Chap. XII.] 



THE CRAYFISH. 



253 



2. The central cell in the ovisac grows at the expense of the 

 others, and becomes the ovum. Certain of the others form a 

 layer round it, and constitute the epithelium of the ovisac (ep.) 



"Hfl. arch. U. 



FIG. 79. CRAYFISH DEVELOPMENT. 



A. Spermatozoon. B. Ovum. ep. fo. Epithelium of follicle, g. s. Ger- 

 minal spot, g. v. Germinal vesicle, m. p. Membrana propria. v. m. Vitelline 

 membrane. 



C. Ovum in which segmentation has produced a blastodermic layer, 6.7. 

 v. m. Vitelline membrane. 



D. Invagination of blastodermic layer. arch. Archenteron. 1)1. Blastopore. 

 ep. Epi blast, hy. Hypoblast. tn. Mesoblast. 



E. Formation of fore and hind-gut. arch. Mesenteron. pr. Proctod&um. 

 st. Stomodseum. 



F. View of developing embryo. ab. Abdomen, m. Stomodseal invagina- 

 tion for the mouth, op. Optic pit. pr. I. Procephaliclobe. 2,3,4. Second, 

 third, and fourth appendages. 



3. The nucleus of the ovum becomes the germinal vesicle (g. v.). 



4. Within it there appear numerous small corpuscles, the 

 germinal spots (g. s.). 



