The pronephros of Ohrysemys marginata. 57 



The pronephros being in so many animals evanescent and often 

 not functional, irregularities in the nature of precocious or arrested 

 development will naturally occur. 



The phenomena which later aft'ect the metamerism of the kidney 

 also effect, and may obliterate, the metamerism of the muscles, etc. 



d) Evidence that the pronephric duct is of meso- 

 dermal origin. 



Felix states that Selachii and Amniotes have an "ektodermalen End- 

 abschnitt". From the observations upon the pronephric duct in Chrysemys 

 I have been led to the conclusion that it is formed entirely from 

 the mesoderm, although the duct, in its middle region only, is con- 

 tiguous with the ectoderm during a part of its growth. There is 

 no doubt as to its isolation from the ectoderm in its posterior region. 



Pkice concludes for Bdellostoma that the posterior region of the 

 segmental duct is apparently formed in exactly the same way as 

 is the anterior end. Wheelee (93) finds chat in Petromyzon also, 

 the wohle of the pronephric duct is of mesodermal origin. In 

 spite of the statements to the contrary by several authorities with 

 regard to the entirely mesodermal origin of the duct in Selachii, 

 there is much evidence to show that the ectoderm takes no part in 

 its formation. It is probable that the real proof of the ectodermal 

 origin of the pronephric duct (in its hinder region) would be the 

 presence of a thickening of the ectoderm, due to active proliferation, 

 in the posterior region of the trunk near the cloaca, into which the 

 hinder end of the developing pronephric duct completely merges. 

 In the figures of those writers who feel so sure about the ectodermal 

 participation, there is no evidence of a thickening of the ectoderm 

 such as we should expect to see. On the contrary, in Dogfish 

 embryos where the duct has almost reached the cloaca, the former 

 is absolutely distinct from the ectoderm and closely associated with 

 tlie mesoderm. 



Whether the same is true for the only other doubtful cases, viz. 

 the Mammalia, must remain an open question for the present. 



The relations of pronephros to meson ephros in 



Chrysemys. 



The most important point which the early development of 



Chrysemys brings out is the relation between pro- and mesonephros. 



There is no appreciable break, in either the time of appearance 



