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 outer 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 coelom 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 

 tubae), the derivative of the pronephric nephrostomes. This receives the eggs, 

 which pass from the ovaries into the coelom. 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. 



