152 THE STORY OF THE LIVING MACHINE. 



This factor of heredity again we must trace 

 back to the machinery of the cell. We have seen 

 in the previous pages evidence for the wonderful 

 nature of the chromosomes of the cells. We 

 can not pretend to understand them, but they 

 must be extraordinarily complex. We have seen 

 proof that these chromosomes are probably the 

 physical basis of heredity, since they are the only 

 parts of each parent which are handed down to 

 subsequent generations. With these various facts 

 of cell division and cell fertilization in mind, we 

 can reach a very simple explanation of funda- 

 mental features of heredity. The following is an 

 outline of the most widely accepted view of the 

 hereditary process. 



Recognizing that the chromosomes are the 

 physical basis of hereditary transmission, we can 

 picture to ourselves the transmission of hereditary 

 characters somethingas 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 equiva- 

 lent to each other and equivalent also to the un- 

 divided egg. If the original chromosomes con- 

 tained 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 



