Chap. 18 REPRODUCTION 335 



fusion. The glands produce secretions that control activities of the system. 



Similarities of Male and Female. The union of a male and female germ 

 cell is the first event in the life of the great majority of multicellular animals. 

 Since each of them has a male and a female parent it is not surprising that 

 females inherit male as well as female characteristics, and that males inherit 

 female ones as well as male. No animal is entirely male or female in its 

 chemical content, its structure, or its behavior. The pars anterior of the 

 pituitary gland of the male liberates the same gonad (sex organ) stimulating 

 hormones as that of the female. The nipples, developed in all female mammals, 

 are also present in the males. 



The characteristics of the opposite sex appear in the sex reversal that 

 occurs in some animals in nature as well as in experiments. The right ovary 

 of most birds is ordinarily only partly developed and the left one produces 

 the eggs. If the left one is removed by careful operation the small and incom- 

 plete right one usually develops into a testis and produces sperm cells. This 

 is because the cortex or outer layer of the bird's fully developed ovary secretes 

 a male-suppressing substance that ordinarily prevents the development of 

 sperm cells. Without it they would form in the medulla or central part of the 

 ovary. In the experiment the active cortex was removed with the functional 

 left ovary; it was undeveloped in the incomplete right one. Thus the male part 

 of the right ovary was no longer repressed. 



Male and Female Cells — Gametes. In many lower plants and animals, all 

 of them aquatic, the male and female cells are often about the same size 

 and shape and both may swim with tail-like flagella. Within the bodies of 

 multicellular animals, constituting essentially aquatic surroundings, the eggs 

 are moved by cilia or by muscular pressure while the sperm cells are agile, 

 persistent swimmers (Fig. 18.3). Eggs are the energy-conserving cells; sperms 

 are the energy-expending cells. Many eggs are enlarged with food stored for 

 the embryo (Table 19.1). We fry eggs for food, but not sperm cells. 



More eggs and sperms are produced than ever fulfill their promise. A bull- 

 frog lays from 10,000 to 20,000 eggs at one time. Counting on one egg 

 matured per month, covering the period between 12 and 45 years, a woman 

 produces about 430 eggs. Yet, within a pair of human ovaries thousands of 

 eggs wait in vain to develop further. George W. Corner quotes an investigator 

 who counted the incompletely developed eggs in both ovaries of a 22-year- 

 old woman and found about 420,000. The numbers of sperm cells are 

 astronomical in the majority of animals. It has been estimated that during 

 his reproductive lifetime, a man produces about four hundred billion sperms 

 or about one billion to each egg released from the human ovaries. 



Fertilization. The union of a sperm and egg which constitutes the beginning 

 of a new individual is fertilization. It may be external and occur in the open 

 water, as it does in starfishes and sea-urchins, most fishes, frogs and toads, 



