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PRINCETON, N. J.
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Division . . . .-
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Digitized by the Internet Archive
in 2011 with funding from
Princeton Theological Seminary Library
http://www.archive.org/details/evolutionofm01haec
THE EVOLUTION OF MAN,
haeckel's evolution of man. plate /.
DEVELOPMENT OF THE FACE (Third Stage).
EXPLANATION OF CHAP. XXI.
M. Man.
B. Bat. b. Sheep.
C. Cat
THE
EVOLUTION OF MAN
A POPULAR EXPOSITION
PRINCIPAL POINTS OF HUMAN ONTOGENY AND PHYLOGENY.
FEOM THE GERMAN OF
ERNST HAECIiEL,
PE0FE8S0K IN THE UN1VEK8ITT OF JENA,
AUTUOK OP "the HI8T0KY OP CKEATION," ETC.
IN TWO VOLUMES.
VOL. I.
NEW YOEK :
D. APPLETON AND COMPANY,
549 & 551 BROADWAY.
1879.
CONTENTS OF VOL. I.
PAGE
List of Plates ... ... ... ... ... ... xiv
List of Woodcuts ... ... ... ... ... xv
List of Genetic Tables .. . ... ... ... ... xviii
Preface to the First Edition ... ... ... xix
Preface to the Third Edition ... ... ... ... xxvii
Prometheus ... ... ... ... ... xxxvii
Faust ... ... ... ... ... ... xxxvii
CHAPTER I.
THE FUNDAMENTAL LAW OF THE EVOLUTION OF ORGANISMS.
General Significance of the History of the Evolution of Man. — Ignor-
ance of it among the so-called Educated Glasses. — 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 Fundamental
Law of Biogeny. — Heredity and Adaptation are the two Formative
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
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
VI CONTENTS.
PACK
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
Marrow) is involved in that of the Psychic Activities, or the Mind 1
CHAPTER II.
THE EAELIER HISTORY OP ONTOGENY.
Caspar Friedkich Wolff.
The Evolution of Animals as known to Aristotle. — His Knowledge of
the Ontogeny of the Lower Animals. — Stationary Condition of the
cientifid Study of Nature during the Christian Middle Ages —
First Awakening of Ontogeny in the Beginning of the Seventeenth
Century. — Fabricius ab Aquapendente. — 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, Spallauzani. — 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. — Tne Theoria Generationis. — Ee-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 ... ... ... 25
CHAPTER III.
MODERN ONTOGENY.
Karl Ernst Baer.
Karl Ernst Baer, the Principal Disciple of Wolff. — The Wurzbnrg 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-
CONTENTS. Vll
PAOB
salis. — The Four Types of Evolution in the Four Main Groups of
the Animal Kin.£?dotn. — Baer's Law of the Typo of Evolution and
the Degree of Perfection. — Explanation of this Law by the Theory
of Selection. — Baer's Successors : Eathke, Johannes Miiller, Bis-
choff, Kolliker. — The Cell Theoiy : Schleideu, Schwann. — Its Appli-
cation to Ontogeny : Eobert Remak. — Retrogressions in Ontogeny :
Reichert and His. — Extension of the Domain of Ontogeny : Darwin 48
CHAPTER IV.
THE EARLIER HISTORY OF PHYLOGENY.
Jean Lamarck.
Phylogeny 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 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
Philosopliie Zoologique. — First Scientific Statement of the Doctrine
of Descent. — Modification of Organs by Practice and Habit, 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
" Formation and Transformation of Organisms." — Goethe's Theory
of the Tendency to Specific Differences (Heredity) and of Meta-
morphosis (Adaptation) ,,. ... ... ... ... 70
CHAPTER V.
MODERN PHYLOGENY.
Charlks Darwin.
Relation of Modem to Earlier Phylogeny. — Charles Darwin's Work on
the Origin of Species. — Causes of its Remarkable Success. — The
Theoiy of Selection : the Interrelation of Hereditary Transmission
and Adaptation in the Struggle for E.\istence. — Darwin's Life and
Voj'age Round the Woi'ld — His Grandfather, Erasmus Darwin. —
Charles Darwin's Study of Domestic Animals and Plants. — Com-
Vm CONTENTS.
parison 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 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 Induc-
tive Law of Biology. — Foundation of this Induction. — Palaeon-
tology.— Comparative Anatomy. — The Theory of Rudimentary
Organs. — Parposelessness, or Dysteleology. — Genealogy of the
Natural System. — Chorology. — CEkology. — Ontogeny. — Refutation
of the Dogma of Species. — The " Monograph on the Chalk
Sponges;" Analytic Evidence for the Theory of Descent ... 93
CHAPTER VI.
THE EGG-CELL AND THE AMCEBA.
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-
cell, compared with a highly Differentiated Mind-cell or Nerve-cell
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-celled
Organisms. — The AmcEba3. — Organization and Vital Phenomena. —
Their Movements. — Amoeboid Cells in Many-celled Organisms. —
Movements of such 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
CHAPTER VII.
THE PROCESSES OF EVOLUTION AND IMPREGNATION.
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
CONTENTS. ix
PAGE
Standard of Measurement for the Grade of tlie State. — Parallel
between the Processes of Individual and of Eace Development. —
The Functions of Evolution. — Growth. — Inorganic and Orf^anic
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 Physiology,
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-csll. — 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. — Disappear-
ance of the Germ-vesicle. — Mouernla. — Reversion to the Monera-
furm.— The Cytula ... ... ... ... ... ...148
CHAPTER VIII.
EGG-CLEAVAGE AND THE FORMATION OF THE GERM-LAYERS.
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. — Genn-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-
gastrula (Annjhigastrula) 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 siiper-
ficialis) and Vesicular Gastrula {Fei-6-Gastrula) of Articulates
(Arthropoda). — Permanent Two-layered Body-form of Lower
Animals. — The Two-layered Primajval Parent-form ; Gastraea. —
Homology of the Two Primary Germ-layera in all Intestinal
Animals (Metazoa). — Significance of the Two Primary Germ-
layers. — Origin and Significance of the Four Secondary Germ,
layei-s.— The Exoderm or Skiu-laycr gives rise to the Skin-sensory
CONTENTS.
PAGE
Layer and the Skin-fibrons Layer.— The Entoderm or Intestinal
Layer gives rise to the Intestinal-fibrous Layer and the Intestinal-
glandular Layer ... ... ... ... .. ... ... ... ISi
CHAPTER IX.
THE VERTEBRATE NATURE OF MAN.
Relation of Comparative Anatomy to Classification. — The Family -rela-
tionship of the Types of the Animal Kingdom. — Different Signi-
ficance and Unequal Value of the Seven Animal Types. — The
Gastraa Theory, and the Phylogenetic Classification of the Animal
Kingdom. — Descent of the Gastraaa from the Protozoa. — Descent
of Plant-animals and Worms from the Gastrsea. — 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 {coriwni). — The Outer-skin (epidermis). — Body-cavity (cceZoma).
■ — 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 Secondai'y Germ-layers ... 244
CHAPTER X.
THE CONSTRUCTION OF THE BODY FROM THE GERM-
LAYERS.
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 Germ-area. — Light Germ-area (area ji3cZZ«ci(ia.) 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
CONTENTS. XI
TAGU
separates the Germ-shield into a Right and Left Ilalf. — 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
Homy Layer, Spinal Canal, and Primitive Kidney. — Formation of
the Ca3lom and the First Arteries. — The Intestinal Canal proceeds
from the Intestinal Furrow. — The Embryo separates from the Germ-
vesicle. — 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 ... 274
CHAPTER XI.
GENERAL STRUCTURE AND ARTICULATION OF THE
INDIVIDUAL.
Essential Agi'eement between the Chief Palingenetic Germ Processes
in the case of Man and in that of other Vertebi-ates. — The Human
Body, like that of all Higher Animals,dcvelops fx-om Two Primary and
Fom* 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
Anns. — 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 Vertebra) (Trunk-Segments, or Metamcra). — The Con-
struction and Origin of the Vertebral Column. — Vertebral Bodies
and Vertebral Arches. — Skeleton-plate and Muscle-plate. — Forma-
XH CONTENTS.
FAGB
tion of the Skull from the Ilead-plates. — Gill-openings and Gill-
arches. — Sense-organs. — Limbs. — The Two Front Limbs and the
Two Hind Limbs 328
CHAPTER XII.
THE GERM-MEMBEANES AND THE FIRST CIRCULATION OP
THE BLOOD.
The 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 Higher Mammals, and in
its Earlier Stages not even fi-om those of all Higher Vertebi-ates. —
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 Cii'culation of the
Blood, in the Allantois : Navel-arteries and Navel-veins. — Divisions
of Human Germ-history ... ... ... ... ... ... 3G3
CHAPTER XIIL
THE STRUCTURE OF THE BODY OF THE AMPHIOXUS AND
OF THE ASCIDIAN. .
Causal Significance of the Fundamental Law of Biogeny. — Influence
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. — Phylogenetic
Hypotheses are Reliable and Justified. — Importance of the Amphi-
CONTENTS, Xlll
PAGE
oxns 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
Eespiratory 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 (Petroniyzon). — Comparison of Amphioxus and Ascidian.
— Cellulose Tunic. — Gill-sac. — Intestine. — Nerve-centres. — Heart.
— Sexual Organs ... ... ... ... ... ... 406
CHAPTER XIV.
GEEM-HISTOEY OF THE AMPHIOXUS A^D OP THE
ASCIDIAN.
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
FuiTow, and betvFeen 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 Intestinal-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 forms
a Notochord between the Medullary and Intestinal Tubes. —
Ketrogressivo Development of the same. — The Tail with the
Notochord is shed. — The Ascidian attaches itself firmly, and
envelops itself 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 (Chordonia). — General
Comparison and Significance of the Amphioxus and the Ascidian 439
LIST OF PLATES.
Plate T. (Frontispiece). Development of the face in Mammals
(Man, Bat, Cat, Sheep) in three difierent stages
Explanation vol. ii. 346
Plate II. (between p. 240 and p. 241). Total egg-cleavage. Gas-
trulation of holoblastic eggs (primordial and nnequal cleavage)
Explanation 240
Plate III. (between p. 240 and p. 241). Partial egg-cleavao-e.
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 thi.s from the four secondary germ-layers ... Explanation 321
Plates V. (between p. 320 and j). 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. 3G2 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 362
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 405
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
LIST OF WOODCUTS.
FlGtrHE PAGT',
1. Human egg-cell . . 122
2. Iluuian liver-cell . . 124
3. Epithelium cell from tongue 124.
4. Thorny cells of epidermis 125
5. Hnman bone-cells . 126
6. Enamel cells of tooth . 12G
7. A minil-cell . . .128
8. Blood. cells in process of
division .... 131
9. Active lymph-cells . . 132
10. rrimitivo eggs of various
animals .... 13i
11. Mammalian egg-cell . . 136
12. Egg-cell of Hen . . 139
13. An Amoeba . . . 142
14. Egg-cell of a Chalk-sponge 144
15. Blood-cells absorbing mat-
ter .... 145
10. Blood-cells dividing . .159
17. Sperm-cells (seed-cells) . 173
18. Fertilizationof mamnialiaa
egg .... 175
19. Mononila of Slammal . 179
20. iloncron dividing . . 180
12. Cytula of Mammal . . 181
FIGURE PAGE
22. Germination of a Coral . 190
23. Gastrulaof Gastrojihysema 193
24. Gastrula of Sagitta . . 193
25. Gastrula of Uraster . . 193
26. Gastrnla of Nauplins . 193
27. Gastrula of Limnceus . 193
28. Gastrula of Amphiosus . 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 Ealjbit . . 210
37. Cytula of Eabbit . . 210
38. Rabbit-egg with two cells 210
39. Eabbit-egg with four cells 212
40. Eabbit-egg with eight cells 212
41. Gastrula of Eabbit . , 213
42. Egg of an Osseous Fish . 217
43. Gastrnla of an Osseous Fish 219
44. Egg-cell of Hen . . 223
45. Egg-cleavage of Bird . 225
IG. Mulberry-germ of Chick . 228
47. Bladder-germ of Chick . 228
48. Invaginated germ of Chick 228
XVI
LIST OF WOODCUTS.
FIGURE
49. Gastrala of Chick .
50, 51. Four secondary germ-
layers ....
52—56. Diagrammatic longitu-
dinal and transverse sec-
tions througli tlie ideal
Primitive Vertebrate
57, 58. „ „
59.
60.
61. „ „
62-69. Diagrammatic trans-
verse sections through
the most important germ-
forms of the ideal Primi-
tive Vertebrate
70. Diagrammatic transverse
sections through various
mammalian germs (ex-
plaining the separation
of the intestine from the
yelk-sac).
71. Gastrula of Mammal .
72. Intestinal germ-vesicle of
Mammal
73. Transverse section through
the intestinal germ-
vesicle of Mammal
74. Exoderm-cells of the above
75. Entoderm-cells of the above
76. Transverse section through
germ-area .
77-81. Intestinal germ-vesicle
of Babbit
82, 83. Germ-area of Babbit .
84. „ „
So. ]. J,
276
283
288
289
289
290
290
293
294
296
297
298
PAGK
ficui:e
PAGE
228
86.
Sole-shaped germ-shield
of Dog ....
298
236
87.
Sole-shaped germ-shield
' of Chick
298
88.
Transverse section
through germ-shield
300
256
89.
n »
301
259
90.
n n
302
263
91.
» »
304
264
92.
j> »
306
267
93.
)> »
309
94. Development of egg-mem-
branes .... 312
95. Transverse section
through germ of Chick . 317
96,97. „ „ 318
98. „ „ 319
99. „ „ 331
100. Separation of the intes-
tine from the yelk-sac
(diagrammatic) . . 333
101. Longitudinal section
through embryo Chick . 336
102. Longitudinal section
through bead of an
embryo Chick . . 337
103-105. Lyre-shaped Chick
embryo . . . 342
106, 107. Germ-disc or germ-
area of Rabbit . . 344
108,109. „ „ 345
110, 111. Skeleton of Man . 351
112. Transverse section
through germ of Chick . 352
113. Human neck-vertebra . 354
114. Human chest-vertebra . 354
115. Human lumbar-vertebra . 354
LIST OF WOODCUTS.
XVll
FIGURE
116, 117. Head of embryo
Cliick ....
118. Head of embryo Dog
119. Rudiments of the limbs .
120.
121. Lyre-shaped germ of Dog
122. Human germs from the
second to the fifteenth
week ....
123. 124. Anatomy of human
germs (four and five
weeks) ....
125. Head of Nose-ape .
126. Head of Julia Pastrana .
127-131. Human eggs and
germs from second to
sixteenth weeks .
132, 133. „ „
134..
135. Chick germ with allantois
136. Dog germ with allantois .
137.
138. Pregnant human uterus
with egg - membranes
and navel-cord . .
356
356
357
359
367
368
370
374
374
376
377
378
380
381
382
384
139.
Development of egg-mem-
branes ....
385
140.
Development of amnion .
387
141.
>i
388
142.
)i »
389
143,
144. Development of heart
392
145,
146. „
393
147.
» »
395
118.
First circulation of the
blood ....
396
149.
)) »»
397
150.
)) ))
398
151.
Amphioxus lanceolatus .
420
152.
Transverse section
through Amphioxus
424
153.
An Ascidian .
431
154.
Another Ascidian .
434
155.
Gastrula of Amphioxus .
444
156.
Gastrula of Sponge
445
157.
Transverse section
through Amphioxus larva
447
158-
-160. „
452
161.
Transverse section
through Vertebrate
457
162.
Appendicularia
459
LIST OF GENETIC TABLES.
TABLE PAGT?
I. Systematic Survey of the main brandies of Biogeny ... 24
II. Systematic Survey of the constituent parts of the one-
celled germ-form before and after fertilization ... 183
III. Systematic Survey of the most important differences 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
VI. 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 develojiment of the human
organ-systems from the germ-layers ... ... 327
VIII. Systematic Survey of the most imjiortant 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 ... ... ... 4G5
X. Systematic Survey of the relationship in form of the
Ascidian and Amiihioxus on the one hand, of the Fish
and Man on the other, in a fully developed condition 466
XI. Ontogenetic cell pedigree of the Amj)hioxus ... ... 467
PEEFACE TO THE FIRST EDITION.
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."
XX PREFACE TO THE FIRST EDITION.
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 historj^ 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 Eeymond, 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 of
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 w4iich, not only
is another limit, assumed to be different (but in reality
identical), indicated for human knowledge, but the con^
elusion is finally draw^i, that man will never pass over
these limits : " We shall never know that ! Ignorabimus ! "
As the unanimous thanks of the Ecclesia militans have
PREFACE TO THE FIRST EDITION. XXI
been gained by the author of this " Ignorahimus,'' 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 Lam*entian 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
miitabimur ! Semper ignorahimus ! "
With this Ignorabimus the Berlin school of Biology
tries to stop science in its advance along the paths of
evolution. This seemingly humble but really audacious
" Ignorahimus" is the " Ignoratis" of the infallible
Vatican and of the " black international " which it leads ;
that mischievous host, against which the modern civilized
XXU PREFACE TO THE FIRST EDITION.
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
si)ite 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 of
reason.
In this mighty " war of culture," affecting as it does
PREFACE TO THE FIRST EDITION. XXlll
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
Pioman 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 fm'nishes the 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
XXIV PREFACE TO THE FIEST EDITION.
undertook in publishing these was indeed much harder than
that incurred in the "Natural History of Creation;" for
while 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 emx^iric 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
PREFACE TO THE FIRST EDITION. XXV
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.
Ernst Heinrich Haeckel.
Jena, July 13, 1874<.
PEEFACE TO THE THIED EDITION.
When, two j^ears ago, I published the first edition of the
"History of the Evohition 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 of miracles, by suj^er-
natural creative acts. These adherents of the Creation
Theory I need not answer; for Anthropogeny, as the
special aj^plication of the Theory of Descent to Man,
naturally starts from the recognition of this latter theory :
ten years ago, in my Gcncrelle Morpliologie, and again in
the " Natural History of Creation," I explained my own
conception of this in sufficient detail.
XXVm PREFACE TO THE THIRD EDITION.
I cannot, however, refrain from defending my stand-
point against those naturahsts, 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 derHeuti-
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 Kdlliker 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
PREFACE TO THE THIRD EDITION. XXIX
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 logical
"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 Amphioxus), 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 Gastraeads from a one-celled
organism (resembling the undifferentiated Amoeba). Proofs,
as I believe, of these assertions are given in Chapters
XIII.-XXV. 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
XXX PREFACE TO THE THIRD EDITION.
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 Einged
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 of
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 Eiitimeyer, 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 Vogt 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
PREFACE TO THE THIRD EDITION. XXXI
mock at pliilosopliic 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 MorpJiologic, 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 i)rogress 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
XXXU PREFACE TO THE THIRD EDITION.
task of accumulating dry details to wider views, and thus to
gain sympathy with allied aims. On the other side, the
sterility of sach 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 hack 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. 387.)
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
PREFACE TO THE THIRD EDITION. XXXIU
of B'logcny, has impugned the very foundation on which
Anthropogeny rests. Most of his objections are, it appears
to me, refuted by the expLanations ^Yhich I have given
in this third edition as to the very important relations
of PaHngenesis and Kenogenesis. (Compare especially
Chapters I., 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
opijosed to the most recent researches of Van Beneden and
Eauber, of whom the former in the case of the Piabbit, the
latter in the case of the Chick, describes a kenogenetic
gastrula-form, which, in accordance with the Gastrsea
theory, may easily be referred to the palingenetic gastrula
of the Amphioxus. Kolliker says finally : "As the last 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 ^\rn.\)\Q petitio jprindiyii. 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 Wurzburg anatomist there
explains with reference to germ-history, his " essential
agreement in fundamental concej)tions " 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 Wcge der
XXXIV PREFACE TO THE THIRD EDITION.
Heutigen Entwickelungsgeschiclite "). 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) ; for
between the two there is the most intimate causal
connection.
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
PREFACE TO THE THIRD EDITION. XXXV
half a century ago accompanied bis wcll-kno^Yn " History
of the Evokition 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 of
" 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 last 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 VHI., IX.,
XYI., and XIX. especially a^Dpear 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 jiresent day,
leaves 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
XXXVl PREFACE TO THE THIRD EDITION.
of hmxican 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.
Ernst Heinrich Haeckel.
Jetia^ October C, 1876.
PROMETHEUS.
Enveil tliine heaven, Zeus, with vaporous cloud,
And practise, like a boy beheading thistles,
Ou oaks and mountain summits ;
Yet must thou let my earth alone to stand,
And these my dwellings, which thou didst not build,
And these my flocks, for whose bright glow
Thou enviest me.
I know not aught more wretched
Beneath the sun than you, ye Gods !
Who nourish pitcously.
With tax of sacrifice and reek of prayer ; your glory
Would starve, if children were not yet, and suppliants,
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 sun,
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 3 thanks for
saving
That slumbering one above.
Why should I honour thee ?
Hast thou e'er lighten'd the woes of the laden ones?
Hast thou e'er dx'ied the tears of the sorrowful?
It was not thou who welded me to manhood.
But Time the almighty. Fate the everlasting,
My Lords and thine.
XXXVill FAUST.
Dost fondly fancy I shall hate my life,
And hie me to the Avaste, 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 I! Goethe.
FAUST.
Earth's narrow circle is well known to me ;
What is above the eye can never see.
Fool, who peei"s 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 ears 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 day ;
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.
Goethe.
THE EVOLUTION OF MAN.
CHAPTER I.
THE FUNDAMENTAL LAW OF THE EVOLUTION OF
ORGANISMS.
General 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 Series of Evolution. — The Evolution of
the Tribe determines the Evolution of the Germ. — Ontogeny as au
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 tbe 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 Physiologj'. — 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.
"The History of the Evolution of Organisms consists of two kindred and
closely connected parts : Ontogeny, which is the history of the evolution of
individual organisms, and Phylogeny, which is the historyof the evolution
of organic tribes. Ontogeny is a brief and rapid recapitulation of
Phylogeny, dependent on the j'hysiological functions of Heredity (roproduc-
2 THE EVOLUTION OF MAN.
tion) and Adaptation (nutrition). The individual organism reproduces in
the rapid and short course of its own evolution the most important of the
changes in form through which its ancestors, according to laws of Heredity
and Adaptation, have passed in the slow and long course of their pateonto-
logical evolution." — Haeckel's Generelle 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." ^
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
GENERAL IGNORANCE OP THE HISTORY OF EVOLUTION. 3
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" have never seen such a
human germ, or embryo, in the early stages of development,^
nor are they aware that it is not at all different 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
fui'ther 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 (Marsupialia), 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 beai-s
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. Tlie 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 hiirher desrree, if we
refer these remarkable facts to their final causes, and recognize
4 THE EVOLUTION OF MAN.
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 myself 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,
OBJECT OF THE HISTORY OF EVOLUTION. 5
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,^
so-called, or more correctly with Ontogeny,^ 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,^ 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. AU 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
6 THE EVOLUTION OF MAN.
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 afford 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 ^ (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, " the first principle of Biogeny." '
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,
somewhat 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 forms
THE INFLUENCE OF PHYLOGENY ON ONTOGENY. 7
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 fonns 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
5 THE EVOLUTION OF MAN.
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
two 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
creation 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
find, — and not less so in the Phylogeny of man, — a number
CONNECTION BETWEEN PHYLOGENY AND ONTOGENY. 9
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
onto£fenetic 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 Gastrtea. A
later embiyonic form of the human being points with equal
certainty to a primitive worm-like ancestral form which is
related to the sea-squu-ts or Ascidians of the present day.
But the low animal forms Mdiich constitute the ancestral
line between the unicellular amoeba and the gastrtea, 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 pi-incipal objects to
lO THE EVOLUTION OF MAN.
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 Palingenetic process^ (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 bistory of the developed
ancestors. The term Kenogenetic process^ (or vitiation of
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
PALINGENESIS AND KENOGENESIS. II
of the greatest importance to scientific Phylogeny, which,
from the available empiric material supplied by Ontogeny,
by Comparative Anatomy, and by PalfBontology, 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 palseontological
evolutionary incidents in the history of the tribe. On the
other hand, this is not the case with the following germinal
12 THE EVOLUTION OF MAN".
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 qf 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 (Kenogenesis) is introduced by changing adapta-
tion."!"
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 Hetcrotopy ; if the
latter, Heterochrony.
HETEROTOPY AND HETEROCHRONY. 1 3
" 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 fjerm-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 gi-eatly affected by a displacement in position, which
is connected with the transition of embryo cells from one
gerra-layer into another.
The kenogenetic " displacements in time," or " Hetero-
chronisms," are equally significant. They are seen in the
fact that in the germ-history (Ontogeny) the sequence in
which the organs appears diflers 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,
14 THE EVOLUTION OF MAN.
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 the
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 animals permanently represent in the system of the
animal kingdom forms which appear transiently in tlie
evolution of individuals of higher groups. But formerly
HEREDITY AND ADAPTATION. I 5
it was impossible to understand and interpret aright this
remarkable resemblance. Dai'win's greatest merit is that
he has now enabled ns 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 oi'ganic 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 phj'siological 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 wliicli a human being passes in its embryonic
evolution ; it was impossible to comprehend the reason of
the curious series of various animal-like forms which appear
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 ogg, and that liis evolution
was only an unfolding of the form, a simple process of
growth. But this is not at all the case. On the contrary,
1 6 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 eveiy
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 learn the efiicient causes {causce
efficientes) of individual evolution ; by the aid of this theory
we first perceive that such mechanical causes alone suffice
to efiect the evolution of the individual organism, and
that the co-operation of designing, or teleological causes
(causoB 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, as 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 of
MONISM AND DUALISM, 1/
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 of
the Doctrine of Evolution, the most wonderful problems,
hitherto deemed unapproachable, of the organization of
man and animals have admitted of a natural solution, of a
mechanical explanation, by non-purposive causes. It has
enabled us to substitute everywhere unconscious causes
acting from necessity', for conscious purposive causes.^^
If the recent progress in the Doctrine of Evolution had
accomplished only 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, Avhich has heretofore prevailed in speculative
philosophy, must ultimately prevail throughout philosophy.^^
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 Icam the most important facts in
thLs field of research.
The significance of the facts of Ontogeny is so great and
so evident tliat 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 with the fact that every human being develops
from an egg, and that this egg is a simple cell, like the egg-
1 8 THE EVOLUTION OF MAN.
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 diflerent 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 Ontogenj^ 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 difiers 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 b}^ 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
facts in favour of the monistic philosophy can be given than
INTERRELATION OF FORMS AND FUNCTIONS. 1 9
these efforts on the part of the dualistie 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
facts, " 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, I 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 perfonned 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 of
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.
20 THE EVOLUTION OF MAN.
But I must here add at once, that our knowledge of the
evolution of functions is as yet far from being 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 oriranic 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.^^
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
DEFECTIVE STATE OF PHYSIOLOGY. 21
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,
tlie lungs, and the organs of generation, affords us also most
important information as to the evolution of the respective
functions of these organs.
22 THE EVOLUTION OF MAN.
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 form
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,
similar to the permanent form of this organ in the lower
Fishes. Step by step this simple brain develops still
further, passing through forms corresponding to those of
ONTOGENY AND PIIYLOGENY. 2$
the Amphibia, Beaked Animals (Ornithostoma), 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 is
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
supernatural.
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
able to solve this the highest of all problems.
24
THE EVOLUTION OF MAN.
TABLE I.
List of the principal branches of Biogeny, or the History or Organic
Evolution, with reference to the four chief stages of Organic In-
dividuality— Cell, Organ, Person, and Race.'*
I.
First 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 tlie in-
dividual organism).
Germ-history of
Forms.
(^Morphogeny.')
