66 



SKULL OF A 43-MM. HUMAN FETUS. 



Hertwig. Hence it would seem quite likely that 

 the crescentic ridges never come to occupy the 

 position of the inferior nuchal lines, and hence they 

 can not represent them. 



In the mature occipital bone the crescentic 

 ridges are represented rather imperfectly, but are 

 nevertheless recognizable as the lateral delimita- 

 tions of the condylar fossa. Their lateral ex- 

 tremities are here fairly distinct, reaching the 

 jugular tubercle, but their posterior extremities 

 are lost in the region of the posterior margin of the 

 foramen magnum. 



The paraforaminal area, or condylar fossa, is the 

 depressed area bounded by the crescentic ridge, 

 the paracondyloid process, and the occipital neural 

 hemiarch. Its size here is relatively much greater 

 than that of the corresponding region of the mature 

 bone. As yet the superior articular process of the 

 atlas does not he far enough out, as figure 4 shows, 

 to reach the fossa. Its floor is unperforated, but 

 is very thin. The corresponding convexity upon 

 the ental surface forms the posterior boundary of 

 the jugular recess. 



The supraoccipital fundament includes the dor- 

 somedial parts of the two squamae, joined together 

 above the superior occipital incisure by the tectum 

 posterius. The tectum is practically plane and its 

 ental surface looks almost directly forward. Its 

 lower border presents a descending process, which 

 projects into the superior occipital incisure (figs. 

 4, 14). It is of a younger type of cartilage than 

 that of the tectum above it. In a 30-mm. human 

 embryo a similar process has been described by 

 Fawcett (19106). The ascending process has 

 been described. The cartilage of the tectum is 

 very thin, as is seen in the midsagittal section 

 shown in figure. 14. 



The ossification center for the supraoccipital is 

 single at this stage. Its right half is seen from the 

 front in figure 14, and figure 4 gives a view of it 

 from the rear. It is somewhat butterfly-shaped 

 and terminates laterally in sharp down-turned 

 points which do not pass beyond the occipito- 

 parietal commissure. From these lateral extremes 

 the outer border curves downward and inward to 

 the dorso-lateral angles of the superior incisure 

 of the foramen magnum. The ascending and 

 descending processes are not included in the ossifi- 

 cation, which, however, involves the entire remain- 

 ing tectum posterius with the exception of a 

 narrow edge along the superior border. No actual 

 bone has yet been deposited, but there has evi- 

 dently been considerable calcification of the 

 cartilaginous matrix, judging by the staining 

 reaction. 



It is quite evident that this area of calcified 

 cartilage has remained unstained in Bolk's (1904) 

 van Wihje preparations of the occipital region of 

 the human skull, and he has accordingly mis- 

 interpreted this region as membrane. This failure 

 to recognize an imperfection in his method has led 

 Bolk into a number of errors. Thus he is in error 

 when he states that it is highly improbable that 

 the supraoccipital ossification is of the true 

 endochondral type. It is as distinctly endo- 

 chondral as any ossification could be. This 

 region is unique in that the calcification of the 

 cartilage takes place very early. From his prep- 

 arations Bolk gains the impression that bony 

 development begins in the center of the supra- 

 occipital before cartilaginous development is 

 accomplished here, the failure to chondrify being 

 due to the rapid development of the brain. He 

 does not explain why this rapid brain development 

 should not also have a similar effect in retarding 

 the development of the "Knorpelspange" just 

 above, which is merely the uncalcified upper 

 margin of the tectum posterius, as I have pointed 

 out. 



It is of interest, however, that this upper margin 

 does remain uncalcified and unossified for such a 

 long period for, as Bolk has shown, it is present 

 throughout his series of four human chondro- 

 crania. In the last, however, it is becoming 

 thinner and more attenuated. I would suggest 

 that this cartilaginous edge may be retained to 

 favor growth of the supraoccipital here. A 

 similar reason may underlie the persistence of the 

 twin nodules of cartilage which are present at the 

 apex of the superior occipital incisure, in Bolk's 

 preparations, and which he states agree in position, 

 in one case at least, with the bones of Kerckring. 

 These nodules, in Bolk's figure 1, are in the same 

 position as the descending process in No. 886, 

 apparently. It seems to me quite likely that they 

 are not isolated masses of cartilage situated in 

 membrane, but that they are connected with the 

 calcified cartilage of the tectum above. 



It is of interest to observe that Fawcett (19106) 

 questions Bolk's findings when he says (p. 306) : 

 "I must confess the appearances in his figures 

 scarcely explain what is seen in this cranium." 

 Some other recent authors have not been quite so 

 critical. 



OCCIPITAL VERTEBRA. 



The neural hemiarch of the occipital vertebra 

 does not stand out so distinctly as that of la. 

 Behind the condyle it is bent to form the posterior 

 condylar notch. In figure 4 it is quite distinctly 



