SKULL OF A 43-MM. HUMAN FETUS. 



67 



outlined, terminating in the dorsal foraminal 

 prominences. The ental surface shows, just lateral 

 to the hemiarch, a groove which is rather indefi- 

 nitely marked except just behind the jugular 

 tubercle. The dorsal foraminal prominences are 

 much farther apart than in la, and the foramen 

 occipitale magnum is correspondingly larger. 

 There is, throughout the entire extent of the 

 hemiarch, a direct connection of its cartilage with 

 that of the adjoining squama. In the region of 

 the aforementioned preossification center its 

 material is of a distinctly more advanced type 

 than that of the neighboring squama. 



It is of interest to note that Lewis has found the 

 tip of the arch, in a 21-mm. human embryo, 

 separated from the squama, this tip projecting 

 dorsally into the mesenchyme. Kernan (1916) 

 does not mention this separation in his 20-mm. 

 human embryo. In three other human embryos 

 examined by Lewis, of 20 mm. length, there was no 

 separation of the occipital hemiarch from the 

 squama, though there was a difference in the 

 character of the cartilage of these parts, the 

 squama being of a younger type. Even in the 19 

 mm. stage, Lewis found a greater amount of 

 fusion between these structures than in No. 460, 

 so that there seems to be some variation here. 

 No. 460, it may be noted, was a negro skull. 

 Because of this separation of the occipital neural 

 hemiarch from the squama in No. 460, and of the 

 difference in the character of the cartilage here 

 and in the other embryos examined, together with 

 his observation that "there is also a more gradual 

 transition as regards the degree of differentiation 

 from the cartilage of the transverse process into 

 the squama than from that of the lamina," Lewis 

 favors the view that the squama arises by upward 

 extension from the transverse process of the oc- 

 cipital vertebra rather than from the occipital 

 neural hemiarch, as I formerly suggested. 



FORAMEN OCCIPITALE MAGNUM. 



The foramen occiptale magnum is relatively 

 larger than in la. Its lateral contour shows the 

 dorsal and ventral foraminal prominences (figs. 

 5, 6). Seen from without, the plane of the inter- 

 condyloid incisure looks downward. The region 

 lying between the prominences faces almost 

 directly backward and also a little downward. 

 Viewed from the side it is seen to present a distinct 

 dorsal concavity, corresponding to the posterior 



condylar notches. At these points the foramen 

 is widest (fig. 1). The superior occipital incisure, 

 whose plane is directed backward and slightly 

 upward, is filled with the spino-occipital mem- 

 brane. It is much wider than in la. The 

 superior occipital incisure may persist as a well- 

 marked notch in adult skulls of certain dogs. Dr. 

 A. H. Schultz has shown me three very striking 

 examples in skulls of bull dogs and pugs. These 

 are Nos. 71, 381, and 382 of the Schultz collection. 



CERVICAL VERTEBRAE. 



The cervical vertebrae (figs. 3, 4, 5, 6) are well 

 developed. The distance between the tips of the 

 hemiarches of the atlas is the same as that between 

 the dorsal foraminal prominences, but below that 

 the hemiarch tips gradually become more closely 

 approximated, those of the seventh vertebra 

 being separated by a comparatively short interval. 

 A comparison with Lewis's figures of the same 

 region gives a graphic demonstration of the closure 

 of this part of the spinal canal and foramen 

 magnum. 



The atlas presents a distinct anterior arch or 

 hypochordal bar, which is separated from the dens 

 epistrophei by a thin sheet of connective tissue 

 (fig. 11). The late al masses are stout and show 

 concavities upon the upper surface for the con- 

 dyles of the occiput. The costo-transverse 

 foramen on the left side is closed, but that on the 

 right side lacks a very short piece of the costal 

 process which, however, appears to be forming. 



The epistropheus presents a stout dens, from 

 whose tip emerges the notochord, as shown in 

 figure 11. The bodies of the vertebra? form a line 

 which is almost straight and which makes with the 

 basal plate an angle of 125°. The corresponding 

 angle in the Lewis 21-mm. embryo was 110°, as 

 measured by me from his figures. This angle is 

 probably variable. 



TYPE OF CARTILAGE. 



With the exception of the ossification centers, 

 which have already been described, the occipital 

 region is made up almost entirely of a mature type 

 of cartilage. The character of this varies some- 

 what in the different regions. The paracondyloid 

 process is tipped with young cartilage. The 

 condyle, too, reveals a younger type of cartilage 

 at the region of the future articular surface. 



