The pronephros of Chryseinys margiuata. 13 



8,39-4,3 y 3,37—4,1 



4,4 -4.13 VI 4,3 -4,11 



4,14—4,22 VII 4,13-4,21 



4,24—4.32 VIII 4,22—4.30 



4,33—4,41 IX 4,32-4,40 



4,43—5,7 . X 4,42-5,7 



5,8-5,17? XI 5,8-5,17? 



Gut begins 2,2; neurenteric canal blocked up (6,15 to 6,30 

 approx.) ; heart extends from 2,30 to 3,23 approx. ; foregut ends 3,24. 



On the left side of the body the rudiment of the kidney is in 

 the form of a thickening of the somatic layer of the lateral plate 

 mesoderm extending from 4,4 to 4,34: after this section there is a 

 backward continuation of the thickening in the form of a solid 

 cord of cells lying freely between mesoderm and ectoderm but staining 

 deeply as do the mesoderm cells. There seems to be no doubt that 

 this cord of cells is formed from mesoderm by proliferation, or 

 splitting, or backward growth of the duct itself. In the regions 

 between successive protovertebrae the kidney rudiment is more 

 marked than in the regions opposite the protovertebrae. The 

 solid cord of cells is distinctly flattened posteriorly; it does not 

 fuse with the ectoderm; its end appears to consist of an aggre- 

 gation of cells which have recently been proliferated from the neigh- 

 bouring mesoderm in the intersegmental regions, where there is an 

 unevenness of the contour of the somatic mesoderm which suggests 

 that this proliferation has occurred. The duct ends about 5,3. 



Hence on the left side the segments affected at this stage 

 are VI, VII, VIII whereas the duct extends over segments IX 

 and X. 



There is a slight trace of a connection of duct and underlying 

 mesoderm in segment IX (4,34 and 4,35). 



On the right side, the somatic thickening of the mesoderm 

 appears first at 4,1 and is continuous back as far as 4,31. A careful 

 examination of the kidney rudiment in segments VI, VII and VIII 

 will reveal the fact that the rudiment seems to get further and 

 further from the general coelom as one passes from the anterior region 

 of segment VI to the posterior region of segment VIII. The rudiment 

 is throughout this region the same- distance from the respective 

 protovertebrae, and so one can only explain the change by supposing 



