As the part of the urethral canal immediately succeeding the termination of the vasa defe rentia is the analogue of the vagina, some mo dification of this part might be anticipated in the male corresponding with the extraordinary kali' and developernent which characterise the vagina in the female : accordingly we find that the combined prostatic and membranous or muscular tract of the urethra is proportionally longer and wider in the Marsupial than in any other Mammiferous quadrupeds (fig. 135, b). It swells out immediately beyond the neck of the bladder, and then gradually tapers to its junction with the spongy part of the urethra : it is not, however, divided like the vagina. Its walls are thick, formed of an external thin stratum of nearly transverse muscular fibres ; and a thick glandular layer, the secretion of which exudes by innumerable pores upon the lining membrane of this part of the urethra. In a male Kangaroo I found that a glairy mucus followed compression of this musculo-prostatic tract of the urethra : the canal itself is here slightly dilated.
Three pairs of Cowper's glands (c, c, c, fig. 135) pour their secretion into the bulbous part of the urethra : the upper or proximal pair are not half the size of the two other pairs in the Kangaroo, but are relatively larger in the Koala and other Marsupials : the two lower pairs are situated, one on each side the lateral division of the bulb of the urethra; their ducts meet and join, above this part, with the duct of the smaller gland t each gland is inclosed by a muscular capsule.
The penis consists of a cavernous and a spongy portion, each of wh ich commences by two distinct bodies. The separate origin of each late ral half of the spongy body constitutes a double bulb of the urethra (e, e, fig. 135), and the' ac celerator urine,' as it is termed, undergoes a similar division into two separate muscles, each of which is appropriated to compress its par ticular bulb. The two bulbous processes of the corpus spongiosum soon unite to surround the urethra, but again bifurcate to form a dou ble glans penis in the multiparous Marsupials, in which most of the ova are impregnated in both ovaria, as the Phalangers, Peranieles, Opossums, tke. ( b, b, fig. 136).
This modification of the opposite extre mities of the corpus spongiosum, called bulb' and ' glans,' was detected by Cowper in his dissection of a male Opossum ; and, in his account of the anatomy of that animal in the Philosophical Transactions for the year 1704, lie says, " As the bulb of the urethra in man is framed for the use of the glans, to keep it sufficiently distended when required, so it seems it is necessary to have two of these bulbs, inclosed with their particular mus cles in this animal, to maintain the turgescence of its double or forked glans when the penis is ereeted."—Vol. xxiv. p. 1585.
The force of this ingenious 'reasoning on the correlation of the bulb to the glans might seem to be invalidated by the fact that in the um parous Marsupials, as the Kangaroo, the glans penis (f, fig. 135) is single, and yet the bulb double
but in this circumstance we may perceive an example of the retention of a typical structure at the deeper seated part of a system of organs, when not incompatible with a slight modifi cation of a peripheral segment of the same system ; it being by no means obviously neces sary to abrogate the division of the urethral bulb simply because the blood accumulated in each division was to be driven in a concen trated current upon a single, instead of a dou ble glans penis.
The intermediate strictures of the glans be tween the two extremes above instanced are presented by the Ursine Dasyure, Koala, and Wombat. In the Koala (fig.135, B) the glans penis terminates in two semicircular lobes, and the urethra is continued by a bifurcated groove along the mesial surface of each lobe. In the Wombat (fig. 135, C)there is asirnilar expansion of the urethra into two divergent terminal grooves, but the glans is larger, cylindrical, and par tially divided into four lobes:* the chief struc ture of interest in this part of the Wombat is the callous external membrane of the glans, and its armature of small recurved, scattered horny spines, which do not occur in any other Marsupial animal. The small retroverted ppa pilla:on the infundibuliform glans of the koala and on the bifurcate glans of the Phalan gers and Petaurists are not horny.
In the Perameles lagotis not only is the glans bifurcate, hut each division is perforated, and the urethral canal is divided by a vertical septum for about half an inch before it reaches the forked glans. From the septum to the bladder the canal is simple, as in other Marsupials. The bifurcations of the glans in the Opossums and Phalangers are simply grooved.
If the experiments of Ilaighton and the detection, by Drs. Bischoff and Barry, of sper matozoa upon the ovary itself after coitus, had not rendered the question of the neces sity of the contact of the semen with the ovarium for impregnation almost independent of the aid of Comparative Anatomy, the diffe rences of structure above described in the urethra and glans penis of the Marsupial animals would have gone far to explode the once prevalent notion of an aura seminalis' fertilizing the ovum through the 'medium of the circulating fluid : for why, on such an hypothesis, should the impregnation of two ovaria, each communicating with a distinct oviduct, uterus, and vagina, as in the Opos sum, require two conduits of the semen in the male, one for each vaginal—and wherefore, in the case of an uniparous Marsupial, in which the fecundating stream need ascend only to a single ovarium, as in the Kangaroos and Potoroos, is the penis terminated by a single glans ? The spermatozoa of the Perameles have a single barb at the base of the head, which is sub-elongate and compressed ; in other respects, as in size and proportion of the filamentary tail, they resemble the spermatozoa of the Rabbit.