunic. 
The Sea-squirt has, therefore, received the name " micro- 
cosm." 117 

In order correctly to understand the internal organiza- 
tion of the Sea-squirt, and thoroughly to compare it with the 
Amphioxus, we must place ourselves in the same position to 
it as to , the latter (Plate XI. Fig. 14, on the left side ; the 
mouth extremity is turned upward, the back to the right, 
the abdomen to the left). The posterior end, corresponding 
to the tail of the Amphioxus, is usually adherent, often by 
means of root-like processes. The ventral and dorsal sides 
are internally very different, but are often externally undis- 
tinguishable. On opening the thick tunic, in order to note 
the internal organization, we observe first a very consider- 



able cavity, filled with water; this is the gill-cavity, or 
respiratory cavity (Fig. 153, cl; Plate XI. Fig. 14, cl). It 
is also called the mantle or tunic cavity, because it receives, 

Fig. 153. — Structure of an Ascidian 
(viewed from the left side, as in Plate 
XII. Fig. 14) ; the dorsal side is turned 
towards the right, the ventral side to- 
wards the left, the mouth-opening (o) 
upwards ; at the opposite, tail extremity, 
the ascidian is firmly attached to some 
substance below. The gill-intestine 
(&)•), which is pierced by many open- 
ings, continues below as the stomach- 
intestine. The large intestine opens 
through the anus (a) into the gill- 
cavity (cl), from which the excrement 
is removed with tbe inhaled water 
through the mouth of the tunic (a') ; m, 
tunic. (After Gegenbaur.) 

not only the water for respir- 
atory purposes, but also ex- 
crement and the sexual pro- 
ducts. The greater part of the 
respiratory cavity is occupied 
by the latticed gill-sac (br). 
The latter is in its whole posi- 
tion and constitution so like 
the gill-body of the Amphioxus, that many years ago, 
before anything was known of the real relationship of the 
two animals, the English naturalist, Goodsir, called attention 
to this striking similarity. In the Sea-squirts also the 
mouth -opening (0) leads directly into this gill-sac. The 
water breathed in passes through the openings of the 


latticed gill-sac into the gill-cavity, and is removed from 

there by the respiratory pore or excretory opening (a). A 
ciliated groove traverses the ventral side of the gill-sac, the 
same "hypo-branchial groove " which we found before in the 
Amphioxus at the same place (Plate XI. Fig. 14, y, 15, y). 
The food of the Sea-squirt, like that of the Amphioxus, con- 
sists of small organisms, Infusoria, Diatomacece, parts of 
dismembered sea-weeds and sea-animals, etc. These pass 
with the inhaled water into the gill-sac, and from the end of 
this into the digestive part of the intestinal canal, first into 
an extension answering to a stomach (Fig. 14, rag). The 
small intestine connected with it usually forms a loop, 
curving around toward the front, and opens in a vent (Fig. 
153, a), not directly out, but first into the gill-cavity ; from 
here the excrement is removed, together with the inhaled 
water and the sexual products, through the common ex- 
cretory opening (a). The latter is sometimes called gill- 
pore, or respiratory pore (joorus branchialis), sometimes the 
cloacal opening (Plate XL Fig. 149). In many Sea-squirts, a 
glandular mass, representing the liver, opens into the intes- 
tine (Fig. 14, lb). In some, there is another gland near the 
liver, which is supposed to be the kidney (Fig. 14, u). The 
real body -cavity (coeloma), which is filled with blood and 
surrounds the stomach, is very small in the Ascidian, as in 
the Amphioxus, and equally in both cases is usually con- 
fused with the gill-cavity, which is filled with water. 

In the mature Sea-squirt there is no trace of a noto- 
chord, an inner bony axis. This adds interest to the fact, 
that the young animal, as it emerges from the egg, has a 
notochord (Plate X. Fig. 5, ch), above which lies a rudimen 
tary medullary tube (Fig. 6, m). In the mature Sea-squirt, 


this tube is entirely shrivelled up, and forms a little knot of 
nerves lying near the front above the gill-sac (Fig. 14, m). 
It answers to the so-called upper throat-ganglion, or the 
" brain " of other Worms. Special organs of sense are either 
entirely wanting, or exist in the very simplest form, as 
eye-specks and taste papilla?, which surround the mouth 
(Fig. 14, au, eyes). The muscular system is very feebly 
and irregularly developed. Immediately below the thin 
leather-skin (corium) with which it is intimately connected, 
is a thin pouch-shaped muscular membrane, as in the lowei 
Worms. On the other hand, the Sea-squirt has a cen- 
tralized heart, and appears in this respect to be more highly 
organized than the Amphioxus. On the ventral side of 
the intestine, at a considerable distance behind the gill-sac. 
lies a spindle-shaped heart (Fig. 14, hz). It permanently 
retains that same simple pouch-shaped form which the 
rudimentary heart of the Vertebrate possesses for a very 
short time. (Cf. the heart of the human embryo, Fig. 
144, c, p. 392.) This simple heart of the Ascidian, how- 
ever, exhibits a remarkable peculiarity. It contracts in 
alternate directions. While in all other animals the pul- 
sation of the heart takes place constantly in a given 
direction, usually from back to front, in the Ascidians it 
alternates between opposite directions. First, the heart 
contracts in the direction from back to front, then, after 
standing still a minute, it begins to pulsate in the opposite 
direction, driving the blood from front to back; thus the 
two great vessels proceeding from the opposite ends of 
the heart act alternately as arteries and veins. This is a 
peculiarity which appears only in the Mantle- Animals 



Of the other important organs, we have yet to mention 
those of reproduction, which lie at the posterior extremity 
of the body-cavity. All the Sea-squirts are hermaphrodites. 
Each individual has a male and a female gland, and is thus 
capable of self-fertilization. The mature eggs (Fig. 154, o) 
fall directly from the ovary (o) into the gill-cavity. The 
male sperm, on the contrary, is carried from the testes (t) 
into the same cavity by a special seed-duct (vd). Here 
impregnation takes place, and here in many Sea-squirts 
developed embryos are found (Plate XL Fig. 14, z). These, 
with the water that has been inhaled, are then thrown out 
at the gill-pore (q) ; they are thus "born alive." 

Many Sea-squirts, especially of the smaller species, 

Fig. 154. — Structure of an Ascidian (observed from 
the left side, as in Fig. 153, and Fig. 14, Table XL) : 
sb, gill-sac ; v, stomach ; i, large intestine ; c, heart ; 
t, testes ; vd, seed-duct ; o, ovary ; o', matured eggs in 
the gill-cavity. The two little arrows indicate the en- 
trance and exit of the water through the two openings 
of the tunic. (After Milne Edwards.) 

multiply, not by sexual reproduction, but 
asexually by the formation of buds. Great 
numbers of the individuals thus produced 
from buds remain permanently attached to 
each other, thus forming large masses, or 
comes like the well-known coral societies. 
Among these social or compound Ascidians, 
those species are peculiarly interesting in 
which the mass seems to be beautifully 
combined of many star-shaped groups. Each 
star-shaped group consists of a larger or 
smaller number of individuals, of which every one possesses 


its independent organization and its own mouth-opening. All 
the individuals together have, however, but a single common 
gill-pore, which is situated at the central point of the star- 
shaped group. These star-shaped compound ascidian group* 
(Botryllus, Polyclinvm,, etc.) throw much light on the 
Phylogeny of one of the most remarkable races of animals, 
the Star-animals (Echinoderma). The parent-forms of 
these are the Star-fish, or Asterids, which are, like the 
compound Ascidians, star-shaped societies formed of Worms 
connected by a common central intestinal opening. 11 * 

If we now once more glance back at the entire organiza- 
tion of the simple Ascidians, Sea-squirts {Phallusia y Cyn- 
thia, etc.), and compare it with that of" the Amphioxus, we 
find that the two present few points of resemblance. The 
developed Ascidian is indeed like the Amphioxus in some 
important points of internal structure, especially in the 
peculiar construction of the gill-sac and intestine. But 
it seems so far removed in most other particulars of its 
organization, and is so dissimilar in outward appearance, 
that the very near relationship of the two organisms is only 
revealed by study of their germ-histories. We will now 
consider and compare the individual development of the 
two animals, and shall in this way find, to our great sur- 
prise, that the same embryonic animal form develops from 
the egg of the Amphioxufl as from the egg of the Ascidian, 


Platb X. — Germ-history of the Ascidian and of the Amphioxus 
(Principally according to Kowalevsky.) 

