CHONDROCRANIUM OF A 20 MM. HUMAN EMBRYO 619 



tiles. It would be desirable to have further information regard- 

 ing the history of the lateral bar in man, both as regards its con- 

 stancy and the details of its formation, before accepting as final 

 its interpretation as part of the primitive praefaciaUs commissure. 



In this connection, Voit's observations on the rabbit are of 

 importance. Here the lateral commissura praefacialis is con- 

 nected plainly with the tegmen tympani and joins the pars 

 cochlearis distal to the geniculate ganglion and, in particular, 

 ventral to the great superficial petrosal nerve. This process is 

 represented in my embryo by a triangular flange of cartilage pro- 

 jecting from the pars canalicularis in the angle of the external 

 genu of the facial nerve (fig. 3, 5). From its position it can, in 

 the embryo, be inferred to be equally capable of reaching the 

 pars cochlearis on either the dorsal or the ventral aspect of the 

 great superficial petrosal nerve. 



The pars canalicularis is a three-sided pyramid with its base 

 ventral, apex dorsal, on the dorsal aspect of the pars cochlearis. 

 The surfaces are lateral, cranio-mesial, caudo-mesial. Each 

 border of the triangular outer surface marks the course of one of 

 the semicircular canals, to wit: the anterior border that of the 

 anterior vertical canal, the posterior border that of the posterior 

 vertical canal, the inferior border that of the external canal 

 (fig. 2). The apex corresponds to the highest point in the arch 

 of the anterior vertical canal. The external canal makes a well 

 defined prominence on the outer surface. The course of the 

 others is less clearly indicated in the surface relief. 



The cranio-mesial surface is framed by the arch of the anterior 

 vertical canal. Beneath this is a well marked fossa subarcuata. 

 This surface is bounded caudally by a ridge formed by the union 

 of anterior and posterior semicircular canals, the crus communis. 

 This ridge is the dividing Hne between the two mesial surfaces. 

 Upon this ridge is the opening of the ductus endolymphaticus 

 (fig. 1, 10). 



The inferior edge of the external surface is extended downward 

 to bound the outer side of the facial groove. It presents pos- 

 teriorly a small mastoid process. Cranial to the mastoid process 

 is a prominent ridge, the crista parotica. At the cranial end of 



