^2lS 



arch originating from semi-segment la has been enlarged in a lateral 

 direction ; whilst in D the foramen arcuale medianum is indicated 

 as the very probable natural limitation of semi-segment \a opposite 

 to semi-segment \b. 



A indicates consequently the minimal degree, D the maximal 

 degree of activation of the osteogenetie potency in the semi-segment la. 



On purpose I have not represented in these schemes the share 

 that semi-segment la would have in the structure of the massae 

 laterales. As long as this part of the atlas does not show a relief, 

 by which we could indicate Ihe boundary-line between the semi- 

 segments la and \b, I do not think myself justified to insert it 

 in a scheme, however prob.able llie view may be theoretically. 



Briefly expressed the following lias been demonstrated in this 

 communication : 



1^' . As was ascertained by Bolk, we have to see in the occurrence 

 of ponticuli posleriores and laterales in the human atlas nothing 

 else than an atavistic variation, as the form of the atlas occurring 

 normally with man has originated by reduction from the mammal- 

 atlas, in which the mentioned ponticuli usually occur constantly. 



2. The ponticuli posteriores and laterales, whether they occur as 

 a variation, as with man, or are constant, as with most of the 

 mammals, belong to the semi-segment la 



In all cases in which the mentioned ponticuli are extant, the atlas 

 is certainly not equivalent to Ihe other vertebrae, as the formula 

 for Ihe atlas must then be Sla -\- SlA -\- Slla. Consequently de Buklkt's 

 supposition that elements that originally were situated cranially, have 

 assimilated with the atlas, is correct. 



3. The variations of the atlas designated as foramina arcualia 

 medialia or niediana are most likely the proof, that also Ihe part 

 of the arcus posterior, in so far as it is situated cranially from the 

 mentioned foramina, extending between the two places of insertion 

 of the ponticuli posteriores must be reduced to the semi-segment la. 



4. The fact that the two mentioned ponticuli belong to the semi- 

 segment la is the proof, that activation of the osteogenetie potency 

 existing in this semi-segment is possible. 



15 



Proceedings Royal Acad. Amsterdam. Vol. XV lU. 



