334 TEXT-BOOK OF EMBRYOLOGY. 



each side (Fig. 296) . The outgrowths are at first hollow and communicate with 

 the pharyngeal cavity; later they become solid and (in embryos of 14 mm.) lose 

 their connection with the parent epithelium. They elongate and grow caudally 

 in the mesodermal tissue until (in embryos of 16 mm.) their caudal ends lie 

 ventral to the carotid arteries (Fig. 298). In embryos of 29 mm. their caudal 

 ends rest upon the cephalic surface of the pericardium, their cephalic ends 

 reaching to the isthmus of the thyreoid. The two parts eventually fuse to a 

 considerable extent, but the gland as a whole always consists of two distinct 

 lobes. 



The gland continues to enlarge, at the same time becoming lobulated by the 

 ingrowth of connective tissue, until the child is two or three years old. At this 

 time it is situated in the anterior mediastinum, usually in the medial line. After 

 this it begins to atrophy and becomes a mass of fibrous and fatty tissue through 

 the growth of the interlobular septa and their encroachment upon the lobules. 

 The classical view that the thymus begins to atrophy after the second or third 

 year and is quite degenerated in the adult has recently been somewhat offset 



Lat. thyreoid 

 (postbr. body) 



FIG. 299. Diagram of the branchial groove derivatives in man. Verdun. 



by the view that comparatively slight changes take place in it until puberty. 

 According to the latter view, degeneration goes on after puberty at a rate which 

 varies widely in different individuals, and the thymus may persist as a functional 

 organ up to the age of sixty years. 



The histogenesis of the thymus has been a subject of much study and con- 

 troversy, not only in regard to its origin, but also in regard to its change from 

 an epithelial to a lymphoid structure and the regressive changes in the latter. 

 It has almost certainly been proven to be of entodermal origin. It is at first an 

 epithelial mass which later becomes broken up into lobules by the ingrowth of 

 connective tissue. In regard to the histological changes which it undergoes, 

 the older views are in general that a " pseudomorphosis " takes place; that is, 

 the epithelial elements are replaced by lymphoid cells which wander in from 

 the neighboring blood vessels, HassalPs corpuscles being remnants of the 

 epithelium. Later other investigators looked upon the changes as a " transfer- 



