The pronephros of Chryseniys marg-iiiata. 45 



1. The portion which joins together the pronephric tubules — 

 this he distinguishes as the "Sammelgang". 



2. The succeeding portion — which he says is almost always of 

 mesodermal origin, and which he consequently calls the "mesodermale 

 Endabschnitt". This may extend back to the cloaca, or (3) may 

 intervene. 



3. The "freie oder ectodermale Endabschnitt" — a portion of 

 which may be derived from the ectoderm (Selachii and Amniotes). 



Chrysemys embryos show that the front portion of the pronephric 

 duct fSammelgang), which extends in the earlier stages of the de- 

 velopment of the pronephros over segments VI, VII and VIII is 

 formed from a continuous groove of the lateral plate mesoderm, 

 not by the fusion of the distal ends of the anterior pronephric 

 tubules (cf. EiJCKEET, v. Wijhe, C. Earl, Wheeler and Brauer). 

 Closure of this groove in places gives rise to pronephric tubules; 

 distally it does not close, and thereby gives rise to a continuous 

 pronephric duct in the region of segments VI, VII and VIII. 



Thus the pronephric tubules are formed in connection with this 

 part of the pronephric duct at the same time and from the same 

 rudiment (primitive kidney groove). Dean (Bdellostoma) says the 

 duct is formed after the pronephric tubules. 



In segments IX and X at least (and possibly in segment XI 

 also) the rudiment of the pronephros appears before the tubules 

 as a solid cord of cells continuing' backwards without break from 

 the duct in segments VI, VII and VIII. The duct in this region 

 also shows early traces of connection with the mesoderm at intervals, 

 and at a somewhat later stage this mesodermal connection becomes 

 a continuous one. The duct in segments VI to XI is always clearly 

 diiferentiated from the ectoderm. 



See Plate 1 and 2 Figs. 8, 9, 10, 17, 18, 19. 



At a still later stage the continuous connection between duct 

 and mesoderm in segments IX and X becomes interrupted, the 

 persisting connections giving rise to pronephric tubules. 



In the region behind the pronephros we find the duct arises as 

 a cord of cells between ectoderm and mesoderm: this grows 

 gradually backwards towards the posterior end of the bodj^, as de- 

 tailed in the preceding account. 



The three interesting points in connection with this backward 

 growth will be dealt with in turn: 



1. How is the backward growth of the duct effected? 



