492 COMPARATIVE ANATOMY 



Placental Mammals passes through stages which resemble succes- 

 sively those which are permanent in Crocodiles and Chelonians 

 and in Monotremes. It arises from a " genital prominence " on the 

 ventral wall of the cloaca. A channel passes along the border 

 facing the cloaca to the opening of the urinogenital sinus : this 

 condition is usually retained throughout life in the case of the 

 clitoris, while in the male (and occasionally in the female also 1 ) 

 the groove becomes closed to form a canal continuous with 

 the urinogenital canal or urethra, which is thus considerably 

 lengthened. In addition to the paired, erectile corpora cavernosa 

 there is a median corpus spongiosum or corpus cavernosum urethrce 

 in connection with the penis (Fig. 370) : corpora cavernosa are also 

 present in the clitoris, and the corpus spongiosum is represented 

 by the so-called bulbi vestibuli of the vulva. 



In many Mammals a bone (os penis) becomes developed in 

 the septum between the corpora cavernosa (e.g. many Monkeys, 

 Rodents, Bats, Carnivores, Whales, Lemurs, Apes). In some there 

 is also a bone or cartilage in the clitoris. The penis may bear 

 horny papillae and even calcified plates and spines (e.g. certain 

 Rodents), and the glans is provided with a special kind of tactile 

 corpuscles. 



(For the glands of the external genital organs, cf. p. 484). 



ADRENAL BODIES 2 



The adrenals are so called owing to the fact that they are 

 usually situated in close proximity to the kidneys, right and left 

 of the vertebral column. Hence they are usually treated of in 

 connection with the urinogenital organs a procedure which is 

 only justified by the difficulty of knowing in what other connection 

 to describe them : they have no organic or genetic relation to 

 these organs. 



Both morphologically and developmentally the adrenals consist 

 of two distinct parts, one of which arises from the coelomic 

 epithelium and the other in connection with the rudiments of the 

 sympathetic nervous system ; thus both mesoderm and ectoderm 

 take part in their formation. 



The constituents formed from the splanchnopleuric coelomic 

 epithelium originate in the form of numerous epithelial buds or 

 ridges at the dorsal margin of the base of the mesentery, which 



1 The formation of a complete canal in the clitoris (e g. in many Insectivores, 

 Rodents, and Lemurs) results in an entire separation of the urethra from the 

 urinogenital canal, so that the latter is no longer concerned with the passage of 

 the urine. 



2 The term adrenal body is here used to include the two primarily distinct 

 interrenal and suprarenal organs as described in the text, so as to prevent the 

 confusion arising by the use of "adrenal" and "suprarenal" as synonymous 

 terms. 



