THE FEMALE GAMETOPHYTE 83 



tion to the daughter nuclei may occur. The high numbers 

 reported for the endosperm are doubtless to be explained by 

 the triple fusion. 



In the great majority of cases the gametophyte number has 

 been counted only in the mother-cells, and the sporophyte num- 

 ber in the tissues of the ovule or young embryo. Still, occa- 

 sional counts throughout the entire life-history show that the 

 reduced number that occurs in the division of the mother-cell 

 is maintained up to the time of fertilization, whether the inter- 

 val be short, as in Angiosperins, where only from three to five 

 nuclear divisions intervene between reduction and fertiliza- 

 tion, or long, as in the liverworts, where the gametophyte is the 

 more permanent generation and the sporophyte is a compara- 

 tively temporary structure. 



Why the number of chromosomes should lie so constant, and 

 why a reduction in number should take place, are the most 

 important questions in this connection. The constancy of the 

 numbers has led many to believe that the chromosome is a 

 permanent organ of the nucleus, just as the latter is a perma- 

 nent organ of the cell ; but no one would assign such a reason 

 for the constant recurrence of six stamens in a lily. There is 

 other evidence in favor of the individuality of the chromosomes, 

 but it does not seem to be sufficient. The physiological advan- 

 tages are evident, for the constancy in number enables each 

 parent to transmit an equal number of chromosomes to the off- 

 spring, and the reduction prevents the constant geometrical 

 increase in the number of chromosomes which would otherwise 

 occur. Strasburger 108 says: " The morphological cause of the 

 reduction in the number of the chromosomes and of their equal- 

 ity in number in the sexual cells is, in my opinion, phylogenetic. 

 I look upon these facts as indicating a return to the original 

 generation from which, after it had attained sexual differentia- 

 tion, offspring were developed having the double number of 

 chromosomes. Thus the reduction by one-half of the number 

 of chromosomes in the sexual cells is not the outcome of a 

 gradually evolved process of reduction, but rather it is the reap- 

 pearance of the primitive number of chromosomes as it existed 

 in the nuclei of the generation in which sexual differentiation 

 first took place. . . . The reduction in the number of chromo- 

 somes takes place, in the higher plants, in the mother-cells of 



