THE GERM-LAYERS AND EVOLUTION
In his papers of 1866 and 1867 Kowalevsky had remarked upon the
widespread occurrence of a certain type or fundamental plan of early
embryonic development, characterised by the formation, through
invagination, of a two-layered sac, whose cavity became the alimentary
canal. This developmental archetype was manifested in, for instance,
_Sagitta_,[424] _Rana_,[425] _Lymnaea_,[426] _Astacus_,[427]
_Phoronis_,[428] _Asterias_,[429] _Ascidia_,[428] the _Ctenophora_,[428]
and _Amphioxus_.[428] He noticed also that the invagination-opening
often became the definitive anus. Further instances of this mode of
development were later observed by Metschnikoff[430] and by
Kowalevsky[431] himself, but it was left to Haeckel to generalise these
observations and build up from them his famous Gastraea theory. This was
first enunciated in his monograph of the calcareous sponges,[432] and
worked out in detail in a series of papers published in 1874-76.[433]
Haeckel maintained that the "gastrula" stage occurred in the development
of all Metazoa, and that it was typically formed, by invagination, from
a hollow sphere of cells or "blastula." This typical formation might be
masked by cenogenetic modifications caused chiefly by the presence of
yolk. The gastrula stage was the palingenetic repetition of the
ancestral form of all Metazoa, the Gastraea.
From the Gastraea theory there followed at once two consequences, (1)
that ectoderm and endoderm, invagination-cavity (_Urdarm_) and
gastrula-mouth (_Urmund_ or _Protostoma_), were, with all their
derivatives, homologous, because homogenous, throughout the Metazoa, and
(2) that the descent of the Metazoa had been monophyletic, since all
were derived from the ancestral Gastraea. Huxley's suggestion (_supra_,
p. 208) that the outer and inner layers in Coelentera were homologous
with the ectoderm and endoderm of the germ was thus fully confirmed and
greatly extended.
The great importance of the Gastraea theory lay in the fact that it
linked up, by means of the biogenetic law, the germ-layer theory with
the doctrine of evolution. It supplied an evolutionary interpretation of
the earliest and most important of embryogenetic events, the process of
layer-formation. Upon the Gastraea theory or its implications were
founded most of the phylogenetic speculations which subsequently
appeared.
Upon the Gastraea theory Haeckel based a system of phylogenetic
classification which was intended to replace Cuvier's and von Baer's
doctrine of Types. This took the form of a monophyletic ancestral tree.
Its main outlines are given on p. 290 in graphic form, combined and
modified from the table on p. 53 of the 1874 paper and the genealogical
tree given in the _Kalkschwaemme_.[434]
_Monophyletic Genealogical Tree of the Animal Kingdom, based upon the
Gastraea Theory and the Homology of the Germ Layers_.
_______________________________________________________________________
| | | . |
| | | m |
| | _Vertebrata_. | o |
| . | | | l |
| m | _Arthropoda_. | | e |
| r | | | | o |
| e | | | | c |
| d |_Echinoderma_. | | _Mollusca_. | |
| d | | | | | | a |
| n | | | Sagitta. \______ | ______/ | | |
| e | | | | \|/ | . d |
| | | | | | | a n |
| y | | | | Nematoda. Himatega. | i a |
| b | | | | | | | r |
| | | | | | | | a d |
| | | | | | | | t o |
| | \______________|______|_ __|____________|_____/ | a o |
| | \/ | m l |
| | | ae b |
| | _Coelomati_ | H |
| | (worms with body-cavity}. | | h |
| | \ / | t |
| | \ / | i |
| | \ / | W |
| |________________________________\/_____________________|______|
| . | | | |
| ) d | | | . |
| s e | _Zoophyta_ | Plathelminthes. | m |
| l n | (Coe;enterata). | | | o |
| a i | | | | l |
| m l | Acalephae. \______________ |_____/| e |
| i | | \/ | o |
| n , | Spongiae. | _Acoelomi_ | c |
| a t | | | (Worms without | |
| u | Archispongia. Archydra. body cavity). | o |
| t g | | | | | n |
| u | | | | | |
| G e | \______ ______/ | | | d |
| ( u | \/ | | a n |
| r | Protascus. Prothelmis. | i a |
| t | | | | r |
| | | | | a d |
| A | Gastraea radialis Gastraes bilateralis | ae o |
