190 Henry Fox 



The pharynx anteriorly is in contact with the stomatodeal plate (St.). 

 As a whole it is a relatively wide, dorso-ventrally flattened sac. Only 

 two pairs of i^haryngeal pouches are present as wide lateral diverticula 

 of the pharjTix. Of these the first (Ph. P. I) alone reaches the ecto- 

 derm and joins with it for a short distance. The second pair (Ph. P. 2) 

 are only barely indicated as faint outbulgings, the one on the left being 

 the more distinct of the two. 



The pericardium is of small size, in striking contrast to the enormous 

 bulk it attains in later stages. It contains the inner portions of the 

 great vitelline veins (v. v.), which are joined together only at their 

 extreme anterior ends. Only one pair of fully developed aortic arches 

 is present — the first or mandibular (ao. i). These extend dorsally 

 in front of the first pouch and join the paired dorsal aortse. Two 

 prominent out-bulgings from the sides of the vitelline veins are prob- 

 ably destined to form the common trunk from which the remaining 

 aortic arches subsequently arise. 



A cat eml^ryo, No. 413 of the Harvard collection, shows the next 

 stage in advance (Fig. 57). The posterior part of the body is still 

 approximately straight, but the head portion is strongly flexed upon it 

 and is of relatively much greater extent than before. 



Anteriorly the stomatodeal plate has disappeared. The phar}Tix is, 

 as before, a wide flattened sac, but its width in its anterior portion is 

 somewhat greater than in its posterior part. Its floor is somewhat 

 deeper than before and close to the mouth is produced into a deep 

 median groove — the median oral groove (M. GE.). 



The hypophysis (HYP) appears as a blunt protuberance from the 

 dorsal side of the pharynx close to its anterior extremity. Three pairs 

 of phar}Tigeal pouches are now present. The first two pairs reach tlie 

 ectoderm and join with it for a considerable extent (see light areas of 

 Ph. P. 1 and 2, Fig. 57). Of the third pair, the pouch on the right 

 side reaches the octoderm, while that on the left is still removed by a 

 slight interval from it. 



The first pharyngeal pouch forms a relatively large transverse fold. 

 The greater part of its lateral margin is in contact with the ectoderm. 

 The area of contact is widest dorsally and diminishes progressively in 

 width toward the ventral side. The extreme ventral part of this margin 

 extends a slight distance below the region of contact as a free edge, 

 which then turns suddenly inwards as the ventral margin of the pouch. 

 This part of the pouch projects slightly below the floor of the pharynx 

 and thus forms a ventral diverticulum of the pouch. 



