200 MORPHOLOGY OF THE ORGAA'S OF VERTEBRATES. 



are developed in the larger lymph trunks, preventing any back 

 flow. 



In connection with the lymph system lymphoid tissue is de- 

 veloped, especially around 

 the genital organs of the 

 ichthyopsida, where, as in 

 the amphibia and reptiles, 

 this forms the prominent 

 'fat bodies.' Aggrega- 

 tions of such lymphoid tis- 

 sue give rise to lymph 

 glands, which are variously 

 distributed through the ver- 

 tebrate body. Of these 

 the most prominent is the 

 spleen, the cells of which 

 are said to arise, in the 

 tadpole, from the ento- 

 derm. It usually is some- 

 what close to the stomach, 

 and in Protopterus (Fig. 40, sp) it is still enclosed in the gas- 

 tric walls. Among other lymph glands may be mentioned the 

 tonsils, which occur at the beginning of the pharynx in reptiles, 

 birds, and mammals. 



Fig. 210. Urogenital organs and fat 

 bodies of adult frog. C, cloaca ; F, fat body ; 

 M, mesonephros ; P', postcava ; T, testis ; U, 

 ureters. 



