ANATOMY OF A 17,8 MM. HUMAN EMBRYO 39 



compact epithelial masses (not marked off in the figures) differing 

 in structure from the rest of the anlagen. The cephalic portion 

 of each cylinder is closely applied to the lateral wall of the com- 

 mon carotid artery, and is the part described by Katschenko 

 ('87) as the nodulus thymicus, and by Fox ('08) as the carotid 

 gland. The recent work of Hammar ('11) substantiates the view 

 that this part eventually becomes separated from the thymic 

 cylinder and (coming to lie at the caudo-dorsal border of the 

 lateral part of the thyreoid gland) forms with its fellow of the 

 other side the caudal pair of para-thyreoids. The cephalic ends 

 of the thymic cylinders also show two hollow projections, a medial 

 one, extending toward the pharynx and ending blindly, dorsal to 

 the common carotid artery ; and a more lateral process extending 

 cephalad and ending blindly alongside the ventro-medial surface 

 of the vagus nerve where the latter is crossed on its lateral side 

 by the hypoglossal. The former or medial of these processes is 

 evidently the remains of the thymo-pharyngeal duct. The lat- 

 eral process seems to be the remains of the cervical sinus fused 

 with the third pharyngeal pouch, as maintained for corresponding 

 structures by Katschenko ('87), Fox ('08), and Hammar ('11), 

 and not an outgrowth from the thymus as conjectured by Pren- 

 ant ('94) and Bell ('05). Katschenko from a study of this 

 structure in the pig, maintained that it formed a considerable 

 portion of the head of the thymus, a view since corroborated by 

 Prenant ('94) who, however, considers it of entodermal origin. 

 Fox ('08) found that in the cat it apparently atrophied early in 

 development, and that in later stages of the rabbit it had largely, 

 if not entirely disappeared. He is inclined to think that, when it 

 does persist, it does not form an integral part of the thymus, but 

 merely an associated structvire. 



The fourth pharyngeal pouch of either side, exclusive of the 

 so-called ultimobranchial (postbranchial) body (Greil '05) is 

 represented by a solid epithelial mass {P-thijr. IV) seen in plates 2 

 and 6. These masses, which represent the cephalic pair of para- 

 thyreoids, are situated dorsal to the lateral lobe of the thyreoid 

 and are entirely separate from the pharynx. That on the right 



