ORGANS OF GENERATION IN THE VERTEERATA. 101 



AVES. 



In the males of this class, the testes, two in number, are placed 

 high in the abdomen, beneath the kidneys; they are subject to re- 

 markable variation of size, according to the period of the year their 

 office is required; thus, in January the testicle of the sparrow is 

 about this size, o, and by April it attains the size of a large pea. 

 The vasa deferentia, seldom much curved, pass down and terminate 

 separately on a rudimentary penis in the urethro-sexual pouch. 

 The epididymis is quite rudimentary, and varies much in colour, 

 being black, yellow, or green. Coitus is usually effected simply by 

 an eversion of the cioacag, therefore the intromittent organ is only 

 rudimentary:, in those birds, however, which copulate in water, as 

 the drake, swan, &c, it necessarily attains a larger size. It is 

 sometimes double as in serpents, and is always grooved along its 

 upper surface, for the passage of the semen. The ovarium and 

 oviduct are confined to the left side, they primarily, however, exist 

 on both sides, but rarely continue their development on the ri<rht. 

 A clitoris is present in the females of those species whose males°are 

 provided with a penis. 



MAMMALIA. 



The male organs of generation in this class consist of the testicles, 

 vasa deferentia, penis, urethra, vesiculas seminales, the prostatic 

 and Cowper's glands, and with one interesting exception, supplied 

 by the monotremata, the genito-urinary systems are quite distinct 

 from the digestive. In by far the greater number of mammalia the 

 testicles descend into the scrotum as in man ; in some, however, as 

 the amphibious mammalia, the cetacea, the elephant, and the orni- 

 thorhynchus, they never leave the abdomen, and in others, as the 

 bat, the mole, and the hedge-hog, among the insectivora; the rat, 

 the guinea-pig, the porcupine, and the squirrel, among the rodentia, 

 they descend during the rutting season, and return into the abdomen 

 after it is over. The communication between the tunica vaginalis 

 and the peritoneum remains unclosed in those animals in which the 

 testicles descend, except man, and where these glands occasion- 

 ally pass into and out of the cavity, the communication is very 

 free. The vasa deferentia are very serpentine in those ani- 

 mals where the testicles remain in the abdomen, and in ruminants, 

 the elephant, and especially the horse, they are greatly dilated near 

 their termination in the urethra. 



Vesiculce Seminales.— There seems to be no general law for the 

 presence or absence of these accessory bodies. They are met with 

 ID the camel, elephant, bull, ram, horse, boar, guinea-pig, rabbit, 

 hedge-hog, &c. In these animals they have either no communica- 

 tion, or a very imperfect one, with the vas deferens. In man and 

 the simiae only is the communication free and direct. The prostate 



