THE LATER DEVELOPMENT OF THE FROG 193 



their appearance. This is the embryonic or larval pro- 

 nephric system or larval kidney, known also as the head kidney. 

 This kidney is limited to early larval life and is replaced during 

 the tadpole stage by an excretory organ which remains the 

 definitive kidney of the adult; this is the mesonephros, which, 

 it should be noted, retains as its efferent duct, the duct of the 

 original pronephros . We have then to describe the development 

 of the pronephros and the pronephric duct, the development 

 of the mesonephros, and the disappearance of the pronephros. 

 We shall see how, during the later stages, the arrangement of 

 these parts is complicated by the relation between the excre- 

 tory and the reproductive systems. Since these organs are 

 symmetrically paired we may describe only the organs of one 

 side. 



A. THE PRONEPHROS AND THE PRONEPHRIC DUCT 



In a preceding section we described the position and rela- 

 tions of that part of the mesodermal somite known as the inter- 

 mediate cell mass or nephrotome, and said that this formed a 

 rudiment of the pronephric system. The first indication of the 

 pronephros is seen before the nephrotomal region has separated 

 from either the myotome or the lateral plate, as a solid thicken- 

 ing of the somatic mesoderm anteriorly (Fig. 70, B). This 

 thickening, which begins before the cavity (ccelom) of the 

 lateral plate and nephrotome appears, gradually extends 

 posteriorly along the nephrotomal level, and finally reaches to 

 the region opposite the cloaca, although this is not until the 

 anterior part of the rudiment has become quite markedly dif- 

 ferentiated. Anteriorly, in the region of somites 2-4, the 

 pronephros itself is formed, while the posterior remainder forms 

 the pronephric or segmental duct. As the thickening of the 

 anterior pronephric region becomes marked, the rudiment here 

 begins to extend ventro-laterally, like an epaulet, over the outer 

 surface of the dorsal margin of the lateral plate (Fig. 70, C). 

 Spaces appear in this cell mass, about the same time that the 

 ccelom appears in the lateral plate; in the lateral or distal part 



