210 Henry Fox 



species agrees with the other types. In some of his figures I am noi 

 certain whether Hammar means to include the submeckelian fold as 

 a part of the recessus anterior or to limit the latter to the dorsal apex. 

 His descriptions seem to me to favor the latter alternative. He applies, 

 at any rate, no distinctive term to the fold, and accordingly I have felt 

 free to call it the submeckelian fold. 



The foregoing remarks make it apparent that the same essential type 

 of development of the tympanic pouch and Eustachian tube holds in 

 species belonging to four different orders of mammals, i. e., Eodentia 

 (Lepus), Ungulata (Sus), Carnivora (Felis), and Primates (Homo). 

 So far as known, other species, which have been much less thoroughly 

 investigated, agree with this type. Accordingly, it seems reasonable to 

 suppose that the same type prevails in the majority of ordinary placental 

 mammals and that it represents the typical development of the struc- 

 tures in the class. In forms which are adapted to a special environ- 

 ment (Cetacea, for example) or which are farther removed from the 

 main phylogenetic series (Edentata) it may show important modifica- 

 tions. So far as I am aware, these forms have not yet been investi- 

 gated in regard to this point. The Marsupials and Monotremes have 

 also not been sufficiently investigated to allow of any assertions being 

 made concerning them. It may be added that a figure by Maurer, show- 

 ing an early stage of the phar^mx in Echidna, bears a striking likeness 

 to that of my 6.5 mm. pig and 6.2 mm. cat. 



B. THE MODIFICATIOXS AND FATE OF THE SECOND PHARYXGEAL POUCH. 



(b') The Eetrogressive Modifications of the Pouch. 



We left the second pharyngeal pouch fully and typically developed 

 in a cat of 6.2 mm. (Figs. 58-60). Its form at that stage is that of a 

 postero-laterally projecting, vertical fold, which is connected by its 

 entire peripheral margin with the ectoderm of the corresponding groove. 

 At its dorso-lateral angle it is produced into a slight elevation forming 

 a dorsal apex (D.A.2) similar to that of the preceding pouch, but 

 considerably less prominent. On its ventral side the pouch is continued 

 as a prominently projecting ventral diverticulum (V.D.2). The deep 

 portion of the latter is limited to the lateral half of the i^harynx, its 

 internal border forming a free edge (Fig. 60). At the base of this 

 edge the diverticulum is continued mesially as a low fold similar to the 

 same part in the first pouch. Like the first pouch, the second has four 

 borders and three surfaces. The borders are antero-lateral, postero- 

 internal, lateral and Central. The surfaces are antero-lateral, medio- 



