FROM SIMPLE CELL TO COMPLEX ANIMAL 35 



follow closely the figures n and 12, in order to get the steps 

 described. 



When the full-grown primary oocyte and spermatocyte are 

 ready to divide at the beginning of the third stage, the chromo- 

 somes in a given nucleus unite and fuse in pairs. Thus when 

 division begins there is only one-half the usual number, but each 

 chromosome is of double value (bivalent). As the primary cells 

 divide into the secondary oocytes and spermatocytes, the bival- 

 ent chromosomes behave as single chromosomes in ordinary 

 nuclear division and split, and one-half of each bivalent chro- 

 mosome passes to each daughter nucleus. In this way each 

 secondary oocyte or spermatocyte contains one-half the 

 usual number of single (or univalenf) chromosomes. By this 

 device of first uniting and then separating whole chromosomes 

 instead of splitting the single chromosomes into halves, as 

 occurs in ordinary nuclear divisions, an actual reduction of the 

 number of chromosomes is secured. Furthermore it may be, 

 while these chromosomes are fused, there is an actual interchange 

 and modification of the substance of each. 



The secondary cells divide once more in forming the mature 

 eggs and sperm. In this last division the chromosomes simply 

 split as they do in ordinary indirect nuclear divisions. 



In all these steps in the division of nuclear material the his- 

 tory of ova and sperm is essentially the same. There are, 

 however, very striking differences in the way the cytoplasm, and 

 the cell as a whole, behaves. In the history of the sperm cells 

 the protoplasm divides equally in both divisions, and thus each 

 primary spermatocyte produces four small spermatids. Each 

 spermatid gradually changes, without further division, into a 

 spermatozoon. 



In the divisions of the primary oocyte, on the contrary, one 

 of the daughter nuclei enters a kind of bud in the wall of the cell 

 and carries very little cytoplasm with it. It is clearly an abor- 

 tive cell, and is called a polar body. The second nuclear division 

 results in another abortive egg-cell or polar body. The first 

 polar body sometimes divides into two. All the polar bodies 

 finally disintegrate. Thus, since one of the nuclei has almost all 

 the cytoplasm and the others almost none at all, there is only one 



