GENERAL PRINCIPLES 53 



ejaculation does not exhaust the semen in the bull's testes, since ten ejacula- 

 tions may be collected from a single bull in a two-hour period. There is 

 some decrease in the volume of the ejaculate after the first few. 



After the semen is collected it is cooled gradually to about 40° F. for 

 storage or shipment. The semen may be diluted and injected into the uterus 

 of a cow and fertilization effected as long as seven days after its collection. 

 The dilution of the semen is about 1:10, so that an average ejaculate of 4.5 

 milliliters would yield 45 milliliters for injection. About one-half to one 

 milliliter of this diluted semen is sufficient for insemination of one cow. The 

 economy provided by artificial insemination is obvious. In one season one 

 ram sired 2500 lambs by this method. 



General principles 



It should be noted that the observations and experiments on fertilization 

 lead us to the same general principles that we found in the process of 

 ovulation and that we shall find again in later embryonic development. We 

 are usually dealing with systems all ready to begin functioning — systems 

 which simply need a stimulus of some sort to release substances already 

 present but blocked in some way. This principle of a release mechanism 

 holds true in fertilization. We have seen it apply similarly to the case of the 

 release of pituitary substances, and we shall see it again when we come to 

 discuss the formation of the primitive nervous system in the vertebrate 

 embryo. 



While one of the functions of fertilization is to stimulate the egg to 

 develop, it is well to consider the effects of adding a new set of chromosomes 

 to the egg. We have seen in parthenogenetic development that the egg 

 chromosomes suffice in many instances to carry the egg to the adult stage. 

 Thus we may ask why this sort of reproduction is not more common. Most 

 organisms have evolved some sort of sexual reproduction. Even those species 

 in which budding predominates exhibit sexual reproduction as well. Actually 

 the union of two sets of chromosomes from different individuals results in 

 greater variation in structure and function of the offspring as compared with 

 the little variation resulting from development with only one set of chromo- 

 somes. This provides a greater opportunity for the action of evolutionary 

 factors. If sexual reproduction had not evolved, the organism may not have 

 survived. Even species in which both male and female reproductive systems 



