io6 The Science of Life. 



the tide of research and the spray of controversy. The 

 little word cell, one of the least fortunate of scientific 

 terms, which once seemed to express a simple fact, has 

 now to cover a perplexingly intricate microcosm. We 

 cannot do more here than make an outline-map of the 

 territory. 



The cell is a structural unit or unit-area, a unified 

 living 1 corpuscle of complex substances. Within this unit 

 it is convenient to distinguish certain parts, (a) The 

 general cell -substance or cytoplasm has a complex 

 structure, and consists in part of living matter (proto- 

 plasm), in part of obviously lifeless inclusions (meta- 

 plasm). (b) Within the cytoplasm is the nucleus, again 

 a little world, with readily stainable chromatin sub- 

 stances, and illusive unstainable achromatin. (c) In at 

 least a large number of animal cells, especially when 

 they are about to divide, two small bodies known as 

 centrosomes are demonstrable, each surrounded by a 

 sort of halo of delicate rays the astrosphere. (d) In 

 most plant cells there is a very definite cell-wall round 

 each unit, and this is often traversed by distinct inter- 

 cellular bridges of protoplasm which link cell to cell. 

 In the animal cell the wall is usually much less definite, 

 but the intercellular bridges are very common. 



Only a few cells grow to a relatively large size, such 



as the giant Gregarine, parasitic in the Lobster, which 



may measure three quarters of an inch in 



Cell-division. .*.. , ,, 



length. Such cases are rare, and most cells 

 remain microscopic. The process of cell-division is thus 

 of fundamental interest, since it is the general mode of 

 organic growth. By absorbing food and water a cell 

 increases in size, and thus contributes to the increased 

 size of the organism, but the cell's increase has usually 

 narrow limits, therefore the growth of the organism 

 necessitates cell-division. The brain of man and higher 

 animals is a noteworthy exception, inasmuch as the 

 nerve-cells do not divide after birth (except in very rare 

 cases of injury). 



There are two chief modes of cell-division, technically 

 known as direct and indirect, or amitotic and mitotic. 

 The former is much the less frequent, and much the less 



