CARTILAGINOUS SKULL OF A 21 MM. HUMAN EMBRYO. 



are of the same degree of differentiation and no distinction between them can be 

 seen. 



In embryo No. 128 (Carnegie Collection), 20 mm. in length, the resemblance 

 which the occipital neural arch and its transverse process bear to those in the 

 cervical region is very striking. The cartilages of the occipital and cervical neural 

 arches show much more advanced differentiation than the thin transverse processes. 

 The muscle intertransversarius, between the transverse processes of the second 

 and the first cervical vertebrae, is in line with and serially related to the rectus capitis 

 lateralis, which passes between the transverse process of the first cervical vertebra 

 and that of the occipital. In this embryo (No. 128) it can be seen also how the 

 squama of the occipital is a continuation upward from the transverse process 

 rather than from the lamina. The laminae of the occipital vertebra are also fused 

 along the entire length to the squamae, but are readily distinguishable from the 

 latter by their much more advanced cartilaginous differentiation. The tips of 

 these laminae are connected with the tips of the laminae of the atlas by the inter- 

 spinous ligament, as in embryo No. 460. 



In embryo No. 240 (Carnegie Collection), 20 mm. in length, the cartilage of 

 the occipital neural arch on each side is likewise fused with the squama throughout 

 its entire length. The lamina also shows more advanced differentiation than 

 the squama. In embryo No. 431 (Carnegie Collection), 19 mm. in length, the 

 tips of the occipital neural arches are separated from the squama on either side 

 by the condensed mesenchyme forming the perichondrium. The extent of the 

 fusion of the laminae and the squamae is, however, more extensive than in embryo 

 No. 460. These observations indicate that the occipital neural arch is more marked 

 in this stage than in the adult, and that probably an extreme degree of accentua- 

 tion in the embryo would precede any marked manifestation of an occipital vertebra 

 in the adult. The manifestation of an occipital vertebra in adult skulls has been 

 described by Gladstone, Kollman, and others. Such variations remind one of the 

 conditions found in embryos at this stage, and we shall probably find that embryos 

 vary as much, if not more, than adults and that such embryonic variations always 

 precede the adult variations. 



The relation and attachment of the dorsal membrane to the tips of the occipital 

 hemiarches are similar to its relation and attachment to the tips of the neural 

 arches of the cervical vertebras, in that in both cases the dorsal membrane is con- 

 tinuous with the perichondrium on the medial side of the tip of the neural arch. 

 Its attachment to the upper border of the squama is somewhat different, since here 

 it is merely continuous with the thin edge. The tip of the occipital hemiarch is 

 connected with the tips of the vertebral hemiarches by a distinct band of condensed 

 membrane, the interspinous ligament described by Bardeen (fig. 9). 



The transverse or jugular process springs from the occipital hemiarch at the 

 junction of the roots and lamina and projects laterally back of the jugular foramen. 

 We have already noted its serial relationship with the vertebral transverse processes. 

 The lateral extremity of the jugular process has a knob-like enlargement, and into 

 this are inserted the rectus capitis lateralis muscle and the occipito-mastoid muscle 

 (fig. 24). 



