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 coelomic 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 ccelom. 
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 pheochrome 
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 
