ABBREVIATIONS 



Am., aortic arches 



Ar.car., carotid artery 



Ao.r., aortic root 



Br., bronchus 



D.ao., dorsal aorta 



Oes., oesophagus 



Par. Ill, IV, parathyreoids, derived 



from the third and fourth pouches, 



respectively 

 Ph., pharynx 

 Ph.div., pharyngeal diverticulum 



S.a., systemic arch 



Thy., thymus 



Thr., thyreoid 



Tr., trachea 



U.b., ultimohranchial vesicle or body 



Vag., vagus nerve 



V.a.5, fifth visceral arch 



Ves., vesicular portion of parathy- 

 reoids 



V.-p. 2, 3, 4, 5, visceral pouches, second 

 to fifth 



PLATE 1 



EXPLANATION OF FIGURES 



1 Frontal section through the posterior pharyngeal region of an embryo 

 Chelydra serpentina 5 mm. long. X 80. 



2 Frontal section through the corresponding region of an embryo Chry- 

 semys marginata 6.5 mm. long, showing developing fifth visceral arch and the 

 differentiated fourth and fifth visceral pouches. X 80. 



3 Same embryo as figure 2, section taken farther ventrally, showing relation 

 of fifth pouch to ultimobranchial vesicle. X SO. 



4 Frontal section from an embryo C. serpentina 7.5 mm. long, passing 

 through the main body of the ultimobranchial vesicle and showing also progres- 

 sive development of the fifth visceral pouch. X 80. 



5 Frontal section from an embryo C. serpentina 9 mm. long, showing 

 connection between fourth visceral pouch and ultimobranchial vesicle and also 

 a further step in development of the fifth visceral pouch and corresponding vis- 

 ceral arch. X 80. 



320 



