1901.] NATURAL SCIEN'CES OF rillLADELPIHA. 229 



ventral portion. In the fifth section posterior to this (fig. 6) the 

 cleft is cut throughout the greater part of its dorso -ventral extent, 

 and hence appears as a broad, solid mass extending out from the 

 side of the pharynx and reaching nearly to the skin, where it all 

 but meets a slight papilla projecting inward from the latter 

 {Hyih.). This figure also reveals another feature of the cleft- 

 outgrowth which is of particular importance. Il will be noted that 

 it is the upper portion of the cleft-fold which approaches most 

 nearly the skin, whereas the ventral portion recedes gradually 

 from it as we descend. About the middle of this ventral portion 

 is a small indentation in the outer edge occupied by a small 

 blood-vessel (x. ). It will be also noticed that the proximal (inner) 

 portion of the cleft is situated at a. higher level than in fig. 5. 

 From this it follows that the line of origin of the fold from the 

 pharynx extends from below upward and backward. 



Posterior to this region the hyomandibular fold bends more 

 sharply backward and accordingly in transverse section appears 

 considerably narrower (fig. 7, Hipn., right side). We next obtain 

 the c:mdition shown in fig. 5 (left side), where the fold (Hym.) ig 

 cut approximately at right angles to its surface and hence appears 

 extremely narrow. Below the fold is a large oval mass, the 

 Anlage of the hyoidean muscles, i.e., deprei«sor mandibuhe and 

 depressor ossis hyoidei (h.m.'). Spemanu has noticed a relation 

 between the subsequent development of these muscles and the 

 degeneration of the hyomandibular fold. I have found the same 

 relation to exist in Bnfo, but shall call attention to it later. The 

 fold next enlarges somewhat, and then, gradually receding moi*e 

 and more from the exterior, blends imperceptibly with the pharyn- 

 geal wall. These stages are show'n consecutively in figs. 6 and 7 

 (left sides). 



The transverse sections also show some structural features, which 

 are of impurtance in tracing certain stages in the subsequent his- 

 tory of the cleft-fold. Anterior to the cleft is the efferent man- 

 dibular aortic artery, a branch from the carotid. At the present 

 stage this vessel is rather difficult to trace, but with some care can 

 be worked out. Since, owing to the general antei'o-ventral direc- 

 tion taken by the plane of the hyomandibular fold, the anterior 

 wall of the latter faces forward and also upward, it follows that in 

 transverse section structures anterior to the fold will be seen dorsal 