Fig. 1-6. — Germ-history of an Ascidian. 

Fig. 1. — A parent-cell (cytula) of an Ascidian. In the bright-coloured 
protoplasm of the parent-cell lies eccentrically a bright spherical kernel 
(nucleus), and in the latter a darker nucleolus. 

Fig. 2. — An Ascidian egg in the process of cleavage. The parent-cell 
has divided by repeated bisection into four similar cells. 

Fig. 3. — Membraneous germ-vesicle of an Ascidian (Blastula). The cells 
resulting from the cleavage of the egg form a spherical bladder filled with 
fluid, the wall of which consists of a single layer of cells. (Cf . Fig. 22, F , Q.) 

Fig. 4. — Gastrula of the Ascidian resulting from the blastula (Fig. 3) 
by inversion (invagination). The wall of the primitive intestine (d), which 
opens at o by the primitive mouth, consists of two layers of cells ; the inner 
intestinal layer formed of larger cells, and the outer skin-layer, of smaller. 

Fig. 5. — Larva of the Ascidian swimming freely. Between the medullary 
tube (m) and the intestinal tube (d) the notochord is inserted (ch), which 
passes throughout the long rudder-like tail to its point. 

Fig. 6. — Transverse section through a larval Ascidian (Fig. 5), through 
the posterior part of the trunk just in front of the beginning of the tail. 
The section is just the same as that of the Amphioxus larva (Fig. 11, 12). 
Between the medullary tube (m) and the intestinal tube (d) lies the noto- 
chord (ch) ; on both sides are the lateral musoles of the trunk (r). 

Fig. 7-13. — Germ-history of the Amphioxus. 

Fig. 7. — Parent-cell (cytula) of the Amphioxus. (Cf. Fig. 1.) 

Fig. 8. — An amphioxus-egg in the process of cleavage. (Cf. Fig. 2). 

Fig. 9. — Blastula of the Amphioxus. (Cf. Fig. 3.) 

Pig. 10.— Gastrula of the Amphioxus. (Cf. Fig. 4.) 

Fio. 11. — Young larva of the Amphioxus. The notochord (eh) lies 
between the medullary tube (m) and the intestinal tube (i). The medullary 
tube has an opening at the anterior extremity of the body (ma). 


Fio. 12. — An older larva of the Amphioxus. On both sides of the 
medullary tube (m) and of the notochord (ch) a longitudinal row of muscle- 
plates (mp) is visible ; these mark the embryonic vertebrae, or metamera. 
An organ of sense has developed in front (ss) . The wall of the intestine 
(d) is much thicker below on the ventral side (du) than above on the dorsal 
side (do). The anterior part of the intestinal canal widens into the gill- 

Fig. 13. — Transverse section through a developed Amphioxus (Fig. 15) 
a little behind the centre of the body. Above the intestinal tube (d) is the 
dorsal blood-vessel, or main artery (£), and below it the ventral blood-vessel, 
or the intestinal vein (v). At the inner wall of the gill-cavity (c) lie the 
ovaries (e), and outside these the side canals (u). The dorsal muscles (r) 
are divided into several parts by inter •muscular ligaments (mo) ; /, dorsal 

Plate XL — Structure of the Ascidiaic, of the Amphioxus, and of top 

Laeva op the Petromyzon. 

For the sake of comparison, all the three animals are placed in the same 
position and are represented of .the same size. The view is from the left 
side. The head extremity is turned upward, the tail downward ; the dorsal 
side to the right, the ventral side to the left. The enveloping membrane is 
removed from the left side of the body, to show the inner organization with 
the organs in their natural position. 

Fig. 14. — A simple Ascidian (Monascidia) , magnified six times. 

Fig. 15. — A developed Amphioxus (magnified four times). 

For the sake of giving a more distinct view, the Amphioxus in Fig. 15 is 
drawn about twice its actual breadth. In reality, its breadth amounts to 
but half of the length as represented here. 

Fig. 16. — Young larva of a lamprey (Petromyzon Planeri), eleven days 
after emerging from the egg, magnified 45 times. (After Max Schultze.) 
The larva of the lamprey, which undergoes a peculiar transformation at a 
later period, was formerly considered as a distinct species under the name of 

The meaning of the letters is the same in all the figures. 

Of the Meaning of the Letters in Plates X. and XI, 

a, anus 

tn % , spinal marrow 

au, eye 

ma, anterior opening of the modal 

6, ventral muscles 

lary tube 

c, gill -cavity 

nib, muscular ligaments 

eh, notochord (spinal axis) 

mg, stomach 

el, oloaoal cavity 

mh, mouth-cavity 

ea, notochord sheath 

rrvp, muscle-plate 

d, intestinal tube 

mt, mantle 

do, dorsal wall of the intestine 

n, nose (nose-pit) 

du, ventral H „ „ 

o, mouth-opening 

e, ovary 

p, ventral pore 

en, endostyle 

q, cloaca! opening 

/, fin-seam 

r, dorsal muscles 

g, ear-vesicle 

8, tail-fins 

h, horn-plate 

si, seed-duct 

hd % testes 

sm, opening of the seed-duet 

hk, remtiole 

8S, organ of sense 

hv, auricle 

t $ aorta (dorsal blood-vessel) 

hz, heart 

th, thyroid gland 

», eggs 

u, side canals 

fc, gills 

v, intestinal vein (ventral blood 

fco, gill-artery 


^ leather-plate 

to, root -fibres of the asoidian. 

lb, liver 

e, boundary between the gill.intes- 

lb', anterior end of the liver 

tine and the stomach-intestine 

le, liver vein 

y, hypo-branchial groove 

m, medullary tabo 

•, embryos of the ascidian 

m^ brain- bladder 


Ontogeny of the Ascidian (1-6) and of the Amphioxus (7 13). 



