CHONDROCRANIUM OF A 20 MM. HUMAN EMBRYO 017 



face of the pars canalicularis joins the lateral surface of the pars 

 cochlearis at an angle to form this facial groove which lies at the 

 bottom of the ventro-lateral otic recess of Macklin. This con- 

 tains the anlagen of the otic ossicles. 



The cochlear portion is ventral, cranial and mesial to the 

 canalicular portion (figs. 1 and 3). It is oval, the long axis 

 pointing cranially and mesially. Lateral, mesial and dorso- 

 caudal surfaces are distinguished. 



The lateral and mesial surfaces are divided by a rounded border 

 which passes from the peri-lymphatic foramen below, around the 

 cranial pole, to the superior otic notch above. The first part of 

 this border deviates outward to form the promontorium (fig. 

 3, 29). It divides the peri-lymphatic foramen from the region 

 of the foramen ovale. The lateral surface is smooth and convex. 

 Dorsally, at the junction with the ventral surface of the canalic- 

 ular portion, is the facial groove. The outlines of the fenestra 

 vestibuli cannot yet be made out, as the foot plate of the stapes 

 is buried in indifferent tissue (fig. 3). There is trace of the sulcus 

 caroticus found by Macklin in the 40 mm. stage in the sections 

 (fig. 4,^0). 



The dorso-caudal surface is taken up by a large opening, the 

 perilymphatic foramen (fig. 3), which includes the future fenestra 

 cochlearis and the ductus perilymphaticus. The division of 

 these two by a processus interperilymphaticus, well marked in 

 the 40 mm. stage of Macklin, has here scarcely begun (fig. 3, 11), 

 The outer edge of the common opening juts laterad to form the 

 promontory. 



The mesial surface is divided by its junction with the basal 

 plate into ventral and dorsal sections. The ventral is smooth, 

 convex, and forms the external wall of the ventral basi-capsular 

 fissure (fig. 3, 9). Above the union with the basal plate the 

 surface is smooth, convex, and continuous dorsally with the sur- 

 face of the canaHcular portion. Cranially the opening of the 

 internal acustic foramen appears (fig. 1). This is rather wide 

 open cranially and mesially. This condition is what we would 

 be led to expect from an examination of the models of Levi. In 

 his model of the 17 mm. chondrocranium, the internal acustic 



