306 CHAELES EUGENE JOHNSON 



The third pouch also has severed its connection with the ecto- 

 derm and appears as an elongate, rather thick-walled longitudinal 

 vesicle, extending from the tip of the anterior horn of the hyoid 

 to a point opposite the middle of the posterior horn. The 

 cephalic end of the pouch lies medial to the anterior horn, while 

 the caudal end is lateral to the posterior horn. In length, the left 

 pouch extends through nineteen sections (285At), the right through 

 seventeen sections (255iu). A very short pharyngeal stalk or 

 entodermal duct, now closed, extends through the sixth to the 

 eighth sections, inclusive, on the left and through the fifth to the 

 seventh on the right. On each side the pouch is crescentic in 

 cross-section (fig. 7), but anterior to the pharyngeal stalk the 

 convex side is ventral while posterior to the stalk it is dorsal. 

 The walls of the vesicle are generally of uniform thickness 

 anterior to the pharyngeal attachment, but here and there the 

 epithelium shows a tendency to fold, and at the anterior end 

 solid buds of cells have formed; likewise on the ventrolateral 

 surface of the vesicular wall there is a conspicuous ridge, formed 

 evidently by local proliferation, extending from the anterior end 

 of the pouch to its pharyngeal stalk. This ridge is symmetrical 

 on the two sides of the body and, together with the cell proHfera- 

 tion noted on the anterior wall of the pouch, is apparently the 

 beginning of thymus formation. Caudal to the pharyngeal 

 stalk the ventral wall of the pouch is decidedly thicker than the 

 dorsal, and from the dorsolateral ^vall there projects outwardly 

 a sohd cellular peg which evidently represents the point of separa- 

 tion from the ectoderm. 



The fourth visceral pouch is detached and far removed from 

 the surface epithelium. It is a small, more or less rounded 

 vesicle, with irregular surface contour and with slit-hke cavity. 

 The ventrolateral wall is thickened, especially in its middle 

 portion. The entire vesicle extends through eight sections 

 (120 ju). It is attached to the ultimobranchial vesicle by a short, 

 narrow stalk which contains the last traces of a cavity. The 

 two sides of the body exhibit practically identical conditions. 

 A differentiation into thymus and parathyreoid portions is not 

 with certainty recognizable. 



