CHAPTER XXIV 



FERTILIZATION 



In a general sense fertilization may be defined as the union of the 

 sperm cell with the egg cell, though, as will be seen, the process involves 

 several steps, takes a certain length of time, and there may be a question 



B 



D E p 



Fig. 46. — Diagrams showing the successive steps in the fertilization of the egg cell of a 

 sea urchin, which is mature when the sperm cell enters. {From Wilson, " The Cell,'' by the 

 courtesy of The Macmillan Company.) A, the entrance of the sperm cell; the maturity of 

 the egg cell is indicated by the two polar bodies. B, the approach of the two pronuclei, 

 the centriole of the sperm cell and the aster developed about it preceding the male pro- 

 nucleus. C, the meeting of the two pronuclei; the centriole has divided. D, the formation 

 of two asters about the two centrioles, now on opposite sides of the two pronuclei, which 

 are undergoing fusion. E, the fusion nucleus representing the two pronuclei during a 

 period of pause, while the asters are reduced in size. Fertilization may now be said to be 

 complete. F, the first cleavage division, which follows the pause, at the beginning of the 

 anaphase. 



as to when the union is actually consummated. Two phenomena are 

 involved : the activation of one cell by the other and the union of corre- 

 sponding chromosomes from the two parents. The former effect is 

 paralleled by artificial parthenogenesis. Loeb discovered in 1899 that the 

 eggs of starfishes and sea urchins could be caused to develop by artificial 



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