. The cell; outlines of general anatomy and physiology. y with whichmaterials are taken up and given out. This internal increase ofsurface may be compared with the external increase of surface,which is shown in the formation of many-branched pseudopodia(Fig. 10), and indeed it answers the same purpose. In animal-cells, on the contrary, the formation of vacuoles andthe secretion of sap only take place extremely rarely, for instance,in notochordal cells; on the other hand, adventitious substances,such as glycogen, mucin, fat globules, albuminous substances, etc., ITS CHEMICO-PHYSICAL AND MORPHOL
. The cell; outlines of general anatomy and physiology. y with whichmaterials are taken up and given out. This internal increase ofsurface may be compared with the external increase of surface,which is shown in the formation of many-branched pseudopodia(Fig. 10), and indeed it answers the same purpose. In animal-cells, on the contrary, the formation of vacuoles andthe secretion of sap only take place extremely rarely, for instance,in notochordal cells; on the other hand, adventitious substances,such as glycogen, mucin, fat globules, albuminous substances, etc., ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 35 are more frequently found; these either distend the cell or renderit somewhat solid. When there has been a considerable develop-ment of such substances, the protoplasm may again assume afrothy appearance, as in Actinosphoerium (Fig. 15), or it maybecome transformed into a network structure, as in a Tradescantiacell (Fig. 13), the only difference being that the interstices arefilled with substances denser than sap. K<i di n Nacv. nM Fig. 15.—Actinosphcerium Eichhomi (after R. Hertwig, Zoologie, Fig. 117): M medullarysubstance, with nuclei (n) ; E peripheral substance, with contractile vacuoles {cv); Nanutrient substances. The most perfect examples are often seen in animal egg-cellsThe exceedingly large size, wrhich is attained by many of these, isnot so much caused by an increase of protoplasm, as by the storingup of reserve materials, which vary very much as to their chemicalcomposition, being sometimes formed and sometimes unformedsubstances, and which are intended for future use in the economyof the cell. Very often the egg-cell appears to be almost entirelycomposed of such substances. The protoplasm only fills up thesmall spaces between them, like the mortar between the stones of 36 THE CELL a piece of masonry (Fig. 16) ; if a section be made of an egg, theprotoplasm is seen to be present in the form of a delicate net-work, in the larger and smaller
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