The pronephros of Chryserays marginata. 47 



an ectodermal participation was probable, but an examination of 

 older embryos minimized this belief. 



The ectoderm is distinctly grooved in the region immediately 

 above the growing duct (Plate 1 Fig. 16), and since the somatic meso- 

 derm lies close beneath the ectoderm, the duct between comes to lie 

 in the ectodermal groove above mentioned. Further, since the cells 

 at the posterior end of the duct are not at this stage arranged 

 in a solid cord, they present the appearance of merging into the 

 ectoderm above. 



In all cases the cells of the duct can be distinguished from the 

 ectoderm both by their deeper stain and by the arrangement of the 

 nuclei, the nuclei of the lowest ectoderm cells having an epithelial 

 arrangement. But the most important evidence against an ectodermal 

 participation in the duct formation is to be found in embryos with 

 18 or more protovertebrae. In these embryos the ectodermal 

 groove is quite distinct, its contour is unbroken, and the duct is not 

 only differentiated by staining deeply (like the mesoderm below), but 

 in many sections at the extreme posterior end of the duct, mesoderm 

 and duct are undeniably fused, and can be no further distinguished 

 from each other. 



In embryos with 23 protovertebrae the duct reaches back 

 on each side to the gut, and fuses with the gut wall in the region 

 of the primitive streak (viz. at the lips of the former blastopore), 

 where mesoderm cells are being actively proliferated from endoderm, 

 and where it is impossible to cells differentiate endoderm from meso- 

 derm cells. 



The point of fusion of duct with gut wall in the primitive streak 

 region is an important clue to the explanation of the meaning of 

 the opening of duct into cloaca. So far as I am aware this point 

 has not been noticed in any other vertebrate: its observation is 

 rendered easy in Chrysemys owing to the comparatively late per- 

 sistence of the neurenteric canal etc. 



The problem of the fusion of the duct with the gut is thus a 

 simple one in Chrysemys, and the fact that the region of fusion is 

 common to both mesoderm and endoderm explains the opening of 

 pronephric duct into the cloaca in a simple and not unexpected 

 manner (see Plate 2 Fig. 24, 25, 26). 



Thus the evidence for an entirely mesodermal origin of the 

 pronephric duct in Chrysemys is overwhelming as regards the front 

 and hind ends, and the contiguity of duct and ectoderm for a time 



