THYMUS 



171 



The thymus is thus in its first origin bilateral. A pocket develops from the 

 third cleft on each side, which extends downwards as a thick- walled tubular 

 prolongation along the carotid artery. The pocket persists as the thymus vesicle 

 in the proximal section of each rudiment (fig. 216). From the lower end of the 

 tube solid epithelial buds are given off, and from these lateral buds again come 

 off, so that this part of the gland acquires a ramified lobular appearance like an 

 acinous gland. The acini are, however, solid, and remain so. The two rudiments 



par. IV. lot. thyr. 



K'&?39||5^ 



*$m&3&?r i 



I 

 par. III. 



FKJ. 216. SECTION THROUGH THE BRANCHIAL DERIVATIVES IN A HUMAN EMBRYO OF 15'5 MM. 



(T. H. Bryce.) 



<'.., carotid artery ; vg, vagus nerve ; j.v., jugular vein ; mes. thyr., mesial thyroid ; lat. thyr., lateral 

 thyroid ; thym., thymus ; par. IV., parathyroid of the fourth pouch ; par. III., parathyroid of the 

 third pouch. It appears here as a cellular mass closely related to the thymus rudiment ; in the 

 adjoining section it becomes free, and forms a rounded cord having exactly the same structure as the 

 parathyroid of the fourth pouch. 



are brought into close contact with one another in front of the trachea (fig. 215), 

 and unite to form a single-lobed body, which comes to lie in the anterior 

 mediastinum in close relationship with the pericardium. 



The surrounding vascular connective tissue forms a capsule to the gland, extends between 

 the several buds so as to divide it up into lobules, and also sends processes carrying vessels with 

 them into the interior of the lobules. Each lobule becomes differentiated into a cortical and 

 medullary zone, but before this the gland loses its epithelial structure and assumes the appear- 



