CARTILAGINOUS SKULL OF A 21 MM. HUMAN EMBRYO. 



303 



of course, a great help towards understanding the cartilaginous skull. Such a 

 literature for any of the lower vertebrates is scarcely existent, and therefore very 

 little is now to be gained through a comparison of the cartilaginous skulls of differ- 

 ent species. 



It is well to bear in mind the great difference in size between the cartilaginous 

 skull of the embryo and that part of the adult skull ossified in cartilage. Table 2 

 gives a few comparative measurements. Unfortunately, the embryonic measure- 

 ments are all from the one cartilaginous skull under consideration ; it would be most 

 desirable to have several such specimens, since the size of the skull varies consider- 

 ably in embryos of corresponding ages. To realize the extent of this variability 

 one has but to compare the length of the basal plate, from the foramen magnum 

 to the hypophysial canal, in embryos of approximately the same age. Similar 

 differences exist in the adult, and the adult measurements here used are the aver- 

 ages for six skulls. 



TABLE 2. 



These figures serve to indicate in a general way the greater immaturity of 

 the anterior end of the cartilaginous skull at this stage. The adult basioccipital 

 is about 13 times as long as the cartilaginous basioccipital; the adult nasal septum 

 about 30 times as long as the embryonic nasal septum. In agreement with this 

 we find that the basioccipital consists of more advanced cartilage than the nasal 

 septum. In one of the following paragraphs I have stated reasons for believing 

 that the alar process of the sphenoid at a later stage becomes incorporated into 

 the body of the sphenoid; this is partly substantiated by comparing the ratios for 

 the distances between the alar processes of the embryo and between the lingulae 

 of the adult with the width between the outer edges of the optic foraminae or the 

 distance between the foraminse rotundas (the latter formed in precartilage in the 

 embryo) in both embryo and adult. These ratios are the same in each case, namely, 

 1:14, thus indicating that if the rate of growth in each of these regions of the 

 sphenoid is the same the alar process must be looked upon as ultimately forming the 

 lateral part of the body of the sphenoid. If, however, we compare the width of 

 the basisphenoid in the embryo (not including the alar processes) with that of the 



