PLATE 2 



EXPLANATION OF FIGURES 



6 Transverse section through the ectodermal duct of the second visceral 

 pouch in an embryo Chelydra 9.5 mm. long. X 300. 



7 Section through the third visceral pouch of the same embryo as figure 6, 

 showing early step in the development of the thymus. X 230. 



8 Transverse section through the fourth pouch derivative of an embryo 

 Chelydra with carapace 9.5 mm. long. X 300. 



9 Transverse section through the left ultimobranchial vesicle of an embryo 

 Chelydra 10.5 mm. long. X 230. 



10 Transverse section through the developing thymus of the right side of an 

 embryo Chelydra 10.5 mm. long. X 230. 



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

 branchial body of the right side of an embryo Chelydra 10.5 mm. long. X 300. 



12 Transverse section through a part of the left lateral wall of the left ultimo- 

 branchial vesicle and attached fourth pouch derivative, from an embryo Chelydra 

 with carapace 9.5 mm. long. Same embryo as figure 8. X 300. 



322 



