The Pharyngeal Pouches in the Mammalia 219 



authors, while agreeing in certain respects, are hopelessly discordant in 

 others. The most satisfactory account is that given by Hammar of 

 the development in man, Hammar also describes a few stages in the 

 formation of the tonsil in several other species of mammals. In the 

 main my results are in harmony with his. The only statement of his 

 to which I cannot subscribe is that the "hiemengang" (== my filiform 

 process?) is an ectodermal derivative. As his figures show, this struc- 

 ture occupies the same relative position as my filiform process. In 

 Hammar's view this is formed by the passive deepening of the ecto- 

 dermal groove produced by the growth of the hyoid region. He also 

 pictures it as protruding above the margin of the pouch as a dorsal 

 organ. In the pig, on the other hand, this structure is perfectly con- 

 tinuous with the ventro-lateral angle of the pouch and appears as a 

 prolongation of the latter. In none of the specimens examined by me 

 did I notice any communication between the lumen of the filiform 

 process and the exterior. As for a dorsal organ projecting above the 

 ventro-inferior edge of the pouch, I find no evidence of it. Accordingly, 

 I am disposed to think that the filiform process and the "kiemengang" 

 are independent structures. The latter would then be absent in the 

 pig, while the former would be lacking in the human species. This 

 view is supported by the observations of Piersol on the rabbit. He 

 speaks of the ventro-lateral angle as continued in a blind tube. This 

 evidently corresponds to the filiform process. He later speaks of the 

 latter as cutting off from the pouch and undergoing changes reminiscent 

 of the thymus. I find nothing of this in the pig. In the latter the 

 process simply disappears — at least I have not seen any trace of it 

 later than a 17 mm. animal. On the other hand, Piersol describes a 

 long epithelial tube, which according to him arises from an insinking 

 of the ectodermal groove. This corresponds to the "hiemengang" as 

 described by Hammar. Piersol states that it originates while the 

 filiform process is still present (twelfth day), but later (fourteenth 

 day) it disappears without leaving a trace. If the facts as described 

 are correct, the rabbit shows the filiform process and "Tciemengang" as 

 independent formations. The latter is evidently an extremely tem- 

 porary structure. As already mentioned, I saw no trace of it in the 

 pig, and in this I am in agreement with Eabl and Kastschenko, both of 

 whom studied the same animal. The short duration of the "kiemen- 

 gang" may perhaps have led to its being overlooked in this animal. 



