CHAPTER ELEVEN. 

 FERTILIZATION. 



1. Review of Conjugation. In an earlier chapter a 

 study was made of the union of gametes that are wholly- 

 similar. This sort of union is called conjugation, and is 

 quite common among the lower plants and animals. In 

 these gametes there is no outward sign of the differences 

 which we think of as belonging to males and females. 

 We now wish to study the union of eggs and sperms, 

 such as were described in the preceding chapter, where 

 there is a very striking difference in the gametes. These 

 differences are sex differences and we call the offspring 

 sex-cells. Their union we call fertilization. 



2. The Effects of the Special Development of Eggs and 

 Sperms. In Chapter Ten it was shown that, with all their 

 differences, eggs and sperms agree in one important 

 particular. In both cases, when ripe, they have in their 

 nuclei only one-half the number of chromosomes usual in 

 the cells of the species. There are some interesting results 

 that seem to come from this reduction in the chromosomes. 

 Under ordinary circumstances neither of these cells has 

 the power of dividing again, or of developing any further 

 in any way. There are exceptions to this, but it is true as 

 a general thing. In some of the gametes which are alike 

 we saw that they might resume development separately if 

 they did not happen to unite. In the case of eggs and 

 sperms, unless they unite, both will die. Union, then, has 

 become almost imperative in these special cells. The sperm 



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