XIII 



PHYLUM CHORDATA 



201 



much the same way as in Elasmobranchs, except that, owing to 

 the smaller proportion of yolk, the resulting blastoderm (U.) and 

 the embryo formed 

 therefrom are propor- 

 tionally much larger, 

 and the yolk sac (y. s.) 

 correspondingly smaller 

 than in the two pre- 

 vious classes. Epiboly 

 takes place as in Elas- 

 mobranchs, the blasto- 

 derm gradually growing- 

 round and enclosing 

 the yolk (C-F). The 

 embryo (cmb.) arises 

 as an elevation grow- 

 ing forwards from the 

 thickened edge of the 

 blastoderm, and, as it 

 increases in length, ap- 

 pears as a clear colour- 

 less band (H,ew&.) wind- 

 ing 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 Cyclo- 

 stomes, from a fold 

 of ectoderm, the walls 

 of which are in appo- 



y.s 



FIG. 822. Nine stages in the development of Salmo 

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

 ing, bl. blastoderm ; emb. embryo ; r, thickened edge 

 of blastoderm ; ys. yolk-sac. (A G after Henneguy.) 



sition. Gradually the 



head and tail become 



free from the yolk, and 



at the time of hatching the yolk-sac (I, y. s.) is a shoe-shaped 



body sessile upon the ventral surface of the transparent embryo. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Teleostomi are Pisces in which the primary cranium is 

 always complicated by the addition of membrane bones, of which 

 a pair of parietals and one of frontals above, and unpaired vomer 

 and parasphenoid below, are the most constant. The chondro- 

 cranium is always more or less ossified by cartilage bones, and the 

 upper and lower jaws are both bounded by membrane bones. The 



