The kidneys, ureters, bladder and urethra remain comparable in all mammals excepting the monotremes where there is no urinary passage through genital openings, the urine being ejected through the vesicoanal pouch, a sort of cloaca. In the bird the first two parts remain as in mammals while the ureters terminate in a hernia-like pouch of the rear wall of the cloaca. It would seem from this arrangement that ureteral transmission of infection from the anal area to the kidneys would be facilitated in the lowest mammals and in the Aves. Variations in size, shape and position exist to a minor extent in the higher orders but in all forms, the system remains a post-peritoneal structure. Differences in construction are to be seen for example, in the single pyramidal kidneys of marsupials and certain rodents, in the lobulated organ of bears, cattle and seals, in the twisted viscus of horses, but these gross appearances do not destroy the uniform scheme upon which the functionating unit is built. The single-lobed kidney discharges all its collecting tubules into one calyx while the multiple pyramids of the lobulated organ are fitted with individual calices which in turn empty into the pelvis proper; this is true whether the lobulations are retained, as in the bear, or are smoothed out in the course of development as in man. The secretory tubule remains in essentially the same form in all kidneys; the modern idea of its anatomy may be found in the work of Huber[32]. The most decided example of the lobulated kidney is to be found in the class Aves, wherein the organ consists of two or three large lobes lying in concavities of the sacrum, each lobe being made up of tiny lobules. The latter appear to the unaided eye as fairly distinct divisions whether viewed on the exterior or by cross section. There is a cortex and a medulla to each, the separation being clear in a large specimen, vague in a small one. These lobulations are quite well observed when the kidney is full of urates, a common finding in birds. Magnification of the avian kidney reveals an apparently simpler tubular arrangement than is found for mammals[33][34], yet the relation of vascular plexuses and secreting tubules remains similar.