RELEASE OF EGGS 15 



somes during maturation, resulting in two types of eggs — male-determining 

 eggs and female-determining eggs. In other species this segregation occurs 

 in the sperm, resulting in two types of sperm. 



At first sight, the most striking thing that occurs during maturation is 

 the growth in the size of the egg. Most of the primitive germ cells are about 

 10 fi* (0.01 mm.) in diameter, the size of an average body cell. In the 

 mammal such cells increase to a size which in some species may reach a 

 diameter of 200 fx- In the frog the increase is up to a diameter of 2000 ^, 

 and in the birds the diameter of the egg reaches about 40,000 p. This tre- 

 mendous increase in size is due largely to the deposition of proteins, fats, 

 and carbohydrates, which form a food reserve used as an energy source 

 and as a source of raw materials for the synthesis of protoplasm. 



All eggs have some yolk, which in general is similar to that of the 

 hen's yolk. Yolk is commonly in the form of granules, and these are various 

 combinations of fat, protein and carbohydrate. The large size of eggs is 

 due not so much to an increase in the amount of protoplasm as to an increase 

 in the amount of stored, inert food materials. 



Release of eggs 



After the egg has secreted all of its yolk and passed through the necessary 

 phases of maturation it is ready for ovulation. Since the time of ovulation 

 must bear some relation to the breeding cycle of the animal involved, the 

 timing and control of ovulation are important problems. There appear to 

 be, in general, two quite different controls for ovulation. One is an environ- 

 mental control as a result of which animals ovulate for a brief period once 

 a year according to the season. The other type of control is an internal con- 

 trol which establishes some sort of a rhythm so that the animals ovulate 

 once a day, once every five days, once a month, or once in any given period. 

 But that period has no relationship to season. There are a few exceptions: 

 The rabbit ovulates shortly after copulation with the male. In this case 

 the control is neither environmental nor internal; a particular stimulus, copu- 

 lation, releases the egg from the ovary. 



Let us summarize the process of ovulation in a frog, referring to Figure 

 7, which shows diagrammatically the female reproductive system of the 

 ordinary frog. The large ovary, containing some 2000 eggs, is attached by 



* A micron (/j.) is one-thousandth of a millimeter. 



