414 THE AMERICAN NATURALIST. [Vol. XXXVII. 



has become the conviction that the chromosomes carry the keys 

 to many and probably the most important problems of develop- 

 ment and heredity. We do not know what the chromosome 

 does but its characteristic activities during nuclear division and 

 its behavior at critical periods in the life history are so remark- 

 able that the assumption of its importance in these events is 

 quite justified. The most attractive theory of reduction phe- 

 nomena assumes that specific characters are largely defined by 

 the amount and nature of the chromatin in the nucleus and that 

 a species, to keep true, must so provide that the chromatin con- 

 tent is relatively stable from generation to generation. 



Reduction of the chromosomes at some period of the life his- 

 tory is almost universal among higher animals and plants but we 

 should note an important difference between the two groups in 

 the manner in which this is accomplished. Briefly seated for 

 animals, the reduction occurs just before the formation of the 

 sexual cells (gametes) which have in consequence one half the 

 number of chromosomes characteristic of the organism. The 

 fertilization of the animal egg by the sperm brings the male and 

 female nuclei together and as a result of their fusion the number 

 of chromosomes becomes again normal. 



In plants above the thallophytes the history is very different 

 and in sharp contrast to that of the animal. There is no reduc- 

 tion at the time when the gametes are formed. The gametes 

 have consequently the same number of chromosomes as the sex- 

 ual plant (gametophyte). Their fusion gives to the sexually 

 formed spore double the number characteristic of the gameto- 

 phyte. This fact is believed to be largely responsible for the 

 peculiarities of the asexual generation that follows. The sporo- 

 phyte runs through its vegetative development, without any 

 change in the double number of chromosomes, to the time of 

 spore formation when the sporogenous tissue (archesporium) is 

 differentiated. There is then a period of growth during which 

 some or all of the archesporial cells become spore-mother-cells. 

 And during that preparation for spore formation (sporogenesis) 

 the number of chromosomes is reduced by half, becoming again 

 the number of the gametophyte. The reduced number first 

 appears in the nuclear divisions inside of the spore-mother-cell 



