DEVELOPMENT OF THE CEREBRAL CORTEX 113 



The question arises, and it is a most important one, whether 

 all these nuclei proceeding from the matrix represent both neuro- 

 blasts and spongioblasts or whether they are spongioblasts alone. 

 His considered the distinctive characteristic of the neuroblast in 

 the younger embryo to be the darker stain taken by the cell at 

 the end from which the nerve process arises. But he distinctly 

 states that in the middle of the third month this is no longer to 

 be depended on. He agrees that the cortical layer exclusively, 

 or very nearly exclusively, now holds nerve cells, but in the inner 

 zone of the intervening layer the tangentially directed cell bodies 

 should be considered gUa cells. 



In the first or external* layer of the cortex (the molecular layer 

 of Meynert) the few scattered nuclei present may be safely looked 

 upon as spongioblasts. At least if any neuroblasts are present 

 they are so few in number they may safely be ignored. But 

 between the nuclei here present and the naked nuclei swarming 

 in the different layers of the cortex, even at birth, I can detect 

 no characteristic differences. Some of the nuclei in the broad 

 streams going out from the matrix and in the ' Ubergangschicht' 

 have partly developed cell bodies and distinct protoplasmic 

 processes, but the great majority are naked nuclei and correspond 

 in every way to the majority of those already arranged in the 

 cortical layers. Some are doubtless spongioblasts, but it is 

 hardly possible that they represent more than a small percentage. 

 It seems quite evident that many neuroblasts reach the cortex 

 where they arrange themselves in their ultimate position before 

 they have developed any protoplasmic processes or possess any 

 demonstrable cell bodies. His suggests that the protoplasmic 

 process may serve as a locomotor apparatus, but the majority 

 of the nuclei are clearly able to reach their goal without it. He 

 speaks of the route followed by the cells in their migration from 

 the matrix to the cortex as always radial, and in this respect 

 differing from that taken by the neuroblasts of the spinal cord 

 and the medulla oblongata. This is apparently true in the earher 

 stages and would apply up to the end of the fourth month of foetal 

 life, at which stage he thought the migration was completed. 

 But as the brain develops, the space between the matrix and 



