160 GENERAL BIOLOGY 



little or no influence on the formation of the egg, it being already com- 

 plete by the time she herself 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 does ripen, an interesting process takes place, it 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 can 

 be seen no more in a short time. The stainable nuclear material breaks 

 up into a number of chromosomes just as does the chromatin of the 

 Paramoecium, and one-half of these chromosomes remain in the larger 

 portion, the other half passing 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 

 does except that the sperm divide equally as to size, thus forming two 

 definite, living sperm-cells where there was only one before again, this 

 is just 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 every plant and animal that lives comes into existence in prac- 

 tically the same way, that is, through a single cell of the father and a 

 single cell of the mother uniting, we see that this is nature's way of 

 bringing together the normal number of chromosomes needed to make a 

 complete individual. This again, means that each individual thus 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, each and every one of us would 

 be quite unlike our parents, because each would be less than either par- 

 ent, instead of each taking one-half from each parent and thus becoming 

 a complete human being like both. As our parents can give us only the 

 single egg-cell and the single sperm-cell, everything else being merely 

 food and environment, it follows that everything we can possibly inherit 

 as to our physical and mental makeup must be in the chromosomes that 

 these eggs and sperm contain ; for, it is only the chromosome part that 

 intermingles, divides, and causes new cells to form. 



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 are speaking 

 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. 



The first group or sheet of cells becomes a hollow sphere called a 



