A CASE OF CYCLOPIA 241 



Notwithstanding these malformations, the cranial nerves from 

 the oculomotor caiidad and most of the fiber systems with their 

 nuclei could be identified in the brain stem. There was a com- 

 plete absence, however, of the pyramidal system throughout. 



Naegeli calls attention to von Monakow's (16) classification 

 of fiber systems and nuclei into phylogenetically old and phylo- 

 genetically young groups, and points out that only the former 

 are to be identified throughout in his case. 



He concludes from the striking resemblance between the ar- 

 rangement of structures in the forebrain region in his case and 

 those obtaining in the forebrain of teleosts, and also from the 

 complete absence of so-called phylogenetically young fiber sys- 

 tems, that the condition of the brain in cyclopia may represent 

 an arrest of development at a phylogenetically early stage. 



It is evident from the above that the prosencephalic disturb- 

 ance here noted, dependent upon the cyclopian condition, was 

 not so extensive as in the present case. 







CONCLUSION 



In the case I have reported, there is a very marked superficial 

 resemblance between the forebrain vesicle and the telencephalon 

 in teleosts. The resemblance, however, is only superficial, for 

 the thickened basal structures present in this case are altogether 

 pallial. In teleosts the basal nuclei form the bulk of the thick- 

 ened base of the telencephalon, while in this case these basal 

 structures are absent. 



The absence of the so-called phylogenetically young fiber sys- 

 tems is sufficiently explained by the lack of complete develop- 

 ment of the suprasegmental neurones whose processes make up 

 the bulk of such systems normally. There can be no doubt 

 that the very slight attachment of the thalamic mass to the 

 cerebral vesicle had much to do with the growth inhibition of 

 the cortical neurones. 



The presence of a well marked hippocampal formation while 

 the rhinencephalon, as before defined, is entirely wanting, may 

 be explained on mechanical grounds. Such a finding offers the 



