PRIMORDIAL CRANIUM OF THE CAT 293 



but not united with, the third ventral root, arises from the 

 spinal cord between the ' atlas and occipital arch. In smaller 

 specimens further evidences of a dorsal root and ganglion were 

 observed. This component was best developed in an embryo of 

 15 mm. (W. U. C. ser. 52, si. 22-24) in which the dorsal root 

 joined the third ventral root before reaching the hypoglossal 

 foramen. 



Atlas and epistropheus 



In connection with the occipital region some observations on 

 the epistropheus and atlas should be mentioned. These verte- 

 brae have not reached their full chondrogenous development, 

 the neural arches specially being very incomplete (fig. 9). The 

 dens epistrophei is relatively very much -longer in the embryo 

 of the present stage than in the adult. Its relations are also 

 different in that it projects beyond the anterior arch of the 

 atlas so far craniad as to enter a httle way into the cranial 

 cavity through the occipital foramen (figs. 9, 14, 19). The 

 atlas is remarkably massive. Just cephalad of the neural arches 

 is a pair of processes of special interest. These project dorsad 

 from the spot where the neural and anterior arches meet (the 

 future lateral mass), inchning a little toward the median plane, 

 and end in blunt extremities. These atlantal processes, which 

 are the cartilaginous precoursors of the little bridges of bone 

 of the adult atlas, completing the circumferences of the foramina 

 atlantaha, form, with the neural arches, a notch on each side 

 lodging the gangUon of the first spinal nerve and the vertebral 

 artery. The articular surfaces of the atlas for the occipital 

 condyles are formed at the meeting place of the neural and ante- 

 rior arches. Two parts enter into the formation of the articular 

 surfaces: (1) a thickening at the junction of the neural and 

 anterior arches, which meets the basal portion of the occipital 

 condyle; (2) the atlantal process which articulates with the 

 lateral part of the condyle. 



