536 



GENERATION. 



[CHAP, xxxvi, 



corpus spongiosum, and no prolongation of the urethra, but the seminal 

 secretion passes into the female along the groove formed by the union of the 

 two cavernous bodies. 



Aves. The generative organs of birds exhibit a close analogy to those of 

 the higher reptiles, the penis is even less developed, except in the struthious 

 birds (ostrich) and the swimmers. The ovary is racemose and single, the 

 right with its .oviduct being permanently atrophied ; a singular violation of 

 symmetry which, is confined to birds. In this class of vertebrata, incubation, 

 that singular substitute for utero-gestation, attains its highest perfection : it 

 appears to arise from the concurrence of these three exigencies the neces- 

 ,,sary size and early maturity of the young, the necessity of warmth to their 

 development, and the incompatibility of utero-gestation with flight. 



Mammalia. It is from the possession of a remarkable accessory organ of 

 generation, that this important class of the highest organised beings takes its 

 name. After all organic connection has ceased, the young are still dependent 

 on the parent for nourishment, and are supported by the secretion from a 

 special gland with which the female is furnished for the purpose the mam- 

 mary gland. The mammalia are divided into the monotremata, the marsu- 

 2)ialia, and the placental mammalia. The monotremata are as yet imperfectly 

 known, -but they present a curious connecting link between the oviparous and 

 mammalian type ; they derive their name from possessing but one aperture, 

 that of the cloaca, common to the generative, urinary, and digestive canals, 

 and in this respect resemble birds; they are represented by the ornithorynchus 

 and echidna of Australia. In the male, the penis is perforated by a urethral 

 canal, through which the semen, but not the urine, passes : in the female, the 

 ovaries are racemose, the ova large, and containing all the elements of an 

 egg, and there are two uteri, opening by distinct apertures into the cloaca. 

 The marsupialia (called so from the possession of a marsupium, or pouch, 

 in which the young is lodged and suckled after its discharge from the uterus), 

 are ovo-viviparous, that is, the young are brought forth alive, but they never 

 have any placental connection with the parent. Not only are there two uteri, 

 but two vaginae, which terminate by two separate orifices in a sort of genito- 

 urinary cloaca.* About thirty-nine days after conception, the young is ex- 

 pelled into the marsupium, where it becomes attached to one of the nipples 

 by its mouth, and continues thus to draw nourishment from the mother for a 

 period of eight months : this peculiar method of foetal nourishment necessi- 

 tates a very advanced and disproportionate development of the organs of 

 assimilation, which is the most remarkable characteristic of the embryo 

 marsupial. The placental mammalia, in the general structure of their genera- 

 tive organs, resemble man. The testicle consists of seminiferous tubules 

 arranged in bundles, and enclosed in a fibrous capsule. The penis is com- 

 posed of two corpora cavernosa arising from the ischia, a corpus spongiosum 



* In the male, the testicles are contained in a scrotum placed above, and 

 not below, the penis, in a situation analogous to the marsupium in the female, 

 and supported, like it, by two marsupial bones ; the vasa deferentia open into 

 the urethra, which, invested by its corpus spongiosum, passes through the 

 centre of the penis, and which now, for the first time, we find forming a com- 

 plete canal, leading from the bladder to the extremity of the intromittent 

 organ. 



