SEX 



299 



called fertilization. The essential thing about fertilization is 

 the uniting of two different nuclei into one. What the meaning 

 of this process is in the life of organisms we do not know 

 with certainty. We know some of the effects of the process, 

 and we can tell what conditions lead up to it in some cases. 



FIG. 131. Reproduction in rockweed, or bladder wrack 



a, expansions of the rockweed containing the gamete organs ; b, section of an egg-bearing 

 organ ; c, the large gamete, or egg, with large, distinct nucleus and food granules ; d, the 

 small gamete, or sperm, having the shape of a pear and bearing motile cilia. Sperms 

 swarm around an egg until one of them unites with the egg. After the conjugation the 

 zygote develops into a new individual 



349. Male and female. The gametes that are so unlike as 

 we have seen them to be in the rockweed are distinguished 

 by special names. The large gamete is sometimes called the 

 oo sphere, or egg cell. The small one is called the spermatozoid, 

 or the sperm cell. We sometimes distinguish the two gametes 

 by calling the large one the female and the small one the male. 



Most of the familiar plants and animals reproduce by means 

 of male and female gametes, forming zygotes. This kind of 

 reproduction is called sexual reproduction, in distinction from 

 reproduction by spores, which is called asexual ; that is, without 

 sex. There are many animals and very many plants that repro- 

 duce both sexually and asexually (see Chapter LXI). 



