28 CHARLES EUGENE JOHNSON 



V extends anteromedially across the ventral surfaces of the 

 jugular vein and vagus. This limb is a solid cylindrical mass and 

 is the anlage of the thymus III. It represents a rapidly pro- 

 liferating area, including the anterior and dorsolateral walls of 

 the pouch. A continuation of the thymus anlage forms the apex 

 of the V, and the ectobranchial duct is connected chiefly, if not 

 wholly, with this part of the pouch, but the exact limits of ecto- 

 derm and entoderm cannot be definitely made out. The other 

 limb of the pouch is a somewhat flask-shaped vesicle with walls 

 of uniform thickness in the enlarged part ; the neck portion is the 

 drawn-out pharyngeal end of the pouch, which is hollow through- 

 out and extends caudo-medially across the space between the 

 third and fourth aortic arches. Another embryo of the same 

 age shows the next step in the development of this pouch. It 

 has here separated from the ectoderm and straightened out, lying 

 lengthwise in the neck, just ventral to the jugular and the vagus 

 and lateral to the third aortic arch. The thymus portion can 

 now be followed as a ridge-like thickening, nearly to the posterior 

 extremity of the pouch. The vesicular portion is somewhat 

 flattened laterally and its narrow dorsal and ventral walls con- 

 sist as yet of un thickened epithelium; the medial wall, on the 

 other hand, has become distinctly thickened and constitutes the 

 first trace of the parathyreoid body III. 



The fourth visceral pouches at six and three-quarter days are 

 completely detached from the ectoderm, and that of the right 

 side has also separated from the pharyngeal wall. These 

 pouches are scarcely half the size of the third, and have a rather 

 short, club-shaped form, the anterior ends tapering more or less 

 and pointing toward their former ectodermal connections. The 

 pouches have a relatively large central cavity and the anterior 

 wall is somewhat thickened, but otherwise no regional differen- 

 tiations are noticeable. 



On the left side a rather misleading situation has arisen in the 

 relations of the third pouch and the postbranchial body (figs. 

 1 and 2). The latter is still attached to the pharyngeal wall 

 by a slender remnant of the entobranchial duct, but lies further 

 cephalad than in the preceding stages, being now opposite the 



