THE SHAPES AND SIZES OF ANIMALS 125 



cells enclosing a central space filled with fluid. 

 The blastula now becomes flattened on one side 

 and the flattened side becomes doubled up within 

 the rounded part, so that the larva now assumes 

 the form of a hemispherical cup, the walls of which 

 consist of two layers of cells, outer and inner, 

 between which is a narrow chink-like space con- 

 taining fluid, which is really the last disappearing 

 remnant of the blastula cavity of the earlier stage. 

 The cup-shaped larva is spoken of as a gastrula. 

 A gastrula developing in this fashion is said to 

 be formed by invagination. Such an invaginate 

 gastrula is of very wide occurrence, occurring as an 

 early larval stage in members of all the large 

 groups of the animal kingdom above the Protozoa 

 i.e., in Sponges, Ccelenterates, Echinoderms, Worms, 

 Molluscs, Arthropods, and Vertebrates. 



The second mode, referred to above, in which a 

 gastrula is formed is by what is called delamination. 

 The starting-point, the egg, is the same as before, 

 and so also is the gastrula itself, for the delaminate 

 and invaginate gastrulae, though formed in entirely 

 different ways, cannot always be distinguished 

 from each other. In the development of the 

 delaminate gastrula the egg segments, giving rise 

 to a solid heap of cells, the morula ; and this 

 becomes a blastula as before. Each cell of the 

 blastula now divides into inner and outer parts, so 

 that the blastula wall becomes double, consisting 

 of outer and inner layers of cells, surrounding a 

 central cavity filled with fluid. By perforation of 

 one pole the cavity is placed in communication 



