PLATE 4 



EXPLANATION OF FIGURES 



17 Transverse section through the fourth pouch derivative and the ultimo- 

 branchial vesicle of the right side of an embryo Trionyx sp. with carapace 9 mm. 

 long. X 230. 



18 Transverse section through the same structures of the right side of an 

 embryo Chrysemys 6 mm. long. X 230. 



19 Transverse section through the fourth pouch derivative of the right side 

 of an embryo Trionyx with carapace 13 mm. long. X 230. 



326 



