216 



ZOOLOGY 



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- 

 pyle, through which the 

 sperms find 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- 

 mincd disc, in which seg- 

 mentation takes place 

 (Fig. 870, ^,5). in much 

 the same way as in 

 Elasmobranchs, except 

 that, owing to the 

 smaller proportion of 

 yolk, the resulting blas- 

 toderm (hi) 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 grov/ing 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. 

 Tliere is no open medullary groove, the nervous system being 

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



amh 



2/s 



Fig. 870. — Nine stages in tlio development of Saliuo 

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

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

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



