504 



(3) The pronephric region of the kidney is coincident with the 

 condition of most widely separated somatic and splanchnic layers of 

 the nephrotome : there are indications of pronephros in the heart 

 region although they disappear at an early stage in development. 



Growth of the pronephric duct caudalwards. 



There is no doubt that in Chrysemys the pronephric duct arises 

 anteriorly from the somatic layer of the nephrotome in segments YII 

 to X. It is probable that the growth of the duct caudalwards is 

 effected by a reinforcement from the mesoderm, the ectoderm taking 

 no part in the process (see Fig. IV). 



In Fig. V — a section across the posterior region of the duct at 

 a rather later stage (18 proto vertebrae)— the duct is widely separated 

 from the ectoderm and is fused with the- mesoderm just in front of 

 the neurenteric canal region. 



pr. d. 



h.g. 



end. prol. 



end. prol. 



Fig. VI. Transverse section of Chrysemys with 23 protovertebrae. On left 

 side of figure the right pronephric duct is shown in connection with the wall of 

 the gut ; on the right side of the figure the left pronephric duct is shown at a more 

 anterior level. The section is across the region immediately in front of the neur- 

 enteric canal. 



end. prol. mesoderm produced by proliferation of endoderm; h.g. hind-gut; 

 ntcd. notochord ; pr. d. pronephric duct ; som. somite ; sp. cd. spinal cord. 



A short distance in front of the neurenteric canal — where the 

 mesoderm and endoderm are indistinguishably fused — the posterior 

 end of the pronephric duct fuses with the wall of the gut (see fig. VI). 



