THE DEVELOPMENT OF VERTEBRATA. 285 



plasm of the body, we recognise that the reproductive 

 cells are distinct from, the rest of the body, and that 

 when they are separated from the parent they give rise 

 to new individuals. In most cases two germ cells unite 

 in fertilisation before they are capable of development, and 

 therefore they are called gametes. 



In these gametes, and, as we have seen, probably especi- 

 ally in the chromosomes of their nuclei, are properties 

 which give rise to the peculiarities or "characters" of 

 the individuals developed from them. The individuals 

 may to some extent be modified by the conditions of life 

 and by functional exercise, but we can usually distinguish 

 many characters which are congenital, or due only to the 

 properties of the gametes. When such characters are 

 carried on from generation to generation they are said 

 to be inherited, or developed by heredity, but when a 

 new character appears it is a variation. 



8. Distinction between Soma and Gametocytes. It 

 is admitted by all biologists that the gametes in a given 

 adult organism are derived by a continuous succession of 

 cell divisions from the fertilised ovum or zygote from 

 which the organism was itself developed. In the early 

 stages of development the embryo consists of a number 

 of embryonic cells of which none can be distinguished 

 as reproductive cells. At a certain stage of development 

 some of the cells become distinct as reproductive cells, the 

 rest forming the soma or body. 



9. Special Characters of Reproductive Cells or 

 Gametocytes. It may perhaps be urged that the repro- 

 ductive organs are differentiated in the same way as other 

 organs, such as the brain or the liver, and that the cells 

 of the other organs are capable of multiplication and 

 growth as well as the reproductive cells. In the lower 

 animals and in plants the organs are not so definitely 

 differentiated, and continuous growth takes place in 

 various parts of the body, producing shoots, buds, etc. ; 

 also parts other than reproductive cells separated from 

 the body may produce new individuals. 



