The pronephros of Chrysemys marginata. 



neurenteric canal open from 5,2 — 5,7. 



It will be seen that in embryos A and B and in older stages 

 the first protovertebra is not so clearly differentiated as are those 

 succeeding it, hence there is difficulty in ascertaining the exact 

 limits of the first metaotic protovertebra (I) in many cases. 



Traces of the kidney or its duct in A. 



Right side. 



The most anterior limit of the 

 primitive kidney groove on this 

 side is 2,10, from which it is 

 visible at least to 2,18. At inter- 

 vals backwards to 2,38 it is more 

 or less discernible but after 2,38 

 it is most marked, gradually de- 

 creasing in size and distinctness 

 back to 3,16 after which it is not 

 differentiated from the proto- 

 vertebra. 



Left side. 



The somatic layer of the meso- 

 derm begins to thicken noticeably 

 at 2,14. 



From 2,16 to 2,22 the "rudi- 

 ment", or "primitive kidney groove" 

 as I propose to call it, is distin- 

 guishable, and again from 2,26 to 

 2,28; also from 2,31 to 2,35; it is 

 feebly developed from 2,38 to 3,2, 

 and at its maximum from 3,2 to 

 3,20 after which it gradually de- 

 creases until it is quite unrecog- 

 nizable after 3,22. 



(See Plate 1 Figs. 1, 2 and 3.) 



Thus the left kidney rudiment begins and ends more posteriorly 

 than the same structure on the right side. The regions between 

 segments often show the rudiment of the groove more developed 

 than in the segments themselves. The coelom extends on the left 

 side from 2,6 to 3,23 (approximately) and from 2,6 to 3,13 approxi- 

 mately on the right side. 



On the left side the segments affected are III, IV, V, VI, VII 

 and on the right side II, III, IV, V, VI 



(Segments in italics are those in which the kidney rudiment 

 is well defined). 



