286 ZOOLOGY. 



All this is perfectly true, and must be included in a 

 complete general view of reproduction and heredity; but 

 at the same time the reproductive cells, especially in the 

 higher animals, have this peculiarity that they are set 

 free from the parent before they develop into new indi- 

 viduals or at any rate they begin to develop as single 

 cells not as pieces of tissue, that they retain the primitive 

 undifferentiated condition of embryonic cells and are not 

 formed from the latter by special changes, and that they 

 are the only cells in the body which are capable of the 

 rejuvenating process known as conjugation or fertilisation. 



It is very important to have a clear conception of this 

 process of development in relation to the problems of 

 reproduction and evolution. Development is in all cases 

 the formation of a large mass of cells by the division or 

 " segmentation " of the original ovum. The organs and 

 tissues of the body are formed by the differentiation of 

 groups of these cells. How this differentiation is brought 

 about, how one group is caused to form the brain, another 

 the muscles, and so on, is a much debated question which 

 we have not space to discuss here. The important point 

 for our present purpose is that all the cells are not used in 

 the formation of the body, but a certain group of cells 

 remain undifferentiated, retaining all the properties of the 

 original ovum from which they are descended : these are 

 the reproductive cells or gametocytes, which multiply by 

 cell division but always remain undifferentiated primitive 

 cells, and ultimately give rise to the gametes which pro- 

 duce the next generation. We have thus the important 

 modern conception of the distinction between soma com- 

 posed of somatic cells on the one hand, and the reproductive 

 cells or gametocytes contained in the reproductive organs 

 on the other. 



The gametocytes then are not produced by the soma but 

 have only a fraternal relation with the somatic cells, and 

 the developmental properties of the ovum of a frog, for 

 example, are not derived from that frog, but are due to 

 the persistence or continuation of the properties of the 

 ovum from which both the soma and the gametocytes of 

 the frog are descended. The relation of the individuals 



