DEVELOPMENT OF THE CEREBRAL CORTEX 115 



withstanding the proportional increase in the superficial area due 

 to the development of fissures and convolutions. By this the 

 superficial area must be made more than double that of the plane 

 surface. 



In the recently published work by Keibel and Mall,* Streeter 

 states that the "migration is most active during the third month 

 and continues well into the fourth." In the English edition he 

 says "at this period" (end of the fourth month) "the wandering 

 of the cortical neuroblasts is completed." In the German edi- 

 tion the statement 'um dieser Zeit' is not quite so definite. He 

 says further "The ependyma does not appear as active as hereto- 

 fore although it apparently is still giving off spongioblasts that 

 are to form the neuroglial elements of the white substance. The 

 cortical or pyramidal layer has taken up all its wandering neuro- 

 blasts from the deeper layers and is sharply marked off from the 

 subjacent intermediate layer." He assumes that all the new 

 elements given off by the matrix after the end of the fourth month 

 are spongioblasts, although His expressly states it is impossible 

 at this stage to distinguish between spongioblasts and neuro- 

 blasts in the primitive granular form. The question may arise 

 here as to whether or not the matrix may not again become active 

 after passing into the so-called resting stage. This might explain 

 why what appears to be its great activity in later stages has been 

 overlooked. 



Probably at some period not long after birth the matrix is 

 exhausted and no longer produces new elements. The indications 

 are, that proceeding from the caudal end of the neural tube cere- 

 bralwards, the matrix is actively productive of new elements in 

 successive stages; that is, as one segment becomes exhausted and 

 goes into a state of rest the next contiguous segment becomes 

 active, and so on until the last and highest segment takes up the 

 work and gives birth to the cells that form the cerebral cortex. 

 The development of many of these elements goes on rapidly 

 during the migration to the cortex. His has estimated from care- 

 ful measurements of the thickness of the cortical layer at various 



■• Human embryology. Keibel and Mall, 1911-1912. Lippincott, Philadelphia, 

 U. S. A. 



