The pronephros of Chrj'semys marginata. 



Embryo B. 



This embiyo is slightly older than the preceding one. 



Stained in bulk in Boraxcarmine for 45 hours. After clearing 

 in xylol I was able to count 6 protovertebrae on each side. The 

 sections were mounted upon 5 slides, 36 on each slide. 



Body begins 1,1. 



Left side of embryo. 



. Gut begins 1,32. 

 The mesoderm is first visible 1,26. 



Right side of embryo. 



Gut begins 1,24. 

 Mesoderm first visible 1,16. There 



No trace of segmentation behind is no trace of segmentation behind 



The neuropore is open in this embrj'o; the "rudiment" of the 

 heart extends from 2,7—2,15; the foregut ends 2,15; and the neur- 

 enteric canal is open from 4,23 — 4,29. 



Traces of the kidney 



Left side. 



The condition of the kidney at 

 this stage is in the form of a 

 groove which I have called the 

 "primitive kidney groove". 

 The groove on this side begins 

 at or about 2,23, but is most 

 conspicuous in the region from 

 3,7 to about 3,20 and ends about 

 3,23. 



On both sides the primitive 

 kidney groove loses its nature 

 posteriorly and takes the form of 



or its duct in B. 

 Right side. 



There is a slight thickening of 

 the somatopleuric mesoderm in 

 the lateral plate region, and this 

 tends to become a groove. Traces 

 of this are visible as far forward 

 as 2,14. This appearance is more 

 pronounced in regions between 

 protovertebrae than it is about 

 the middle of the body-segments. 



Thus the groove is very distinct 

 in 2,15 and 2,16; 2,25 and 2,26; 

 2,31 and 2,32 and is well developed 



