UROGENITAL SYSTEM. 331 
openings of numerous collecting tubules (p. 309). In section the 
substance of the kidney shows two different textures, recognizable 
to the naked eye. There is an quter cortical and an inner medul- 
lary substance, the two interlocking as a series of pyramids. These 
different appearances are due to the fact that the cortex contains the 
renal corpuscles and convoluted tubules, while the medulla is com- 
posed of the straight tubules of Henle’s loops and of the collecting 
system. ~ 
The ureters are free for most of their course from the kidney to the 
urinary bladder, into which they enter instead of the cloaca. The 
bladder, in the monotremes and marsupials, is solely allantoic in 
nature, but in the placental mammals a portion of the cloaca is also 
included in it. From the bladder a single tube, the urethra, leads to 
the exterior. The mammalian urine contains urea instead of uric 
acid, a resemblance to the amphibia and a contrast to the sauropsida. 
REPRODUCTIVE ORGANS OF THE SEPARATE GROUPS. 
CYCLOSTOMES.—The gonads, which are usually unpaired, are supported by 
a fold of the peritoneal membrane (mesorchium or mesovarium, p. 122). The eggs 
and sperm escape into the ccelom and are carried thence by way of the abdominal 
pores. The myxinoids have hermaphroditic gonads, the anterior part being female, 
the posterior testicular; but one sex predominates. Nansen believes that the sexes 
alternate in function (proterandric hermaphroditism). The eggs of the petromy- 
zonts are small, those of the myxinoids are larger and are enclosed in a horny shell, 
with anchoring hooks at either end. 
ELASMOBRANCHS,—In the elasmobranchs, as in all other vertebrates, the 
gonads are at first paired and symmetrical, though occasionally one side or the other 
may be reduced or become degenerate or those of the two sides may fuse. Thus in 
some skates only the left gonad may be functional. Elsewhere in the group they are 
paired and lie far forward, attached to the dorsal wall of the coelom. The Miillerian 
ducts of the two sides in the female meet in front in a common opening (ostium 
tube), the derivative of the pronephric nephrostomes. This receives the eggs, 
which pass from the ovaries into the ceelom. The different parts of the duct are 
specialized, the upper part serving as a shell gland, forming the capsule for the 
eggs. This is horny and in most species is provided with tendril prolongations at 
the four corners, by which the eggs (‘skate barrows’) are attached to submerged 
objects. Some species of both sharks and skates are viviparous. In these the 
lower part of the Miillerian duct (oviduct) serves as a kind of uterus. In some 
species the lining of this uterus is covered by vascular villi, by which nourishment 
and oxygen are conveyed to the growing young which escapes in approximately the 
perfect shape. The eggs of elasmobranchs are very large, those of some 
species exceeding even those of the ostrich in size. 
