gthwise,
half of each entering each of the two cells arising from the cell
division. From this method of division of the chromosomes it follows
that the daughter cells would be equivalent to each other and equivalent
also to the undivided egg. If the original chromosomes contained
potentially all the hereditary traits handed down from parent to child,
the chromosomes of each daughter cell will contain similar hereditary
traits. If, therefore, the original fertilized egg possessed the power
of developing into an adult like the parent, each of the daughter cells
should likewise possess the power of developing into a similar adult.
And thus each cell which arises as the result of such division should
possess similar characters so long as this method of division continues.
But after a little in the development of the egg a differentiation among
the daughter cells arises. They begin to acquire different shapes and
different functions. This we can only believe to be the result of a
differentiation in their chromatin material. In the cell division the
chromosomes no longer split into equivalent halves, but some characters
are portioned off to some cells and others to other cells. Those cells
which are to carry on digestive functions when they are formed receive
chromatin material which especially controls them in the performance of
this digestive function, while those which are to produce sensory organs
receive a different portion of the chromatin material. Thus the adult
individual is built up as the cells receive different portions of this
hereditary substance contained in the original chromosomes. The original
chromosomes contained _all_ hereditary characters, but as development
proceeds these are gradually portioned out among the daughter cells
until the adult is formed.

From this method of division it will be seen that each cell of the adult
does not contain all the characters concealed in the original
chromosomes of the egg, although each contains a part which may have
been derived from each parent. It is thought, however, that a part of
the original chromatin material does not thus become differentiated, but
remains entirely unchanged as the individual is developing. This
chromatin material may increase in amount by assimilation, but it
remains unchanged during the entire growth of the individual. It thus
follows that the adult will contain, along with its differentiated
material, a certain amount of the original physi