236 PROCEEDINGS OF THE ACADEMY OF [March, 



It now remains for us to })oint out the differences between the 

 hyomandibular fold in the present and preceding stages and, if 

 possible, to ascertain how such differences have been produced. 

 In the first place, one will recall that the outer border of the fold 

 in the last stage described a gentle curve, arching from above 

 downward and forward, and that throughout the greater part of its 

 length this boi'der approached very closely the external epiblast. 

 In the present stage the arch described by the outer border ie 

 interrupted about its middle by a club-shaped " diverticulum," 

 which, again, is the onl}^ portion of the fold which approaches 

 closely the external epithelium. The remainder of the outer bor- 

 der lies a considerable distance below the skin. Again, a compari- 

 son of coronal sections shows that the anterior extension of the 

 fold is more marked than in the earlier stage. Hence there are 

 at least three differences to be accounted for, i.e., (1) the recession 

 of the outer border of the fold from the external epithelium; (2) 

 the formatiou of the blind, distal " diverticulum," which still 

 retains the original position of the fold near the skin, and (3) 

 the more anterior dii'ection taken by the fold. 



In order to account for these changes it is evident that at least 

 two factors must be borne in mind. These are (1) the growth 

 process — i.e., the general increase in size of the parts in accordance 

 with the growth of the individual — and (2) the differentiation of 

 new structures. First, as regards the recession of the outer border 

 of the fold from the skin: By comparing the figures illustrating 

 the two stages, one will observe that a considerable increase in the 

 transverse diameter of the head has taken place, whereas little, if 

 any, increase has occurred iu the vertical plane. On the other 

 hand the pharynx has not increased concomitantly in size, but, 

 instead, has undergone an actual decrease, so that it is not only 

 relatively, but also absolutely, smaller in size than in the earlier 

 stage. However, between the points of origin of the hyoman- 

 dibular fold the pharyngeal cavity i-etains approximately its orig- 

 inal width, a feature due to the fact that in this region it forms a 

 pair of shallow evaginatious (fig. 16). The smaller size of the 

 pharyngeal cavity can be readily made out in the transverse sec- 

 tions (compare figs. 5-7 with 10-14). This reduction is in all 

 probability connected with the increase in amount of mesenchyme 

 and particularly with the segregation of the latter to form the Anla- 



