503 



From a comparison of embryos about this stage the following 

 general characteristics are observable : 



(1) The pronephric duct does not extend posteriorly into the 

 region where somatic and splanchnic layers of the nephrotome are not 

 yet differentiated. Moreover, the duct invariably keeps ahead of the 

 progress of segmentation backwards. 



an. pr. d. 



ntcd. sp. cd. 



som. I. p. 



spl. I. p. end. 

 Fig. IV. Transverse section of a Chryserays embryo with 11 protovertebrae. 

 On the left side of the figure segment XII, right side, middle region ; on right of 

 figure segment XII left side, posterior region. Lettering as in fig. I. 



Fig. V. Transverse section of Chryseuijs with 18 protovertebrae. The left 

 side of the section passes posterior to the right pronephric duct, the right side of 

 the section cuts across the posterior region of the left pronephric duct. 



coel. general body cavity ; pr. d. pronephric duct ; som. somite. 



(2) The region where tlie duct is present between ectoderm and 

 mesoderm is coincident with that in which the somatic and splanchnic 

 layers of the nephrotome are differentiated but not separated by 

 a cavity. 



