70 T. H. Buhlend, 



to affect the newly-acquired region of the neplirotomes and divide 

 it into chambers widely open to the coelom (inner pronephric chambers) ; 

 the glomus too is only partially subdivided, but sufficiently so to 

 give origin to the term "internal pronephric glomeruli". If further 

 specialized and subdivided and their coelomic connections longer, 

 the inner pronephric chambers would become Malpighian capsules, and 

 their coelomic passages nephrostomal canals. 



The structures found at the front end of the kidney in Ganoids 

 etc. seem to be explained in this way. But to many the best 

 evidence that pro- and mesonephros are merely conditions of a 

 homodynamous structure are such statements of Felix as the 

 following: 



„Aus diesen Darstellungen folgt wohl ohne weiteres, daß man 

 vielleicht das einzelne Kanälcheu, sei es nun Vornieren-, Urnieren- 

 oder Nachnierenkanälclien, einigermaßen bestimmt charakterisieren 

 kann, daß es aber unmöglich ist, eine solche Charakteristik auf eine 

 ganze Summe von Kanälchen auszudehnen, welche in einer Vorniere, 

 Urniere oder Nachniere vereinigt sind." 



The factors in the evolution from pronephros to 

 mesonephros in Chrysemys. 



]. The arrested development of the tubules. 



From a consideration of the order of appearance of the tubules 

 it is clear that it is only in the anterior region of the pronephros 

 (in Chrysemys) that the tubules are diiferentiated at the same time 

 as the pronephric duct. Behind this region — in the posterior part 

 of the pronephros and throughout the mesonephros — the tubules 

 do not join the duct until some time after the duct has extended 

 beyond them in its growth. If we consider, and there is reason- 

 able supporting evidence, that the kidney in its pronephric condition 

 extended through practically the whole length of the coelom back to 

 the cloaca, we are compelled to admit that the majority of the 

 tubules have suffered an arrested development. 



Whether this changed time of tubule formation is the cause of 

 the morphological modification of the tubules which suffer an 

 arrested development (as compared with typical pronephric tubules) 

 is not quite clear, but it would seem that it is the environment 

 which affects both morphological structure and time of appearance 

 of the tubules. 



I 



