232 PROCEEDINGS OF THE ACADEMY OF [March, 



cularis, having passed beyond the transversely placed commissure 

 quadrato-cranialis anterior (palato-pterygoid). Of the quadrate 

 the outer, distal portion of the processus muscularis is most dis- 

 tinct at the present stage. Above this process are the muscles of 

 mastication (^.m.) already mentioned, while to its outer or ventral 

 surface are attached two muscles, the depressor mandibula? 

 (m.d.m.^ and depressor ossis hyoidei (m.d.h.). Between these 

 two muscles courses the ramus hyomandibularis of the facial nerve 

 (vii h.). Both of these muscles belong to the hyoid or second 

 visceral-arch and have been differentiated out of the common 

 muscle mass of that arch. 



The quadrate in the region posterior to that just considered 

 blends gradually and imperceptibly with the surrounding mesen- 

 chyme. This can be followed by examining the figures consecu- 

 tively. 



We will now turn to the consideration of the hyomandibular 

 fold in the present stage. In fig. 12 the rhomboidal cavity of 

 the pharynx is sharply prolonged at its right veutro-lateral angle, 

 and from the wall of the cavity immediately above this prolonga- 

 tion a narrow, solid cord, rep^'esenting an extension of the wall, 

 extends upward and outward in close contact with the ventral sur- 

 face of the processus muscularis (Eu.). Just internal to its blind, 

 distal extremity can be seen a small vessel interposed between the 

 cord and the cartilage. This vessel is the mandibular aortic arch. 

 Ventral to the cord is a semicircular mass of procartilage, in the 

 hollow of which is placed the depressor mandibulffi. This is the 

 Anlage of the hyoid, a more complete view of which can be 

 obtained in fig. 13 {H.'). The hyoid, like the mandible, is a stout, 

 thick bar placed transversely beneath the floor of the pharynx and 

 separated from its fellow in the mid-line by a less compact tissue. 

 Anteriorly the two ai-e se])arated by the thyroid gland outgrowth 

 (TA.). At its outer extremity the hyoid turns sharply upward 

 as a flattened plate with a concave outer surface in which is lodged, 

 as already mentioned, the depressor mandibuhw Its inner surface 

 is closely applied to the outer and ventral wall of the hyomandib- 

 ular fold {Eu.). 



In the region posterior to that shown in fig. 12 the hyomandibu- 

 lar fold presents much the same appearance as in tiie last stage 

 (compare figs. 13 and 14 with G and 7). It will be noticed, how- 



