BODY-CELLS AND GERM-CELLS 121 



lodgement within it,'and, if successful, discomfort, disease, 

 or death, is almost invariably caused.^ The larger 

 enemies, or parasites, have been known for ages ; while 

 the smaller, but far more dangerous foes, the germs of 

 disease, have only been appreciated in comparatively 

 recent times. Now, however, they attract a very large 

 amount of attention, and the germ theory of disease has 

 led to the most fruitful advance ever made in the history 

 of medicine and surgery. 



The cells, or units, which compose the body of one of 

 the higher animals differ greatly in structure according 

 to the part they play in the economy of the organism. 

 Thus, in man, the upper skin, or epidermis, is composed 

 of layers of cells, becoming horny scales on the surface. 

 The epidermic cells are continuous with those lining the 

 digestive tract and passing up the ducts into the various 

 glandular organs. The connective tissues which bind 

 the various structures together and make up many parts, 

 such as tendons and the lower skin or dermis, are also 

 composed of cells and fibrous elements derived from 

 cells. The supporting tissues, bone and cartilage, are 

 composed of cells and structures derived from cells ; and 

 the same is true of the great contractile tissues, striped 

 and unstriped muscular fibre, and of the elements of the 

 nervous system — nerve-cells and nerve-fibres. Out of 

 many of these elements the complex organs are built 

 up, with the addition of peculiar or specific cells of their 

 own. 



All the varied units which compose the Metazoan body 

 may be classified under two chief heads. There are the 

 cells which are concerned with maintaining the life of the 

 individual — the body-cells or somatic cells ; and there are 

 those concerned with maintaining the life of the species 

 — the reproductive cells or germ-cells. 



In the higher animals, the latter are aggregated in 



* An example of benefit rather than injury may be found in the lactic 

 acid bacillus which, according to Metschnikoff, plays a valuable part in 

 digestion. Metschnikoflf believes that the custom of drinking sour milk, 

 common to many races, is founded on an empirical knowledge of the 

 beneficial effects wrought by this bacillus, 



