44 



EMBRYOLOGY 



the same time fertilization establishes a chromosome balance which deter- 

 mines the sex of the fertilized egg. Note, however, that other factors, such 

 as temperature of the developing embryo, food, and the age of the adult, also 

 determine sex. We shall consider this problem somewhat later. 



Sperm and egg gamones 



The first question we shall take up is how the sperm and the egg come 

 together. We will use as a pattern the fertilization reaction in the sea urchin 

 egg. In this case the egg yields an extract which has very definite effects upon 

 sperm, and extracts of sperm contain substances which have very definite 

 effects on eggs. These substances are similar to hormones and, because they 

 are found in the gametes, they are called gamones. In the egg we find sub- 

 stances called gyno gamones, and in the sperm similar substances are called 

 andro gamones. 



In the egg there are at least two gynogamones, called gynogamones I and 

 II. Number I, when added to a sperm suspension, causes activation of swim- 

 ming movements of the sperm. Freshly discharged sperm are often relatively 



+ CYNOGAMONE E 



AGGLUTINATED SPERM 



FREE SWIMMING SPERM 



JELLY 



+ ANDROGAMONE E 



JELLY 0ISSOLVE0 



NUCLEUS^ EGG 



Fig. 20. Some effects of gamones. 



