378 LIFE : OUTLINES OF GENERAL BIOLOGY 



of oxygen to combine with at least a part of the food materials; 

 and excretion, the discharge of the products of the reactions. Thus 

 there are six great properties maintained by three great processes; 

 and in the Ama'ba there is a balance between them. There is an 

 absence of over-emphasis on any one phase to the exclusion of the 

 rest ; and this means that the one-celled individual is physiologically 

 complete in itself to a degree that cannot be said of most of the 

 cells composing a multicellular organism. 



Casually examined, the Amceba is a blob of viscid protoplasm of 

 irregular and changing shape, containing numerous dark granules, 

 clear spaces or vacuoles, and a central body of more definite shape 

 and solid appearance, the nucleus. Very similar in appearance are 

 the leucocytes or white blood cells of the Vertebrates; indeed, these 

 are hardly less independent or less complete in themselves than the 

 Amaba. But specialisation is the dominant note in the cells of the 

 Metazoa (just as specialisation for particular conditions of life is 

 seen in the more highly organised Protozoa), and these specialisa- 

 tions of cells deserve to be noticed. 



In the blood of mammals the Amotba-like white cells are far out- 

 numbered by the red blood corpuscles, small disc-like cells whose 

 chief function — that of carrying oxygen combined with the pigment 

 with which they are highly charged — quite dominates over the 

 other activities of the cell, so that there is not even a nucleus. 

 There may be five million of these cells in a cubic millimetre of blood. 

 Less specialised are the cells of epithelial tissues, which are found as 

 coverings in the body, in part externally, but also lining the ali- 

 mentary canal, the air passages, the blood-vessels, and so on. Their 

 chief adaptation is that of presenting at the surface a compact 

 pavement, which may, in certain cases, be ciliated. Or again, the 

 cells may be glandular, with the power of forming special sub- 

 stances (such as digestive juices), which they pour out into the 

 cavity they surround. Much more specialised are the nerve-cells. 

 These have a central portion of very variable size and shap)e sur- 

 rounding the large nucleus, and a series of fine processes, often also 

 several short branching connections to neighbouring cells, and 

 finally a single nerve-fibre of very great length, along which the 

 efferent or outgoing nervous impulse travels. Then there are the 

 cells of smooth or unstripcd muscle, which are -elongated spindles, 

 an oval central part with the nucleus and extending from this in 

 opposite directions the fibre-like portions with the power of con- 

 traction. Striped or skeletal muscle, again, is still more highly 

 specialised; the fibres may be over an inch long and angular in 

 section; close to the surface may be seen the nuclei, of which there 

 may be even many hundreds in a single fibre, though each fibre 

 behaves as a single cell. In the connective tissues, which include 

 bone, cartilage, and fat, there is usually a light scaffolding of 



