UROGENITAL SYSTEM 807 



mesomeric cells furnishing the nephrogenous tissue from which the 

 definitive organs develop." 



"Three views are held as to their relations one to another. Accord- 

 ing to one they are parts of an originally continuous excretory organ 

 (holonephros) which extended the length of the body cavity. This has 

 become broken up into the separate parts which differ merely in time of 

 development and function, with minor modifications in details. A 

 second view is that they are three separate organs, while a third regards 

 them as superimposed structures which occasionally overlap (birds, 

 gymnophiona) and thus are not, strictly speaking, homologous but 

 rather homodynamous. The first view has the most in its support, but 

 for convenience the three structures are kept distinct." 



It is of considerable value to trace the successive series of these 

 excretory structures in the different types of animals. It will be re- 

 membered that in some of the forms studied, such as the earthworm, 

 there was an excretory organ in practically every segment of the animal's 

 body. It will be further remembered that insistence has constantly been 

 laid upon the fact that the so-called higher animal forms have practi- 

 cally every structure that the lower forms possess, plus something addi- 

 tional. This is well exemplified in the study of the nephridic organs. 



The nephridic organs of the amniotes pass through a three-fold 

 development. The first excretory organs which grow, form the 

 pronephros or head-kidney, the next succeeding being known as the 

 mesonephros or Wolffian body, while the last to form, which becomes 

 the permanent kidney of the higher forms, is called the metanephros. 



While all three are closely related both in their development and their 

 structure, there is a difference in their origin and in some of the details. 



THE PRONEPHROS. 



As its name implies, that is the first of the excretory organs to 

 appear. A review of the embryology of the excretory system must be 

 had at this point. 



As the myotome is being formed from the epimere, the dorsal end of 

 each mesomere closes. This forms a sac which opens into the coelom. 

 Each of these is called a nephrotome and lies a little behind the head. It 

 is from these nephrotomes that the pronephros is formed. The number 

 of pronephridic organs varies from one in the teleosts, to a dozen or more 

 in the caecilians. The usual number, however, in the higher forms is 

 two. From the somatic walls of these nephrotomes there is an out- 

 growth toward the ectoderm. This forms slender pronephric tubules as 

 in the amphibia, or solid cords which later have a lumen from within 

 them as in elasmobranchs and amniotes. They thus all become tubules, 

 the proximal ends of each communicating with the metacoele by way of 

 the cavity in the nephrotome. The opening to the metacoele is called a 

 nephrostome, and as already noted, there will be as many tubules and 

 nephrostomes as there are somites. The distal ends of the nephrotomes 



