FIG. 216 GD 



218 



219 



Y CH 



Figs. 216-230. Development of shark, Scyllium (mainly). (All but 216 after BAL- 

 FOUR.) 216. Egg freed from case showing germ disc GD. 217. Germ disc at second 

 cleavage. 218. Germ disc at fifth (?) cleavage. 219. Vertical section of similar stage. 

 220. Vertical section of slightly older germ disc. 221. Blastula. 222. Early gastrula. 

 223. Blastoderm showing early growth of embryo. 224-226. Slightly later stages of growth 

 of embryo. 227. Stage showing early embryo and mode in which the blastoderm sur- 

 rounds yolk. 228. Early embryo viewed as a transparent object. 229, 230. Transverse 

 sections of early embryo. 



A. Anal invagination. A U. Auditory vesicle. BP. Dorsal lip of blastopore. C. 

 Coelenteron. CF. Tail folds. CH. Notochord. CP. Cephalic plate. EC. Ectoderm. 

 EN. Entoderm. G. Gut. GD. Germ disc. GS. Gill slits. H. Heart. HE. Head 

 eminence. M, Central nervous system. M' '. Yolk nuclei, merocytes. MES. Mesoblast. 

 NC. Neurenteric canal. OP. Optic vesicle. PS, Primitive segments. .S. Mouth pit, 

 stomodaeum. SC. Segmentation cavity. 



194 



