1084 



vestigators. As far as the animals investigated bj me are concerned, 

 an increase in the number of neurones by means of mitotic division 

 of nerve cells seems thus to be concluded during the first month of 

 post-embryonic life. Donaldson's') statement: "Moreover, in the case 

 of the albino subjected to modifying conditions after 30 days of age, 

 tJie mimher of neurones is already complete at this age, so that the 

 changes induced are again merehj of size""), uidess some neurones 

 should have been destroyed," is an assertion that 1 cannot agree 

 with, as far as my material is concerned, and I am inclined to 

 think that it does not describe the conditions in any animal. If 

 one gives a strict definition of a neurone as being a nerve-cell 

 with its processes, one of which is an axon and the others dendrites, 

 and one adds to this the generally accepted condition, which by 

 means of the evidence put forward about it, has almost become 

 a certainty, namely that one cell in the ventral horn does 

 not send more than one axon out into the ventral root and that 

 the axons do not show any T-division on tiieir way through this 

 root, the considerable post-embryonic increase in the numl)er of 

 axons in this i-egion, which has been shown above to be an actual 

 fact, is a proof of tiie real existence of an increase in the number 

 of neurones during a considerably longer period of development than 

 the one given by Donaldson. 



The Wistar school (Donaldson and others) have, as has been stated 

 above, with their splendid statistical and experimental investi- 

 gations found, by means of the methods they have used (staining 

 of medullary sheaths), that post-embryonic growth in the nerve roots 

 is principally merely an advancing myelinisation. The most important 

 of all the changes that take place during this process, namely the 

 post-embryonic growth in the number of axons, has quite escaped 

 theii- notice. There was therefore no need to look foi' an increase 

 in the number of neurones going on for a longer lime postembiyonally 

 than the time during which the mitosis in the cenli-al nervous system 

 showed clearly that an increase of this kind really existed. But is 

 mitosis the only way in which an increase or a new formation of 

 the cells in the central nervous system can take place? 



Scattered statements in the literature exist to the effect that a new 

 formation of nerve-cells may also take i)lace by means of 



') Donaldson, H. H., Hatai, S. and King, H. D. Postnatal growth of the Brain 

 under several experimental conditions. Studies on the albino rat. Journ. Nerv. and 

 Mental Disease. Vol. 42, 1915. 



') The italics are mine. 



