316 CARTILAGINOUS SKULL OF A 21 MM. HUMAN EMBRYO. 



into the basisphenoid, and one can only approximate the future line of junction 

 between the two bones which ossify in the cartilage by the relation to the apex of 

 the otic capsule and the entrance of the notochord. The basioccipital lies in a 

 plane almost exactly horizontal to the long axis of the embryo. This elongated 

 quadrilateral plate is thinner at the center, slightly concave on the cranial surface, 

 and slightly convex on the pharyngeal surface. It becomes thicker where it joins 

 the sphenoid. The rounded caudal end is bent downward and notched in the 

 midline at the foramen magnum. The caudal part of the basioccipital gives much 

 more of a clue to its vertebral origin than the flattened cranial part. The caudal 

 part is thickened and on the cranial surface is incompletely divided into two lateral 

 masses by a deep median groove. These correspond, perhaps, to the bilateral 

 masses which fuse to form the bodies of the cervical vertebrae. Levi finds in his 

 13 mm. embryo two bilateral cartilaginous centers medial to the hypoglossal roots. 

 They probably represent, as Macklin suggests, the bilateral centers for the occipital 

 vertebra^. Their position in the 13 mm. embryo corresponds to the position of the 

 lateral masses in this 21 mm. embryo. These masses are united across the midline 

 beneath the notochord, and anteriorly are continued into the broad flat plate of 

 the basioccipital. The center of each mass contains the most highly differentiated 

 cartilage of the entire chondrocranium. 



In the 14 mm. embryo described by Levi the otic capsule and the basioccipital 

 are apparently separated by a wide gap. Levi probably did not include all the 

 precartilaginous tissue and blastema of the otic capsule, since in embryo No. 109 

 (Carnegie Collection), 11.5 mm. in length, the blastema of the otic capsule is in con- 

 tact with the basioccipital. It is only in younger stages, 9 mm. length, that I find 

 the blastema of the otic capsule separated from the basioccipital. The union of the 

 blastema, then, precedes that of the cartilage in this region. In embryo No. 460 

 the anterior half of the lateral border of the basioccipital is fused with the cochlear 

 part of the otic capsule along a crescentic line. The line of fusion can still be recog- 

 nized by the differences in the degree of differentiation of the two cartilages. The 

 cochlear part consists of young cartilage and the nuclei are closer together than in 

 the basioccipital. The border of the basioccipital is, however, not so far advanced 

 as the central part. The two cartilages were so completely fused in the 17 mm. 

 embryo described by Levi that he was unable to recognize any histological border 

 between them. Embryo No. 128 (Carnegie Collection), 20 mm. in length, although 

 supposedly smaller and younger than the 21 mm. embryo (No. 460), shows a more 

 advanced condition in the cartilaginous differentiation of this region, and it is 

 impossible to find a histological border between the basioccipital and the cochlear 

 part except at either end of the line of fusion. In another 20 mm. embryo (No. 22, 

 Carnegie Collection) there is only the very slightest indication of the line of fusion . 

 There is probably some variation in the rate of differentiation of the cartilages of 

 this region in embryos of the same age and size. 



