292 ROBERT J. TERRY 



the positions of the planes of the dorsal and ventral divisions 

 differ from one another, that of the ventral, condylar part is 

 transverse, whereas that of the dorsal division is very obhque. 

 The latter faces ventro-caudad and forms an angle, open cau- 

 dally, of approximately 145 degrees with the plane of the ventral 

 part of the foramen (fig. 3). 



The occipital condyles (figs. 2, 3, 19) are a pair of oval emi- 

 nences only slightly raised above the level of the ventro-lateral 

 margins of the foramen magnum. Each condyle extends at 

 first laterally from the incisura occipitahs anterior along the 

 caudal margin of the basal plate to the level of the hypoglossal 

 canal, then dorso-laterally upon the lateral occipital arch. The 

 basilar portion is broader but shorter than the lateral which 

 stretches out upon the occipital arch as far as the deep notch 

 mentioned above (p. 291). The lateral and basal parts of the 

 condyle are connected with the lateral mass and ventral arch of 

 the atlas by intervening mesenchjrtna; an articular cavity is 

 not present. 



The tectum posterius (figs. 1 and 2) is a slender transverse 

 bridge of cartilage arching over the brain and connecting the 

 parietal plates from side to side. It is far from the auditory 

 capsules caudally and dorsally, standing closer to the occipital 

 side walls. The tectum expands laterally in joining the parietal 

 plates, becomes narrow in its middle opposite the medulla ob- 

 longata. The caudo-ventral margin of the tectum is con- 

 cave from side to side, bounds the incisura occipitalis posterior 

 and affords attachment to the spino-occipital membrane; the 

 cephalic, and at the same time dorsal, margin is straight, pre- 

 senting no indication of a processus ascendens. 



The term incisura occipitalis posterior (fig. 1) is given ten- 

 tatively to the bay extending from the foramen magnum to 

 the tectum posterius. Its lateral boundaries are the caudal 

 margins of a pair of cartilaginous plates continuous with the 

 occipital walls ventrally, and with the laminae parietales an- 

 teriorly. This notch is filled by the spino-occipital membrane. 



Hypoglossal nerve. Three ventral roots unite to form a single 

 nerve in the embryo of 23.1 mm. A dorsal root opposite to, 



