238 PROCEEDIXGS OF THE ACADEMY OF [^larcll, 



part represents a region slightly dorsal to the middle portion of the 

 distal border. Below this the edge recedes to a slight extent from 

 the exterior and in its middle portion forms a slight, barely per- 

 ceptible depression. If we now conceive that in the future growth 

 of the animal all the lower portion of the distal edge remains 

 stationary and that the middle depression deepens considerably, 

 while the upper portion alone remains in proximity to the ecto- 

 derm, then we should obtain a condition very similar to that shown 

 in fig. 12, except that complete outward extension of the fold is 

 not shown in the figure (see instead fig. 10). In fig. 12 the 

 arrested ventral portion can be seen as an extension of the right 

 inferior angle of the pharynx, while the concavity between it and 

 the plate-liiie hyomandibular fold is the much-deepened depression 

 (see also fig. 11). In the latter figure the lower portion of the 

 fold can be seen as a blunt extension from the ventre -lateral wall 

 of the pharynx, while the flattened, oval mass external to and 

 above it is the dorsal portion, or, as we have temporarily termed 

 it, the " diverticulum." More posteriorly, as shown in fig. 12, 

 this " diverticulum " becomes continuous Avitli the proximal por- 

 tion of the fold, and accordingly the area embraced between these 

 two portions anteriorly represents the depression, which we saw 

 beginning in fig. 6. One will notice that in this area a muscle — 

 the depressor mandibulse (m.cZ.m.) — has just attained attachment 

 to the Anlacje of the quadrate cartilage, while external to it its 

 companion muscle, the depressor ossis hyoidei Qm.d.h.), has 

 acquired attachment to the tip of the processus muscularis. The 

 ''diverticulum" lies between these two muscles and, as already 

 mentioned, extends anteriorly between them until it reaches the 

 anterior surface of the outer muscle (depressor ossis hyoidei), 

 around which it curves outward (fig. 10, also 17 and 18). In the 

 behavior of these two muscles lies the clue to the solution of the 

 problem under consideration. One will recall that both of these 

 muscles belong originally to the hyoid arch, and consequently their 

 acquirement of attachment to ^he quadrate is a later affair. In 

 stage II the original hyoidean muscle-mass, from which these two 

 are subsequently differentiated, extends in its long axis almost 

 vertically and is situated entirely behind the hyomandibular fold 

 (see figs. 2-7). Later, however, as the muscle increases in size 

 its long axis becomes extended in an obliquely anterior direction, 



