502 



At this stage it is quite impossible to distinguish pronephric 

 tubules, the latter only becoming differentiated when the segmentation 

 process has made its impress felt upon the nephrotome region. 

 Although it has been stated that the first permanent trace of the 



som. I. p. som. rttcd. sp. cd. 



ect. an. pr. d. 



spl. I. p. 



Fig. II. Transverse section of a Chrysemys embryo with 12 protovertebrae. On 

 left of figure is shown posterior end of segment VII right side ; on right of figure is 

 shown the middle region of the same segment on the left side. Lettering as in Fig. I. 



kidney does not occur in front of segment VII, yet on the left side 

 of tlie embryo the groove extends forwards as far as the front end 

 of segment VI, and as far forwards as the middle of segment V on 

 the right side. 



Erom the above figs, it will be clear that the embryonic Chelonian 

 shows more primitive features at this stage than does the Chick, not 



sp. cd. 



an. pr. 



Fig. III. Transverse section of a Chrysemys embryo with 11 protovertebrae. 

 On left of figure is seen segment IX of the right side, posterior region ; on right of 

 figure segment X of left side, posterior region. Lettering as in figure I. 



only in the more obvious origin of the pronephric duct, but also in 

 widely communicating somite and lateral plate regions. The last- 

 named condition is even more pronounced at a slightly later stage. 



