CHAPTER FIFTEEN. 

 SOME EGGS THAT ARE NOT FERTILIZED. 



1. The Fate of Eggs. As we have seen, some of the 

 eggs formed by plants and animals are fertilized by the 

 sperms and then may develop into mature plants or 

 animals. But a great many fail to be fertilized, and such 

 eggs, no matter how perfect they are, cannot preserve 

 their life under ordinary circumstances. Such finally die 

 and decompose. Investigators have found that they can 

 take the eggs of many of the lower animals while they 

 are in good condition, even though they have not been 

 fertilized, and, by changing the external conditions about 

 them in certain ways, stimulate them to begin development 

 as though they had been fertilized. (See page 65.) 



2. Parthenogenesis. There is one other exception which 

 is so remarkable as to demand attention. Quite a number 

 of organisms regularly produce eggs that develop into 

 the adult without fertilization by the male cell. We do 

 not know whether they are stimulated in some special 

 way while in the body of the parents, or have the power 

 within themselves to develop without any stimulus. This 

 development without fertilization is parthenogenesis. The 

 circumstances under which it takes place are different in 

 different organisms. In some cases, as in the queen bee, 

 the mother can, apparently, determine at any time 

 whether the eggs shall be fertilized or not. In other cases, 

 as in some of the rotifers (microscopic animals), the non- 

 fertilized eggs ("summer eggs") are laid at certain seasons 



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