458 



ELEMENTARY BIOLOGY 



germ cells, however, the chromosomes divide in a different 

 way. In a cell that is to form an egg cell half of the chromo- 

 somes separate out ; the remaining chromosomes then split 



4 5' 67 



FIG. 244. The nucleus in cell division 



/, Diagram of a cell with the chromatin in a tangle, or network ; before cell division the 

 chromatin assumes the form of a continuous thread. The thread breaks up into a 

 definite number of pieces, or chromosomes, 2 ; the number of these is constant for any 

 given species of plant or animal. The chromosomes arrange themselves in a central 

 ring, j ; the membrane inclosing the nucleus disappears ; fine threads appear to connect 

 the chromosomes with tiny bodies at opposite ends of the cell. Each chromosome 

 splits in two lengthwise, 4. The members of each pair move from each other to opposite 

 ends, 5. The half-chromosomes form two new tangles, 6, and gradually lose their definition. 

 The new masses of chromatin become the nuclei of two new cells, 7 



lengthwise (see Fig. 245). As a result of these two divisions, 

 which usually follow each other in rapid succession, 1 the egg 

 cell contains one fourth of the chromatin present in the cell 

 from which it was formed, and only one half of the usual 

 number of chromosomes. The division which separates the 



1 It is believed that in some cases the splitting of the chromosomes takes 

 place before the separation into the reduced numbers. 