2. Germ-hi>tory of
Functions.
(^I'hysiogeny )
1. Germ-history of the cells (and cytods)
and of the tissues composed of the cells.
Jlistogeny.
2. Germ-history of the organs, and of the
systems and apparatus composed of the
organs. Organogeny.
3. Germ-history of the persons (called
" the history of the evolution of bodily
form "). Blastogeny.
4. Germ-his'ory of races (or of social
aggregates composed of persons : fa-
milies, communities, states, etc. Cor-
\ mogeny.
The gcrni-history of the functions, or the
history of the development of vital
activities in the individual, has not yet
been accurately and scientifically in-
vestigated.
II.
Second branch of Biogeny,
or of the history of the
evolution of organisms:
Tribal History, or •<
rhylogony (history of
the pala.'ontological evo-
lution of 01 ganic
species).
!. Tribal history
of Forms.
(Morphophyly.)
1. Tribal history of the cells (hardly at-
tempted as yet). Histophyly.
2. Tribal history of organs (an unrecog-
nized main object of comparative ana-
tomy). Organophyly.
■{ 3. Tribal history of persons (an unrecog-
nizt'd main object of the natural system
of classification). Blastophyly.
4. Tribal history of races (or of social
aggregates composed of persons : fa-
milies, commuuilies, states, etc. Cor-
mophyly.
4. Tribal history
of Functions.
(^I'hysiophyly.')
^The tribal history of the functions, or the
history of the pala>outological develop-
ment of vital activities, has, in the case
of most organisms, not yet been ex-
amined. In the case of man, a large
I ait of the history of culture falls under
tlu:> head.
CHAPTER II.
THE EAKLIER HISTORY OF ONTOGENY.
Caspar Friedrich Wolff.
Tlie Evolution of Animals as known to Ai-istotle. — His Knowledge of the
Ontogeny of the Lower 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-
bricins 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 Spcrm-aninial or the Egg as the Pre-formed
Individual. — Animalculists : Lecuwenhoek, Hartsoeker, Spallanzani. —
Ovulists : Hallcr, Leibnitz, Bonnet. — Victoiy of the Theory of Evolution
owing to the Authority of Haller and Leibnitz. — Caspar Friedrich Wolff.
— Ilis Fate and Works. — The Theoria Generationis. — Ke-formation, or
Epigencsis. — 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.
" He who wishes to explain Generation must take for his theme the
organic body 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 bo 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' — under.'Stauds the thing not only historically but
truly philosophically, and he has a philosophic knowledge of it. Our own
26 THE EVOLUTION OF MAN.
Theory of Generation is to be such a philosophic comprehension of an organic
body, very different from one merely historical." — Caspar Friedrich Wolfp
(1764).
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 Generationis. This was
the first foundation-stone for a true history of animal
germs. In 1809, exactly fifty years later, Jean Lamarck
published the Fhilosophie Zoologique, 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
ARISTOTLE ON DEVELOPMENT. 2/
History of the Evolution of Man, we must rapidly glance 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
History."
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 ")}^
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 aa 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.
28 THE EVOLUTION OF MAN.
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.-^^ 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
ARISTOTLE AS A NATURALIST. 29
conceives the evolution of the individual to be a new
formation, in which the several parts of the body develop
one after the 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 pi'oved. 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
30 THE EVOLUTION OF MAN.
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
embryo.
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
EARLIER STUDENTS OF EVOLUTION. 3 1
science from the iron chains of dogmatism, that human
Anatomy and the History of the Evolution of Man could
move again more fi'eely, 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 (IGOO), 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
ova ("Everything living comes from an egg"). 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 Pulli, and De Ovo Incuhato (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
32 THE EVOLUTION OF MAN.
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. (C£ 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 di'awn 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
of 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
IMPORTANCE OF THE CHICK. 33
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, owincf 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.
Throuo-hout the whole of the first half of the eighteenth
centuiy, during which time the systematic Natural History
34 THE EVOLUTION OF MAN.
of animals and plants received so great an impulse from
Linnseus' famous Sy sterna JSfaturce, 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 ah-eady 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.^' 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
THEORIES OF PRE -FORMATION AND ENCASEMENT. 35
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 fenude 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
36 THE EVOLUTION OF MAN,
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
o-enerations, previously formed and encased one in another,
existed in every organism ; a conception on which was also
founded the curious Prolepsis Theory of Linn pens.
The two opposite theories of encasement soon began a
ANIMALCULISTS V. OVULISTS. 37
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 inquiry whether
the angels lived in the East or in the West of the heavenly
regions." ^^
The Animalculists, or the Believers in Sperm, looked
upon the moving seminal threads as the real animal germs,
and 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,
Hartsoeker, and S})allanzani were the chief defenders of
this theory of Pre-delineation.
The opposite party, the Ovulists (Ovists), or Believers
in Eggs, who adhered to tlie older Theoiy of Evolution,
38 THE EVOLUTION OF MAN.
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-kno^vn worh, Eleone/nia Physiologice, adopted the Theory
of Evolution (Theory of Pre-formation) in a most decided
manner, in these words : " There is no coming into being !
HALLER AND LEIBNITZ. 39.
{Nulla est epigenesis). No part of the animal body was made
previous to another, and all were created simultaneously
{Nulla in cor pore animali p>ars ante aliam facta est,
et 07)ines simul creatce 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
Theodicoe, for instance, he says : " I think that souls, which
will some day be human souls, as in the case of those of
40 THE EVOLUTION OF MAN.
otlier 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,
one 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.^^
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
CASPAR FRIEDRICH WOLFF. 4 1
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 Theor'ia General lonis, 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 Wolff's 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 : Xulla est
epigenesis ! It is not surprising that the entire body of
physiological scholars of the second half of the eighteenth
42 THE EVOLUTION OF MAN.
century submitted to the dictum of this physiological pope,
and opposed Epigenesis as a dangerous innovation. More
than half a century elapsed before Wolff's labours met with
their deserved acknowledgment. Only after Meckel, in the
year 1812, had translated into German another most im-
j)ortant publication of Wolffs, " On the Formation of the
Intestinal Canal" (published 1764), and had drawn atten-
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 difliculties, 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-
HISTORY OF WOLFF. 43
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 Epi genesis
as one of the most dangerous heresies ; just as is the case
now with the Theory of Descent. Although Coihenius,
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 Wolfi" was obliged to accept a summons
with which the Empress Catherine of Russia honoured him
in 1706. 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.^^
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
44 THE EVOLUTION OF MAN.
phases of its evolution an internal structure and an external
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 we
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 in 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 Wolff,
who actually proved the most important facts known in
this the darkest portion of Embryology. From this verj^
difficult investigation he even drew the correct conclusion
that the entire embryonic body of all higher animals, as
well as of birds, is for a while a flat, thin, leaf- shaped
plate, which at first appears simple, but subsequently
as if composed of several layers. The lowest of all these
WOLFF ON GERM-LAYERS. 45
layers, or 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 17G8 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 comiected 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 j^lan; 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
46 THE EVOLUTION OF MAN.
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 Wolff's
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
plants.
We therefore find in Wolff distinct traces of those
theories of which, at a much later period, other gifted
WOLFF AS A MONISTIC rillLOSOPHER. 47
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.
CHAPTEH III.
MODEEN ONTOGENY.
Karl Ernst Baer.
Karl Ernst Baer, the Principal Disciple of Wolff. — The Wiirzbarg 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 Dorsalis. — 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 Miiller,
Bischoff, KoUiker. — The Cell Theory : Schleiden, Schwann. — Its Appli-
cation to Ontogeny : Robert Remak. — Retrogressions in Ontogeny :
Reichert 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 cairy the proof of it into all branches of research
would be an almost endless task." — Kakl Ernst Baek (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
THREE PERIODS IN THE HISTORY OF ONTOGENY. 49
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 1850, 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
besinnine: of the third division is as recent as the time of
Dar^vin.
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 Pre -formation, 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 WolfJ's 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
50 THE EVOLUTION OF MAN.
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,
stai"ted 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 con-
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. Dalton,
a skilful artist, was engaged to prepare the necessary copper-
plates.
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 Organism." 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 ^'^ 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 Wolff's theory. He was able to
define clearly Wolft's 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 Wolfil 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
52
THE EVOLUTION OF MAN.
liveliest interest in them after his departure from Wurzbiirg,
began his own much more comprehensive researches in
1819, and nine years later published, as the fruit of these
researches, 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.^^ It is the firm founda-
tion on which the whole histoiy 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 OAvned. He studied
at Dorpat from 1810 to 1814, and then went to Wlirzburg,
where Dollinger not only initiated him into Comparative
Anatomy and Ontogeny, but also exercised over him, by
his OAvn 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.
BAERS WORK. 53
There, however, he forsook ahiiost 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 Wolff's 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 the
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
54 THE EVOLUTION OF MAN.
the skin, or dermal layer, and the flesh, or muscular layer.
From the uppermost of these two lamellse, 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 process
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
DISCOVERY OF THE HUMAN EGG. 55
Among the many and great services which Baer ren-
dered in detail to Ontogeny, especially to that of 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
is 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
ago — 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, tlie 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 characteristic
organization of this group of animals in which ]\[an is
included, was that of the Chorda Dor salts. This is a long,
thin, cylindrical cartilaginous cord, which in ail Vertebrates
56 THE EVOLUTION OF MAN.
passes lengthwise through the whole body of the embryo.
It is developed at a very early stage, and is the first forma-
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 of
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-
FOUR TYPES OF DEVELOPMENT. 5/
tebrates, is essentially similar, notwithstanding the great
variety of outward forms. Baer, however, independently
and almost simultaneously, furnished proof that the four
groups develop from the egg by entirely different processes,
and further, that the order of the series of embryonic forms
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 kinodom, 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
gi'adation of development from the lowest animal to the
highest. Cuvier and Baer proved that this conception is
totally erroneous, — and that, on the contrary, there are four
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 increasinf<-
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
58 THE EVOLUTION OF MAN.
elementary parts, and in the segments of its composite
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 Morphologic (vol. ii. p. 10), the type of
evolution is the mechanical result of Heredity ; the degree
of perfection is the mechanical result of Ada^Dtation.
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
FRESH IMPETUS GIVEN TO ONTOGENY. 59
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 (1840), 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
Miiller, of Berlin, on Star-animals {Echinodei^mna), are espe-
cially noteworthy ; also those of Albert Kolliker, of Wiirz-
burg, on Cuttle-fishes {Cej)halopoda); of Siebold and Huxley,
on Worms and Plant-animals ; of Fritz Miiller (Desterro), on
the Crustacea ; of Weismann, on Insects, etc. The number of
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
60 THE EVOLUTION OF MAN.
ontogenetic works are those of Kowalevsky, E. Ray Lan-
kester, and Eduard van Beneden, to which we shall presentl}^
again refer.^^
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
Miiller, applied this discovery directly to the animal body.^^
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, chiefly 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
egg hold in the Cell Theory ? Is it itself a cell, or is it
THE CELL THEORY. 6 1
composed of colls ? These were the important questions
which the Cell Theory at once raised in the study of Em-
bryology.
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
coiTCct idea of the History of Evolution, of the Cellular
Theory as a whole, and of a sound view of the structure
and development of tissues in particular. The inaccuracy
of Rcichert'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 " enveloping-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
62 THE EVOLUTION OF MAN.
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
could 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 of
the layers differentiate themselves, or assume various forms ;
and then there is a further diflferentiation, or, in other
words, a division of labour of the ceils within the layers
themselves, and this latter differentiation produces all the
various tissues of the body.
These are the very simple principles of Histogeny, or
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
64 THE EVOLUTION OF MAN.
the same four layers which Baer had previously assumed.
The upper part of the middle layer after its cleavage
(Baex's " Flesh-stratum "), Bemak calls the skin-lamella
{Hautplatte, or better, Hautfaserplatte); it forms the outer
wall of the body (the true skin, cutis vena, the muscles,
bones, and the like). The lower part (Baer's " 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 Bemak 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 Bemak's doctrines, or
to remodel them altogether. Beichert, of Berlin, and Wil-
helm 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 Bhylogeny,
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
Beichert and His as a great gain.
His, in 1868, in a large book, on " The Early Evolution
of the Chick in the Egg," detailed his entirely erroneous
HIS AND GOETTE. 6$
views in a very learned form, and under the banner of a
new and very exact mathematical and physical method, he
has i^cently expressed the same views in a general form in
his book on " 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 " impoitant 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 Boinbinator igneus, as the Basis of a Com-
parative Morphology of Vertebrates " (Leipsic, 1875). This
monograph is the biggest existing contribution to the
literatiu-e 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 con-esponding degree of excellence
in the voluminous text, will be sadly disappointed. Not
only is the whole account most obscure, confused, and
66 THE EVOLUTION OF MAN.
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 "
(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
HUXLEY AND KOWALEVSKY. 6/
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
{3Iedu8ce). 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
fiesh 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 {Echinodet^Tna), 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
68 THE EVOLUTION OF MAN.
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 of 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 (Arthropoda), as of Vertebrates.
All these animals may be comprised under the head of
Intestinal Animals (Metazoa), in distinction from the
Primaeval Animals (Pivtozoa), 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 the
animal kingdom.
DARWIN. 69
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 " Gastr?ea
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.^*
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 w^hich 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, Gei-m-history, or Ontogeny, with the second
part. Tribal-history, or Phylogeny.
CHAPTER IV.
THE EARLIER HISTORY OF PHYLOGENT.
Jean Lamarck.
Phylogeny before Darwin. — Origin of Species. — Karl Linnaeus' Idea of
Species, and Assent to Moses' Biblical History of Creation. — The
Deluge. — Palseontology.- — George Cuvier's Theoiy 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
and of Metamorphosis (Adaptation).
" It would be an easy task to show that the characteristics in the organi.
zation 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 ascendant over the other animals. The absolute advantage they
NEW ERA BEGUN BY DARWIN. 7 1
enjoyed, and the now 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 Lamauck (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 gi'owth.
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
Wolff's 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 forai 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 lies placed in our hands the
72 THE EVOLUTION OF MAN.
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 to
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 of
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
Ontogeny."
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
LINN^US' ''SYSTEMA NATURAE." 73
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 Dai'win 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 fii'st presented by Linnaeus, who, in 1735,
in his Systema Katurce, 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
74 THE EVOLUTION OF MAN.
Botany, as the most important collective term, although
incessant strife has been waged as to the particular meaning
of the term. Linnseus 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-
loofical ideas, Linnseus 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
CUVIERS 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.''^^
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
^6 THE EVOLUTION OF MAN.
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
of the earth, the stratified surface, bore evidence of having
THEORY OF CATASTROPHES. JJ
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 paloeontological 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 havino; 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
yS THE EVOLUTION OF MAN.
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 of
CONSERVATISM OF BIOLOGY. 79
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
thouofht worth while to allude to Lamarck's valuable
Philosophle 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
laboui-s. 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 a
naturalist and a philosopher, in the field of inorganic nature
aided essentially in constructing a "Natural History of
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 of
the Heavens," made a most successful and important "at-
tempt to treat the constitution and the mechanical origin
of the entire imiverse according to Newtonian principles,"
or, in other words, to treat it mechanically, to conceive
8o THE EVOLUTION OF MAN.
it monistlcally : and this attempt of his to explain the
origin of the entire world by means of naturally working
causes (causce eficientes), forms to this day the basis of
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 {causce 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
fact, 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
nature.
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 to
these passages, as I there showed, he might even be declared
KANT. 8 1
an adherent of the Theory of Descent. Several very sig-
nificant expressions, to which Fritz Schultze, in his interest-
ing work on " Kant and Darwin,^'' has lately again called
attention, actually enable us to recognize Kant ^' 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 {causce 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 {causoB JiTiales) 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
82 THE EVOLUTION OF MAN.
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 bravery 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 sur les corps vivants, which contains the first germs of
HISTORY OF LAMARCK. 83
his Theory of Descent. In 1809 appeared the important
Philosophic Zoologique, the principal work in which he
elaborated this theory. In 1815 he gave to tlie world his
comprehensive treatise on the Natural History of Inver-
tebrates (Hisfoire naturelle cles 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.^
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 con-
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 while to devote a syllable to this the greatest advance.
In short, Lamarck's Zoological Philosophy shared the fate
84 THE EVOLUTION OF MAN.
of Wolff's 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 the}' 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
PhilosopJiie Zoologique, I shall here briefly mention some of
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 (causoe ejfficientes), 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
Lamarck's theories. 85
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 theoiy 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
same 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
of animals and plants result, according to him, from the
86 THE EVOLUTION OF MAN.
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-oj)erate 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 fui-ther development and
LAMARCK ON THE "APE QUESTION." 87
increased use of its organs, and to the fact that it trans-
mitted the improvements thus acquired to its descend-
ants. Lamarck considered tlie 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 i^air 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
body 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 Ape's language of mere sounds grew into
the word-language of Man, and abstract ideas were accu-
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
88 THE EVOLUTION OF MAN.
ideas of Lamarck contain the first and oldest germs 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
was 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 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.^^ 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, ".^Mr Morphologie,"
GOETHE AS A NATURALIST. 89
abound in most excellent ideas. Some of tliem 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 we
have gained, fearlessly to assert that the moi'e 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"
(179G). 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-
dom."
That Goethe, in these and other utterances, did not
speak merely figuratively, that he grasped the internal
relation and connection of orjjanic forms in a genealofiical
sense, is yet more evident in remarkable separate passages in
which 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
90 THE EVOLUTION OF MAN.
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-
tion.
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"
(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
GOETHE VIRTUALLY AN EVOLUTIONIST. 9 1
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 tioie prophet of the theory of descent.
The numerous sentences which I have prefixed, as mottos
to the chapters of the Generelle Ilorphologle, clearly
show how far Goethe had advanced in his conception of
the innate genealogical connection of the diverse organic
92 THE EVOLUTION OF MAN.
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.
CHAPTER V.
MODERN PHYLOGENY.
Charles Dauwin.
Relation of Modern to Earlier Phylogeny. — Charles Darwin's Work on the
Origin of Species. — Causes of its Eemarkable 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. — 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 Eolle. —
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.— Palseontology. — Comparative Anatomy. — The Theory of Rudl-
mcntary Organs. — Purposelessnes^, or Dysteleology. — Genealogy of the
Natural System. — Chorology. — Qikology. — 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 parJy recall in imagination
the former condition of our early progenitors ; and can approximately place
them 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
ears, probably arboreal in its habits, and an inhabitant of the Old World.
94 THE EVOLUTION OF MAN.
This creature, if its whole structure had been examined by a naturalist,
would have been classed among the Quadrumana, as surely as would the
commou 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
Geoffroy, 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
DARWIN S AKGUMENT. 95
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 Darwinian
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
Sti-uggle for Existence. The gist of Darwin's theory,
properly so called, is this simple idea : that tlie 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 accordino: 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
96 THE EVOLUTION OF MAN.
animals in an undomesticated state. This Struggle for
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.^"
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-
DARWIN S LIFE. 97
cular movements of the cells in the ganglia of his powerful
brain, which had not made their appearance in his son
llobert. 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
leadere 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
chapter.
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-
98 THE EVOLUTION OF MAN.
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, wliich 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
Beckenham.
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 is
NATURAL SELECTION. 99
exhaustively discussed, and is established by the weightiest
evidence. Having fully expressed my opinion of this book
in my Gencrelle Morpliologie, 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.^^
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 fii'st 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
lOO THE EVOLUTION OF MAN.
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
Man.
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
HUXLEY AND VOGT. lOI
theory to Man was made by Huxley, wlio now holds the
first place among English zoologists.^^ 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 18G3, 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
evolved.
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 of
Evolution.
Gustav Jaeger ^ and Friedrich Rolle ^* must be men-
tioned among zoologists who, after the publication of
Darwin's work, took up the Theory of Descent, advanced
it, and drew the right logical conclusion, that Man is
descended from the lower animals. Friedrich Rolle, in 18G6
102 THE EVOLUTION OF AUN.
published a work on " Man, his Descent and Civilization, iu
the light of the Darwinian Theory."
At the same time, in the second volume of my GenereUe
Morphologie der Organismen, which appeared in 1866, I
made the first attempt to apply the Theory of Evolution to
the entire classification of orcjanisms, includinsj Man.^^ 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
j^lace 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 GenereUe Moiyhologle, 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.
At last, in 1871, Daiwin himself published a very in-
INFLUENCE OF SEXUAL SELECTION. I03
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," ^^ 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 hio-her animals. Accordinjx 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 jesthetic 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 of Creation,"
and pp. 244-247 in the second volume of the Generelle
Movphologle.)
In all essential points Darwin approves of the general
outline of the genealogical tree ffiven in the Generelle Mor-
104 THE EVOLUTION OF MAN.
fholofjle 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 strono^est
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 aU 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 may be concisel}'' stated
as follows : the Descent of Man from lower anmials is a
special deductive laio, necessarily following from the general
inductive laiu of the entire Doctrine of Descent. This
sentence formulates the relation most clearly and simply.
BIOLOGY AN INDUCTIVE SCIENCE. IO5
The Doctrine of Descent is really notliinc^ 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
I06 THE EVOLUTION OF MAN.
Archaeology. However much particular items of such induc-
tive knowledge may in time be improved and modified,
their general significance, as a whole, remains quite un-
touched.
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. Modem 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
a very late period that the higher placental Mammals
PALiEONTOLOGY AND COMPAEATIVE ANATOMY. I07
appear, and among the latest and youngest forms of the
latter is Man. Both the perfection of forms and their
vai'iety 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
degi'ee 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 variet}'-,
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 I'iveted 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
7
I08 THE EVOLUTION OF MAN.
all 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.
The 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
w^ell with that of all other Mammals, — such as Apes, Gnawing
animals {Rodentia), Hoofed animals (Ungidata), Whales,
and Oppossums, — that the complete dissimilarity of the
outward form is as nothing in the balance against it. We
learn also from Comj)arative 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
understood.
The recognition of this truth raised Comparative
Anatomy itself to a higher rank, so that Gegenbaur,^' the
" DTSTELEOLOGT. IO9
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
us have in our bodies various muscles which we never use ;
for example, the muscles in the external ear and the parts
immediately sun'ounding it. These outer and inner ear
muscles are of great use to most Mammals, especially such
no THE EVOLUTION OF MAN.
as have the power of erecting the ears, hecause 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
ago 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 stiU in many
herbivorous animals, such as Apes and Rodents, in which
it is of considerable size, and of great physiological im-
portance.
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-
RUDIMENTARY ORGANS. Ill
parativc 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 philosophical
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 also shown in its true light by these dysteleological
facts. Thus viewed, the " moral ordering of the world " is
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 ti'ies to
keep his spiritual flock in ecclesiastical leading-strings, can
112 THE EVOLUTION OF MAN.
any longer tell the fable of " the moral ordering of the
world." It exists neither in nature nor in human life,
neither in natural history, nor in the history of civilization.
The terrible and ceaseless " Strucjcrle 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
ignored.
The Natural System of Organisms, which classifies all
the various forms in larger and smaller groups, according to
the degree of similai'ity 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
Descent.
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
THEORY OF MIGRATIONS. II3
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 grou})s 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 of
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 show-
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 diftcrent 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
114 THE EVOLUTION OF MAN.
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 QEkology 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." 1 1 5
to 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.
Il6 THE EVOLUTION OF MAN.
It is true that in the year 1857 a celebrated and able,
but very untrustworthy and dogmatic naturalist, Louis
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
think 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,^^ 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,
1875.)
VARIABILITY OF SPECIES. 11/
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 satisfactoiy 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," ^ which appeared in
1872. In it I closely 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 ^^■ider 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 with
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
Il8 THE EVOLUTION OF MAN.
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, Paheontology 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 "
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
animals.
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.
INSTANCE OF THE ORIGIN OF SPECIES. 1 19
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 have been obtained.
CHAPTER VI.
THE EGG-CELL AND THE AMCEBA.
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 E^ential Constituent Parts
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 Amoebse. — Organization and
Vital Phenomena. — Their Movements. — Amoeboid Cells in Many-celled
Oi'ganisms. — Movements of such Cells, and Absorption of Solid Matter. —
Absorbent Blood Corpuscles. — Comparison of Amceba 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 must be regarded as one-celled
beings, similar to the Amoebse 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 'horrible, shocking, and immoral' doctrine, the unalterable fact, which
can be proved at any moment under the microscope, that the human egg
TUE HUMAN EGG-CELL. 121
is a simple cell, which is in no way different to those of other mammals,
must equally be pronounced ' horrible, shocking, and immoral.' " — Stamm-
BAUM DES Menschengeschlechts (1870.)
In order clearly to understand Ontogeny, or the evolution
of the individual Man, the most significant of the many
Avonderful and varied facts which meet us must first
be brought into prominence, 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 whole
form and constitution, not essentially different from those
of other Mammals, though there is some difierence between
the eersf-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 Avas 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
122
THE EVOLUTION OF MAN.
present position of the cell theory, and as to the views
commonly held in connection with it.
"^-^UUdLbjilliilUU^
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
eggcell 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
thread-shaped, moving sperm-cells jDass, in the process of impregnation, into
the egg-yelk.
In order rightly to appreciate the Cell Theory, which
THE CELL A UNIT OF LIFE. 1 23
has been regarded during the last thirty -five yeai's as the
true basis of all morphological and physiological know-
ledge in Zoology and Botany, it is especially necessary ti
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
views 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 "
(Lehensheerd). It is, perhaps, most accurately described as
the organic unit of form of the lowest grade, as an indi-
vidual of the first order {GenereUe Morphologie, vol. i.
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
124
THE EVOLUTION OF MAN.
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.^^
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 wall, 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 (6) has two kernels.
Fig. 3. — Three epithelial cells from the mucous membrane of the tongue.
SIMPLE CELLS. 1 25
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-G, may be compared as examples.
/J,^!!f?
Fig. 4.— Five thorny, or heckle-cells, the edges of which fit into each
other, from the epidermis ; one (h) 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 (cytohlastus) ; 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
( Urscldeim 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
126
THE EVOLUTION OF MAN.
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-
diverse.
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.
NERVE-CELLS. 12/
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-
128
THE EVOLUTION OF MAN,
STRUCTURE OF NERVE-CELLS. 1 29
Pig. 7. — A large branched norvo-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 fibrilla)), which are
embedded in the inter-cellular substance, and which pass out into the
branched processes of the cell. An nnbranched process (a) passes over
into a nerve vessel. (After Max Schultze.)
cell, an elementary origan 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, 6). 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 large 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
130 THE EVOLUTION OF MAN.
its minute and complex structure, is in form only a single
cell.
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 imderstood. 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, which 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.^ 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
CYTODS AND CELLS. I3I
iiidiviJual 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-
functions which the entire organism accomplishes. Every
one of these little beings grows and feeds itself indepen-
dently. 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. — Blootl-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 kei-nels, which become nioro
widely separated from each other (c) . Finally
a complete separation between the two parts
is effected at the point where the original cell
was i)inched in, so that tliere are now two
cells (/). (After Prey.)
creep about, if it has room for free motion, and is not pre-
vented by a solid covering ; from its outer surface, it sends
8
132
THE EVOLUTION OF MAN.
out and draws back again, finger-like processes, thereby
modifying its form (Fig. 9). Finally, the young cell has
Fro. 9.— Active cells from the
inflamed eje of a Frog (from the
watery moisture of the eye, the
humor aqums). The naked cells
move freely and creep about ;
like AmoebaB and Ehizopods 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
amccboid lymph-cells is not visible,
being covered by 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 lingering here over the extremely 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 cell. This
is of the highest significance, because the whole Science of
NUCLEUS AND NUCLEOLUS. 1 33
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
vitelliis, or yelk, and the nucleus the germinal vesicle,
(vesicida germinativa). As a rule, tlie nucleus of the egg
is of a soft, often vesicidar 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
THE EVOLUTION OF MAN.
