776 THE EXCRETORY AND REPRODUCTIVE SYSTEMS 



scribes the origin of this duct in the frog from a thickening of the somato- 

 pleuric layer of the nephrotomic plate in segments 4-9. This somatopleuric 

 thickening separates, becomes canalized, and grows caudally to join the dorsal 

 area of the cloaca, a union which is accomplished at about the time of hatching 

 (fig. 258F'). The pronephric tubules in their development unite with the 

 cephalic end of this duct. 



As the development of the pronephric kidney advances it is to be observed 

 that one large glomus is formed, projecting into the restricted coelomic 

 chamber or nephrocoel which is shut off partly from the common peritoneal 

 cavity by the expanding lungs (fig. 346D). Each ciliated nephrostome opens 

 into this nephrocoelic chamber (fig. 346F). {Note: Reference may be made 

 to figure 335A-C which shows the well-developed renal portal system inserted 

 into postcardinal vein in relation to the pronephric kidney. The postcardinal 

 vein breaks up into a series of small capillaries which ramify among the coiling 

 pronephric tubules (see figure 346C) to be gathered up again into the posterior 

 cardinal vein as it opens into the common cardinal vein.) 



d. Chick 



The pronephric tubules of the pronephric kidney of the chick are rudi- 

 mentary, occupying a region of the nephrotomic plate, from the fifth to the 

 sixteenth somites. However, all of the tubules do not appear simultaneously. 



The pronephros begins to form at about the stage of 12 to 13 pairs of 

 somites (stage 11, Hamburger and Hamilton, '51, or at about 40 to 45 hrs. 

 of incubation), and small aberrant tubules are formed (fig. 345E) which 

 grow caudally to give origin to the pronephric duct as indicated in figure 344A. 



Fig. 345. Developing kidney tubules. (A & B) General structure of adult human 

 kidney. (A) This diagram represents a single renal unit in relation to blood vessels, 

 collecting duct and the minor calyx. Arrows denote direction of excretional flow. The 

 position of A in drawing B is shown by the elongated oblong in B. (A is redrawn, some- 

 what modified, from Glendening, 1930, The Human Body, Knopf, Inc., N. Y.) (B) 

 Human kidney, part of wall removed, exposing pelvis and other general structures. (Re- 

 drawn from Maximow and Bloom, 1942, A Textbook of Histology, Saunders, Phila- 

 delphia, after Brauer.) (C) Including C-1 to C-6. Stages in the development of a 

 mesonephric renal unit in the frog, Rana sylvatica (C to C-6 redrawn from Hall, 1904, 

 Bull. Mus. Comp. Zool. at Harvard College, vol. 45). C represents a section through a 

 developing mesonephric tubule showing cellular condensation in relation to pronephric 

 (mesonephric) duct. C-1 to C-6 are diagrammatic figures of a developing renal unit from 

 right side of body. The somatic or lateral portion of the tubule is shaded by lines, the 

 splanchnic portion is unshaded. (D) Diagrammatic representation of a section through 

 pelvic kidney of Necturus maculosus. (Redrawn and modified from Chase, 1923, J. 

 Morph., 37.) A tubule of the ventral series is shown with a peritoneal canal and ciliated 

 nephrostome which opens into the coelomic cavity. A tubule of the dorsal series also is 

 depicted. The latter type of tubule lacks a ciliated nephrostome opening into the coelom. 

 (E) Pronephric tubule in the chick. Section passes through somite 11 of embryo of 16-17 

 somites. (F) Section through mesonephric kidney of 96 hr. chick embryo, partly sche- 

 matized. (G) Schematized section through mesonephric kidney of six to seven day chick. 



