CHAPTER FOUR 



Fertilii^ation or tlie Prod S Ej 



Completion of Maturation of the Egg Symmetry of the Egg, Zygote, and 



Penetration and Copulation Paths of Future Embryo 



the Spermatozoon 



"The unfertilized egg dies in a comparatively short time, while 

 the act of fertilization saves the life of the egg and allows it to give 

 rise, theoretically at least, to an unlimited series of generations. . . . 

 Fertilization makes the egg immortal." (J. Loeb, 1913.) 



Fertilization generally is regarded as a two-fold process, involving 

 first the activation of the egg. This can be accomplished by the sperm 

 or by a variety of parthenogenetic agents. Second, there is the mixing 

 of hereditary (nuclear and chromosomal) potentialities, a process 

 known as amphimixis. Insemination means simply the exposure of 

 the eggs to spermatozoa. The most accepted evidence for activation is 

 an elevation of the vitelline membrane away from the egg and its 

 transformation into what is then called a fertilization membrane. 

 The vitelline membrane can be lifted from the ovarian eggs of the 

 frog in distilled water or calcium-free Ringer's solution at pH 7.5 

 to 10.2. This purely physical change seems to be the result of pro- 

 gressive cytolysis or coagulation of the egg cortex, and the membrane 

 itself has the consistency of thin cellophane. There are, of course, 

 physiological changes in the egg accompanying fertilization which 

 are so dynamic that even the microscopic surface pigment granules 

 become violently active. The ultimate proof of successful fertilization 

 is, of course, the diploid condition of the chromosomes of the 

 zygote. 



As the frog's egg is shed into the water, the male (in amplectic 

 embrace) simultaneously and by active expulsion movements sheds 

 clouds of spermatozoa over the eggs. Fertilization in Anura is there- 

 fore external. The jelly, deposited on each egg as it passed through the 

 oviduct, swells almost immediately thereafter, partially protecting 

 the egg against multiple sperm entrance. The swollen jelly appears to 

 be arranged in three layers, being twice the thickness of the egg diam- 



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