ORIGIN OF THE SEX CELLS 113 



2. Mature sperm cells are very small and very active cells; mature 

 egg cells, owing to their large size, are passive. 



3. Because all of the sperm cells are functional and also because of a 

 much greater number of multipUcation divisions, the total number of 

 sperm cells produced is enormously greater than the number of egg cells. 

 The number of egg cells which become mature and may be fertihzed is 

 only a small fraction of the number actually produced. The rest act as 

 nurse cells and contribute their substance to those which are to develop. 

 A good example of this is seen in the fresh-water hydra, in the ovary of 

 which a large number of egg cells are developed ; only one of these, how- 

 ever, becomes fully mature and capable of producing another individual. 



139. Division of Labor between the Germ Cells. — It will be observed 

 that in spermatogenesis and oogenesis there has been a division of labor 

 between the two types of sex cells. In order that fertilization shall occur 

 the two cells must come together, and to assure the development of the 

 embryo a large store of food is provided. If both cells had a store of 

 food neither would be able to move effectively and their union could not 

 occur, but the accumulation of a sufficient store of food in one while the 

 other becomes small and active makes it possible for the latter to seek 

 out the former and to unite with it. That practically every mature egg 

 cell will be fertilized is also insured by the enormous number of sperm 

 cells produced as compared with the number of egg cells. This greater 

 number, however, imposes no proportionately greater strain upon the 

 energy of the individual, since frequently there is no more material in 

 100,000 sperm cells than in one egg cell. In a certain species of sea 

 urchin the volume of the individual egg cell is equal to that of 500,000 

 sperm cells. 



140. Variations in Gametogenesis. — In the details of this process 

 many variations occur in different animals. All of the sex cells which 

 the animal produces may mature at the same time; this is the rule in 

 insects, which, after the maturation and fertihzation of the eggs, deposit 

 them as rapidly as possible and soon die. Maturation periods may 

 occur at intervals and the animal live through several breeding seasons. 

 Many mammals exhibit this phenomenon. Birds have an annual breed- 

 ing season. At other times than during the breeding season the sex 

 organs in these animals are quiescent and the maturation of the sex cells 

 is arrested. In still other animals, particularly in the male sex, matura- 

 tion goes on continually and the animal can breed at any time. Under 

 domestication the breeding season may be greatly extended. The 

 domestic hen in the original state had a restricted breeding season and 

 laid only a limited number of eggs. Under domestication the number 

 has been increased until 361 eggs have been produced in one year, which 

 means practically continuous sexual activity. 



