272 The Nature of Biological Diversity 



segmental structures, migration of differentiated nerve cells plays an 

 important role in the molding of nerve centers. These movements are 

 more difficult to analyze than the ones in the hrain stem since the 

 active displacement of cell populations occurs at the same time as 

 gross morphological changes in the shape of the hrain. In the follow- 

 ing discussion we shall outline only two of these movements, which 

 occur in the cerehellar cortex and in the avian telencephalon. 



Migratory Movements in the Cerebellum. The complex changes in 

 size and shape in the Purkinje cells during their late differentiation 

 were first described by Cajal (10) and nothing can be added to his 

 exact analysis of the process. Until recently it had not been settled 

 whence these cells originate and how they appear during the early 

 developmental phases of their differentiation [unpublished observa- 

 tions (11)]. We traced these cells in our series of avian brains in 

 stages between 7 and 8 days of incubation. They appear as elongated 

 neuroblasts in proximity to the deep subependymal germinal layer. 

 Since they react intensely to silver and exhibit the characteristic 

 fusiform shape of cells in a migratory phase, it was easy to explore 

 their position during the following days and trace their active dis- 

 placement from the deep to the cortical cerebellar layer. During the 

 early phases they are closely packed in dense cellular rows; as they 

 reach the cortical layer they spread in a fan-like fashion and settle 

 in the layer which will be known as the Purkinje layer in the fully 

 developed organism (Figs. 4, 16 to 19) . 



The origin of the Purkinje cells from the germinal layer was re- 

 cently described by Uzman (12), who utilized the technique of 

 labeling the migrating cells with tritiated thymidine. His observations 

 establish beyond doubt the migratory movements of these cells. This 

 technique can be very useful to detect the migratory movements in 

 nerve cell populations. In future research both this technique and 

 the silver method, which has the advantage of staining selectively 

 the migrating cells, should be combined. 



Migratory Movements in the Cerebral Hemispheres. The mass mi- 

 gration of one of the avian telencephalic centers was described in 

 detail in a previous publication (6). Since it does not substantially 

 differ from other migratory movements of differentiated nerve cells, 

 we shall mention here only its main features. 



The mass migration of the nucleus, which we designated as nucleus 

 epibasalis centralis, starts at 8 days. It consists of two dense neuroblast 

 columns moving along the lamina medullaris dorsalis. The cells ex- 

 hibit the characteristic elongated shape of migrating neuroblasts and 

 the intense silver affinity. One can trace the two moving columns and 



