220 REPRODUCTION 



plied equally with the characters borne by the chromosomes of 

 both parents. 



When the grandmother cells of the spores are formed their 

 nuclei are constituted as in Fig. 126, A, like the other cells of the 

 body. But now when the grandmother cells begin division a 

 paternal and its homologous maternal chromosome become as- 

 sociated in a pair (Fig. 126, E and F), and so for all of them, 

 and the members of a pair are sent to opposite poles as shown 

 in Fisf. 126, G. It would now follow that when the mother cells 

 divide and form the spores, two of the four spores would have 

 only maternal and two only paternal chromosomes (Fig. 126, H 

 and I) descended from any single pair in the prophases of the 

 grandmother cell (Figs. 126, E and 123, B). 



It can now be seen that when the paternal and maternal 

 chromosomes become segregated in the division of the grand- 

 mother cells the process is the reverse of what happens when the 

 egg is fertilized; in the latter process the number of chromo- 

 somes is doubled, and in the former it is halved, or, what is the 

 same thing, the original number is restored. But it will be 

 seen that the question is not simply one of the number of 

 chromosomes, it is also a question of their kind, for even though 

 the two parents belong to the same species, or even to the same 

 variety, they are certain to differ in some manner or degree. 



Two Generations in the Life-cycle. — It is now evident that 

 there are two generations in the life-cycle of such an organism 

 as we have been studying, namely, the x generation and the 

 2X generation, x signifying a chromosome number. The 2X 

 generation begins when x chromosomes in the sperm cell fuse 

 with X chromosomes in the egg cell, thus forming the fertilized 

 egg which grows to be the mature fern plant or sporophyte, or, 

 as we may now call it, the 2X generation. The x generation 

 begins when in the division of the grandmother cell of the spores 

 as many chromosomes (x) as are suppHed by the sperm cell are 

 sent to one pole and as many (x) as are contributed by the egg 

 cell are sent to the other to form the nuclei of the spore mother 

 cells. A mother cell of the spores is therefore the one-celled 