J 2 r<^'-.\ /r\ A3
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 germ-vesicle), and in the latter is a
nucleolus (germ-spot) ; in the nucleolus a germ-point (nucleolinus) is often
visible. Fig. A 1 — A 4. The primitive egg of a Chalk Sponge (Leuculmis
echinus), in four conseci^ive conditions of motion. Fig. B \-^B 8. The
primitive egg of a Hermit-crab (Chondracanthtts cornutus), in eight conse-
cutive conditions of motion (after E. van Beneden). Fig. C 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. Pi-imitive egg of a Hen. Fig.
■F, Primitive human egg.
EGG-CELLS. 1 35
the germinal point (punctum germinativuTn). 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 germfiinativa).
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 -^^J of a line, or 02 millimetres. If the mam-
mahan 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 ^'o of a line (I- — jL
of a millimetre). It has always the same spherical form,
136
THE EVOLUTION OF MAN.
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 haman 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 egg-yelk, by the granular cell-substance (protoplasm), con-
sisting of innumerable yelk-granules 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 peUucida, or chorion). This is
traversed by veiy 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
egg-yelk.
IDENTITY OF ALL PRIMITIVE EGGS. 1 37
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 diflcrent
in these different Mammals. On the contrary, we must
assume that such diflerences, 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 Qgg (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 Qgg 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
138 THE EVOLUTION OF MAN.
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, h). From the tread a thin cord of white
yelk passes through the yellow to the 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 (meinbrana viteUina, 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.^
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-
DEVELOPMENT OF THE EGG-CELL.
139
spondingly 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 hlastodermicus). 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 ('7) and
is surrounded by a thin yelk-membrane (a).
The cell-kernel, or germ-vesicle, lies in the
upper part, in the tread (6). From this the
white yelk passes into the centre of the
yelk-cavity (d'). 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 layer of transparent
albumen first forms round the yellow 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 very
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
c('ll. But, as in the case of Birds, a considerable quantity
140 THE EVOLUTION OF MAN.
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.
Bvit 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 of
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 probably
the common one-celled ancestral organism of the whole
animal kingdom, including Man. But this last hypothesis is
by no means as certain as the former conclusion.
AMCEB^. 141
The inference from the one-celled germ-form to 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 Amceba.
The name AmoebjiB 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 livinij Amoebse
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,
slim}', half-fluid body, which consists of protoplasm, we can
only see a small solid or vesicular substance, which is the
142
THE EVOLUTION OF MAN.
nucleus. This imicellular 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
Fig. 1.3.— a creeping Amoeba (mnch en.
larged). The form-valae of the whole or-
ganism is that of a simple naked 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-
cleolus.
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 (Pseiidopodla), because they
perfonn 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 Amnebse 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-
AMCEBOID LIFE. 1 43
sule, wliich 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
made to eat. If finely pulverized colouring matter, such as
cai'mine 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 digestino; 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 AmcebsB
144
THE EVOLUTION OF MAN.
are quite visible to the naked eye ; but the greater number
arc microscopic. Our reasons for regarding the Amoebse 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 Amoebse (Fig. 14), even
Fig. 14. — Egg-cell of a Chalk Sponge (Oljn-
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 Amceba.
within the parent organism. In this
condition they were observed by earlier
naturalists, and were mistaken for
Amcebse, 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 {Mediisoi) ;
the esffifs 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
WHITE BLOOD-CORPUSCLES.
145
entirely sober conclusion, which regards the Amoeba as the
particular one-colled organism which gives us an approxi-
mate representation of the ancient one-celled ancestral
form common to all many-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 I showed that these colourless blood-
corpuscles, like independent Amoeb?e, 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 Amoeba3, 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 ab.sorption of the
colouring matter into the blood-cclla, which was accomplished exactly as by
Amoebae. (See " Monograph of Radiolaria," 1862, pp. 104, 105.)
146 THE EVOLUTION OF MAN.
are able to move, to eat, and to act entirely like Amoeba;
(Fig. 9).
The 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 tlie 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
form.
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
UNICELLULAR HUMAN GERM. I47
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 illusti'ated in the Amoeba.
The extant Amoebte 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 oiijanism 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 originally only simple cells of one common
structure.
CHAPTER VII.
THE PROCESSES OP EVOLUTIOX AND IMPREGNATION.
Development of the Many-celled from the One-celled Organism. ^ — ^Tho Cell-
hermit and the Cell-state. — The Principles of the Formation of the
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 Kace 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 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 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-
rnla. — Reversion to the Monera-form. — 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 appi'oached one of the greatest of the mj-steries 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
THE MULTICELLULAR ORGANISM. 1 49
by means of tho seed, of the physical and mental chai-actcristics of the
father, aifcct all the questions which the human mind has ever raised in
regard to existence." — Rudolph ViRCHOW (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 difiicult
problem of the History of Evolution is thus now reduced
to the simple question : " 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
150 THE EVOLUTION OF MAN.
same principles as a civilized state, in which the several
citizens have devoted themselves to various services directed
towards common ends. This coniparison 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 coniparison 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 d priori, before we call
observation, d 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
THE CELL COMMUNITY. 15 I
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 didven 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 qntirely 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 himt, others
began to cultivate the ground, others devoted themselves
to religion and medicine, which now began to develop,
152 THE EVOLUTION OF MAN.
and so on. In short, the ever-increasing division of labour
specializes the people into various ranks or castes, which
always tend to become more sharply defined 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
measured.
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 organic 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-
SPECIALIZATION OF CELLS. 1 53
duction ; and others, again, have formed themselves into
protecting organs for the little community, and so on. In
short, various classes or castes 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
civilized.
We may follow the comparison further. It may be
asserted d 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.**
At present we will rather examine whether the d priori
views on Phylogeny which we preconceived, are in accord-
154 THE EVOLUTION OF MAN.
ance with the facts which Ontogeny places befoi'e us ;
whether in the evolution of the individual organism
from the egg-cell, the same phenomena appear, which we
have presupj^osed 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 of
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 d 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 of
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
egg-cell.
At first, all these numerous cells which arise by the con-
LIFE-PHENOM EN A. I 5 5
tinuous 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
Biogeny.
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 ;
(G) 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
functions.
Of all vital phenomena, growth may be regarded as the
one which |)lays the chief part in the evolution of the
individual organism, and as tlie really fundamental function
of evolution. The bearing of this function on the evolu-
9
156 THE EVOLUTION OF MAN.
tion of the germ is so great, that Baer expressed the most
general result of his researches in the following proposition :
" The history of tlie 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 indisj^ensable to the
evolution of the single cell as to that of the entire many-
celled orofanism. The usual method of nutrition in the
158 THE EVOLUTION OF MAN.
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
NUTRITION AND REPRODUCTION. I 59
that belonging to it as an individual, so that a part is thus
elevated into a (new) whole" {Generelle MorplioJogie, vol.
ii. p. IG). 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 size exceeding that
of the individual, — compound growth, on the other hand,
is the residt 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. 10.— 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 (h, c, (7). The protoplasmic body then
becomes pinched in between the two kernels,
which separate more and more from each
other (c). Finally the cell parts into two, at
the point where it was pinched in (/). (After
Frey.)
having, by the absorption of nutrition, already reached its
usual size, exceeds that measure, it divides into two cells
(Fig. IG). Just in the same way in many-celled animals (for
example. Corals), when the individual grows beyond the
l60 THE EVOLUTION OF MAN.
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.^^
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.^''
(Cf Chap. XXV.)
Immediately connected with reproduction is a fifth
HEREDITY AND ADAPTATION. * l6l
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 VIH. p. 175, we
will not stop to examine them here. (See also Generelle
Morphologic, 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 impiilse
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 higher
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 of
1 62 THE EVOLUTION OF MAN.
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 Gastraea. 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 ceils
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. AU these different kinds
of cells originally proceeded by differentiation or specializa-
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. Noav, 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
ATAVISM. 163
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 ceU 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 stage 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
164 THE EVOLUTION OF MAN.
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 oi'iginally 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
essential functions of evolution— as the true formative forces
INACTIVITY OF PHYSIOLOGY. 1 65
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 regai'ded 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 applicalile. Among the latter are the choro-
logical and oecological 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 phj-siological
functions of evolution has been almost entirely acquired
hy means of morphological, not physiological research,
1 66 THE EVOLUTION OF MAN.
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 differentiation
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 improved and baseless
hypothesis."
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,
DU BOIS-REYMOND. 1 67
is in actual process oi" 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
conn)leted in the course of many ontogenetic stages.
Again, this same want of 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
is as irrational as would be a dispute as to whether the
mouse is a great or a small animal. The elephant will of
course declare the mouse to be a tiny creature, while the
louse, living on the skin of the mouse, must regard the
la,tter 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 time.
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,
1 68 THE EVOLUTION OF MAN.
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.^'' 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 individual
so in that of the species, each new form arises directly from
that which preceded it ; and here also the physiological
process of development always preserves its continuitji.
Even in those extreme cases where a new foi'm does indeed
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 "
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 this
family of goats. In this case a new variety of goats bear-
ing four horns has " suddenly " arisen, and under favourable
SUDDEN VARIATION. 1 69
conditions this young lie-goat may become the founder of
an entirely new four-horned race, or (by correlative adapta-
tion and constant* 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 of
correlative adaptation, the hairy skin covering both these
protuberances, changes into a hard, bare horny sheath,
analoo-ous 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
afibrds 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
I/O THE EVOLUTION OF MAN.
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 Amoebfe, Gregarinse, Flagellata, Forami-
niferse, Radiolaria, Myxomycetfe, etc. In these cases
there is no form of impregnation : the multiplication of
individuals, and the preservation of the species depend here
simply on asexual 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 " [Farthenogenesis) never
occurs. This we must explicitly afiirm 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.^
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 o,^^.
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 Midler on the fertilization of flowers
by insect agency, are especially interesting from this point
FERTILIZATION. ~ I/I
of vJew.*^ 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 (ovidum), and the male is called the sperm-
cell, or seed-cell (zoosper77imm, 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-
ceU 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
1/2 THE EVOLUTION OF MAN.
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-cells
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
SPERMATOZOA.
173
the "pin-shaped form" which occurs in Man and other
Mammals (Fig. 17).
A \ K\ H
r - M I J[
Fig. 17. — Seed-cells or sperm-cells from the semen of various Mammals.
The broad side of the flattened, pear-shaped nucleus portion of the sperm-
cell (the so-called " /teat? of the sperm-animalcule") is represented in the
drawings marked I; the narrow side in those marked II : k, kernel of the
sperm-cell; m, central poi-tion (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
174 THE EVOLUTION OF MAN.
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 /.;) is only the longish round or oval cell-nucleus ;
the body, the central' portion (m), is only an aggregation
of 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 other
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 aflbrd 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 tliat closer
study has shown that the whole process of fertilization is
extremely simple, and entirely without any special myster3\
Essentially it consists merely in the fact that the male
sperm-cell coalesces with the female egg-cell. Owing to its
sinuous movements, the very mobile sperm-cell finds its way
THE FERTILIZED EGG-CELL.
175
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 j)urpose "
burying themselves in the protoplasm of the yelk-mass,
where, in a spirit of self-sacrifice, they completely efiace
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
tlie 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 in, therefore, of a nature entirely
different from that of the unfertilized egg-cell. For since
I
176 THE EVOLUTION OF MAN.
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 of
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- M
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-ceUed organism. The very complex
molecular movement of the protoplasm in this parent-cell,
PARENT-CELL AND PARENT-KERNEL. 1 77
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. Tlie
life of the parent-cell (Cytula) is tJie product or resultant
of the paternal activities, transmitted through the spei^mfi-
cell, together loith 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 — tliat 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-
178 THE EVOLUTION OF MAN,
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.^'' 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 {cytiila, 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 Hert-
THE MONERULA.
179
wig, we assume tliat the germ-vesicle does not completely
disappear, but that the germ-spot (iiiideoliis) 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 e-erm-stao-e, 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.^^ (Fig. 19.)
r Fin. 19.— Monernla of a Mammal (Eabbit) . The fertilized egg-cell, after
the disappearance of the germ-vesicle, is a simple globe of protoplasm (d).
The outer membrane is fonned by the modified zona pellucida (z), together
with a mucons layer (h) secreted on the outside of the zona,. A few single
sperm-cells (s) are still visible in the membrane.
We regard it as a fact of the gi-eatest interest that the
liuman child, like that of every other animal, is, in this
tirst 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
10
l80 THE EVOLUTION OF MAN.
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- ■
duee themselves by division (Fig. 20). These Monera
Fig. 20. — A Moneron (Protamoeha) iu the act of reproduction. A. The
whole MoneroD, 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 beo-ins 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 commences from this
simplest form. The entire human child, with all its great
CONSTITUENTS OF MONERULA.
ISI
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
chano-es which it undero-oes 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
b
Fig. 21. — Pareut-cell or cytula of a Mammal (Rabbit) : l, parent-
kernel ; n, nucleolus of the latter ; p, protoplasm of the parent-cell ; z
modified zona jidlucida ; s, sperm-cells ; /(, external albuminous membrane.
of the globular egg, the homogeneous yelk, which when
in this condition we call the Monerula, in allusion to the
Monera-form.
Although morphologically we can see no defined 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 ))rotoplasm of the paternal
1 82 THE EVOLUTION OF MAN.
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, ])) seems to originate ; from the mixture of the two
forms (3, 4) the parent-kernel (cytococcus) seems to origin-
ate (Fig. 21, A;) .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.^^
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
niove'ments of the cytula are therefore the resultant of the
united plastidule movements of the female egg-cell and of
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 on
the "Perigenesis of Plastidules" (1876), I have explained
the important bearing of this conception in explanation of
the elementary processes of evolution.
( i83 )
TABLE II.
Review of the Constituent Parts of the One-celled Germ-organism, before
and after fertilization.
Cf. the works of Eduard Strasburger (" Ueber Zellbildung, Zelltheilung
and Befruchtung," 2ud Edition ; Jena, 1876) ; of Oscar Hertwig (" Beitrage
zur Kontniss der Bildung, Befruchtung, und Theilung des Thierischen Eies;"
1875) ; of Leopold Auerbach (" Organologische Studien ; " 1874) ; and of Otto
Biitschli (" Studien liber die ersten Entwickelungs-Vorgiinge der Eizelle,"
etc.; 1876).=*
I. The Fertilizing Male,
II. The Fertilized Female,
III. The New Cell, the
or Paternal Sexual
or Maternal Sexual
product of the Concre-
CeU.
Cell.
scence of I. and II,
The Sperm-cell.
The Egg-cell.
The Parent-cell.
Spcrmule.
Ovule.
Cytula.
Syn. Thread - cell.
Syn. The unfertilized
Syn. The fertilized
Seed-aniinalcule. Sper-
egg.
egg. The first cleavage-
matozoa. Zoosperm.
Fig. 1, p. 122.
globule. The oldest
Fig. 17, p. 173.
Fig. 10, p. 134.
cleavage-cell. Segnien-
tella prima.
Fig. 21, p. 181.
Constituent Parts.
Constituent Parts.
Constitiaent Parts.
I. A. Protoplasm of the
II. A. Protoplasm of the
III. A. Protoplasm of
Sperm-cell.
Egg-cell.
Parent-cell: Cleavage-
yelk.
(Spermoplasma.)
(Ovoplasma.)
(Cytuloplasma.)
The central portion
Yelk, egg-yelk, Lecy-
Protoplasm of the
and the tail of the seed-
thus, vitellus.
first cleavage - globule
thread, together with
(the product of the
the outer sheath of the
amalgamation of I. A.
" head."
and 11. A.
I, B. Kernel (nucleus)
II. B. Kernel (nucleus)
III. B. Kernel (nucleus:)
of the Sperm-cell.
of the Egg-ceU.
of the Parent-cell.
{Sji' nnococcus.)
(Ococorcus.)
(Cytulococcus.)
Spcriu-kernel (Hert-
Germ-vesicle, or Pur-
Cleavage-kernel (Hert-
wig). " Head of the
kinje's vesicle (Vesicnla
wig). Germ - kernel
sperm-auinial" (M'ith
Genninativa) , contain-
(Strasburger). Kernel
the exception of the
ing the germ-spot
of the first cleavage-
thin outer sheath).
(Macula Genninativa),
globule (product of tho
or the nucleolus, which,
amalgamation of the
according to Hertwig,
sperm. kernel and the
becomes the egg-kemel.
egg-kernel ?).^*
CHAPTER VIIL
EGG- CLEAVAGE AND THE FORMATION OP THE GERM-
LAYERS.
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 (Invagination) of the Germ-vesicle.
— Formation of the Gastrula. — Primitive Intestine and Primitive
Mouth. — The Two Primary Germ-layers ; Exoderra and Entoderm. —
Kenogenetic Form of Egg-cleavage. — Unequal Cleavage (segmentatio
inequalis) and Hood-gastrula (Amphigastrula) of Amphibia and
Mammalia. — Total and Partial Cleavage. — Holoblastic and Meroblastic
Eggs. — Discoidal Cleavage (segmentatio discoidalis) and Disc-gastrula
(Discogastrula) of Fishes, Eeptiles, Birds. — Superficial Cleavage (seg-
mentatio sitperficialis) and Vesicular Gastrula (Peri-Gastrula) of Ar-
ticulates (Artliro'poda) . — Permanent Two-layered Body-form of Lovrer
Animals. — The Two-layered Primaeval Parent-form ; Gastraea. —
Homology of the Two Primary Germ-layers in all Intestinal Animals
(Metazna). — 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 gives 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-
FORMATION OF GERM-LAYERS. 1 85
liar orgauization. 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(1828).
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 Primseval Animals, or Protozoa, are peculiar in
this respect. These latter include the Monera, Amoebae,
Gregarinse, Flagellata, Rhizopoda, Infusoria, and others.
All 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 of
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 (Molliisca,) Star-animals
{Echinoderma), Ai-ticulated animals (Arthvopoda), and Ver-
tebrates.^^
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
1 86 THE EVOLUTION OF MAN.
which will be explained later, are called Cleavage-cells or
Cleavage-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 germinal
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 Primseval Animals, or Protozoa).
It was only after I had established the Gastrsea 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.^^
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
EARLIEST MAMMALIAN GERM-PUOCESSES. 1 87
differ from the correspondingly developed embryos of other
higher IMaramals (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."
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. (Of 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
• specially 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.
1 88 THE EVOLUTION OF MAN.
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 XI.). 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 (Lininceus), 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 (Olyiithus). 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}^
After the Monerula (Fig. 22, A) has changed into the
parent-cell, or cytula (B), the latter divides into two similar
cells (C). 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, lie in one plane. Each of these now again
EGG-CLEAVAGE. 1 89
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.^^ These
repeated and similar bisections finally result in the produc-
tion of a globular mass 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. S.^o)
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 (blastoderma) ; the cells (all of one kind), a simple
stratum of which forms the germ-membrane, are called the
geiTu-membrane-cells (celluke hlastodertniccB); and the entire
hollow ball, the walls of which are composed of these cells,
is called the germ-membrane-vesicle, or, briefly, the germ-
vesicle, or vesicular-germ (blastula ; formerly called the
vesicula hlastodermica).^^ The inner cavity of the ball,
which is filled with clear liquid or jelly, is called the
cleavage-cavity (cavitm scgmentm'ium), or the germ-cavity
(hlastocceloma).
%#■
INVERSION. 191
Fig. 22. — Germination of a Coral (Monoxenia Danviiiii) : A, Monerula;
B, Parent-cell (Cytida) ; C, two cleavage. cells ; D, four 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, Gastrula, or Germ-cup, seen from
outside.
Ill 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-
fii.he, 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
192 THE EVOLUTION OF MAN.
germ- 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 paiis assume a different form and size ; the inner
cells become rounder ; the outer become longer (Fig. 22, /).
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 (Gastrida, Fig. 22, /, in
longitudinal section ; K, surface view).^^
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, /, A''), 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.
It is certainly a most highly interesting and significant fact,
that animals of the most diverse tribes, Vertebrates, Soft-
bodied Animals (Molluscct), Articulated animals (Arthro-
foda), Star-animals {Echinodernia), Worms, and Plant-
animals {Zoophyta) 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
THK GASTRULA
193
it necessary tluit we sliould most carefully examine the
structure of its body. Ordinarily it is invisible to the
Fig. 2k Fig. 25. Fig. 26. Fig. 27.
Fig. 23.
Fig. 28.
Fig. 23. — (.4) Gastrula of a Zoophyte (Gastrophysema). (Haeckel.)
Fig. 24. — (B) Gastrula of a Worm (Sagitta). (After Kowalevsky.)
Fig. 2.5. — {€) Ga.strula of an Echinoderm (Starfish, Vraster). (After
Alexander Agassiz.)
Fig. 26. — (D) Gastrula of an Arthropod (Nauiolius).
Fig. 27. — (E) Gastrula of a Mollusc (Pond-snail, Limna'us). (After Karl
Rabl.)
Fig. 28.— (F) Gastrula of a Vcrtebiate (Lancelot, Amphioxus). (After
Kowalevsky.)
In all, d indicates the primitive intestinal cavity; 0, the primitive moath;
s, the cleavage-cavity ; j, the entoderm, or intestinal layer ; e, the exoderm,
or skin-layer.
194 THE EVOLUTION OF MAN.
naked eye, or, at most, under favourable circumstances, it
is seen as a tiny speck, usually ^V — iV *^^' ^^ most ^ — 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.*'^
I shall call the central cavity of the Gastrula-body the
primitive intestine {'protogaster), and its opening the pri-
mitive mouth ('protostoina). 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 appeal' 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 prnnitive 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.^*
The two cellular layers which surround the cavity of
STRUCTURE OF THE GASTRULA.
195
the primitive intestine, and alone constitute the wall of
the latter, are of very great significance. For these two
^\'hich alone constitute the whole body, are, in fact, the
two primary germ-layers, or primitive germ-layers (blas-
ioplnjlla). 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
form.s 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) : ^.external view.
B, in longitudinal section through the axis ; (j, primitive intestine (primitive
intestinal cavity) ; o, primitive mouth (primitive mouth-opening) ; 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) .
196 THE EVOLUTION OF MAN.
between the two germ-layers ; this cavity, filled with blood
or lymph, is the body-cavity {coeloma).^^
The two primary germ-layers, the outer or serous, and
the inner or mucous layer, were fii-st 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 laj^er 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-
luscct), Star-animals {Echinoderraa), and Articulated animals
{Arthropoda), form from the same two primary germ-
layers. 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.
BLASTODERMIC CELLS. I97
Ordinarily the cells of the Gastrula-gerrn, which com-
pose the two primary germ-layers, akeady present recog-
nizable difierences. 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 (blastoderma), which
forms the wall of the globular germ-vesicle, or Blastula
(Fig. 22, F, G), consisted solely of a single layer of similar
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 unifoi'mity 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, //), a nature difiering from
198
THE EVOLUTION OF MAN.
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 lai'ger, 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
{Arcliigastrida). 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 Medusse and Hydra-polyps ; in low Worms of dif-
ferent classes (Sagitta, Fig. 24 ; Ascidia, Plate X. Fig. 1-4) ;
again, in many Star-animals {Echinoderina, Fig. 25) ; in
low Articulated-animals {Arfhropoda, Fig. 26), and Soft-
bodied Animals {Mollusca, Fig. 27) ; lastly, in the lowest
Vertebrate {ArrqMoxus, Fig. 28; Plate X. Fig. 7-10).
BELL-GASTRULA. 1 99
Although the animals which we have named belong to
the 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 unlcnown. 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
IH'Ocesses. The more or less varying Gastrula-form, which
results from this kenogenetic egg-cleavage, may be called,
generally, the secondary, modified Gastrula, or Metagastrida.
200 THE EVOLUTION OF MAN.
Among the many and various kenogenetic or vitiated
forms of egg-cleavage and gastrulation, I again distinguish
three different chief forms : 1. Unequal cleavage {segraen-
tatio incBqualis, Plate II. Fig. 7-17) ; 2. Discoidal cleavage
(segmentaiio discoidaUs, Plate III. Fig. 18-24) ; and 3.
Surface cleavage {seginentatio suj^erjlcicdis, 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 (Discogastrida, Plate III. Fig. 24);
surface cleavage results in a Bladder-gastrula {Perigastrida,
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.)
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 difiicult to investigate the first incidents in
the eo;o;-cleavaoje and p'astrulation 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 pu])lished, 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
LATER FORMS OF GASTRULA. 201
of the Guinea-pig (1852), and of the Roe-deer (1854). But it
was only quite recently that Eduard van Beneden, an emi-
nent Belo'ian zooloo-ist, 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-gastrula. 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, however, 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.^^
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 ma.sses of jelly, which, in
the case of the Frog, form thick lumps, while those of the
Toad furm long strings. When the opaque, brown, grey,
202 THE EVOLUTION OP MAN.
or black-coloured egg is minutely examined, the upper
half appears darker than the lower. In some kinds, the
centre of the upper half is blacker, while the corresponding
centre of the lower half is of a whiter colour.*''^ 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).^^ 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.^'' 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 segments.
FORMS OF CLEAVAGE.
203
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 ajipears, perpendicularly to the two
Fig. 31. — Tlie cleavage of a Frog's egg (10 times enlarged) : A, the
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 colls (8 animal and 8 vegetative) ; O, 24 cells (16 animal and 8 vege.
tative); H, 32 cells; I, 48 cells; A', 64 cells; L, 96 cleavage-ccUs ; 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 eipiator, and should, tlierefore, rather be compared to the
11
204 THE EVOLUTION OF MAN.
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 northern
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. 81, 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 IG (F).
Parallel to the first, horizontal ring-furrow, a new ring-
furrow now appears, nearer the northern pole ; this, there-
fore, Ave 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, ili). 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-sphei'e, and only 32 cells in the lower, brighter half:
160 cleavage-cells in all. The inequality between the two
hemispheres increases yet further ; and while the northern
REPEATED DIVISION OF CELLS. 205
heinispliere 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 is 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, drawings 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
whicli 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.
206
THE EVOLUTION OF MAN.
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).
'f'h - -^^^^JO
'':s^-
W-ff^o F
P
Fig. 32-35. — Fonr longitudinal sections of the segmented egg of a Toad,
in four successive stages of evolution. In all, tbe letters indicate the same
parts : F, cleavage-cavity ; D, the roof of this cavity ; R, dorsal half of
the germ ; B, intestinal half ; P, the yelk. plug (white circular space at the
lower pole); s, yelk-cells of the entoderm (the gland-germ of Remak) ;
N, primitive intestinal cavity (protogaster, or Eusconi'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 (P) ; l; A-', section
through the swollen circular lip or edge of the primitive mouth (the so-*
called anus of Rusconi). The dotted line between h and A' indicates the
former connection between the yelk-plug (P) and the central mass of yelki
cells (r). In Fig. 35 the egg has turned round 90°, so that the dorsal hal
of the germ (R) is seen above ; the intestinal half (B) is now turned down-
ward, (After Strieker.)
AMPHIBIAN GASTRULA. 20/
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, K). This is the primitive intestinal cavity or
stomach-cavity of the Gastrula, the Protofjastcr. It was
first observed by Rusconi in the eggs of Amphibia, and
is accordingly called Eusconi'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 (B). 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 ))lug
up the entire wide opening of the primitive mouth, and
there form the so-called yolk-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 properistoTiia,
Fig. 35 /v, k'). Presently the primitive intestinal cavity (N)
extends gradually at the cost of the cleavage-cavity {F) ;
and, finally, the latter entii-ely 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 (M). The
cleavage-cavity lies in the anterior, the yelk-plug in the
posterior part of the body.*"^
When the primitive intestine is complete, the Frog-
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
208 THE EVOLUTION OF MAN.
we saw before (Fig, 23-29). In the latter, the Bell-gastrula
(Archigastrida), 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-
pliigastrida) 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 intestina,l
cavity {N). Yelk-cells fill the greater part of the latter;
and they also fill the primitive mouth (yelk-plug, P). Both
the intestinal layer (0) 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 (A mphi gastrula)
we are unable to distinguish sharply between the different
ej)Ochs, which, marked by the mulberry -germ and the germ-
vesicle, we saw followed each other in the case of the Bell-
gastrula {Archigastvvbla). 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 Ainphigastrula to the genuine, palin-
genetic origin of the Archigastrula of the Amphioxus.
