The pronephros of Chrysemys marginata. 87 



Fig. 5. Embryo B (section 3,12). Shows how the protovertebra 

 gradually merges into the more lateral part of the mesoderm in this 

 region, viz. end of segment VI and middle of segment YII. 



Fig. 6. Embryo C (section 4,6). Transverse section through the 

 region of the 6th protovertebra. 



Note the position of the primitive kidney groove with regard to the 

 nephrotome. The groove now projects dorsally into a groove in the 

 ectoderm. 



Fig. 7. Embryo C (section 4,17). A transverse section passing 

 between segments VII and VIII. Distinguish nephrotome from lateral 

 plate region. 



Fig. 8. Embryo C (section 4,31). Shows the condition at the hinder 

 end of the kidney outgrowth on the animal's right side. Note the sickle- 

 shaped appearence of the kidney rudiment. 



Fig. 9. Embryo C (section 4,32). This section is the one im- 

 mediately behind that shown in Fig. 8. The duct is now free from 

 the underlying mesoderm on the right side. 



Fig. 10. Embryo C (section 5,8). Through the region of proto- 

 vertebra X on each side. Notice the pronephric duct on the right side 

 is scarcely recognizable, although it has grown back to this level on the 

 animal's left side. 



Fig. 11. Embryo E (section 3,5). Section through the region of 

 protovertebra V. On the left side a trace of the primitive kidney groove 

 remains. 



Fig. 12. Embryo E (section 3,24). This, and the three following 

 figs, represent successive sections in the middle and hind region of proto- 

 vertebra VII. The groove-like condition of the kidney rudiment is very 

 obvious, and also its relation to the coelom. 



Fig. 13. Embryo E (section 3,25). The section behind that shown 

 in Fig. 12. 



Fig. 14. Embryo E (section 3,26). Section succeeding that shown 

 in Fig. 13. 



Fig, 15. Embryo E (section 3,27). Passes through the region be- 

 tween protovertebrae VII and VIII on the right side. Notice traces of 

 the effect of rotation outwards of the distal end of the kidney groove in 

 Figs. 14 and 15. 



Fig. 16. Embryo H (section 4,6). Between segments VIII and IX 

 on left and through seg. IX on right side. Note the condition of the 

 kidney rudiment and the ectodermal groove above it. 



Plate 2. 



Fig. 17, Embryo H (section 4,7). Succeeds the section shown in 

 Fig. 16. 



The duct is now free on the embryo's right side. 



