THE CELL AND PROTOPLASM. 113 



cells, which introduces into the offspring chro- 

 matic material from two parents. But if the two 

 reproductive cells should unite at once the num- 

 ber of chromosomes would be doubled in each gen- 

 eration, and hence be constantly increasing. To 

 prevent this the polar cells are cast out, which re- 

 duces the amount of chromatic material. The 

 union of the two pronucleii is plainly to produce 

 a nucleus which shall contain chromosomes, and 

 hence hereditary traits from each parent and the 

 subsequent splitting of these chromosomes and 

 the separation of the two halves into daughter nu- 

 cleii insures that all the nucleii, and hence all cells 

 of the adult, shall possess hereditary traits derived 

 from both parents. Thus it comes that, even in 

 the adult, every body cell is made up of chromo- 

 somes from each parent, and may hence inherit 

 characters from each. 



The cell of an animal thus consists of three 

 somewhat distinct but active parts the cell sub- 

 stance, the chromosomes, and the centrosome. Of 

 these the cell substance appears to be handed 

 down from the mother; the centrosome comes, at 

 least in some cases, from the father, and the chro- 

 mosomes from both parents. It is not yet certain, 

 however, whether the centrosome is a constant 

 part of the cell. In some cells it cannot yet be 

 found, and there are some reasons for believing 

 that it may be formed out of other parts of the 

 cell. The nucleus is always a direct descendant 

 from the nucleus of pre-existing cells, so that there 

 is an absolute continuity of descent between the 

 nucleii of the cells of an individual and those of 

 its antecedents back for numberless generations. 

 It is not certain that there is any such continuity 

 of descent in the case of the centrosomes ; for, 



