114 E. LINDON MELLUS 



the cortex becomes filled with new growths and the nuclei can 

 no longer pursue the purely radial direction. 



I have not been able to find any satisfactory explanation of the 

 formation of the 'Ubergangschicht.' With the exception of 

 the claustrum, for which it appears to form the anlage, it is only 

 a transitory formation, although distinct traces of it persist at 

 birth and perhaps longer. Preparations from the occipital lobe 

 in the eight months brain have been described. In those prepara- 

 tions (fig. 1) the ' Ubergangschicht' is still present as a layer of 

 closely packed nuclei almost completely surrounding the ventricle 

 but separated from the matrix by a pale layer containing only 

 scattered nuclei. Several streams of nuclei lead from the matrix 

 to the 'Ubergangschicht', but instead of taking a radial direction 

 they run for some distance along the wall of the ventricle and par- 

 allel to it, and then leaving the ventricular wall join with other 

 streams to form this closely packed layer. From the external 

 border of the 'Ubergangschicht' its component parts appear to 

 be migrating in a radial direction towards the cortex. WTiy nuclei 

 destined to take part in the formation of the cerebral cortex- 

 should collect in a distinctly formed band which apparently 

 persists through many months of intra-uterine life before pro- 

 ceeding on their further journey is a question even harder to 

 answer than that of the propulsive force which carries them to 

 their appointed destination. His states that the formation of 

 the cortex goes on not only during the entire third but also the 

 greater part of the fourth month. I do not find any statement of 

 his that the process is complete at that time, but it is generally 

 understood that such was his belief. All authorities seem to 

 agree in fixing the time of the completion of the migration of the 

 neuroblasts somewhere about the end of the third or fourth month 

 of foetal life. According to Jackson^ the volume of the central 

 nervous system at the third month is about 7 cc. and at birth 376 

 cc. In the interval between these periods the volume of the 

 central nervous system has increased more than fifty-three fold 

 and the cerebral cortex has nearly doubled in thickness not- 



' Prenatal growth of the human body. C. M. Jackson. Am. Jour. Anat., vol. 



9, lyoit. 



