DEVELOPMENT OF THE THYMUS 321 



derm. This process was not present in any of the embryos I 

 examined. 



The vesicula cervicahs (fig. 2, V.c.) hes lateral to the third 

 pharyngeal pouch, between the cephalo-dorsal portion of the 

 parathyroid {Pt. 3) and the caudo-ventral part of the ganglion 

 nodosum (Gji.) and a short distance anterior to the hypoglossal 

 nerve (N.XII). Its general shape is fusiform, with its long 

 axis almost perpendicular to the surface ectoderm. The middle 

 third is solid, but each extremity contains a cavity. The expan- 

 sion of the central portion is due to a proliferation of the cells 

 of its anterior wall which presses tightly against the dorsal sur- 

 face of the parathyroid. The pressure against the parathyroid 

 has apparently caused the obliteration of the cavity of the 

 vesicula cervicalis in its central portion. Its inner third lies 

 closely along the ventral side of the ganglion nodosum into which 

 its curved end projects and with which it apparently is fused. 



The parathyroid {Pt. 3) is now a massive structure lying 

 lateral to the third pharyngeal pouch {S.h.3), anterior to the 

 hypoglossal nerve with which it is in contact, ventral to the 

 ganglion nodosum and dorsal to the carotid artery (A.c). Its 

 general shape is that of a hemisphere with its flat side turned 

 toward the vagus nerve and the vesicula cervicalis. A depres- 

 sion in both the vesicula cervicalis and the parathyroid mark 

 the points of most intimate contact between the two structures. 



The entodermal anlage of the thymus (T.e.) is now, through- 

 out its greatest extent, a solid cord of cells, and still attached to 

 the third pharyngeal pouch. Its anterior end lies closely against 

 the parathyroid and the hypoglossal nerve. From its point of 

 origin it extends caudally, and, with the exception of about one- 

 fourth of the posterior portion, lies ventro-laterally to the carotid 

 artery and the vagus nerve. The caudal portion makes a rather 

 sharp turn in a ventro-mesial direction and occupies the upper 

 part of the pericardial region. The diameter of the caudal part 

 is considerably greater than that of the remaining portion. This 

 is due to the presence of a large lumen and thick walls in this 

 region. In the central and anterior portions only a few slight 

 traces of a lumen persist. The vesicula cervicalis and the an- 



