236 Henry Fox 



made upon pigs, and the large size which the outer free lobe of the 

 fundus attains in these creatures probably led him to consider it as an 

 important part of the dorsal extremity of the thymus. Its similarity 

 in histological structure to the thoracic thymus was another fact upon 

 which he based his view as to its importance. 



To me these facts do not warrant the ascription of the ectodermal 

 structure under consideration to the thymus. First, as regards the 

 prominence of the outer lobe of the fundus, it is evident from my 

 observations on the cat that this condition is not general among mam- 

 mals. Even in the pig it does not seem to be absolutely constant, since 

 Kastschenko himself speaks of an animal of 30 mm., in which he could 

 find no trace of a thymus superficialis. In the sheep, according to 

 Prenant's account, it is evidently similar to that in the pig. It might 

 be inferred from these facts that a prominent fundus prsecervicalis is 

 limited in late foetal life to the ungulates,^ but is probably more or less 

 early atrophied in other forms. 



With regard to the histological structure of the so-called thymus 

 superficialis, it will suffice to state that its similarity in this regard to 

 the thymus is no greater than that which any branching epithelial mass 

 shows. In specimens which I examined the lobuli of the fimdus prsecer- 

 vicalis bore as strong a resemblance to those of the salivary glands as to 

 the same parts in the thymus. The resemblance is therefore unimpor- 

 tant. 



I would therefore conclude that the fundus praecervicalis is to be 

 looked upon as an associate of the thymus, but not as an integral 

 part of it. 



Prenant evidently considers the vesicula thymica of Kastschenko as 

 including all the derivatives of the fundus praecervicalis. He regards 

 it apparently as of endodermal origin and hence as a part of the thymus. 

 He says, "La tete du thymus se developpe aux depens de la 3d poche 

 entodermique et d'un diverticule de cette poche; celui-ci, qui est sans 

 doute identique a la vesicule thymique Kastschenko, s'enforce dans 

 le ganglion du vague." This statement is erroneous. The vesicula 

 th}anica of Kastschenko is not a diverticulum of the third pouch, but 

 represents the inner blind recess of the fundus prascervicalis, which I 

 prefer to call the vesicula prsecervicalis. 



Verdum correctly considers his "le fond du troisieme sillon ecto- 

 dermique" as corresponding to the "vesicule thymique" of Kastschenko. 



^Verdun, however, states that it disappears entirely in an 18 mm. calf. 



