216 



ZOOLOGY 



SECT. 



through which the ova make their way to the exterior. There is 

 reason for thinking that these pores are to be looked upon as 

 degenerate oviducts, and in no way homologous with 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 micro- 

 pyk, through which the 

 sperms rind access : it 

 is formed of a super- 

 ficial layer of proto- 

 plasm surrounding 

 mass of transparem 

 fluid yolk of a pah 

 yellow colour. At one 

 pole the protoplasm ac- 

 cumulates to form an 

 elevated area or ger- 

 minal disc, in which seg- 

 mentation takes place 

 (Fig. 870, A, )i 

 the same way as 

 Elasmobranchs, except 

 that, owing to th( 

 smaller proportion o 

 yolk, the resulting bias 

 toderm (bl.) and th< 

 embryo formed there 

 from are proportionally 



_ -^^f-.) 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 (emb.) 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, cmb.) 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 



e.rrtb 



n 



y.s 



y.s 



FIG. 870. Nine stages in the development of Salrno 

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

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

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



