DEVELOPMENT OF THE THYMUS 323 



ventral aspect. Its caudal portion is greatly flattened but as 

 it approaches the nerve it gradually assumes a cylindrical form 

 which also is the shape of the vesicula cervicaUs lateralis. A 

 part of its flattened caudal portion dips into the ganglion nodosum 

 while a portion lies in close contact with the thymus. Fusion 

 between the thymus and the vesicle has apparently not yet 

 taken place, for the boundary of both can still be clearly deterr 

 mined. The lumen of the vesicula cervicalis is for the most 

 part obliterated. Only slight traces here and there in its course 

 persist. It is largest in the portion that dips into the ganglion 

 nodosum. Here the lumen is large and the wall of this portion 

 of the vesicle is no thicker than that of earlier stages. Appar- 

 ently no cell proliferation takes place in this region. The sur- 

 face of the entire vesicula cervicalis is more or less irregular. 

 An idea of its shape can best be obtained by referring to figure 

 4 in which the hypoglossal nerve and a part of the ganglion 

 nodosum were removed, thus almost entirely exposing it. 



The parathyroid {Pt. 3) is an elongated and very irregular 

 mass of cells that is tightly packed in between the cervical 

 vesicle and vagus nerve on its dorsal aspect, and the carotid 

 artery on its mesial surface. Its caudo-mesial and caudo-lateral 

 portions are in contact with the thymus while its anterior portion 

 is on a Ifevel with the arch of the vesicula cervicalis over the 

 hypoglossal nerve. 



The thymus {T.e.) is considerably longer than in the preceding 

 stage. Its cephahc and caudal ends have about the same relative 

 position to the other structures as in the 14.5 mm. embryo. Its 

 greater length at this stage is due to growth which has kept 

 pace with the growth of the pharynx. It is still connected with 

 the third pharyngeal pouch by a greatly attenuated cord of cells. 

 Its anterior portion (figs. 3-4) is fused to the caudal aspect of 

 the parathyroid from which it extends caudally. As in the pre- 

 ceding stage, the caudal portion makes a sharp turn in a ventro- 

 mesial direction and lies over the upper portion of the peri- 

 cardium. The caudal portion of the right thymus extends across 

 the mesial plane while the same region of the left thymus lies 

 to the left of the mesial plane and extends farther caudally than 



