GASTR^A THEORY 289 



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 Gastrsea theory there followed at once two 

 consequences, (i) that ectoderm and endoderm, invagination- 

 cavity ( Urdarni] and gastrula-mouth (Unnundor Protostoina), 

 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 Ccelentera 

 were homologous with the ectoderm and endoderm of the 



o 



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 implica- 

 tions 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 Kalkschw dunned 



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, 



ibid., ix., pp. 402-508, 1875. "Die Physemarien, Gastneaden der 

 Gegemvart," and " Nachtrage zur Gastrasa-Theorie,' ; ibid., x., pp. 55-98, 

 1876. Republished in Biologiiche Studien, 2nd part, Studicn zur 

 Gasirtza-Thcoric, 270 pp., 14 pis., Jena, 1877. 

 1 See Ann. Mag. Nat. Hist. (4), xi., p. 253. 



