142 OUTLINES OF EVOLUTIONARY BIOLOGY 



part in bringing the gametes together. So also, of course, do 

 many other factors in various plants and animals, but we must 

 distinguish between factors which act directly upon the gametes, 

 such as chemotaxis, and those which act indirectly through the 

 soma or body of the organism, as for example through the visual 

 and olfactory senses of the higher animals. 



It is probable, however, that chemotaxis itself is but a secondary 

 factor which serves to bring the gametes within the range of one 

 another's direct influence. Thus in Coccidium (Fig. 39) the 

 chemotactic action seems to be exhausted after a certain number 

 of spermatozoa have been attracted to the neighbourhood of the 

 ovum and a fresh attraction appears to be exerted by the ovum 

 itself or by its nucleus. 



The term cytotropism, or cytotaxis, has been applied to the 

 attraction which, according to some observers, is sometimes 

 set up between two adjacent cells, and something of this kind 

 probably comes into play in the mutual attraction of gametes. 

 It can probably act only at very short distances, and hence the 

 necessity for some preliminary means of attraction such as 

 chemotaxis. That chemotaxis alone is not a sufficient explana- 

 tion of the phenomenon in question is suggested by the case of 

 Spirogyra. The conjugation of the gametes in this plant has 

 already been described in Chapter VIII. It will be remembered 

 that the process may take place between the cells of two filaments 

 lying close together, parallel with one another (Figs. 43 and 

 44), and is then inaugurated by those of the male filament. 

 Each of these cells which happens to lie opposite to a cell of the 

 female filament puts forth a hollow protuberance of its wall, which 

 is presently met by a similar protuberance from the wall of the 

 female cell, the two projections uniting to form a canal through 

 which the protoplasmic body of the male gamete creeps inside the 

 cell- wall of the female gamete to conjugate with the latter. It 

 sometimes happens, however, that, owing to inequalities in the 

 sizes of the cells, there may be a cell in one filament which lies 

 between two cells of the opposite filament and for which there is 

 no mate, all the adjacent cells being coupled. In such cases the 

 solitary cell (Figs. 43 and 44, S.C.), if it exhibits any of those re- 

 markable activities which are shown by the conjugating cells on 

 either side of it, merely makes preliminary advances which are 

 prematurely checked, as though there were a competition for 

 partners in which it was unsuccessful. Here it is obvious that 



