192 Henry Fox 



A ventral view (Fig. 3) shows a number of important features. 

 Projecting from each side of the pharynx are the four pharyngeal 

 pouches, each with its ventral diverticulum. That of the first pouch 

 (Y. D. 1) fornls a low narrow ridge extending from the infero-lateral 

 angle of the pouch inwards and slightly backwards quite to the median 

 line, where it joins the corresponding ridge of the opposite side. There 

 is thus formed a complete transverse V-shaped fold, the apex of 'the 

 fold being the meeting point of the two opposite limbs. The ridge 

 corresponding to the median oral groove begins immediately in front 

 of this apex. The shallow impression between corresponds to the tuber- 

 culmn impar of His. Just behind the apex is the median thyroid. The 

 latter consists of two lobules joined to each other and to the pharynx 

 by a slender epithelioid cord. 



The ventral diverticula of the next two pouches are much deeper than 

 that of the first, but are largely confined to the lateral half of the 

 pharynx. A faint ridge, however, extends from the base of the second 

 diverticulum to the median line, where it is joined by a similar ridge 

 from the third pouch (see Fig. 60 of the next series for this condition). 

 The two sets of ridges thus converge to form a rather low protuberance 

 immediately above the thyroid and in front of the tracheal ridge. 



The presence of these inner low ridges connecting the opposite ventral 

 diverticula of the second and third pouches shows their essential agree- 

 ment in this respect with the first pouch. Only, in the case of the two 

 former, the lateral half of each ridge is produced far below the level 

 of the inner portion, while in the first pouch the depth (its height) of 

 the ridge is throughout approximately uniform. 



Owing to the form of the ventral diverticula of the second and third 

 pouches there is left between their opposite lateral halves a considerable 

 space, in which is lodged the apical portion of the heart along with the 

 large arteries radiating from it (Fig. 3). The prominent aortic arches 

 at this time are the third (carotid), fourth (aorta typica) and the fifth 

 (pulmonary). The latter has a small posteriorly directed branch — the 

 later pulmonary artery (Fig. 3, Pul.), The first aortic arch is much 

 reduced in size and has lost all connection with the dorsal aorta. The 

 second is also extremely reduced and is only connected with the dorsal 

 aorta by an extremely narrow (apparently functionless) vessel. 



Immediately back of the common origin of the aortic arches begins 

 a sharp median ridge, which deepens posteriorly. It represents the 

 future lar}Tix and trachea. 



