30 L. BOLK 



riability as to the age at which the skull bones unite in Prosimiae. 

 In monkeys the ossa frontalia unite and a persisting metopical 

 suture is an individual and rare exception. Finally, in Anthro- 

 poids a metopical suture in an adult skull has never been seen. 



The history of the metopical suture therefore is a somewhat 

 complicated one. Originally the suture was always present, 

 later it disappears, and finally in man it reappears as a not in- 

 frequent variation. 



I wish to emphasize, that in consequence of this behavior of 

 the frontal suture in the course of evolution, two possibilities 

 must be taken into consideration when trying to account for 

 its reappearance in man. Firstly this reappearance can be ex- 

 plained as due to a quite new influence acting only in man, namely 

 the increased development of the brain which prevents the two 

 frontal bones from uniting. But there is another point of view 

 of a more physiological nature, claiming our full attention in no 

 lesser degree. In primitive Primates the metopical suture per- 

 sisted. In the further course of evolution certain causes, to which 

 I intend to return, exerted their influence in such a way that both 

 frontal bones were compelled to unite and the metopical suture 

 disappeared. Now, I believe, the possibility presents itself that 

 the metopical sutm-e in man reappears, just because the factor, 

 which once caused its disappearance in monkeys, no longer 

 exerts its influence in the human skull. From this point of view 

 the problem has not yet been examined. 



In the foregoing it is made clear that the metopism of the 

 human skull is the starting point of some very interesting prob- 

 lems, to which I will shortly refer in the order in which they are 

 treated on the next pages. Firstly the question about the fre- 

 quency of the anomaly in Dutch skulls will be discussed, then 

 the question whether the metopical suture occurs more frequently 

 in brachycephalic skulls, and whether it is true that a persisting 

 frontal suture is of some influence upon the shape of the skull. 

 Thereupon we will examine if there exists any relation between 

 metopism and intellectual development, particularly if it is true 

 that the anomaly is more frequent in large skulls, containing a 



