Interpretation of Facts 161 



of the two ovaries, though only about 100 to 200 of these actually ripen 

 and pass out of the body during the sexual life of the individual. This 

 means that the mother has little or no influence on the formation of the 

 egg, because it is already complete by the time she is born. 



The eggs lie dormant and do not begin to ripen until sexual life 

 begins (averaging from twelve to fifteen years in the human being). 

 But, when an egg ripens, an interesting process takes place. The egg 

 is expelled from the ovary immediately and, just as Paramoecia split 

 into two parts, so does the egg. But the egg does not divide equally. 

 A little piece, called the polar body, separates from the main part of the 

 egg. This polar body may divide again, but even so, it deteriorates and 

 cannot be seen in a short time. The stainable nuclear material breaks 

 up into a number of chromosomes just like the chromatin of the Para- 

 moecium. One half of these chromosomes remain in the larger portion, 

 the other half passes into the polar body to deteriorate with that part. 



The head of the male cell (spermatozoan) is practically all nuclear 

 material and goes through approximately the same process as the egg, 

 except that the sperm divides equally as to size, thus forming two definite, 

 living sperm cells where there was only one before — again, this is quite 

 like Paramoecia. And here, too, the chromosomes divide equally, so 

 that each sperm has only one half the full number of chromosomes it 

 had before it divided. 



As practically all plants and animals come into existence in prac- 

 tically the same way, through uniting a single cell of the male and a 

 single cell of the female, we see that this is nature's way of bringing 

 together the normal number of chromosomes needed to make a com- 

 plete individual. Thus, each individual comes into possession of one 

 half the traits or capacities of each parent cell (not necessarily one half 

 of the traits or capacities of the parent) from which he sprang. Were 

 this not true, all of us would be quite unlike our parents, because we 

 would be less than either parent, instead of taking one half from each 

 parent, thus becoming a complete human being. As our parents can 

 give us but the single egg cell and the single sperm cell, everything 

 else being merely food and environment, everything we can possibly 

 inherit must be present in the fertilized egg. The precision with which 

 the cell divides its chromatin equally between the two daughter cells 

 in mitosis indicates the importance of the chromosomes. In fact, it has 

 been shown that these bodies are the all important part of the cell from 

 the point of view of heredity, as they are the carriers of the genes. The 

 cytoplasm seems to play but a small part in the role of heredity. 



For anyone wishing to study life, therefore, the study of chromo- 

 somes looms up as the most important factor. 



The laboratory study of the fertilized cell, of which we speak, has 

 shown that each such fertilized cell divides into two cells ; these two 

 into four; each of these four into two, making eight; these eight into 

 sixteen ; and so on indefinitely until the entire body has finished its 

 growth. 



