WHAT MAKES CELLS DIVIDE? 167 



The multitudes of cells which form our bodies have 

 been evolved from a single pair of cells. The maternal 

 ovum is a single cell, and always remains as such until 

 it is fertilised by the paternal spermatozoon, which in 

 its turn is also a single cell. The conjugation of the two 

 at once causes cell-division to take place within the egg. 

 Multiplication occurs, and where there was one cell 

 there are now two; and each daughter cell divides and 

 divides until generation after generation of new cells 

 are produced, and this cell-reproduction ultimately 

 leads to the formation of the new individual. The 

 basis of the formation of new beings is the reproduction 

 of cells by their division in response to the conjugation 

 of an original pair of cells. We had therefore to ask 

 ourselves why this conjugation should cause cell- 

 division; but unfortunately the answer was unknown. 



Throughout our lives, although we cannot actually 

 feel it, the cells in our bodies are continually repro- 

 ducing themselves by division by mitosis, and individual 

 cell-death is also constantly taking place. It is true 

 that some cells, such as some cells of the nervous 

 system, prpbably live throughout the length of our 

 lives, but myriads of other cells are constantly dividing 

 to help to build up the tissues. "Birth" and death 

 are continually going on among the individual units 

 of ourselves. When a tissue is sectioned and examined 

 microscopically it will frequently be seen that some of 

 the cells are in the act of division by mitosis; but when 

 we asked what makes the division occur, and what 

 makes cells multiply to build up the tissues, we could 

 only say once more that the reason was quite unknown. 



