SKULL OF A HUMAN FETUS OF 40 MM. 413 



cells are still larger and vacuolated, and where it fuses with the 

 mandibular bone there is what appears to be a beginning center 

 of endochondral ossification. Here the ground substance is 

 much more deeply staining, and at one point a bud of osteo- 

 blastic tissue appears to be invading the cartilaginous mass. 

 Elsewhere, however, the cartilage and covering bone are separated 

 by a narrow interval, containing connective tissue, and nowhere 

 else in the cartilage are there any indications of ossification. 



Just beneath the posterior expansion of the shaft is seen the 

 small, flattened tympanic bone (fig. 2), while medial and above the 

 latter, lying with its long axis parallel to that of the Meckelian 

 cartilage, and immediately applied to it, is the slender goniale. 

 Above and lateral to the posterior extremity of the shaft is to 

 be seen the squamo-temporalis. 



Ventrally the mylohyoid muscle is attached to the perichon- 

 drium of Meckel's cartilage, but in the middle third its attach- 

 ment is to the inner table of the mandible and its dorsal pro- 

 longation, this connection being established just above the 

 cartilage. Hence it would appear that the mylohyoid ridge of 

 the mature bone indicates the original position pf Meckel's 

 cartilage. 



By reference to the Hertwig model it is seen that in the inter- 

 val between the 40 nam. and 80 mm. stages there has been a 

 modification of the curve of the cartilages, the dorsal curvature 

 having been eliminated in the older stage, so that the proximal 

 portion of the cartilage is forced away from the cochlea and the 

 angle at the symphysis considerably widened. It would appear 

 that in the process of development the cartilage grows more 

 rapidly in length than in thickness, and hence with advancing 

 age it becomes progressively more slender. This is brought 

 out clearly when my model is compared with the earlier Low 

 ('09) models on the one hand and with the Hertwig model on 

 the other, the cross-section of the rod in the latter being less 

 when compared with its length than is the case in my model, 

 and much less than that of the earlier Low models. 



Evidence of beginning ossification and resorption of the 

 cartilage opposite the interval between the lateral incisor and 

 canine tooth germs is first noted by Low ('09) in the 31 mm. 



