1901.] NATURAL SCIENCES OF PIIILADELPIHA. 239 



tlie superior border faciDg forward. At this time the commou 

 muscle divides into an inner and anterior mass, the depressor man- 

 dibular, and an outer and posterior mass, the depressor ossis 

 hyoidei. With subsequent growth both muscles extend forward 

 more and more until one of them — the depressor mandibulse — 

 invades the area intervening between the skin and the inferior por- 

 tion of the distal edge of the hyomandibular fold at the point 

 indicated by the slight depression shown in fig. 6 (x). Here its 

 anterior extremity comes into close relation with the segregating 

 Anlage of the quadrate at a point just in front of and below the 

 fold. The outer muscle — the depressor ossis hyoidei — also acquires 

 attachment to the quadrate Anlage, but at a point above and pos- 

 terior to the fold. 



I have just mentioned that the depressor mandibulae extends for- 

 ward in the space between the lower portion of the distal border 

 of the hyomandibular fold and the skin. With this invasion an 

 effective barrier is interposed betw'een the two ; and as a result of 

 the subsequent increase in size of the muscle and of the extension 

 in Avidth of the head, it follows that this lower portion of the 

 hyomandibular fold will be arrested in its growth and will conse- 

 quently come to be more and more removed from the exterior. 

 At the same time the segregation of the mesenchyme to form carti- 

 lage Anlagen interposes additional barriers to the outward growth of 

 the fold. Hence it is possible to understand why it is that the 

 lower part of the fold should lie so far beneath tiie ectoderm as 

 shown in the present stage (figs. 11, 16, 17). On the other hand, 

 the dorsal portion of the hyomandibular fold — i.e., that which 

 forms the " diverticulum " — is situated above the depx'essor man- 

 dibul?e, so that the latter does not interfere with its normal growth 

 and as a result this portion of the fold still retains its proximity to 

 the skin. With the increase in width of the head it has been carried 

 outward with the skin. In its proximal portion, however, this 

 part also has been encroached upon by the developing depressor 

 mandibulffi, and as a result it presents the form of a long-drawn- 

 out cord, narrow and flattened in its proximal part and swollen in 

 its terminal part, where it is not encroached upon to any great 

 extent by the surrounding structures. 



Along the dorsal edge of the fold no well-marked changes, so 

 far as I have been able to determine, seem to have taken place. 



