FIG. 231 



232 



233 



234 



235 



236 



237 



238 



240 



242 



239 



244 



245 



246 



247 



Figs. 231-247. Development of lung-fish, Ceratodus. (After SEMON.) X 4-7. 

 231. Egg immediately before cleavage. 232, 233. First cleavage, seen from above and 

 from the side. 234. Second cleavage, seen from above. 235, 236. Third cleavage, 

 seen from above and from the side. 237. Blastula. 238, 239. Gastrulae showing 

 closure of bListopore. 240. Early embryo, seen from the side. 241. Early embryo 

 showing medullary folds (head). 242. Tail region of same embryo. 243. Tail region 

 of slightly later stage. 244. Head region of same embryo. 245-247. Later embryos. 



AV. Auditory vesicles. BP. Blastopore. GS. Gill slits. J\f. Mouth pit. A/F. 

 Medullary folds. O. Olfactory lobes. OP. Optic vesicles. PN. Primitive kidney, 

 proncphros. PS. Primitive segments. Y. Yolk mass. 



199 



