30 SEX AND HEREDITY 



terms of the Vegetable Kingdom. The female cell, or 

 egg, is relatively large and non-motile ; the male cell is 

 relatively small, and in more primitive forms it is motile 

 in water, but in more advanced Plants of the Land that 

 motility is finally lost. 



It thus appears that though very primitive organisms 

 may be sexless, there is a fundamental unity of the 

 method of sexual propagation in Plants, when once it 



Fie. 17. 



Fusion of male and female gametes of Lily. (After Mottier.) A, shows the 

 vermiform male nucleus applied to the egg-nucleus (Liliitin Martagon) ; B, shows 

 the egg-cell of Lilium candiduin with the two sexual nuclei fusing. The nuclear 

 membranes have disappeared at the place of contact. 



had become fully established. In certain of the rudi- 

 mentary examples of sex the gametes may be indistinguish- 

 able as male or female. From such simple beginnings 

 we have been able to trace the steps of differentiation of 

 sex. Indications of a gradual increase in size of the 

 female gamete, or ovum, and of its loss of motility have 

 been seen ; while the male spermatozoid remains relatively 

 minute and active. A reasonable biological explanation 

 of this has also been offered. A full sexual differentiation 

 of this nature was already attained in the more advanced 



