THE LIVING MACHINE BUILDING FACTORS. 175 



inheritance. There are plenty of puzzling pro- 

 blems connected with heredity, but the fact of 

 heredity is one of the foundation stones of bio- 

 logical science. Upon it must be built all theories 

 which look toward the explanation of the origin 

 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 physi- 

 cal 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 ceH 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. 



Kecognizing 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 equiva-- 

 lent to each other and equivalent also to the un- 

 divided egg. If the original chromosomes con- 



