378 



and the Alispheiioid. The rotation of the septum nasale has conti- 

 nued ; the nasal cavity now lies obliqnelj'^ iindei' and anteriorly to 

 the caviim cranii. This rotatory movement apparently results from 

 the further upward Hexing of the frontal part of the basis cranii. 

 The ethmoidal angle now becomes distinctly visible, right in front 

 of the centrum of ossification of th<3 Alisphenoid. 



Fig. 10. Embryo of 43 mm. The ethmoidal angle has reached 

 its definite value for the skull of the adult rat, the fiontal part of 

 the basis cranii has now become the anteiior wall of the cranial 

 cavity, the Jiasal cavity is situated before the cranial cavity, the 

 skull has become prognathous. 



It is evident, then, that the transformation from the orthognathous 

 into the prognathous skull-type in the mammalian embryos is a 

 regular process in which two succeeding phases are recognizable. 

 In the first phase a straightening of the basis cranii takes place; 

 the sphenoidal angle disappears. Its disappearance it attended with 

 a change in the direction of the septum nnsale, which is now placed 

 obliquely to the base of the skull. After this the second fundamental 

 alteration in the basis cranii commences, viz. the formation of the 

 ethmoidal angle, the anterior (ethmoidal) portion of the base being 

 turned up together with the septum nasale, which is attached to it. 

 Consequently a part of the base of the fetal skull becomes the front 

 wall of the cranial cavity. 



I shall not enter into details concerning the various mammalian 

 embryos that I have examined but will only add a few general 

 remarks. 



From the foregoing it is sufficiently evident that the orthognathous 

 skull of man is to be considered as a persisting early fetal form. 

 In stating this fact we have at the same time disproved the current 

 opinion, that the sphenoidal angle, Avhich is so characteristic of the 

 human skull, is due to the intense development of the human brain. 

 This angle, indeed, is not only a feature of all fetal mammalian 

 skulls, but it occurs even in the chondrocranium of Reptiles. It is 

 an essential character of, let me say, the primordial cranium of 

 vertebrates in general. I shall noi discuss this point any further. 



The question now arises whether the intense growth of the 

 Hemispheres has had no influence whatever on the anatomical relations 

 of the skull, apart from the necessarily considerable enlargement of 

 the cerebral crane. Such an influence, and even a very remarkable ' 

 one, can indeed be demonstrated, as may be seen in comparing 

 Fig. Jl and 12. 



Fig. 11 shows the median section through the head of a dog's 



