MITOSIS. 



of the chromatic material. The achromatic structures were successively 

 the centrosome surrounded by archoplasm; the diplosome in a centro- 

 sphere; two centrosomes connected by a spindle and each surrounded by 

 polar radiations; the division of this amphiaster, as it is called, into two cen- 

 trospheres each with its.centrosome; and, finally, the reduction of the cen- 

 trosphere to archoplasm. Each new cell ordinarily receives half of the 

 protoplasm, spindle, centrosome and chromatic material of its parent, 

 and becomes a cell of the same sort. 



The process of mitosis requires probably about half an hour, but 

 the time is variable and it may last several hours. In the blood cells of 

 amphibia it is said to take two hours and a half. Mitoses will be found 



Close monospireme Loose monospireme 



(viewed from (viewed from above 



the side). i.e., from the pole). 

 Polar side. 



Monaster (viewed from the side). 



Monaster (viewed 

 from above). 



Dyaster. 



Beginning, Completed, 



Division of the protoplasm (Dispiremes). 



FIG. 15. MITOTIC FIGURES FROM THE EPITHELIUM OF THE ORAL CAVITY OF TRITON 

 ALPESTRIS. X 560. 



in all well preserved, rapidly developing tissues. They are abundant 

 in embryos; and if numerous in tumors they furnish evidence of rapid 

 growth and malignancy. After death, if the tissues are not hardened by 

 cold or reagents, it is thought that mitoses may go on to completion, as 

 they are absent from specimens which are not properly preserved. 



Varieties of mitosis. In connection with the formation of sexual 

 cells (the ova and spermatozoa) there occur two successive mitotic divisions 

 of a unique sort. A cell which had itself been formed by ordinary mitosis, 

 in preparing for division converts its chromatic material' into one half of 

 the usual number of chromosomes. It divides into two cells, each with the 

 reduced number, and these divide once more in the same way. Thus 



