CARTILAGINOUS SKULL OF A 21 MM. HUMAN EMBRYO. 317 



OCCIPITAL VERTEBRA. 



Each exoccipital corresponds for the most part to a vertebral neural arch or 

 hemiarch, which is so distinct in this embryo that we may properly apply the term 

 occipital hemiarch. The roots, laminae, and transverse processes can be readily 

 recognized. The roots are broad and thick and correspond to the roots or pedicles 

 of the vertebral neural arches. On the right side, as already noted, there are three 

 roots dividing the cranial end of the hypoglossal canal into two parts, a condition 

 not uncommon in the adult. The caudal root on each side is the largest and con- 

 sists of more advanced cartilage, and it is questionable whether the anterior roots 

 should be looked upon as forming part of the occipital vertebra. It has often been 

 suggested that they are part of an anterior, still more rudimentary vertebra. The 

 laminae extend outward and upward from the roots, forming part of the lateral 

 border of the foramen magnum. They are much thickened, rounded, tapering 

 cartilages, quite distinct, and in the dorsal part entirely separate in this particular 

 embryo from the squamal cartilage. The cartilage of the occipital arch is more 

 differentiated than the squamal. During the further course of development, as 

 Macklin has pointed out, the occipital neural hemiarches grow around the central 

 nervous system and approach each other in the midline. It is not clear whether 

 they actually meet and fuse, or whether a small part of the squamal cartilage may 

 not intervene. The squamal cartilages, which likewise grow around the central 

 nervous system, meet and fuse before the occipital hemiarches.' The ossicle of Kerck- 

 ring, which develops in this region, may represent a separate ossification center 

 of the nuchal plate intervening between the tips of the occipital neural hemiarches, 

 or this center may possibly be looked upon as the ossification of the fused occipital 

 spinal epiphyses. Schultz described a skull with two bilateral ossicles in this region, 

 and suggests that they may correspond to the epiphyses of the spinous processes. 



In this embryo the separation of the dorsal part of the lamina of the occipital 

 vertebra from the squama of the occipital, and the more intimate relation of the 

 latter with the transverse process, would seem to indicate that the squama is an 

 extension upward from the transverse process rather than an outgrowth from the 

 occipital neural lamina. The relation of the squama to the transverse process 

 and to the occipital neural lamina is clearly shown in figures 1, 7, and 16. The 

 occipital transverse process forms part of the caudal and lateral margins of the 

 jugular foramen and continues up into the squama and alar lamina without 

 line of demarcation. The alar lamina, as will be seen in figure 16, constitutes the 

 ventral and more medial part of the squama adjoining the lamina of the occipital 

 and the medial part of the transverse process. There is also a more gradual transi- 

 tion as regards the degree of differentiation from the cartilage of the transverse 

 process into the squama than from that of the lamina. 



In embryo No. 22 (Carnegie Collection), 20 mm. in length, the squama of the 

 occipital is fused with the lamina throughout the entire length of the latter on 

 both sides of the embryo, although throughout most of this length the younger 

 cartilage of the squama can easily be distinguished from the thickened, older 

 cartilage of the lamina. Towards the apex of the arch, however, the two cartilages 



