220 Henry Fox 



I may add here that the term kiemengang was first applied by Eabl 

 to the endodermal structure here called filiform process. Hammar con- 

 siders Eabl's account as contradicted by his results as determined in 

 the human species, and accordingly applies the same term to the ecto- 

 dermal structure. In case two independent structures, one endodermal 

 and the other ectodermal, are found to exist in mammals, it will be 

 necessary to return to Eabl's original use of the term. 



My rabbit and cat series throw no light on this puzzling matter. In 

 the former I did not examine sufficiently early stages, while in the 

 latter the stages which would show the structures under consideration 

 were lacking in the collection. 



The remaining results agree fairly closely with those of Hammar. 

 The form of the early tonsil in the forms studied by me differs somewhat 

 from that in man, though the difference is a minor one. In regard to 

 the rabbit of 21 days, Hammar does not speak of it as having an infra- 

 tonsillar sinus. My specimen, on the other hand, shows a fold which, 

 in my opinion, corresponds to this sinus. As, however, I was unable 

 to examine forms in which the latter is undoubtedly present, I will not 

 urge this homology. 



The above comparisons would indicate that, although the chief fea- 

 tures in the history of the second pouch agree in all species of mammals 

 studied, there are considerable differences in detail. The matter of 

 the filiform process and kiemengang would illustrate this. In later 

 stages, as Hammar shows and as my specimens indicate, the tonsillar 

 fold differs considerably in its structural details in various species of 

 mammab. 



C. THE METAMORPHOSES OF THE THIRD PHARYNGEAL POUCH AND ITS 



DERIVATIVES. 



(c') The Elongation of the Ventral Diverticulum and the Forma- 

 tion of the Thymus. 



When typically developed, as in the cat of 6.2 mm. (Figs. 58-60), 

 the third pharyngeal pouch bears a considerable resemblance to the 

 second. Like the latter, it bears a deep ventral diverticulum (V.D.3), 

 which likewise is limited to the lateral half of the pharynx, its inner 

 half forming only an inconspicuous ridge. The deep part of the diver- 

 ticulum is only slightly prolonged in a ventro-mesial direction. 



The lateral margin is joined to the ectoderm for almost its entire 

 extent, in this respect also resembling the preceding pouch (see clear 

 area in Fig. 7). 



