114 



ZOOLOGY 



SECT. 



those on the one side are numerous, clear, narrow, arranged 

 parallel with one another, and provided with cilia at their free 

 ends ; while those on the other are fewer and larger, of rounded 

 shape, coarsely granular and devoid of cilia : between these two sets 



FIG. 82. Development of Sycon raphanus. a, ovum ; 6, c, ovum segmented 6, as seen from 

 above, c, lateral view; d, blastula ; c, amphiblastula ; /, commencement of invagination ;. 

 g, gastrula attached by its oral face ; /(, i, young sponge A, lateral view ; i, as seen from above. 

 (From Sollas, after Schulze.) 



of cells is a cavity in which are a few cells the beginning of the 

 middle layer. The clear cells next become pushed in or in- 

 vaginated within the granular cells (Fig. 82, /) so that the embryo 

 becomes converted into a double-walled cup the gastrula the 

 outer layer of the wall of which is formed by the granular ectoderm 

 cells, and the inner by the clear endoderm. The flagella of the 



