1901.] NATURAL SCIENXES OF rHILADELPHIA. 233 



€ver, that the fold is considerably narrower than in the precednig 

 stage, and also that its distal extremity is much farther removed 

 from the external surface. This condition Avill be more fully coii- 

 sidered presently. The narrowing of the fold, however, is more 

 apparent than real. If one will bear in mind the statement 

 already made that the fold extends downward and obliquely for- 

 ward, a true explanation of the difference will suggest itself. 

 Naturally a section which passes through in the same plane as that 

 of the fold will show the latter as a broad mass. This explains 

 the appearance of the fold as shown in fig. 6 (right side). In this 

 figure the section on the right side passes through the eye, whereas 

 on the left side it passes some distance behind the eye. Hence the 

 section traverses the right side in an obliquely forward direction, 

 thus coinciding in the main with the plane of the fold. In the 

 same specimen the fold on the left side is cut throughout trans- 

 versely, so that, except in its most posterior portion, it appears as 

 a narrow, two-layered lamina. 



It is in its distal anterior portion that the hyoraandibular fold 

 has undergone its greatest modification. In fig. 12 the fold is 

 continuous with the wall of the j)harynx. In fig. 11, which is the 

 third section anterior to that of fig. 12, this connection no longer 

 exists. The fold appears as a solid, somewhat flattened cord 

 (Ea.), closely underlying the upper, outer extremity of the pro- 

 cessus muscularis. Its internal surface is in intimate contact with 

 the mandibular aortic arch (m.a.), while externally the two muscles 

 of the hyoid arch — i.e., depressor mandibulaj (m.d.m.) and depres- 

 sor ossis hyoidei (m.d.h.) — approach it very closely. The proxi- 

 mal portion of the anterior part of the fold can be seen in the 

 figure as a relatively broad diverticulum from the wall of the 

 outer, inferior angle of the pharynx (Hym.). 



Anterior to the region just considered this cord-like extension of 

 the fold extends forward a short distance and then bends sharply 

 outward in front of the two muscles just mentioned (fig. 18, Hym., 

 right side). In this region it enlarges considerably and finally 

 terminates as a blind, bulbous sw'eDing in the mesenchyme a short 

 distance below the external epithelium. This part is shown in fig. 

 10 (Tym.), also in fig. 17 (Ttjm.). 



Perhaps a clearer conception of the state of the fold may be 

 gained by a comparison with some coronal sections. In fig. 16 we 



