CONCERNING VERTEBRATE CEPHALOGENESIS. 



HOWARD AYERS. 



In this short resume of some of the results of my investiga- 

 tions in vertebrate cephalogenesis, I shall not give a complete 

 account of the morphological data upon which the conclusions 

 are based ; but shall introduce only such facts as seem necessary 

 for this presentation. 



A more detailed account with a critical consideration of the 

 literature bearing upon the subject is reserved for a more 

 extended and illustrated publication which I have in prepara- 

 tion. 



The problem of the origin of the vertebrate head, more espe- 

 cially the brain, from the invertebrate type is, so far as our 

 knowledge yet reaches, an insoluble one ; but given the verte- 

 brate type of body and central nervous system, we are in posi- 

 tion to clearly demonstrate its phylogenetic outcome as repre- 

 sented in the mammalian head and brain. 



Starting with the central nervous system of Amphioxus, we 

 have to deal with an organ which affords many points of contrast 

 with the axial nervous system of higher vertebrates and which 

 serves, as some authorities think, to bridge over the chasm 

 between the invertebrate type (arthropod and annelid) and the 

 vertebrate. 



With the exception of an undetermined small number of 

 anterior segments, the nerve cord of Amphioxus is divided up 

 into a series of physiologically equal segments as Steiner has 

 shown ; but I feel confident from anatomical facts that further 

 and more detailed experimentation will show that there are some 

 modifications to be introduced into Steiner's results, and that 

 there are differentiations physiologically which the crude meth- 

 ods of experiment used by him excluded from the physiological 

 reaction. Steiner's results on other vertebrates would lead to a 

 conclusion differing from that which he has published with 

 regard to Amphioxi^s. For in his experiments on Amphioxus 



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