Anatomy of the Ascidian (14) and Amphioxus (i,-,) 





Relationship of the Vertebrates and Invertebrates. — Fertilization of the 
Amphioxus. — The Egg undergoes Total Cleavage, and changes into a 
Spherical Germ-membrane Vesicle (Blastula). — From this the Intes- 
tinal Larva, or Gastrula, originates by Inversion. — The Gastrula of the 
Amphioxus forms a Medullary Tube from a Dorsal Furrow, and 
between this and the Intestinal Tube, a Notochord : on both Sides the 
latter is a Series of Musole-plates ; the Metamera. — Fate of the 
Four Secondary Germ-layers. — The Intestinal Canal divides into an 
Anterior Gill-intestine, and a Posterior Stomach-intestine. — Blood- 
vessels and an Intestinal-muscle Wall originate from the Intestinal, 
fibrous Layer. — A Pair of Skin-folds (Gill-roofs) grow out from the 
Side -wall of the Body, and, by Coalescence, form the Ventral Side of 
the Large Gill-cavity. — The Ontogeny of the Ascidian is, at first, iden- 
tical with that of the Amphioxus. — The same Gastrula is Developed, 
which forms a Notochord between the Medullary and Intestinal Tubes. 
— Retrogressive Development of the same. — The Tail with the Notochord 
is shed. — The Ascidian attaches itself firmly, and envelopes itself in 
its Cellulose Tunic. — Appendicularia, a Tunicate which remains through, 
out Life in the Stage of the Larval Ascidian and retains the Tail-fin 
with the Chorda (Chordoma). — General Comparison and Significance of 
the Amphioxus and the Ascidian. 

u In the formation of its most important organs, the Amphioxus remains 
throughout life at that lowest stage of development, which all other Verte- 
brates pass rapidly through during the earliest period of their embryonic 
existence. We must therefore regard the Amphioxus with peculiar reverence 
48 that animal, which among all existing creatures is the one alone capable 


of giving us an approximate idea of our oldest Silurian vertebrate ancestor*. 
But the latter are descended from Worms, the nearest blood-relatives of 
which an the Ascidians of the present day." — The Pedigree of the Human 
Race (1868). 

The peculiarities in the structure of the body, which dis- 
tinguish Vertebrates from Invertebrates, are so striking, 
that the relationship of these two main groups of the animal 
kingdom formerly threw great difficulties in the way of 
•systematic classification. When, in accordance with the 
Theory of Descent, the relationship of the various groups 
of animals began to be regarded as not figurative, but as 
really genealogical, this question came to the front, and 
seemed to offer one of the greatest obstacles to" the success 
of the theory. Even at an earlier period, when without this 
fundamental thought of the true genealogical connection 
of the relationships between the great main groups of the 
animal kingdom, the so-called " types " of Baer and Cuvier 
were studied, investigators believed they had found, here 
and there among Invertebrates, points connecting these 
with Vertebrates ; some single species of Worms, in par- 
ticular, appeared to approximate in the structure of their 
bodies to the Vertebrates ; as, for example, the oceanic Arrow- 
worm (Sagitta). But the attempted analogy was shown, 
by closer investigation, to be untenable. After Darwin 
gave an impulse to a true tribal history of the animal 
kingdom, by his reform of the Theory of Descent, this very 
relation seemed to offer one of the greatest difficulties. 
When, in 1866, I attempted, in my Generelle Morphologic, 
to carry out the Theory of Descent in detail, and to apply 
it to the natural system, no part of my task demanded 
more care than the connection of the Vertebrates with the 


But just at this time the true connection was discovered 
in an entirely unhoped-for and most unexpected quarter. 
Toward the end of the year 1866, among the treatises 
of the St. Petersburg academy, two works appeared by 
the Russian zoologist, Kowalevsky, who had spent a long 
time at Naples, and had occupied himself in studying the 
individual evolution of some of the lower animals. A fortu- 
nate accident had led Kowalevsky to study almost simul- 
taneously the individual evolution of the lowest Vertebrate, 
the Amphioxus, and that of an Invertebrate, the direct 
relationship of which to the Amphioxus had not been even 
guessed, namely, the Ascidian. Greatly to the surprise of 
Darwin himself, and of all zoologists interested in that 
important subject, there appeared, from the very commence- 
ment of their individual development, the greatest identity 
in the structure of the bodies of those two wholly different 
animals, — between the lowest Vertebrate, the Amphioxus, 
on the one hand, and that misshapen lump adhering to 
the bottom of the sea, the Sea-squirt, or Ascidian, on the 
other hand. In this undeniable ontogenetic agreement, the 
existence of which, in an astonishing degree, can be proved, 
the long-sought genealogical link was, of course, directly 
found, according to the fundamental law of Biogeny, and 
that group of Invertebrates, which is most nearly allied to 
the Vertebrates, was clearly recognized. There can be no 
longer any doubt, especially since Kupffer and several other 
zoologists have confirmed and continued these investiga- 
tions, that of all classes of Invertebrates, the Mantle-animals 
(Tunicata), and of the latter, the Ascidians, are most nearly 
allied to the Vertebrates. We cannot say the Vertebrates 
are descended from the Ascidians ; but we may safely assert, 


that of all Invertebrates, the Mantle-animals, and among 
the latter the Ascidians, are the nearest blood-relations to 
the primaeval parent-form of Vertebrates. An extinct species 
of the very varied Worm tribe must be assumed as the 
common parent-form of both groups. 

In order fully to appreciate this extraordinarily im- 
portant circumstance, and especially in order to gain a secure 
basis for the desired genealogical tree of Vertebrates, it is in- 
dispensable to note minutely the germ-history of these two 
remarkable animals, and to compare the individual develop- 
ment of the Amphioxus stage by stage with that of the 
Ascidian. (Cf. Plate X., and p. 436.) We will begin with 
the Ontogeny of the Amphioxus (Plate X. Figs. 7-12). 
Kowalevsky had already spent several months in Naples 
with the express intention of studying the wholly unknown 
germ-history of the Amphioxus, before he succeeded in 
observing the mature eggs in the first stages of development. 
He says that the Lancelet begins to deposit its sexual products 
in the month of May, in the warm evening hours, between 
seven and eight o'clock. 119 He noticed that at this time, 
the male animal first ejected a whitish fluid, the sperm, and 
that, somewhat later, the female, attracted by the sperm, 
also deposited its eggs in the water. 

According to other observers the deposit of the sexual 
products is said to take place through the gill-pore (porus 
branchialis). The eggs are simple roundish cells. They 
have a diameter of only fa of a millimetre, are, therefore, 
only half as large as mammalian eggs, and offer no special 
peculiarities (Plate X. Fig. 7). The active elementary 
bodies of the male seed, the pin-shaped " seed-animals," or 
sperm-cells, all resemble those of most other animals. (Cf. 


Fig. 17. p. 173.) Fertilization is accomplished in this way : 
the moving whip-cells of the sperm approach the egg, and 
with their head-portion, that is, the thickened portion of 
the cell which encloses the nucleus, they force their way 
into the yelk-mass or cell-substance of the egg. 

Either before or immediately after fertilization, the egg- 
cell loses its original kernel, and appears for a time in the 
form of a kernel-less cytod, as a monerula. (Cf. Fig. 19, p. 179.) 
A new kernel soon, however, originates in the impregnated 
yelk; this is the parent-kernel, and the monerula thus changes 
into the parent-cell (cytula, Fig. 21, p. 181.) This now 
undergoes a regular and total cleavage, the details of which 
in a coral (Monoxenia) we have described in detail (cf. Fig. 
22). The repeated bisection of the parent-cell into 2, 4, 8, 
16, 32, 64 cells and so on, gives rise to the globular, black- 
berry or mulberry-shaped body which we called the " mul- 
bp.rry-germ' (morula, Fig. 22, E). Fluid collects in the 
interior of this globular mass, composed entirely of one sort 
of cleavage-cells, and the result is the formation of a spheri- 
cal vesicle, the wall of which is composed of a single layer 
of cells (Plate X. Fig. 9). We called this vesicle the mem- 
branous germ-vesicle (blastula). Its contents form a clear 
fluid ; the wall, which consists of a single layer of cells, is 
the germ-membrane, or blastoderma (Fig. 22, F, G). 

