REPRODUCTION AND DEVELOPMENT IN CHORDATES 



equally to the nuclear constituents of the zygote (Fig. 5.11). Ihe union of 

 two pronuclei, each containing one chromosome of each of the homologous 

 pairs characteristic of the species, restores the diploid number of chromosomes 

 and is known as amphimixis (p. 41). Each zygote, consequently, has the 

 same number of pairs of chromosomes that each of its parents has, and each 

 parent contributes one chromosome of each pair. Meiosis and amphimixis 

 furnish the physical basis for an understanding of heredity (pp. 185-194). 



Meiosis and spermiogenesis occur in the seminiferous tubules of the testis, 

 and the spermatozoa are mature when they are shed by the male and pene- 

 trate the eggs. The female germ cells are not always mature when the 

 spermatozoa enter. The primary oocytes, surrounded by their follicles, grow 

 and store nutrients in the ovary, and in some animals both meiotic divisions 

 occur before ovulation; ova are liberated and fertilized. In other animals 

 the primary oocyte is ovulated, and the spermatozoon enters before either 

 meiotic division has occurred (Fig. 5.9). Meiosis begins in many vertebrates 

 just before the time of ovulation, and the spermatozoon enters either the 

 primary or secondary oocyte, which then completes its meiosis. When it 

 enters an immature egg, the spermatozoon remains quiescent until oogenesis 

 is completed. The nucleus of the spermatozoon, or male pronucleus, then be- 

 comes rounded and vesicular like the female pronucleus before amphimixis 

 takes place. 



As long ago as 1785, when Lazaro Spallanzani mixed lemon juice and 

 vinegar, among other things, with frogs' eggs in an attempt to stimulate 

 them to develop, biologists sought to secure activation by artificial means. 

 Finally, in 1899, Jacques Loeb succeeded in finding a method of artificial 

 parthenogenesis, as the experimental activation of an egg that normally unites 

 with a spermatozoon is called. A few years later Loeb was able to rear to 

 maturity frogs that had developed from artificially activated eggs; these frogs 

 had a mother but no father. It is now known that a variety of methods will 

 initiate development in eggs that normally develop only after fertilization. 

 George Lefevre, Sr., in 1907 used dilute organic acids (Fig. 5.12); others have 

 used heat, shaking, pricking of the surface of frogs' eggs, and various chemi- 

 cal agents. Most of the studies on artificial parthenogenesis have been on the 

 eggs of invertebrates, but in recent years attempts have been made, with some 

 success, to activate mammalian eggs artificially. It has been stated that 

 parthenogenesis occurs normally in some insects, notably bees and aphids, and 

 among the rotifers. Parthenogenesis, whether normal or artificial, can be 

 induced only when the egg cell is in a certain stage, which may be called 

 a fertilizable condition, and corresponds only to the activation aspect of 

 fertilization. There is only one parent, and amphimixis cannot occur. 



Fertilization depends not only on the fertilizable condition of the egg but 

 afso on the ability of the spermatozoa to come in contact with the egg. The 

 flagellate spermatozoon of vertebrates swims by lashing its tail; fertilization 

 can occur only in liquids. The lower vertebrates, such as the frogs, typically 

 copulate as the eggs are passed from the body of the female, so that the 



143 



