SKULL OF A 43-MM. HUMAN FETUS. 



61 



septa. The lower contour lines of the two limbs, 

 as seen in figure 10, meet at an angle of 115°, as 

 in 7a. The prechordal limb in No. 460 is relatively 

 shorter, and the corresponding angle, as measured 

 by me on Lewis's figure 5, is 125°. 



The basioccipital and basisphenoidal elements 

 of the basal plate are united by continuity of 

 cartilage at the undefined spheno-occipital com- 

 missure. Here the plate is thinnest, as figure 11, 

 of the midsagittal section, shows. In this figure, 

 too, it is apparent that the anterior end is much 

 thicker than the posterior and that the upper 

 surface presents a deep antero-posterior con- 

 cavity, located in advance of the center. 



At the junction of its anterior and middle thirds 

 the plate is narrowed, due to the encroachment of 

 the cochlea?. The caudal end of the curved 

 basicochlear commissure projects outward beyond 

 the cranial. The cut edge of this union is seen on 

 the basal plate in figure 10, and on the cochlea in 

 figure 15. The dorsal and ventral basicochlear 

 grooves are not so deep as those of la. 



The dorsal surface of the basal plate (figure 1) 

 is shallowly concave from side to side throughout 

 almost its entire extent. Anteriorly, however, 

 this concavity becomes very narrow and indistinct 

 as the surface rises upon the basisphenoid, and 

 disappears at the root of the dorsum sella?. The 

 ventral surface presents a corresponding low 

 transverse convexity, which falls away laterally 

 into the ventral basicochlear grooves, flanked by 

 the protruding cochlear portions of the otic 

 capsules. The ventral surfaces of the cochlea? and 

 of the basal plate lie practically in the same level 

 and thus combine to give to the ectal surface of 

 the base of the skull a flattened appearance in 

 contrast to the roomy concavity of the corre- 

 sponding ental surface. Posteriorly, the diverg- 

 ing limbs pass uninterruptedly into the primordia 

 of the exoccipitals. 



NOTOCHORD. 



The notochord was modeled in relief upon the 

 midsagittal section of the basal plate and epi- 



stropheus and is illustrated in figure 11. Com- 

 mencing below, it proceeds through the body of 

 the epistropheus and dens, emerging from the 

 apex of the latter. The cartilage of the dens 

 projects a little farther forward than the point of 

 exit of the chorda and upon this cartilaginous tip 

 the chorda rests; it then springs across the very 

 narrow gap between the dens and the adjoining 

 caudal edge of the basioccipital and here shows 

 some thickening. It now proceeds along the 

 dorsal surface of the basioccipital, in the midline, 

 for 900 micra, being buried in perichondrium. 

 The anterior end of this part is compressed dorso- 

 ventrally. It then traverses the plate as shown in 

 the figure, here perforating the anterior end of the 

 beginning ossification center for the basioccipital. 

 Within the cartilage it is a very slender thread 

 and at its point of exit it loses its continuity, there 

 being here a break of something less than 100 

 micra. Shortly after emerging below the basal 

 plate it comes into contact with the long, attenu- 

 ated pharyngeal bursa and follows a course upon 

 the dorsal edge of this. Especially at the ex- 

 tremities of the bursa it is contorted and varicose. 

 Leaving the pharyngeal bursa, the chorda lies 

 immediately above the epithelial roof of the 

 pharynx for ' a considerable distance. It then 

 turns sharply, as an attenuated and somewhat 

 contorted cord, to approach the body of the 

 sphenoid, which it enters almost at right angles to 

 the surface. Finally, it curves forward to ter- 

 minate in the body of the sphenoid near its dorsal 

 surface and some distance short of the crista 

 transversa. This basisphenoidal portion of the 

 chorda is a little wider than that just outside the 

 cartilage and its terminal end is somewhat 

 nodular and irregular in direction. 



In its course the notochord of No. 886 thus 

 resembles closely that in Huber's (1912) human 

 embryo J, No. 47, 32 mm. long, shown in his 

 figure 10. 



OCCIPITAL REGION. 



The occipital region is a homogeneous mass of 

 cartilage whose caudal boundary is formed by the 

 margin of the primitive foramen magnum. The 

 cranial boundary is marked by cartilaginous 

 unions with the otic region and the foramina 

 separating these. Proceeding from the front 

 late ally and backward, we note the following 

 commissures: spheno-occipital, basicochlear, cap- 

 sulo-occipital, and occipitoparietal. Behind the 



basicochlear commissure a section of the boundary 

 is formed by the posterior margin of the jugular 

 foramen. The lateral half of this, which is thin 

 and forms the concave lip of the sigmoid sulcus, 

 runs almost directly outward and so makes an 

 angle with the medial thicker and more rounded 

 half. It is at this angle that we find the anterior 

 end of the jugular tubercle. The boundary is 

 then continued as the capsulo-occipital com- 



