202 SCIENCE FROM AN EASY CHAIR 



allied species. The salamander has twenty-four of 

 them ; some worms have only two, some insects thirty- 

 six, some plants eight, others twelve, and so on. When 

 the V-shaped pieces have thus taken up their position 

 in the dividing cell, each splits longitudinally, so as to 

 form two V-shaped pieces lying one over the other (Fig. 

 42d). Then the halves separate and travel away from 

 each other. In this way two circlets, each made up by 

 the correct number of V-shaped pieces, come into place 

 at opposite sides of the cell (Fig. 42*?). After this the 

 protoplasm becomes nipped in between the two circlets 

 so as to separate the cell into two halves, each with its 

 circlet of exactly the correct number of V-shaped pieces 

 of " chromatin " formed by the splitting of those of the 

 parent cell (Fig. 42 /). It is in this way that the nuclei 

 of the new cells are accurately provided with not merely 

 half of the nuclear chromatin of the mother cell, but 

 with half taken from all parts of it, owing to the thread- 

 like form of the chromatin and the longitudinal splitting 

 of the thread. 



Fertilisation of the egg-cell by the sperm-cell con- 

 sists essentially in the junction or fusion of the nuclear 

 chromatin threads of the egg-cell with the nuclear 

 chromatin threads of a single sperm-cell or spermatozoon, 

 which sinks into the egg-cell and fuses with it. This 

 has been witnessed and studied with the greatest care. 

 The leading fact of interest is that the egg-cell and the 

 sperm-cell have only half the number of V-shaped 

 nuclear pieces which the ordinary cells of the same 

 animal or plant possess. Thus a salamander's ripe egg 

 and ripe spermatozoid have each only twelve V-shaped 

 pieces not twenty-four. This is brought about by the 

 parent cells, which divide to form the egg-cell of the 

 female and the spermatozoid of the male, not splitting 

 their V-shaped nuclear bits ; consequently, the number is 



