4 • FERTILIZATION: THE NATURE OF 

 THE STIMULUS FOR DEVELOPMENT 



w 



e have seen that the egg at the time of ovulation is organized in a 

 general way with respect to the three germ layers but that this organization 

 can be changed by chemical treatment. We also learned in the course of the 

 discussion (p. 29) that the sperm may start the expression of this organ- 

 ization by stimulating any part of the egg to develop. Fertilization, then, 

 exercises an activating effect on the egg. Another and quite different aspect 

 of fertilization is the addition of the paternal chromosomes. However, as 

 we saw in some of the experiments just considered, the full complement of 

 chromosomes is not necessary for early development. Therefore we shall 

 consider chiefly the activating effect at fertilization, and we shall not go 

 into any detail on the effect of adding the male chromosomes to the female 

 chromosomes. 



For this activation process the sperm is not entirely necessary. In many 

 forms, fertilization can be imitated by treating the egg with a wide variety 

 of agents, such as certain acids, alkalies, ultraviolet light, and a number of 

 special chemical compounds. The embryos resulting from such activation 

 are necessarily haploid and have only the maternal chromosomes. This activa- 

 tion by artificial means is termed artificial parthenogenesis. Since the haploid 

 embryos are essentially normal, the addition of nuclear materials from the 

 sperm simply restores the diploid number of chromosomes to the egg. At 



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