PLATE 1 



EXPLAXATION OF FIGURES 



1 Transverse section through the postbranchial body and the posterior 

 portion of the third visceral pouch of a grebe embryo of six and three-quarter 

 days. X 50. 



2 A section from the same series, through the posterior end of the post- 

 branchial body, showing its close association with the pharyngeal end of the 

 third pouch. X 210. 



3 Transverse section of the thymus and parathyreoid derivatives of the 

 fourth visceral pouch of a grebe embryo of seven days. X 210. 



4 A section from the same series, taken near the caudal end of the pouch. 

 X 210. 



5 Transverse section near the caudal end of the third pouch of a grebe embryo 

 of seven days. X 210. 



6 A nearly horizontal section through the thymus derivatives of the third 

 and fourth pouches of a grebe embryo of eight and one-half days. X 50. 



7 Transverse section of a grebe embryo of five days, showing the fourth, 

 fifth, and sixth pouches of the left side. X 50. 



8 Transverse section of a grebe embryo of five and one-half days, showing 

 the same pouches as figure 8, the section cutting the posterior wall of the sixth 

 pouch. X 50. 



9 Transverse section of a grebe embryo of twelve days, showing the posi- 

 tion of the postbranchial body with respect to the thyreoid, parathyreoid, and 

 carotid glands. The section is taken just behind the parathyreoid IV, which 

 therefore is not shown. X 30. 



ABBREVIATIOXS 



Ao.3,Jt,5,6, third, fourth, fifth, sixth Thy.III,IV, thymus of the third and 



aortic arches the fourth visceral pouch 



C.gl., carotid gland Thr., thyreoid 



Car., carotid artery V.a.V,VI, fifth and sixth visceral 



Par. 1 1 1,1 V, parathyreoid of the third arches 



and the fourth visceral pouch. Vag., vagus nerve 



P.b., postbranchial body V.p.III,IV,V,VI, third, fourth, fifth, 



Phar., pharynx sixth visceral pouches 



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