The pronephros of Chrysemys marginata. 



75 



Theoretical. 



It is of course useless to formulate a theory of the evolution 

 of the kidney based upon the examination of one species only. 



The following theoretical considerations apply not only to 

 Chrysemys but also — so far as I am able to judge at present, — to 

 the kidney development in the Dogfish, Chick, Crocodile and Lepi- 

 dosteus. 



Pronephros and mesonephros are both conditions of the same 

 organ, and differ because the tubules of which they are composed 

 for the most part develop at different times and therefore meet with 

 a different environment when they do appear. 



sp. cd prof pr. d pr. tub 



nep 



nep 

 Textfiff. F. 



Textfig. G. 



Textfig. F. Notice that in this figure the pronephric tubule nephrostome 

 opens into the general coelom, the nephrotome being confined to the region between 

 protovertebra and nephrostome. d. ao dorsal aorta, gl glomus, gls glomerulus. 

 ms. t mesonephric tubule, nep nephrotome (containing in some sections a cavity-the 

 nephrocoel). neph nephrostome. ^^r. d pronephric duct. ^;r. tub pronephric tubule. 

 prot protovertebra with myocoel. sj}. cd spinal cord. 



Textfig. G. This diagram shows the pronephric condition between two segments. 

 Although the nephrotome cavity is absent the pronephric tubule is not precluded 

 from opening into the coelom. 



Textfig. F shows diagrammatically the relations of a typical 

 pronephric tubule to the protovertebra, to the nephrotome and to the 

 general coelom, at an early date. 



Textfig. G represents the condition of a pronephric tubule which 

 occurs in the region between two body-segments. 



Textfig. H indicates the relations of a mesonephric tubule — 

 an outgrowth from the somatic mesoderm of the nephrotome. Its 

 changed relations are due to the fact that more of the coelom (viz 



