242 D. DAVIDSON BLACK 



strongest evidence that the condition of the brain in cyclopia 

 cannot represent an arrest of development at a phylogenetically 

 early stage. 



The observations in connection with the histological structure 

 of the cortex, which will be more fully recorded elsewhere, have, 

 I think, shown the value of a close investigation into the finer 

 anatomy of the abnormal nervous system. It is in such cases 

 as this that the key to many of the problems concerning the devel- 

 opment of the normal brain may be found. 



My conclusions can best be summarized as follows: 



1. That the central nervous system in this case of cyclopia 

 does not show any evidence that may be taken to indicate an 

 arrest of its development at an early phylogenetic stage. From 

 a study of Naegeli's paper, and from what I have seen in other 

 cyclopian brains in my possession, I am led to the conclusion 

 that any apparent indication of such a reversion is purely super- 

 ficial in character and may be explained on mechanical grounds. 



2. That the persistence of so-called phylogenetically old neu- 

 rone systems in the brain, and the absence of phylogenetically 

 young systems, is due solely to the absence during development 

 of certain portions of the forebrain in the ventral region (mid- 

 line), and to the interference with the mechanics of growth 

 caused thereby. 



3. That the condition of development of the primary fore- 

 brain will present a new mechanical problem in each case of 

 cyclopia, depending upon the extent of the primary absence of 

 tissue anlagen, and that the extent of this 'lesion' cannot always 

 be judged by the condition of the cyclopian eye. In contrast to 

 this, the condition of development in the brain stem in each 

 case, provided no other malformations are present, will be ver}- 

 similar; and such abnormalities as are present will be due to 

 the absence of fiber systems of the suprasegmental type. 



4. That the condition of cortical development is such as to 

 confirm the opinion already expressed by Bolton and Moyes (3), 

 namely : that the sensory or afferent fibers to the cortex develop 

 in all probability before the motor or efferent fibers from the 

 cortex. 



