24 EMBRYOLOGY 



menstrual cycle is so variable, and because menstruation may be delayed by 

 such different factors as worry or change in climate, tests such as these have 

 been very useful in determining pregnancy during the first month after 

 fertilization. 



In the analysis of causes of ovulation we find that the principal factors 

 are what we might call release mechanisms. The pituitary contains the hor- 

 mone, but something like light acting on the retina must release the hormone 

 from the gland. In the frog the eggs are completely mature in the ovary and 

 await their release by the application of a hormone in very low concentration. 

 This illustrates a general principle in development— namely, that each stage 

 produces something which stimulates or induces the succeeding stage to 

 appear. Thus during development there is a progressive series of changes, 

 each change related to the preceding one. The result is a chain of reactions 

 extending from the release of pituitary hormone to the fully formed adult. 

 This principle of stimulation or induction will be of importance in our dis- 

 cussion of fertilization and of the formation of various organs of the body 

 through contact with stimulating tissues. 



