506 



The prouephric tubules are not all quite similar; but vary in 

 a gradational manner from before backwards. 



The following diagrams show the nature of the gradation (Fig. VII). 



These slight differences in different regions of the pronephros 

 are due to the following causes : 



(1) A shifting dorsal wards of the tubule opening. 



(2) The formation of a space between the somatic and splanchnic 

 layers of the nephrotome in its ventral region into which 

 the tubules come to open. 



(3) The tendency which the coelomic epithelium ventral to the 

 pronephros acquires for fusing with the vascular glomus, thereby 

 producing either an internal glomerulus or else both an 

 internal and external glomerulus. 



sp. cd. 



pr. d. 



Figs. VIII, IX, X. Transverse sections through a Chrysemys embryo with 

 23 protovertebrae. Figs. VIII and IX are separated by one section. Figs. IX and 

 X represent successive sections ; on the left side in Fig. VIII middle of segment 

 X right side is shovrn ; on the right side the anterior region of segment X left side. 

 Fig. IX is two, and Fig. X three sections further back. 



e.V. cardinal vein; d.ao. dorsal aorta; gl. glomus; nep. nephrotome; other 

 lettering as in Fig. I. 



Figs. VIII, IX and X illustrate the change which takes place at 

 the hind end of the pronephros. 



I examined an embryo Scyllium canicula (7 m.m. long) and 

 found the same changes clearly visible in the region of the groove- 

 like anterior end of the Wolffian duct (segments VIII and IX on 



