3 66 THE POPULAR SCIENCE MONTHLY. 



that is, they contain a central kernel, the nucleus, which is more dense 

 than the surrounding protoplasm, and of a slightly different chemical 

 composition. These bodies, which circulate among the tissues with 

 the blood forming a part of it, manifest independent movements, 

 thrusting out and drawing in portions of their mass in true amoeboid 

 fashion ; they devour solid particles of matter which come in their 

 way their smaller comrades, the red corpuscles, not always escaping 

 their voracity. This they do by flowing around and inclosing them, 

 as already described. They have also been observed by Klein to mul- 

 tiply by division, like the monera. The white blood-corpuscle, identi- 

 cal, apparently, with the amoeba, which may be found in the standing 

 water of pools attached to the surface of leaves, and in many other 

 similar situations, is a true cell the morphological unit from which 

 all organisms, whether low or high, originate, and by whose multipli- 

 cation, development, and differentiation, all the tissues of their bodies 

 are produced. 



The history of the growth and development of every animal 

 whether moner, mollusk, or man is a history of cell-multiplication 

 and cell-differentiation ; and the most highly endowed individual of 

 them all possesses no property, no faculty, no power, which is not at 

 last foreshadowed in the formless, structureless, protoplasmic cell from 

 which they are all alike derived. Is the nervous tissue of man in the 

 highest degree irritable and automatic that is, sensitive and self-act- 

 ing ? So is protoplasm, though in an almost infinitely less degree. Is 

 muscular tissue eminently contractile, serving for the production of 

 the varied and complicated movements of all parts of the body ? 

 Protoplasm is also capable of slight spontaneous motions of its entire 

 mass. Are the various glands of the body actively secretory and ex- 

 cretory ? So is protoplasm within the narrow limits of its chemical 

 necessities. Equally, also, with the highest tissues and organisms, it 

 reproduces its kind. 



The complex body of any one of the higher animals may, then, be 

 considered as consisting of certain tissues, each of which has not only 

 been derived from protoplasm but each of which corresponds, in its 

 perfected function, to some one of the fundamental properties of pro- 

 toplasm, to the special manifestation of which it is devoted for the 

 benefit of the organism as a whole, on the important principle first 

 spoken of by Milne-Edwards as the physiological division of labor. 

 Division of labor among the tissues, however, as among the members 

 of a community, has its limits. While every tissue has some leading 

 quality, some special function, developed to the highest degree in the 

 interests of the organism as a whole (contraction in muscle-tissue, se- 

 cretion in gland-tissue, and so on), yet each tissue retains in its own 

 private interests, as it were, vestiges of all the other protoplasmic prop- 

 erties belonging to their common ancestor. Hence, all the tissues are 

 assimilative to the extent of keeping up their own nutrition ; all are 



