662 C. JUDSON HERRICK AND JEANNETTE B. OBENCHAIN 



ficial areas of diffusely arranged cells, an arrangement seen, for 

 instance, in the lateral geniculate body of the recessus metatha- 

 lamicus. 



The different cell laminae vary in thickness and in the number 

 of contained neurones. Where these thickenings are localized 

 and not very extensive they produce ventricular eminences, the 

 interverning sulci representing simply lines of less extensive pro- 

 liferation of neurones and of relatively indifferent physiological 

 type. Thus are formed the lobus medius and lobus ventralis 

 thalami and some other ventricular eminences. In some of the 

 larger areas of this sort the increase of the thickness of the brain 

 wall may be so marked as to cause an externally visible eminence 

 on the lateral surface of the brain also, as in the case of the lobus 

 subhabenularis and primordium hippocampi. But if the enlarge- 

 ment of the cell plate is carried still further, there results a total 

 fold of the brain wall, marked by an external eminence and 

 a ventricular recess, as seen in the tectum mesencephali, and 

 elsewhere.^ 



In any comparison of the ventricular sculpturing of a cyclo- 

 stome brain with other species, these functional factors must 

 first be determined and compared and the embryological develop- 

 ment must also be investigated in order that the influence of 

 vestigial factors may be recognized. We are very far from a 

 sufficiently complete knowledge of the cerebral structure of any 

 ichthyopsid types to justify final conclusions and our morpholog- 

 ical interpretations of these structures should be regarded as pro- 

 visional only until our anatomical knowledge is more complete. 



CONCLUSIONS 



This inquiry was directed primarily toward a detailed exam- 

 ination of the external and ventricular surfaces of a lowly organ- 

 ized type of brain and of the underlying deep structures, as an 

 aid to the understanding of the functional factors which have 

 operated in the evolution of brain form. In the cerebrum of 

 petromyzonts (i.e., that portion of the brain in front of the isth- 



- In this connection attention should be called to the recent illuminating 

 analysis of the morphogenetic factors operative in the evolution of the mam- 

 malian cerebral cortex by Kappers ('13, pp. 368.-372). 