It is far harder to do this in the case of Mammals,
although the course of egg-cleavage and gastrulation in
DEVELOrMENT OF MAMMALIAN EMBRYO. 209
these is, on the whole, very similar to that of Ampliiliia.
Until recently the growth of the mammalian embryo was
entirely wrongly explained ; and it is only lately (1875)
that Yan Beneden, whose views we adopt here, pointed out
its real significance.*"^ His studies were directed towards
the emhryo 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 YII. Fig.
K, M), there is not the slightest reason to doubt that the
esfof-cleavaoce 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
tw^o move apart. After this the protoplasm of the parent-
cell, 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 (Fig. 88).
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
smalk-r cleavage-cell takes a much deeper colour from car-
mine, osmium, etc., than does the larger. The two cells
thus already betray their relations to the two primitive
2IO
THE EVOLUTION OF MAN.
Fig. 36. — Monerula of a Mammal (Rabbit). The fertilized egg-cell after
loss of the germ-vesicle is a simple ball of protoplasm (d). The outer
envelope of this is formed by the modified zona 'pellucida {£) and by a mucous
layer (/t), -which is deposited on the outside of the zona. A few sperm-cells
are still visible in this mucous layer (s).
Fig. 37. — Parent-cell, or cytula, of a Mammal (Eabbit) : h, parent-kernel,
or nucleus ; n, nucleolus ; p, protoplasm of the parent-cell ; 2, modified
zona 'pellucida ; h, external albuminous envelope; s, sperm-cells.
Fig. 38. — Commencement of cleavage in the mammalian egg (Rabbit).
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.
EXODERM AND ENTODERM CELLS. 211
germ-layers. The brighter and liarder cleavage-cell (Fig.
38, e) is the mother-cell of the exoderm ; the darker and
softer cleavage-cell (Fig. 38, i) is the mother-eel of the
entoderm. All the cells of the outer germ-layer, the skin-
layer, are produced from the exoderm mother-cell (Fig.
38, c ; 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
yelk.
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.
30; 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. 89, i). The
line which connects the central points of the two latter
cleavage-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 ;
212
THE EVOLUTION OF MAN.
we therefore find that there are now eight cleavage-cells,
the descendants in the third generation of the parent-cell
(Fig. 40). Four larger, Ijiighter, 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 th*e 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 (Rabbit) : e, the
two exoderm-cells (larger and brighter); i, the two entoderm-cells (smaller
and darker) ; z, zona 'pellucida ; h, outer albuminous envelope.
Fig. 40. — Egg of Mammal (Eabbit), with eight cleavage-cells: e, four
exoderm-cells (larger and brighter) ; i, four entoderm-cells (smaller and
darker); 2, zona pellucida ; /i, outer albuminous covering.
Now, however, the eight cleavage-cells alter their
original position, and lose their globular form. One of the
FORMATION OF GASTKULA.
213
four exoderm-culls makes its way into the middle of the
cell-mass, and, together with its three fellows, forms a P3^ra-
mid (or tetrahedron). The four exoderm-cells arrange
themselves in the form of a cap over the point of this
])yramid (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 e^s. While in the original
(or primordial; egg- cleavage, the rhythm advances in regular
geometrical progression (2, 4, 8, 16, 32, 04, 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, IG, 24, 32, 48, 64, 90, 100, etc. (Cf Talkie V.)
Fig. 41. — Gastrula of a
Mammal {Amphigastrula of a
Rabbit), in lonpjitudinal section
through the axis : e, exoderm-
cells (64 brighter and smaller) ;
i, entoderni-ccUs (32 darker and
larger) ; d, central entoderm-
cclls, filling up the primitive in-
testinal cavity ; o, external ento-
dcrm-cells, plugging the primi-
tive mouth-opening (j-elk-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 tlian the
more inert entoderrn-cells. The latter always remain
behind the former, and are overgi-own l)y them. This pro-
214 THE EVOLUTION OF MAN.
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).^^
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 IG, 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 (82 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 {Am2jMgastrula)'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
MAMMALIAN IIOOD-GASTRULA. 215
a dark brown colour when treated with osiiiic 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
onl}' 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 is thus clearly indi-
cate d.*"^
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. (Of.
Fig. 22 and 28.) All these and many other classes of
animals agree in that, in their egg-cleavage, the luJiole 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-
l)oda), in Insects, most Spideis and Crabs ; among Soft-
bodied animals (Mollusca), in Ccphalopods or Cuttle-fishes.
2l6 THE EVOLUTION OF MAN.
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 forniativus, or Tnorpho-
lecithus) is the nucleated egg-cell, capable of evolution, which
divides in the process of cleavage, and produces the nu-
merous cells which constitute the embryo. The nutritive
yelk (vitellus nutritivus, or trojjholecithus), 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 eml3ryo 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 yeJk 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 Eemak ;
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
DEVELOPMENT OF FISH-GASTRULA. 21/
Osseous Fishes (Teleostei), tlie evolution of wliich I studied
at Ajaccio, in Corsica, in 1875, were of tlie 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.™ 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 (O'Gi — 0 66 mm.). Within a thin,
structureless but firm egg-membrane (chorion, Fig. 42, c) lies
Fig. 42. — Egg of an oceanic Osseous
Fisb : J), protoplasm of the parent. cell ; fc,
kernel of parent-cell ; n, clear albumin-
ous ball of nutritive yelk ; /, fat-globule
of the latter ; c, external egg-membrane, / ' , \
or chorion. 1 1
a large albuminous ball, which c
is quite transparent and as clear \ ■''/""■
as water {n). At both poles of -<v ^i■i£:«i'-^' -^ ^
the axis of this ball there is a
groove-like indentation. In the gToove at the upper pole,
which, in the floatinsf eo-cr is turned downwards, lies a
simple, lentil-shaped cell, containing a kernel (Fig. 42, ^)).
In the unfertilized ei?j?, this is the oritrinal eijor-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
2l8 THE EVOLUTION OF MAN.
small cell alone is the formative yelk, and is the only part
concerned in the cleavage process, which does not extend
to the nutritive yelk.'^'^
The cleavage of the parent-cell, or the formative yelk,
proceeds entirely independently of the nutiitive yelk, and
in quiet, regular, geometric progression. (C£ Plate III. Fig.
18-24.) Only the formative yelk, with the contiguous
portion of the nutritive yelk (ii), 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 sej^a-
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, 82, 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 globidar
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 dii'cction,
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, s).
This is the cleavage-cavity, and corresponds to the cleavage-
cavity in the centre of the prMngentic Blastula (Plate II.
THE DISC-GASTRULA. 219
Fig. 4). The sliglitly 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 .cavit}^ 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 celhdar
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. — Discgastrtila (Biscn-gas-
trida) of an Osseous Fish : e, exodorm ;
i, eutoderm ; ic, swollen edge, or primi-
tive mouth-edge ; n, albuminous ball
of nutritive yelk ; /, fat-globule with-
in tho 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
tlie Disc-gastrula (Disco-gastrula, Fig. 43). The cell-mass of
this Gastnda forms a thin, circular disc. The lower concave
220 THE EVOLUTION OF MAN.
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 sijigle layer of larger cells (Fig. 24, i), which are
characterizied by a softer, less transj^arent, 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 (j^roperi stoma), the
entoderm, and the exoderm pass into each other without
clear limits (Fig 43, tu).
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-gas trula {Arclii-
gastrida, 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-
THE NUTRITIVE YELK. 221
gastrula (Disco-gastrula, Fig. 43). We may therefore infer
that the latter is directly, or through the intermediate stage
of the Hood-gastrula, descended from the original BeU-
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 kcnogenetic 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-vcsiclcs, 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.'^ This
is by no means the case. The nutritive yelk, wdiat-
ever its size, always remains a lifeless store of nutritive
matter, wliich, 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
fimnative yelk, from the parent-cell. This is equally true
222 THE EVOLUTION OF MAN.
of the small Osseous-fish which we have been examininsr,
and of the huge eggs of the Primitive Fishes (Selachii), 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 tlie 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.
(Of Fig. 10 E,Y). 134.) As in all Skulled-animals {Craniota)
the original egg-ceU or primitive egg {'protoviim) is com-
pletely covered by a continuous layer of smaller cells, as
though by an epithelium. This skin-coat, or epithelium, is
PROTOVUM AND METOVUM.
223
the so-called Graafian follicle ; immediately under tliis 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 lyrotovuin 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.'^'^ The accumulation of the large
yellow-yelk mass within the ball of protoplasm forces the
kernel {yesiciila germinativa), which is contained in the
latter, quite to the upper surface of the yelk-mass. Here
the kernel {ves'icida germinativa) is surrounded by a small
quantity of protoplasm ; and these two together form the
lentil-shaped " formative yelk " (Fig. 44, h). This appears
on 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 cicatricida. 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 (resicM^a germi-
nativa), together with the protoplasm of tlio
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 {d'). The two kinds of yelk are,
however, not sharply distinguished.
thread-like cord of white nutritive yelk (cZ), which contains
no particles of yellow yelk, and is softer than the yellow
nutritive yelk, passes fi-om the tread directly to the
224 THE EVOLUTION OF MAN.
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, 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 discoidaUs, Fig. 45) entirely
similar to that of the egg of the Fish (Plate III. Fig. 18-24).
Two similar cleavage-cells (A) first arise from the parent-
cell. These part into 4 (i?), into 8, 16 (C), 82, 64 cells, and
so on. As before, the division of the kernel always precedes
the division of the cells. The planes of division between
CLEAVAGE OF THE BIRDS EGG.
225
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 cicatricida, 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-cells
are parted by a second furrow (perpendicular to the first) into four cells.
C. 10 cells have originated from the ■!• cleavage-cells, owing to the fact that
between the first two bisecting furrow.'?, 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 10 radial furrows and
about 4 concentric ring-fuiTOWS. E. A stage with 01' radial fuiTows and
about 0 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 mulbcrry-gorm (Morula). The separation of
the kernel always precedes the formation of the furrows.
226 THE EVOLUTION OF MAN.
an eiglit-rayed star. A circular fui-row 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 {(J). 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. '^^ 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 e^gg is, however, thinner and
flatter than that of the Qgg of the Osseous Fish (Plate III.
Fig. 21).
In the Hen's &gg, 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 {id), is
again formed ; and a cleavage-cavit}'' (s) is formed between
this germ-membrane (Blastoderma, Fig. 47, h) 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 {Discogastrula), resembling that of the Osseous Fish
GASTRULA OF CHICK. 22/
(Plate III. Fig. 24). While, however, in the latter case the
nutritive yelk is attached directly to the lower surface of
tlie 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, s, 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
exoderm.
The germ-disc (Blastodiscus), which in an iinincubated,
freshly-laid Hen's egg lies at the tread, or cicatricula, is
thus already a complete Disc-gastrula (Discogastrula, Fig.
49). 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 z)J'^
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
]2
228
THE EVOLUTION OF MAN.
superficial cleavage (segmentatio siiperjicialis, Plate III,
Fig. 25-30). This form is entirely unrepresented among
Vertebrates. It, however, plays the most important part
Fig. 46—19. — Gastnilation 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); h, clcava2:e. cells.
Fig. 47. — (B) 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 ; iv, thickened edge ;
s, cleavage-cells.
Fig. 49. — (D) Gastrnla (Discogastrula) of Chick : d, primitive intestinal
cavity.
CLEAVAGE OF ARTICULATES. 229
in the very extensive articulated tribe (Arthropoda), in
Insects, Spiders, Centipedes, and Crabs. The Gastrula
which I'esults 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 es^.
The cleavage, as it affects only the former, not the latter,
is naturally entirely superficial ; the provision, wliich 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, Penevus.) The parent-ceU, 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, smaU, similar cells,
which lie side by side in a single layer over the whole
surface of the egg, forming a superficial germ -membrane
230 THE EVOLUTION OF MAN.
(Blastodei'Tna, Fig. 28, h). This germ-membrane is a simple,
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 11. 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
germ.
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 (o).
The inverted portion of the germ-membrane, the cells of
which enlarge and assume a slender cylindrical form, consti-
tutes tJie intestinal layer and surrounds the cavity of the
primitive intestine. The superficial, uninverted portion of
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
RELATION OF THE CLEAVAGE-FOKMS.
231
form, that of the Bell-gastrula (Archl-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 nntritive-yelk in the centre of the egg."^^
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 tliat 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
ess-cleavage is as follows : —
I. Palingenetic
cleavage.
II. Kenogenetic
(modified by
adaptation)
cleavage.
il. Original cleavage (BeIl-'\
gastrula). A. Total cleavage (with-
*■ V out any independent
/2. Unequal cleavage (Ilood- nutritive yelk).
gastrula) . j
1 3. Discoidal cleavage (Disc-^
I gastrula). B. Partial cleavage (with
r an independent nu-
4. Superficial cleavage (Blad- tritive yelk).
^ der-gastrula). J
The lowest known intestinal animals {Metazoa), that is to
232 THE EVOLUTION OF MAN.
say, the low 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
primceval, sirifiple parent-form, the tuhole body of which
consisted throughout life, as now in the case of the lowest
Plant-animals, only of tivo 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 Gastrsea (i.e. primitive intes-
tinal animal).^^
According to the Gastrtea 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
HOMOLOGY OF THE GERM-LAYERS. 233
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 imier 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,
etc.
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 ceU-stratum, and in addition to this appears only
to produce the reproductive cells.^°
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
lie 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. Trustwortliy
234 THE EVOLUTION OF MAN.
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.''^
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. Iil
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 Reichert,
His, and other confused observers have asserted.''^ 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
MIDDLE GERM-LAYERS. 235
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 laj'^er. 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
23^
THE EVOLUTION OF MAN.
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 throngli the embryo of an Earth-worm : hs,
skin-sensory layer ; hm, skin-fibrous layer ; df, intestinal-fibrous layer ; dd,
intestinal-glandular layer ; a, iutestinal cavity ; c, body-cavity, or Cxloma;
n, nerve-centres ; u, primitive kidneys.
Fig. 51. — Corresponding section of the larva of Ampliioxus (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 : —
B. The four secondary germ-layers.
A. The two primary germ-lay m:
I. The outer or animal germ- /
layer (the skin-layer, or ;
exoderm). (
II. The inner or vegetative V
germ-layer (the intesti- /
nal layer, or entoderm). /
1. Skin-sensory layer (skin-stratum, Baer).
2. Skin-fibrous layer (flesh-stratum, Baer).
3. Intestinal-fibrous layer (vascular stra
turn, Baer).
4. Intestinal-glandnlar layer (mucous stra-
tum, Baer).
OEIGIN OF MIDDLE GERM-LAYERS. 23/
Much recent research by Kowalevsky, Ray-Lankester,
Van Beneden, and others has justified this " Four-layer
Theory " of Bacr, For instance, it can be plainly shown
that in the Earth-worm (Fig. 50), in the AmjDhioxus (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 (As),
and the skin-fibrous layer (Inn) : similarly the intestinal
or inner layer separates into the intestinal-fibrous layer
(df), and the intestinal-glandular layer (dd). The body-
cavity, or coeloma (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 (^inesoderma). 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,
that 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
sliall presently find, that only a single middle layer
(Remak's " motor-germ inative 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
238 THE EVOLUTION OF MAN.
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 deofree 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 (Echinoderma), and again in
the higher Worms (Fig. 50). This fact in Comparative
Ontogeny is of the greatest phylogenetic significance. In
all cases, these four secondary germ-layers develop from
the two primary germ-layers ; it is only in the lower Plant-
WOLFFS KNOWLEDGE OF THE GEEM-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 tjq^e," the proof of which
was not furnished till half a century later by Baer."
(Cf. p. 46.)
Bemah's three gcrm-laye) s.
I
Cater, or
upper
layer
Inner, or
under
layer
I. Outer, or nnper
germ-laj'er (sen-
sory layer)
II. Middle germ,
layer (motor-ger-
minative layer)
III. Inner, or under
germ-layer (tro-
phic layer)
The four secondary
germ-laye7's.
1. Skin. sensory
layer
2. Skin-fibrons layer/
3. Intestinal-fibrous \
layer
Intestinal-glan-
dular layer
The two primary
germ-layers.
Animal layer,
Exoderm, or skin-
layer.
Ycgctativo layer,
Entoderm, or intes-
tinal layer.
Modified ontf)gcnetic process.
Original phylogenetic process.
240 THE EVOLUTION OF MAN.
EXPLAITATION OF PLATES II. AND III.
Egg-cleavage and Gastrulation.''*
These two plates are intended to illustrate, by means of diagrammatic
sections, the most important differences in animal egg. cleavage and gas-
tralation. Plate II. represents holoblastic 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. cell
{Cytula) ; /, cleavage. cells (Segmentella) ; m, mulberry-germ (Morula); i,
germ-vesicle {Blast ula) ; g, cup-germ {Gastrula) ; s, cleavage-cavity; d,
primitive intestinal cavity ; 0, primitive mouth ; n, nutritive yelk ; i, intes-
tinal layer (Entoderm) ; e, skin-layer (Exoderm).
Fig. 1-G. — Original or primordial egg-cleavage of the lowest Vertebrate
(Amphioxus). Fig. 1, parent-cell (Cytula) ; Fig. 2, cleavage-stage with
4 cleavage -eel Is ; Fig. 3, mulberry -germ (Morula) ; Fig. 4, germ-vesicle
(Blastula) ; Fig. 5, the same, in process of inversion (Invaginatio) ; Fig. 6,
Bell-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; i, 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-
gastr ula (Amphigastrula) .
Fig. 18-24. — Discoidal egg-cleavage of an Osseous fish (Motella ? Cottus ?).
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 ; Fig. 21, mulberry-germ
(Morula) ; Fig. 22, gorin-vesicle (Blastula) ; Fig. 23, the same, in process of
inversion; Fig. 24, Disc-gastrula (Discogastrula).
Fig. 25-30. — Superficial egg-cleavage of a Crab (Peneus). Fig. 25,
parent-cell (Cytula) ; Fig. 26, cleavage-stage with 4 cells ; Fig. 27, cleavage-
stage with 32 cells ; Fig. 28, mulberry -germ (Morula), and at the same
time the germ-vesicle (Blastula); Fig. 29, Bladder-gastrula (Perigastrula) ;
Fig. 30, Nauplias-gcrm ; the pharynx-cavity has formed in front of the
primitive mouth (d), owing to an inversion from without.
V
5 '4
.iiiifi'ilIllHllllliiiHi
■ii'J-i:i.JJau:.iiii!iiiiiiiHiiiiii!iiiiHiiLiiiiiiiiiii!:iiiaj;i
iiiiiiii !|!|ii
|]|lilili!iilllll!:
(
, -^
■>~N.
1
"■^'Qi^^&a.
TABLE III.
List of the most important differences in Animal Egg-clearago and
Gastralation."*
The letters a<-/ indicate the 6 animal tribes (the primitive animals being
excluded) : a, Plant -animals ; h, Worms ; c, Soft-bodied animals (Mollusca) ;
d, Star-animals (Ecliinoderma) ; e, Articulates (Arthropoda) ; /, Vertebrates.
Total Cleavage.
Segmi n ta t io total is.
Holoblastic Eggs.
Gastrula with-
out
nutritive yelk.
Hologastrula.
I. Original Cleavage.
{Segmentatio primordialis.)
Archiblastic eggs.
Bell-gastrula,
{Arch igastrula .)
(Plate I.I. Fig. 1-6.)
' a. Most low Plant-anim.ils (low
Sponge.'!, Ilydrapolyps, Me-
dusa-, Coral.s).
b. Many low Worms (S.gitta,
Phoronis, Ascidia, many
Nematodes, etc.).
c. A few low Soft-bodied .animals
(Mollusca) — Terebiatula, Ar-
giope, Pisidium.
d. IMost Star - animals (Echino-
derma).
e. A few low Articnl.ates (some
Branchiopods, Copepods, Tar-
digrades).
/. The Skull-Iess Vertebrate (Ani-
\ phioxus).
II- Unequal Cleavacje.
(Segmen ta t io innqnalis.')
Amphiblastic eggs.
Hood-gastrula.
(Am2}higaslrHla-')
(Plate II. Fig. r-17.)
a. Numerous Plant-animals (many
Sponges, Medusa?, Corals,
Sipiioiiopliores, Ctenophora;).
6. Most Worms.
c. Most Soft-bodied Animals (Mol-
lusca).
d. Individual Star-animals (vivi-
parous species and a lew
others).
e. A few low Articulates (Arthro-
poda)~\>oi\i Crustaceans and
Traclieates.
/. Cyclostoma, Ganoids, Amphibia,
Mammals (all :).
II.
Partial Cleavage.
Segmentatio partialis.
Meroblastic eggs.
/ III. Discoidal Cleavage.
{.Segmentatio discoidalis.)
Discoblastic eggs.
Disc-gastrula.
(Disco-gastrula.)
(Plato III. Fig. 18-21.)
Gastrula with
nutritive yelk.
ikrogaslrula.
c. Cuttle-fish, or Cephalopods.
e. Some Articulates {Arthropoda),
Millepedes, Scorpions, and
others.
/. Primitive Fishes (Selachii),
Osseous Fishes, Heptiles,
Birds (and Monotremes .').
IV. Superficial Cleavage.
{.Segnumtatio superjicialis .)
Periblastic egys.
Bladder-gastru'a.
(Peyigaitrula.)
(Plate III. Fig. io-ao.)
f a. A few Sponges (?).
Alcyonium (.-).
b. Individual Worms (.>).
e. Tlic great m.njority of Articu-
lates (.1 rth ropoda) — Crus-
1 taccans, Myriopods, Spiders,
^ Insects.
TABLE IV.
Systematic Survey of the five earliest germinal stages of Animals with
reference to the four different type-forms of Egg-cleavage.
A. Total Cleavage.
{Segmentatio totalis.)
a. Original or primor-
dial cleavage.
I. a. Archi-
monerula.
(Fig. 22 A, p. 191.)
A cytod in which
the formative and
nutritive yelk are not
distinct.
II. a. Archi-
cytula.
(Plate II. Fig. 1.)
Parent-cell -nhich
has arisen out of the
archi-monerula by the
formation of the pa-
rent-kernel.
III. a. Archi-
morula.
(Plate II. Fig. 3.)
A solid (generiilly
globular) heap of
similar cells.
IV. a. Archi-
blastula.
(Plate II. Fig. 4.)
A hollow (usually
globular) vesicle, the
wall of which consists
of a single layer of
similar cells.
V. a. Archi-
gastrula-
Bell-g;istrula.
(Plate II. Fig. 6.)
Fig. 23-28, p. 193.
Primitive intestine
empty, without nu-
tritive yelk. Pri-
mary germ -layers
one-layered.
6. Unequal cleavage.
B. Partial Cleavage.
(Segmentatio partialis.')
I. 6. Amphi-
monerula.
A cytod which
includes fomiative
yelk at the animal
pole, nutritive yelk
at the vegetative
pole: the two are
not very distinct.
II. &. Amphi-
cytula.
(Plate II. Fig. 7, 12.)
Parent-cell which
has arisen out of the
amphi-monerula by
the formation of the
parent-kernel.
III. b. Amphi-
morula.
(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. b. Amphi-
blastula.
(Plate II. Fig. 10.)
A roundish vesicle,
the wall of which at
the animal pole con-
si^ts of smaller cells,
at the vegetative
pole of larger cells.
V. b. Amphi-
gastrula.
Hood-gastrula.
(Plate II. Fig. 11,17.)
Fig. 32-35, p. 206,
Fig. 41.
Primitive intestine |
partly tilled with
segmented nutritive
yelk. Germ-Iaycrs
often many-layered.
I. c. Disco-
monerula.
A cytod which
includes formative
yelk at the animal
pole, nutritive yelk
at the vegetative
pole : the two are
quite distinct.
II. c. Disco-
cytula.
(Plate III. Fig. 18.)
Parent-cell which
has arisen out of the
disco-monerula by
the formation of the
parent-kernel.
III. c. Disco-
morula.
(Plate III. Fig. 21.)
A flat disc, com-
posed of similar cells
on the animal pole
of nutritive yelk.
IV. c. Disco-
blastula.
(Plate III. Fig. 22.)
A roundish vesicle,
the small hemisphere
of which consists of
cleavage - cells, the
larger of nutritive
yelk.
V. c. Disco-
gastrula.
Disc-gastrula.
(Plate III. Fig. 24.)
Fig. 43, p. 219,
Fig. 49, p. 228.
Primitive intestine
filled with unseg-
mented nutritive
yelk. Flat germ-disc.
I. d. Peri-
monerula.
A cytod which in-
cludes formative yelk
in the outer wall,
nutritive yelk in the
centre.
II. d. Peri-
cytula.
(Plate III. Fig. 25.)
Parent-cell which
has arisen out of the
peri-monerula by the
formation of the pa-
rent-kernel.
III. d. Peri-
morula.
(Plate III. Fig. 27.)
A closed vesicle : a
cellular stratum sur-
rounds the whole cen-
tral nutritive yelk.
IV. d. Peri-
blastula.
(Plate III. Fig. 28.)
A closed vesicle : a
cell layer surrounds
the whole nutriiive
yelk (= Peri-morula).
V. d. Peri-
gastrula.
Bladder-gastrula.
(Plate III. Fig. 29.)
Cleavage-cavity fill-
ed with unsegmented
nutritive yelk. Pri-
mitive intestine dif-
ferent.
TABLE V.
Systematic Survey of some of the most important rhythmical variations in
Egg-cleavage.^*
Only the first column (AmpUioxus) presents the original, palingenetic
cleavage- hythm, in regular geometrical pi'ogression. All the other columns
show the descended kenogenetic modifications.
c = parent-cells. s = cleavage-cells. e = exoderm-cells.
i = entoderm-cells.
I.
I.anceJet
(Amphi-
oxus).
n.
Ampliihian
(Frog).
ni.
Mammal
(Rabbit).
rv.
Snail
(Trochus).
v.
Worm
(Fabricia).
VI.
Worm
(Cyclogena).
Ic
U
Ic
Ic
Ic
Ic
2s
2s
2s
(le + 10
2s
2s
(le + 10
2s
(le + 10
4s
4s
4s
(2e + 20
4s
3s
(2e + 10
3s
(2e + 10
8s
8s
(le + 4i)
8s
(4c -f 40
8s
(4e + 40
5s
(4e + 10
4s
(3e+ 10
12s
(8c + 4i)
12s
(8c + 40
12s
(8e + 40
6s
4e + 20
5s
(lc+ li)
16s
16s
(8e -f 8i)
16.S
(8e + 80
20s
(IGe + 40
10s
(8c + 20
6s
(5c + 10
1
24s
(IGe 4- 80
24s
(IGe + 80
24s
(IGe + 80
lis
(8e + 30
7s
(Ge + li)
328
32s
(IGc + IGO
32 s
(IGc + IGO
40s
(32e + 80
19s
(IGe + 30
8s
(7e + 10
48s
(32e -f 160
48s
(32c + IGO
44s
(32e+ 120
21s
(IGc + 50
9s
(8c + li)
648
64s
(32e + 320
64s
(32e + 320
76s
(G4e + 12i)
37s
(82e + 5i)
10s
(9e + 10
96s
(Gle + 320
968
(64e + 32i)
84s
(64e + 20i)
38s
(32e + 60
128s
160s
(128e + 320
148s
(128e + 20i)
70s
(Gle + Gi)
CHAPTER IX. I
^
THE VERTEBEATE NATUEE OF MAN. ]
Relation of Comparative Anatomy to Classification. — The Family-relation- ' '
ship of the Types of the Animal Kingdom. — Different Significance <
and Unequal Value of the Seven Animal Types. — The Gastroea Theory,
and the Phylogenetic Classification of the Animal Kingdom. — De- '
scent of the Gastrcea from the Protozoa. — Descent of Plant-animals
and Worms from the Gastrjea. — Descent of the Four Higher Classes of :'
Animals from Worms.— The Vertebrate Nature of Man. — Essential and l!
