THYMUS GLAND. 



227 



of the second visceral clefts, which extends inwards from the integument, as well 

 as from the covering of the corresponding visceral swellings. All these parts 



FIG. 185. THYMTJS AND THTBOID OF A Yorxc STOEK. 

 T, trachea ; B, bronchi ; Oe, oesophagus ; H t heart ; Tin, thymus ; Tr, thyroid. 



are gradually withdrawn inwards and separated from the surface at the 

 boundary between head and neck. 1 



1 His states that in human embryos the gill-clefts never become perforated, but 

 simply blind slits are formed from "both epiblast and hypoblast. The same holds 

 good also for the tympano-Eustachian passage. 



Q 2 



