208 THE ENDODERMAL DERIVATIVES 



Summary of Embryonic Development of the 11 mm. Frog Tadpole: 

 Endodermal Derivatives 



The derivatives of the primary digestive tube, from anterior to posterior, are: 



Mouth — with the exception of that part from oral aperture to point of the 

 internal nares (choanae) . 



Pharynx — dorso-ventrally flattened cavity into which mouth leads. 



Visceral clefts — junction of endodermal lining of visceral pouch with ecto- 

 dermal invagination from side of head to form clefts associated with 

 visceral pouches II, III, IV, and V. Pouch VI never forms a cleft. 

 By the time the clefts are open they are covered by the operculum so 

 that they lead into the opercular chamber ventro-laterally. External 

 gills degenerating. 



Thyroid gland — now separated from floor of pharynx, dividing posteriorly 

 into two lobes. Found just anterior to heart as paired pigmented 

 masses. 



Thymus gland — derived from dorsal part of first and second visceral pouches 

 (first degenerating) and migrating posteriorly to position near the 

 ear. 



Parathyroid glands (epithelioid bodies) — from ventral portions of third and 

 fourth visceral pouches. Difficult to locate. 



Ultimobranchial bodies — small pigmented masses at level of trachea ventral 

 to sixth visceral pouches. Difficult to locate. 



Trachea and lungs — laryngo-tracheal groove from median ventral floor of 

 pharynx near visceral clefts leads into tubular trachea which bifurcates 

 into two primary bronchi. These expand posteriorly into lungs. 



Oesophagus — gut from laryngo-tracheal groove to stomach, bending behind 

 liver. 



Stomach — identified by folded lining and thick muscular walls. 



Duodenum — from pyloric end of stomach to coiled intestine. 



Liver — highly vascularized and enlarged organ which incorporates vitelline 

 veins. Original liver diverticulum becomes gallbladder with common 

 bile duct (ductus choledochus) leading into dorsal wall of duodenum. 



Pancreas — within curvature of stomach, duct joining common bile duct. 



Intestine — two complete spiral coils posterior to duodenum. Cloaca as yet 

 unformed but pronephric ducts open into posterior end of hindgut. 