| | | (sedens). (repens). | n o |
| a . | | | | A l |
| o s | | | | | b |
| z r | \_______________ _______________/ | |
| a e | \/ | o |
| t y | _Gastraea_ | N |
| e a | (Ontogeny : Gastrula). | |
| M l | | | |
| | | | | |
| m | | | |
| r | | | |
| e | | | |
| g | | | |
| | | | |
| y | | | |
| r | | | |
| a | | | |
| m | | | |
| i | | | |
| r | | | |
| P | | | |
| | | | |
| o | | | |
| w | | | |
| T | | | |
|______| _________|_________________________|______|
| | | |
| | __________| |
| | | |
| . | | |
| t | Planaeada Acinetae. Ciliata. |
| u | (Ontogeny : Planula). | | |
| g | | \_________ _________/ |
| > | | \/ |
| i o | | Infusoria. |
| / n | | | |
| < | | | |
| a , | Synamoebae Gregarinae | |
| o s | (Ontogeny : Morula). | | |
| z r | | | | |
| o e | | \_____ ______/ |
| t y | | \/ |
| o a | | Amoebina. |
| r l | | | |
| P | \____________ _____________/ |
| > m | \/ |
| i r | _Amoebae_ ? ? ? |
| < e | (Ontogeny : Ovulum). | | | |
| g | | | | | |
| | | | | | |
| o | _Monera_ Monera. |
| N | (Ontogeny : Monerula). |
| | |
|______|______________________________________________________________|
The scheme is in many respects an interesting and important one. The
great contrast between the Protozoa, or animals with neither gut nor
germ-layers, and the Metazoa, which possess both structures, is for the
first time clearly brought out. The derivation of all the Metazoa from a
single ancestral form, the Gastraea, leads to the conclusion that the
types are not distinct from one another as Cuvier and von Baer supposed,
but agree in the one essential point, in the possession of an
_archenteron_ (Lankester, 1875), and an ectoderm and endoderm which are
homologous throughout all the Metazoan phyla. Finally, in the separation
of the sponges, Coelenterata and Acoelomi as animals lacking a body
cavity or coelom[435] from the four higher phyla, which are essentially
Coelomati, there is contained the germ of a conception which later
became of importance.
Somewhat similar views as to the importance of the germ-layer theory for
the phylogenetic classification of animals were published by Sir E. Ray
Lankester in 1873.[436] He distinguished three grades of animals--the
Homoblastica, Diploblastica, and Triploblastica. The first included the
Protozoa, the second the Coelenterata, the third the other five phyla,
distinguished by the possession of a third layer, the mesoderm, and a
"blood-lymph" cavity enclosed therein. He used the germ-layer theory to
prove the essential unity of type of all the Triploblastica.
The Gastraea theory gave point and substance to the biogenetic law, and
enabled Haeckel to state much more concretely the parallelism existing
between ontogeny and phylogeny. He was able to assert that five
primordial stages, each representing a primitive ancestral form,
recurred with regularity in the very earliest development of all
Metazoa.[437] These were the monerula, cytula, morula, blastula, and
gastrula (see Fig. 15). The monerula was the fertilised ovum after the
disappearance of the germinal vesicle;[438] it was the equivalent of
the primordial anucleate Monera which are the ancestors of all
animals. The ovum after the nucleus had been re-formed became the
cytula, which was the ontogenetic counterpart of the amoeba. The
morula, a compact mulberry-like congeries of segmentation-cells,
corresponded to the synamoeba, or earliest association of
undifferentiated amoeboid cells to form the first multicellular
organism. The blastula, or hollow sphere of segmentation cells,
usually ciliated, was reminiscent of the planaea, an ancestral
free-swimming form whose nearest living relation is the spherical
_Magosphaera_. The gastrula, finally, is the two-layered sac formed
from the blastula, typically by invagination of its wall. It repeats
the organisation of the gastraea, which is the common ancestor of all
Metazoa, and finds its nearest living counterpart in the simple
"sponges" _Haliphysema_ and _Gastrophysema_.[439] The ancestral line
of all the higher animals begins with the five hypothetical forms of
the moneron, amoeba, synamoeba, planaea, and gastraea.
