1901.] NATURAL SCIENCES OF PHILADELPHIA. 237 



(jen of the cartilages and muscles. The cartilages are laid down 

 close to the wall of the pharynx, and with their increase in size the 

 latter is naturally reduced; while at the same time the increase in 

 amount of the general mesenchyme accounts for the increase in 

 width of the entire head. 



As a consequence of the retarded growth of the pharynx and of 

 the increase in width of the head it follows that, unless there is 

 sufficient rapidity of growth in the fold to compensate for the 

 arrested growth of the pharynx, the distal extremity of the 

 hyomandibular fold will be removed more and more from the exte- 

 rior and that ultimately it will come to lie quite deeply. To such 

 causes, I think, must be attributed the recession of the hyoman- 

 dibular fold from the exterior. The head has increased in width, 

 while the pharynx has remained stationary, and even been reduced 

 in size, so that its appendage, the fold, quite naturally recedes from 

 the ectoderm. 



But this explanation suggests another problem: Why does the 

 fold not exhibit sufficient rapidity of growth to enable it to retain 

 throughout its original position near the external epithelium, as in 

 the case of the remaining visceral-clefts, and, moreover, why does 

 it retain this position at one point, i.e., where the blind, bulbous 

 "diverticulum" terminates? This question brings us to our 

 second topic — the formation of the " diverticulum." This part is 

 not, I consider, a new formation, but merely that portion of the 

 fold which has managed by its normal growth to retain its original 

 position near the ectoderm. In this connection I wish again to 

 call attention to the condition in stage II. The distal border then 

 formed a gentle arch, which for a considerable part of its extent 

 was in close proximity to the skin. However, at both its dorsal 

 and ventral extremities this border recedes progressively more and 

 more from the exterior until finally it blends at both ends with 

 the lining of the pharynx. Hence in sections the dorsal and 

 ventral portions of the distal border are seen at varying levels 

 below the ectoderm, while the crown (of the arched plate) is situ- 

 ated near the latter (compare figs. 2-7). Fig. 6 is instructive in 

 this connection. Take the fold as shown on the right side. It will 

 be seen that the distal edge is in close proximity to the ectoderm 

 for a considerable part of its length. However, the upper portion 

 of this border is closer to the ectoderm than the remainder. This 



