204 MANUAL OF ANIMAL BIOLOGY 



through the capillaries contained in the Malpighian bodies and 

 those surrounding the convoluted portion of the tubules, urea and 

 various inorganic salts, both dissolved in water, are removed from 

 the blood, passed through the tubules, and finally reach the blad- 

 der through the ureters. 



The composite waste product, urine, consists approximately of 

 96% water, 1.8% dissolved salts, and 2.2% urea. The last is a 

 white crystalline substance, containing more than 46% nitrogen, 

 and has the chemical formula of (NH 2 ) 2 CO. It is, of course, readily 

 soluble in water. It will be remembered that the actual synthesis 

 of urea is accomplished by the liver cells ; its removal from the 

 blood, by the kidney cells. Both the manufacture and the removal 

 of urea must be carried on continuously. Any impairment of 

 these functions by which the metabolic wastes of the cells are 

 removed will result very quickly in a general systemic poisoning. 



5. Reproductive System 



The sexes in Frogs are separate, but there is no great differenti- 

 ation in the external structural characters between the male and 

 female. The fore limb of the male is somewhat stouter and the 

 first digit of each hand is somewhat larger. During the breeding 

 season in the spring this digit becomes further enlarged. (W. pp. 

 188-195.) 



Male Reproductive Organs. The male organs of reproduc- 

 tion consist of a pair of testes in which the sperm are developed, 

 and numerous fine ducts, known as the vasa efferentia, which 

 convey the mature sperm cells from the testes into the kidney 

 tubules. Each testis is a yellowish, capsule-shaped body which 

 lies on the ventral side and near the anterior end of each kidney. 

 The vasa efferentia extend into the kidney substance and there 

 connect with the canal system leading to the urogenital canal 

 which extends from each kidney to the cloaca. The urogenital 

 canals serve as common ducts for the wastes from the kidneys and 

 the sperm from the testes. Attached to each testis is a yellow fat 

 body. The latter are not directly concerned in reproduction, but 

 serve as storehouses for excess nutriment received during the 

 summer season. (W. f. 132.) 



A microscopic examination of a testis shows that it consists 

 of many greatly twisted tubules, together with blood vessels, 

 nerves, and connective tissue elements. The tubules open near 



