48 BIOLOGY 



terial from both of the cells which entered into the combination. 

 Now inasmuch as, after this combination, the offspring which 

 arises from the cell thus formed by the union of the two parental 

 cells inherits characteristics from both parents, and inasmuch 

 as the only part of the original sex cells which enters into the 

 union is the chromatin, it follows that the chromatin material 

 itself is the bearer of heredity, and that in these little chromatin 

 threads, minute as they are, there must be a complexity suffi- 

 cient to contain the features of inheritance that are handed on 

 from generation to generation. 



These facts give at least some idea of the separate properties 

 of cell substance and nucleus. The cell substance by itself has 

 the functions of irritability and conductility ; but not of assimi- 

 lation, growth, or reproduction. These latter functions can be 

 carried on only when a nucleus is present. 



WHAT IS MEANT BY PROTOPLASM 



It has become evident by this time that the original con- 

 ception of protoplasm has quite disappeared. Indeed, if we 

 ask to-day just what is meant by protoplasm, the question 

 becomes very difficult to answer. We can no longer look upon 

 it as simply the jelly-like substance within the cell in which the 

 nucleus lies embedded, for it is evident that although this 

 substance has the properties of irritability and conductility, it 

 does not have the properties of assimilation and growth. If we 

 wish still to call protoplasm the physical basis of life, we must 

 extend the term to include the nucleus as well as the sub- 

 stance outside of the nucleus, since without the nucleus, 

 protoplasm is unable to carry on life processes. If, however, 

 we include, in this term protoplasm, the centrosome, and the 

 nucleus with its chromosomes, it becomes evident that proto- 

 plasm has quite lost its original significance. It is no longer 

 the homogeneous substance, and can no longer be looked upon 

 as a chemical compound, but is on the other hand a mechanism 

 with a number of distinct, though closely correlated parts. 



