Embryology. 



139 



this stage. At any rate it has been found to occur in all 

 the main divisions of the animal kingdom, as a glance at 

 the accompanying figures will serve to show (Fig. 4 2) *. 

 Moreover many of the lower kinds of Metazoa never 

 pass beyond it ; but are all their lives nothing else than 



FIG. 43. Gastrula of a Chalk Sponge. (After Hackel.) A, External 

 view. B, Longitudinal section, g, digestive cavities; o, mouth ; 

 i, endoderm ; e, ectoderm. 



gastrulae, wherein the orifice becomes the mouth of 

 the animal, the internal or invaginated layer of cells 

 the stomach, and the outer layer the skia So that 

 if we take a child's india-rubber ball, of the hollow 



1 In most vertebrated animals this process of gastrulation has been 

 more or less superseded by another, which is called delamination ; but 

 it scarcely seems necessary for our present purposes to describe the 

 latter. For not only does it eventually lead to the same result as- 

 gastrulation i.e. the converting of the ovum into a double-walled sac, 

 but there is good evidence among the lower Vertebrata of its being pre- 

 ceded by gastrulation ; so that, even as to the higher Vertebrata, 

 embryologists are pretty well agreed that delamination has been but a 

 later development of, or possibly improvement upon, gastrulation. 



