DIFFERENTIATION OF STRUCTURE 25 



certain of the cells are set aside for reproduction and 

 others for supplying food. 



It is just possible that from some such group of cells 

 as Volvox, nature passed from the simple uni-cellular 

 form, where each cell was practically as good as its neigh- 

 bour, to the multi-cellular stage, where each cell or set 

 of cells was wholly dependent for its well-being on the 

 close connection with other cells. It is easy to imagine 

 how this may have come about, and the formation of 

 a body or soma been reached. 



In the lowest forms of multi-cellular organisms, we 

 find already a definite distribution of the work that 

 must be performed for the good of the organism as a 

 whole. Certain cells are formed into an outer or pro- 

 tective layer, and others into an inner layer, the main 

 function of which is nutritive, and the simpler sponges 

 supply beautiful examples of this. The structure of 

 these simple forms becomes still more complicated, for 

 example in Hydra, where we find that special cells in 

 the outer layer are set aside for such purposes as defence, 

 or it may be for locomotion, or even for the transmission 

 of stimuli. Moreover in Hydra, and even in the sponges, 

 there is a gradual development of an amorphous layer 

 between the outer and the inner layers, and the gradual 

 development of this means that the other cells are being 

 placed usually in a position less suitable for carrying on 

 the duties detailed to them. 



The increasing complexity of structure is apparently 

 a matter of necessity. As specialisation increases, means 

 must be taken to insure that the various types of cells 

 receive a suitable supply of nourishment, and so the 

 organs of locomotion and of digestion develop ; and as 



