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 (juniculi precervicales) 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 bodies 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 post branchial 
bodies, are histologically entirely different from the thyroid proper. 
They are described below. 
Visceral Pouches. The second visceral pouch leaves ino 
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 IT 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 
