52 ESSENTIALS OF ZOOLOGY 



generated. The chromosomes lying in the equatorial plane of the 

 spindle now undergo an equatorial division, each being divided into 

 two corresponding halves. The actual division pf the chromosomes 

 here constitutes the metaphase stage. Following this stage each of 

 the chromosomes resulting from the division migrates along the 

 spindle fibers toward its respective centriole, or pole. The stage dur- 

 ing which occurs the migration of the chromosomes from the equa- 

 torial position to the poles is called the anaphase. 



As the chromosomes near the poles of the spindle they crowd very 

 close to each other. At this time a constriction of the cytoplasm be- 

 gins in the plane of the equatorial plate. This is the beginning of 

 the telophase stage. The cytoplasm perfects its constriction and 

 divides into two parts. A nuclear membrane forms to enclose each 

 chromosome group, and immediately the chromosomes begin to sepa- 

 rate from the group, although certain ones still clump together. 

 The chromosomes progressively lose their identity and their stain- 

 ing qualities. The nucleus resumes its granular appearance of the 

 resting cell. One or more nucleoli soon become evident. The for- 

 mation of the new nuclear membrane excludes the centrosome, so 

 it takes its normal position just outside the nuclear membrane in 

 the cytoplasm. At about this time the centriole divides into two. 

 These two new cells resulting from the division are spoken of as 

 daughter cells. These cells have each received the same quantity and 

 quality of chromatin material. 



Following the organization of these daughter cells, which are in 

 the resting stage as far as division is concerned, growth is rapid 

 until they reach their typical limit of size. For most average cells 

 under optimum conditions, it is stated that this requires less than 

 two hours. Then after a further period of from one to twelve 

 hours, another mitotic division may take place. The universality 

 of this process in all types of organisms, both plant and animal, 

 and the regularity of the occurrence of the phases of the process 

 suggest that it is of vital significance. The great precision with 

 which the chromatin is divided between the two cells seems to 

 indicate that this is a most significant step. Chromatin is recog- 

 nized as the material which makes possible the inheritance of quali- 

 ties from cell to cell and, in case of sex cells, from generation to 

 generation. The purpose of the splitting of the chromosomes in 

 the metaphase stage seems to be to provide each daughter cell with 



