THE PROBLEMS OF DEVELOPMENT 5 



microscopes the egg appeared to be a comparatively unorganized structure. 

 The view that the egg protoplasm is relatively unorganized is called the 

 epigenetic theory of development and is diametrically opposed to the prefor- 

 mation theory. The epigenetic theory states that the egg starts as a simple 

 structure which gradually changes into new structures according to certain 

 principles. The study of the gradual change in form during development 

 and of the factors that bring about the series of developmental changes we 

 call embryology. 



The problems of development 



The problems of embryology are many. They can be organized according 

 to the order in which processes take place in development. There is a series 

 of problems that center about the process of fertilization. At this period we 

 may ask several questions. First of all, what determines whether a cell 

 becomes egg or sperm? Next, what is the mechanism of the control and the 

 timing of the release of the egg from the ovary? When is the egg ready 

 for fertilization? How is it released from the ovary? Simpler problems, such 

 as the length of life of the sperm, have both theoretical value for the 

 embryologist and practical value for the breeder of animals. How long will 

 an egg live after it is released from the ovary? How long will a sperm live 

 and retain its fertilizing power after ejaculation ? 



Another important series of problems arises in connection with the chem- 

 ical conditions for fertilization — the acidity or alkalinity of the sperm fluids 

 and of the vaginal fluids of the female. There is also the study of certain 

 hormones, called gamones, liberated by the egg and sperm respectively. 

 Gamones liberated by the egg influence the behavior of the sperm, and those 

 liberated by the sperm influence the behavior of the egg. Practical problems, 

 such as the investigation of spermicides — effective killing agents for sperm — 

 also belong here. 



The unfertilized egg after release from the ovary soon becomes a resting 

 cell. No visible changes take place until it is fertilized. Then a number of 

 changes in the nucleus and cytoplasm occur, leading to the first cell division. 

 In one sense, then, the sperm stimulates the egg cell to divide. What is the 

 nature of this stimulus ? What conditions does the sperm produce in the egg 

 which cause the latter to undergo cell division? Or, why does the egg not 

 divide without the sperm? Can we extract in some way a substance that would 



