372 



THE DEVELOPMENT OF CERTAIN FEATURES OF THE CEREBELLUM. 



sixth month of intrauterine life to the third month of postnatal life. This result 

 corresponds very closely to that of Berliner, as given above. We have, therefore, 

 in the thickness of the molecular layer an index of the stage of development of the 

 cerebellum, an index which will be applied in the consideration of the microcephalies. 



The greater thickness of the molecular layer of the flocculus from the sixth 

 month of antenatal to the third month of postnatal life is additional evidence in 

 favor of Edinger's view that the flocculus, as well as the vermis, is phylogenetically 

 old. It is interesting to note that between birth and two years of age the percentage 

 increase in thickness of this layer of the flocculus and vermis is the same, and just 

 50 per cent of that in the hemispheres for the same period (see table D). 



As in the development of the Purkinje cells, the adult condition of the molecular 

 layer of the cerebellar cortex is practically reached in the second year, the growth 

 thereafter being only 1 to 2 per cent. 



TABLE E. Average thickness of molecular layer. 



The brains reported in table E were all without pathological change except 

 for their small size. A comparison of this table with table C shows that here, in 

 some instances, we have a very great deviation from the normal. In some of the 

 microcephalic specimens, as in the 1-year-old child with spina bifida, this deviation 

 may be interpreted as an arrest of development, inasmuch as in this brain four layers 

 of cells in the outer nuclear layer in the vermis and five to six in the hemisphere still 

 persist. This is a condition which, according to Biach, is normal in a child of 6 

 weeks. The thickness of the molecular layer of vermis and hemisphere, it will be 

 noted, is about midway between the normal for a child of 16 days and that of a 

 child of 3 months. We have, therefore, a persistence of two conditions the num- 

 ber of cellular layers in the outer nuclear layer and the thickness of the molecular 

 layer. Each may be considered as an index of development and both speak for an 

 arrest of development in this brain at about the sixth week of postnatal life. This 

 same arrest of development was observed in another case of microcephalism not 

 included in the table. 



In the other instances of microcephalism there is no indication of the persistence 

 of a condition normal at an earlier period of development. The outer nuclear layer 

 is entirely absent. The thickness of the molecular layer in some parts, as in the 

 flocculus of the 22-months-old child, is very much below the normal; in other parts, 

 as in the vermis and hemispheres of the adult, it is very much above the normal. 

 In these cases the deviation from normal must be attributed, not to arrest in 

 development, but to an atypical development and an under development as a whole. 



