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 alge, 
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 
D 
