28 HOW ANIMALS DEVELOP 



an ordinary cell consists of a bag made of the nuclear 

 membrane filled with rather liquid protoplasm. When 

 the cell is about to divide into two the nuclear 

 membrane disappears, and out of the liquid contents 

 there are built up a number of little solid lumps, 

 which if the cell is killed can be stained very deeply 

 with many dyes, and are therefore called chromosomes, 

 from the Greek words for "colour" and "body." 

 Different chromosomes are often different in shape, 

 so that they can be recognized, and it is very 

 important to notice that they always occur in pairs, 

 so that each cell has two of each kind. The number 

 and shape of the chromosomes in the cell is fixed for 

 any particular species, but is different in different 

 species ; some have as few as four, others up to one or 

 two hundred. But as the chromosomes are always in 

 pairs of similar ones the number must always be even. 

 When the chromosomes become visible at the begin- 

 ning of an ordinary cell-division, each one is already 

 split longitudinally into two half-chromosomes lying 

 side by side. As the cell divides, these two halves 

 separate from each other, and one half goes into each 

 of the two cells which are formed. When the division is 

 over they count as whole chromosomes, and gradually 

 disappear into a normal fluid nucleus (Fig. 2). 



The cell-division which results in the formation of 

 the germ-cells seems superficially very different from 

 the ordinary divisions, but it has recently been 

 realized that the whole difference follows from one 

 single slight alteration in the way the division begins. 

 The difference is this: that when the chromosomes 



