396 ROBERT J. TERRY 



SUMMARY 



1. The basal plate of the occipital region in the cat is derived 

 from a pair of parachordal cartilages and from two, probably 

 three, hypochordal commissures or arches. The parachordals 

 are continuous with the lateral occipital arches and, topo- 

 graphically, are to be compared with the lateral masses of 

 the atlas. Chondrification of the perinotochordal sheath in the 

 occipital region forms the apex of the dens epistrophei, which, 

 in certain respects, is comparable with a vertebral centrum. 



2. The observations on the developmental processes of the 

 occipital region in the cat indicate that, in principle, they are 

 comparable with those of an atypical vertebra of the atlas kind. 

 The basal plate of the occipital region falls into the category of 

 arch structures, not centra. 



3. In the atlas, the beginning of an atlantal foramen is very 

 early indicated by a notch, traversed by the first spinal nerve, 

 the sides of which are formed by the neural arch and a process 

 of the lateral mass. The possible significance of this atlantal 

 process was discussed. 



4. The occipital condyles are located primarily on that part 

 of the basal plate derived from the parachordal cartilages and, 

 in accordance with the interpretation of the form of the occipital 

 element, belong to arch structures and not to centra. 



5. The position of the lateral occipital arches and the plane 

 of the foramen magnum, parallel with the floor of the nose in 

 embryos of 23.1 mm., is correlated with flexures, in a sagittal 

 plane, of the cranium as a whole. 



6. The fissures around the otic capsule, between it and the 

 basal plate and walls of the chondrocranium in the 23.1 mm. 

 embryo, are vestiges of the spaces primarily existing between 

 these independently arising cranial elements. 



7. The incisura occipitahs posterior of the cat embryo is in- 

 termediate in relative size between that of the reptile (Lacerta) 

 and that of man. There is apparently a tendency for this space 

 to increase in area from lower to higher forms. 



