REPRODUCTION 



45 



rarely becomes anus. The future motor mechanism, muscle, is derived 

 indirectly from the gastrula layers. 



The gastrula is strongly suggestive of the two-layer body plan of a 

 coelenterate. A simple coelenterate such as Hydra, two-layered through- 

 out, including even the tentacles, can be regarded as a somewhat elab- 

 orated gastrula, the Hydra "mouth" corresponding to the blastopore 

 (Fig. 39). This resemblance, together with the fact that a gastrula stage, 

 modified in one way or another, occurs nearly universally in the develop- 

 ment of metazoan animals, gave rise to Ernst Haeckel's "gastraea" 

 theory which proposed that gastrula-like animals (essentially coelen- 

 terates) must have been the ancestors of all Metazoa. According to 

 this theory, the occurrence of the gastrula form in the ontogeny of a 



Fig. 39. — Diagrams showing structural similarity of a coelenterate and a gastrula. 

 A, Hydra, longitudinal section; B, gastrula, axial section. A, archenteron, prospective 

 digestive cavity; BP, blastopore'; E, enteric (digestive) cavity; EC, ectoderm; £ A'', 

 endoderm; M, mouth; T, tentacle. 



vertebrate is a "repetition" of the coelenterate stage in phylogeny. 

 This may very well be true but it is not necessary to hold this view in 

 order to account for the gastrula stage in ontogeny for some such form 

 as the gastrula is the necessary precursor of any adult metazoan which 

 has a skin (ectoderm) and a digestive tube (endoderm). 



In Amphibians. In the amphibian the vegetal wall of the blastula 

 (Fig. 7,$E-G) is so thick that the vegetal hemisphere is, in. effect, solid. 

 It consists of large cells heavily laden with inert yolk. Such a wall 

 cannot readily bend inward as does the corresponding thin and labile 

 layer of the Amphioxus blastula. 



In the amphibian three processes going on simultaneously effect 

 gastrulation. The beginning of gastrulation is seen when a crescent- 

 shaped groove (Fig. 40 A, /) forms at a certain place on the surface of 

 the blastula. It lies just on the vegetal side of the equator determined 

 by the animal and vegetal poles and extends transversely to the median 



