200 



ZOOLOGY 



SECT. 



is placed in the utriculus close to the ampullae of the anterior 

 and horizontal canals. 



Urino-genital Organs. The kidneys (Fig. 817, M., and Fig. 

 821,72.) are of great size, extending the whole length of the dorsal 

 wall of the abdomen, above the air bladder, and partly fused 

 together in the middle line. They are derived from the meso- 

 nephros of the embryo. Their anterior ends (Fig. 817, kd, Fig. 

 821, 72) are much dilated and consist in the adult of lymphatic 



tissue, thus ceasing to discharge a renal 

 function. The ureters (meson ephric ducts, 

 ur.) unite into a single tube, which is 

 dilated to form a urinary bladder (Fig. 

 817, u. bL, Fig. 821, v f ), and discharges 

 into the urino-genital sinus. 



The gonads are of great size in the 

 sexually mature fish. The testes (Fig. 

 817, ts.) are long, smooth, pinkish, paired 

 organs, extending the whole length of 

 the abdominal cavity ; each is continued 

 posteriorly into a duct (v. df.) which opens 

 into the urino-genital sinus, and the homo- 

 logy of which with the ducts of the 

 primitive nephridial system is still un- 

 certain. 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. 



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 (Fi^. 822, A,E) in 



UT 



FIG. 821. Salmo fario. The 



kidneys and adjacent parts. 

 (?, pre-caval vein ; R (to the 

 right) kidney ; R (to the left), 

 degenerate anterior portion 

 of kidney ; rr, efferent renal 

 vein ; #. subclavian vein ; u, 

 ur. ureter ; v, bladder. (From 

 Gegenbaur's Comparative Ana- 

 tomy.) 



