96 



COMPARATIVE ANATOMY 



mesomeres become transformed into kidney tubules. The process begins 

 in the more anterior mesomeres and progresses posteriorly. Certain 

 differences in mode of development and in eventual structure compel the 

 distinction between an earlier and more anterior system of tubules, the 

 pronephros, and a later and more posterior system, the mesonephros. In 

 Anamnia the mesonephros becomes the adult kidney and the pronephros 

 disappears except that in a few fishes it is the definitive and only kidney. 

 In amniotes, following development of a pronephros and a mesonephros, 



the tubule-forming process continues 

 backward, but with some modifi- 

 cations and compKcations, to form 

 a third kidney, the metanephros, 

 which becomes the adult kidney. 

 The tubular epididymis, associated 

 with the testis of the adult amniote, 

 is a part of the embryonic mesone- 

 phros which otherwise disappears 

 except for certain vestiges which are 

 apparently of little functional 

 importance. 



In Anamnia each mesomere at 

 first forms a single renal tubule. In 

 what is conceived to be the more 

 primitive method of development of 

 the tubules, the cavity is the original 

 cavity of the mesomere and is there- 

 fore a part of the embryonic coelom 

 and continuous with the coelomic 

 space of the hypomere. In amniotes, 

 however, the mesomere material may be not only unsegmented but 

 devoid of any original cavity. In such case the tubule shapes up as a 

 solid cord, later hollowing out. 



Meanwhile, as the pronephric tubules form, the mesomere material 

 on each side of the embryo gives rise to a longitudinal tube (Fig. 64) which 

 extends backward from the pronephric region to the wall of the cloaca into 

 which the cavity of the duct finally opens. The pronephric tubules of 

 each side are joined to the corresponding pronephric duct (Fig. 70) thus 

 putting the coelom into communication with the exterior by way of the 

 cloaca. The coelomic openings or nephrostomes (Figs. 64B, n, and 

 70, 7zs) of the pronephros are ciliated. The arrangement apparently 

 serves for drainage from the coelom to the exterior. 



The mesonephric tubules develop in many more segments than do the 

 pronephric (Figs. 70 and 71). They acquire connexion with the already- 



PiG. 60. — Transverse section of the 

 embryo of a lizard (Lacerta muralis) 

 having twenty-eight pairs of mesodermal 

 somites; cut through the tenth pair; 

 much enlarged, a, dorsal aorta; m, 

 myotome — muscle-forming part of meso- 

 dermal somite; n, notochord; o, omphalo- 

 mesenteric vein; sc, neural tube (spinal 

 cord) ; sk, mesenchyme derived from 

 sclerotomic part of sorr;ite. (From 

 Kingsley.) 



