8 Farmers' Bulletin 1167. 



with the sperm cell. What 20 will it be? Chance alone can tell, 

 except that there will be one from each pair. Here comes the sperm 

 cell, to mature which a similar process was gone through. Chance 

 again determined that 20 out of the original 40 "chromosomes should 

 disappear and no determination can be made to show which 20 

 would prevail. But this is not all. The sperm cell does not come 

 alone. It .comes in swarms, but only one sperm cell penetrates the 

 cell membrane and fertilizes the egg. What determines which of 

 the thousands of sperm cells will be the one? Chance again. 



The chromatin is the chief, if not the sole, determining factor in 

 the development of the identity of the new individual, and the 

 chromosomes are the agents by which this identifying process is 

 carried out. When we think of the great number of combinations 

 of chromosomes possible before fertilization is actually effected, we 

 begin to understand why it is that members of the same family 

 often do not look alike and why we have so many disappointments 

 in breeding animals. 



If chance is the sole factor in determining what chromosomes shall 

 persist, where does intelligent breeding come in? Of what advan- 

 tage is it to exercise care in matings? Why not leave it all to 

 chance ? 



The simple answer to these questions is that we can lessen the 

 probability of undesirable working of the laws of chance by increas- 

 ing the uniformity of the chromosomes. That is done by careful 

 selection to a certain type and by working with a group of animals 

 whose ancestry is known. The highest examples of such methods 

 are found in the work of breeders of purebred livestock where un- 

 broken and known lines of descent run back for a great many genera- 

 tions, sometimes for 50 years or more on a single farm under the 

 ownership of a single human family. The discussion of the subject 

 of selection will be found under that heading later. 



THE GESTATION PERIOD. 



From the time the egg is fertilized, developments are rapid. The 

 fertilized egg divides and subdivides with multitudes of subdivisions, 

 until in time the embryo " begins to look like something," and we 

 can make out the form of the animal and its parts. The time be- 

 tween the fertilization of the egg and the birth of the young animal 

 is known as the period of gestation. This period varies with the 

 kind of animal. The period for the common domestic animals is 

 shown below, followed by a table to determine how long a time may 

 be expected to elapse between service and birth. To make a practical 

 application of the table count forward the correct number of days 

 from the date of service. 



