. The biology of the amphibia. Amphibians. THE UROGENITAL SYSTEM 267 bling roughly the sweat glands of mammals, and, like them, they are excretory organs. The anterior part of such a kidney rudi- ment theoretically, if not actually present, develops in most lower vertebrates into a functional organ, the pronephros, before the posterior section, the mesonephros, differentiates. The former is the typical kidney of the young larvae, and in forms such as Eleutherodactylus, which hatch from the capsules as metamor- phosed individuals, it shows a very early degeneration. In other Amphibia it functio
. The biology of the amphibia. Amphibians. THE UROGENITAL SYSTEM 267 bling roughly the sweat glands of mammals, and, like them, they are excretory organs. The anterior part of such a kidney rudi- ment theoretically, if not actually present, develops in most lower vertebrates into a functional organ, the pronephros, before the posterior section, the mesonephros, differentiates. The former is the typical kidney of the young larvae, and in forms such as Eleutherodactylus, which hatch from the capsules as metamor- phosed individuals, it shows a very early degeneration. In other Amphibia it functions up to the time of independent feeding or a little later. Both pronephros and mesonephros arise from the mesomere, a portion of the mesoderm lying between the myotomes and the non-segmented lateral plates of the embryo (Fig. 97). In uro- deles two or three or at most four segments (Megalobatrachus,. FiGo 97.—Diagram of the development of the pronephric canal. Gl., glomeru- lus; , lateral plate; , pronephric canal; Mes., mesomere; My., myotome, , nephrostome; , neural tube. {After Felix.) Mibayashi, 1928) of this mesomere give risg^€© tubules, usually one to each segment. These collective^?6j^n>%ne pronephros. The tubules open proximally into tj^^dj^cllvity in the form of nephrostomes, while distally tM?'en^s%end caudally and fuse to form a common pronep1?nc^i^S;1b. This grows posteriorly immediately under the ectoderm and finally fuses with the cloaca. In Salientia, as in most urodeles, only two or three tubules enter into the formation of the pronephros, while in caecilians the num- ber involved is from 10 to 13. In the wall of the body cavity of the larva, adjacent to the proximal openings or nephrostomes of the pronephric tubules, a series of branches from the dorsal aorta push out a fold of the peritoneum to form a sinus, the glomus, which serves as the arterial blood supply. In some forms, at least, these blood vessels are originally metamerically
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