SEXUAL REPRODUCTION 325 



growing into new filaments. From this it appears as if 

 sex in its simplest form has originated by a change in 

 the behavior of ordinary swarm spores and that the 

 essence of the process is the fusion of two reproductive 

 cells. It is an interesting fact that these gametes always 

 show a decided preference for fusing with those from 

 some other filament rather than with those from the 

 same cell or filament. The precise chemico-physical re- 

 lations at the bottom of this behavior are not yet known. 

 Thiat it is an advantage, however, is pretty clear, from 

 the fact that so many plants and animals have evolved 

 this method of reproduction. It certainly gives a greater 

 opportunity for variation by bringing together the char- 

 acters of two parents instead of one. 



(Edogonium (Fig. 83) has a method of sexual reproduc- 

 tion in which the egg (i.e. female) gamete is very much 

 larger than the sperm (i.e. male) gamete. A particular 

 cell or cells in a filament swells up so as to become nearly 

 spherical instead of the elongated cylindrical form found 

 in the ordinary vegetative cells. Its contents then with- 

 draw slightly from the cell-wall and round up to form an 

 egg gamete. A spot on the wall softens and presently 

 forms a perforation through which the sperm gamete can 

 enter. In some other part of the same filament or, in 

 some species, in another filament, some cell divides 

 rapidly to form a number of very short cylindrical cells. 

 In each of these the nucleus divides once. The cytoplasm 

 is then divided between these two nuclei and each half be- 

 comes organized to form a sperm. When these sperms 

 are mature they escape through the broken cell-wall and 

 swim about by means of a crown of cilia at the smaller 

 end. They are attracted to the perforation in the wall 

 of the cell (oogonium) containing the egg, probably by 

 means of some substance diffusing away from it and serv- 

 ing as a directive stimulus. This is a case of what was 

 called a tropism in Chapters XII and XIV, where we 

 learned that a variety of external conditions may act as 



