20 HEREDITY 



centrosome, and which apparently plays an important 

 part in initiating the phenomena of cell- division. 



Now it is beyond reasonable question the nucleus 

 that is the essential part of the cell. The rest of the 

 cell body has probably no more than a nutritive 

 function. The changes in the nucleus which pre- 

 cede, accompany, and succeed the act of reproduc- 

 tion must later be described. But we may be 

 permitted here to indulge in certain abstract con- 

 siderations as to the intimate structure of the 

 nucleus. We have already noticed the existence 

 of a special portion of the nucleus which is readily 

 stained by suitable dyes, and which is therefore 

 called the chromatin. Every division of this and 

 every other cell of any of the higher animals and 

 plants is initiated by the breaking up of the nuclear 

 chromatin into a number of separate rod-like bodies 

 known as chromosomes. When these chromosomes 

 are examined by the microscope, after treatment 

 with suitable dyes, they are found to contain a 

 number of smaller bodies, each of which must be 

 regarded as a complex structure, " possessed of a 

 historic architecture that has been elaborated slowly 

 through the multitudinous series of generations that 

 stretch backwards in time from every living indi- 

 vidual." These minute bodies, or microsomes, can- 

 not further be resolved by the microscope, but 

 Weismann assumes that each of them in its turn 

 consists of a number of still smaller bodies, which 

 he calls deterriiinants. He has his own names for 

 the larger units, but it would be of little profit to 

 quote them. The concept of determinants, however, 

 is worthy of note, for Weismann argues that these 