[Illustration: FIG. 15.--The Five Primary Stages of Ontogeny. (After
Haeckel.) 1. Monerula. 2. Cytula. 3. Morula. 4. Blastula. 5. Gastrula.]
We may take the following account[440] of the phylogeny of the human
species, from the gastraea stage onwards, as typical of Haeckel's
speculations on the evolution of the higher forms. The progenitors of
man are, after the Gastraeada:--
1. Turbellaria.
*2. Scolecida. (Worms with a coelom, probably represented
at the present day by _Balanoglossus_.)
*3. Himatega. (Evolved from Scolecida by formation of
dorsal nerve-tube and chorda, and resembling tailed
larvae of Ascidians.)
4. Acrania. (With metameric segmentation. Including
Amphioxus.)
5. Monorrhina. (Cyclostomes.)
6. Selachia.
7. Dipneusta.
8. Sozobranchia. (Amphibia with permanent gills.)
9. Sozura. (Tailed Amphibia.)
*10. Protamnia.
*11. Promammalia.
12. Marsupialia.
13. Prosimiae.
14. Menocerca. (Tailed apes.)
15. Anthropoides.
16. Pithecanthropi.
17. Homines.
It will be noticed that except for the hypothetical forms (marked with
an asterisk), which are themselves generalised classificatory groups,
the ancestral forms belong to long-recognised classes. The whole course
of the evolution follows well-worn systematic lines. This is typical of
Haeckel's phylogenetic speculations.
A more abstractly morphological scheme of the evolution of Vertebrates
is given in the _Systematic Phylogeny_ of 1895.[441] The ontogenetic and
ancestral stages are arranged in parallel columns thus:--
Cytula. Cytaea (Protozoa).
Morula. Moraea (Coenobium of Protozoa).
Blastula. Blastaea (_Volvocina_, etc.).
Depula (invaginated blastula). Depaea.
Gastrula. Gastraea (cf. _Olynthus_, _Hydra_, and
primitive Coelentera).
Coelomula (with one pair Coelomaea (cf. _Sagitta_, _Ascidia_,
of coelom-pockets). and primitive Helminthes).
Chordula (with medullary Chordaea (_cf._ Ascidian larva and
tube and chorda). larva of Amphioxus).
Spondula (with segmented Prospondylus (Primitive Vertebrate).
mesoderm).
This scheme differs from the earlier one chiefly in taking into account
certain advances, notably as regards the cytology of the fertilised ovum
and the true nature of the coelom, which had been made in the interval
of some twenty years.
Haeckel's Gastraea theory, though it exercised a great influence upon the
subsequent trend of phylogenetic speculation, was by no means
universally accepted _telle quelle_. Opinions differed considerably as
to the primitive mode of origin of the two-layered sac which was very
generally admitted to be of constant occurrence in early embryogeny. Ray
Lankester, in his paper of 1873, and more fully in 1877,[442] propounded a
"Planula" theory, according to which the ancestral form of the Metazoa
was a two-layered closed sac formed typically by delamination, less
often by invagination. He denied that the invagination opening (which he
named the blastopore) represented the primitive mouth,[443] holding that
this was typically formed by an "inruptive" process at the anterior end
of the planula, which led to the formation of a "stomodaeum." A similar
process at the posterior end gave rise to the anus and the "proctodaeum."
