ACCESSORY SEX ORGANS 299 



Very few eggs and no sperms can develop into new in- 

 dividuals without fusion with the gamete of the other 

 sex. In some aquatic animals this is very simple, since 

 the mass of eggs which the female lays in the water 

 attracts the male, probably by the stimulation afforded 

 by some chemical substance which they give off, and 

 causes him instinctively (see Chap. XV) to deposit the 

 sperms in contact with them. In land animals this is 

 not possible, and it becomes necessary for the female to 

 retain the eggs in a special organ where the male may 

 deposit the sperms in order to permit their fusion with 

 one another. 



Fertilization consists in the fusion of two cells, an 

 egg and a sperm, each of which is a highly organized unit 

 without the power of further division. At the beginning 

 of the process the sperm-cell attaches itself to the surface 

 of the egg (Fig. 71) and apparently is carried inward by 

 movements of the egg protoplasm. During its continued 

 journey into the interior the sperm nucleus gradually en- 

 larges through the absorption of fluid until finally it 

 becomes of the same size as the egg nucleus with which 

 it comes in contact. 



Results of Fertilization. — The important results of 

 fertilization are two-fold. In the first place the union 

 of the two sex-cells, each alone incapable of division, 

 supplies a stimulus that impels the fertilized egg to 

 undergo a large number of divisions resulting in a new 

 individual. In the second place each of these sex-cells 

 carries a definite number of chromosomes characteristic of 

 the species. Let the number be four ; the nucleus formed 

 by their union thus contains eight. At every cell division 

 these are accurately halved and distributed so that each 

 cell of the body receives an equal number of paternal and 

 maternal chromosomes. The high importance of this fact 

 will be considered in connection with the subject of 

 heredity, but at this point it may be pointed out that 

 chromatin is the bearer of hereditary traits and the off- 



