CHROMOSOMES 257 



species, it follows that either during the formation of 

 the gametes, or at some one or other of the cell 

 divisions leading up to their formation, there must occur 

 a reduction in the number of chromosomes to one-half 

 of their former number. In the case of the higher 

 animals this reduction takes place during the two cell 

 divisions which directly lead up to the formation of the 

 gametes themselves. In plants, on the other hand, the 

 reduction takes place during the formation of those cells 

 which are known as spores. From these, after a certain 

 number of intervening cell generations, the gametes 

 take their origin. These intervening cell divisions in 

 plants are characterized in every case by the appear- 

 ance of the reduced number of chromosomes. In 

 the higher plants, in fact, a generation is, as it were, 

 interposed between the reducing division and the 

 actual formation of the gametes. For the spores are 

 themselves unicellular reproductive bodies like the 

 gametes, but differ from the latter in the fact that 

 they develop without undergoing conjugation, and 

 give rise to a larger or smaller mass of tissue consisting 

 of cells with the reduced number of chromosomes. 

 From the fact that the cells of this gamete-bearing 

 generation contain half as many chromosomes as 

 those of the spore-bearing generation with which it 

 alternates, the generation produced from the spores 

 has been spoken of as the ^-generation in contrast 

 with the ordinary, or 2X-, generation. In animals the 

 ^-generation is reduced to a single generation of cells 

 only, which is represented by the gametes themselves* 



\Ve must next proceed to examine the actual 



17 



