The Pharyngeal Pouches in the Mammalia 209 



the venlro-lateral portion of the tympanic pouch and the neighboring 

 dorso-internal surface of the external auditory tube. At first the two 

 surfaces are separated by a considerable interval filled with connective 

 tissue. This interval later becomes narrower until it is reduced to an 

 exceedingl}^ thin layer — the membrana propria of the definitive mem- 

 brane. The formation of the tympanic membrane begins on the ventro- 

 lateral surface of the pouch, but subsequently it extends dorsalwards 

 so as to include the portion containing the manubrial fossa. 



After its differentiation the j)ouch as a whole increases in width both 

 laterally and longitudinally. Its posterior portion extends backwards 

 as a prominent projection (posterior recess). The margins become 

 upraised and thus the pouch as a whole assumes a cup-like form. The 

 concavity on the dorsal side, corresponding to the promontory, lies close 

 to the latero-ventral surface of the auditory capsule. 



As already mentioned, my results make it highly probable that the 

 developmental history of the first pharyngeal pouch is in all important 

 respects similar in the three types studied. This probability is still 

 further heightened when the results are compared with those obtained 

 by other investigators. Thus Piersol has described and figured tlie 

 earlier stages in the rabbit. They agree in every important particular 

 with the corresponding stages in the cat and pig. 



The most complete comparison can, thanks to the work of Hammar, 

 be made with the human species. Hammar figures nearly every stage 

 from the typical first pharyngeal pouch to the end of foetal life. I 

 have carefully compared Hammar's descriptions and figures with mine 

 and find that in every important particular they are applicable to the 

 types examined by me. It is in fact difficult to recognize any importaiJt 

 differences, at least as late as the stage when the tympanic pouch and 

 Eustachian tube have been fully differentiated. In the ease of the 

 human sjDecies the tympanic pouch during the later fcetal life gives 

 rise to several outgrowths from its dorso-lateral margin. Prom one of 

 these the mastoidal cells arise as a complex series of buds. In the rabbit 

 these outgrowths had not formed at as late a stage as that of an animal 

 of 21 days. Wliether they are present at the same stage of development 

 in the other two forms I am, at present, unable to say. The latest stages 

 of each, which I was enabled to examine, showed no trace of them. 



Hammar does not lay as much stress as I on the submeckelian fold. 

 He describes its formation correctly, but apparently fails to note its 

 separation from the vestibular folds and its later lateral expansion. 

 His figures, however, leave no doubt that in these particulars the human 



