74 



ZOOLOGY 



Fertilization 

 by union of 

 a sperm cell 

 with an egg 

 cell 



Partheno- 

 genesis, or 

 reproduc- 

 tion from 

 unfertilized 

 egg cells 



certain seaweeds, multitudes of minute cells are thrown 

 out, all exactly alike. These join in couples and com- 

 pletely fuse, after which they develop into new plants. 

 This union is also fertilization, and unlike the Para- 

 mecium, the cells participating will not develop or con- 

 tinue their race without it. 



3. In the higher forms of life, the cells which take 

 part in fertilization are not alike. The sperm cell pro- 

 duced by the male is very different in appearance from 

 the egg cell produced by the female, although each con- 

 tains the essential contribution of chromosomes. Fer- 

 tilization is no longer optional, as it were ; it is obliga- 

 tory. In the highest plants, indeed, certain vegetative 

 methods of reproduction are still possible ; and when 

 impossible under natural conditions, they may still take 

 place through man's influence, by cuttings or grafts. 

 Nevertheless, the seeds will not develop without some 

 stimulus, something of the nature of fertilization. In 

 the higher animals reproduction by the union of sperm 

 and egg cells is the invariable method, and we think of 

 fertilization as necessary for the continuance of life. 



4. We think of the process of fertilization as consist- 

 ing essentially of the union of the protoplasm of two 

 cells derived from different individuals. These cells, 

 called gametes, have the simplex or reduced number of 

 chromosomes ; they make up the full number when 

 united to form -the zygote. This seems clear enough, 

 but we are puzzled when we find that in many animals, 

 even such highly organized ones as insects, partheno- 

 genesis takes place ; that is, reproduction from unferti- 

 lized egg cells. This is not like the simple division of 

 the protozoan, or the vegetative propagation of the 

 plant ; here we have an egg cell, apparently made for 

 fertilization, and it develops without it ! It is, as it 



