PRIMORDIAL CRANIUM OF THE CAT 345 



chordal commissure of young cartilage. This lies at a level 

 cephalad of that of the anterior root bundle of the hypoglossal 

 nerve. Cartilage afterwards forms dorsad of the notochord in 

 the region of the primary commissure, so that the former comes 

 secondarily to he within the basal plate. I am aware of the 

 fact that some observers have described this anterior commis- 

 sure of the parachordal in other forms as lying dorsad of the 

 notochord, but in cat I have found it primarily hypochordal. 

 Weiss, however, found the primary process of cartilage forma- 

 tion hypochordal in the apparently unsegmented division of the 

 basal plate. The next cartilaginous union to be estabhshed be- 

 tween the parachordal cartilages in cat is the hypochordal arch 

 uniting their caudal extremities. This has formed somewhat 

 earUer than the anterior arch of the atlas, next to which it stands. 

 The posterior hypochordal commissure is in the same transverse 

 plane as the primitive lateral occipital arch. Between the two 

 hypochordal arches, anterior (primary) and posterior, there 

 remains a sheet of mesenchyma, stretching from side to side 

 beneath the notochord, and continuing into the medial edges 

 of the parachordals. The direction in which chondrification 

 proceeds in this tissue is lateromesad. In the middle of this 

 tissue a third conamissural process is indicated where the medial 

 edge of each parachordal sends a projection toward the mid- 

 plane, producing a constriction in the vacuity between the 

 parachordals. These two symmetrical processes are directed 

 ventro-mesad to a hypochordal plane and lie at a level corre- 

 sponding to the middle of the future hypoglossal foramen. In 

 the smaller embryos (9 mm.) four membranous arches in the 

 occipital region were noted by Kernan, who states that the two 

 cranial have a tendency to fuse. 



When, in the cat, the parachordal plates have formed, the 

 three root bundles of the hypoglossal lie against their lateral 

 margins, in the angle formed by the lateral occipital arch. The 

 foramen is later completed by the formation of a cartilaginous 

 bar in front of the nerve roots, which extends laterally and 

 dorsally from the anterolateral corner of the parachordal, 

 to unite with the primary lateral occipital arch beyond the 



JOURNAL OF MORPHOLOGY, VOL. 29, NO. 2 



