FIG. 249 



250 



251 



252 



261 ^£_^^ MES 262 Q|_ H T M 263 



264 



Figs. 249-268. — Development of Ganoids, Acipenser and (last four figures) Lepi- 

 dosteiis. X about 12. 249. Egg immediately before cleavage. 250. Second cleavage. 

 251. Third cleavage. 252. Blastula. 253. Vertical section of blastula. 254. Early 

 gastrula. 255. Late gastrula. 256. Vertical section of late gastrula. 257. Early 

 embryo. 258. Sagittal section of same stage. 259, 260. Head and tail regions of 

 slightly later embryo. 261. Transverse body section of hinder body region of same 

 stage. 262, 263. Head and tail regions of late embryo. 264. Embryo immediately 

 before hatching. 265. Lepidosteiis' blastula. 266. Vertical section of early gastrula. 

 267. Late gastrula. 268. Embryo, showing mode of separation from yolk. 



BP. Dorsal lip of blastopore. C. Coelenleron. EC. Ectoderm. EN. Entoderm. 

 F. Pectoral fin. OS. Gill slits. H. Heart. HE. Head eminence. KV. Kupffer's 

 vesicle. LC. Marginal limit of coelenteron. M. Mouth pit. MC. Medullary canal. 

 MES. Mesoblast. NC. Neurenteric canal. OL. Olfactory pits. OF. Optic vesicles. 

 PN. Primitive kidney, pronephros. PS. Primitive segments. SC. Segmentation 

 cavity. T. Tail eminence. VL. Ventral lip of blastopore. Y. Yolk, yolk mass. 

 YP. Yolk plug. 



20^ 



