774 THE EXCRETORY AND REPRODUCTIVE SYSTEMS 



of the mesonephros is well illustrated in figure 335B-E relative to the devel- 

 oping venous system in anuran larvae. The mesonephric kidney also functions 

 in the embryos of elasmobranch fishes, reptiles, birds, and mammals. In the 

 mammals its efficiency as a renal organ appears to be correlated with the 

 degree of intimacy existing between the extra-embryonic and maternal tissues 

 in the placenta. When this relationship is intimate (fig. 373D) as in rats, 

 mice, humans, etc., the mesonephric kidneys are less developed, and therefore 

 probably less functional, than in species such as the pig. In the pig the pla- 

 cental relationship between embryonic and maternal tissue is not so close as 

 in the species mentioned above (fig. 373B), and the mesonephric kidneys 

 are very large and well developed. 



c. Metanephros and Opisthonephros 



As indicated on p. 773 the metanephros is the kidney of the adult form of 

 reptiles, birds, and mammals, while the opisthonephros is the mature kidney 

 in fishes and amphibians. As the definitive or adult form of the body is achieved 

 in both of these groups, the mature form of the kidney assumes the renal 

 responsibilities. 



3. Development and Importance of the Pronephric Kidney 



a. General Considerations 



Observation and experimentation upon the developing urinary and genital 

 systems of gnathostomous vertebrates suggest that the pronephric kidney, 

 and particularly its duct, the pronephric duct, are most important in the later 

 development of the excretory and reproductive systems (Gruenwald, '37, '39, 

 '41). The pronephric kidney therefore may be regarded as fulfilling two im- 

 portant functions in the gnathostomous vertebrates, namely: 



( 1 ) It operates as an early renal organ in free-living larval species, and 



(2) It is a necessary precursor in the development of the reproductive 

 system and the later excretory system. 



The pronephric kidney develops from the anterior portion of the nephro- 

 tomic plate at about the level of the developing heart and stomach region 

 (fig. 344A and B). This area of the nephrotomic plate becomes segmented 

 into separate nephrotomes (fig. 344 A and B). During the differentiation of 

 each nephrotome in the pronephric area, the connection between the nephro- 

 tome and the dermo-myotome disappears, and a small dorso-lateral outgrowth 

 from the middle portion of the nephrotome occurs (fig. 344B, 1 and 2). This 

 cyHndrical outgrowth proceeds dorso-laterally toward the developing skin 

 and then turns posteriad and grows caudally (fig. 344B, 3). In the next pos- 

 terior nephrotome, it meets a similar rudimentary tubule with which it unites 

 (fig. 344B, 3 and 4). The area of union formed by these combined tubules 