These processes take place so rapidly in the Amphioxus, 

that in from four to five hours after impregnation, that is, 

about midnight, the spherical blastula is complete. On one 

side of the latter appears a groove-like depression, by which 

the vesicle is turned into itself (Fig. 22, H, p. 190). This 

furrow grows constantly deeper, while the spherical form of 

the vesicle changes into an oval or ellipsoid shape (Fig. 165). 



At last, the inversion is complete, so that the inner part of 
the wall, that which has been inverted, lies on the inside oi 
the outer, the uninverted part. In this way an almost 
hemispherical hollow body is formed, the thin wall of which 
is composed of two layers of cells. The hemispherical form 
soon again changes into an almost spherical or oval shape, 
in consequence of the inner cavity becoming considerably 
enlarged, while its opening becomes narrower (Plate X. 
Fig. 10). The form which the embryo of the Amphioxus 
has now attained in this way is a true Gastrula or intes- 
tinal larva ; is indeed a gastrula of that original and 
simplest form which we have already distinguished as the 
Bell-gastrula or Archigastrwla (p. 191, Fig. 22,1, K). 

Fig. 155. — Gastrula of Amphi- 
oxus, in longitudinal section : d, 
primitive intestine ; o, primitive 
mouth ; i, intestinal layer, or ento- 
derm ; e, skin-layer, or exoderm. 

As in all those lowly 
organized animals which 
form a primitive Bell-gas- 
trula of this sort, the body 
of the Amphioxus, which 
has but one axis, is merely 
a simple intestinal pouch 
the inner cavity of this is the primitive intestine (proto- 
gaster) (Fig. 155, d, Fig. 156, g) its simple opening is the 
primitive mouth (protostoma, o). The wall is at once 
the intestinal wall and the body- wall. It is composed of 
two cell-strata, of the two well-known primary germ-layers. 
b The inner stratum, or the inverted portion of the blastula, 



which immediately surrounds the intestinal cavity, is the 
entoderm, the inner or vegetative germ-layer, from which 
are developed the wall of the intestinal canal and all its 
appendages (Fig. 155, 156, i). The outer cell-stratum, the 
part of the blastula not inverted, is the exoderm, the outer 
or animal germ-layer, which furnishes the rudiment of the 
body-wall, the skin, the flesh, the central nervous system, 
etc. (e). The cells of the inner stratum, or entoderm, are 
considerably larger, duller, darker, and more adipose than 
those of the outer stratum, or exoderm, which are clearer, 
brighter, and less rich in fatty particles. Thus, even during 
the process of inversion, a differentiation takes place between 
the inner inverted stratum and the outer uninverted. The 
cells of the outer layer are soon covered with fine bright 
hairs : fine, short, thread-like appendages, grow from the 
protoplasm, which keep up a constant vibratory motion. 

Fig. 156. — Gastrula of a Chalk-sponge (Olvnthus; : A, from the outside; 
B, in longitudinal section through the axis ; g, primitive intestine ; o, primi* 
tive month - i, intestinal-layer, or entoderm ; e. skin-layer, or exoderm. 


By the motions of these delicate vibratory hairs, the gastrula 
of the Amphioxus, like that of many other animals of low 
organization, after it has broken through the egg-coveringb 
rotates and swims in the ocean (Fig. 156). 

In the course of further development the roundish Bell- 
gastrula of the Amphioxus lengthens, and at the same time 
it becomes rather flatter on one side parallel to the longi- 
tudinal axis. The flattened side is afterwards the dorsal- 
side ; the opposite ventral side remains roundly arched. In 
the middle of the dorsal surface appears a shallow longi- 
tudinal furrow or channel (Fig. 157), and on each side of 
this channel the surface of the body rises in the shape of 
two parallel ridges or longitudinal swellings. I need 
hardly say, that this channel is the primitive groove, or 
dorsal furrow, and that these swellings are the dorsal 
swellings or spinal swellings which form the first rudiments 
of the central nervous system, the medullary tube. These 
two swellings grow higher and higher ; the groove becomes 
deeper and deeper. The edges of the two parallel swellings 
incline towards each other, and finally coalesce, and thus 
the medullary tube is completed (Plate X. Fig. 11, m). The 
formation of the medullary tube from the outer skin takes 
place, therefore, on the naked dorsal surface of the independent 
Amphioxus larva in exactly the same way as in the embryo 
of Man and of other Vertebrates within the egg-envelopes. 
In both cases, also, the nerve-tube finally separates entirely 
from the horny plate. The fact is peculiar, that at that 
end of the body which afterwards is to be the anterior or 
mouth end of the Amphioxus, the medullary tube remains 
open at first, and has an external opening (Fig. 11, ma). 

Even at the time when the first trace of the dorsal furrow 


appears, the two primary germ-layers of the Amphioxus 
larva split up into the four secondary germ-layers (Fig. 157, 
transverse section). Round the inner vegetative layer of 
the intestinal tube there arises, in consequence of a fission 
of the cells of the latter, a second external cell-stratum, the 

Fig. 157. — Transverse section through 
a larval Amphioxus (after Kowalevsky) : 
/is, skin-sensory layer ; hm, skin-fibrous 
layer; c, ccelom-fissnre (rudimentary 
body-cavity) ; df, intestinal-fibrous layer; 
dd, intestinal glandular layer; a, primi- 
tive intestine (primitive intestinal cavity). 
Above, the dorsal furrow is seen between 
the two dorsal swellings. 

intestinal-fibrous' layer (df) ; from this originate the 
muscles and the fibrous membranes of the intestinal tube, 
and the blood-vessels. The original inner cell-stratum 
must now be called the intestinal-glandular layer (dd). 
Analogously, the outer animal germ-layer falls, in con- 
sequence of a fission in its cells, into two strata, an outer 
skin-sensory layer (hs) and an inner skin-fibrous layer (hm). 
The former gives rise to the outer skin (epidermis) and the 
medullary tube ; the latter to the leather-skin (corium) 
and the trunk-muscles. A space forms between the skin- 
fibrous layer and the intestinal-fibrous layer, in which a 
colourless liquid collects, thus forming the body-cavity 
(cceloma, c). It is a fact of great moment for the germ- 
layer theory that, here in the Amphioxus, the origin of the 
skin-fibrous layer from the animal, and that of the intestinal- 
fibrous layer from the vegetative germ-layer is plainly 

As soon as the four secondary germ-layers have formed 



a cylindrical cord, pointed at both ends, and composed of 
large, light-coloured vesicular cells, appears in the middle 
line of the skin-fibrous layer, directly over the intestinal 
tube (d) and below the nerve-tube (m), (and therefore 
along the long axis of the body). This is the chorda 
dor satis, or notochord (Plate X. Fig. 11, 12, ch). The lateral 
portions of the skin-fibrous layer, which lie on both sides 
of the notochord, and which we may in this case also call 
" side-layers," or " side-plates," split into two strata, a thin 
leather-skin (corium) and an underlying muscle-plate. 
The latter soon breaks up into a number of homogeneous 
sections, lying one behind another. These are the side 
muscles of the trunk, which indicate the first articulation 
or metameric structure of the body (Fig. 12, mp). 

