460 THE AMERICAN NATURALIST [Vol.XLIX 



elements, of various shapes and sizes that are found 

 within the nucleus; they are especially demonstrable as 

 deeply staining bodies, definite in number for each cell at 

 the period of division. In many cases in both plants and 

 animals they have been found to be made up of small 

 particles, the chromomeres, and various investigators 

 have expressed the belief that these, too, are definite in 

 number and play an important part in the larger collective 

 entity, the chromosome. 



Almost from their discovery, the chromosomes have 

 had an especially important part assigned to them in the 

 drama of heredity because of the previous philosophical 

 deductions of Weismsfnn. Weismann reasoned that if 

 there were no reduction of heritable substance in the life 

 cycle of an organism, it would pile up indefinitely because 

 of the nuclear fusion at fertilization. He, therefore, pre- 

 dicted the discovery of some mechanism by which the 

 character conserving substance would be divided. A few 

 years later his prediction was verified in its important 

 details by actual observation of the chromosome reduc- 

 tion in the formation of germ cells in Ascaris. From this 

 discovery and from the facts that a specific number was 

 found for the cells of each species, that all the cells of an 

 individual appeared to possess the same number (except 

 when they were halved at gametogenesis), that they were 

 apparently permanent organs, that they were longitudi- 

 nally halved in division so as to give each daughter cell 

 the same number as well as an exact half of each chromo- 

 some possessed by the mother cell, investigators were 

 early tempted to place upon chromosomes the whole 

 burden of inheritance. 



Our observations regarding chromosomes and the re- 

 duction divisions in plants now rest on a basis of cyto- 

 logical investigation of over 250 species, representing 

 over 150 genera and divided among the four great groups 

 of this kingdom. Montgomery's 1906 list of chromosome 

 numbers in animals represents investigations on 185 spe- 

 cies, comprised in about 170 genera, distributed among 



