66 COMPARATIVE ANATOMY 



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 almost inevitable precursor of any adult metazoan which 

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



In Amphibians. Compared to gastrulation in Amphioxus, the corre- 

 sponding process in the amphibian is very complex. In Amphioxus the 

 entire wall of the blastula is thin. The wall of the vegetal half is only 

 slightly thicker than that of the animal half. The blastocoele is very 

 nearly spherical. In the frog the vegetal wall of the blastula (Fig. 39, E-G) 

 is so thick that the vegetal hemisphere is, in effect, solid. It consists of 



A ^ B 



Fig. 43. — Diagrams showing structural similarity of a coelenterate and a gastrula 

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

 digestive cavity; BP, blastopore; E, enteric (digestive) cavity; EC, ectoderm; EN, 

 endoderm; M, mouth; T, tentacle. 



large cells heavily laden with inert yolk. It is not to be expected that such 

 a wall could 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. 44.4 ; /) 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 

 plane determined by the first cleavage. The equator and a zone extending 

 superficially somewhat into the vegetal hemisphere are marked by espe- 

 cially rapid cell-proliferation. It is in this particularly active region, 

 the "germ ring," that the groove appears. Figure 44.4 'represents a section 

 in the median plane of an embryo at this stage. The groove (I) is the 

 result of an invagination which occurs near where the upper thin wall and 

 lower thick wall of the blastula join. The outer layer bounding the 

 invagination consists of smaller cells which have moved inward from 