By these separations the gastrula of the Amphioxus has 
changed into a vertebrate body of the simplest form, with 
the characteristic disposition of the fundamental organs 
which belongs exclusively to Vertebrates. Directly below the 
skin we find, at the dorsal side of the medullary tube, on the 
ventral side of the intestinal tube, and between the two 
tubes, the firm axis of the body, the notochord; and, on 
either side of this, the regular series of muscle-plates. If 
we now look at the larva of the Amphioxus from one side 
(Plate X. Fig. 11, 12), we see that on the top lies the 
medullary tube, still open anteriorly (ma) ; directly under, 
this lies the strong notochord (ch), and under this the 
much broader intestinal tube (d). The latter also has an 
opening at one end, the original mouth of the gastrula (o). 
It is, however, a very singular and important fact that this 
primitive mouth does not afterwards become the permanent 
mouth-opening of the Amphioxua On the contrary, it soon 


closes. The future permanent mouth is formed only second- 
arily, from the outside, and at the opposite end of the body 
(near ss, Fig. 12). At this point, a groove-like depression 
originates in the outer skin (epidermis), and this grows 
inwards and breaks a way through into the closed intestine. 
Similarly, the anal opening forms behind (in the neighbour- 
hood of the closed gastrula-mouth). We saw that in Man 
and in all higher Vertebrates mouth and anus originate 
as shallow grooves in the outer skin ; and that these also 
break through inwards, thus gradually communicating with 
both blind ends of the intestinal tube. (Cf. p. 338.) 

Between the intestinal and the nerve tubes we find the 
notochord as a cartilaginous cylindrical rod, traversing 
the entire length of the larval body. On each side of the 
notochord lie the muscle-plates, already broken up into 
a number of separate pieces, or primitive vertebral seg- 
ments (10 to 20 on each side) ; these are separated from 
each other by simple oblique, parallel lines of demar- 
cation. In the fully-formed animal each of these divid- 
ing lines describes an acute angle forwards (Plate XI. Fig. 
15, r). The number of separate muscle-plates indicates 
the number of metamera of which the body consists. At 
first this number is small, but it afterwards increases 
considerably in the direction from front to rear. This 
is owing to that same terminal budding in virtue of 
which the chain of primitive vertebral segments grows 
in the human embryo. Here, too, the foremost metamera 
are the oldest, and the terminal ones the most recent. To 
each metameron corresponds a definite segment of the 
medullary tube and a pair of spinal nerves, which pass from 
it out to the muscles and to the skin. Of all the organic 


systems of the body, it is in the muscle-system that arti- 
culation first appears. 130 

While these characteristic differentiations are taking 
place in the two lamellae of the animal germ-layer — -while 
the medullary tube and the outer skin (epidermis) are 
separating from the skin-sensory layer, and the notochord 
and the muscle-plates from the skin-fibrous layer, equally 
important processes, characteristic of the vertebrate type 
are taking place in the vegetative germ-layer. The inner 
lamella of this — the intestinal-glandular layer — undergoes 
but few modifications; it produces only the internal cell- 
coating, or epithelium of the intestinal tube (d). But the 
outer lamella, the intestinal-fibrous layer, produces both 
the muscular covering of the intestine and the blood- 
vessels. Probably simultaneously, two main vessels ori- 
ginate from this layer : an upper, or dorsal vessel, corre- 
sponding to the aorta, situate between the intestine and the 
chorda dorsalis (Figs. 13, t, 15, t); and a lower, or ventral 
vessel, answering to the heart and the intestinal vein, on 
the lower edge of the intestine, and between it and the 
ventral skin (Figs. 13, v t 15, v). Moreover, at this time 
the gills, or respiratory organs, also develop in the anterior 
portion of the intestinal canaL The whole anterior or 
respiratory section of the intestine changes into a gill-body, 
which is pierced by numerous openings, so that it resembles 
a lattice-work, as in Ascidia. The cause of this is that the 
foremost portion of the intestinal wall adheres in places 
to the external skin, and that, at these points of adhesion, 
openings form in the wall and extend from outside into 
the intestine. At first these gill-openings are but very few, 
but soon they are numerous, appearing first in one row, 


then in two rows, one behind the other. The foremost 
gill-opening is the oldest. Finally, a lattice-work of fine 
gill-openings appears on each side. 

We must call special attention to the fact that at first, 
in the embryo of the Amphioxus, as in that of all other 
Vertebrates, the side wall of the neck is perforated in such a 
way by openings, that there is an open passage through the 
latter from the external skin into the anterior intestine 
(Fig. 158, K). The inhaled water, which is taken in to the 
gill-intestine through the mouth, passes out directly through 
the gill-openings. While the number of these gill-openings 
is increasing very rapidly, over the upper row of these a 
longitudinal fold rises, on each side, on the side-wall of the 
body (Fig. 159, U). The narrow body-cavity prolongs itself 
in these longitudinal folds (Lh). Both side-folds grow 
downward and hang as free gill-roofs. The free edges of 
these then incline towards each other and coalesce in the 
middle line of the ventral side, thus forming the ventral 
seam or Raphe (Fig. 160, R). The gill-pore alone remains 
open (Fig 15, p). Thus originates a closed gill-cavity 
answering exactly to that of Fishes, and at the same time 
identical with that of the Ascidians. The gill-cavity of the 
Ascidian, the Amphioxus, the Fishes, and the larval Am- 
phibia, are to be regarded as homologous parts. This large 
gill-cavity, filled with water and communicating freely 
with the surrounding water, must be distinguished from 
the small body-cavity, filled with lymph and without any 
external communication. The latter, the codoma (Figs. 
158-160, Lh), in the adult Amphioxus is very narrow and 
very small in size (Fig. 152, Lh). When the gill-cavity 
of the Amphioxus is complete, the respiratory water. 




'/ Zh 

Figs. 158-160. — Transverse section through an early larval form of 
Amphioxus. (Diagrammatic, after Kolph.) (Cf. Fig. 152, p. 424.) In Fig. 
158 there is a free passage from without into the intestinal cavity (D), 
through the gill -openings (K). In Fig. 159 the lateral longitudinal folds 
of the body-wall, the gill-roof, are forming, growing downwards. In Fig. 
160 these side-folds have grown towards each other and their edges have 


ooalesced in the middle line of the ventral side (R) . The respiratory water 
now passes from the intestinal cavity (D) into the gill-cavity {A). In all, 
the letters indicate the same parts : N, mednllary tube ; Ch, notochord ; 
My side-muscles ; Lh, body-cavity ; O, portion of the body-cavity in which 
the sexual organs afterwards form ; D, intestinal cavity lined by the intes- 
tinal-glandular layer (a); A, gill-cavity ; X, gill-openings ; b = E, outer skin, 
or epidermis ; E xt the same as the inner epithelium of the gill-cavity; B tt 
the same as the outer epithelium of the gill-cavity. 

which was taken in at the mouth, passes out, no longer 
directly through the gill-openings, but through the gill-pore 
(jp. branchialis). That portion of the intestinal canal which 
is situated behind the gill-body becomes the stomach- 
intestine, and forms on the right side a single purse-like 
protrusion, which becomes a blind liver-sac. This digestive 
portion of the intestinal canal is enclosed in the narrow 

