Embryology. 139 
thisstage. At any rateit has been found to occur in all 
the main divisions of the animal kingdom, asa glance at 
the accompanying figures will serve to show (Fig. 42) '. 
Moreover many of the lower kinds of Metazoa never 
pass beyond it; but are all their lives nothing else than 
Fic. 43.—Gastrula of a Chalk Sponge. (After Hiackel.) A, External 
view. B, Longitudinal section. g, digestive cavities; 0, mouth, 
z, endoderm ; é, ectoderm. 
gastrulz, 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more or less superseded by another, which is called delamination; but 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 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 impiovement upon, gastrulation. 
