CELL DIVISION 



15 



after the spireme has been completely formed, it breaks up 

 into a number of lengths, which retain the curves and bends 

 that existed in the spireme, generally appearing in the form 

 of U's or V's (Fig. 3). These separate lengths of the spireme 

 are known as the chromosomes. The number of chromo- 

 somes appearing in a cell during the process of division is 

 different in different species of animals and plants, but is 

 constant in the cells of the same species. Thus, in man 

 32 chromosomes appear in a dividing cell ; in a mouse, 24 ; 

 in a newt, 24 ; in Artemia, a crustacean, 168 ; in Ascaris 

 megalocephala univalens, 2 ; and so on. 



While this is happening within the nucleus, the centro- 

 somes are travelling further and further apart, the threads 



FIG. 2. Spireme. Radia- 

 tions from centrosomes. 



FIG. 3. Spireme broken 

 into short lengths, the 

 chromosomes. Spindle 

 is being formed. 



between them which form the spindle lengthening out con- 

 siderably, so that eventually the centrosomes with their 

 radiations and the spindle, between them occupy a large 

 area of the cytoplasm (Fig. 3). At this stage the nuclear 

 membrane disappears, so that there is nothing dividing the 

 ground substance of the nucleus from that of the cytoplasm 

 (Fig. 4). They are in fact left to mingle freely. We now 

 have the two centrosomes, with the spindle between them, 

 and the chromosomes, free inside the cell, the nuclear 

 membrane no longer separating the chromosomes from the 

 spindle. At this stage each of the chromosomes attaches 

 itself to a fibre of the spindle, the two centrosomes generally 



