314 CARTILAGINOUS SKULL OP A 21 MM. HUMAN EMBRYO. 



cartilage is considerable precartilage extending along the inner wall towards the 

 apex (fig. 15). Part of the outer wall of the orbit, consisting of the zygomatic 

 blastema and the zygomatic process of the frontal blastema, and also the zygo- 

 matic arch, are shown in figure 9. In lateral views of the adult skull the upper 

 border of the zygomatic arch is horizontal, and it is therefore easier to compare 

 figure 9 and the other lateral views with the adult if the figures are rotated so as 

 to bring the zygomatic arch into a horizontal position. Attention has already 

 been called to the fact that the angle made by projecting the line of the upper 

 edge of the zygomatic arch on to the basioccipital is almost exactly the same in 

 the embryo and in the adult. 



By rotating figure 7 to correspond to the usual adult position it will be seen 

 that the occipital squama projects backward from the exoccipital and mastoid, 

 and that, since it covers only part of the medulla and cerebellum, it is concerned 

 with the wall of the posterior cranial fossa. It is from this lateral view that one 

 gets the impression that the squama extends into the jugular process, or more 

 strictly, into that part of the exoccipital which continues into the lateral part of 

 the jugular process, rather than into the occipital neural arch which lies in a deeper 

 plane. The occipital neural arch has attained only a small proportion of the growth 

 necessary to inclose the medulla. The tip of the squama, on the other hand, has 

 grown farther around. The condition here is not as far advanced as that found by 

 Levi in a supposedly younger embryo. 



The lateral aspect of the jugular process is quite prominent, and to it are 

 attached two muscles, the rectus capitis lateralis (serially related to the inter- 

 transversarii) and the occipitomastoid muscle (fig. 14). The former needs 

 no special comment. The latter muscle is not found in the adult and is infre- 

 quent in embryos. Its presence would seem to indicate either that the mastoid 

 process was separate or movable, or that the temporal and occipital cartilages 

 were at one time movable on each other. We do find that the occipital and 

 mastoid cartilages are separated by young cartilage or precartilage in the region 

 below the mastoid foramen. Above this, however, no line of separation can be 

 found, but this does not necessarily mean that the two cartilages were articulated 

 by a movable joint. The significance of the mastoid process is described elsewhere. 

 The canalicular part of the otic capsule forms a conspicuous part of the lateral 

 wall of the chondrocranium. A slight bulging on the lower part of its lateral 

 surface indicates the position of the lateral semicircular canal. The styloid process 

 and incus lie beneath the lateral edge of the otic capsule almost flush with the 

 lateral surface. 



Between the canalicular part and the occipital cartilage is the mastoid cartilage. 

 It is fairly clearly defined in the region between the otic capsule and the exoccip- 

 ital below the mastoid foramen. In the region above the otic capsule there is 

 no line of demarcation between the squama and mastoid. The mastoid process 

 projects from the lower edge of the mastoid cartilage. In this embryo it consists 

 of blastema; in other embryos, as will be stated farther on, a small cartilaginous 

 nodule is sometimes embedded in the blastema. To the mastoid process are 



