ATTEACTION OF GAMETES 141 



contains very little deutoplasm. This is correlated with the fact 

 that it develops within the body of the parent at the expense of 

 food material derived from the blood of the latter. There is 

 reason "to believe, however, as we shall see later on, that the 

 small size of the mammalian egg is a secondary feature. 



The plant egg-cell may also be loaded up with food material, 

 so as to attain a large size, as in the green alga, Chara, 

 where the contrast between the minute flagellate spermatozoon 

 and the relatively gigantic ovum, filled in this case with starch 

 grains, is very striking. In the higher plants, however, where, 

 as in the case of the Mammalia, the developing embryo is not 

 dependent for its nutrition upon food supplies stored in the egg- 

 cell, the latter remains quite small, as, for example, in the fern 

 (Fig. 52) and the flowering plant (Fig. 55, e). 



According to some authorities one of the most important 

 differences between ovum and spermatozoon in animals lies in the 

 fact that the centrosome of the former disappears finally during 

 the process of maturation, the centrosome of the zygote being 

 contributed by the spermatozoon alone. In view of the fact, 

 however, that a definite centrosome is not usually recognizable at 

 all in the higher plants we cannot attribute very great importance 

 to its supposed absence in the animal ovum, and we shall also see 

 presently that centrosomes appear in developing eggs which have 

 not been fertilized by spermatozoa. 



We have already had occasion to refer to the existence of 

 some attractive force whereby the male and female gametes 

 are brought together in conjugation. Many observers main- 

 tain that this is simply a case of positive chemotaxis, or 

 the chemical stimulation of the protoplasm of one gamete by 

 a specific secretion of the other in such a way as to cause 

 them to respond by approaching one another (or by the male 

 .gamete approaching the female). 



It is a well known fact that certain spermatozoa are attracted 

 by specific chemical substances. Thus the free-swimming sperma- 

 tozoa of ferns and mosses are attracted by weak solutions of 

 malic acid and cane sugar respectively, and those of Coccidium 

 are attracted by nuclear matter discharged from the ovum in the 

 process of maturation. 



There can be no doubt that, whether the attracting substance 

 be secreted by the germ cells themselves or by some other part 

 $ the organism, chemotaxis sometimes plays a very important 



