The development of the urinogenital organs of the lamprey. Q\ 



Felix has called attention to this fact in Teleostei, but no form could 

 be a better illustration than Petromyzon. By the time the meso- 

 nephros is to develop in Petromyzon^ the mesoderm has passed far 

 beyond the embryonic stage and is practically in the same condition 

 as in the adult animal. The myotomes have separated from the lateral 

 mesoderm and have descended to the mid-ventral line; the somato- 

 pleure and splanchnopleure have formed the splanchnic and somatic 

 peritoneum and retroperitoneal mesenchyme, and we still look in vain 

 for anything comparable to the mesonephros of the Selachian. The 

 material for the rudiments of the mesonephric tubules must therefore 

 lie either in the peritoneum or in the retroperitoneal tissue of the 

 region in which they subsequently arise. In Petromyzon, I believe 

 with FüRBRiNGER and Vialleton, that they lie in the peritoneum. 

 In other forms the undiflerentiated cells which are to give rise to 

 the tubules may be left in the mesenchyma when peritoneum and 

 retroperitoneal tissue differentiate from the common epithelial meso- 

 derm layer. I can see nothing unusual or surprising in this fact, 

 nor can I believe that Felix is justified in his tirade on those authors 

 who have derived the mesonephric tubules from the peritoneum in 

 forms which he has not studied. The mesonephros simply does not 

 make its appearance till sometime after the intermediate cell-mass is 

 no longer recognizable as such. During development the portion of 

 the original coelom, so faithfully retained in the Selachian, is occluded 

 and disappears, if, indeed, it has ever been present. The lumen of 

 the mesonephric tubule and Malpighian body, when it appears, has been 

 claimed not to be a portion of the body cavity. One would suppose 

 that some of the authors who press this claim had never studied any 

 embryonic organs except kidneys, or they would certainly know that 

 the homologies of embryonic cavities are determined by their walls 

 and not by their vacuities. Surely no one doubts the homology of 

 the neurocœle in Petromyzon and Selachians, notwithstanding the fact 

 that the central nervous system of the former arises as a solid chord 

 and acquires its lumen later. Nor should the late appearance of the 

 lumen in the mesonephric tubule in forms like Petromyzon prevent 

 us from homologizing it with the persisting coelomic cavity of the 

 Selachian mesonephric tubule. 



The origin of the mesonephros of the Teleost as described by 

 Felix is not so readily reduced to the Selachian schema. Felix 

 has, however, attempted such a reduction in the following manner 

 (1897 b, p. 419 and 419) : "Nun haben wir bei den Salmoniden keine 



