210 



ZOOLOGY 



SECT. 



(y.s.) correspondingly smaller, than in 

 A B 



em. 



y.s 



y.s 



FIG. 881. Nine stages in the development of Salmo 

 fario. A H, before hatching ; I, shortly after 

 hatching. W. blastoderm ; emb. embryo ; r. thickened 

 edge of blastoderm ; y. s. yolk-sac. (A G after 

 Henneguy.) 



tail become free from the yolk, and at 

 yolk-sac (7, y.s.) is a shoe-shaped body 

 surface of the transparent embryo. 



the two previous classes. 

 Epiboly takes place as 

 in Elasmobranchs, the 

 blastoderm gradually 

 growing round and en- 

 closing the yolk (C-F). 

 The embryo (emb.) arises 

 as an elevation growing 

 forwards from the thick- 

 ened edge of the blasto- 

 derm, and, as it in- 

 creases in length, ap- 

 pears as a clear colour- 

 less band (//, emb.) 

 winding round the 

 yellow yolk, and 

 kept in close contact 

 with it by the enclosing 

 zona radiata. There is 

 no open medullary 

 groove, the nervous 

 system being formed, 

 as in Lampreys, from a 

 fold of ectoderm the 

 walls of which are in 

 apposition so as to form 

 a keel-like ridge. The 

 endoderm and meso- 

 derm are formed as a 

 result of a process of 

 infolding of the pos- 

 terior edge of the 

 blastoderm (Fig. 882). 

 Gradually the head and 



the time of hatching the 

 sessile upon the ventral 



ec 



en+ma 



FIG. 882. Longitudinal section of blastoderm of Salmo, at about the stage represented in 

 D of Fig. 881. ec. ectoderm ; en + ms, infolding giving rise to endoderm and mesoderm. 

 (After O. Hertwig.) 



