352 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



chorion. In the cotyledonary placenta the villi are grouped in small 

 areas (cotyledons) with spaces of naked chorion between them. This 

 form is characteristic of the ruminants. The deciduate type includes 

 the zonary and the discoidal forms. In the zonary placenta (eden- 

 tates, sirenians, elephants, hyracoids and carnivores) the villi form a 

 girdle around the placental sac, the ends of the chorion being free from 

 them. In the discoidal forms (insectivores, rodents, bats, edentates, 

 primates) the villi are restricted to one side of the chorion. 



ADRENAL ORGANS. 



Under this heading are included two sets of structures, interrenals 

 and suprarenals, of uncertain morphology and function. The names 

 are given in allusion to the fact that they are usually closely associated 

 in position with the nephridial structures, though they have no other 

 relation to them. The two differ in structure and probably in function 

 and are very distinct in the lower vertebrates but in amphibia and 

 amniotes they are united in a common structure, the interrenals forming 

 the cortex, the suprarenals the medulla of the mammalian adrenals. 



The interrenals arise from the ccelomic epithelium but it is as yet 

 uncertain as to the details, some thinking that they are connected with 

 the pronephros, others with the mesonephric structures, while still others 

 regard them as distinct in origin. They are at first either isolated 

 clusters of cells or longer bands of cells near the dorsal margin of the 

 mesentery, sometimes bilaterally symmetrical and in the lower verte- 

 brates extending through the length of the cpelom. 



The suprarenals find their anlage in the sympathetic ganglia, from 

 which certain cells early separate. Among these are peculiar cells 

 which are called chromaffin cells (chromaphile or phaeochrome 

 cells) because of their staining brown or yellow with chromic acid 

 salts. These usually are closely associated with the blood-vessels, 

 either the dorsal branches of the segmental arteries or the postcardinal 

 veins. 



In the fishes the two organs are separate, the suprarenals often 

 being more or less metameric in character, and in close relations to 

 the vessels of the mesonephros. The interrenals form more compact 

 organs between the nephridia of the two sides. In all tetrapoda the 

 two organs are more closely associated, the tissues of the two being 

 mixed in the adults of the amphibia and reptiles, while in the mammals 



