824 MAMMALIA. 



regarded as forming quite a distinct and separate group in the animal 

 creation, serving to accomplish another step in that grand transition by 

 which the physiologist is conducted from the oviparous to the placental 

 Vertebrata. 



(2433.) The MARSTJPIALIA are, strictly speaking, ovoviviparous ; that 

 is to say, the uterine ovum never forms any vascular connexion with the 

 maternal system, but after a very brief intra-uterine gestation the em- 

 bryo is expelled in a very rudimentary and imperfect condition, even its 

 extremities being as yet but partially developed ; and in this helpless 

 state the foetus is conveyed from the uterus into a pouch or marsu- 

 pium, formed by the integument of the abdomen, there to be nourished 

 by milk sucked from the mammary glands, until it arrives at such a state 

 of maturity as enables it to assume an independent existence. 



(2434.) We may naturally expect, therefore, that, with habits so re- 

 markable, the structure of the generative apparatus, both in the male 

 and female Marsupial, will oifer important peculiarities; and these 

 accordingly first present themselves for description. 



(2435.) We select the Kangaroo as an example of the entire group, 

 beginning, as we have hitherto done, with the organization of the male 

 organs of generation. 



(2436.) The first circumstance that strikes the attention of the 

 anatomist in a male Marsupial is the extraordinary position of the testes, 

 which, instead of being situated behind the penis, as in most placental 

 Mammals, are placed in front of that organ in a kind of scrotum that 

 occupies the same place as the pouch of the female, and is in like 

 manner supported by two marsupial bones derived from the pubes, 

 around which the cremaster muscle winds in such a manner as to enable 

 it powerfully to compress the testicles during the congress of the sexes. 

 The vasa deferentia derived from the testes open into the commencement 

 of the urethra, which now, for the first time, forms a complete canal 

 leading from the bladder to the extremity of the penis. The auxiliary 

 glands that pour additional secretions into the urethra are of great size, 

 and more numerous than those met with in the human subject. In the 

 first place, the commencement of the urethral tube is embraced by a 

 bulky and conical pros ta te, to which succeed three pairs of large secreting 

 organs (Cowper's glands), each enveloped in a musculo-membranous 

 sheath, apparently intended to compress their substance, and thus effi- 

 ciently discharge their secretion into the canal of the urethra, there to 

 be mixed up with the seminal fluid. 



(2437.) But perhaps the most decided peculiarities that characterize 

 the males of Marsupial quadrupeds are met with in the construction of 

 the penis itself. The two roots or crura of the corpora cavernosa are 

 not, as in the higher Mammals, attached to the branches of the ischium 

 by ligamentous bands, but each swells into a large bulb enclosed in a 

 powerful muscular envelope. The bulbous portion of the urethra is 



