CHONDROCRANIUM OP A 20 MM. HUMAN EMBRYO 611 



jugular foramen. Their cartilage is continuous for a short dis- 

 tance dorsal to this point, the line of union being known as the 

 coramissura occipito-capsularis (fig. 2, 33). This hne of union 

 extends to the apex of the otic capsule, where there is a break in 

 the cartilage known as the fissura capsulo-parietalis. The in- 

 terruption found by Macklin in his 40 mm. embryo and desig- 

 nated by him fissura capsulo-occipitalis is also present in this 

 embyro. 



From the tip of the paracondyloid process there extends 

 mesaid and craniad (figs. 2, 3, 4, 5), a thin process of cartilage 

 which reaches the basioccipital, passing laterad and craniad to 

 the hypoglossal canal. This was found by Macklin in his 40 

 mm. embryo, where it bounded laterally and cranially a foramen 

 which he termed paracondyloid foramen. It corresponds in 

 position to a ridge of bone which in the adult passes from the tip 

 of the jugular process, mesiad and craniad to basioccipital. It 

 represents, as Macklin suggests, a costal process, not that of 

 the occipital vertebra, but of one of the unsegmented vertebrae 

 since its connections to the basioccipital and occipital wing are 

 craniad and laterad to the hypoglossal foramen. 



The paracondyloid process was described by Levi in all the 

 embryos examined by him and figured prominently in his models. 

 He interpreted it to be the tip of the neural arch of the occipital 

 vertebra. Macklin has sufficiently demonstrated the erroneous- 

 ness of this view, and considers it to represent the transverse 

 process and possibly the costal process of that vertebra. 



As has been pointed out the paracondyloid process is but the 

 free extremity of the lamina alaris, and this has been shown to 

 be a bar of cartilage uniting at intervals with a parallel ridge of 

 the occipital. It is therefore suggested that in the lamina alaris 

 are represented the costal and transverse processes of several 

 vertebrae. The outer bar represents the fused costal elements; 

 and may be termed the costal bar. The points of union corres- 

 pond to the bases of the transverse processes, the areas of sepa- 

 ration indicate the intervals between the processes of successive 

 vertebrae. If this interpretation prove correct, three such 

 processes can be enumerated. 



