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‘Figs. 249-268. — Development of Ganoids, Acifenser and (last four figures) Lefi- 
dosteus. X about 12, 249. immediately before cleavage. 250. Second cleavage. 
251. Third cleavage. 252. Blastula, 253. Vertical section of blastula, 254. Early 
gastrula, 255. Late OY gee 256. Vertical section of late gastrula. 257. Early 
obs 258. Sagi section of same stage. 259, 260. Head and tail regions of 
slightly later embryo. 261. Transverse body section of hinder body region of same 
—. 262, 263. Head and tail regions of late embryo. 264. Embryo immediately 
hatching. 265. idosteus’ blastula. 266. Vertical section of early gastrula. 
267. Late gastrula. 268. bryo, showing mode of separation from yolk. 
BP. Dorsal lip of blastopore, C. Coelenteron. ZC. Ectoderm, ZN, Entoderm. 
¥. Pectoral fin. GS, Gill slits. AY. Heart. HZ. Head eminence. AV. Kupffer's 
vesicle, LC. Marginal limit of coelenteron. A/, Mouth pit. A/C. Medullary canal. 
MES. Mesoblast. NC. Neurenteric canal. OZ. Olfactory pits. OP. Optic vesicles. 
PN. Primitive kidney, pronephros. PS. Primitive segments. SC. Segmentation 
cavity. 7. Tail eminence. VZ. Ventral lip of blastopore. Y. Yolk, yolk mass. 
YP. Yolk plug. 
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