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resorption. So the figures 3 and 4 illustrate a rotation of the septum 

 nasale, and consequentl}^ of the facial skull. From its original sub- 

 cerebral position (orthognathism) it shifts into a j)receiebral position 

 (|)rogna(hi8m). That in connection with this rotation plagiostom}' 

 changes into teleostomj we will pass over in silence, although this 

 phenomenon would give amj)le scope for interesting observations. 



It has thus been shown that the chondrocranium of Reptiles, in 

 its early phase of development, resembles the orthognathous type. 

 Now we are going to demonstrate that the process of development 

 in Mammals bears a great resemblance to that of Reptiles. 1 have 

 studied the ontogenesis of the skulls of a number of Mammals, and 

 in all of them I met witli the phenomena that I am going to 

 describe for the skull of Mus decumanus. 



Fig. 5 represents the median section of an embryo of Mus 

 decumanus of 11.5 mm. In this stage the primordial cranium is 



Fig. 5. 



Fig. 6. 



sufficiently differentiated. We will confine ourselves to the skeleton, 

 omitting all further remarks that the following series of figures might 

 suggest. In this stage the Hypophysis has become a closed vesicle, 

 which, however, still adheres to the epithelium of the mouth. Behind 

 the Hypophysis lies the basicranial plate, which in Mus is subchordal 

 over its whole length. Frontal to the Hypophysis lie the prechordal 

 plate presenting a slight broadening dorsad, which is homologous 

 with the strongly developed Septum orbitale in Reptiles. At its lower 

 surface the Septum nasale is fastened. There is no denying that the 

 basicranial plate and the prechordal plate form an angle. This 



