" SOMA " AND " GERM CELLS " 185 



of the growth process : it is discontinuous growth, as 

 we saw in Chapter V, because it is growth conditioned 

 by the separation of a part or parts of the individual 

 to form new individuals. The protoplasm of the 

 individual organism continues to exist and to increase 

 in bulk, but it can only increase beyond a certain 

 limit of size if it separates to form two or more new 

 individuals. Under favourable conditions of life this 

 process continues indefinitely, so that the protoplasm 

 of which the organism is composed is immortal in the 

 sense that it need never die so long as the conditions 

 remain favourable to its continued life, though it 

 dies of course as soon as the conditions become suffi- 

 ciently unfavourable. But in the higher organisms — 

 in the great majority of multicellular animals and 

 plants-^the functions of nutrition and growth are, as 

 we know very well, separated from the function of 

 reproduction. The feeding and growing body does 

 in most cases regularly die whether the general condi- 

 tions of Ufe continue favourable or not. It dies, as 

 we say, of old age : it is a soma or " mortal " body. 

 On the other hand, the reproductive cells (germ cells) 

 which it produces grow, under certain conditions, into 

 new individuals of the species. The organisms we are 

 now going to consider will help us to understand how 

 this separation of functions came to arise. 



Chlamydomonas. — The organisms belonging to this 

 genus are very common in pools, rain-water tanks, 

 etc. Each is a minute green cell (Fig. 20, a), oval or 

 sometimes rather oblong in shape, with a basin-shaped 

 chloroplast occupying the hinder end of the cell, con- 

 taining usually one conspicuous pyrenoid, and enclosing 

 central colourless cytoplasm containing the spherical 

 nucleus. The front end of the cell also consists of 



