224 GENERAL ZOOLOGY 



structures involved in mitosis are the spindle, the centrosomes, 

 centrioles, and astral radiations, all of which together with the 

 chromosomes may be involved in a final explanation of the 

 mechanics of the process. Since tissues or cells ordinarily used 

 for the study of mitosis consist of material that has undergone 

 special treatment with reagents and stains prior to mounting on 

 glass slides, it has been claimed that some of the structures seen 

 in such preparations are possibly artifacts caused by coagulation 

 or precipitation by reagents. These objections have been 

 answered by demonstrating that chromosomes as well as a 

 spindle and centrosome areas can actually be seen in living cells 

 under proper conditions. When living cells undergoing mitosis 

 are dissected, the spindle appears as a hyaline material to which 

 the chromosomes adhere as jellylike masses. The spindle is not 

 an artifact though the fibers may be. The centrosome areas 

 are more viscous than the surrounding cytoplasm of dividing 

 cells, showing that they too are actual structures in the cell. 



Division of somatic cells occurs throughout the developmental 

 stage of Metazoa. In the adult animal, cell division is confined 

 to certain cells whose function is to replace loss. Thus the outer 

 layer of the human epidermis is being continually rubbed off and 

 replaced by cells pushing up from the deeper layers of the 

 epidermis, in the lowest of which, cell reproduction takes place 

 throughout life. The red blood corpuscles of mammals also have 

 a short life in the circulation and are replaced by cells of the bone 

 marrow. Mitotic divisions may occur sporadically among other 

 tissue cells. It is said that the nerve cells of adult mammals 

 never undergo cell division. The number of chromosomes 

 appearing in somatic mitosis is a constant one for the species, 

 although there may be a difference, also constant, between the 

 sexes of the same species. This point will be discussed in con- 

 nection with the germ cells. 



Amitosis. — Though simple and more direct than the mitotic 

 method of cell division, amitosis is a much rarer form of cell 

 division. There has been much controversy about its occurrence 

 and nature, which need not be entered into here. Amitosis has 

 been described in various tissues under normal conditions, and in 

 degenerating and pathological tissues, from which it seems clear 

 that it is not the ordinary method of cell division. Figures of cells 

 dividing amitotically show each constituent of the cell, beginning 



