The Pharyngeal Pouches in the Mammalia 195 



flattened and is produced outwards as a thin process which reaches the 

 ectoderm. The ventral diverticulum projects almost directly forwards. 



The median thyroid is of a relatively large size. It has lost all 

 connection with the pharynx and lies at a lower level than in the 

 preceding specimen. 



Owing to the more rapid growth of the mandibular and hyoid arches 

 as compared with that of the arches posterior to tliem, the originally 

 almost transverse plane of the second and third pouches becomes 

 postero-lateral. Their originally anterior and posterior surfaces thus 

 become antero-lateral and postero-internal, respectively. Their lateral 

 margins thus come to project backwards. 



The increased antero-postcrior growth of the mandibular arch leads 

 to a change in the direction of the tubo-tympanal border of the first 

 pouch. The latter is at first almost transverse, but later assumes a 

 more anterior direction. The more antero-mesial direction of this 

 border in the present stage as compared with that in the preceding 

 shows the beginning of the change. As growth continues the border 

 progressively lengthens, thus giving an increased width to the basal 

 portion of the pouch. These relations are clearly indicated in the 

 dorsal view (Fig. 59). 



Figs. 59 and 60 show the relations of the more posterior pouches to 

 the now fully formed sinus prsecervicalis — relations which are of con- 

 siderable importance in view of later developments. Owing to the 

 great increase in bulk of the hyoid arch the posterior border of the 

 latter projects backwards. The third and fourth arches increase but 

 slightly in bulk and thus remain at a considerably lower level than the 

 arches in front. The sinus is thereby formed as a deep recess, the bottom 

 being formed by the arches mentioned. Just within the anterior margin 

 of the sinus opens the second pharyngeal groove. The third groove 

 occupies the middle of the inner walls. Dorsally it meets the upper 

 extremity of the fourth groove. From this point the latter turns 

 strongly downwards, backwards and inwards to where it meets the fourth 

 pouch. As the latter lies at a considerably lower level than the other 

 pouches, this part of the sinus projects inw^ards as a prominent, pointed 

 process. 



The ventral view (Fig. 60) shows some additional features. On the 

 right side the section is taken at a slightly lower level than on the 

 opposite side, and accordingly it shows the entire exterior of the first 

 two arches, together with the ventral extension of the first pharyngeal 



