DEVELOPMENT OF EXCRETORY SYSTEM 773 



word nephrotome is applied to each segmented mass or bridge of 

 mesoderm, developed within the nephrotomic plate, which connects 

 the somite to the unsegmented lateral plate mesoderm or hypomere. 

 See figure 344B.) The early development of the kidney tubules in 

 the hagfish, Polistotrema (Bdellostoma) stout i (Price, 1896), and in 

 the elasmobranch fish, Sqiialus acanthias (Scammon, '11), tends to 

 simulate holonephric conditions. 

 (2) Pronephros, mesonephros, metanephros, and opisthonephros are terms 

 for types of kidneys. Actually, during the development of all gnathos- 

 tomous vertebrates, the nephrotomic plate on either side produces 

 not one holonephros but instead three types of kidneys which are 

 adapted to three different developmental and functional conditions. 

 These kidneys develop antero-posteriorly in three general regions of 

 the nephrotomic plate (fig. 344A). The most anteriorly developed 

 kidney is called the pronephros; the kidney which develops from the 

 midregion of the nephrotomic plate is the mesonephros; and that which 

 arises from the caudal end of the nephrotomic material is the meta- 

 nephros. Kerr ('19) attaches the name opisthonephros to the kidney 

 which arises posterior to the pronephros in the late larvae of fishes and 

 amphibia. The opisthonephric kidney takes its origin from the entire 

 caudal portion of the nephrotomic plate. It therefore represents the 

 nephrogenic tissue of the posterior part of the embryonic mesonephric 

 kidney plus the nephrogenic material which enters into the formation 

 of the metanephric kidney of reptiles, birds and mammals. 



b. Types of Nephrons or Renal Units Produced in Developing 

 Vertebrate Embryos 



Four main types of renal units are produced during kidney development 

 in various vertebrate species. Consult figure 344C-F. 



2. Functional Kidneys During Embryonic Development 



During embryonic development, the following types of functional kidneys 

 occur in the gnathostomous vertebrates. 



a. Pronephros 



The pronephric kidney is functional in all species producing free-living 

 larval forms. In these larvae it operates not only to remove waste materials 

 but is essential also in the removal of excess water, thus preventing edema 

 (Howland, '16, '21; Swingle, '19). Free-living larvae are found in teleost, 

 ganoid and lung-fishes, and in the amphibia. 



b. Mesonephros 



In all free-living larvae the pronephros is succeeded by the mesonephros 

 during the larval period. The decline of the pronephros and the ascendancy 



