373 



Now, when we test tlie skulla of the various classes of mammals 

 by the criteria just mentioned, it appears that the whole class of 

 the Primates, so not only man, is characterized by an orthognathous 

 skull, in contradistinction to all the other mammalian classes. Applying 

 the degree of prominence of the jaws as a criterion for prognathism 

 is an erroneous method, which e.g. has led to the classification of 

 apes among the prognathous forms. Though their jaws may be ever 

 so much developed, the base of the skull never presents an ethmoidal 

 angle, while the nasal cavity is never situated before the cranial 

 cavity and in younger individuals there is even a sphenoidal angle. 

 The strongly developed facial part of the skull in several apes, 

 however, reminds us forcibly of a prognathous skull. These forms 

 I will, therefore, distinguish as pseudoprognathous. 



In the foregoing the principle has been established for an inquiry 

 into the relation between prognathism and orthognathism. The 

 object of such an inquiry must be the answer to the question: 

 which skull-type is the primitive one and which is the specialized 

 type. F'irst of all I will report the result of my examination of 

 embryos of a number of mammals. It is the following: the fetus 

 of all mammals is initially orthognathous, i.e. has a sphenoidal angle 

 lacks an ethmoidal angle and the nasal cavity is subcerebral. Now, 

 whereas this condition persists in apes partly and in man completely, 

 in the other mammals the fetal orthognathous skull passes gradually 

 into the prognathous type; first the sphenoidal angle disappears, 

 then the ethmoidal angle is developed and coincidently the nasal 

 cavity rotates; its subcerebral position passes into a precerebral 

 position. So it becomes evident that the orthognathous condition in 

 man, which is the special feature of the human physiognomy, reveals 

 itself again as a persisting fetal property. 



Before demonstrating this in a series of embryos, I will briefly 

 dwell on the fact that this transformation of the orthognathous skull 

 into the prognathous type is a process with which we are confronted 

 already in Reptiles, so that it has evidently been inherited by the 

 Mammals from their reptilian ancestors. 



Fig. 3 represents a median section through the head of an embryo 

 of Lacerta, length of the head 4 mm. The chorda is still present, 

 the vertebrae are not differentiated, likewise the cranio- vertebral 

 joint is still incomplete. Of the chondrocranium the basicranial plate 

 enclosing the Foramen can be recognized. This plate extends frontad 

 as far as the Hypophysis cerebri, which is still attached to the 

 epithelium of the roof of the mouth. In front of the Hypophysis lies 

 the prechordal plate. The latter presents two enlargements the one 



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Proceedings Royal Acad. Amsterdam. Vol. XXV. 



