GROWTH OF THE CEREBRAL CORTEX 541 



loose network, but always show the general direction of the cur- 

 rent of migration. 



The (one or two) transitional layers lie between the ental sub- 

 layer of the lam. mult, and the ventricular wall, separated by 

 pale bands which in turn are bridged by slender chains of cells. 

 These transitional layers are probably the loci where the indif- 

 ferent cells or neuroblasts perform their rotation. The cells of 

 the transitional layers have small nuclei and more or less rich 

 protoplasm and their arrangement suggests every stage of rota- 

 tion. In brains a few days before birth (brain weight, 0.15 to 

 0.17 gram) this layer is clearly divided into two layers by a light 

 band, as seen in figure 8, but in older brains the ental layer di- 

 minishes, or may even disappear leaving a single layer. The 

 relatively broad layer poor in cells lying between this layer and 

 the ventricular wall is prettily striped by the chains of the mi- 

 grating cells which run radially at wide intervals. The light 

 band lying between the transitional layers and the ental sub- 

 layer of the lam. mult, is narrow. In contrast to the small cells 

 in the transitional layers, the cells of the lam. mult, are large and 

 better stained. 



In the newborn brain these transitional layers can be seen 

 always lying between the ventricular wall and the cortical layers 

 proper, so that in the sagittal sections they extend from the 

 frontal tip where the cerebral cortex goes into the olfactory bulb, 

 to the beginning of the cornu Ammonis. In frontal and hori- 

 zontal sections, those transitional layers are also seen where the 

 ventricular wall is close to the cerebral cortex. Where the cortex 

 overlies interbrain structures, the transitional cell layers are not 

 distinct. 



These transitional layers disappear three or four days after 

 birth and are not to be seen any more in brains of over 0.5 gram, 

 in which the indifferent cells or neuroblasts radiate in loose chains 

 from the ventricular wall directly to the ental sublayer of the 

 lam. mult., without forming distinct transitional layers. The 

 cells making the chains are less crowded and majority of them 

 small-sized as seen in the younger brains. 



