206 ORIGIN OF SEX AND OF THE SOMA 



for illustration because of the existence of several forms 

 which make up a closely connected series ; and mid- 

 way in this series Pleodorina shows the actual first 

 appearance of the soma, P. illinoiensis being a very 

 slight modification of the Eudorina type. 



The vast majority of multicellular animals have a 

 well-marked soma, i.e. a body consisting of tissues whose 

 cells are not germ cells and do not reproduce the 

 species by spore or gamete formation ; but in many 

 of the lower invertebrates new individuals are produced 

 by budding of these somatic tissues which are not too 

 highly specialised for particular vegetative functions. 

 In the higher animals this process falls into abeyance, 

 and the life of the somatic tissues is strictly limited to 

 the service of the individual and comes to an end 

 with the life of the individual. 



In plants, however, the power of reproducing the 

 species is much more often retained by the cells of 

 the vegetative body. In the filamentous green algae, 

 for instance, referred to in the preceding section, all, 

 or nearly all, the cells of the multicellular thread of 

 which the body is composed retain the power of form- 

 ing zoospores and gametes, and thus may become 

 " germ cells." In the bulky algae (seaweeds) with 

 massive tissues, as well as in all the higher plants, this 

 power is lost by the general body cells, but the plant 

 may be reproduced " vegetatively " by budding, as in 

 many of the lower invertebrate animals, new plants 

 being thus formed apart from the germ cells proper. 

 This power of " vegetative reproduction " is retained 

 by some at least of the body cells of practically all 

 plants, even the highest and most complicated forms, 

 as we shall see in later chapters. 



Taken together these facts show us that the dis- 



