ANIMAL CELLS. 



983 



in some way, from aggregations of protoplasm, but still, it is submit- 

 ted, always under the influence of preformed nuclei or nucleoli. In 

 the so-called exogenous development, rupture of the wall of the old 

 cell occurs, the nuclei escape, and a new cell is developed from each. 

 (Bennett.) The endogenous mode of formation consists in the increase 

 in size of the old cell, a cleavage of the nucleus into two, and the de- 

 velopment of two cells within the original cell-wall. 



Animal cells undergo various changes in size and form (Fig. 122). 

 Their nuclei, for example, may increase in size and alter in shape, 

 but not to such an extent as the cells themselves ; in many cells they 

 become obscure ; and in some cells, as in those of elastic tissue, they 

 finally disappear. Their envelopes may expand and burst, or they 

 may shrivel up and dry. The. cells may dissolve or disappear, or be- 



Fig. 122. 



Fig. 122. Forms of animal nucleated cells, a, flat and round cell of red and white blood corpuscle. 6, 

 ramified cell of gray nervous matter, c, cells multiplying by binary division of nucleus and cell, and 

 embedded in a solid matrix of cartilage, d, cells of areolar connective tissue, showing tbvir splitting into 

 fibrillse. e, elastic fibre cells. /, ramified pigment cell, g, muscular fibres, h, capillary vessel. 



come the seat of special deposits. Sometimes the substance of a cell 

 undergoes but little apparent change ; but usually, their contents are 

 modified according to the tissue which they form, or the secretion 

 which they prepare, , b. Thus, some cells, as those of bone, dentine, 

 and enamel, become loaded with earthy matter ; others, as the red 

 corpuscles, with iron in combination, and coloring substance ; others, 

 as those of adipose tissue, with fat ; some with mucus or horny mat- 

 ter. Their relations to each other may undergo many modifications. 

 In the fluids of the body, as e. g., the blood, the cells remain separate. 

 In the solid tissues, they may be simply connected together by a 

 minute quantity of intercellular substance, as in the epithelium and 

 epidermis. Sometimes they elongate, and form fibres in various ways, 

 d ; or, after lengthening, they may subsequently join with similar cells, 

 and give rise to the formation of tubes. The septa between these may 

 be absorbed, and the coalesced and arborescent branches of different 

 hollowed cells may lead to the production of capillary, A, or lym- 

 phatic plexuses. Moreover, animal cells seetn to exercise a singular 

 power over the intercellular substance, blastema, or matrix, which 

 often appears very large in quantity, in proportion to the cells them- 

 selves, as, for example, in cartilage, c. This matrix may be watery, 

 soft and gelatinous, firmer and tenacious, still more solid and hyaline, 

 or hard and opaque. It presents great varieties in chemical compo- 



