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) l . 

 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; 

 *, 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 skin. 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 more or less superseded by another, which is called delamination ; but 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 desaibe 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. 



