MICHIGAN ACADEMY OF SCIENCE. 55 



There must be soiuetliiiig inliereut witliiu the cells that detei-niiiies 

 just wiiat each cell shall develop into in its own jiarticulai' euvirou- 

 meut. In plants generally and to a large extent in the lower ani- 

 mals any group of cells is able to regenerate the whole individual, 

 thus showing that the directive forces for all the structures of the 

 individual are present in each cell and not distributed respectively 

 among the various tissues. It is a fact that, in many plants as 

 well as in most animals, the amount of cytoplasm that is carried 

 into the egg Avith the sperm is very small or sometimes entirely 

 lacking. Indeed, the important feature of fecundation appears to 

 be the union of the male and female nuclei. This fact as well as 

 the features of mitosis and meiosis to which attention has already 

 been called makes it seem doubly certain that it is not only the 

 nucleus but the chromosomes within the nucleus, that decide how 

 the cells shall develop, and this means how the individual will be 

 constructed. 



If these chromosomes are of so great importance to the cell as 

 the foregoing would indicate, how do they act? We must first 

 of all consider their position within the nucleus. The "resting" 

 nucleus as distinguished from the nucleus in the process of di- 

 vision may be described as a large vacuole of nuclear sap bounded 

 by a tough plasma membrane of cytoplasmic origin, such as is 

 always found where cytoplasm comes in contact with a body of 

 water (e. g. at the exterior of the protoplast as well as the "tono- 

 plast" around the large central vacuole). Suspended in the nuclear 

 sap lies the tangled semi-fluid nuclear network consisting of a 

 delicate thread on which are strung at various points irregular 

 lumps of semi-fluid chromatin. A large drop of reserve protein 

 (the nucleolus) chemically closely akin to chromatin is also sus- 

 pended in the nuclear sap, apparently in more or less intimate 

 proximity to the nuclear network. The latter may criss-ci'oss 

 through the central portion of the nucleus or perhaps more often 

 lie near its circumference. 



A closer examination of the relation of the chromatin lumps in 

 the resting nucleus to the chromosomes that appear during mitosis 

 makes it almost certain that the fine thread with the scattered 

 chromatin masses on it is to be looked upon merely as 

 made up of chromosomes stretched out and that these separated 

 chromatin lumps are identical with the closely crowded deeply 

 staining chromatin bodies visible in the chromosome. If now the 

 chromosomes are the bearers of heredity it must be these scattered 

 chromatin masses in the resting nucleus that have this function. 



