ALIMENTARY TRACT AND ITS APPENDAGES 307 



considerable morphological importance, being of very ancient 

 origin and common to the whole series of vertebrates. They are 

 the thyroid body or gland, the thymus, the postbranchial or 

 suprapericardial bodies, and certain epithelial vestiges. 



Fate of the Visceral Clefts. The times of opening and closing 

 of the visceral clefts have been already given (pp. 176 and 177). 

 The later history of the first visceral pouch has been described 

 (p. 297). The second, third, and fourth pouches retain their 

 connections with the corresponding ectodermal grooves for a 

 long time during the thickening of the visceral arches. The con- 

 sequence is, that not only the pouches, but also the ectodermal 

 furrows, are drawn out into long epithelial tubes, and the original 

 closing plate is thus deeply invaginated. In the case of the 

 second cleft the tube ruptures and begins to degenerate on the 

 sixth day, leaving no remnants. In the case of the third and 

 fourth clefts the ectodermal components become solid on the 

 sixth day, and form strands (funiculi prcecervicales) connecting 

 the entodermal pouches with the sinus cervicalis. These strands 

 are subsequently broken through and disappear. Parts of the 

 entodermal pouches, however, persist in the thymus, supraperi- 

 cardial l^odies and other epithelial remains. (See below.) 



Thyroid. The thyroid sac (median thyroid of authors) loses 

 all connection with the pharyngeal epithelium on the fourth day, 

 and on the seventh day it becomes divided in two massive lobes 

 placed bilaterally (see Fig. 178). These then migrate backwards 

 on each side of the trachea towards the hinder end of the deriva- 

 tives of the third visceral pouch (Verdun) and become lodged 

 in the junction of the subclavian and common carotid arteries, 

 where they are found in the adult just internal to the jugular vein. 



The so-called lateral rudiments of the thyroid, or postbranchial 

 bodies, are histologically entirely different from the thyroid proper. 

 They are described below. 



Visceral Pouches. The second visceral pouch leaves no 

 derivatives in the adult ; during the fourth day, however, a con- 

 siderable thickening of the epithelium appears on its dorsal and 

 posterior aspect, near its opening into the pharynx; though this 

 disappears very soon, it may be considered to represent the 

 thymus II of Selachia and Anura. 



The third visceral pouch loses its connection with the pharynx 

 by atrophy of its internal portion between the seventh and eighth 



