OP AETS AND SCIENCES. 123 



pharyngeal, end of tlie fissure. This line of contact is coextensive with, 

 the cleft, so that the latter is closed throughout its whole length (Figs. 

 9, 10, 11),* although a complete fusion of its walls can hardly be said 

 to have taken place at this time. The second cleft has not yet broken 

 through the pharyngeal walls, so that for a short period subsequent to 

 the formation of the first cleft there is no direct communication throuirh 

 the walls of the neck between the jiharyngeal cavity and the outside 

 world. The pharyngeal portion of the walls of the first cleft at this 

 stage is composed of two layers of cells, a superficial layer of flattened 

 epithelium, and a deep (or mucous) layer consisting of wedge-shaped 

 or somewhat modified columnar cells. On the posterior or hyoid wall 

 the deep layer is several cells in thickness, but on the anterior or man- 

 dibular wall this layer is usually only two, and at most not more than 

 three, cells deep. 



In cross sections through the centre of the cleft of an embryo 8 mm. 

 long (Fig. 12), the ventral half of the external division of the cleft is 

 separated from the internal portion by a thick partition consisting of 

 three layers : the ectoderm lining the external portion of the cleft, ihe 

 entoderm lining the internal portion of the cleft, and the mesoderm in- 

 tercalated between them. The deepest portion of the external moiety 

 of the cleft, however, which lies at its dorsal extremity and which co- 

 incides with the ventral angle of the depression of Hunt, is separated 

 from the inner portion of the cleft by only a kw cells which have been 

 proliferated from the hyoid wall (Fig. 13). I believe this affords con- 

 siderable evidence that tlie cleft is gradually closed from its ventral 

 toward its dorsal edge by an upgrowth of tissue which results from a 

 very complete fusion of the hyoid and mandibular arches. 



The regions of the external portion of the cleft which play the most 

 important part in the formation of the external meatus are indicated 

 in the sections (Fig. 12) by « and b. They arc a dorsal depression (a), 

 which lies at the extremity of the cleft, and a ventral depression (b), 

 which lies anterior to the primary elevation of the hyoid. From 

 the same sections (Fig. 12) the inner portion of the cleft (c) is to be 

 seen directed obliquely from the pharynx dorsally and outward. Since 

 the inner portion of the cleft and the pharynx meet at an angle, the 

 latter affords a well-marked indication of the extent of each. These 

 characters are important in determining the cleft, and orienting its 

 position in the successive stages of its metamorphosis. 



Cross sections through the cleft of an embryo 12 mm. long (Fig. 14) 

 show that the dorsal depression (a) has become deeper and more nar- 



* Consult Explanation of Figures. 



