44 MORPHOLOGF OF THE CELL, 



of the \Ahole protoplasmic body, undergoes further distortions and entire change of 

 form. But new strings also make their appearance ; ridge-like portions arise from the 

 peripheral plasma, or even on the thicker strings, and finally become detached in such a 

 way that the two ends of the new string remain united with the rest of the protoplasmic 

 substance ; they do not grow up as branches with one free extremity. (Hanstein, /. c. 

 p. 2 2 1.) In the same manner threads disappear; both ends, remaining in connexion 

 with the rest of the protoplasmic body, coalesce with it. The strings form, together 

 \vith the central masses of protoplasm which contain the nucleus and those which clothe 

 the cell-wall, a connected system, single portions of which may change their position with 

 respect to one another. 



Besides these displacements of large portions of the protoplasm of a cell endowed 

 with circulation— in consequence of which the parietal protoplasm at any one spot 

 accumulates or diminishes, and the mass of protoplasm in the cell-cavity which con- 

 tains the nucleus wanders about, and alters the grouping and form of the strings to 

 correspond to its own — under high magnifying power another form of movement 

 then comes in view, which is undoubtedly of the same origin, although the exact 

 mode is unknown. In the parietal plasma, in the mass which contains the nucleus, 

 but most distinctly in the strings, the very small granules interspersed among the pro- 

 toplasm, and generally also small grains of chlorophyll, are to be seen in ' streaming ' 

 movement, which under high magnifying power may even appear very rapid. It must 

 not, however, be overlooked that when the cell is magnified, say five hundred times, 

 the rapidity of the movement is also apparently increased five hundred-fold. Within 

 even a very thin string, the granules not unfrequently flow in opposite directions near 

 one another. Granules of chlorophyll often appear to be in motion on the surface of 

 thin strings; it may nevertheless be assumed with certainty that they also are enclosed 

 in the substance of the string, but, being very prominent, are covered by only a very 

 thin lamella of it. 



Those mass-movements of larger portions of protoplasm on which the various 

 internal grouping of the protoplasmic body of the cell depends, may be compared to 

 the displacements of the mass of the body which, in the case of naked Amoebae, change 

 the external contour, and cause its creeping motion ; in the case of circulating proto- 

 plasm the firm cell-wall hinders the external change of contour as well as the change 

 of place of the whole ; but the large internal sap-cavity allows of similar displacements 

 of larger portions in the interior. The ' streaming ' movement, which is visible by 

 means of the imbedded granules, occurs in the creeping naked protoplasm of the 

 Amoebae as well as in that enclosed in a cell-wall. 



(c) The Nucleus. That the nucleus, which is never absent from the Muscineae and 

 Vascular plants, but more often from the Thallophytes, presents itself as a product of 

 differentiation of the protoplasm, i. e. must be regarded as a formed portion of the pro- 

 toplasm itself, is sufficiently evident, not only from its chemical behaviour (vide supra, 

 under a), but also from its participation in the processes of cell-formation (cf. sect. 3); 

 and this need not be further demonstrated. On the other hand, it must be made clear 

 that, once formed, it constitutes a characteristically formed portion of the cell which, 

 to a certain extent, has a mode of development of its own. At first the nucleus is 

 always a homogeneous roundish body of protoplasmic substance ; subsequently its sur- 

 face becomes firmer without its taking the form of a special skin ; in the interior arise 

 usually two or three (sometimes more) larger granules, called Nucleoli, which, however, 

 are often entirely wanting. The nucleus has, at the time of its origin in the young cell, 

 usually already attained its permanent size, or nearly so ; its growth is never proportional 

 to that of the cell ; in young tissue-cells (Fig. i) it usually occupies a large portion of the 

 cell-cavity ; in fully grown cells its mass is increasingly small in proportion to that of 

 the whole cell. Usually a further development remains also in the sharper bounding by 

 a firmer outer layer and the formation of small vacuoli and nucleoli ; only rarely does 

 it grow for a longer time ; more vacuole-fluid collects in the interior ; its substance may 



