Page:Popular Science Monthly Volume 85.djvu/250

246 the nuclear membrane during division. In general the degree of differentiation may be measured by the degree of unlikeness btweenbetween [sic] different cells, and by the completeness with which the protoplasm of different cells is prevented from intermingling.

All the phenomena of life, including heredity and development, are cellular phenomena in that they include only the activities of cells or of cell aggregates. The cell is the ultimate independent unit of organic structure and function. The only living bond between one generation and the next is found in the sex cells and all inheritance must take place through these cells. Inherited traits are not transmitted from parents to offspring but the germinal factors or causes are transmitted, and under proper conditions of environment these give rise to developed characters. Every oosperm as well as every developed organism differs more or less from every other one and this remarkable condition is brought about by extremely numerous permutations in the distribution of certain parts of the sex cells in maturation and fertilization. Sex is an inherited character dependent upon an alternative distribution of certain chromosomes of the nucleus. There is much evidence that the factors for all sorts of alternative characters are associated with the chromosomes. The differentiation of the oosperm into the developed organism is accomplished in part by the associations and dissociations of germinal units which lead to the formation of new materials and by the segregation and localization of these materials in definite cells.

Germ cells and probably all other kinds of cells are almost incredibly complex. We know that former students of the cell greatly underestimated this complexity and there is no reason to suppose that we have fully comprehended it. What Darwin said of the entire organism may now be said of every cell.

An organic being is a microsome—a little universe, formed of a host of self-propagating organisms, inconceivably minute and numerous as the stars in heaven.