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.— i
The Leather-skin (corium). — The Outer-skin (epidermis). — Body- a
cavity (cwlorna). — The Intestinal Tube. — The Gill-openings. — The •
Lymph-vessels. — The Blood-vessels. — The Primitive Kidneys and y
Organs of Eeproduction. — The Products of the Four Secondary Germ- I
layers. ~
" 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 j
himself. For if we look for the purpose of our existence, we cannot possibly j
find it ontside ourselves. We are here for our own sake." — Karl Eknst ;
Baer (1824).
A DIFFICULT task now lies immediately before us in this
history of our individual development ; we must trace the
complex hyiman body with all its various parts, — organs,
IMPORTANCE OF COMPARATIVE ANATOMY. 245
limbs, etc. from the simple Gastrula. The two primaiy
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 thu
same result.
246 THE EVOLUTION OF MAN.
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.^^ 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),^^ of the
animal kingdom, are: (1) the Protozoa; (2) the Plant-animals
{Zoophytes) ; (3) the Worms (Vermes) ; (4) the Soft-bodied
animals (Mollusca) ; (5) the Star-animals (Echiiioderma) ;
(6) the Articulated-animals {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 of
THE DOCTRINE OF TYPES, 247
my Gasti'«2a Theory ,^^ on which I base the monoj)liyletic
genealogy of the animal kingdom, and which I am con-
vinced must supersede the Theory of Types which now
prevails. According to this Gastr^ea Theory, which I
enunciated in the " Monograph on the Chalk Sponges '
(vol. ii. pp. 4G5-4G7), 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, Ai'thropods, 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
typca They are more specialized and typical than the
Primitive-animals, and less typically organized and charac-
terized than the four higher tribes.
The Gastriiea 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 (protogaster), the primitive
intestinal or stomach cavity, in its most simple form. In
the Gastrjca itself, and in the extant Gastreads {Hallpliy-
sema, Gastrophysenia), 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
248 THE EVOLUTION OF MAN.
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 Amoebse and Gregarinse, 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.*^^ ■
We must therefore primarily divide the Avhole 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 Gastreea,
the previous existence of which may be, even at this day,
proved with certainty by means of the Gastrula. This
PROTOZOA AND METAZOA. 249
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. (C£ Plates II.,
III., and Fig. 22-28, pp. 191, 193.) The Gastrula at the
present day presents a correct picture of the primitive
Gastrtea, 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 Hydi-a, 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 GastriTsa. In the
Linna;an 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
vai-ious parasitic Worms, Tape-worms, Round-worms,
TrichiniB, etc. Different as all these worms appear, in their
perfect state, they can all be traced back to the Gastriea.
(Cf. Table XVIII. in Chap. XVII.)
250 THE EVOLUTION OF MAN.
We must look for the original parent-form of the four
hioher tribes of animals amono; 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 (Molliisca — 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 Gastroea Theory, at
the present stage of zoological classification and of embryo-
logical knowledsfe. If I am right in assertino^ the orio^inal
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 tAvo 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.
MONOPHYLETIC GENEALOGY. 25 I
In specially observing the position of Man in the animal
system, 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 (MoUasca), or Star-animals (Echinoderma). There-
fore by far the gi'eater part of the animal kingdom may be
entirely overlooked in our future investigations, whether
Ontogenetic or Phylogenetic. We have nothing farther 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-
13
252 THE EVOLUTION OF MAN.
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 miglit 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
VERTEBRATE NATURE OF MAN. 253
important and essential to tlie reader, are shown by the
History of Evohition and Comparative Anatomy to be oi
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 existinix Vertebrate, retaining the form tlirouohout
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 Lancelet, or Amphioxus (Plates X. and XI.). 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, lanccloate leaf. It is, therefore,
called the Lancelet. The narrow body is compressed on
both sides, is similarly pointed in front and at the back,
254 THE EVOLUTION OF MAN.
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 bearinof
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
(Ascidia) of which it is immediately connected in its de-
velopment and bodily structure.
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 XL) Li 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 XL
with Fig. 52-56.)^
THE NOTOCHORD. 255
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 tlie 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
tlie 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
256
THE EVOLUTION OF MAN.
' Fifi. 53.
Fig. 54.
Fig. 55.
Fig. 56.
Fig. 52. — The ideal Primitive Vertebrate type, seen from the left side :
mr, medullary tube ; x, chorda ; na, nose ; au, eyes ; g, ear-vesicle ; md,
mouth; l; 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.
BILATEEAL FORM OF VERTEBEATES. 2$/
Fig. 51'. — Trausvcrse section of the same in the anterior part (through
the gill-body, at /.;/, Fig. 53).
Fig. 55. — Transverse section of the same in the central part (in the
ncighboui'hood 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
sections.
same relative position and connection ; but their positions
in relation to the central plane of section are exactly re-
versed ; the left half reseml)les 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 vential 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.
Plates IV. and Y.^)
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, tlierefore,
specially employed in the formation of the dorsal half, and
the two inner in the formation of the vcnti-al half Each
of the two halves develops in the form of a tube, and
surrounds a cavity in which another tube is enclosed.
258 THE EVOLUTION OF MAN.
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, r>i). 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, 'nw). 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 XL Fig. 15, r)!). 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, no), a pair of eyes (au),
and a pair of auditory vesicles (g) of the most simple cha-
racter.^^ Some of these organs of sense are not represented
THE rillMITIVE VERTEBRATES.
259
in the Amphioxus, probably in consequence of secondary
reversion. (Of. Chap. XIII.)
Fig. 57. — Transverse section through the
anterior part of the primitive vertebrate type :
mr, sj)inal tube; x, chorda (notochord) ; msi,
dorsal muscles ; kb, gill-veut ; k, gill-intestine.
On both sides of the spinal tube
of all Vertebrates, and the notochord
"vvhich 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).
Fig. 58. — Transverse section through the
central portion of the ideal Primitive Verte-
brate : /, skin-fold, forming fin ; mr, spinal tube ;
ar, chorda ; msi, dorsal muscles; ms2, ventral
muscles ; a, aorta (in the mesentcr}-) ; ma,
stomach-cavity ; c, body-cavity (visceral cavity) ;
hz, heart.
Outside this wall we find the outer firm coverincr
of the whole body, called the leather-skin {comum, or
26o THE EVOLUTION OF MAN.
cutis, 111). The lower layers of this tough and thick
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 {epiderr)iis, oJt). 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-sti-atum, 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
cwloma. 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 coelom, when developed, is
I
INNER STRUCTURE OF PRIMITIVE VERTEBRATES. 26 1
separated into two distinct cavities, which arc completely
divided by a transverse partition, the muscular midriff, or
diaphragm. The first, or chest-cavity, contains the cesopha-
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 ccelom, 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 (coaloma), 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 coslom,
and having two openings; a mouth-opening for taking in
food (Fig. 59, 60, mcV), and an anal opening for discharg-
ing w^aste-matter or excrement (af). '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-
^L pouches (Fig. 59, GO, I) were probably present even in the
^■parent-form of Vertebrates. The walls of the intestinal
^B canal and of all these appendages, consist of two very
^H different parts or layers ; the inner cellular covering is the
^B intestinal-glandular layer, or the fourth germ-layer ; the
^T 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.
262 THE EVOLUTION OF MAN.
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-jEibrous 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 coeloma, 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 mor})hological 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 chai'acteristic 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 (Jcs), are most closely connected to the primitive
THE INTESTINAL CANAL IN PRIMITIVE VERTEBRATES. 263
respiration of Vertebrates. All lower Vertebrates, the
Anipliioxus, Lampreys, and Fishes, continually take in
water through the mouth, and let it pass out through
Fig. 59. — The ideal Primitive Vertebrate, seen from the left side : na,
nose ; au, eye ; g, ear ; md, mouth ; ks, gill-openings ; x, chorda ; wr,
spinal tube ; leg, gill-vessels ; Ic, gill-intestine ; hz, heart ; ms, muscles ;
ma, stomach ; v, intestinal vein ; c, body-cavity ; a, aorta ; I, liver ; d, small
intestine ; e, ovarj' ; h, testes ; n, kidney canal ; af, anus ; lli, 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 " (7.;^), 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, liowcver, one of the most interest-
ing proofs of the descent of thesa three higher classes of
Vertebrates from the Fishes.
264
THE EVOLUTION OF MAN.
Not less interestiiifj and siafnificant is the circumstance
that the later respiratoiy 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 lungs 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; h, gill-body; ks, gill-openings; kg, vascular
gill-arcbes; 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 ; /, skiu-fold, acting as float.
THE VASCULAR SYSTEM IN PRIMITIVE VERTEBRATES. 265
the system of blood-vessels and the system of lymphatic
vessels. The cavities of the former contain the red blood ;
those of tlic latter, the colourless lymph. To the lymphatic
system belongs the coelom (the so-called pleuro-peritoneal
cavity) ; and also numerous lymphatic ducts which extend
through all the organs, absorbing the juices wliich 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
intestines, 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 lA,inged- 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 tlic 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 cliannels 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. GO, v) lies below the intes-
266 THE EVOLUTION OF MAN.
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 " (leg) j^ass 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 (]iz). 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, wei'e originally hermaphrodite, that each indi-
vidual was capable of reproducing itself indopi n lently, and
that the separation of the sexual organs took place at a
SEXUAL ORGANS IN THE PRIMITIVE VERTEBRATE. 26/
later period. Wc may, therefore, assume that the primitive
Vertebrate possessed both ovaries (Fig. GO, 61, e) and
testes (h).
Fig. 61. — Transverse section through the
l)osterior part of the ideal Primitive Vcrtebi'ate :
/, float ; mr, spinal tube ; x, notochord ; ms,
muscles; e, ovaries; n, primitive kidney ducts;
a, body-arteries ; d, intestine ; v, 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.^'' 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 rei)eated 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.
268 THE EVOLUTION OF MAN.
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 lymg 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 wliich 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
METAMERA, 269
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, they
are morpliohxjically unimj^ortant, 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
notocliord, 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,
(Cf. 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
primaries.
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
270 THE EVOLUTION OF MAN.
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 {epideriDiis) ; 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 ejndermis
and the inner coelom, 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 the
RELATION OF THE ORGANS TO THE GERM-LAYERS. 2/1
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 coelom-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 endocoelar, that
is, the inner, or visceral coelom-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-vessels,
which open into the latter, are formed, like the heart, the
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
272 , THE EVOLUTION OF MAN.
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. Its
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. (Of. Plates IV., V.)
TABLE VI.
Systematic Snrvcy of tlic principal organs of the ideal rrlmitive Vertebrate,
the hypothetical parent-form of Vertebrates, and of their development
from the germ-layex's.
Primary Germ-
layers.
Secondary Germ-
layers.
Most important Organs of
ths Primitive Vertebrato.
Skin-layer
(Animal gprtn-layer,
Baer).
Lamina dermal is, IT.
Excderma.
Skin-sensory layer
QSkin-stratum, Baor),
or
Senrory layer.
Lamina neurodentialis, 11.
II.
Skin-fibrous layer
(Flesh-straluiu, Baer),
or
Flesh-laysr.
Lamina inodomalis, H.
i ■
Outer skin (Epid^'.rmis'). A
simple cell-covering of the
outer surface.
Spinal tube (Tubus medul-
laris) (with the organs of
sense : the nose, eyes, organs
of hearing).
Primitive Kidneys {Protone-
phra) Ca pair of simple ducts,
primitive kidney ducts).
True skin (^Corium) (^Cutisani
subcutis').
Sluscles of the trunk (dorsal
and ventral muscles).
Notochord {Otorda dorsalis).
Kxocoelar, or Parietal Coelora-
epithelium (the inner oell-
covering of the body-wall).
Jlale sexual glands (^Testes}.
Cooloma, or Body-cavity. A space between the body-wall and the
intestinal wall, between the exoderm and the entoderm, filled with lymph
(colourless blood).
n.
Intestinal laver
(Vepetatlve Rorm-
l.iyor, Baer).
Iximina gastralis, II.
Entodeima.
in.
Intestinal-fibrous layer
(Vascular stratum, Baer),
or
Vascular laver.
Lamina inoi/astralis, H.
IV.
Intfistinal-glandular layer
(Mucous stratum, Baer),
or
Mucous layer.
iMm na mycugaslralis, U.
Female sexual glands {pvary").
Kiidocci'lar, or Visceral Cce-
lom-epitheliiim (the outer
cell-covering of the intestinal
tube.)
Principal Wood-vessels (primi-
tive artery or dorsal vessel,
and the primitive vein or
ventral vessel).
^lesentery.
IMuscular inteslinc wall (fi-
brous intestinal wall).
Intestinal epithelium (innel
cell-covering of the intestinar
tulx!).
Intestinal plnndnlar epithe-
lium (livir-cells and other
intestinal gluudulur cells).
CHAPTEr. X.
THE CONSTRUCTiON OF THE BODY FROM THE GERM-
LAYERS.
The Original (Palingenetic) Development of the Vertebrate Body from
the Gaatrnla. — Relation of this Process to the Later (Kenogenetic)
GeiTuination, 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 Tubes. — Contemporaneously with the Completion
of the Yelk-sac, the Germ-layers flatten, and only later again assume
a Tubular Form. — Origin of the Disc-shaped Mammalian Germ-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 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 Ccelom and tlie First Arteries. — The Intestinal Canal proceeds from
the Intestinal Furrow. — The Embryo separates from the Germ-vesicle.
— 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 AValls 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,
»
GASTRULA OF THE MAMMAL. 275
which likewise coalesce at the edges. Thus two main tubes are formed, one
above the other. During the formation of these, the embiyo separates into
strata, so that the two main tubes are composed of subordinate tubes which
enclose each other as fundamental organs, and arc capable of developing
into all the organs." — Karl Ernst Baer (1828).
The mammalian egg, in the stage of development in which
■vve 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 com] ilex 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 yei 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 refeiTcd. It is only
in a few unimportant points that the Amphioxus appears to
14
276
THE EVOLUTION OF MAN.
Fig. 62-69. — Diagrammatic transverse sections through the most im-
portant germ-forms of the ideal Primitive Vertebrate (Fig. 52-61).89
Fig. 62.—^. Transverse section through the Gastrula; two-layered germ.
Fig. 63.— B. Three-layered germ.
Fig. 64.— C. Four-layered germ (four secondary germ-layers).
BELL-GASTRULA OF AMPHIOXUS. 277
Fig. 65. — D. The body-cavity appears between the skin-layer and the
intestinal layer.
Fig. 66. — E. The notocliord appears between the spinal furrow and the
intestine.
Fig. 67. — F. The primitive kidneys and primitive vertebrae appear ; the
spinal tube is closed.
Fig. 68. — G. The rudiments of the sexual organs appear near the primi-
tive kidneys. The primitive vertebrae surround the notochord and the
spinal 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) ; x, male germ-glands
(rudimentary testes) ; a, aorta (primitive artery) ; vd, intestinal vein
(primitive vein) ; vc, cardinal vein ; ch, notochord ; uw, primitive ver-
tebriE ; IV, vertebi"iB ; r»i, dorsal muscles ; bni, 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 (/(/) is horizontally shaded.
The skin-sensory layer (lis) 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 oi
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
2b, p. 193) affords a safe starting-point. A sei'ies of dia-
278 THE EVOLUTION OF MAN.
grammatic transverse sections through those germ-forms
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 (mesoderma, Fig. 63 mh), 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
{mfiesoderifna), 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 (Jis) and the skin-fibrous layer {hf), and
correspondingly, the entoderm, or intestinal layer, into the
intestinal-fibrous layer (df) and the intestinal-glandular
layer (cW).
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 onf) ;
these are the spinal swellings (medullary or dorsal swell-
ings). Their two parallel edges bend toward each other
(Fig. QQ mf) and finally coalesce, so that the trench
becomes a tube ; this is the spinal tube (Fig. 67 tut).
Along the longitudinal axis of the body, a solid cylindrical
DEVELOPMENT OF MAMMALIAN GASTRULA. 2/9
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 rin) and the ventral muscles (bin).
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 (coeloma, Fig. 65-61) c). The intestinal tube lies
freely in this, being only supported along the length of the
notochord by a band of the intestinal-fibrous layer, which
afterwards extends into the mesentery (Fig. 68 g). Two
narrow canals, filled with blood, form within the intestinalr
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
skin (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
2 So THE EVOLUTION OF MAN.
their 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 " noto-
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 of a form
THE MOST IMPORTANT PROCESSES IN GASTRULATION. 28 1
with a single axis, it assumes the bilateral vertebrate form.
8. The body-cavity (coeloma) 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-
fibrous layer) .^°
These chief, fundamental, and palingenetic acts in the
282 THE EVOLUTION OF MAN.
evolution of the individual, the assumption of which is
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 {Cyclostovii) , 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-membi-anes 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 tiU 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
Avhicli is at first circular, afterwards oval, and which in the
hen's egg we have learned to call the tread (cicatricida),
lies at a particular point on the outer surface of the large
globular nutritive yelk. When germination begins, the flat
2:erm-disc arches outwards and detaches its outer surface
THE 'TREAD," OR CICATRICULA. 283
from the large yelk-ball which lies beneath ic. The flat
layers become tubes, in consequence of their edges inclining
Fig. 70. — Separation of the disc-shaped mammahan 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 (kh). B. The spinal tube
(mr) appears in the centre of the germ-disc, and underneath fhis the
notochord (cli). C. The intestinal-fibrous layer (df) has grown round
the intestinal-glandular layer (dd). D. The skin-fibrous layer (/i/) and the
iutestinal-fibi'ous layer (df) separate in the outer wall; the intestine (cf)
begins to separate itself from the navel-vesicle (?i6). E. The intestinal tube
(mr) is closed ; the body-cavity (c) begins to form. F. The primitive verte-
brae (w) separate ; the intestine (d) is almost entii'ely closed. G. The
primitive vertebrae (iv) begin to grow round the spinal tube ()/!)•) and the
notochord {ch) ; the intestine (d) is separate from the navel-vesicle (nb).
H. The vertebrae (w) have grown round the spinal tube (nir) and the noto-
chord ; the body-cavity (c) is closed, the navel-vesicle has disappeared. The
amnion and serous membrane are omitted.
In all, the letters indicate the same parts : h, horn-plate ; mr, spinal
tube; hf, skin-fibrous layer; u;, primitive vertebrge ; c/i, notochord ; c,ho6.j-
cavity (cceJooia) ; df, intestinal-fibrous layer; dd, intestinal-glandular layer;
d, intestinal cavity; nh, navel-vesicle.
284 THE EVOLUTION OF MAN.
towards each other and coalescing (Fig. 70). The germ
growing at the expense of the nutritive yelk, the latter
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 globidar
sac, the yelk-sac, or navel-sac (saccus vittelinus, or vesicula
umhilicalis, Fig. 70 nh). 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 (uinbilicus) 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
flat 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 Gastrsea Theory, the full
significance of which now becomes evident, teaches us that
THREE STAGES IN VERTEBRATE PHYLOGENY.
285
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 all Vertebrates, as of all Invertebrate
animals ; and the flat 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. ^^ The curving of these
germ-layers, which actually occurs, and their coalescence
into tubes is, therefore, not original and primary, but a much
later, tertiary 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 :
Primary
Secondary
Tertiary
(Palingenetic)
(Kenogenetic)
(Kenogenetic)
Process of Germ-
Process of Germ-
Process of Germ.
formation.
formation.
formation.
From the first, the
The germ. layers
The germ-layers form
germ -layers form closed
spread themselves out
a flat germ-disc, the
tubes.
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 a closed
yelk-sac from the cen-
tube.
tre of the intestinal
tube.
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
286 ■ THE EVOLUTION OF MAN.
germ or embryo, but as essentially a part of the latter, a
part of its intestinal tube. According to this view, the
primitive intestine (jirotogaster) 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 Gastr^ea 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.'^'^ 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.
GROWTH OF THE MAMMALIAN GASTRULA. 287
In Mammals the corresponding germinal processes are
very complex and peculiar. Till quite recently they were
entirely wrongly explained ; the recently published researches
of Eduard van Beneden ^^ which placed them, for the first
time, in a true light, enabled us to bring them into agree-
ment with the principles of the Gastrsea 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
sm'face 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 w^alls 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 (Amjihigastrula) of
the Mammal is complete (Fig. 71), it changes into a large
globular vesicle, filled wdth 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 (d). The mam-
malian germ now forms a solid ball, consisting of a quantity
288
THE EVOLUTION OF MAN.
of dark, multilateral eiitoderm-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 noAv 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 Eabbit)
in longitudinal section through
the axis : e, exoderm-cells (64,
lighter.coloiu'ed and smaller) ; i,
entoderm-cells (32, darker and
larger) ; d, central entoderm-cells,
occupying the primitive intes-
tinal cavity ; o, jjeripheric ento-
derm-cells, plugging the primi-
tive mouth-opening (the yelk-
plug in the "anus of Eusconi").
This vesicular condition of the mammalian germ was
detected two centuries ago (1677) by Regner de Graaf He
discovered small, globular, transparent vesicles, \\ith 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
THE PROCHORION.
289
re-discovered by Baer ; those of the Rabbit were afterwards
more minutely described by Bischoff, in 1842. They may
l)e found in the uterus (matrix) of the Rabbit, the Dog, and
other small Mammals within a few days after im.pregnation.
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.^'-^ (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. 72 (the surface) and in Fig. 73 (in
Fig. 72. — Intestinal germ-vesicle (Gastrocystis) of a Eabbit (the so-called
" Germ-vesicle," or vesicula Uastodei-mica, of other writers) : a, external
egg-membrane (chorion) ; b, skin -layer (exoderma), 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 gorm-vosicl&
section). The thick, external, structureless membrane which
surrounds this is a modification of the original egg-mem-
brane (zona pplhicida, p. 135), with the addition of an
290
THE EVOLUTION OF MAN.
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 hexaoonal; a lioht-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
rabbit.
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 "
(Keimhlase, Bischofi") ; the " sac-germ " (Baer) ; the " vesi-
cular embryo," or the " germ-membrane vesicle " {vesicula
blastodermica, or, briefly, hlastosphcBi'a). 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
THE INTESTINAL GERM-VESICLE. 29 1
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 entkely difierent germ-forms. The latter (the
Blastida) precedes gastrulation. The former (vesicula
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-layers.
On the other hand, the globular wall of the mammalian
" germ- vesicle " is the specialized skin-layer {exoderma),
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 ulterior 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 tlie developing intestinal cavity.
On all these ground.s, which have been very recently
bi'ought 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
292 THE EVOLUTION OF MAN.
is formed of both primary germ-layers, we will adopt Van
Beneden's name, and call it the intestinal germ-disc (Gas-
trodisGUs).
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 hlastodermicus), some-
times the " embryonic spot " (tache embryonnaire), but
more usually the germ-area {area gevminativa). 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 of
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 of
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 {''niesoderma) , appears
in the middle of the circular disc, between the two earlier
strata. According to most observers, the mesoderm arises
trom the iimer primary germ-layer ; according to others, on
GERM-AREA. 293
tlie contrary, it arises from the outer of the two ; ^^ 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. 76. — 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 (tliQ several cell- stratum of the middle layer) ;
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 overgrow^s 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 (prochorion), which
294
THE EVOLUTION OF MAN.
Fig. 77. — Egg from the uterus of a Kabbit (4 mm. in diameter). The
germ-membrane vesicle (b) has slightly retreated from the smooth outer
6gg-membrane (prochorion, a). The circular germ-area (c) is visible in the
centre of the germ-membrane, and at the edge of the former (at d) the inner
DEVELOPMENT OF MESODERM. 295
stratum of the germ-vcsiclo 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 (h). The outer
egg-membrane {pi'ochorion) becomes knotty, or warty (a).
Fig. 80. — The same, seen fi-om one side. The letters as in Fig. 79.
Fig. 81. — Egg froui the uterus 01 a Rabbit (8 mm. in diameter). Nearly
the whole of the germ-membrauo vesicle is already double-layered (6) ; ouly
below (at (?) 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 (j^rochorion) 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 RejDtile
(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.
296 THE EVOLUTION OF MAN.
The first modification of the cliseoidal, 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
Pig. 82. — Circular germ-area of a Eabbit, distingnished into the 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. 83. — 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 loellucida) ; the darker ring we
shall call the dark germ-area {area opaca). (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
THE GERM-SHIELD.
297
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. 8J-. — Gerni-area or
germ-disc of a Kabbit (about
ten times magnified). As
the delicate, half-transparent
germ-disc lies on black
ground, the light germ-area
appeal's as a darker ring, the
dark germ-area (situated on
the outside), on the contrary,
as a white ring. The oval
germ. shield, situated in the
centre, also appears wliitish ;
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
tlie 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 "
(notaspisy* Remak called it the "double shield," because
it arises from a shield-shaped thickening of the outer and the
middle genn-layers. In most book.^ this germ-shield is
298
THE EVOLUTION OF MAN.
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 ah^eady 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 enlarged) . 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 (6). 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 Remak,
"embryo-rudiment " of other authors). In the centre is the dorsal farrow,
on either side ai'e 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 central line
a delicate, white streak appears, which soon becomes pro-
THE PRIMITIVE STREAK. 299
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 tliis point.
(Cf Fig. 88, 80 ) In the centre of the primitive streak an
even, dark line, the so-called primitive groove, becomes
defined (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-
t'lva) 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 aptl}'' 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
millimetres.
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 tlie 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
15
300
THE EVOLUTION OF MAN.
studying these sections, one by one, in every stage of the
evohition, 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
germ-shield.
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-
FiG. 88. — Transverse section throngh 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 (wy) ; 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-
INTERCHANGE OF CELLS BETWEEN THE TWO GERM-LAYERS. 3OI
tiulinal axis of the sole, all three soles coalesce, and here form
the thick axial band (Fig. 88, xy). This coalescence is very
siijnificant. It causes an exchano^e 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, 111, 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 gi'oove (Fig. 89, n) becomes visible in
Fig. 89.— Transverse section through the gcrm-shicld of a Chick (in a
stage rather later than in Fig. 88). The letters indicate the same parts as
in Fig. 88. In the middle of the axis-band (y) the chorda dorsalis, or noto-
chord, becomes defined (x). (After Waldeyer.)
L
302 THE EVOLUTION OF MAN.
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 (/) 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 (h).
"■/> ei
Fig. 90. — Transverse section througli 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 (/i), from which the outer skin
with its appendages arises; (2) the thicker, axial spinal plate (tn), which
gives rise to the spinal tube {tiihus medullaris) ; this originates from the
dorsal furrow (Bf), the deepest part of which forms the primitive groove
{Pv). The boundaries between the spinal plate (m) and the horn-plate (It)
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-layei'S (sp). The inner portion of
these side-plates soon becomes defined as the primitive vertebral band
(uwp). The tiny fissure in the side-plates is the first rudiment of the
future body-cavity (uich). The inner germ-layer (the intestinal-glandular
layer) (d d) is not yet modified. (After KoUiker.)
