338 THE INTERNAL ENVIRONMENT OF THE BODY Part IIT 



every egg becomes fringed by sperm cells headed toward the eggs with tail- 

 pieces vibrating. In less than an instant this activity passes, the sperms 

 cease moving all at once as if a quick shadow passed over each egg and 

 stopped them. Actually, one sperm has pierced the egg membrane and is on 

 its way to the nucleus and as this occurs a special barrier, the fertilization 

 membrane, instantly forms around the egg and shuts out the competing 

 sperms. With the union of the sperm and egg nuclei that soon follows, the 

 inheritance of the coming individual is decided and its sex determined. With- 

 out ado or hesitancy the single cell goes through the process of division 

 into two cells, repeating this again and again. Thus a new sea-urchin begins. 

 Fertilization is a kind of junction between the existence of a sperm and 

 an egg, each of them prepared by meiotic divisions, and a new individual in 

 which mitotic divisions (Chap. 3) and differentiation are preeminent. These 

 processes are discussed under their respective names. 



Special Types of Sexual Reproduction 



The bisexual method of reproduction is the usual one in higher animals, in 

 all of the vertebrates, and in many invertebrates, jellyfishes, nearly all insects, 

 starfishes, sea-urchins, and their kin. Several varieties and irregularities of 

 sexual reproduction occur. 



Hermaphroditism. In some species, each individual normally produces 

 both eggs and sperm cells at the same time, and is called an hermaphrodite. 

 Such animals belong to a few groups of invertebrates, among them planarians 

 and other flatworms, earthworms, leeches, and snails. Among vertebrates, 

 hermaphroditism occurs only rarely. Even in hermaphroditic species, pairs of 

 animals mate and cross-fertilization occurs. In earthworms, the reproductive 

 organs are so located that the eggs of one worm can be only fertilized by the 

 sperm of another worm (Chap. 28). 



Although rare, hermaphroditic frogs, birds and even mammals are known; 

 some of these animals have one testis and one ovary, or some other combina- 

 tion of the primary organs. More often, the animal is a partial hermaphrodite 

 having the primary organs of one sex and the ducts and external genitalia of 

 the other. Hermaphrodites with both testicular and ovarian tissue are ex- 

 ceedingly rare in man, 



Freemartins. A freemartin is a sterile cow which was born a twin of a bull 

 calf. Her ovaries are usually testislike but contain no developing sperms; 

 the vasa deferentia and other masculine ducts are represented but the exter- 

 nal genitalia are mainly female. The twins are known to come from separate 

 eggs. The sterile condition of the freemartin is believed to occur because the 

 membranes (chorions) of the twins are fused in such a way that the blood 

 vessels are joined and there is a common circulation between them. Thus, 

 the hormone of the testes of the bull calf passes into the body of the heifer 



