MITOTIC DIVISION OF CELLS 



73 



The shape of the chromosomes varies much in different cases ; 

 they may be more or less spherical, but they are frequently short, 

 rod-like bodies, often shaped like a V (Fig. 32, A). Each one is 

 composed, like the spireme thread from which they are derived, 

 of an aggregation of chromatin granules, held together by a 

 linin basis. The chromatin granules are sometimes arranged 

 like the beads on a necklace (Figs. 32, B ; 77), and are known as 

 chromomeres. The number of the chromosomes also varies 

 greatly, from as low as two in a variety of the horse-worm 

 (Ascaris) to as many as one hundred and sixty-eight in the shrimp 

 Artemia. In cases where the chromosomes are very small each one 

 may perhaps be equivalent to only a single chromomere. 



Either before or after taking up its position in the equatorial 



FIG. ^.-^Sperm-mother- cells of a Salamander, during Mitosis. In A the 

 chromosomes are shown ; in B the spireme thread is split lengthwise, 

 and also shows very clearly the chromomeres of which it is made 

 up. (Prom Weismann's " Evolution Theory," after Hermann and 

 Driiner.) 



c, dividing centrosome ; chr, chromosomes ; Jd, chromomeres ; zk, cytoplasm. 



plate, each chromosome splits longitudinally into two parts 

 (Fig. 31, D), in fact the splitting can sometimes be observed in 

 the spireme thread even before it breaks up transversely into 

 chromosomes (Fig. 32, B). The result of this splitting is that 

 the number of chromosomes is doubled ; but the daughter chromo- 

 somes very soon separate into two equal groups, one of which 

 moves towards each centrosome (Fig. 31, E). Each group contains 

 one of the two halves of each parent chromosome. 



Having migrated to opposite poles of the spindle the two 

 groups of daughter chromosomes there form the foundations of two 

 new nuclei (Fig. 31, F). The chromosomes break up into granules 

 again ; a new nuclear membrane is formed, whereby a portion of 

 the general cytoplasm is separated off to form the linin network 

 and ground-substance of the nucleus ; the asters and nuclear 



