THE FROG — A REPRESENTATIVE VERTEBRATE 



413 



by a hormone from the pituitary gland (section 192). This mechanism 

 for discharging eggs from the ovary is quite different from that in mam- 

 mals, in which an accumulation of liquid within the follicle causes it 

 to rupture. The eggs pass into the pleuroperitoneal cavity, and are 

 carried anteriorly by the action of peritoneal cilia (present only in 

 females) toward the openings (ostia) of the paired oviducts. As the eggs 

 are carried down these highly coiled tubes by the beating of cilia within 

 the ducts, they are covered with several layers of a jelly-like albumin 

 secreted by certain oviducal cells. Just before entering the cloaca, each 

 oviduct expands to form a thin-walled ovisac where the eggs are stored 

 for a short time until mating takes place. 



Sperm are produced in numerous, microscopic seminiferous tubules 

 within the testes. During the breeding season, under the stimulus of a 

 pituitary hormone, the mature sperm leave the testis through minute 



1 f-H^ 



f t, < \ ?fs 



« 





Fai jbody 



Testis~r 



Adrenal 



^land 



Renal vein 



Wolffian duct ^ 



Entrajice of 

 Wolffiajiduct'l 



Cloaca-^ 



""'§»l Posterior 



vena. Cava. 



Testis 

 mesentery 



*f- Kidney 

 4 

 -^^Vestigial 



oviduct 



Urinary bladder 



%r Seminal vesicle 



Entrance of 



'■'%. ve^sti^ial 



oviduct 



Figure 21.15. Ventral view of the urogenital system of a male frog. The vestigial 

 oviduct shown in this figure is not always present. 



