ORGANIC FORM AND ARCHITECTURE 705 



spindle") or from centrosome to equator (the "equatorial spindle"). 

 In this arrangement there is a curious likeness to that of iron filings 

 round a magnet or arranging themselves on a plate imdcr electrical 

 influence. Then the cell constricts and separates the two poles from 

 one another, thus soon forming two daughter-cells, each with its 

 normal number of chromosomes, and each with a centrosome. 

 The chromosomes gradually sink back into the resting network 

 state, and there is a restoration of the apparently simple siait4s quo 

 after a complex and meticulously precise process of division. 



TYPES OF CELLS. — Many of the Protozoa are so highly organised 

 that it seems clearer to speak of them as non-cellular rather than 

 as single cells; but the Amoeba at least is so simple that it may 

 fairly be regarded as the most generalised type of cell, displaying 

 within itself all the great functional properties, e.g. of contractility, 

 irritability, secretion, growth, reproduction. 



Casually examined, the Amoeba 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 

 Amoeba. 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 highlj^ organised cell (or cell-equivalent) of the 

 Protozoa) and these specialisations of cells deserve to be illustrated. 



In the blood of mammals the Amoeba-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 pig- 

 ment 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 

 alimentary 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 formmg special substances 

 (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 shape surrounding 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 

 unstriped muscle, which are elongated spmdles — with an oval central 



VOL. I zz 



