CHAPTER LIV. 



THE UROGENITAL SYSTEM. 



As has already been noted, not only in the frog but in several of the 

 type-forms studied, there is an intimate connection between the excretory 

 and the reproductive systems. In fact this is so intimate that it is im- 

 possible to take up either subject without touching upon the other. For 

 this reason it is customary to treat both under the head of the Urogenital 

 System. The excretory organs consisting of the paired kidneys, or 

 nephridic organs and their ducts, serve the purpose of casting out of the 

 body the waste matter containing nitrogen,, and occasionally other 

 substances. 



The gonads (ovaries or testes) are the reproductive glands. To any 

 and all of these, accessory structures are frequently added. The nephri- 

 dic organs proper have already been quite fully described in the frog, a 

 review of which is essential to the understanding of that which follows. 



It will be remembered that the kidneys are parenchymatous glands, 

 being composed of a soft, more or less spongy tissue in which there is a 

 profuse quantity of blood. The great quantity of blood is sent through 

 the tiny venules which anastamose with the arterial capillaries in the 

 Malpighian corpuscles (Fig. 16.) 



Some of the typical parts which go to make up the kidneys of higher 

 forms are lacking in certain groups of animals. In the amniotes, neph- 

 rostomes are never formed, although they do occur in most ichthyopsida. 

 In the pronephros the Malpighian corpuscle is rudimentary or lacking at 

 all stages, while there is no differentiation of convoluted tubules and 

 Henle's loop. 



Professor Kingsley's excellent account of the urogenital system 

 is followed here. 



Theoretically the function of the various parts of the nephridial 

 tubules is in outline as follows : In the primitive condition the nitro- 

 genous waste, is elaborated in the liver, collected in the coelom and, 

 together with the coelomic fluid is passed outward through the neph- 

 rostomes and the tubules, which act merely as ducts. "Higher in the 

 scale the parts become more differentiated and specialized. The renal 

 corpuscles form a filtering apparatus by which water is passed from the 

 blood-vessels of the glomerulus into the tubules near their beginning, 

 and this serves to carry out the urea, uric acid, etc., secreted by the 

 glandular portions of the walls of the tubules (convoluted tubules, 

 ascending limb of Henle's loop)." 



"All three nephridia arise from the mesomeric somites or from the 

 Wolffian ridge which appears on either side of the median line where the 

 mesomeres separate from the rest of the wall of the body cavity, the 



