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angle, wliicli we also found in Lacerta, is the sphenoidal angle that 

 we know to be the typical feature of the orthognathous skull. Whereas 

 the base of the skull is directed almost quite horizontally, tfje axis 

 of the septum nasale is directed |)erpendicularly. Therefore in 

 this stage of development the nasal cavity of Mus is subjacent to 

 the cranial cavity. The skull of this young embryo of Mus possesses, 

 therefore, two features, which are characteristic of the orthognathous 

 skull, viz. a sphenoidal angle and a subbasal situation of the nasal 

 cavity. That the latter condition is not the consequence of the intense 

 development of the cerebral hemispheres, is borne out by the fact 

 that in an early stage of development of Reptiles we find the same 

 direction of the septum nasale. The condition in Mus, just described, 

 is iidierited from the reptilian ancestors of Mammals, which in 

 their turn have inherited it from more primitive vertebrates. Plagio- 

 stomy, to which we referred heretofoi'e, and which, to some extent, 

 is encountered in the represented embryo of Mus, indicates in^what 

 direction we have to look for an explanation of this condition. 



Accordingly we conclude that orthognathism is the characteristic 

 of the young fetal mammalian skull. Now let us see how the prog- 

 nathous type is developed from the primitive type. 



Fig. 6 illustrates the median section through the head of an embryo 

 of 13.5 mm. in length. The chorda begins to disappear, the Hypo- 

 physis lies within the cranial cavity, but is still attached to the 

 mouth-epithelium. The base of the chondrocranium begins to stretch, 

 but the sphenoidal angle is still recognizable. The axis of the septum 

 nasale is still perpendicular to the prechordal plate. 



Fig. 7. Fig. 8. 



Fig. 7. Embryo of 20 mm. The basis cranii is stretched, the 



