DIFFERENTIATION OF STRUCTURE 23 



Similarly, an examination of the green seaweed 

 Enteromorpha shows us that while the body is in the 

 main composed of a vast number of more or less 

 similar green cells, some at the base of the tubular Entero- 

 body are colourless and concerned with the fixing morpha. 

 of the organism to the substratum, while some of 

 the general body-cells are capable of taking on 

 reproductive functions. The contents of these 

 latter cells divide into minute ovoid bodies, each 

 provided with four cilia and each capable of pro- 

 ducing a new Enteromorpha. 



Differentiation of structure thus appears in forms 

 of quite low r grade, but it is only in plants and ani- 

 mals of much higher rank that we meet with complete 

 illustrations of the principle hinted at in Hydra and 

 in Enteromorpha. In plants like a rose or an elm 

 and in animals like a frog or a rabbit we have to mffer- 

 deal with organisms composed of countless myriads ^u!ti- ed 

 of cells, some more intimately connected with the cellular 

 duty of nutrition, even, it may be, with that of isms."" 

 forming one special secretion to be used in the 

 digestive process, some simply protective, as in the 

 case of the external cells of the body, some purely 

 supporting, as those which form the skeleton, some 

 having for their function the elimination of excreta 

 or waste materials or the formation of reproductive 

 cells, and so on, and in every case the form and 

 structure of the cells in question are specially adapted 

 for the carrying out of the functions allotted to them. 

 Moreover, we find that many cells of the same kind 

 are associated together to form tissues. Thus we 

 have in the animal, epidermal tissue, muscular or 

 contractile tissue, glandular tissue, nervous tissue, Tissues, 

 and so on, and in the plant also glandular tissue, 

 conductive tissue, epidermal tissue, &c. &c. Lastly, 



