558 THE HUMAN EMBRYO. 



still further interrupted by the formation of the cartilaginous 

 body of the hyoid, which lies exactly in its path. 



The paired lateral thyroid rudiments early separate from 

 the epithelium, and form a pair of lobed masses lying at the sides 

 of the larynx, and of considerably larger size than the bifurcated 

 median rudiment. At a later stage they shift still further back, 

 so as to lie alongside the trachea, and then fuse with the median 

 rudiment to form the definite thyroid body. The median 

 rudiment gives rise to the isthmus of the adult thyroid, and 

 probably to parts of the lateral lobes as well ; the greater part 

 of the lateral lobes, however, are formed from the much larger 

 lateral rudiments. 



At an early stage the lobes are excavated by a number of 

 detached cavities, which become the vesicles of the adult thyroid 

 From the history of their development it follows that the epi- 

 thelial walls of these vesicles are of hypoblastic origin. 



The duct or stalk of the middle thyroid rudiment usually dis- 

 appears in great part ; detached portions of it not uncommonly 

 persist as accessory suprahyoid or epihyoid bodies, or as cysts. 



Occasionally the upper part of the stalk persists as a tube, 

 the lingual duct, extending from the foramen caecum, on the 

 dorsum of the tongue, backwards and downwards towards the 

 body of the hyoid, or actually reaching this in some cases. 



The lower or posterior part of the stalk may also occasionally 

 persist, forming the so-called pyramid of the thyroid, a some- 

 what pyriform body, enlarged and saccular at its lower or 

 posterior end, and tapering upwards to a fibrous cord which is 

 attached to the dorsal surface of the hyoid bone. The pyramid is 

 apparently formed by persistence and enlargement of one of the 

 two branches into which the stalk bifurcates at its lower end. 

 When present, it is visually single, but cases have occurred in 

 which two pyramids were found, due apparently to persistence 

 of both branches of the bifurcation. 



6. The Thymus. 



The thymus is a paired organ of epithelial origin, developed 

 in connection with the second and third branchial clefts, and 

 perhaps the first branchial cleft as well. 



It appears about the middle of the fifth week ; but as to the 

 precise mode of its formation there is still some doubt. Born 



