46 T. H. BURLEND, 



There is no doubt that the mesoderm plays a prominent part 

 in this growth because the ectodermal and mesodermal layers are 

 differentiated by the stain, — the mesoderm assuming a much 

 deeper tint: this property is shared by the duct also (see Plate 2 

 Figs. 17, 21). 



Furthermore, there is no direct evidence that the duct grows 

 backwards independently; the extreme posterior end consists of a 

 few loosely scattered cells which do not suggest a process of active 

 cell-division in this region; whereas the evidences of a cellular 

 connection between duct and underlying mesoderm are numerous in 

 segments XI, XII and XIII etc. Again, the somatic mesoderm of 

 the nephrotome in these segments has a definite outline except in 

 the intersegmental regions, where the outline is ill-defined and where 

 it seems quite likely that cells pass off to reinforce those which 

 already constitute the developing duct. 



In some embryos which I examined the posterior end of the 

 duct consisted of a flat mass of cells between ectoderm and meso- 

 derm, as yet quite unarranged, but in later embryos arranged in the 

 form of a solid cord of cells. 



If there be a proliferation of the somatic mesoderm at the 

 intersegmental regions (as my sections seem to indicate) there is no 

 definite cellular connection between duct and mesoderm in segments 

 XI, XII and XIII until after the time when the duct has grown 

 back past these segments. This is exactly the condition in segments 

 IX and X of the pronephros also. It certainly seems as though 

 mesoderm cells break off from the somatic mesoderm betw^een suc- 

 cessive somites and thereby participate in the growth of the duct: 

 this process is exactly what we might anticipate — the attempt to 

 form a primitive kidney groove in a region where the environment 

 has changed. 



Thus my examination of Chrysemys embryos led me to regard 

 the backward growth of the duct as effected not so much by in- 

 dependent growth, but by proliferation mainly from the interseg- 

 mental regions of the somatic mesoderm, whereby the pronephric 

 duct rudiment is augmented. 



2. Does the ectoderm take any part in the duct formation? 



The only evidence favouring this view^ is to be found in the 

 region extending from about XIII to about XX. 



Casual observation of the earlier stages led me to believe that 



