ANATOMY OF A CYCLOSTOME BRAIN 663 



mus) the neurones are arranged for the most part very simply in 

 a compact layer of central gray. This layer, however, is broken 

 up into a series of detached cell plates, each with a characteristic 

 type of form, size and arrangement of neurones, and this differ- 

 entiation has in the main been functionally determined. 



In the different parts of the adult brain of Ichthyomyzon 

 various forms of functional differentiation of the primordial cen- 

 tral gray can be observed and provisional conclusions can be 

 drawn regarding the probable sequence of the ontogenetic and 

 phylogenetic developmental stages of the corresponding parts of 

 higher brains. These conclusions must be controlled by further 

 study, especially of the details of embryonic development. The 

 observations here recorded suggest that the first step in the func- 

 tional differentiation is an increase in the number of neurones 

 in the affected region, accompanied usually by adaptive changes 

 in their form and internal structure. 



A very slight thickening of the central gray may be mani- 

 fested by a localized eminence on the ventricular surface which 

 may be accompanied by an increase in the amount of neuropil 

 in the overlying stratum album causing a similar eminence on 

 the lateral surface of the brain. Further differentiation may be 

 accompanied by the migration of neurones laterally from the 

 central gray into this neuropil, thus giving rise to a more super- 

 ficial nidulus (nucleus) among the terminals of the afferent tract 

 related to the area in question, in accordance with the doctrine of 

 neuro-biotaxis (Kappers). An early phase of this type of differ- 

 entiation is seen in the lateral geniculate body of Ichthyomyzon. 



Further increase in the number of neurones in the differen- 

 tiated area results in an outward folding of the entire wall of the 

 neural tube, thus producing a lateral evagination, as illustrated 

 in the tectum mesencephali and cerebral hemisphere of Ichthyo- 

 myzon. Here too there may follow a lateral migration of neu- 

 rones away from the central gray, producing a layer of super- 

 ficial gray matter. Both of the latter processes attain their 

 maximum in the mammalian cerebral hemisphere. 



The details of the form changes involved in the process of 

 evagination of the cerebral hemispheres of higher brains are as 



