TISSUES 13 



Amoebae where before we had only one; and no doubt if they 

 have a good conceit of themselves, each thinks itself the mother 

 of the other. Division follows normally only on the growth of 

 an Amoeba to a certain size, which is larger than the average. 

 Well fed cultures divide more rapidly than poorly fed ones. 



Many different species of Amoeba have been recorded, and 

 their life-history, which is often more complex than that 

 described above, has been studied; but there is always a 

 "snag" somewhere, and many organisms that look like 

 Amoebae are simple amoeboid stages in the life-history of 

 higher organisms. Thus the freshwater Hydra has an egg which 

 is indistinguishable in structure from the typical Amoeba. Only 

 by breeding and rearing in cultures can the real nature of 

 these amoeboid organisms be determined. 



Tissues 



We have seen that the Amoeba is a single cell, and this single 

 cell can perform many functions. It can feed and breathe 

 and move and excrete and reproduce; but in higher multi- 

 cellular organisms these functions are assigned to, and carried 

 on by, cells especially adapted to these ends. In a primitive 

 state of society, amongst savages, each man is his own hunter, 

 fighter, butcher, baker, builder, gardener, and, in fact, uni- 

 versal provider; and with the exception that he cannot be his 

 own undertaker he manages to carry out all the functions 

 which in higher and more civilized societies become the work 

 of specialized or expert craftsmen. The latter is the case with 

 organisms that consist of many cells — the Metaphyta, multi- 

 cellular plants, and Metazoa, multi-cellular animals. There 

 are muscular cells which bring about movement, nerve cells 

 which are responsible for irritability and automatism, cells 

 for digestion, cells for breathing, and reproductive cells. 

 Accumulations of cells, similar in structure and in function 

 {i.e. doing the same things), are called tissues. Thus we have 

 storage-tissues in plants, muscular or nervous tissues in 

 animals and so on. The tissues are built up into organs such 

 as roots and leaves in plants, or the heart and brain in higher 

 animals, and the organs are again built up to form with others 

 the body of the plant or animal. 



