But, with one or two individual
exceptions, as plants and animals evolve, the union of two elements,
male and female, is needed to start the amazingly complex process of
building a new individual. Thus in flowers the stamens, the pollen
bearers, provide the male element which, through the intermediary of
the pistils, fertilizes the egg in the vesicle. In the higher animals
the egg or ovum is produced by the female, and is fertilized by the
sperm-cell produced by the male. The necessary union between these two
essential elements is attained in various ways. Thus the female salmon
deposits her eggs on a convenient spot in the bed of a stream and the
attendant male salmon then projects over them the spermatozoa. In the
higher animals there is a further development, and special organs are
evolved to ensure the conjunction of the two elements. I have not space
to describe in detail the effect of this union of the two cells,
generally spoken of as fertilization. It may be found fully recorded
step by step in any biological manual. Very briefly, the sperm-cells,
which are active, freely moving units, swarm round the egg-cell and one
of them eventually enters it.
Pages:
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236