SEXUAL REPRODUCTION 225 



in the egg, and that the fusion of the two 

 nuclei is an essential part of the whole 

 process. 



The result of fertilisation is invariably to 

 start a series of chemical changes in the egg. 

 The first of these changes commonly results 

 in the secretion of a membrane over its outer 

 surface, and then a period of quiescence 

 usually intervenes before any further visible 

 development begins. After the lapse of a 

 certain time, which may vary within rather 

 wide limits, the fertilised egg commences to 

 "develop." 



The lines along which development proceeds 

 differ greatly in different groups of plants. 

 In the simpler ones, such as Chlamydomonas, 

 no apparent growth takes place, but the 

 zygote divides, giving rise to a number of 

 separate cells which escape as zoospores from 

 the zygote membrane, and finally grow into 

 as many different individuals. A somewhat 

 similar course is pursued by many other algae, 

 but in some of them the production of motile 

 zoospores is postponed until after an embryo, 

 composed of a larger or smaller number of cells, 

 has been formed. 



In the higher plants, from the mosses 

 upwards, the zygote gives rise to a plant 

 quite unlike that from which the gametes 

 were produced. This plant forms repro- 

 ductive bodies known as spores, and when 

 the spores in their turn germinate, they 

 give rise to another very dissimilar cellular 