The question as to whether delamination or invagination was to be
considered the more primitive process was discussed in detail by
Balfour,[444] without, however, any very definite conclusion being
reached. He held that both processes could be proved in certain cases to
be purely secondary or adaptive, and that accordingly there was nothing
to show that either of them reproduced the original mode of transition
from the Protozoa to the ancestral two-layered Metazoa (p. 342). He by
no means rejected the theory that the Gastraea, "however evolved, was a
primitive form of the Metazoa," but, having regard to the great
variations shown in the relation of the blastopore to mouth and anus
(pp. 340-1), he was inclined to think that if the gastrula had any
ancestral characters at all, these could only be of the most general
kind. Balfour's attitude perhaps best represents the general consensus
of opinion with regard to the Gastraea theory.
From the same origins as the Gastraea theory arose the theory of the
coelom. The term dates back to Haeckel in 1872, and the observations
which first led up to the theory were made by the men who supplied the
foundations of the Gastraea theory--A. Agassiz, Metschnikoff and
Kowalevsky. But it was not Haeckel himself who enunciated the coelom
theory.
It will be remembered that Remak introduced in 1855 the conception of
the mesoderm as an independent layer derived from the endoderm. The
pleuro-peritoneal or body-cavity was formed as a split in the "ventral
plates" of the mesoderm. Haeckel's "coelom" corresponded to the
"pleuro-peritoneal cavity" of Remak, but his view of the origin of the
mesoderm brought him much closer to von Baer's conception of the origin
of _two_ secondary layers from ectoderm and endoderm respectively than
to Remak's conception of the mesoderm as a single independent layer.
Much uncertainty reigned at the time as to the exact manner of origin of
the mesoderm;[445] some held that it developed from the ectoderm, others
that it originated in the endoderm, while still others, and among them
Haeckel, considered that part of it came from the ectoderm and part from
the endoderm (pp. 23-4, 1874).
The solution of the problem came from those observations on the
development of the lower forms to which we have just alluded.
The early history of these discoveries and of the theory which grew out
of them has been well summarised by Lankester,[446] and may conveniently
be given in his own words:--
"As far back as 1864 Alexander Agassiz ("Embryology of the Star-fish,"
in _Contributions to the Natural History of the United States_, vol. v.,
1864) showed in his account of the development of Echinoderma that the
great body-cavity of those animals developed as a pouch-like outgrowth
of the archenteron of the embryo, whilst a second outgrowth gave rise to
their ambulacral system; and in 1869 Metschnikoff (_Mem. de l'Acad.
imperiale des Sciences de St Petersbourg_, series vii., vol. xiv.,
1869), confirmed the observations of Agassiz, and showed that in
Tornaria (the larva of Balanoglossus) a similar formation of
body-cavities by pouch-like outgrowths of the archenteron took place.
Metschnikoff has further the credit of having, in 1874 (_Zeitsch. wiss.
Zoologie_, vol. xxiv., p. 15, 1874), revived Leuckart's theory of the
relationship of the coelenteric apparatus of the Enterocoela to the
digestive canal and body-cavities of the higher animals. Leuckart had in
1848 maintained that the alimentary canal and the body-cavity of higher
animals were united in one system of cavities in the Enterocoela
(_Verwandschaftsverhaeltnisse der wirbellosen Thiere_, Brunswick, 1848).
Metschnikoff insisted upon such a correspondence when comparing the
Echinoderm larva, with its still continuous enteron and coelom, to a
Ctenophor, with its permanently continuous system of cavities and
canals. Kowalevsky, in 1871, showed that the body-cavity of Sagitta was
formed by a division of the archenteron into three parallel cavities,
and in 1874 demonstrated the same fact for the Brachiopoda. In 1875
(_Quart. Journ. Micr. Sci._, vol. xv., p. 52) Huxley proposed to
distinguish three kinds of body-cavity: the schizocoel, formed by the
splitting of the mesoblast, as in the chick's blastoderm; the
enterocoel, formed by pouching of the archenteron, as in Echinoderms,
Sagitta and Brachiopoda; and the epicoel.... Immediately after this I
put forward the theory of the uniformity of origin of the coelom as an
enterocoel (_Quart. Journ. Micr. Sci._, April, 1875).... My theory of
the coelom as an enterocoel was accepted by Balfour and was greatly
strengthened by his observations on the derivation of both notochord and
mesoblastic somites from archenteron in the Elasmobranchs, and by the
publication in 1877 by Kowalevsky of his second paper on the development
of Amphioxus--in which the actual condition which I had supposed to
exist in the Vertebrata was shown to occur, namely, the formation of the
mesoblast as paired pouches in which a narrow lumen exists, but is
practically obliterated on the nipping-off of the pouch from the
archenteron, after which process it opens out again as coelom" (pp.
