2OS THE EVOLUTION OF MAX. 



we saw before (Fig. 23-29). In the latter, the Bell-gastrula 

 (Archigastrula}, the body has but one axis. The primitive 

 intestine is empty, and the opening of the primitive mouth 

 is wide. The skin-layer and the intestinal layer consist 

 each of a single cell stratum. The two lie close together, 

 for the cleavage-cavity has entirely disappeared during the 

 process of unfolding. The amphibian Hood-gastrula (Am- 

 phigastrula) is entirely different (Fig. 32-35; Plate II. 

 Fig. 11). In this the cleavage-cavity (F) continues for a 

 considerable time side by side with the primitive intestinal 

 cavity (N). Yelk-cells fill the greater part of the latter: 

 and they also fill the primitive mouth (yelk-plug, P). Both 

 the intestinal layer (0) and the skin-layer (a) consist of 

 several strata of cells. Finally, the general outline of the 

 entire Gastrula, instead of having only one axis, has three : 

 for the three axes which characterize the bilateral body 

 of the higher animals, are indicated by the eccentric evo- 

 lution of the primitive intestinal cavity. 



In the evolution of the Hood-gastrula (Amphigasti^ula) 

 we are unable to distinguish sharply between the different 

 epochs, which, marked by the mulberry -germ and the germ- 

 vesicle, we saw followed each other in the case of the Bell- 

 gastrula (Archigastrula). The Morula-stage (Plate II. Fig. 

 9) is as indistinctly separated from the Blastu la-stage 

 (Fig. 10), as the latter is from the Gastrula (Fig. 11). But 

 in spite of this, we shall not have much difficulty in retra- 

 cing the whole kenogenetic or vitiated course of evolution 

 of this amphibian Ainphigastrula to the genuine, palin- 

 genetic origin of the Archigastrula of the Amphioxus. 



It is far harder to do this in the case of Mammals, 

 although the course of egg-cleavage and gastrulation in 



