CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



together so that each seems doubled. Soon afterward the fibrils of the achro- 

 matic spindle begin to contract, and thus separate the halves of the chromosomes 

 in such a way that one-half of each is turned toward one pole, and the other 

 half toward the other. As this continues, the two groups, which are equal in 



central _.. 

 particle, 



Polar r*u&*0en, 

 (Cyt*t*r) 

 afav&tlon, sphere* 



dear area 

 of nucleus- 



cen&vtZ 

 jUxrUdo 



FIG. 17. Monaster Stage of Karyokinesis. 



(Rabl.) 



size, draw away from each other and from the equator, each group being 

 formed of daughter chromosomes. 



Anaphase. The two groups (daughter chromosomes) now gradually ap- 

 proach their respective poles, or centrosomes, and the equator becomes free. 

 On reaching the pole, each group gathers in a form which is similar in arrange- 

 ment to the monaster and is known as the diaster. During this time the cell 

 body becomes slightly constricted by a circular groove at its equatorial plane. 



Telophase. Soon afterward the fibrils of the chromatic spindle which 

 connect the two groups begin to grow dim and finally disappear. The daugh- 



FIG. 18. Stages of Karyokinesis. (Rabl.) A. Commencing separation of the split chromo- 

 somes. B. The separation further advanced. C. The separated chromosomes passing along 

 the fibers of the achromatic spindle. 



ter chromosomes assume the form of threads twisted in a coil and develop 

 each a nuclear membrane and a nucleolus, forming a daughter nucleus. The 

 nuclei enlarge and the nuclear threads assume the appearance of the resting 

 state of the nucleus. Meanwhile, the constriction about the body of the cell 

 has become deeper and deeper until the protoplasm is divided into two equal 

 parts, or daughter cells, each with its daughter nucleus, and the process of 

 karyokinesis is completed. 



