UROGENITAL SYSTEM. 307 



adenoid tissue (reticular connective tissue crowded with leucocytes). 

 Well-known among these structures are the so-called fat bodies (cor- 

 pora adiposa) connected with the gonads of the amphibia, and the 

 'hibernating glands' of some rodents and insectivores, which con- 

 sist of richly vascularized masses of fat. In the lymph nodules this 

 adenoid tissue is enmeshed in a rete mirabile of lymph vessels. In 

 the blood-lymph glands there is a somewhat similar relation to blood- 

 vessels as well, for the details of which reference must be made to 

 histological text-books. These lymph structures, which occur at 

 various points of the body, are apparently places for the formation of 

 leucocytes (lymphocytes). 



The spleen, attached to the mesentery near the stomach and pan- 

 creas, is intermediate in some respects between lymph and blood- 

 lymph glands and is the largest lymph structure in vertebrates. It 

 is developed in the walls of the alimentary canal and is said to have 

 an entodermal origin. Later it separates from the stomach and 

 assumes its definitive position. It serves, apparently, as a place for 

 the disintegration of the red blood corpuscles in addition to functioning 

 as a leucocyte-forming organ. 



The tonsils (p. 247) belong to the category of adenoid glands. 

 There are two kinds of these, the pharyngeal and the palatine tonsils, 

 the latter occurring between the inner ends of the Eustachian tubes of 

 amniotes, the palatine (best developed in mammals) are paired struc- 

 tures on either side of the pillars of the fauces. Other tonsil-like 

 structures occur at different points of the floor and roof of the mouth 

 of the tetrapoda. 



THE UROGENITAL SYSTEM. 



In several phyla of the animal kingdom there is an intimate relation 

 between the reproductive and excretory organs, the ducts of the latter 

 serving either to carry the products of the gonads directly to the ex- 

 terior or acting as brood organs where a portion of the development 

 of the egg takes place. This close association of the two systems is 

 especially marked in most vertebrates and hence this section is headed 

 Urogenital System, because of the difficulty of treating the two com- 

 ponents separately. 



The urinary or excretory organs have for their purpose the elimina- 

 tion of the nitrogenous waste (and occasionally other products) from 



