The pronephros of Chrysemys marginata. 65 



kranialwärts den mesodermalen Endabschnitt des primären Harn- 

 leiters fort, etc." 



c) The evidence that tlie pronephric tubules are 

 not m et a m eric. 



At no time in the development of Chrysemys is the pronephros 

 a metameric structure. This is important, and certainly strengthens 

 the theoretical view that the pronephros and duct arise from a 

 continuous primitive groove of the body cavity lining, which becomes 

 closed at intervals, the remaining parts becoming tubules and 

 the distal continuous part persisting as a duct. 



Other evidence of this kind is furnished by: 



Shipley (1887) concludes that the pronephros of the Lamprey 

 has not a metameric origin. Wheeler (1899) however supports 

 quite the opposite view. 



Price says that the pronephros of Bdellostoma is metameric 

 except in the segments 11 — 20. Rückert and Rabl state that the 

 pronephros in the Dogfish is metameric: on examination of Scyllmm 

 embryos of various stages I am not prepared to believe that the 

 pronephros in Selachii is segmental, and I am certain that anj'one 

 who consults the only papers already published upon this subject 

 will be disinclined to accept such a fundamental statement as proved 

 upon the evidence available. The work of Rückert, van Wijhe, 

 and Rabl implies that some doubt existed in their minds as to 

 where the protovertebra ends and the nephrotome begins; also they 

 did not distinguish clearly between nephrotome and lateral plate 

 region. In my observations I have been guided by Sedgwick: the 

 nephrotome region is segmental, the lateral plate region is not. 



Gregory (1900) examined embryos of Aromochelys and Platypeltis: 

 an examination of this authors results tends to show that the 

 kidney in these forms develops much less regularly than does the kidney 

 of Clirysemys (and Chelone). The author gives the pronephric segments as 

 from 4 — 10, and states that there are from 7 to 14 pronephric tubules. 

 The mesonephros is said to reach forward to the 6^^ segment; the 

 mesonephric funnels which lie in the pronephric region open into the 

 coelom. I have tried to find some relation between her account and 

 my own, and I think that she must regard the pronephric tubules which 

 open into the lower end of the nephrotome as mesonephric, otherwise 

 I see no signs of pro- and mesonephros overlapping in my specimens. 

 Furthermore, from Miss Gregory's figures I feel convinced that she 

 has made the same mistake which other workers upon kidney 



Zool. Jahrb. XXXVI. Abt. f. Anat. 5 



