346 • ROBERT J. TERRY 



nerve roots. The three roots are united in passing through 

 the hypoglossal notch of the primitive cartilaginous occipital 

 element, and there is no indication of a subdivision of the mar- 

 gin into smaller notches for each root, as appears to be the case 

 in an earlier stage and before cartilage has formed. This con- 

 dition varies in maimnals; in rabbits for example, (Noordenbos, 

 Voit) two canals are present on each side. That two canals 

 occur occasionally in the human skull as an anomaly is well 

 known. It will be recalled that the hypoglossal nerve in cat 

 embryos shows a tendency to conform with a spinal nerve in 

 presenting a posterior root and ganghon in relation to the third 

 motor root. The presence of this ganglion was discovered by 

 Vulpian ('62) and I can confirm this observation here. 



The presence of more than two cartilaginous hypochordal 

 commissures in the development of the occipital basal plate has, 

 I believe, not been recognized. Also, the recognition of the 

 primary hypochordal nature of the anterior commissure appar- 

 ently rests on only one other observation, that of Weiss, in the 

 rat. Until further distribution of these phenomena be dis- 

 covered, or the present observation confirmed, it would seem of 

 Uttle profit to attempt an interpretation of their significance. 

 The antero-posterior succession of a series of transversely placed 

 bar-hke structures in the base of the skull at once suggests the 

 idea of segmentation. A relation of these bars laterally with the 

 hypoglossal nerve roots may or may not be a primary one. But 

 in utilizing the results of chondrocranial study in any discussion 

 of the segmentation of the cranium, it must be borne in mind 

 that such evidence can be of high value only when the relation 

 of chondral to blastemal developmental processes is known. It 

 seems highly probable that the cartilaginous commissures here 

 described and the membranous bars observed by Kernan ('15) 

 are two developmental phases of one and the same process, 

 which subsequent investigation will elucidate. 



In regard to the condensed tissue sheath about the occipital 

 notochord found in cat, it may be said that a number of obser- 

 vations made on different animals point to the presence of such 

 a layer as probably of general occurrence. Froriep ('86, p. 92) 



