228 PROCEEDINGS OP THE ACADEMY OP [Marcll, 



skin more closely than in fig. 2. In fig. 4 we see the fold at it& 

 dorsal origin from the pharyngeal wall (Hym.). Here it is to be 

 seen as a rather wide, shallow, blunt diverticulum of the latter. 

 In the sections intervening between this and fig. 3 the distal end 

 progressively moves peripherally as we pass down until it comes to 

 occupy the position shown in the latter figure. Hence the dorsal 

 edge of the cleft is higher in its proximal portion than in its distal 

 part. *In fig. 4 it will also be noticed that the proximal portion of 

 the cleft in its dorsal portion approaches very closely to the origin 

 of the second visceral -cleft. In the other two figures the cleft is 

 separated throughout by a considerable interval from the second 

 cleft. It follows from this that as it descends the plane of the 

 first cleft moves forward also. 



Grouping the facts so far obtained we find the hyomandibular 

 cleft as a solid, two-layered diverticulum of the pharyngeal wall,^ 

 which extends outward and somewhat forward to a point a short 

 distance removed from the external ectoderm. Here it terminates 

 in a blunt, rounded edge, extending downward and slightly for- 

 ward and presenting throughout its course no well-marked indenta- 

 tions or depressions. Above and below, however, the outer edge 

 gradually recedes more and more from the skin until it blends 

 imperceptibly with the lining of the pharyngeal cavity. The edge 

 thus has the form of a gentle arch. In general the cleft outgrowth 

 is elongated dorso-ventrally, but it also is directed obliquely for- 

 ward. This forward direction is more pronounced in its dorsal 

 than in its ventral portion. In its lower portion the cleft is widely 

 separated from the second visceral-cleft, but in its dorso-posterior 

 portion it approaches the latter very closely, particularly in its 

 proximal, internal part. 



These observations are further confirmed by transverse sections 

 (Pis. VI, VII, figs. 5-7). In fig. 5 the hyomandibular cleft can be 

 seen as a short, blunt diverticulum from the inferior, outer angle of 

 the pharynx (^Hym.). The cleft here is cut through its antero- 



with processes or appendages of the body. In the present paper, however, 

 I designate by "distal" Ihatpoition of the hyomandibular fold (or of its 

 deri\ative. the Eustachian cord) vvhicli is farthest removed from its con- 

 nection with the pharynx, wliile I eniph>y the term "proximal " to denote 

 tliat part of the same structure which is nearest the point of orijiin from 

 the pharynx. My use of the-e terms in connection with the structure men- 

 tioned is due to the necessity of liaving some fixed term to apply to each of 

 its extremities, the relative position of which vary in the difiereut stages. 



