780 THE EXCRETORY AND REPRODUCTIVE SYSTEMS 



The renal units empty their products into these collecting ducts in the 

 mature form of the kidney. 

 (5) Whereas the functional pronephric kidney is confined to those species 

 which develop free-living larvae, the mesonephric kidney is functional 

 in all vertebrate embryos with the possible exception of a few mam- 

 malian species. 



a. Squalus acanthias 



The mesonephric tubules in the embryo of Squalus acanthias and in other 

 elasmobranch fishes originate in a manner similar to the pronephric tubules. 

 That is, a single tubule arises from each nephrotome of the nephrotomic plate. 

 In doing so, the nephrotome loses its connection with the developing somite 

 or dermo-myotome, and its dorso-lateral aspect thickens and grows laterad 

 in the form of a tubule. This tubule comes in contact, and fuses, with the 

 pronephric or segmental duct (fig. 344B, 11; G.l, G.2). The latter then be- 

 comes the mesonephric or Wolffian duct. In the 20.6-mm. embryo of Squalus 

 acanthias according to Scammon ('11), 37 pairs of these tubules are present, 

 extending along the mesonephric duct to the cloaca (fig. 347A). Later, this 

 primitive segmentation is lost, and many tubules are developed in each seg- 

 ment. The anterior portion of the kidney soon degenerates; the nephrostomal 

 connections of the mesonephric tubules with the coelom established during the 

 development of the tubules are lost; and the mesonephric tubules assume the 

 general morphology shown in figure 344G.3). As shown in figure 3440.3, a 

 series of collecting ducts eventually develops to connect the mesonephric tubules 

 with the mesonephric duct. Renal units eventually arise in the nephrogenous 

 tissue overlying the cloaca. This area corresponds to the metanephric region 

 of higher vertebrates, and the mature kidney of Squalus acanthias thus be- 

 comes a combination of caudal, mesonephric, renal units, associated with 

 metanephric units. The mature kidney thus is an opisthonephros. (See Kerr, 

 '19, also p. 773). In the adult kidney, segmentally arranged nephrostomes may 

 be observed in a limited area along the medial side of the kidney, although 

 they do not connect with the renal units. 



b. Frog 



The mesonephric renal units in the frog begin to arise at about the 10-mm. 

 stage. As in the shark embryo, the early origin of the mesonephric renal units 

 is segmental. An intermediate zone of the nephrotomic plate between the 

 developing mesonephros and the pronephric kidney does not develop renal 

 units. Coincident with this fact those units which arise more posteriorly in 

 the nephrotomic plate are developed better than those which arise anteriorly. 



The renal units arise as cellular condensations of mesodermal cells within 

 the cellular mass of the nephrotomic plate (fig. 345C-1). These cellular con- 

 densations elongate, become canalized, and assume a union with the meso- 



