216 



ZOOLOGY 



SECT. 



through wliich tlio ova make their way to the exterior. There is 

 reason for thinking that these pores are to be looked u])on as 

 degenerate oviducts, and in no way homologous Avith the 

 abdominal pores of Elasmobranchs. 



Development. — Impregnation is external, the male shedding 

 his milt or seminal fluid on the newly-laid eggs. The ovum 



is covered by a thick 

 membrane, the zona 

 radiata, perforated by 

 an aperture, the riiirro- 

 pf/Ie, through which the 

 - sperms tind access : it 

 is formed of a super- 

 ficial layer of proto- 

 plasm surrounding a 

 mass of transparent 

 fluid yolk of a pale 

 yellow colour. At one 

 pole the protoplasm ac- 

 cumulates to form an 

 elevated area or ger- 

 minal disc, in which see:- 

 mentation takes place 

 (Fig. 870, yl,^) in much 

 the same way as in 

 Elasmobranchs, except 

 that, owing to the 

 smaller proportion of 

 yolk, the resulting blas- 

 toderm (Jbl) and the 

 embryo formed there- 

 from are proportionally 

 much larger, and the 

 yolk - sac {y.s.) corre- 

 spondingly smaller, than 

 in the two previous 

 classes. Epiboly takes 

 place as in Elasmo- 

 branchs, the blastoderm 

 gradually growing 

 round and enclosing the yolk (C-F). The embryo {emh) arises 

 as an elevation growing forwards from the thickened edge of 

 the blastoderm, and, as it increases in length, appears as a clear 

 colourless band (H, emh) winding round the yellow yolk, and 

 kept in close contact with it by the enclosing zona radiata. 

 There is no o^^en medullary groove, the nervous system being 

 formed, as in Lampreys, from a fold of ectoderm the walls of 



e-Tizb 



y.s 



y.s 



Fio. S70. — Nine stages in the development of Salmo 

 fario. A— H, before hatching ; I, shortly after l:aii-h- 

 ing. hi. blastoderm ; emh. embryo ; r. thickened edge 

 of blastoderm ; y. s. yolk-sac. (A — G after Hcnneguy.) 



