THE FOREGUT 



Visceral arches 



195 



IV V 



External gills 



Hyoid arch 

 Mandibular arch 



Visceral groove 



III IV V 

 Aortic arches 



Frontal (horizontal) reconstruction of the external gill stage of the frog larva. 

 (Redrawn and modified after Huettner.) 



posterior) pair rarely open as pouches, remaining as mere cords of 

 endoderm. By definition we use the term "pouch" for the endodermal 

 evagination, "groove" for the ectodermal invagination, and "cleft" 

 for the combination of ectoderm and endoderm which meet when these 

 grooves and pouches break through. Visceral pouches I to V inclusive 

 appear in this order and grow laterally toward the head ectoderm, 

 all but the first and fifth, meeting a corresponding groove and break- 

 ing through to form clefts. Occasionally a sixth visceral pouch is de- 

 veloped, but it is generally vestigial. 



These clefts can be classified in tabular form as follows: 



Pouch (Endoderm) 



Arch (Mesoderm) 



Cleft (Ectoderm and 



Endoderm) 



Groove (Ectoderm) 



Visceral pouch I 



'Does not perforate. 



Visceral arch I — mandib- 

 ular 

 (Aortic arch I) 



* Visceral cleft I 



(Hyomandibular— 

 Eustachian tube. 

 Ectodermal plate — 

 tympanic membrane) 



Visceral groove I 



