DEVELOPMENT OF EXCRETORY SYSTEM 775 



grows caudalward to the next nephrotome to unite with its tubule (fig. 

 344B, 5), etc. As a result, the fused portions of the pronephric tubules give 

 origin to the pronephric or segmental duct (fig. 344B). 



The above method of origin of the pronephric duct has been described 

 for elasmobranch fishes, reptiles, birds, and mammals. A different method 

 of pronephric duct origin occurs in the amphibia and teleosts where the 

 pronephric duct apparently arises by a longitudinal splitting of the nephro- 

 tomic plate (Field, 1891; Goodrich, '30). The pronephric duct, once formed, 

 continues to grow caudalward above the nephrotomic plate until it reaches 

 the caudal end of the plate. In this area, the growing end of the pronephric 

 duct turns ventrally and joins the cloaca (figs. 344A; 346F). 



The entire pronephric portion of the nephrotomic plate is never realized 

 in the formation of pronephric tubules. The number of tubules actually formed 

 varies greatly and is confined generally to a limited number of nephrotomes 

 in the middle or posterior pronephric area. 



b. Shark, Squalus acanthias 



Jn Squalus acanthias, a considerable nephrotomic area, overlying the caudal 

 portion of the developing heart in segments 5-11, may produce suggestive 

 indications of pronephric tubule formation. However, generally only three to 

 five pronephric tubules are definitely formed. The distal ends of these tubules 

 unite to form the pronephric or segmental duct and the latter grows caudal- 

 ward to join the cloaca. The pronephric tubules are aberrant and soon dis- 

 appear, but the pronephric duct remains and when joined by the mesonephric 

 tubules it becomes known as the Wolffian or mesonephric duct (fig. 347A). 



c. Frog 



In the frog, Rana sylvatica. Field (1891) describes the origin of the pro- 

 nephric kidney from a thickening and outgrowth of the somatopleuric layer 

 of the nephrotomic plate in segments 2-4. Three tubules arise from this 

 thickened area, one tubule in segment two, another in segment three, and 

 a third in segment four. 



A cross section of the developing second pronephric tubule at a time when 

 the neural tube is wholly closed and a short while before hatching is shown 

 in figure 346A. At about the time of hatching the second pronephric tubule 

 is well advanced, as indicated in figure 346B, and the fully developed first 

 pronephric tubule of an embryo (larva) of about 8 mm. is shown in figure 

 346C. The entire pronephric kidney of one side consisting of three tubules 

 viewed from the ventral aspect at the 8 mm. stage is presented in figure 346E. 

 The general plan of the pronephric kidney at the 18 mm. stage is pictured 

 in figure 346F. Figure 346D lies in plane A-D of figure 346F. 



Contrary to the manner of origin of the pronephric duct from the distal 

 ends of the pronephric tubules in the embryo of Squahis acanthias, Field de- 



