146 CAUSES OF VARIATION 



" chromomeres " of others, the " microsomes " of still others, 

 and the " ids " of Weismann. 



Lying generally in the cytoplasm just outside the nucleus will 

 commonly, but not always, be found an extremely minute highly 

 staining body, the centrosome, about which, when division is 

 about to occur, the near-by matter is thrown into radiating lines 

 like iron filings about the poles of a magnet, giving the whole a 

 kind of starlike appearance. 



These are the portions of the cell most concerned in cell 

 divisions, and their special characters are most pronounced and 

 the differences most distinct just previous to the act of division, 

 and least well marked in the cell during its " resting stage " 

 between divisions. 



The actual process of cell division whereby one cell gives 

 rise to two and by which growth is attained is essentially as 

 follows : 



When division is about to take place the chromatin matter 

 (granules) assumes the appearance of a fine network running 

 through the mass of the nucleus, the granules looking like beads 

 strung upon a thread. This network commonly, though not 

 always, condenses into a ribbon or thread (the spireme), which, 

 however, speedily breaks up transversely into a definite num- 

 ber of segments, generally in the form of short rods, straight or 

 curved (the chromosomes). Whether or not the reticular or net- 

 work form passes through the spireme stage, the result is always 

 the same ; namely, the chromatin matter becomes divided into 

 a definite number of chromosomes. Here are two remarkable 

 and significant facts ; first, the number of chromosomes is con- 

 stant in all individuals of the same species ; and second, " in all 

 species arising by sexual reproduction the number is even!' 1 



While the chromatin matter has been engaged in breaking up 

 (or down) to form the chromosomes, another significant process 



1 Wilson, The Cell, p. 67. The author here gives the number of chromosomes 

 characteristic of certain species as follows: some of the sharks, 36; mouse, 

 salamander, trout, and lily, 24; ox, guinea pig, and onion, 16; grasshopper, 12; 

 Ascaris, 4 or 2 ; the crustacean Artemia, 168; man, 16 or possibly 32. In this 

 connection it is worthy of note that varieties of the same species often differ in 

 the number of their chromosomes, the significance of which variation has not 

 yet been determined. 



