THE FROG : VISCERA AND VASCULAR SYSTEM 65 



lymph sac above the ccelom and below the backbone. 

 Each consists of a mass of twisted uriniferous tubules, held 

 together by connective tissue and richly supplied with 

 blood vessels. Each tubule begins blindly in the substance 

 of the kidney as a thin-walled Malpighian capsule, whose 

 side is indented by a cluster of blood vessels, the glomerulus, 

 the rest of the tubule being glandular. The glomeruli 

 receive blood only from the renal artery, the tubules also 

 from the renal portal vein. The tubules open into collecting 

 tubes, which run across the kidney to enter the main duct of 

 the organ or Wolffian duct. 1 This lies along the outer edge 

 of the organ and passes backward to open into the dorsal 

 side of the cloaca. Water is excreted in the glomeruli, 

 and solids, chiefly urea, in the glandular part of the tubules. 

 The urine is held in the bladder and voided at intervals. 

 The organs in which the ova and spermatozoa of 



animals are formed are known as gonads. 

 Reproduction. Those in which -- spermatozoa arise are testes; 



those in which ova arise are ovaries. The male 

 organs of reproduction dT*rhe-feQg_are the testes and their 

 ducts. The testes are a pair of ovoict~~bodies slung from 

 the surface of the kidneys, by a fold of the peritoneum known 

 as the mesorchium. Each consists of a mass of seminiferous 

 tubules, in which the spermatozoa are formed (Fig. 402). 

 They communicate by about half a dozen small ducts, the 

 vasa efferentia'm the mesorchium, with the collecting tubules" 

 of the kidney, along which, and, through the Wolffian ducts, 

 the sperm passes to the cloaca^or the male frog has not 

 separate ducts for sperm (vasa deferentid) and for urine, but 

 passes these fluids to the exterior through the same passage. 

 In the male each Wolffian duct has attached to it a sac, 

 the vesicula seminalis, in which the sperm is stored until 

 it is used for fertilising the eggs of the female. In the 

 female, the ovaries correspond in position to the testes, 

 the membrane by which each of them is slung being known 

 as the mesovarium. They ate pleated folds of peritoneum 

 containing eggs in various stages of ripeness held together 

 by connective tissue. In the breeding season they increase 

 in size and shed the ripe eggs into the body cavity, where, 



1 Often called the ureter, although it does not correspond to the 

 ureter of man. 



