4 SEX AND HEREDITY 



sexual cells, which are developed for the purpose. This 

 fusion is called " Syngamy." 



In very simple sexually produced organisms, whether 

 Animals or Plants, these parental cells are both alike. 

 They are then called Isogametes, and in most primitive 

 organisms they are motile through water. Such gametes 

 are believed to represent a rudimentary and primitive 

 state. A more advanced type is that where the two are 

 unlike. The difference is first seen in size, and this is 

 usually followed also by difference in motility. The 

 smaller is as a rule more actively motile, and it is called 

 the MALE GAMETE, or micro- gamete, or spermatozoon, or 

 spermatozoid. The larger is as a rule non-motile, and is 

 called the FEMALE GAMETE, or macro-gamete, or ovum, that 

 is the egg. All these terms are equally applicable to the 

 phenomena shown in Animals and in Plants, and they 

 will be used in the descriptions which follow (Fig. 2). 



The most obvious result of sex, as seen in the higher 

 Animals and in many of the higher Plants, appears to be 

 an increase in the number of individuals. In horses, in 

 cattle, and in Man this is the only method of propagation. 

 The same is the case with some Plants, such as the Pines 

 and Firs, and certain large brown Seaweeds. But while 

 this is true for the most advanced Animals, and for some 

 highly organised Plants, it does not apply for organisms 

 lower in the scale. In these, whether Animals or Plants, 

 there are other methods of increase in numbers, for 

 instance by various types of budding. Such budding is 

 very common indeed among the Highest Plants. It 

 would then be a mistake to think that for living things 

 generally multiplication is only by a sexual process, or 

 that multiplication and sexuality are necessarily con- 

 nected. A study of the lower organisms shows clearly 



