Recognizing that the chromosomes are the physical basis of hereditary transmission, we can picture to ourselves the transmission of hereditary characters something as follows:  As we have seen, the fertilized egg contains an equal number of chromosomes from each parent (Fig. 42).  Now when this fertilized cell divides, each of the rods splits lengthwise, 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 physical basis of heredity which still retains its original powers.  This undifferentiated chromatin material originally possessed powers of producing a new individual, and