16-18).
The enterocoelic theory was taken up by O. and R. Hertwig as an
essential part of their _Coelomtheorie_.[447] In a lengthy series of
monographs these workers made a comparative study of the mode of
formation of the middle layer, and arrived at a coherent theory of its
origin. They distinguished in the middle layer two quite distinct
elements, the mesoblast proper, formed by the evagination of the walls
of the archenteron, and the mesenchyme, formed by free cells budded off
from the germ-layers. The following passage gives a good idea of their
views and of the phylogenetic implications involved:--"Ectoblast and
entoblast are the two primary germ-layers which arise from the
invagination of the blastula; they are always the first to be laid down,
and they can be directly referred back to a simple ancestral form, the
Gastraea; they form the limits of the organism towards the exterior and
towards the archenteron. The parietal and visceral mesoblast, or the two
middle layers, are always of later origin, and arise through evagination
or plaiting of the entoblast, the remainder of which can now be
distinguished as secondary entoblast from the primary. They form the
walls of a new cavity, the enterocoel, which is to be regarded as a
nipped-off diverticulum of the archenteron. Just as the two-layered
animals can be derived from the Gastraea, so can the four-layered animals
be derived from a Coelom form. Embryonic cells, which become singly
detached from their epitheliar connections we consider to be something
quite different from the germ-layers, and accordingly we call them by
the special name of mesenchyme germs or primary cells of the mesenchyme.
They may develop both in two-layered and in four-layered animals. Their
function is to form between the epithelial limiting layers a secreted
tissue (_Secretgewebe_) or connective tissue with scattered cells, which
cells can undergo, like the epithelial elements, the most varied
modifications.... This secreted tissue in its simple or in its
differentiated state, with all its derivatives, we call the mesenchyme"
(p. 122).
The important point for us is that, just as all Metazoa were considered
by Haeckel to be descended from the Gastraea, so all Coelomati were held
by the Hertwigs to be derived from an original coelomate _Urform_. In
both cases an embryological archetype becomes a hypothetical ancestral
form.
The Coelom theory was considerably modified, extended and developed by
later workers, particularly as regards the relations to the coelom of
the genital organs and ducts and the nephridia, but no special
methodological interest attaches to these further developments.[448] We
shall here focus attention upon one interesting line of speculation
followed out in this country particularly by Sedgwick--the theory of the
Actinozoan ancestry of segmented animals. Its relation to the Coelom
theory lies in the fact that Sedgwick regarded the segmentation of the
body as moulded upon the segmentation of the mesoblast, which in its
turn, as Kowalevsky and Hatschek had shown, was a consequence of its
mode of origin as a series of pouches of the archenteron. In other
respects Sedgwick's speculations link on more closely to the Gastraea
theory, for one of his main contentions is that the blastopore or
_Urmund_ is homologous throughout at least the three metameric phyla. In
following up Balfour's observations on the development of
_Peripatus_,[449] Sedgwick was struck with the close resemblance existing
between the elongated slit-like blastopore of this form (giving rise to
both mouth and anus), with its border of nervous tissue, and the
slit-like mouth of the Actinozoan (functioning both as mouth and anus),
round which, as the Hertwigs had shown, there lies a special
concentration of nerve cells and nerve fibres. He found another point of
resemblance in the gastric pouches of the Actinozoa, which he
homologised directly with the enterocoelic pouches of the Coelomati. He
was led to enunciate the following theses:--[450] (1) that the mouth and
anus of Vermes, Mollusca, Arthopoda, and probably Vertebrata, is derived
