Form and Function

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Much doubt was cast upon the Chordate affinities of the Enteropneusta by
Spengel in his monograph of the group,[422] but when the development of
the coelom came to be more thoroughly worked out in _Balanoglossus_ and
Amphioxus, the striking resemblance in this respect between the two
forms gave additional support to the Batesonian view.[423]

[386] The stages in the development of microscopical
technique are well summarised by R. Burckhardt,
_Geschichte der Zoologie_, p. 121, Leipzig 1907.

[387] "Entwickelungsgeschichte des Amphioxus lanceolatus,"
_Mem. Acad. Sci. St Petersbourg_ (Petrograd) (vii.),
xi., No. 4, 1867, 17 pp., 3 pls.

[388] "Weitere Studien ue. die Entwickelungsgeschichte des
Amphioxus lanceolatus," _Arch. fuer mikr. Anat._, xiii.,
pp. 181-204, 1877.

[389] Particularly by Hatschek (1881) and Boveri (1892).

[390] "Entwickelungsgeschichte der einfachen Ascidien,"
_Mem. Acad. Sci. St Petersbourg_ (Petrograd), (vii.),
x., No. 15, 1866, 19 pp., 3 pls. "Weitere Studien ue. die
Entwicklung der einfachen Ascidien," _Arch. f. mikr.
Anat._, vii., pp. 101-130, 1871.

[391] _Descent of Man_, i., p. 205, 1871.

[392] _Arch. f. mikr. Anat._, vi., 1870, and viii., 1872.

[393] _Archives de Biologie_, 1884, 1885, and 1887.

[394] _Bull. Acad. Sci. St Petersbourg_ (Petrograd) xiii.,
1869, and _Zeits. f. wiss. Zool._, xxii., 1872.

[395] _Mem. Acad. Sci. St Petersbourg_(Petrograd)(7),
xix., 1873.

[396] Giard, _Arch. zool. exper. gen._, i., 1872, and
Lacaze-Duthiers, _ibid._, iii., 1874.

[397] For the later history of the Amphioxus-Ascidian
theory the reader may be referred to A. Willey's
well-known work, _Amphioxus and the Ancestry of the
Vertebrates_, New York and London, 1894, and to Delage
et Herouard, _Traite de Zoologie concrete_, Tome viii.,
Paris, 1898.

[398] "Studien zur Urgeschichte des Wirbelthierkoerpers,"
_Mittheil. Zool. Stat. Neapel_, 1882-1907.

[399] Leydig (_Vom Baue des thierischen Koerpers_,
Tuebingen, 1864), who, in a measure, forestalled Dohrn
and Semper by comparing Vertebrates with reversed
Arthropods, specially insects, supposed the old mouth to
pass between the _crura cerebri_.

[400] _Zeits. f. wiss. Zool._, xliv., 1886.

[401] Quoted by E. B. Wilson, _Wood's Holl Biological
Lectures for 1894_, p. 121.

[402] _Cf._ Metschnikoff, _Quart. Journ. Microsc. Sci._,
xxiv., pp. 89-111, 1884.

[403] "Die Stammesverwandschaft der Wirbelthiere und
Wirbellosen," _Arb. zool.-zoot. Instit. Wuerzburg_, ii.,
pp. 25-76, 1875; "Die Verwandschaftsbeziehungen der
gegliederten Thiere," _Ibid._, iii., pp. 115-404,
1876-7.

[404] Abuse of Cuvier also dates from the early days of
evolution, see Radl, ii., pp. 12-17.

[405] "On the origin and history of the urino-genital
organs of Vertebrates," _Journ. Anat. Phys._, x., 1876.
The conclusions of Balfour and Semper were adversely
criticised by M. Fuerbringer (_Morph. Jahrb._, iv.,
1878), and were negatived by later research.

[406] _A Monograph on the Development of Elasmobranch
Fishes_, London, 1878.

[407] _A Treatise on Comparative Embryology_, vol. ii., p.
311, London, 1881.

[408] _Loc. cit._, vol. ii., p. 327.

[409] "On the Ancestral Form of the Chordata," _Q.J.M.S._,
xxiii., 1883. "The Relation of the Nemertea to the
Vertebrata," _ibid._, xxvii., 1887. Hubrecht gives the
credit for the first indication of the relationship of
Nemertines and Vertebrates to Harting (_Leerboek van de
Grondbeginselen der Dierkunde_, 1874).

[410] "Monographie der Capitelliden des Golfes von
Neapel," _Fauna u. Flora des Golfes von Neapel_, Monog.
xvi., Berlin, 1887.

[411] _Mitt. Zool. Stat. Neapel_, vii., 1887.

[412] _Nature_, xxxvi., p. 162, 1887.

[413] "Nebendarm und Chorda dorsalis," _Nachr. Ges. Wiss.
Goettingen_, p. 390, 1885.

[414] "Embryologische Studien an Wuermern u. Arthropoden,"
_Mem. Acad. Sci. St Petersbourg_ (Petrograd), (7), xvi.,
1870. And in _Arch. f. mikr. Anat._, vii., p. 122, 1871.

[415] "The Old Mouth and the New," _Anat. Anz._, iii.,
1888. _Nature_, xxxix., 1889.

[416] "Recherches sur la Morphologie des Tuniciers,"
_Arch. de Biol._, vi., 1887.

[417] "Die Stellung u. Bedeutung der Morphologie," _Morph.
Jahrb._, i., pp. 1-19, 1876.

[418] "Anatomie des Balanoglossus," _Mem. Acad. Sci. St
Petersbourg_ (Petrograd), (7), x., 1866.

[419] _Zeit. f. wiss. Zool._, xx., 1870. For a recent view
of the relation of the Enteropneusta to the Echinoderma,
see J. F. Gemmill, _Phil. Trans._ B., ccv., pp. 213-94,
1914.

[420] In a series of papers published in 1884-6, the
speculative results being discussed in his memoir on
"The Ancestry of the Chordata," _Q.J.M.S._ (n.s.), xxvi.,
pp. 535-71, 1886.

[421] Reprinted in _Zoological Articles_, London, 1891.

[422] "Die Enteropneusten des Golfes von Neapel," _Fauna
und Flora des Golfes von Neapel_, Monog. xviii., Berlin,
1893.

[423] See Macbride, "A Review of Prof. Spengel's Monograph
on Balanoglossus," _Q.J.M.S._, xxxvi., 1894, and "The
Early Development of Amphioxus," _Q.J.M.S._, xl., 1898.

CHAPTER XVI

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).

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