22 THE CELL 



the body is deposited. Eggs likewise contain an abundant supply of proteid, 

 lecithin and fat which are to serve as nourishment for the developing embryo. 



Other inclosures which occur in greater or less abundance in animal cells, 

 are : fat droplets in the cells of the mammary glands and of the intestinal mucosa 

 during absorption; pigment granules in the pigment cells of the skin and of 

 the choroid coat in the eye; glycogen granules in the liver cells, etc. In the 

 naked cells which are able to take up solid particles from the surrounding 

 medium we observe often small Algae, Bacteria, Infusoria and the like (Figs. 

 19, 20, 21 and 22), which serve as nourishment for the cell, and, after digestion 

 is completed, indigestible shells, skeletons, envelopes, etc. Again small cavities 

 filled with fluid (vacuoles) are present in the protoplasm of certain animal cells. 

 Among these are to be mentioned especially the so-called contractile vacuoles, 

 i. e., drops of fluid which are pressed out of the protoplasm by the contraction 

 of a surrounding sheath, only to be re-collected from the protoplasm in the same 

 place again when the contraction ceases (Figs. 21, 24, 28). 



Finally, there are found within the protoplasm of animal cells certain plant 

 cells, Algse, which do not serve their host as nutritive material, but merely live 

 in company with it (symbiosis}. They are of the greatest importance to the 

 life of the animal cell in which they occur, since through the activity of their 

 chlorophyll bodies, they supply it with the necessary oxygen, thereby rendering 

 it independent of the oxygen contained in the surrounding medium. We have 

 the most beautiful instance of symbiosis in a lichen, which is nothing more than 

 an aggregate individual consisting of a fungus and an alga. 



Reference must be made to works on cytology and histology for a discussion 

 of the ultimate structure of the protoplasm, nucleus and centrosome, as well as 

 for the changes in these accompanying cell division. 



2. THE VITAL PHENOMENA OF CELLS 

 A. INTRODUCTORY SURVEY 



The vital activity of all cells, both plant and animal, consists of two oppo- 

 site processes, assimilation and dissimilation. We include under assimilation 

 all the synthetic processes, of whatever kind, going on in the cell or under 

 its influence; under dissimilation all the disintegration processes going on in 

 the cell or under its influence. 



A. Assimilation is of two kinds, namely growth of protoplasm, i. e., for- 

 mation of living substance, and syntheses of new substances not living. 



Our knowledge of the growth of protoplasm is still very meager. We 

 can observe how the cell increases in size, and how it multiplies after it has 

 reached a certain size, but the inner mechanism of these processes is still quite 

 obscure. Somewhat more satisfactory is our knowledge of the syntheses of 

 organic nonliving substances accomplished by cells. In fact the synthesis 

 which is quite the most important of all, namely, the formation 'of starch in 

 the green parts of plants, is known with tolerable exactness, and may be 

 described briefly as follows: 



There always occur in the neighborhood of the nucleus of plant cells small, 

 colorless, highly refractive bodies, for the most part oval or elliptical in form, 

 which are called trophoblasts and which arise always, like the nucleus and the 



