THE FOREGUT 199 



from the pouches, become invested with mesenchyme, and move to 

 the final position just posterior to the tympanic membrane and ven- 

 tral to the depressor mandibulae muscle. This gland is larger in the 

 younger frogs, attaining its maximum size in frogs of about 20 mm. 

 body length. It is a lymphoid gland, presumably a source of some 

 blood corpuscles, and apparently is essential to the early develop- 

 ment of the frog. 



The Carotid Glands. 



From the ventral ends of the first branchial (second visceral) pouch 

 there arise cell proliferations, at about the time the internal gills 

 appear (9 mm. stage) which develop into the carotid glands. These 

 glands develop and move to the junction of the internal and external 

 carotid arteries. Their function is to regulate the flow of blood, partic- 

 ularly that entering the internal carotid artery. This is accomplished 

 by means of its final spongy consistency. They may aid also in achiev- 

 ing adequate aeration of the blood. 



The Parathyroid Glands (Pseudo-thyroid or "ventraler Krimenust"). 



From the ventral ends of the second and the third branchial (third 

 and fourth visceral) pouches are derived the parathyroid bodies, other- 

 wise known as the pseudo-thyroid bodies. In the adult these are 

 small, rounded, vascular glands which come to lie on either side of 

 the posterior portion of the hyoid cartilage. 



The Ultimobranchial Bodies (Supra-pericardial or Post-branchial 

 Bodies). 



The fourth pair of branchial (fifth visceral) pouches have no 

 glandular derivatives, but a pair of ultimobranchial (post-branchial) 

 bodies arise by cellular proliferations from the rudimentary fifth 

 branchial (sixth visceral) pouches. These organs arise as solid pro- 

 liferations of the pharyngeal wall and come to lie beneath the mucous 

 membrane of the pharynx lateral to the glottis. They shortly separate 

 from the pharynx and acquire cavities. The structure in the adult is 

 somewhat like that of the thyroid gland but the function has not 

 yet been determined. 



The Thyroid Gland. 



The thyroid is an endocrine gland which arises as a single median 

 thickening and evagination from the floor of the pharynx between the 



