HEREDITY 



fertilized, and compare with this the changes 

 which occur in eggs not to Be fertilized. 



In each cell of the ordinary animal there 

 occurs a characteristic number of bodies called 

 chromosomes. We do not know that they are 

 any more important than other cell constitu' 

 ents, but we know their history better. These 

 are contained in the nucleus of the cell, and at 

 the time of nuclear division they are found at 

 the equator of the division spindle. For ex- 

 ample, in the egg of the mouse (Fig. 4,-4), the 

 nucleus is seen to be in the spindle stage, and 

 its chromosomes are gathered together at the 

 equator of the spindle. There each of them 

 regularly splits in two, and one derivative goes 

 to either end of the spindle, and so into one of 

 the daughter-nuclei. Thus each new nucleus 

 has, as a rule, the same chromosome composi- 

 tion as the nucleus from which it was derived. 



But the egg which is to be fertilized under- 

 goes two nuclear divisions in succession, in only 

 one of which do the chromosomes split (see 

 Fig. 4, A-D). In the other division the chromo- 

 somes separate into two groups without split- 

 ting, and each group goes into a different cell 



160 