from the elongated mouth of an ancestor resembling the Actinozoa; (2)
that somites are derived from a series of archenteric pouches, like
those of Actinozoa and Medusae; (3) that excretory organs (nephridia,
segmental organs) are derived from parts of these pouches which in the
ancestral form, as in many polyps, were connected by a circular or
longitudinal canal, and opened to the exterior by pores. This
longitudinal canal was lost in Invertebrates, but persisted in
Vertebrates as the pronephric duct, while the pores remained in
Invertebrates and disappeared in Vertebrates; (4) that the tracheae of
Arthropods, as well as the canal of the central nervous system in
Vertebrates, are to be traced back to certain ectodermal pits in the
diploblastic ancestor comparable to the sub-genital pits of the
Scyphomedusae. These ectodermal pits were all originally respiratory
organs. "The essence of all these propositions," he writes, "lies in the
fact that the segmented animals are traced back not to a triploblastic
unsegmented ancestor, but to a two-layered Coelenterate-like animal with
a pouched gut, the pouching having arisen as a result of the necessity
for an increase in the extent of the vegetative surfaces in a rapidly
enlarging animal (for circulation and respiration)" (p. 47). "I have
attempted to show," he writes further on, "that the majority of the
Triploblastica ... are built upon a common plan, and that that plan is
revealed by a careful examination of the anatomy of Coelenterata; that
all the most important organ-systems of these Triploblastica are found
in a rudimentary condition in the Coelenterata; and that all the
Triploblastica referred to must be traced back to a diploblastic
ancestor common to them and the Coelenterata" (p. 68). The main
assumption was that the neural or blastoporal surface must be homologous
throughout the Metazoa, though it was dorsal in the Chordata, ventral in
the Annelida and Arthropoda. He derived the central nervous system of
the Chordata from the circumoral ring of the common ancestor by means of
the hypothesis that both the pre-blastoporal and the post-blastoporal
parts of it disappeared.[451]
The characteristic relation of the central nervous system to the
blastopore in Annelida and Vertebrates had already been pointed out by
Kowalevsky,[452] who had also sketched a theory of the common descent of
these two phyla from an ancestral form in which the nervous system
encircled the blastopore.
In 1882, before the publication of Sedgwick's papers, A. Lang[453] had put
forward the somewhat similar view that the stomach-diverticula of the
Turbellaria, which he had found to be segmentally arranged in certain
Triclads, were the morphological equivalents of the enterocoelic pouches
of higher animals. This view, however, he soon gave up.[454] Sedgwick's
views found a supporter in A. A. W. Hubrecht,[455] who utilised them in
connection both with his speculations on the relation of Nemertines to
Vertebrates, and with his exhaustive work on the early development of
the Mammalia. He postulated as the far-back ancestor of Vertebrates, "an
actinia-like, vermiform being, elongated in the direction of the
mouth-slit" (p. 410, 1906), and derived the central nervous system from
the circum-oral ring of this primitive form, the notochord from its
stomodaeum, and the coelom from the peripheral parts of the gastric
cavity (p. 169, 1909).
[424] Gegenbaur, _Zeits. f. wiss. Zool._, v., 1853.
[425] Remak, _loc. cit._, p. 183, pl. xii.
[426] Lereboullet, _Ann. Sci. nat._ (4) xviii., pp. 118-9,
1862.
[527] Lereboullet, in Remak, p. 183 f.n.
[428] Kowalevsky, _Mem. Acad. Sci. St
Petersbourg_ (Petrograd), (7), x. and xi., 1866 and 1867.
[429] A. Agassiz, _Contrib. Nat. Hist. United States_, v.,
1864.
[430] _Mem. Acad. Sci. St Petersbourg_ (Petrograd), (7),
xiv., 1869.
[431] "Embryolog. Studien an Wuermern u. Arthropoden,"
_Mem. Acad. Sci. St Petersbourg_ (Petrograd), (7), xvi.,
1870.
[432] _Die Kalkschwaemme_, 3 vols., Berlin, 1872. General
chapters translated in _Ann. Mag. Nat. Hist._ (4), xi.,
pp. 241-62, 421-30, 1873.
