18 ZOOLOGY 



power of absorbing staining fluids such as carmine, hsema- 

 toxylin (which is an extract of logwood), &c. Now in very 

 small animals it sometimes happens that there is only one 

 nucleus present ; it is then possible to cut the body of such an 

 animal into two parts, one of which includes the nucleus and 

 the other does not. The part devoid of a nucleus can live for 

 some time and carry out movements, but it is unable to digest 

 or assimilate food, and in a short time it dies. We conclude, 

 therefore, that the production of the substances which melt 

 solid food (i.e. digestion), and the building up of the di- 

 gested food into new living material (i.e. assimilation), are 

 functions which can be carried out only by the aid of the 

 chromatin. If we apply the term protoplasm to the living 

 substance taken as a whole, and call the body in which 

 the chromatin is stored the nucleus, then the rest of the 

 living substance is termed cytoplasm. Broadly speaking, 

 the amount of nuclear material present bears a more or less 

 fixed proportion to the amount of cytoplasm surrounding it, 

 and it may take the form of a single-branched nucleus or 

 of a number of smaller rounded nuclei. Now there are some 

 animals so small that a single nucleus suffices for all their 

 needs ; but in most animals there are many nuclei, and in all 

 the higher groups of animals (termed collectively the Metazoa) 

 there is an area of cytoplasm surrounding each nucleus which is 

 marked off from the rest of the cytoplasm by a thin membrane. 

 Such an area is termed a cell, and when the living substance of 

 an animal is thus divided up it is said to possess cellular 

 structure. The cells may be compared to the bricks in the 

 wall of a house. Animals devoid of cellular structure are 

 termed Protozoa (lit. first-animals). Most of these are of 

 microscopic size, and very many have only one nucleus. Such 

 animals have been compared to a single cell of the body of 

 a higher animal and have been termed unicellular, but the 

 comparison is misleading. They are closely allied to other 

 Protozoa of slightly larger size and which possess numerous 

 nuclei, and it is quite wrong to compare the tiny sun-animal- 

 cule of our ditches with its two hundred nuclei to a single cell 

 of a higher animal. The proper term to apply to Protozoa is 

 non-cellular. A most distinguished American biologist has 

 recently shown what the meaning of the cellular structure is. 



