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THE AMERICAN NATURALIST [Vol. LI 



cells, and most conspicuoush^ in their chromosome- 

 complexes. The specificity manifested by genus and 

 species, whether it is centered in specific proteins of the 

 cytoplasm or in specific molecules of the chromosomes, 

 forms one great chapter of the specificity problem, while 

 the specificity of different tissues and cells is another. 



It is often stated that the different tissues and cells 

 of an individual of a given species, all have identical 

 chromosome-complexes. If the chromosomes are consid- 

 ered identical in all the cells of an individual, they can 

 not be regarded as responsible for the specificity of his 

 tissues. They can place no restriction upon a wide 

 range of permutability between the various cells in the 

 organism, can put no restraint upon unlimited regen- 

 eration or impede the perpetual proliferation of any type 

 of cells. The assumption of equality of chromosome- 

 complexes in different tissues and of their invariability 

 excludes them from the range of possible carriers of the 

 specificity of tissues and is usually associated with the 

 belief that the specificity of tissues is brought about by 

 segregation of cytoplasmic materials during develop- 

 ment. The possibility is also considered, that environ- 

 ment may act as differential factor. 



OF these two latter factors the segregation of cyto- 

 ].I;i>inic material^ In llic early stage of development leads 

 to tile rnniialion of large cell groups (germ-layers, 

 aiilauo of ()r.<ian>). the differential characters of which 

 are believed to be determined by the presence of definite 

 cytoplasmic materials, transferred to them from the cyto- 

 plasm of the ovum. The differentiation brought about by 

 segregation is regarded as irreversible and though the 

 fells of tlie gcnu-layci-s sliow a great plasticity in their 

 ropoiiM^ to (lifTrrcnt i^ictoi's. there is a well-marked 

 liniitatinii ot' their potencies, it' compared with the first 

 hia-toiiicrc-. It is iK'lieved. however, that the segrega- 

 tion doc- not atTeet the chromosomes and produces merely 

 a dilTcrctitial distribution of the cytoplasmic constituents 

 of the (n-iini among the resulting cell groups. It has been 



