The Pharyngeal Pouches in the Mammalia 235 



both it is located at a considerably higher level than the true carotid 

 gland. 



Piersol in his study of the rabbit does not, like Kastschenko, dis- 

 tinguish between the carotid gland and the dorsal extremity of the 

 thymus. It, however, is probably present, as it is clearly distinguish- 

 able in the later stages of the same animal. Its presence in the earlier 

 stages has been shown by Verdun. 



According to Prenant, the merit of having determined that the carotid 

 gland is a proliferation from the epithelium of the third pouch belongs 

 to de Meuron. Prenant describes and figures correctly the histological 

 structure of the organ. His work is based upon the sheep, but his results 

 are in all respects in harmony with what I have observed in the pig. 



In Verdun's work, "Derives branchiaux chez les vertebres superieurs," 

 the term "la glandule hranchiale III" is applied to this organ. The 

 term "carotid gland" he applies to the conjunctival proliferation sur- 

 rounding the carotid artery at its bifurcation. There has been much 

 confusion in the use of this term. As already mentioned, Steida applied 

 it to both structures, though in the first part of his description, i. e., of 

 the earlier stages, he applies it to the endodermal derivative. I have, 

 therefore, retained it for the latter. The conjunctival swelling itself is 

 no gland and consequently does not deserve to be called one. 



The exact share taken by the ectoderm in the formation of the thymus 

 has been a puzzling problem. His early advanced the view that the 

 entire thymus was an ectodermal structure, but soon abandoned it. 

 Fischelis would apparently consider it as half endodermal, half ecto- 

 dermal. Kastschenko derives the bulk of the organ from the endoderm, 

 but considers that its dorso-peripheral portion, which he designates by 

 the term, thymus superfieialis, is of ectodermal origin. 



Since the last 'investigator the part taken by the ectoderm in the 

 formation of the thymus has been largely ignored. The prevailing 

 opinion regards the thymus as of purely endodermal origin. M}'' own 

 observations, however, corroborate the statements of Kastschenko so far 

 as his facts are concerned, but, unlike the latter, I do not consider the 

 so-called thymus superfieialis as of sufficient constancy or importance to 

 warrant the application to it of this rather pretentious term or to be 

 considered as an actual constituent of the thymus. 



Kastschenko describes correctly the origin of his thymus superfieialis 

 by the constriction of the fundus prgecervicalis.- His observations were 



*rhis is "Le fond du troisiime sillon ectodermique" of Verdun. 