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)
THE SIDE-LAYERS. 303
(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 uivh) is very significant,
for it represents the first rudiment of the future body-
cavity (ccdorna). (Cf Plate IV. Fig. 2, c and 3, c.)
In speaking of these side-layers, which are usually
called " side-])lates," I would say a word or two about
those figurative expressions " layers " and " plates," which
have been universally employed since Baer's time. The
" layers " (lamince), as well as the " plates " (lamellai), 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 tlie 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 (lamince). On the other hand, the
term plates (lamellce) 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
304 THE EVOLUTION OF MAN.
drawn : thus, for instance, the two middle, secondary germ-
layers are usually called the skin-fibre plate and the intes-
tinal-fibre _p/«fe (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 primaiy, and the four secondary germ-layers ; natural!}'-,
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 ofi^ or differentiated from
the four layers, or leaves, as plates; e.g. the muscle-plate,
etc.
After the chorda has entirely separated from the two
side-laj^ers, 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
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 (u'), separate the latter plate from the horn-plate (/i), incline
towards each other. On both sides of the notochord (c/i) the inner portion
of the side-layers (h) has separated itself as a primitive vertebral band
from the outer portion. The intestinal-glandular layer (cl) is not yet
modified. (After Remak.)
THE PRIMITIVE VERTEBRAL CORD. 305
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 maay 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 (laonellce), 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 the 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 gi'owth 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, lu), they finally coalesce. Thus
the open dorsal furrow, the separation at the top of which
jjrows 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 Tnedullaris). Formerly this fact
was regarded with wonder as an inexplicable enigma, but
306 THE EVOLUTION OF MAN.
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 vertebrae, 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
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 {mr), and has pinched itself off from the horn-plate (/i). In the middle
germ-layer, the axial notochord (c/r) is entirely separated from the primitive
vertebral bands (iiio), in the interior of vrhich a transitory cavity (lay/j)
aftervFards forms. The side-layers have split into the outer skin. fibrous-
layer (/ip?) and the inner intestinal -fibrous layer (d/), the two being still
connected by the middle plates (mp). The fissure (sp) between the two is
tlie first rudiment of the body-cavity {cailoma). In the gap between the
primitive vertebral bands and the side-layers, on either side, is the primitive
kidney (xinrj), and on the inside the primitive artery {ao). (After KoUiker.)
outer skin (ejnclerinis), with its horny appendages — nails,
hair, etc. — develops from it. (Cf Plates IV. 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 outer skin ; tliis is the primitive kidney, which
THREEFOLD SEPARATION OF THE MESODERM. 307
accomplishes the excretory functions of the body, and se-
ci'etes 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, ung). 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 thi-ee por-
tions, viz.: (1) the notochord in the central line of the germ-
shield (Fig. 92, c/i) ; (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, h'pl), 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
308 THE EVOLUTION OF MAN.
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, ad). 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 ah'eady 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 ot
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
FORMATION OF THE INTESTINAL TUBE.
309
two processes are in reality quite different. For the spinal
tube closes along throughout its entire length into a cylin-
Fio. 93. — Three diagramTnatic 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) arc 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 thi-ee figures the
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 (cceloma') ; /, intestinal-fibrous
layer; f, 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
right and left edges of the dorsal furrow coalescing. The
310 THE EVOLUTION OF MAN,
closing of the intestinal tube, on the other hand, takes place
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 five stages which are repre-
sented in longitudinal section in Fig. 94. The germ-shield (c),
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, 5 ds). In consequence of the pj'ocesses of growth
I
SEPARATION OF GERM-SHIELD. 3 II
Avhich effect this separation, a furrow-like depression is first
formed round the embryo-body on the upper suiface 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
In order to get a clear and connected view of this
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 originall}'^ consisting only of the two original germ-
layers, the outer and the inner. Thus, over the extent
of the germ-area, the middle gei'm-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 laj'cr 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
now arises. This is the circular wall, which continues to
312
THI EVOLUTION OS MAN.
Fig. 94. — Five diagTammatic 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.
THE AMNION. 313
which separates 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,
ihc wall of which consists of the two primary germ-layers. Between the
outer (a) and inner (i) germ-layers, the middle germ-layer (in) has developed,
co-extensively with the germ-area. In Fig. 2, the embryo (e) is beginning to
separate from the germ-vesicle (ds), while the wall of the amnion-fold Is
developing round it (in front as the head-sheath, is, in rear as tail-sheatb,
ss). In Fig. 3, the edges of the amnion-fold (am) meet above the back of
the embryo and thus form the amnion-cavity (ah) ; while the embryo (e)
separates still more from the germ-vesicle, the intestinal canal (dd) is
developed, and from the posterior end of this, the allantois (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 siguifies embryo; a, outer germ-layer; m, middle germ-layer; i,
inner germ-layer ; am, amnion (ks, hcad-sheath ; ss, tail-sheath) ; ah,
amnion-cavity; as, amnion-sheath of the umbilical cord; Tch=ds, intestinal
gorm-vesicle ; ds, j-elk-sac (navel-vesicle) ; dg, yelk-duct; d,f, 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 membi'ane ; sz, tufts of the foregoing ; ch, tufted membrane or
chorion; r, the space between the amnion and chorion, filled with fluid.
(According 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> <^<^'^)- 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 ; tliey 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. 819, 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,
314 THE EVOLUTION OF MAN.
water-membrane, or amnion (Fig. 94, arn). The embiyo
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 bod}'.
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 sac
(Fig. 94, 3, 4 al), 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, g, 4 els), the remnant of the
original intestinal germ-vesicle (Fig. 94, -^ Jch), 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, g 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
FORMATION OF THE ALLANTOIS AND YELK-SAC. 3 I 5
very wide opening by which the intestinal cavity at first
communicates witli 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. 809).
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 (]i),
from the central part of which it has separated. But, at
a later period, the primitive vertebral plates (two) 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 con-esponding with the central line of
3l6 THE EVOLUTION OF MAN.
the 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 is
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 lamime, by meeting from each
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 tlie formation of
the ventral wall (Fig. 98, hJt). 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 halves,
from the two side-plates, which incline toward each other ;
DEVELOPMENT OF THE DORSAL AND VENTRAL WALLS. 317
these fifrow 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, fi'om the entoderm, and
the ventral wall (and, indeed, the entire body- wall) from
the exoderm.^^
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.
3i8
THE EVOLUTION OF MAN.
d.J/'
Pig. 96.
EXPLANATION OF FIGURES.
319
.<//•
Fi-. 98.
Figs. 95-98. — Transverse sections tbrougli 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 Kollikcr (magnified about 100 times) ;
Fig. 98, after Remak (magnified about 20 times).
h, horn-plate; •>n)*, spinal tube; ^ing, primitive kidney duct; uii, pri-
mitive kidney vesicle; hp, skin-fibrous layer; m = mu = mp, muscle-
plate ; uiv, primitive vertebral plate (rvh, membranous formation of the
vertebral body ; rub, of the vertebral arch ; wq, of the rib, or transverse
apophysis) ; tcwh, 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 coelom ; df, intestinal-fibrous layer ; ao, primitive aortas ; sa,
secondary aorta ; vc, piincipal veins ; d = dd, intestinal-glandular layer ;
dr, intestinal groove. In Fig. 95, the greater pai't of the right half of the
cross-section is omitted, and in Fig. 9G 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 arc not at all familiar with the
320 THE EVOLUTION OF MAN.
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-
senting the complete vertebrate body and its germ, are
carefully compared, the reader will probably obtain a clear
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.^^ (Of 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.
haeckel's evolution of man. plate IV.
TRANSVERSE SECTIONS.
HAECKEL S EVOLUTION OF MAN.
PLATE r
LONGITUDINAL SECTIONS.
1
EXPLANATION OF PLATES IV. AND V.
The two Plates lY. and V. exhibit, partly ontogenetically and partly
phylogenetically, the mode in which the human body arises from the germ-
layers. Plate IV. contains only diagrammatic transverse sections (through
the sagittal and ti'ansverse 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 layei', 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, green. 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.^'
Plate IV. — Diagrammatic Transverse Sections.
Fig. 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 (d) ; the wall of this consists solely of the two primary
geim-layers.
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 rudiments of the earliest organs. (Compare the transverse section
of the embryo Chick at the second day of incubation. Fig. 92.) The spinal
tube (m) and the primitive kidneys (m) are separated from the horn-plate {h).
On both sides of the notochord (ch) the primitive vertebra? («r') and the
side-layers are differentiated. Between the skin-fibrous layer and the intes-
322 THE EVOLUTION OF MAN.
tinal-fibrous layer, the first rudiment of the body-cavity, or the coelom (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 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 (c/i) are already beginning
to be enclosed by the primitive vertebrje (uw), in which the muscle-plates,
bone-plates, and nerve-roots ai-e becoming distinct. The primitive kidneys
(ii) are already completely separated from the horn-plate (It) by the leather-
plate (I) ; c, the coelom ; t, the aortas. The skin-layer, rising around
the embryo, forms the amnion-fuld (am) ; this gives rise to a hollow space
((/) 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
vertebrse (u'Z>), and similarly the notochord and its sheath by the two halves
of the vertebral body (wk). The leather-plate (?) has entirely separated
from the muscle-plate (m^j). The horn-plate (h) has thickened very much
at the head of the posterior limbs (r). The primitive kidneys (u) are pro.
minent in the coelom (c), and lie very near the germ-epithelium, or the
rudimentary sexual glands (A:). The intestinal tube (d) is attached to the
dorsal surface of the body by the mesentery (g), beneath the main artery (f),
and the two principal veins (ii). 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 (/«) have developed into
ovaries, and the primitive kidneys are transferred into oviducts, which open
into the ccelom. The two side protuberances {Ih) 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 (v) ; 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 (?) and the muscle-plate, which lies below
the former, have differentiated from the skin-fibrous layer. The muscle-layer
has separated into an outer circular muscle-stratum and along inner stratum,
and the muscle of the latter has distributed itself into dorsal muscles (») and
ventral muscles (b). The two are separated by the primitive kidneys (w),
EXPLANATION OF PLATE V. 323
which extend from the horn-plato (h) to the coeloin (c). Here the primitive
kidneys have a funnel-shaped opening, through which they carry out the
ovules, which fall from tlie ovaries (k) into the coelom. The intestinal tube
(d) has glands on its surface (liver-vesicles, Ih) . Below it lies the ventral vessel
(the intestinal vein, r), above it the dorsal vessel (the aorta, (). The position
and origin of all these primitive organs is entirely the same in Man and every
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.
Fig. 8. — Transverse section through the human thorax. The spinal tube
(m) is entirely enclosed by the developed circular vertebrae (iv). A curved
rib proceeds right and left from the vertebra, supporting the wall of the
breast (rp). Below, on the ventral surface, between the riglit and left rib,
lies the breast-bone, or sternum (bh). Without, above the ribs, and the
muscles between the ribs, lies the outer skin, formed from the leather-plate
(?) and the horn-plate (h). The greater part of the breast-cavity (or the
anterior part of the coelom, c) is occupied by the two lungs (hi), 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 sternum
lies the heart divided by a partition wall into two halves. The left heart
(hi) contains only arterial, the right (hr) only venous blood. Each half of
the heart is divided by a valved opening into an auricle and a ventricle.
The heart is here represented diagrammatically in its (phylogenetic) original
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. — Dfagrammatic Longitudinal Sections,
Fig. 9. — Longitudinal section through a Gastrula. (Compare Pig. 1,
transverse section.) The intestinal cavity (d) 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
(Prothehni.s), the entire body of which consists of the four secondary germ-
layers. 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
also.
Fig. 11. — Longitudinal section through a low Coelomate Worm. The primi-
tive brain (?u), or the first nerve-centre overlying the throat, has separated
from the liorn-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-
16
324 THE EVOLUTION OF MAN.
cavity (c), wtich has foi'med between the skln-fibrons layer and the intes-
tinal-fibrous layer.
Pig. 12. — Longitudinal section tlwough an hypothetical Worm (Chordo-
nium), which was among the couamon parent-forms of Vertebrates and
Ascidians. The primitive brain ()n) has lengthened into an elongated spinal
tube. Between this spinal tube and the intestinal tube (d), the notochord
(cTi) has developed. The intestinal tube has differentiated into two divisions,
an anterior gill-intestine (with three pairs of gill-openings, ks) which
serves for breathing, and a posterior stomach-intestine (with a liver-
appendage, lb) which serves for digestion. In front, at the head-extremity,
an organ of sense (g) has developed. The primitive kidney (it) opens 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^ — mj) and the spinal marrow (wig). (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 (it-'b) ;
under it the vertebral bodies (wlc) ; under the latter the origin of the ribs is
indicated). In front an organ of sense (g, 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
(Its), the swimming-bladder ( = lungs, lu), the oesophagus (sr), the stomach
{mg), the liver (Ih) with the gall-bladder (i). the small intestine (dd), and
the rectum with the anus (a). Below the throat-cavity lies the heart, with
the aiu'icle (/lu) and the ventricle (hlc).
Fig. 14, — Longitudinal section through a human embryo of three weeks,
showing tne relation of the intestinal tube to its appendages. In the centre
the long-stalked yelk-sac (or the navel-vesicle, ds) projects from the intes-
tinal tube (_ds) ; similarly the long-stalked allautois (ai) projects from the
intestine at the back. The heart (hz) is visible beneath the anterior intes-
tine. Amnion-cavity (aJi).
Fig. 15. — 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 (TOi-^^s), and the spinal marrow (m^). (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 (^fs), the lung (lu), the oesophagus (sr), the stomach
(mgr), the liver (lb), the coil of the small intestine (dd), into which the
yelk-sac (ds) opens, the urinary bladder (hh), and the rectum. Heart (hz).
Fig. 16. — Longitudinal section through developed human female. All
EXPLANATION OF PLATE V. 325
the parts are perfectly developed, but diagrammatically rednced 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^-m^) have been differentiated and transformed in the
manner peculiar to the higher mammals : m-^, fore brain {cerebrum), out-
weighing and covering all the other four brain bladders; wi„, twixt brain
(•• the centre of sight '') ; vi^, mid brain (" the four bulbs ") ; m^, hind
brain (cerehellum) ; m., after brain, or 'prolonged marrow (medulla
oblongata) , passing into the spinal marrow {m^). The brain is enclosed in
the skull (s), the spinal marrow by the vertebral canal: above the spinal
marrow the vertebi'al arches and spinal processes, under it the vertebral
bodies {wk). The intestinal tube has differentiated into the following parts
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 (Zw), the oesophagus (.9?-), the stomach (mg), the liver (lb), with
the gall-bladder (i), the ventral salivary gland, or pancreas (p), the small
intestine (dd), the large intestine (dc), and the rectum with the anus (o).
The body-cavity, or ccelom (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 embryo, indicated here by a small germ-membrane vesicle (e), is
developed. Between the uterus and the os %iubis 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 mammcE
(md), are also originally formed from the horn-plate.
^
326
THE EVOLUTION OF MAN.
ALPHABETICAL LIST
Of the Meaning of the Letters in Plates IV. and V.
N.B. — The skin-sensory layer is indicated by orange, the skin-fibrons
layer by blue, the intestinal-fibrous layer by red, and the intestinal-glandular
layer by green.
a,
anal opening
mj-
— ms, the five brain. bladders
ah,
amnion-cavity
«*i,
fore-brain
al,
allantois (urine sac)
mj.
twixt -brain
am,
amnion
wis,
mid-brain
i,
ventral muscles
m„
hind-brain
U,
breast-boue (sternum)
m^.
after-brain
c,
body. cavity (ccslonia)
mg.
spinal cord (medulla spinalis)
c„
breast-cavity (cavitas pleura!)
md.
milk-glands (mammce)
c„.
venti-al cavity (cavitas pe^-itonei)
mg.
stomach
ch,
notochord (chorda)
mh,
mouth-cavity
d.
intestinal tube (tractus)
mp,
muscle-plate (muscularis)
dc,
large intestine (colon^
n.
principal veins
dd.
small intestine (ileum)
0)
mouth-opening (osculum)
ds.
yelk-sac (uavel-vesicle)
T,
ventral salivary gland (pancreas)
e,
embryo or germ
q,
organ of sense
f,
matrix (uterus)
r,
muscles of the back
9>
mesentery (mesente^-ium)
rp,
ribs (costce)
h,
horn-plate (ceratina)
s.
skull (cranium)
hb.
urinary vesicle (vesica urincB)
sb.
OS pubis
hTc,
ventricle of heart
sh,
throat-cavity (pharynx)
hi.
left (arterial) heart
sr,
gullet (cesophagus)
hr,
right (venous) heart
t,
aorta (main artei'y)
hv,
auricle (atrium)
u,.
primitive kidney (proton ephron)
hz,
heart (cor)
uw,
embryonic vertebra (metameron)
h
gall-bladder (vesica fellea)
*'>
intestinal vein (primitive vein)
h
germ-glands (sexual glands)
vg,
vagina
1(3,
gill-openings (throat-openings)
w.
vertebra
I,
leather-plate (coriuvi)
wh,
vertebral archea
lb,
liver (hepar)
wk,
vertebral bodies
Ir,
windpipe (trachea)
X,
legs, or limbs
hi,
lung (pulmo)
V,
space between the amnion and
',)!,
medullary tube (tubus medul-
the yelk-sac
laris)
z,
midriff (diwphragma)
( 327 )
TABLE VII.
Systematic Survey of the Development of the Oi'ganic Systems of Man from
the Germ-layers. (Cf. Plates IV. and V.)
Outer Primary
Germ-layer.
Skin-layer.
(Animal
Germ-layer,
Baer.)
Exoderma.
Lamina
dermalis, II.
I
a.
First
secondary
germ-layer.
Skin-sensory
layer.
(Skin-stratum,
Baer.)
Lamina
neurodermalis.
H.
Horn-plate.
lAxmella ceratina.
ir.
Marrow -plate.
Lamella medullaris.
III.
Primitive kidney
plate.
Lamella renalis.
1. Outer-skin (epidermis).
2. Appendages of the epi-
dermis (hair, nails, etc.).
3. Glands of the epidermis
(perspiratory, sebaceous,
lacteal glands).
4. Spinal marrow i medullary
5. Brain J tube.
6. Organs of the senses (es-
sential part).
1. Primitive kidneys (?) and
the outlets, which arise
from them for the sexual
products (perhaps from
the skin-fibrous layer ? i)
b. I
Second
secondary
germ-layer.
Skin-fibrous
layer *
(Flesh-stratum,
Baer.)
Lamina
inodtrmalis, U.
IV.
Leather-plate.
Lamella coriaria.
Flesh -plate.
iMmella 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. Exocoelar? (parietal coelom-
epithelium).
12. Male germ-epithelium (ru-
dimentary testes) .' .•■
Body-Cavity (Cceloma): A space between the skin-layer and the intestinal layer, between
the body-wall and the intestinal wall, filled with lymph (colourless blood).
B.
Inner
Primary
Germ-layer.
Intestinal
layer.
(Vegetative
Germ-layer,
Baer.)
Entoderma
Lamina
gattralii, II.
Third
secondary
germ-layer.
Intestinal-
fibrous layer.
(Vascular-stra-
tum, Baer.)
Lamina ino-
gastralis, II.
VI.
Vascular plate.
lAimella vasculosa.
VII.
Mesentery-plate .
Lamella mesenterica.
13. Female germ-epithelium
(rudimentary ovary) ? .'
14. Endocoelar ? (visceral coe-
lum-epithelium).
15. Main blood-vessels (heart,
primitive arteries, prim-
itive veins).
16. Blood-vessel glands (lym-
phatic glands and
spleen).
'17. Mesentery (mesenterium).
18. Intestinal - muscle wall
(and fibrous intestinal
membranes).
Fourth
secondary
germ-layer.
Intestinal-
glandular
layer.
(Mucous-stra-
tum, Baer.)
Lamina myco-
gastralis, II.
VIII.
Mucous plate.
Lamella mucosa.
,19. Intestinal epithelium. (In-
ner cell-coating of the
intestinal tube.)
' 20. Intestinal gland epithe-
lium. (Inner ccU-coat-
ing of the intestinal
glands.)
CHAPTER XL
GENERAL STRUCTURE AND ARTICULATION OF THE
INDIVIDUAL.
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 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 Metameric 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 Skull
from the Head-plates. — Gill-openings and Gill-arclies. — Sense-organs.
— Limbs. — The Two Front Limbs and the Two Hind Limbs.
" The occurrence of an internal skeleton in deiinite 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 metameric 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
vertebrae. The latter, however, must be regarded only as the partial ex-
MAIN FEATURES OF MAMMALIAN GERM-HISTORY. 329
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 Gegenbaur (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 gei*m-history brings various differences
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
higher vertebrate classes. But all these remarkable struc-
tural conditions, which react on the diversified develoj)ment
of other parts, were only kenogenetically acquired at a later
stage, 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
palingenetic, 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
330
THE EVOLUTION OF MAN.
and the four-layered stages). 3. The axial soldering, or
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 kidney
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 coeloma).
8. The rudimentary primitive vessels, or aorti3e (from the
intestinal-fibrous layer). These important germ-processes
result in the formation of ten different parts of the body,
which Ave 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.)
Phylogenetic fission of the germ-layers.
Primitive Organs
(Fig. 99).
Outer primary germ-
layer :
Skin-layer
(Dermal layer, or
Exoderma).
Inner primary germ-
layer :
Intestinal layer
(Gastral- layer, or
Enloderma).
I. Secondary germ-
layer :
Skin-sensory
layer.
n. Secondary germ-
layer :
Skin-fibrous
layer.
'III. Secondary germ-
layer :
Intestinal-
fibrous layer.
IV. Secondary germ-
layer :
Intestinal-
glandular layer.
1. Horn-plate (Ji).
2. Medullary plate
{mr).
3. rrimitive kidney
(wjigr).
4. Chorda {cK).
5. Primitive verte-
bral plate {uw).
6. Sliin-muscle plaie
{hpl).
1. Body-cavity (sp).
8. Intestinal muscle
plate (df).
9. Primitive aorta
{ao).
10. Intestinal gland-
epithelium (dci).
Ontogenetic fission
of the
germ-layers.
A. Upper or
Sensory layer,
Pvemak.
B. Middle or
Motor-germina-
tive layer,
Remak.
C. Lower or
Trophic layer,
Remak.
PRIMITIVE ORGANS OF THE VERTEBRATE. 33 1
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 IV. 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 (mr), which has separated itself from the horn-plate Qi). In the
middle germ-layer the axial notochord (ch) has entirely separated itself
from the two primitive vertebral cords {nw), in the interior of which a
transitory cavity {uwli) afterwards forms. The side-layers have split into
the outer skin-fibrous layer {hpl) and the inner intestinal-fibrous layer
(<Z/), 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 (wfti/), on the inside the
primitive artery {ao). (After Kolliker.)
332 THE EVOLUTION OF MAN.
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. (C£ 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.^^ 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 Gastrsea
Theorj^, 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-vesiele. It is a
MODIFICATION OF INTESTINAL-GLANDULAR LAYER. 333
simple globule, the wall of which consists of a simple layer
of homogeneous cells (Fig. 100, A eld). 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 (kh). B. In the centre of the
germ-disc the medullary fm-row (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 cii'cumference of the genn-disc ; the intestine (d) begins
to separate itself from the yelk-sac or navel. vesicle {nb). E. The intestinal
tube ("!/•) is closed ; the body-cavity (c) begins to form. F. The primitive
vertebrso («•) appear ; the intestine (d) is almost completely closed. G.
The primitive vertebra) (ic) begin to grow round the medullary tube (mr)
and the notochord (ch) ; the intestine (d) is separated from the navel-
vesicle (nh). H. The vertebrae (w) 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 ; tf, primitive TertebraB ; ch, notochord ; c, body-
cavity ; df, intestinal-fibrous layer ; dd, intestinal-glandular layer ; d, in-
testinal cavity ; Hb, navel-vesicle.
334 THE EVOLUTION OF MAN.
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, 5). 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 {nh).
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 meduUary
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
THE TWO INTESTINAL CAVITIES. 335
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 svuface 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 w^ay, 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 sui-face 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
?>2>^
THE EVOLUTION OF MAN.
anterior "intestinal gate" (Fig. 102, at d). Just in the
same way t^e tail curves back against the ventral surface ;
the intestinal wall then encloses posteriorly a similar small
^ y m t i,Tp 7l 1)0 1 3
Fig. 101. — Longitudinal section through the embryo of a Chick (fifteenth
day of incubation). Embryo with arched dorsal surface (black) : d, intes-
tine ; 0, mouth ; a, anus ; I, lungs ; h, liver ; g, mesentery ; v, auricle of
heart; 1<, ventricle of heart ; b, arterial arches; i, aorta; c, yelk-sac; in,
yelk-duct ; u, allantois ; r, stalk of allantois ; n, amnion ; w, 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).
I
THE GEKM- VESICLE BECOMES THE YELK-SAC.
3^7
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. 812.)
Fig. 102. — Longitudinal section through the front half of a chick
(at the end of tho 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 ; hh, heart-cavity ; lik, heart-cap ; Tis, head-
sheath ; kk, head-cap. (After Remak.)
Witli 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 gi-owing intestinal
cavity through a wide connecting aperture, which after-
338 THE EVOLUTION OF MAN.
wards extends into a long narrow canal, the yelk-duct.
Let us suppose we are "svithin 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 sj^eaking, 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
ORIGIN OF THE MOUTH AND ANUS. 339
is afterwards found, and this also grows continually deeper
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 touch the two blind ends
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.
9-i, 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
oi)enings 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 hang^ 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, (Cf 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 sun-ounds the entire body-cavity
(coeloma), and includes the intestine. It develops from the
outer portions of the side-layers. As has been already
340 THE EVOLUTION OF MAN.
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-
plete.
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, un, p. 318).
MODIFICATIONS OF THE VASCULAR SYSTEM. 34I
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 the sole-
k
342
THE EVOLUTION OF MAN.
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 (hh). The spinal furrow is open behind x ; at the posterior
end it is very wide open at z ; mp, medullary plates ; sp, side-plates ; y,
limit between throat-cavity (sh) and head-intestine (vd). — Fig. 104, with
10 pairs of primitive vertebrae. 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 : r, 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 ; ictc, primitive vertebrae.
RUDIMENT OF THE BRAIN. 343
shaped or lyre-shaped germ-shield (Figs. 86, 87, p. 298). In
tlic 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 medidlary 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, lib). 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 nv 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
344
THE EVOLUTION OF MAN.
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
fi'om the dorsal surface, in
four consecutive stages of
evolution (about ten times
the natural size). (After
Bischoff.)
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. peZZitcicZa, 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
ground.
ORIGIN OF THE PRIMITIVE VERTEBRAE.
345
ap])ear 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 iDviini-
tive vertebi'as aud
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-
bra}; in the germ -
area the first
traces of the net
of blood-vessels
appear, bounded
by therena termi.
nalis (a) : b, tail-
sheath, hb, head-
sheath of the
amnion; the folds
in the latter indi-
cate the serous
membrane.
346 THE EVOLUTION OF MAN.
In Fig. 107 there are seven, in Fig. 108 there are
eight, and in Fig. 109 ten pairs of primitive vertebrae. 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 vei'tebra, 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.^^
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 {Arihropoda).
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 j
AETICULATION. 347
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
organi.sm. 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
348 THE EVOLUTION OF MAN.
various functions are distributed among the various kinds of
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 Oi
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 oi
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
(ArthrojJoda), 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.
