476 THE AMERICAN NATURALIST. [Vol. XXXIX. 



interesting subjects of cell research. Reduction phenomena also 

 have a deep phylogenetic significance whose history in plants at 

 least can be traced with a remarkable degree of exactness. 



We are confident that sporogenesis in plants signifies the sud- 

 den return of the organism to the condition of an ancestral 

 sexual generation with the reappearance of a primitive number 

 of chromosomes. The short time consumed in the process and 

 the details and precision of the cell activities show that we are 

 dealing with phenomena whose complicated mechanism can only 

 find explanation in a long phylogenetic history. In the study of 

 reduction phenomena and fertilization we have reached the con- 

 clusion that the chromosomes are intimately concerned with the 

 transfer of hereditary qualities and are probably the chief or 

 even the sole bearers of these characters. And thus we enter 

 upon some of the most far reaching problems of biology, those 

 of heredity, hybridization, and the basis for the remarkable 

 ratios of inherited characters which Mendel first clearly set 

 forth. 



It seems quite certain for both animals and plants that numer- 

 ical reduction of the chromosomes takes place through an asso- 

 ciation of the paternal and maternal chromosomes in pairs to 

 form the reduced number of bivalent chromosomes (dyads). We 

 have presented in Section IV (" Sexual Cell Unions and Nuclear 

 Fusions ") the evidence which indicates that paternal and maternal 

 chromosomes do not unite at the immediate time of nuclear 

 fusion in fertilization. On the contrary, in all higher animals 

 and plants the paternal and maternal chromosomes are believed 

 to remain separate throughout the long series of cell divisions in 

 the new generation up to the time of sporogenesis in plants and 

 gametogenesis in animals, both events being characterized by 

 reduction phenomena. The fusion of the chromosomes takes- 

 place in the growth period which differentiates the spore mother- 

 cell in plants from' the archesporium or the primary gametocyte 

 in animals from the preceding gametogenous tissue. The growth 

 period is one of general protoplasmic accumulation and increase 

 in the chromatin content of the nucleus, and is especially char- 

 acterized by that peculiar activity in the nucleus termed synap- 

 sis. Evidence is accumulating that synapsis is the characteristic 



