516 HISTORY OF THE HUMAN BODY 



nephridial system is made to furnish channels of exit for the 

 germ cells has been already shown (Chapter IX), and the open- 

 ing into the oviduct has been homologized with a prone- 

 phridial nephrostome, while in the male the entire anterior 

 portion of the mesonephros and its duct becomes utilized for 

 the passage of the seminal fluid. 



One of the most fundamental characteristics of vertebrates is 

 the presence of paired gill-slits, extending in two lateral rows 

 along the pharyngeal region and forming direct communica- 

 tions between the pharynx and the exterior; these may be 

 readily derived from nephridia by supposing, first, that the 

 inner ends of these tubes become secondarily connected with 

 the pharyngeal lumen, and secondly, that the tubules become 

 reduced in length until ectoderm and endoderm come in con- 

 tact. Only in some such way can one explain the embryonic 

 development of gill-slits from a series of ectodermic inpushings 

 that meet a similar series of endodermic outpushings, a mechan- 

 ical process that necessitates some reason back of that which is 

 apparent in order to account for the accuracy with which these 

 several folds meet one another. In the embryo of the cyclo- 

 stome Myxine, precisely the form where we would look for the 

 retention of the earliest phases, there still appears at first a 

 fairly long canal between each ectodermic inpushing and its 

 endodermic associate, perhaps a remnant of the nephridial tube. 

 It may also be more than a coincidence that, when genuine 

 nephridia of the pronephrotic system appear immediately 

 posterior to the gill region, none arise in the somites that de- 

 velop the gill-slits. 



Important changes seem to have taken place in both outlets 

 of the alimentary canal, and indications show that vertebrates 

 have acquired both a new mouth and a new anus, although they 

 still retain in the embryo many traces of the older organs. 

 That the mouth of the vertebrates is not the primitive one is 

 shown by a variety of indications, one of the strongest being its 

 exceptionally late appearance in embryonic life. A mouth is 

 one of the most essential of organs, and in other animals, cor- 

 responding to its important function, is one of the first to 



