XIII 



PHYLUM CHORDATA 



209 



nephridial system is still uncertain. The ovaries are also of the 

 full length of the abdominal cavity and are much wider than the 

 testes : they are covered with peritoneum on their inner or mesial 

 faces only, and the numerous ova, which are about 4 mm. in diameter, 

 are discharged when ripe from their outer faces into the ccelome. 

 There are no oviducts, but the anterior wall of the urinogenital 

 sinus is pierced by a pair of genital pores 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. 



CL.S.C 



p.s.c 



rr 



n 



FIG. 879. Salmo fario. The right auditory 

 organ, from the inner side ; the otoliths are 

 shown separately below, a. s. c. anterior semi- 

 circular canal ; and. nv. auditory nerve ; h. s. c. 

 horizontal canal ; ot. 1 3, otoliths ; p. s. c. 

 posterior canal ; sac. sacculus ; ut. utriculus. 



UT 



FIG. 880. Salmo fario. The kid- 

 neys and adjacent parts, d, pre- 

 caval vein ; R (to the right), 

 kidney ; R (to the left), degenerate 

 anterior portion of kidney ; rr, 

 efferent renal vein ; s. subclavian 

 vein ; u, ur, ureter ; v, bladder. 

 (From Gegenbaur's Comparative 

 Anatomy.) 



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 micropyle, through which the sperms find access : it 

 is formed of a superficial layer of protoplasm surrounding a mass of 

 transparent fluid yolk of a pale yellow colour. At one pole the 

 protoplasm accumulates to form an elevated area or germinal disc, 

 in which segmentation takes place (Fig. 881, A, B) in much the 

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

 proportion of yolk, the resulting blastoderm (bl.) and the embryo 

 formed therefrom are proportionally much larger, and the yolk-sac 



