THE TYPICAL CELL 



11 



Indirect Cell Division (karyokinesisi* mitosis\}. This is the 

 usual form of cell division. It consists of a series of changes 

 which chiefly concern the centrosome and nucleus, and which are 

 followed by constriction of the cytoplasm and its final separation 

 into two daughter cells, each of which contains a daughter nucleus 

 and centrosome. 



That condition of any cell during which it is not undergoing 

 mitotic change is described as its resting stage. In this condition 

 the nucleus is surrounded by a distinct nuclear membrane, within 

 which its chromatic fibrils are irregularly disposed. Xear the 

 nucleus is the centrosome, a minute chromatic point which is 

 surrounded by a lighter radiate area, the so-called attraction 

 sphere. It is about the centrosome that the earliest mitotic 

 changes appear. In fact, division of the centrosome itself is fre- 



FIG. 11. DIRECT CELL DIVISION, SHOWING SUCCESSIVE STAGES IN ORDER FROM LEFT 



TO RIGHT. 



(After Nemiloff.) Diagrammatic. 



quently the last occurrence in the formation of a daughter nucleus, 

 a " precocious preparation " for future division. 



The mitotic changes which accompany indirect cell division 

 may be conveniently considered under four heads: prophase, 

 metaphase, anaphase, and telophase. 



PropJiase. The preparatory changes which indicate the ap- 

 proach of cell division begin with the early cleavage of the centro- 

 some, which often even precedes the resting stage. With begin- 

 ning mitotic activity the daughter centrosomes move apart, each 

 surrounded by its clear radiate, or "astral" attraction sphere. 



* Meaning nuclear change. 



f A thread, referring to the appearance of the nucleus. 



