14 HISTOLOGY. 



four cells, each having one half the usual number of chromosomes, arise 

 from the one which first presented this peculiarity. With some modifi- 

 cations but without further division they may become the mature sexual 

 cells. The process of their formation is called maturation, and the two 

 peculiar and final mitoses through which every mature sex cell has passed 

 are called reduction divisions. In the process of fertilization two mature 

 sexual cells, a spermatozoon and ovum respectively, fuse, and the normal 

 number of chromosomes is restored. Thus each parent contributes an 

 equal number of chromosomes to the fertilized ovum and these have been 

 considered bearers of hereditary qualities. The reduction divisions will 

 be further considered under Testis and Ovary. 



An unusual form of mitosis is that in which the centrosome divides 

 into more than two parts and the cell correspondingly divides at once into 

 several. These pluri- or multi-polar mitoses are said to occur normally 

 in parts of certain higher plants; they have been induced by injecting 



a b c 



FIG. 16. MITOSES IN HUMAN CANCER CELLS. (From Wilson, after Galeotti.) 



a, Asymmetrical mitosis with unequal distribution of chromatin; b, tripolar mitosis; 



c, quadripolar mitosis. 



drugs into the skin of salamanders; and are sometimes found in human 

 cancer cells and in the rapidly growing connective tissue of scars. They 

 may lead to an unequal distribution of the chromatic material in the cells 

 which they produce. 



For further information regarding mitosis, and for definition of the 

 many terms frequently employed but not mentioned in this account, the 

 student is referred to Prof. E. B. Wilson's book entitled "The Cell." 



AMITOSIS. 



Amitosis or direct cell division takes place without spindle formation 

 or the rearrangement of nuclear material. The nucleolus, nucleus, and 

 cell body successively divide by fission, or by elongation and constriction, 

 into two parts. The role of the centrosome has not been determined. 

 This form of division is rare and its significance unknown. The suggestion 

 that it is more primitive than mitosis lacks support. Generally it is 

 regarded as a sign of cell degeneration, since it occurs in old cells leuco- 