[433] "Die Gastraea-Theorie, die phylogenetische
Classification des Thierreichs und die Homologie der
Keimblaetter." _Jenaische Zeitschrift_, viii., pp. 1-55,
1874. "Die Gastrula und die Eifurchung der Thiere,"
_ibid._, ix., pp. 402-508, 1875. "Die Physemarien,
Gastraeaden der Gegenwart," and "Nachtraege zur
Gastraea-Theorie," _ibid._, x., pp. 55-98, 1876.
Republished in _Biologische Studien_, 2nd part, _Studien
zur Gastraea-Theorie_, 270 pp., 14 pls., Jena, 1877.
[434] See _Ann. Mag. Nat. Hist._ (4), xi., p. 253.
[435] Term first introduced in _Die Kalkschwaemme_, p. 468,
1872.
[436] "On the Primitive Cell-layers of the Embryo as the
Basis of Genealogical Classification of Animals, and on
the Origin of Vascular and Lymph Systems," _Ann. Mag.
Nat. Hist._ (4), xi., pp. 321-38, 1873.
[437] First distinguished in _Die Kalkschwaemme_, i., p.
465.
[438] Even in the 'seventies it was still believed by many
that the egg-nucleus disappeared on fertilisation. The
true nature of the process was not fully made out till
1875, when O. Hertwig observed the fusion of egg- and
sperm-nuclei in _Toxopneustes (Morph. Jahrb._, i.,
1876).
[439] _Studien z. Gastraea-Theorie_, p. 214, 1877. These
forms were known even in 1870 (Carter, _Ann. Mag. Nat.
Hist._ (4), vi., pp. 346-7), to be Foraminifera. The
figures of supposed collar-cells, etc., do credit to
Haeckel's imagination.
[440] _History of Creation_, Eng. Trans., ii., pp. 278 ff.
[441] _Systematische Phylogenie_, iii., p. 41, Berlin,
1895.
[442] "Notes on the Embryology and Classification of the
Animal Kingdom," _Q.J.M.S._ (n.s.), xvii., pp. 399-454,
1877.
[443] It was "part of the non-historic mechanism of
growth" (_loc. cit._, p. 418).
[444] _Treatise on Comparative Embryology_, ii., chap.
xiii., 1881. For a modern discussion of this problem,
see Hubrecht, _Q.J.M.S._, xlix., 1906.
[445] See Balfour, _loc. cit._, Chapter xiii.
[446] _A Treatise on Zoology_, Pt. ii., 1900. Introduction
by Sir E. Ray Lankester.
[447] _Studien zur Blaettertheorie_, Jena, 1879-80. "Die
Coelomtheorie, Versuch einer Erklaerung des mittleren
Keimblattes," _Jenaische Zeitschrift_, xv., pp. 1-150,
1882.
[448] For an historical account of this work, see
Lankester, _loc. cit._, pp. 21-37.
[449] _Proc. Roy. Soc._, 1883, and _Q.J.M.S._, xxiii.,
1883.
[450] "Origin of Metameric Segmentation," _Q.J.M.S._,
xxiv., pp. 43-82 1884.
[451] See further the same author's article "Embryology"
in the _Ency. Brit._, vol. xi., 11th ed., Cambridge,
1910.
[452] _Arch. f. mikr. Anat._, xiii., pp. 181-204, 1877.
[453] "Der Bau von Gunda segmentata," _Mitth. Zool. Stat.
Neap._, iii., pp. 187-250, 1882.
[454] "Die Polycladen," _Fauna u. Flora des Golfes von
Neapel_, Monog. v., Leipzig, 1884, and "Beitraege zu
einer Trophocoeltheorie," _Jen. Zeits._, xxxviii., pp.
1-373, 1904 (which see for a modern account of theories
of metamerism).
[455] "Die Abstammung der Anneliden u. Chordaten," _ Jen.
Zeits._, xxxix., pp. 151-76, 1905. "The Gastrulation of
the Vertebrates," _Q.J.M.S._, xlix., pp. 403-19, 1906.
"Early Ontogenetic Phenomena in Mammals," _Q.J.M.S._,
liii., pp. 1-181, 1909.