In an early stage of individual development, the struc- 
ture of the body of the Amphioxus larva still corresponds 
essentially with our ideal "Primitive Vertebrate." The 
body afterwards, however, undergoes various modifications, 
especially in the anterior portion. These modifications are 
uninteresting to us at present, because they depend on 
special conditions of Adaptation, nor have they anything to 
do with the hereditary vertebrate type. Of the remaining 
portions of the body of the Amphioxus, we need only 
remark that the germ-glands, or internal sexual organs, do 
not deveope till later, and, as it appears, directly from the 
inner cell-coat of the body-cavity, from the ccelom- 
epithelium. Although no extension of the body-cavity 
is afterwards discernible in the side walls of the gill-cavity, 
in the gill-roofs (Fig. 152), yet such an extension does at 
first exist (Fig. 159, 160, Lh). In the lowest part of this 


extension, the sexual glands originate from a portion of 
the ccelom-epithelium (Fig. 160, 0). In other respects, the 
farther modification of the larva into the adult form of the 
Amphioxus is so simple that we need not now follow it. m 

We will now turn to the history of the development of 
the Ascidian, an animal apparently so much lower and so 
far simpler in its organization, which spends the greater 
part of its life as an unshapely mass, adhering to the bottom 
of the sea. It was most fortunate that Kowalevsky in his 
researches first fell in with those larger Ascidian forms 
which most clearly testify to the kinship between Verte- 
brates and Invertebrates, and of which the larvae, in the 
first stages of development, are exactly similar to those of 
the Amphioxus. This agreement in all the essential charac- 
ters is so great that it is really only necessary to repeat 
word for word what has already been said about the 
Ontogeny of the Amphioxus. 

The egg of the larger Ascidia (Phallusia, Cynthia, etc.) 
is a simple globular cell -f^ to £ mm. in diameter. In the 
cloudy, finely granular yelk a bright, globular germ- vesicle 
(nucleus) about -^ mm. in diameter is seen, enclosing a 
germ-spot (nucleolus). (Fig. 1, Plate X.) Within the enve- 
lope, which surrounds the egg, the parent-cell of the 
Ascidian, after fertilization, passes through exactly the 
same changes as the cytula of the Amphioxus. The special 
incidents in the fertilization and egg-cleavage of the largest 
and most interesting of our Ascidians (Phallusia mam- 
milata) have lately been very accurately studied and 
described by Edward Strasburger. The remarkable details 
of these processes, which do not, however, touch our present 
purpose, are given in the excellent work by that writej 


on " Zellbildung." 122 Here, as in the Amphioxus, the germ- 
vesicle (nucleus) of the egg-cell disappears in great measure 
even before fertilization, while, after the latter process is 
accomplished, the monerula, in consequence of the re-forma- 
tion of a kernel, becomes a cytula. This breaks up by 
primordial cleavage into 2, 4, 8, 16, 32 cells, and so on. By 
continued total cleavage the morula forms the mulberry-like 
heap of like cells. Within this a liquid accumulates, and 
thus a globular germ-membrane vesicle is once more formed, 
the wall of which consists of a single cell-stratum, the 
blastoderm (Plate X. Fig. 3), just as in the case of the 
Amphioxus a true Gastrula, a simple Bell-gastrula (Plate X. 
Fig. 4), is formed from this blastula by inversion. 

Up to this point in the evolution of the Ascidian there 
is no definite ground for assuming its near relationship to 
the Vertebrates ; for a similar Gastrula arises in the same 
way in the most diverse animals of other tribes also. Now, 
however, comes an evolutionary process which is peculiar to 
Vertebrates, and which absolutely demonstrates the kinship 
of the Ascidia and the Vertebrates. From the outer skin 
(epidermis) of the Gastrula originates a medullary tube, 
and, between this and the primitive intestine, a notochord 
— organs which otherwise occur only in Vertebrates, and 
are peculiar to them. The formation of this highly im- 
portant organ takes place in the Gastrula of the Ascidian 
exactly as in that of the Amphioxus. In the Ascidian also 
the oblong-round or oval Gastrula-body, which has but a 
single axis, becomes flat on one side, on the future dorsal 
side. Along the central line of this flat side, a furrow or 
trench forms, the medullary furrow, and on either side of 
this two parallel ridges or swellings arise from the skin- 


Layer. These two medullary swellings coalesce over the 
furrow, thus forming a tube ; in this case also, this nerve 
tube or medullary tube is originally open in front, but 
closed behind. Again, in the Ascidian larva also, the per- 
manent mouth-ope ning is a new formation, and does not 
originate from the primitive mouth of the Gastrula; the 
latter closes, and in its neighbourhood the future anal 
opening is formed by inversion from the outside, at the 
opposite end from the opening of the medullary tube (Plate 
X. Fig. 5, a). 

While these important changes are taking place, exactly 
in the same way as in the Amphioxus, a tail-like appendage 
grows out from the posterior end of the larval body, and 
the larva curls itself within the spherical egg-covering in 
such a way that its dorsal side projects, while the tail is 
bent back upon the ventral side. In this tail now de- 
velops a cylindrical cord, composed of cells, the anterior 
end of which extends into the body of the larva between 
the intestinal and the medullary tubes : this is the chorda 
dorsalis, an organ which, except in this one case, is found 
only in Vertebrates, and of which no other trace is to be 
seen in Invertebrates. Here, again, the notochord consists, 
at first, of a single row of large bright cells (Plate X. Fig 
5, ch); afterwards it consists of several cell-rows. So, too, in 
the Ascidian larva, the notochord develops from the middle 
portion of a cell-stratum, the side portions of which become 
tail-muscles, and which can, therefore, only be the skin- 
fibrous layer. At the same time, a cell-stratum splits ofi 
from the intestinal wall, which afterwards forms the heart, 
the blood and the vascular system, and also the intestinal 
muscles. This is the intestinal-fibrous layer. 



On making a section through the middle of the body in 
this stage (at the point where the tail joins the trunk), we 
find in the Ascidian larva precisely the same characteristic 
disposition of the chief organs as in the larva of the 
Amphioxus (Plate X. Fig. 6). In the middle, between the 
medullary tube and the intestinal tube, is the chorda dor- 
salis ; and on each side of the latter, the muscle -plates of 
the back. The section of the Ascidian larva now differs in 
no essential way from that of our ideal Vertebrate (Fig. 

When it has reached this stage of development, the 
Ascidian larva begins to move within the egg-covering. 
This ruptures the egg-covering ; the larva emerges from the 
latter, and swims freely about in the sea by means of its 
rudder-like tail (Plate X. Fig. 5). These free-swimming 
Ascidian larva have long been known to science. They 
were first observed by Darwin during his voyage round the 
world in 1833. In external form they resemble the larva 
of the frog, the tadpole, and they move about in the water 

Fig. 161. — Transverse section through ideal 
Primitive Vertebrate (Fig. 52). The section 
passes through the sagittal axis and the cross 
axis : n, medullary tube ; x, notochord ; t, dorsal 
vessel ; v, ventral vessel ; a, intestine ; c, body, 
cavity; m l , dorsal muscles; m 2 , ventral mus- 
cles ; h, outer skin. 

like the latter, using their tail as a rudder. This highly 
developed youthful condition of free movement lasts, how- 
ever, only for a short time. A further progressive develop- 


ment yet occurs ; two small sense-organs make their appear- 
ance in the foremost part of the medullary tube : of these 
the one is, according to Kowalevsky, an eye, the other an 
organ of hearing of the simplest structure. A heart also 
develops on the ventral side of the animal, on the lower 
wall of the intestine ; and this is of the same simple form, 
and is situated in the same place as the heart in Man and 
all other Vertebrates. In the lower muscle-wall of the 
intestine a wart-like growth makes its appearance — a solid 
spindle-shaped cord of cell, — the interior of which soon 
becomes hollow : it begins to move by contracting in oppo- 
site directions, now backwards, and then again forwards, as 
in the full-grown Ascidian. In this way the blood-fluid, 
collected in the hollow muscular pouch, is driven in alter- 
nate directions into the blood-vessels, which develop at both 
ends of this tubular heart. A main vessel traverses the 
dorsal side of the intestine, another its ventral side ; the 
former represents the aorta (Fig. 161, t) and the dorsal vessel 
of Worms. The latter represents the intestinal vein (Fig. 
161, v) and ventral vessel of Worms. 

