URINOGENITAL SYSTEM OF SKATE 



605 



-ov. 



thyroid gland lies just in front of the anterior end of the ventral aorta. 

 The thymus is represented by a whitish body dorsal to each of the first 

 four gill-clefts. Each begins as a patch of endoderm, and this is 

 invaded by migratory mesenchyme cells which multiply as lymphocytes. 



Excretory and reproductive systems. — The dark red 

 kidneys lie far back on each side of the vertebral column. 

 They are developed 

 from the hind part 

 of the mesonephros. 

 Several tubes from 

 each kidney combine 

 to form a ureter. The 

 two ureters of the 

 male open into the 

 urinogenital sinus, 

 whence the waste pro- 

 ducts pass out by the 

 cloaca ; in the female 

 they open into little 

 bladders — the dilated 

 ends of the Wolffian 

 ducts — and thence by 

 a common aperture 

 into the cloaca. 



The segmental duct 

 of each side divides 

 into Wolffian and 

 Miillerian ducts. The 

 Wolffian duct becomes 

 in the male the vas 

 deferens, in the female 

 it is an unimportant 

 Wolffian duct ; the 

 Miillerian duct be- 

 comes in the female the oviduct, in the m.ale it is a mere 

 rudiment. 



The muscles and other organs of Elasmobranchs retain 



considerable quantities of nitrogenous waste products. 



There can be no doubt that the body cavity helps in excretion, and 

 gets rid of waste through the two abdominal pores. In some Elasmo- 

 branchs these are replaced by openings (nephrostomes) into the kidney. 

 Occasionally there are both nephrostomes and abdominal pores. 



Fig. 



342. — Urinogenital organs of female 

 skate. — In part after ]\Ionro. 



ap., Aperture of united oviducts: W.D., Wolffian 

 duct; ov., ovary; O.D.G., oviducal gland; 

 E., egg in mermaid's purse ; Bl., bladder at 

 base of Wolffian ducts (arrow into cloaca) ; A'., 

 kidney (arrow from base of oviduct into cloaca). 



