74 



PRINCIPLES OF ANIMAL BIOLOGY 



view the chromosomes form a figure which may resemble a star (Fig. 39). 

 Viewed from the side the chromosomes appear to he in a plane (Fig. 40). 

 In this position the chromatic figure (the chromosomes) may be spoken of 

 as an equatorial plate. The chromatic figure is now at the height of its 

 affinity for stains, and from this point on its ability to take up stains 

 decreases. 



All of the stages from the beginning of the process of cell div^ision to 

 this point are included in the prophases. 



The Metaphase. — In this phase each chromosome splits lengthwise 

 into two equal portions. It is held that in general each chromosome is 

 thus approximate!}^ halved in a quantitative fashion (Fig. 41). And 

 since the chromomercs, which are supposed to contain the elements 

 that control cell activities and heredity, are believed to be arranged in a 



Fig. 41. — Metaphase of cell division. The 

 chromosomes have split lengthwise. 



Fig. 42. — Early anaphase of cell 

 division. The chromosomes .'ire with- 

 drawing toward the poles. 



series along the chromosomes an equal qualitative division of each chro- 

 mosome is thought to occur at the time of splitting. The metaphase is a 

 brief phase covering no more than the period of splitting of the chromo- 

 somes. In some species of animals the splitting of the chromosomes occurs 

 before their arrangement in the equatorial plate. 



The Anaphases. — -Almost immediately after the chromosomes have 

 divided the two new chromosomes formed from each old one begin to 

 draw apart, one going to one pole of the spindle and the other going to 

 the opposite pole. This movement begins almost simultaneously for all 

 the daughter chromosomes, and as a result they pass to the poles as two 

 groups. In an examination of preparations which, ?how man}' mitotic 

 figures it is possible to find many anaphase stages from those in which the 

 chromosome groups are just drawing apart, as in Fig. 42, to a condition 

 in which the chromosomes are more or less denselj^ massed about the ends 

 of the spindle, as in Fig. 43. Thus the anaphases are concerned with the 