When these organs are complete, the progressive Onto- 
geny of the Ascidian is at an end, and retrogression now 
commences. The freely-swimming Ascidian larva sinks to 
the bottom of the sea, relinquishes its power of free loco- 
motion, and becomes fixed. By means of that very part 
of its body which was foremost in locomotion, it adheres 
to stones, marine plants, shells, corals, and other objects at 
the bottom of the sea To secure it to these, several 
excrescences are employed, usually three wart-like bodies, 
which may be observed on the larva, even while it yet 
swima. The tail, which is of no further use, is now lost 



It undergoes fatty degeneration, and is cast off together 
with the entire notochord. The tail-less body becomes a 
shapeless bag, or sac, which, by retrograde metamorphosis 
of its separate parts and by re-formation and modification, 
gradually acquires that remarkable structure which has 
already been described. 

Fig. 162. — Appendicularia (Copelata), 
seen from the left side : m, mouth; fe, gill- 
intestine ; o, oesophagus ; v, stomach ; a, 
anus ; n, brain (upper throat ganglion) ; 
g, ear-vesicle ; /, groove under the gill ; 
h, heart ; t, testes ; e, ovary ; c, notochord ; 
s, tail. 

Among the extant Mantle 
Animals (Tunicata) there is, how- 
ever, an interesting group of 
small animals which retain 
throughout life the tailed, inde- 
pendent ascidian larval stage of 
development, and which, by 
means of their permanent, broad, 
rudder-like tails, move actively 
about in the sea. These are the 
remarkable AppeTidi cularice (Fig. 
162). They are the only extant 
Invertebrates permanently pos- 
sessing a notochord, and are, 
therefore, the nearest allies of 
the extinct Chorda Animals 
(Chordoma), of the primaeval 

Worms which must be regarded as the common parent-form 


of Mantle Animals (Tunicata) and of Vertebrates. The 
notochord of the Appendicularia is a long cylindrical cord 
(Fig. 162, c), which serves to connect the muscles which 
move the flat, rudder-like tail 

Among the various retrogressions which are undergone 
by the Ascidian larva after it has attached itself, the 
degeneration of one of the most important parts of the 
body, the medullary tube, is, next to the loss of the noto- 
chord, of peculiar interest. While in the Amphioxus the 
medulla steadily develops, that of the Ascidian larva soon 
shrinks to the proportions of a small, insignificant nerve 
ganglion, which lies over the mouth -opening, above the 
gill-body, and which represents the exceedingly low mental 
endowments of this animal (Plate XL Fig. 14, m). This 
insignificant remnant of the medullary tube seems to retain 
no likeness to the medulla of Vertebrates, although it 
originated from the same rudiment as the medulla of the 
Amphioxus. The sense-organs, which had developed in the 
anterior end of the nerve-tube, are also lost ; in the full- 
grown Ascidian there is no trace of them. On the other 
hand, the intestinal canal now develops into a very 
capacious organ. This soon breaks up into two separate 
parts — a wide anterior gill-intestine for respiration, and a 
narrow posterior stomach-intestine for digestion In the 
former the gill-openings form in exactly the same way as 
in the Amphioxus. At first the number of gill-openings is 
very small ; it afterwards, however, increases considerably, 
and gives rise to a large, lattice-like perforated gill-body. 
The " hypobranchial groove " originates in the central line 
of the ventral side of this gill-body. The wide gill-cavity, 
which surrounds the gill-body, also develops in the ABcidian 


just as in the Amphioxua. The excretory opening of the 
former corresponds fully to the abdominal pore of the latter. 
In the adult Ascidian the gill-intestine and the heart rest- 
ing on the ventral side of the latter, are almost the only 
organs that recall the original relationship to Vertebrates. 

In conclusion we will glance at the development of the 
curious external gelatinous mantle, or cellulose sac, in which 
the Ascidian is afterwards entirely enclosed, and which 
characterizes the whole class of Mantle Animals (Tunicata). 
Very various and remarkable views have been entertained 
as to the formation of this mantle. For instance, it was the 
opinion of Kowalevsky, that the animal does not itself 
form the mantle, but that the latter is produced by special 
cells from the maternal body, which surround the egg. 
According to this the mantle would be a permanent 
egg-envelope. This is contrary to all analogy, and a 
yriori highly improbable. Another naturalist, KupfTer, 
who has confirmed and extended the researches of the 
former, assumed that the mantle develops from cells which, 
even before the impregnation of the egg-cell, form from the 
outer portion of the yelk, and separate entirely from the 
liner portion This seems very doubtful and unlikely. 
Hertwig's researches, which are confirmed by my own 
observations, first showed that the mantle develops as a 
so-called "cuticula." It is an exudation from epidermic 
cells, which soon hardens, separates from the real body of 
the Ascidian, and condenses so as to form a strong envelope 
round the latter. The matter of these cells is chemically 
indistinguishable from the cellulose of plants. While the 
epidermic cells of the external horn-plate are secreting this 
mass of cellulose, some of them drop into it, continue to 


live in the exuded mass, and aid in constructing the mantle 
In this way the strong external covering is at length 
formed, grows thicker and thicker, and in many adult 
Ascidia constitutes upwards of two-thirds of the entire mass 
of the body. 128 

The farther development of the individual Ascidian is 
of no special interest to us, and we will therefore not continue 
to trace it. The most important result, supplied by Onto- 
genesis, is its perfect agreement with that of the Amphioxus 
in the earliest and most important stages of its germ- 
history. It is only after the medullary and intestinal tubes, 
and, between these, the notochord with its muscles, have 
been formed, that their development takes different direc- 
tions. The Amphioxus pursues a steadily progressive course 
of development, till it entirely resembles the parent-forms 
of the higher Vertebrates, while the Ascidian, on the con- 
trary, enters on a course of retrograde metamorphosis, and 
finally, in the developed state, appears as a very imperfect 
member of the Worm group. 

Those who again review all the remarkable facts which 
we have found both in the structure and in the germ- 
history of the Amphioxus and Ascidian, and who then 
, compare these with the previously ascertained facts of 
human germ-history, will not think that I have ascribed 
exaggerated importance to these highly interesting animal 
forms. For it is now evident that the Amphioxus as the 
representative of Vertebrates, and the Ascidian as the repre- 
sentative of Invertebrates, form the bridge which alone can 
span the deep gulf between these two main divisions of the 
animal kingdom. The fundamental agreement exhibited 
by the Lancelet and the Ascidian in the first and the most 


important points of their embryonic development does not 
only testify their close anatomical form-relationship and 
their connection in the system ; it also testifies their true 
blood-relationship and their common origin from one and 
the same parent form; and hence it at the same time 
throws a flood of light upon the earliest origin of human 
genealogy. 124 

Writing in 1868 " on the origin and genealogy of the 
human race/' I insisted upon the extraordinary importance 
of this circumstance, and declared that we must accordingly 
1 regard the Amphioxus with special veneration as that 
animal which alone of all extant animals can enable us to 
form an approximate conception of our earliest Silurian 
vertebrate ancestors." This proposition has given very 
great offence, not only to unscientific theologians, but also 
to many others, especially such philosophers as still cherish 
the anthropocentric error, and who look on man as the fore- 
ordained object of " creation," and as the true final cause of 
all terrestrial life. The " dignity of humanity," it was said 
in a church newspaper, is, by such a statement as mine, 
"trodden underfoot, and the divine rational conscience of 
man grievously hurt." 

