2Ó4 



If we desist from Ifjiiig to give an aiiswei* to tiie qnestioii aficr the 

 influences that have brougiit al)oiit the reduction of the human atlas 

 ill the above-mentioned part's, tiie inlerprotation of the reoccurrence of 

 the ponticnii laterales and posteriores as regressive variation gives 

 certainly a satisfactory ex|)Uiiialion of this piienonienon, as entering 

 upon fnrtiier delails of tiic problem would iminediatei3- lead ns lo 

 the department of general biology and specially to that of ihc piieno- 

 mena of lieredily. 



The way in whicii dk Burlkt '), the third of the above-mentioned 

 in\esiigators, lias treated the problem differs principally from that 

 of tiie former. In (he views hitherto reproduced tiiere was only an 

 attempt to answer the question after the signification and the origin 

 of the ponticuli posteriores and laterales with the human atlas. 



Dk Bukj.et puts the question in a different way by taking likewise 

 into account with this question the homologa of these elements, as 

 they constantly occur — as has already been mentioned — with many 

 mammals. By doing so the problem assumes a more general nature, 

 and may be formulated as follows: 



"What is the signification of the foramen arcuale and alareofthe 

 mammal atlas and of the parts lying ci'anially from it?" 



When answering this question de Burlet points out the possibility 

 that the arcns posterior atlanlis should not be equivalent to the 

 arcus posterior of the other \ertebrae, in this sense namely, that 

 foreign elements lying originally cranially from it should have 

 assimilated with the arcns posterior atlanlis, and as original source 

 of these elements he indicates the so-called proatlas. 



1 cannot treat de Burlet's view completely within the compass 

 of this communication. The notion proatlas has in the course of 

 time gradually been modified and is even now by no means accu- 

 rately defined, so that an effectual discussion of de Burlet's view 

 that the ponticulus posterior and lateral might be homologised with 

 the proatlas requires necessarily an accurate definition of the proatlas. 

 I hope to do so in a subsequent communication, whicli will be entirely 

 devoted to the Proatlas-problem ; now I can, whilst explaining my 

 own view, only enter upon de Burlet's opinion in so far as he 

 admits the possibility that elements having originally extended cra- 

 nially* from the arcus posterior atlantis should have assimilated with 

 it, and the posterior arch of the atlas should consequently not be 

 homologous with the posterior arch of the other vertebrae. 



1) De Burlet. H. M. — Ueber einen ludimonlaren Wubelkörper an dei- Spitze 

 des Dens Epistrophei bei einem Embryo von Bradypus cuculli. Morpliol. Jabrb. 

 Bd. XLV. H 3. 