When we investigate the origin of the metameric chain
GROWTH OF THE METAMERA. 349
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 all 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-
articulatod germ.^^
As the metamera arise in this way, it will readily be
understood that the anterior primitive vertebrae are earliest
found. Such is indeed the case. The earliest primitive
vertebra?, Avhich are situated about the centre of the germ,
are the first and second neck-vertebrse. Then come the
third and fourth ncck-vertebrse, 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-
350 THE EVOLUTION OF MAN.
vertebrae, to which the ]ibs are attached, five lumbar-
vertebrse, five cross-vertebr?e (inserted into the pelvis), and
four or five tail-vertebrjB. 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
the lyre-shaped germ-shield (Figs. 103, 107) mustre present
the future head. The seven primitive vertebras 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 afiects
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
vertebroe ; 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 have been
removed. (Both figures after H. Meyer.)
i I
352
THE EVOLUTION OF MAN.
These cells are all the products of the skin-fibrous layer.
At a very early period, a small cavity appears in each of
these solid primitive vertebr;©, which cavity, however, soon
again disappears. This " primitive vertebral cavity " (Figs.
95, 96, uwh, i3p. 817, 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
vertebra have separated into the outer muscle-plate (dijj) and the inner
skeleton -plate. The latter below, as the vertebral bodies (w;/i), begins to
surround the uotochord {cli) ■ above, as the vertebi'al arches (trb), 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 (Zip) ; hpr, leather-plate of the dorsal wall ; h,
horu-plate ; a, amnion ; U7ig, primitive kidney duct ; nn, primitive urinary
canal; ao, primitive artery; vc, cardinal vein ; (?/, intestinal-fibrous layer;
dd, intestinal -glandular layer; dr, intestinal groove.
THE SKELETON AND MUSCLE PLATES. 353
indicates an internal separation of the primitive vertebra
into two entirely distinct parts : an inner part, which forms
the skeleton — the skeleton-plate (Fig. 95, uw, Fig. 112, vjh),
and an outer part, which forms the muscle — t^e 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, vjb). 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 (ivh). 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
lamellas 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, ivb) 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 opposite
primitive vertebrae coalesce, from right to left, over the
medullary tube, resulting in the closing of the vertebral
arch. Between each paii' 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
354 THE EVOLUTION OF MAN.
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
Fig. 113 — Third human neck-vertebra.
Fig. 114. — Sixth hnman 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, mp), 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 dii-ect communication with that portion
of the side-plates which develops into the ventral skin and
the ventral muscles.
DEVELOPMENT OF THE SKULL. 355
In front, at the head end of the embryo, the middle
layer (mesoderma) does not split into primitive vertebras and
side-plates, and the original fibrous layer here remains un-
divided, forming the so-called " head-plates " (Fig. 102, k, p.
887). 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
lishes. 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 arc
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 throal-
cavity : these are the gill-openings, or throat-openings.
Between each pair of gill-openings the wall of the throat
356
THE EVOLUTION OF MAN.
cavity grows thicker, and is changed into a bow-shai)ed or
sickle-shaped ridge : these are the gill-arches ; on their
inner side a vascular arch afterwards arises (Fig. 101, p. 336).
1.
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 ; q\, eye-fissures. The first of the three
pairs of gill-openings is separated into an upper jaw process (c) and a lower
jaw process {xi). (After KoUiker.)
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, k, 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
GILL-ARCHES AND GILL-OPENINGS.
357
the side of the gullet. In the higher Vertebrates they after-
wards close. The gill-arclies are transformed partly into the
jaws, partly into the tongue-bone and the bonelets of the
ear (pssicula 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, ^ ^ i k), 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 I'egion and the front
limbs (wing-rudiments) of a Chick, on the fourth day of incubation (about
20 times tho 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 wings (e). The upper of these is the muscle-plate, the
middle is the hind, and the lower is tho 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, Avhich forms a closed
covering round the embryo. (After Remak.)
358 THE EVOLUTION OF MAN.
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, n). 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 aj)pears 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 this stage of develop-
ment, is 3^et 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 difi'erence between
the human embryo and that of Mammals, Bii-ds, Reptiles,
etc. (Plates VI. and VII., upper line ot 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
ORIGIN OF THE LIMBS.
359
not appear till later. This also is a fact of the profoimdest
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 foui'th day of incubation (about 40 times the natura
size) : h, horn-plate ; if, medullary tube ; n, spinal canal ; u, primitive kid-
neys ; X, chorda ; e, hind limbs ; h, allantois canal in the ventral wall ; t,
aorta; r, 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 prima) val, 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
360 THE EVOLUTION OF MAN.
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
Vertebrates.
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. Comj)are, for instance,
carefully and attentively the three consecutive stages of
development, represented in Plate VI. of the Fish {F), Sala-
mander {8), Tortoise {T), and Chick (C), and in Plate VII. the
corresponding embryos of the Hog {H), Calf {C), Rabbit {R),
and of Man (ilf). 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
THREE STAGES IN DEVELOPMENT. 36 1
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
strikinir. The significance of such facts as these cannot be
over-estimated.^°°
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 Mau
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 ci'eation assume a non-
miraculous evolution of organisms.
362 THE EVOLUTION OF MAN.
EXPLANATION OF PLATES VL AND VII.
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 Vertebi'ates 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 — corresiDonding 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, S) ; of a Eeptile (Tortoise, 2') ; and of a
Bird (Chick, C).
Plate VIII. shows the embryos of four Mammals in the three correspond-
ing stages : of a Hog (E), Calf (C), Rabbit (E), and a Man (M). The con-
ditions of the three different stages of development, which the three cross-
rows (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, IL, 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 embr'yonic 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 tm-ned to the right. The letters indicate the same parts, in
all the twenty-four figures, namely : v, fore-brain; 2, twixt-brain ; m, mid-
brain ; h, hind-brain ; n, after-brain ; r, spinal marrow ; e, nose ; a, eye ;
0, ear ; fe, gill-arches ; g, heart ; lu, vertebral column j /, fore-limbs j b, hind-
limbs J s, tail-i""
KABCKKL S EVOMJTION OF MAN.
PLATE VI.
HAKriCBI.'s EV-flLUTION Olf MA>T.
PLATE I'll.
CHAPTER XII.
THE GERM-MEMBKANES AND THE FIRST CIRCULATION
OF THE BLOOD.
The 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 Higher 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 AUantois 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 AUantois : Navel-arteries and Navel-veins. — Divisions of
Human Germ-history.
" Is man a peculiar organism? Docs he originate in a wholly different
way from a dog, bird, frog, or fwh ? 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 ? Or does he develop from a similar embryo,
and undergo tlie same slow and gradual progressive modifications ? The
answer is not for an iustant doubtful, and has not been doubtful for the last
thirty years. The mode of man's origin and the earlier stages of his
364 THE EVOLUTION OF MAN.
development are undoubtedly identical with those of the animals standing
directly below him in the scale ; without the slightest doubt, he stands in
this respect nearer the ape than the ape does to the dog." — Thomas Huxley
(1863).
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 Linnseus, in his Systema NaturcB, 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 essentia] 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
HOMOLOGY BETWEEN HUMAN AND MAMMALIAN GERMS. 365
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 an}^
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 stiU 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. (Cf. 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 impossil)le to lay too 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 higlier 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
366 THE EVOLUTION OF MAN.
germ-layers, in the fission of these, in the construction of the
most important fundamental organs from these germ-layers,
e.tc. 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 diflTerentiated 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
number of new evolutionary conditions have been intro-
duced by adaptation (cf pp. 10-14).^°^
If we apply this law of the ontogenetic connection
ONTOGENETIC RELATION OF ALLIED FORMS. 367
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 pelhuyida) which clearly distinguishes
the mammalian egg from that of all other animals. When
Fig. 121. — Lyre-shaped germ-shield of a dog. HHHHHIi^B
" Double shield " of Remak, " embryonic rudiment ' ^^HBIiliHlll^^l
of other authors.) The dorsal furrow is visible in the ^B' T^B
centre ; on either side are the medullary swellings. ^B "B
the human embryo is fourteen days old, H, f : fl
it, like all other mammalian embryos, is wtk. I|f ^H
in the form of an entirely simple, lyre- ^B> i '^M
shaped germ-shield. Along the middle ^B slj ^M
line of the dor.sal side of this, there ap- ^Bji, i j a^|
pears the rectilineal, groove-shaped medul- |||^||||^^
larj^ 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).io2
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, wlien seen from the .side, the characteristic curvature
of the back, the swelling of the head end, the earliest rndi-
368
THE EVOLUTION OF MAN.
ments of the higher sense-organs, and the rudiments of tho
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, tlie 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 G weeks; VII., of 7 weeks;
VIII., 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-sac, by
means of the yelk-duct, in the centre of the abdomen.
DEVELOPMENT OF THE HEAD, 369
In this stage of development, the extremities, or limbs,
are still entirely wanting ; there is as yet no trace either of
arras 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. Ml., BL, CI., and HI. on Plate VIL)io3
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 tlie 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 abeady 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-
370
THE EVOLUTION OF MAN.
Fig. 123. — Human embiyo of four weeks old, opened on the ventral side.
The walls of the chest and abdomen have been cnt 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 : «•, eye ; 3, nose ; 4, upper jaw ; 5, lower jaw ; 6, the second
gill-arch, and 6" the third; ov, heart (o, right, o', left auricle; v, right, v',
left ventricle) ; b, origin of the aorta ; /, liver (it, 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, z, cut
away) ; n, navel-artery; ti, navel-vein; 7, anus; 8, tail; 9, front limb; 9',
hind limb. (After Coste.)
RUDIMENTS OF THE LIMBS. 37 1
Fig. 124'. — Human onibrj'o of five weeks old, opened on the ventral side
(as in Fig. 123). The chest and ventral walls, with the liver, have been
removed ; 3, outer nasal process ; 4, upper jaw ; 5, lower jaw ; «, tongue ; v,
right, v', left ventricle of heart ; 0', left auricle ; b, origin of the aorta ;
h' h" h"', first, second, third arterial arches ; c, c', c", hollow veins [vencB
ravof); ae, lungs (;/, arteries of lungs); e, stomach; m, primitive kidneys;
(J, left yelk-veins; s, vena X)orkB ; a, right yelk-artery; n, navel-artery;
u, 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). Thi'ee 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
altocjether absent. In this stao'e, 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.^04
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
18
1^2 THE EVOLUTION OF MAN.
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,
VI., 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
DEVELOPMENT OF THE DISTINCTIVE HUMAN CHARACTEIIS. 373
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 naslcus) of Borneo (Fig. 125),
the well-shaped nose of which might well be coveted by men
374 THE EVOLUTION OF MAN,
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 {Semnoipithecus 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.^*^^
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
THE HUMAN EMBRYO. 375
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 aflfords
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 peUucida (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 (cJwrion). 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
formed 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 snnple, 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,
134).
In the human egg, even between the thirteenth and
fourteenth day, this outer egg-membrane, which we shall
Z7^ THE EVOLUTION OF MAN.
2,
Fig. 127.
Fig. 128.
Fig. 130.
Fig. 127. — Human egg between tlie twelfth and thirteenth day. After
Allen Thomson. 1. Not opened ; natural size. 2. Opened, and enlarged.
Within the outer tufted membrane (c/toj-ion) 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 on the fifteenth day, taken from the egg;
enlarged : a, yelk-sac ; b, region of 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. Natm'al size ; opened. The outer tufted membrane
{dxorion') 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; b, yelk-sac ; c, lower jaw process
of the first gill-arch ; A, upper jaw process of the same ; e, second gill-arch
(behind it are two other small arches). Three gill-openings are very plainly
seen; /, rudiments of the fore-limbs; g, ear-vesicle ; /;, eye; i, heart.
THE CHORION.
377
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 suiTOunded by
the tufted membrane (chorion).
Fig. 133. — Human embryo, with amnion and allantois, in the fourth
week. (After Krause.) The amnion (v:) lies pretty close to the body. The
greater part of the yelk-sac (d) has been torn away. Behind this the
allantois (0 is visible, as a pear-shaped vesicle of considerable size. Arms
(/) and legs (b) are just beginning; v, fore-brain; z, twi.\t-brain ; in,
mid-brain ; /(, hind-brain ; n, after-brain ; a, eye ; A:, three gill-arches ; c
heart ; s, tail.
378 THE EVOLUTION OF MAN.
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-
dact, is enclosed in the navel-cord. In this cord, behind the yelk-dact, 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-membi-ane, and at this point forms the placenta.
THE YELK-SAC AND THE PERMANENT INTESTINE. 379
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. 182, 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 plaj's 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 ma}^ 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 occurs
38o
THE EVOLUTION OF MAN.
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, al). Its
Pig. 135. — Longitudinal section throngh the embryo of a Chick (in the
fifth day of incubation). The embryo with curved dorsal surface (black) :
d, intestine ; o, mouth ; a, anus ; I, lungs ; h, liver ; g, mesentery ; v, auricle ;
k, Yentricle ; b, arterial arclies ; f, aorta ; c, yelk-sac ; m, yelk-duct ; iv,
allantois ; r stalk of allantois ; n, amnion ; ic, 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-
THK ALLANTOIS.
381
cavity, and spreads itself out on the inner surface of the
chorion. 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-fivo days old, opened on the ventral side
(as in Figs. 1.34 and 135). Chest and ventral walls have been removed : a,
nose-pits; 6, eyes ; c, under-jaw (first gill-arch) ; d, second gill-arch ; efgh,
heart (e, right,/, left auricle ; g, right, Ii, left ventricle) ; i, aorta (origin of);
kk, liver (in the middle between the two lobes is the cut yelk.vein); /,
stomach ; m, intestine ; n, j'elk-sac ; 0, primitive kidneys : p, allantois ; q,
fore-limbs ; /i, hind-limbs. The crooked embryo has been stretched straight.
(After Bischoff.)
382
THE EVOLUTION OF MAN.
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 first brain-
bladder ; h, second ; c, third ; d, fourth ; e, the eye ; /, the ear-vesicle ; gh,
first gill-arch {g, lowei- jaw, h, upper jaw) ; i, second gill-arch ; W.m, heart
(A-, right auricle ; I, right ventricle ; m, 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, fore-limb ; x, hind-limb.
(After Bischoff.)
Yet even in Man the first rudiment of the allantois is a
stalked pear-shaped bladder (Fig. 133, 0' j^^t as in other
Mammals. ] stated this in 1874!, in the first and second
THE PLACENTA. 383
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. ^'^^
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 anmion-sheath, forms the so-called
navel-cord (Fig. 188, 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 ami those of the child grows very much thimier,
thus giving rise to the remarkable apparatus for nourishing
the embryonic body 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
584
THE EVOLUTION OF MAN.
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 grou23s, the Beaked Animals
Fig. 138. — Egg-membranes
of the human embryo (diagram-
matic) : «!, 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) ; chf,
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, 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 (f). (After
Kolliker.)
(Ornithostoma) and Pouched Animals (Marsiqoialia), 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
SIGNIFICANCE OF THE PLACENTA
/
385
Fro. 139. — Fivo 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
386 THE EVOLUTION OF MAN.
separates the right and left halves ; iu Fig. 5 the germ is seeu fi'om the left
side. In Fig. 1, the prochorion (d), studded with tufts (cJ'j, 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 (mesotleoiia,
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) fi'om 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 ; 7n, 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; dd, in-
testinal-glandular layer ; af, allantois ; rZ=7i7!, region of the heart ; d, yelk-
membrane or prochoi'ion ; d', tufts of the latter ; sh, serous covering ; sz,
tufts of the latter ; ch, tufted membrane or chorion ; chz, tufts of tlie
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 " (JPlacentalia).
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 tlie form of the so-called head-cap, or head-
i
THE AMNION. 387
sheath (Fig. 139, 2 ^^^) ; at the back, it also arches upward
and forms the tail-cap, or tail-sheath (Fig. 139, 2 ss) ; on the
ri<^ht 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 thx*ough 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 furrow below
are still M-ide 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 higlier 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-fiuid (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
tlie 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 sh). The thin wall of the
amnion-.sac, on the other hand, consists of tAvo strata : of an
inner stratum, the horn-plate, and of an outer stratum, the
388
THE EVOLUTION OF MAN.
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 ; sli, notochord ; sa,
aorta ; vc, principal veins ; hh, ventral cavity, not yet closed ; v, anterior,
f/, posterior nerve-roots of the spinal marrow ; mu, muscle-plate ; lip,
leather-plate; /?, horn-plate. (After Remak.)
is, therefore, the outermost layer arising from the fission of
the middle germ-layer {mesodernia). Thus the outer
peripheric portion of the skin-fibrous layer clothes onl}^ 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
SIGNIFICANCE OF THE AMNION.
389
it produces the tufted chorion, the hollow, branched tufts of
which groAY into the depressions in the mucous membrane
of the maternal uterus.
Fig. 142. — Transverse
section through an em-
bryonic Chick iu the
shoulder region (at the
fifth day of incubation).
The sect ion 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.)
In liuinan Phjlogeny 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
390 THE EVOLUTION OF MAN.
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 all 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 aortye," which lie
in the narrow longitudinal cleft between the primitive
THE VASCULAR SYSTEM. 39 1
spinal cords, the side-plates, and the intestinal -glandular
layer (Figs. 92, ao, Fig. 95, 96, ao) ; 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
l)lood-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,^/).
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 fomied in its centre, in consequence of the accumulation
of a small quantity of fluid between the central cells.
Some single colls of the wall separate from the rest and
392
THE EVOLUTION OF MAN.
float about in this fluid. These cells are the first blood-
cells, and the fluid is the first blood. In the same way
Tig. 143. — Longitudinal section through the head of an embryonic Chick
(at the end of the first day of incubation) : u), medullary tube; ch, note-
chord ; d, intestinal tube (with a blind anterior end) ; h, 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 ; hi-, membrane cf the
heart; A;Zr, head-cap of the amnion ; fcs, head-sheath ; /(, horn-plate. (After
Eemak.)
Fig. 144. — Human embryo, of 14 to 18 days, opened at the ventral side.
Under the forehead-process of the head (f) the heart (c) appears in the
heart-cavity (j^) 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 aortge (lying under the primitive vertebras) ; i, terminal
intestine, or large intestine ; o, allantois (h, its stalk) ; v, amnion. (After
Coste.)
THE BLOOD
393
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 honi's. Below the medullary tube, the two primitive aortne (jDa) 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) ; lili, heart-cavity ;
^A', heart membrane; lis, head-sheath, amniou-fold ; /i, horn-plate. (After
Remak.)
Fro. IIG. — Transverse section through the heart-region of the same
Chick (further back than the former). In the heart-cavity (7Ji), the heart
(Ji) is still connected by a heart-mesentery (l\g) with the intestinal-fibrons-
layer {<lf) of the anterior intestine: d, intestinal-glandular layer; up,
primitive vertebral plates; gh, rudiment of the ear- vesicle in the horn-
plate ; hp, first rising of the amnion-fold. (After Remak.)
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
394 THE EVOLUTION OF MAN.
heart, and of the veins, which carry the blood back to the
heart.
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 ccelom (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 (hj)). 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 pair
of aortfe-arches passes up on the inner wall of the first pair
DEVELOPMENT OF THE HEART.
395
of gill-arches, and lies, therefore, between the first gill-arch
(Jc) on the outside, and the anterior intestine ((?) on the
Fig. 147. — Diagramuiatic
transverse section through
the head of an embryonic
Mammal : h, horn-plate ; m,
medullary tube (brain-blad-
der) ; mr, wall of the latter ;
/, leather-plate; s, rudiment-
ary skull ; ch, uotochord ;
k, gill-arch; mp, muscle-
plate ; c, heart-cavity, an-
terior part of the body-
cavity (coeloma) ; d, in-
testinal tube ; dd, intes-
tinal-glandular layer; df,
intestinal-muscle plate; hg,
heart-mesentery ; hw, heart-
wall ; hie, 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 aorttr, which have been already spoken of; they
are also called posterior vertebral arteries (arterice verte-
hrales posteriores). 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-
Tnesentericce), or the yelk-arteries (arterice 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
19
39^ THE EVOLUTION OF MAN.
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 of a Dog, from the
ventral side ; enlarged
about 10 times. In front,
below the forehead, the
first pair of gill-ai'ches 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 aortas
lie between the primitive
vertebrae, and from which
five pairs of yelk-arteries
proceed. (After Bischoff.)
area " {area opaca, or area vascidosa) ; 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 (vence vitellinai) ; they are also often called
omphalic-mesenteric veins (venw ompJialo-mesentericw).
PRIMITIVE CIRCULATORY SYSTEM, 397
Thus the first circulatory system of the blood in the
embryo (Figs. 148-150) occurs in all the higher classes (jf
Fig. 149. — Embryo and germ-area of a Rabbit, in whicb 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 lai'ge 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
Bischoff.)
"VCertebrates 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
398
THE EVOLUTION OF MAN.
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 iwincipalis). The two
branches, which result from the division of this main artery,
Fig. 150. — Embryo and germ-area of a Rabbit, iu which the first system
of blood-vessels is complete, — seen from the ventral side (magnified about
five times). The posterior end of the heart ((?), which is curved iu 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 ^n
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 netwoi'k (lying nearer the surface). The yelk-arteries (/) open
into the two primitive aortas (e). The dark area which surrounds the head
like a halo, rejiresents the recess withiu the head-cap or membrane.
(After BischoS.)
SECONDARY CIRCULATORY SYSTEM. 399
the primitive aortiie, 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 nayel-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 progi'esses 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
life.
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
grow 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 circulation.
When the allantois has grown to the inner w^all of the
chorion, and has changed itself into the placenta, its blood-
vessels alone accomplish the nourishment of the embiyo.
They are called navel-vessels (vasa timbilicalia), and are
originally in pairs : one pair of navel arteries, and one pair
of navel veins. The two navel-veins {vencn umhilicales,
400 THE EVOLUTION OF MAN.
Figs. 123, u, 124, u), wliich cany 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 umhilicales, Figs. 123, n, 124, n), 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 entn*ely new circulation of the blood, limited to the
body of the child, comes into operation simultaneously with
pulmonary respiration.^°^
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
1
TERTIARY CIRCULATORY SYSTEM. 4OI
show that the germ-history of Man (according to the law
of abbreviated heredity) is very rapid and compressed in
the iirst stages of its coufse, 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.^"^
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 wiU become quite
clear when w^e 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.
TABLE VIII.
Systematic Suuvey of the Periods in Human GEUM-niSTouY.
(Cf. Table XXII.)
FIRST MAIN DIVISION OF GEEM-HISTORT.
Kan as a simple Flastid.
The human embryo possesses the form-value of a simple individual of the
first order of a single plastid.
First Stage : Monerula Stage (Fig. 36, p. 210).
The human germ is a simijle 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-cell with the
re-formed kernelj or the parent-cell).
SECOND MAIN DIVISION OF GERM-HISTORY.
Fan as a many-celled Primitive Ani.irfll.
The human embryo consists of many cells, which, however, ar jet 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, tlie other of vegetative cells.
Fourth Sta-'- : Blastula Strge (PI. II. Fig. IG).
The human germ is a vesicle, the wall of which consiit^ of animal cells,
its contents of vegetative cells.
HUMAN GERM HISTORY. 403
THIRD MAIN DIVISION OF GERM-HISTORY.
Man as an iavertebrate Intestinal Animal.
The human embryo possesses the form- value of an individual of the third
order, an unarticulated person (a single metamerou). The primitive in-
testinal cavity is enclosed by two primary germ-layers, from the fission of
which four secondary germ-layers ai'e presently formed.
Fifth Stage : Gastrula Stage (Fig. 41, p. 213, and PI. II. Fig. 17).
The human germ forms an Amphigastrnla, 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 ascidian
larva. Four secondary germ-layers have developed from the two primary
germ-layers, and coalesce along the central line.
FOURTH MAIN DIVISION OF GERMHISTORY.
Man as a true Vertebrate.
The human embryo jDossesses the form. value of an articulated person,
or a metameric chain. The articulation principally affects the bony
system (primitive vertebra?) and the muscle-system. The skin-sensory
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 vertebiso (musclc-phite and bone-plate), and the notocliord. From
the intestinal-fibi'ous lajer proceed the heart with the principal blood-
vessels, and the flesh)' intestinal wall. From the intestinal-glandular layer
the epithelium of the intestinal tube is formed.
Seventh Stage : Acranial 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 metamcra, 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.
404 THE EVOLUTION OF MAN.
Eighth Stage : Cyclostoma Stage (Fig. 132, p. 377, PI. VII. Fig. M I.).
The human germ possesses, iu essential points, the organization of a gill-
less cranial animal (like the developed Myxiuoida 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 swells
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 : Ichthyod Stage (Fig. 134, p. 378, PL VII. Pig. 31 II.).
The human germ possesses, in essential points, the organization of a fish
(or a fish-like SkuUed-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 : Amniotic Stage (PI. VII. 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 allautois 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 Phylogeny in the
following chapters.""')
HAECKELS EVOLUTION OF MAN.
PLATE VIII
HAECKEI.'S EVOLUTION OF MAN.
PLATE IX.
( 405 )
EXPLANATION OF PLATES VIII. AND IX.
(Both Plates are copied from Erdl, " Eniivickelung des Menschen." "")
Plate VIII. Fig. 1. — A human embryo of nine weeks, taken out from the
egg. membranes and magnified three times. (Erdl, Plate XII. Pig. 1-5.)
The skull is still quite transpai'ent, 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 {cerehellum) by a deep indentation. The forehead is
much arclied in front ; the nose is yet very undeveloped ; the eye is still dis-
propox-tionately large and wide open. The npper 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 embiyo is com-
pletely enclosed in the amnion, which is filled with the amnion fluid, as in a
water-bath. The navel cord, Avhich 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
choi'ion-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 deeper
folds. The head and limbs of the embryo are already considerably more
developed than in Fig. 1.
Plate IX. — A human embryo of five months ; natural size. (Erdl, Plato
XIV.) The embryo is enclosed in the delicate transparent amnion, which
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 covei'-
ing is the outer egg-membrane, the chorion.""
CHAPTER XIII.
THE STRUCTURE OF THE BODY OF THE AMPHIOXUS
AND OF THE ASCIDIAN.
Causal Significance of the Fundamental Law of Biogeny. — Influence of
Shortened and Vitiated Heredity. — Kenogenetic Modification of Palin-
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-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). — Move-
ment 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. — Com-
parison of Amphioxus and Young Lamprey (Petromyzon). — Comparison
of Amphioxus and Ascidian. — Cellulose Tunic. — Gill-sac. — Intestine.
— Nerve-centres. — Heart. — Sexual Organs.
" The primitive history of the species is all the more fully retained in
its germ-history in proportion as the series of embryonic forms traversed is
longer ; 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 embryonic states must oe regarded as transferred from a later
to an earlier period of life, or as acquired independently." — Fkitz Muller
(1864).
In turning from the germ-history to the tribal history
of Man, we must constaiitly bear in mind the causal connec-
THE TllIBAL AND GErxM-HISTORY OF MAN. 407
tion which exists between these two main branches of the
history of human evokition. 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
fh'st chapter. According to that first biogenetic principle,
Ontogeny is a short and compressed recapitulation of
Phj'logeny. If this reproduction of tribal history wei'e
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
hig-her oro-anism 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 geim-form of which we are
justified in explaining, without further trouble, as the
reproduction, or the portrait, of an extinct parent-form. But
in most 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
408 THE EVOLUTION OF MAN.
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, " Fur 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 larvcG 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.
