[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 Wirbelthierkorpers,"

_Mittheil. Zool. Stat. Neapel_, 1882-1907.

[399] Leydig (_Vom Baue des thierischen Korpers_, Tubingen, 1864), who, in a measure, forestalled Dohrn and Semper by comparing Vertebrates with reversed Arthropods, specially insects, supposed the old mouth to pa.s.s 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. Inst.i.t. Wurzburg_, 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. Furbringer (_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_, x.x.xvi., p. 162, 1887.

[413] "Nebendarm und Chorda dorsalis," _Nachr. Ges. Wiss.

Gottingen_, p. 390, 1885.

[414] "Embryologische Studien an Wurmern 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_, x.x.xix., 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._, x.x.xvi., 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 inv.a.g.i.n.ation, of a two-layered sac, whose cavity became the alimentary ca.n.a.l. 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 inv.a.g.i.n.ation-opening often became the definitive a.n.u.s. 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 inv.a.g.i.n.ation, 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 repet.i.tion of the ancestral form of all Metazoa, the Gastraea.

From the Gastraea theory there followed at once two consequences, (1) that ectoderm and endoderm, inv.a.g.i.n.ation-cavity (_Urdarm_) and gastrula-mouth (_Urmund_ or _Protostoma_), were, with all their derivatives, h.o.m.ologous, because h.o.m.ogenous, 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 h.o.m.ologous 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 cla.s.sification 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 _Kalkschwamme_.[434]

_Monophyletic Genealogical Tree of the Animal Kingdom, based upon the Gastraea Theory and the h.o.m.ology 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 h.o.m.ologous 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 cla.s.sification of animals were published by Sir E. Ray Lankester in 1873.[436] He distinguished three grades of animals--the h.o.m.oblastica, 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 a.s.sert 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 a.s.sociation 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 inv.a.g.i.n.ation 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.