232 THE GROWTH OF THE BRAIN. 



received proper attention. Among these peculiarities is 

 a modification of portions of the cytoplasm for the 

 production of the neuron. This differentiation is spoken 

 of as polarisation. 



It is probable that all neuroblasts are polarised so 

 that the end of a mononeuric cell furthest from the pri- 

 mitive surface of the body is that from which the neuron 

 grows out, its direction being away from this surface. 

 These relations in the dineuric cells have not been 

 determined. During the development of a mammalian 

 nervous system, many foldings occur, but, provided the 

 cells remain fixed, their polarity is preserved. Some of 

 the neuroblasts, however, migrate, and the relative 

 position of other cells changes in ways unknown. 



Taking all these facts into consideration, it is not 

 remarkable that now and then a neuroblast accom- 

 plishes but incompletely the series of changes marked 

 out for it, and its neuron either develops imperfectly or 

 fails to take the proper direction. It is thus possible 

 that increase in size in a given encephalon may go on 

 almost normally, helped in some case, to be sure, by an 

 increase in the supporting tissues, but without the ac- 

 companying organisation, a phenomenon illustrated by 

 the high weight of the brains of some idiots. It is thus 

 seen that the weight and organisation of the encephalon 

 are not necessarily correlated, a fact which must modify 

 all inferences made directly from encephalic weight to 

 intelligence. 



Since the most important and at the same time the 

 most distant connections of a cell depend on the neuron, 

 it may be considered in more detail. This prolongation 

 of the cytoplasm forming the axis cylinder of the nerve 

 fibre is usually surrounded by a medullary sheath, a 

 secondary structure, the mass of which has already been 

 discussed, and found to be correlated with that of the 