This indignation at my honest and deep respect for the 
Amphioxus is, I am free to confess, quite incomprehensible 
to me. If, on entering a grove of ancient oaks, we express 
reverence for these venerable trees, the life of which has 
endured a thousand years, no one thinks this unnatural 
Yet how high above the oak does the Amphioxus, or even 
bho Ascidian organization, stand in this respect ! And what 
are the thousand years of life of a venerable oak compared 
with the many millions of years the history of which is told 


by the Amphioxus ! But apart from all this, the Amphi- 
oxus (skull- less, brainless, and memberless as it is) deserves 
all respect as being of our own flesh and blood ! At any 
rate, the Amphioxus has better right to be an object of 
profoundest admiration and of devoutest reverence, than any 
one in that worthless rabble of so-called " saints " in whose 
honour our "civilized and enlightened" cultured nations 
erect temples and decree processions. 

The infinite importance of the Amphioxus and the Ascidian 
as explaining the development of Man, and consequently his 
true nature, may be clearly seen from the following sum- 
maries, in which I have stated the principal homologies of 
the highest and of the lowest Vertebrates (Table IX.). The 
table exhibits the undeniable fact that the human embryo 
at an early period of its development agrees in the most 
essential points of its organization with the Amphioxus and 
with the embryo of the Ascidian, while, on the other hand, 
it differs radically from the developed Man. It is, however, 
equally important that we should remember the profound 
gulf which separates the Amphioxus from all other Verte- 
brates. Even yet the Lancelet is represented in all text- 
books of Zoology as a member of the Fish class. When (in 
1866) I totally separated the Amphioxus from the Fishes, and 
divided the entire vertebrate tribe into two chief groups, the 
Skull-less Animals (Amphioxus) and the Skulled Animals 
(all other Vertebrates), my classification was regarded as a 
useless and unfounded innovation. 116 How the matter 
stands is best seen in the appended table (Table X). In all 
essential points, Fishes are more nearly allied to Man than 
to the Amphioxi 

( 4^5 ) 


Systematic Survey of the most important homologies between the human 
embryo, the embryo of the Ascidian, and developed Amphioxus on the 
one hand, and on the other hand, the developed Man. 

Mrnbryo of 





L — Products of the Differentiation of the Shin-layer, 

Naked outer skin 

Naked outer skin 

Naked outer skin 

Hairy outer skin 

Simple medullary 

Simple medullary 

Simple medullary 

Brain and spinal 





Primitive kidney (?) 

Primitive kidney (?) 

PrimitiTe kidney 

Oviduct and 

'excretory canal?) 



' imple thin leather 

Simple thin leather 

Simple thin leather 


skin (Corium) 

skin (Corium) 

skin {Corium) 

thick leather skin 

Simple skin-mus 

Simple trunk- 

Simple muscle- 


oular pouch 

muscle system 






Vertebral oolumn 

No skull 

No skull 

No skull 

Bony skull 

No limbs 

No limbs 

No limbs 

Two pair of limbs 


Separated sexual 


Separated sexual 

sexual glands 


sexual glands 


IL— Products of the Differentiation of the Intestinal layers. 

Simple body cavity 


Dorsal vessel 
Simple liver pouch 


Simple intestinal 

tube with fill- 

Simple body cavity 

Simple tubular 

Simple liver ponch 

Simple intestinal 
tube with gill- 

Simple body cavity 


Simple liver ponch 

Simple intestinal 
tube with gill- 

Distinct chest and 
ventral cavities 

Four- chambered 


Large differs** 
tiated liver 

Differentiated a» 
testinal tube 
without gill- 




Systematic Surrey of the points of connection in form of the Ascidian and 
Amphioxus on the one side, and the FiBhes and Men on the other, in 
oompletely developed conditions. 









Head and trunk 

Head and trunk 

Head and trunk 

Head and trunk 

not distinct 

not distinct 



No limbs 

No limbs 

Two pair of limbs 

Two pair of limbs 

No sknll 

No skull 

Developed skull 

Developed skull 

No tongue -bone 

No tongue-bone 



No jaw-apparatus 

No jaw-apparatus 



(upper * and 

(upper and 

lower jaws) 

lower jaws) 

No vertebral 

No vertebral 

Articulated verte- 

Articulated verte. 



bral column 

bral column 

No ribs 

No ribs 



Brain undifferen- 

Brain undifferen- 

Brain differen- 

Brain differen- 





Eyes rudimentary 

Eyes rudimentary 

Eyes developed 

E} r es developed 

No ear-organ 

No ear-organ 

Ear - organ with 

Ear- organ with 

three semicir- 

three semicir- 

cular canals 

cular canals 

No sympathetic 

No sympathetic 







Intestinal epithe- 

Intestinal epithe- 

Intestinal epithe- 

Intestinal epithe- 

lium ciliated 

lium ciliated 

lium not ciliated 

lium not oiliated 

Simple liver (or 

Simple liver (blind 

Compound liver 

Compound liver 





No ventral sali- 

No ventral sali- 

Ventral salivary 

Ventral salivary 

vary gland 

vary gland 



No swimming 

No swimming 

Swimming blad- 

Lungs (swimming 



der (rudimen- 
tary lungs) 


Kidneys rudimen- 

Kidneys rudimen- 

Kidneys deve- 

Kidneys deve- 


tary (?) 



Simple heart 

Simple tubular 

Heart with valves 

Heart with valvee 



and chambers 

and chambers 

Blood colourless 

Blood colourless 

Blood red 

Blood red 

No spleen 

No spleen 





Thyroid gland 

Thyroid gland 

groove on gill- 

groove on gill- 



I ( 467 ) 


Systematic Survey showing the derivation of the germ-layers of the Aniphioxu* 
from the parent-cell (oytula), and of the main organs from the germ .layer*. 


(Tree showing the ontogenetic descent of the cells in the Amphioxus). 1 ** 

Medullary tube 
Tubus meduUaris 



Outer skin 






Blood -vessels 

Canales Gill-epithelium 
tangueferi Epith. branchiaU 

Leather skin 




-• v- 

Testes Ovaries 

Ttiticuli Ovaria 






















digest ivum 

Skin-sensory Skin-fibrous 

layer layer 

Ijimina neurodermalis) {Lamina inodermalW) 

(Fig. 157, hs) (Fig. 157, hf) 

Intestinal-fibrous Inte^tinal-glan- 

lay -r dular layer 

{Lamina irwgastralis) (Lamina myxogash 
(Fig. 157, dj) (Fig. 157, dd) 


Exoderma (Fig. 155, *) 
flSkiu- layer, or outer germ-layer) 


Ento-'erma (Fig. 155, i- 
(Intestinal layer, or inner germ-layer) 

Gastrula fFig. 22, j,k ~| 

(Cup-germ) LFig. 155. p. 444 J 

Blastula (Fig. 22, F, 0) 

Mo*ula (Fig. 23, B) 

Cytula (Fig. 22, B) 

Monerula (Fig. S2, A)