150 



CAUSES OF VARIATION 



also be similar to that of the mother cell from which they are 

 descended. 



For all cell division within the same tissue this latter is true ; 

 that is to say, in the growth of liver tissue, bone tissue, or any 

 other specific structure the cells appear to be identical and 

 their resulting growths alike. But it must not be forgotten that 

 these different tissues all arose originally from a single cell ; in 

 other words, that some cell divisions are attended by differenti- 

 ation. When ? How ? Here lies the chief mystery of variation. 

 The mechanism of cell division would seem to be specially 

 designed to prevent deviation and to insure absolute transmission 



Fig. 22. Pathological mitosis in epidermal cells of salamander caused by poisons 



A, asymmetrical mitosis after treatment with 0.05% antipyrin solution ; B, tripolar mitosis 

 after treatment with 0.5% potassic iodid solution. After Wilson, from Galeotti 



from mother cell to daughter cell. It does not account for or 

 indeed appear to admit of differentiation of tissues or variation 

 in growth. But differentiation does take place and variation is 

 a fact to be in some way explained. 



Irregularities in cell division. Not all cell division, it is true, 

 proceeds with the regularity and perfection of plan indicated in 

 the description, which is the common method in higher animals 

 and plants and may therefore be regarded as fairly typical. The 

 known abnormal cases are of several distinct kinds : 



I. Asymmetrical mitosis, in which the 'chromosomes are not 

 equally distributed to the daughter cells, most of them massing 



