DEVELOPMENT OF HUMAN PHARYNX 361 



pouches ill certain lower vertebrates. Kolliker ('79), Piersol 

 ('88) and Verdun ('98) described a thymus from the second 

 pouch in the rabbit. Hanson ('11) has more recently shown 

 that no thymus body is derived from the second pouch in this 

 mammal. 



Despite the prevailing opinion and the evidence presented 

 therefor, when critically examined, it is found to offer no sup- 

 port for the localization of thymus anlages in the branchial 

 epithelium. As to the thicker character of the epitheUum of 

 the ventral pocket of the third pouch, it may be stated that 

 growing epithelium tends to be thicker and that simple mechani- 

 cal stresses and strains have clearly a marked effect in causing 

 thinning or thickening of an epithelium, numerous illustrations 

 of which might be given. The thickness of the epithelium can- 

 not in itself, therefore, be taken as a criterion of homology; 

 nevertheless, the tendency to do so is strong; witness the sug- 

 gestion of Hammar ('11) that the thicker character of the epithe- 

 lium on one side of the tubular Complex III in man may mark 

 the localization of the thymus anlage in the epithelium and thus 

 bridge the gap between the condition in mammals and that 

 in lower forms. This fact just alluded to — that in mammals 

 the thymus comes out of the ventral portion of the pouch (so 

 interpreted) while in forms below mammals it develops from the 

 dorsal portion of the pouch, on the assumption of a definite 

 localization of a thymus anlage in the branchial epithelium — 

 is confessedly an objection to the homology of the thymus bodies 

 throughout the vertebrate series, as has been recognized by 

 Maurer ('06), Grosser ('11 b), Hammar ('11), and others. It 

 is only, however, on the assumption of such epithelial localiza- 

 tion that this different mode of origin necessarily opposes such 

 homology, whereas it does give evidence against a primary 

 thymus-forming group of cells in the branchial epithelium. 



The evidence against a distinct thymus anlage becomes mark- 

 edly stronger when the development of the thymus indifferent 

 mammals is considered. In man — as previously stated and 

 as is also well known; confirmed in the work of Hammar already 

 cited — the Complex III in the process of growth and the accom- 



THE AMERICAN JOURNAL OP ANATOMY, VOL. 18, NO. 3 