1
PALINGENESIS AND KENOGENESIS. 4O9
The fact is simply shown by a comparison of the various
histories of individual evolution of higher and lower animals
of the same tribe.^'^
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 wei'e, 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.^*^
In order correctly to distinguish the palingenetic from
the kenogenetic phenomena of germ-history, and from these
rightly to infer the tribal history, Ave must especially apply
ourselves to a comparative study of Ontogeny. It is only
by comparing the germ-history of allied forms that we are
al)lo to discover the traces nf their tribal liistory. For this
410 THE EVOLUTION OF MAN.
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 foims
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 lock-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-j)eriods. 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
THE GEOLOGICAL METHOD. 4I I
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 Fi-shes, which stand
midway between the lower and higher Vertebrates, and
which explain several other phylogenetic periods. Lastly
412 THE EVOLUTION OF MAN.
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
{Zoopliytes) 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 has any
geologist actually seen that these various sedimentary rocky
formations originated in a particular sequence ; and yet all
agree as to this sequence. The reason of this is that only
THE LANCELET AND THE ASCIDIAN. 413
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 certainty, 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 sim})le as that of the geologists. It surpasses the
I'atter in difficulty and complexity in the same proportion as
the organization of Man is higher than the structure of the
rock."2
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 Lancelot
(Amijhioxus), and the other is the Sea-squirt (Ascidia)
(Plates X. and XI.). Both animals are extremely significant.
414 THE EVOLUTION OF MAN.
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 {Echinodermia), 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 {AraiDkioxus). 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 tlie germ-history of the Lancelet and the Sea-
I
HISTORY OF THE LANCELET. 415
squirt, we must first consider these two remarkable animals
in their perfect state and 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 {Liniax), he gave
it the name of Limax 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 lubricwm. 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 Miiller of Berlin, to
whom we are indebted for a very profound and thorough
dissertation upon its anatomy.^^^ Recently our knowledge
of the animal has been greatly extended, and its more
delicate structure especially has become better known.^^*
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.''^
41 6 THE EVOLUTION OF MAN.
Johannes Miiller referred the Lancelet to the class of
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 Vertebrates.
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. )^^^
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.,
such 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 fossuxzed, we
may suppose that all its numerous extinct kindred were
ACRANIA AND CRANIOTA. 417
cquall}' 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
passed.
The perfectly formed Lancelet (Fig. 151) is 5 to G 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 epidevTiiis (Fig. 13, 1). The central
line of the back is traversed by a narrow fin-like ridge
which wiilens behind into an oval tail-fin, and is prolonged
41 8 THE EVOLUTION OF MAN.
underneath into a short, anal fin. The fin-like ridge is 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 h).
In tlie 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 dorsalis),
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, m^). 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 ciliat'ed groove,
STRUCTURE OF THE LANCELET. 419
the single organ of smell. The organ of hearing is entirely
wantinof. 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 canal
divides 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-bod}^, the lattice-like wall of which
is pierced by numerous gill-openings (Fig. 151, d, and Plate
XI. Fig. 15, /.;). 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
hranchialis ; Fig. 151, c). On the ventral side of the gill-
body there is, along the central line, a ciliated groove
(the hypobrancliial groove), which also occurs in Ascidians
and in the larvse 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 apj>le; Fig. 15, y).
Behind the breathing, or respiratory part of the intestinal
canal comes, secondly, the digestive part. The small bodies
20
420
THE EVOLUTION OF MAN.
which the Amphioxus takes up in the water it breathes —
Infusoria, Diatomacese, parts of decayed plants, and animal
Fig. 151. — Lancelet {Aiux>hioxus Imiceolatus), 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, mouth-opening, surrounded by hairs ;
h, anal opening; c, gill-pore (porus hranchialis) ;
d, gill-body ; e, stomach ; /, liver ; <j, small intestine ;
h, gill-cavity ; i, notochord (below this the aorta) ;
fc, aorta-arch ; I, main trunk of the gill-artery ; in,
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 (/), 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
stomach.
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 other Vertebrates have a compressed, thick, purse-
shaped heart, which develops at the throat from the lower
CIRCULATION IN THE LANCELET. 421
wall of the anterior intestine, and from which the blood-
vessels proceed, there is in the Aniphioxus no special central-
ized heart, propelling tlie blood by its pulsations. Instead,
the movement of the blood in the Amphioxus, as in the
Ringed Worms {A'iinelida), is effected by the thin tubular
blood-vessels themselves, which perform the functions of the
heart, contracting and jjulsating through their entire length,
and thus drivinsj 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, ^ ; Plate XI.
Fig. 15, /). The aorta passes between the intestine and the
notochord precisely as in 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, o ; Plate X.
Fig. 15, V ; Plate XL Fig. 13, v). It passes on further over the
pouch-like liver, there forms a kind of cystic vein, weaving
422 THE EVOLUTION OF MAN.
a fine vascular network around the blind-sac of the liver,
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 ol
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
SIDE CANALS AND BODY-CAVITY. 423
we conceive tliat 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 (v. 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
veins.
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, Sj^), 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 signiti-
424
THE EVOLUTION OF MAN.
Fig. 152.— Transverse section through the anterior part of a Lancelet.
(After Rolph.) The outer covering forms the single cell-stratnra of the outer
skin {epidermis, E). Below this lies the thin leather skin (cori'uw), the
inner tissue of which is thickened below (U) ; partition walls of connective
tissue pass inward from it between the muscles (MJ and to the chorda-
sheath ; N, medullary tube ; ch, notochord ; Lh, body-cavity (coeloma) ; A,
gill-cavity; L, upper wall of the latter; E^, inner wall of the same; E^,
outer wall of the same; A'sf, gill-rods; M, ventral muscles; R, Raphe, or
seam formed by the coalescence of the venti'al folds (gill-roofs) ; G, sexual
glands.
1
KIDNEYS. 425
cance. The true body-cavity {Lh) is filled with lymph,
its iiiDcr 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 b}'' 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. 1.52, R).
On each side of this ventral seam, on the inner surface of
the gill-roofs, directly in front of the gill-pore {i^ovus
hranckialis), and over the ventral muscles (J/) 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 ei)ithelial 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 imier wall of
426 THE EVOLUTION OF MAN.
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. hranehialis).
Now on trying to comprehend in one connected view the
results of our anatomic study of tha 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
eveiy 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, vdiich 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. 250) and to compare wit'i 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
I
THE " ROUND-MOUTIIS." 42/
whole century, even beyond Pallas (1778), and only proves
that his notions of Comparative Anatomy and of the history
of evolution arc 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 w^hich 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 Amj^hioxus
with the early forms of development of those Vertebrates
wdiich are most nearly allied to it in the natural system
of this tribe. This is the class of the Round-Mouths
(Ctjdostomi). 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 {Myxhioidce),
which have been made known to us by Johannes Midler's
classic work, " Vergleichendo Anatomic der Myxinoiden."
The other gi'oup is formed by the well-known Lampreys, or
Rock-Suckers {Pdromyzonta), which are eaten as a delicacy.
All these Round-Mouths are usually included in the class of
428 THE EVOLUTION OF MAN.
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 (Pefromyzon, Plate XI.
Fig. 16) is compared with the Amphioxus (Fig. 15). In
both, the notochord (ck) 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 ('/rij), and on each side
of this appears a very simple eye (au) and a simple ear-
vesicle ((/). 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 {hk). 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.^^^
The Amphioxus, which is thus directly connected, on
the one side, with the Fishes through the Round-Mouths
(CydostoTni), 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
THE SEA-SQUIRT, OR ASCIDIAN, 429
nearly related to tlie Mussels, and was therefore classed
with the Soft-bodied Animals (Molliisca). But since 18G6,
when the remarkable germ-history of these animals was
first understood, there has been no doubt that tliey 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 di-ied 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 tlic fish-markets of many Italian seaside towns,
under the name of Sea-fruit (fruttl di -mare). There is
indeed nothing outwardly indicating an animal. When
they arc 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 gi'cat many species, which are
to be found in aU 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-
430 THE EVOLUTION OF MAN.
tiquity, and must have existed during the primfieval
ages.
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-s(piirt has, therefore, received the name " micro-
"117
cosm. ^'■'
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-
STRUCTURE OF ASCIDIAN.
431
able cavity, filled with water; this is the gill-cavity, or
respiratory cavity (Fig. 153, d; Plate XI. Fig. 14, cl). It
is also called the mantle or tunic cavitv, because it receives.
Fig. 153. — Structure of an Asciclian
(viewed fi'om the left side, as in Plato
XII. Fig. l-l) ; 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
(6r), 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 the 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 (Jjr).
The latter is in its whole posi-
tion and constitution so like
the giU-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 (o) leads directly into this gill-sac. The
water breathed in passes through the openings of the
432 THE EVOLUTION OF MAN.
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, DiatortiacecB, 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, my). 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 (porus branchialis), sometimes the
cloacal opening (Plate XI. 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, dt), above which lies a rudimen-
tary medullary tube (Fig. 5, m). In the mature Sea-squirt,
HEART OF THE ASCIDIAN. 433
this tube is entirely shrivelled up, and forms a little knot of
nerves lying near the front above the gill-sae (Fig. li, 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 papillfe, which surround the mouth
(Fig. 14, cm, eyes). The muscular system is very feebly,
and irregularly developed. Immediately below the thin
leather-skin (coriuin) with which it is intimately connected,
is a thin pouch-shaped muscular membrane, as in the lower
Worms. On the other hand, the Sea-squirt has a cen-
tralized heart, and appears in this respect to be more highly
oiganized 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 permanent!}
retains that same simple pouch-shaped form which the
rudimentary heart of the Vertebrate possesses for a very
short time. (Of 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 tlic
two great vessels proceeding from the opposite ends ol'
the heart act alternately as arteries and veins. This is a
peculiarity which appears only in the Mantle-Animal.s
{Tunicata).
434
THE EVOLUTION OF MAN.
Of the other nnportant organs, we have yet to mention
those of reproduction, wliich 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 XI. 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.
Fir,. 154. — Structure of an Ascidian (obsci'ved from
the left side, as in Fig. 153, and Fig. 14, Table XI.) :
sh, 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, tlius 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 ga'oup consists of a larger or
smaller number of individuals, of which every one possesses
KEPRODUCTION IN ASCIDIANS. 435
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 groups
{Botryllus, Folyclhium, 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."®
If we now once more glance back at the entire organiza-
tion of the simple Ascidians, Sea-squirts {Phalhisia, 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 Amphioxus as from the egg of the Ascidian.
EXPLANATION OF PLATES X. AND XI.
Plate X. — Geem-history of the Ascidian and of the Amphioxus.
(Principally accordino to Kowalevssy.)
Fig. 1-6. — Germ-history of an Ascidian.
Fig. 1. — A parent-cell {cytulu) of an Ascidian. In tlae bright-coloured
protoplasm of the parent-cell lies eccentrically a bright spherical kernel
(nacleus), 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 c^^g form a spherical bladder filled with
fluid, the wall of which consists of a single layer of cells. (Cf. Fig. 22, F, G.)
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 (»i) and the intestinal tube (d) lies the noto-
chord {ch) ; on both sides are the lateral muscles of the trunk (r).
Fig. 7-13. — Germ-history of the Amphioxus.
Fig. 7. — Parent-cell (cytida) 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.)
Fig. 10. — Gastrula of the Amphioxus. (Cf. Fig. 4.)
Fig. 11. — Young larva of the Amphioxus. The notochord (ch) lies
between the medullary tube (m) and the intestinal tube (d). The medullary
tube has an opening at the anterior extremity of the body (ma).
EXPLANATION OF PLATES X. AND XL 437
Fig. 12. — An older larva of the Amphioxus. On both sides of the
medullary tube (m) and of the notochord (c/t) a longitudinal row of muscle-
plates {>»p) is visible ; those mark the embryonic vertebra;, or metamera.
An organ of sense has developed in front (ss). The wall of the intestine
((?) is much thicker below on the ventral side (dit) than above on the dorsal
side (do). The anterior part of the intestinal canal widens into the gill-
body.
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 (t), 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 those the side canals (»). The dorsal muscles (r)
are divided into sevei'al parts by inter-muscular ligaments (ml) ; f, dorsal
fin.
Plate XL — Structure op the Ascidian, of the Amphioxus, and of the
Larva of 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 (ilonascidia) , magnified six times.
Fig. 15. — A developed Amphioxus (magnified four times).
For the sake of giving a more distinct view, the Amphioxus iu 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 Planpri), 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 uudor the name of
Ammocoetes.
The meaning of the letters is the same in all the figureB.
ALPHABETICAL EXPLANATION
Of the Meaning of the Letters in Plates X. and XL
a,
aims
TO,
spinal raarrow
au,
eye
ma
anterior opening of the medul.
h
ventral musclea
lary tube
c,
gill- cavity
mh
muscular ligaments
ch,
notochord (spinal axis)
mg
stomach
cl,
cloacal cavity
mh
, mouth-cavity
cs,
notochord sheath
mp
muscle-plate
d,
intestinal tube
mt,
mantle
do,
dorsal wall of the intestine
n,
nose (nose-pit)
dii,
ventral „ „ „
o>
mouth-oponing
6)
ovary
P,
ventral pore
en,
endostyle
Q,
cloacal opening
f,
fin-scam
r,
dorsal muscles
9>
ea,r-vosiclo
S)
tail-fins
h.
horn-plate
si.
seed-duct
hd,
testes
sm,
opening of the seed-duct
hJc,
venticle
ss,
organ of sense
hv.
auricle
t,
aorta (dorsal blood-vessel)
hz.
heart
th,
thyroid gland
i,
eggs
u,
side canals
h.
gills
V,
intestinal vein (ventral blood.
Tea,
gill-artery
vessel)
I,
leather-plate
IV,
root-fibres of the ascidian.
Ih,
liver
X,
boundary between the gill. intes-
Ih',
anterior end of the liver
tine aud the stomach. intestine
Iv,
liver vein
y,
hypo-branchial groove
TO,
medullary tube
z,
embryos of the ascidian
TO.
, brain-bladder
HAECKKLS EVOLUTION OF MAN. PLATE X.
CJNTOGENV OF THE ASCIDIAN (l-6) AND OF THE AmPHIOXUS (7-13).
haeckel's avoLUrioN of man. FI.ATE XI.
AnATOMV (IF THF, ASCIDIAN (14) AND A.MPHIOXUS (15).
CHAPTER XIV.
GER^iI- HISTORY OF THE AMnilOXUS AND OF THE
ASCIDIAN.
Relationsliip of the Vertebrates aud luvertebratcs. — Fertilization of the
Amphioxus. — The Egg undergoes Total Cleavage, and changes into a
Spherical Germ-mcnibrane 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 Muscle-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 Skinfolds (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 tlie Medullary and Intestinal Tnbes.
— Eetrogrcssive Development of the same. — The Tail with the Notochord
is shed. — The Ascidian attaches itself firmly, and envelopes it.self in
its Cellulose Tunic. — Appendicularia, a Tunicate which reuiains through,
out Life in the Stage of the Larval Ascidian and retains the Tail. fin
with the Chorda (Chordonia). — General Comparison and Significance of
the Amphioxus and the A.scidian.
" In the formation of its most important organs, the Ampliioxns remains
thi'oughout life at that lowest stage of development, which all other Verte-
bitites pass rapidly through during the earliest period of their embryonic
existence. We must therefore regard the Amphioxus with peculiar reverence
as that animal, which among all existing crcatui'es is the one aluno capable
440 THE EVOLUTION OF MAN.
of giving us an approximate idea of our oldest Silurian vertebrate ancestors.
But the latter are descended from Worms, the nearest blood-relatives of
which are 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-
woi'm {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 186G, I attempted, in my Generelle Morpholog'ie,
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
Invertebrates.
RELATION OF THE LANCELET TO THE ASCIDIAN. 44 1
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 Vert :!brates, was clearly recognized. There can be no
longer any doubt, especially since Kupffer and several other
zoologists have confirmed and continued these investicja-
tions, that of all classes of Invertebrates, the Mantle-animals
(Tunicata), SiTiid 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,
442 THE EVOLUTION OF MAN.
that of all Invertebrates, the Mantle-animals, and amon<^'
the latter the Asciclians, 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.^^^ 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 (jjorus
hranckialis). The eggs are simple roundish cells. They
have a diameter of only -^^ 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.
GERM-HISTORY OF THE LANCELET. 443
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 (cyhvla, 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, 82, 64 cells and so on, gives rise to the globular, black-
berry or mulberry-shaped body which we called the " mul-
liprry-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 (hlastula). Its contents form a clear
fluid ; the wall, which consists of a single layer of cells, is
the germ-membrane, or hlastoderma (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 hlastula 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. 155).
21
444
THE EVOLUTION OF MAN.
At last, the inversion is complete, so that the inner part of
the wall, that which has been inverted, lies on the inside of
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 Archigastrula (p. 191, Fig. 22, 1, K).
Fig. 155. — Gastrula of Amphi-
oxus, in longitudinal section : d,
primitive intestine ; o, primitive
mouth ; ^, 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 (jproto-
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.
The inner stratum, or the inverted portion of the blastula,
BELL-GASTRULA OF THE LANCELET. 445
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. — Uastrula of a Chalk-sponge (OlyutLus) : A, from tlie outside;
B, in longitudinal section through the axis ; g, primitive intestine ; 0, primir
tive mouth; i, intestinal-layer, or entoderm; e, skiii-layor. or exoderm.
446 THE EVOLUTION OF MAN.
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-coverings
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, ina).
Even at the time wlien the first trace of the dorsal furrow
GERM DEVELOPMENT IN THE LANCELET. 447
appears, the two primary germ-layers of the Ampliioxus
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) :
hs, skin-sensory layer; hm, skin-fibrOus
layer; c, ccsloni-fissnre (rudimentary
body-cavity) ; r/f, 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 (Jiiii).
The former gives rise to the outer skin (epidermis) and the
medullary tube ; the latter to the leather-skin (coriuni)
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
demonstrable.
As soon as the four secondary germ-layers have formed
448 THE EVOLUTION OF MAN.
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
dorsalis, 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 (co7'ium) 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, 'nip).
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 (cJi), 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
inouth-opening of the Amphioxus. On the contrary, it soon
VERTEBRATE NATURE OF THE LANCELET. 449
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
450 THE EVOLUTION OF MAN.
systems of tlie body, it is in the musclG-systom that arti-
culation first appears.-^^"
While these characteristic differentiations are taking
place in the two lamellsB 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,
a,re 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, 15, v). Moreover, at this time
the gills, or respiratory organs, also develop in the anterior
portion of the intestinal canal. The Avhole 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,
RESPIRATION IN THE LANCELET. 45 1
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, jp). 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 coeloma (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,
452
THE EVOLUTION OE MAN.
Figs. 158-160. — Transverse section through an early larval form of
Amphioxus. (Diagrammatic, after Rolph.) (Of. Fig. 152, p. 424.) In Fig.
158 there is a free jjassage from without into the intestinal cavity (D),
through the gill.openings (K). In Fig. 159 the lateral longitudinal folds
of the body-vsrall, the gill-roof, are forming, growing downwards. In Fig.
160 these side-folds have grown towards each other and their edges have
THE LANCELET AND THE PRIMITIVE VERTEBRATE. 453
coalesced in the middle liue 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, medullary tube ; Ch, notochord ;
M, side-muscles; Lh, body-cavity; G, portion of the body-cavity in which
the sexual oi-gans afterwards form ; D, intestinal cavity lined by the intes-
tinal-glandular layer (a); A, gill-cavity ; K, gill-openings ; h=^E, outer skin,
or epidermis ; ii\, the same as the inner epithelium of the gill-cavity j Eo,
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
(p. hrancliicdis). 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
body-cavity.
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 develop till later, and, as it appears, directly from the
inner cell-coat of the body-cavity, from the coelom-
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, Lit). In the lowest part of this
454 THE EVOLUTION OF MAN.
extension, the sexual glands originate from a portion of
the cceloin-epithelium (Fig. 160, G). 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.^^^
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 {Phalliisia, Cynthia, etc.)
is a simple globular cell i^ 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 {Fhallusia mam-
milata) have lately been very accurately studied and
described by Edward Strasbuiger. The remarkable details
of these processes, which do not, however, touch our present
purpose, are given in the excellent work by that writer
GERM-HISTORY OF THE ASCIDIAN. 455
on " Zellbildung." ^^^ Here, as in the Ampliioxiis, 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
Ampliioxus 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
(ejpidermis) 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-
456 THE EVOLUTION OF MAN.
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 Aseidian larva also, the per-
manent moutli-opening 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 Aseidian 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 oil
from tlie intestinal wall, which afterwards forms the heart,
the blood and the vascular system, and also the intestinal
muscles. This is the intestinal-fibrous layer.
FREE ASCIDIAN LARV^.
457
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.
161).
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, medullaiy tube ; x, notochord ; t, dorsal
vessel ; V, vcutral vessel ; a, intestine ; c, body,
cavity; m^, dorsal muscles; m^, 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-
458 THE EVOLUTION OF MAN.
merit 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 dii'ections 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, i) 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 retrogTCssion 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
swims. The tail, which is of no further use, is now lost.
APPENDICULARIA.
459
It undergoes fatty degeneration, and is east 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 : nt, mouth; k, gill-
intestine ; 0, oesophagus ; r, 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 Appendi 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 primroval
Worms which must be regarded as the conmion parent-form
460 THE EVOLUTION OF MAN.
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
gangUon, which lies over the mouth-opening, above the
gill-body, and which represents the exceedingly low mental
endowments of this animal (Plate XI. 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 Ascidian
RETROGRESSIVE DEVELOPMENT. 46 1
just as in the Ampliioxus. The excretory opening of the
former eoriesponds 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
oi'gans 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 d
l^Tiori highly improbable. Another naturalist, KupfFer,
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
inner 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
462 THE EVOLUTION OF MAN.
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.^^^
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 aU 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
THE LANCELET AS THE MOST ANCIENT VERTEBRATE. 463
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.^^*
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
" 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
gTeat 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
tho Ascidian organization, stand in this respect ! And what
are the thousand years of life of a venerable oak compared
with the many milHons of years the history of which is told
464 THE EVOLUTION OF MAN.
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.^^^ 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 Amphioxus.
( 465 )
TABLE IX.
Systematic Survey of the most important homologies between the human
embryo, tiie embi-yo of the Ascidian, and developed Amphioxus on the
one hand, and on the other hand, the developed Man.
Embryo of
Ascidian.
Developed
Amphioxus.
Human
Embryo.
Developed
Man.
I. — Products of the Differentiation of the Shin-layer.
Naked outer skin
Naked outer skin
Naked outer skin
Hairy outer skin
Simple medullary
tube
Simple medullary
tube
Simple medullary
tube
Brain and spinal
maiTow
Primitive kidney (?)
(excretory canal?)
Simple thin leather
Primitive kidney (?)
Simple thin leather
Primitive kidney
duct
Simple thin leather
Oviduct and
sperm-duct
Differentiated
skin (Corium)
skin (Corium)
skin (Corium)
thick leather skin
Simple skin-mus
Simple trunk-
Simple muscle-
(Corium)
Differentiated
cular pouch
muscle system
plate
trunk-muscle
Notochord
Notochord
Notochord
system
Vertebral column
No skull
No skull
No skull
Bony skull
No limbs
No limbs
No limbs
Two pair of limbs
Hermaphrodite
sexual glands
Separated sexual
glands
Hermaphrodite
sexual glands
Separated sexual
glands
11. — Products of the Differentiation of the Intestinal layers.
Simple body cavity
(Coeloma)
One-chambered
Simple body cavity
(Coiloma)
Simple tubular
Simple body cavity
(Cceloma)
One-chambered
Distinct chest and
ventral cavities
Four-chambered
heart
heai't
heart
heart
Dorsal vessel
Aorta
Aorta
Aorta
Simple liver pouch
(?)
Simple liver pouch
Simple liver pouch
Large differen-
tiated liver
Simple intestinal
Simple intestinal
Simple intestinal
Differentiated in-
tube with gill-
tube with gill-
tube with gill-
testinal tube
opeumgs
openings
opemnga
without gill-
openings
466
THE EVOLUTION OF MAN,
TABLE X.
Systematic Survey of the points of connection in form of the Ascidian and
Amphioxus on the one side, and the Fishes and Men on the other, in
completely developed conditions.
Developed
Developed
Developed
Developed
Ascidian.
Amphioxv.s.
Fish.
Man.
Head and trunk
Head and trunk
Head and trunk
Head and trunk
not distinct
not distinct
distinct
distinct
No limbs
No limbs
Two pair of limbs
Two pair of limbs
No skull
No skull
Developed skull
Developed skull
No tongue-bone
No tongue-bone
Tongue-bone
Tongue-bone
No jaw-apparatus
No jaw-apparatus
Jaw-apparatus
Jaw-apparatus
(upper and
(upper and
lower jaws)
lower jaws)
No vertebral
No vertebral
Articulated verte-
Articulated verte-
column
column
bral column
bral column
No ribs
No ribs
Eibs
Ribs
Brain undifferen-
Brain undifferen-
Brain differen.
Brain differen-
tiated
tiated
tiated
tiated
Eyes rudimentary
Eyes rudimentary
Eyes developed
Eyes 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
Sympathetic "
Sympathetic
nerve
nerve
nerve
nerve
Intestinal epithe-
Intestinal epithe-
Intestinal epithe-
Intestinal epithe-
lium ciliated
lium ciliated
lium not ciliated
lium not ciliated
Simple liver (or
Simple liver (blind
Compound liver
Compound liver
none)
intestine)
gland
gland
No ventral sali-
No ventral sali-
Ventral salivary
Ventral salivary
vary gland
vary gland
gland
gland
No swimming
No swimming
Swimming blad-
Lungs (swimming
bladder
bladder
der (rudimen-
tary lungs)
bladder)
Kidneys rudimen-
Kidneys rudimen-
Kidneys deve-
Kidneys deve-
tary (?)
tary (?)
loped
loped
Simple heart
Simple tubular
Heart with valves
Heart with valves
pouch
heart
and chambers
and chambers
Blood colourless
Blood colourless
Blood red
Blood red
No spleen
No spleen
Spleen
Spleen
Hypobranchial
Hypobranchial
Thyroid gland
Thyroid gland
groove on gill-
groove on gill-
body
body
^
( 467 )
TABLE XI.
Systematic Survey showing tlie derivation of the germ-layers of the Ampbioxns
from the parent-cell (cytula), and of the main organs from the germ-layers.
(Tree showing the ontogenetic descent of the cells in the Amphioxus).'^^
IMeJuUarv tube
Tubus medallaris Flesh-mass
Blood-vessels
Musculi
Canales Gill-epithelium
Notochord
sangueferi Epith. branchiate
Choi-da
1
Stomach
Sense-organs
1
epill]
elium
Intestinal
muscle
Liver
epi-
Small
intestine
Leatlier skin
Testes
Ovaries
wall
thelium
epi-
Outer skin
Corium
Testiculi
Ovaria
Peri-
thelium
Epid
ennts
ente
rum
1
Parietal
Visceral
Mesen-
Epithelium
Primitive
Coelom-
Coelom-
tery
diycstivum
kidneys
Kpi-
Epi-
M'sen-
Pro-
thelium
thelium
teriiim
tonephra
Exiicm-
larium
Endocce-
larium
\
-^ ^—
^
Skin-sensory Skin-fibrous
layer layer
{Lamina neurodermalis) (jMmina inodeimalis')
(Fig. 157, /i») (Fig. 157, /t/)
Intestinal-flbrous Intestinal-glan-
layer dular layer
(^Lamina inogaslralis) {Lamina mi/xogaslralii- )
(Fig. 157, ti/) (Fig. 157, dc<)
Exoderma (Fig. i55, e)
(Skin-layer, or outer germ-layer)
Entoderma (Fig. 155, i)
(Intestinal l-iyer, or inner germ-layer)
Gastrula TFig. 22, J, K "]
(Cup-germ) L^'iS- 1^5, p. 444 J
Blastula (Fig. 22, F, G)
(Bladder-germ)
Morula (Fig. 22, £)
(Mulberry-gerui)
Cytula (Fig. 22, /?)
(Parent-cell)
Monerula (Fig. 22, A")
(Parent-cytod)
22
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