38 POPULAR SCIENCE MONTHLY. 



stituent becomes diminished in quantity, as if it had been consumed 

 during the prolonged use of the cell (Hodge, Mann, Lugaro). It is 

 interesting also to note that in hibernating animals in the winter season, 

 when their functional activity is reduced to a minimum, the chromo- 

 phile in the plasm of the nerve cells is much smaller in amount than 

 when the animal is leading an active life in the spring and summer 

 (G. Levi). 



When a nerve cell has attained its normal size it does not seem to be 

 capable of reproducing new cells in its substance by a process of karyo- 

 kinesis, such as takes place when young cells arise in the egg and in the 

 tissues generally. It would appear that nerve cells are so highly special- 

 ized in their association with the evolution of nerve energy, that they 

 have ceased to have the power of reproducing their kind, and the meta- 

 bolic changes, both in cell plasm and nucleus, are needed to enable them 

 to discharge their very peculiar function. Hence it follows that 

 when a portion of the brain or other nerve-center is destroyed, the 

 injury is not repaired by the production of fresh specimens of their 

 characteristic cells, as would be the case in injuries to bones and tendons. 



In our endeavors to differentiate the functions of the nucleus from 

 that of the cell plasm, we should not regard the former as concerned 

 only in the production of young cells, and the latter as the exclusive 

 agent in growth, nutrition and, where gland cells are concerned, in the 

 formation of their characteristic products. As regards cell reproduc- 

 tion also, though the process of division begins in the nucleus in its 

 chromosome constituents, the achromatic figure in the cell plasm un- 

 doubtedly plays a part, and the cell plasm itself ultimately undergoes 

 cleavage. 



A few years ago the tendency amongst biologists was to ignore or 

 attach but little importance to the physiological use of the nucleus in 

 the nucleated cell, and to regard the protoplasm as the essential and 

 active constituent of living matter; so much so, indeed, was this the case 

 that independent organisms regarded as distinct species were described 

 a6 consisting of protoplasm destitute of a nucleus; also, that scraps of 

 protoplasm separated from larger nucleated masses could, when isolated, 

 exhibit vital phenomena. There is reason to believe that a fragment of 

 protoplasm, when isolated from the nucleus of a cell, though retaining 

 its contractility, and capable of nourishing itself for a short time, cannot 

 increase in amount, act as a secreting structure, or reproduce its kind: 

 it soon loses its activity, withers and dies. In order that these qualities 

 of living matter should be retained, a nucleus is by most observers re- 

 garded as necessary (Nussbaum, Gruber, Haberlandt, Korschelt), and 

 for the complete manifestation of vital activity both nucleus and cell 

 plasm are required. 



