The Pharvnoeal Pouches in the Mammalia 191 



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The second pharyngeal pouch, although considerably smaller than the 

 first, is essentially similar to it. It has a ventral diverticulum, which is 

 somewhat less prominent than the same part in the first pouch. 



The third pharyngeal pouch is considerably smaller than its two 

 predecessors. It forms a finger-like outgrowth, which extends outward^ 

 and downwards and, in the case of that on the right side, joins the 

 ectoderm. The left pouch does not quite reach the latter. 



The fourth pharyngeal pouch is only barely indicated by a slight 

 bulging of the walls of the pharynx behind the base of the third pouch. 



The first three pairs of aortic arches are now fully developed and a 

 fourth is beginning to develop. 



In a pig embryo of 6.5 mm. (M- of my collection) and a cat of 6.2 

 mm. (No. 380, Harvard collection) all the pharyngeal pouches and 

 their associated parts are typically developed. The two embryos show 

 almost the same relative stage of development, but that of the pig 

 shows a slightly more primitive condition. It will, accordingly, be 

 considered first. 



The pharynx (Figs. 1, 2 and 3) shows four complete pairs of pharyn- 

 geal pouches, all of which have a more or less extensive contact with 

 the ectoderm of the corresponding grooves. Between the first two pairs 

 of pouches the pharynx is considerably wider than in the region between 

 the last two pairs. Anteriorly the hypophysis (HYP.) projects forward 

 as a blunt protuberance, and immediately back of it arises a minute 

 conical process, the representative of Seessel's pocket. 



The pharyngeal pouches in general have the form of vertical wing- 

 like expansions projecting outwards and slightly backwards from the 

 side walls of the pharynx. Typically, they are joined to the pharynx 

 by a relatively narrow base and only laterally dilate into the wing-like 

 expansions mentioned. Each pouch is attached laterally to the ecto- 

 derm. The extent of this attachment is shown by the clear areas in 

 Fig. 1. As these show, it varies greatly, being most extensive in the 

 second, where it includes almost the entire lateral margin. A similar 

 relation is noted by Hammar in the corresponding stage in man, and, 

 as the figures of the next stage show (see Fig. 58), it holds in the cat. 



A conspicuous feature — shown best in the second and third pouches — ■ 

 is the presence of deep ventral projections to the pouches (V.D. 1-4). 

 They reach to a greater or less extent below the floor of the pharynx. 

 Hammar calls them the ventral diverticula. They appear, from all 

 published figures examined, to be constant at the corresponding stage 

 in all mammals so far investigated. 



