44 MORPHOLOGY OF THE CELL. 



finally become detached, leaving the two ends of the new thread united with the rest 

 of the protoplasm ; they do not grow up as branches with one free extremity. 

 (Hanstein, A c. p. 221.) Some of the threads also disappear, the two ends remaining in 

 connexion with the rest of the protoplasm and coalescing with it. The threads form, 

 with the central mass of protoplasm which contains the nucleus and the layer which 

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

 with respect to one another. 



Besides these displacements — in consequence of which the parietal protoplasm accu- 

 mulates or diminishes at any one spot, and the mass of protoplasm in the cell-cavity 

 which contains the nucleus wanders about, and alters accordingly the grouping and form 

 of the threads — under high magnifying power another form of movement comes into 

 view, which is undoubtedly connected with the former, although the exact mode is 

 unknown. In the parietal protoplasm and in the mass which contains the nucleus, 

 but most distinctly in the threads, the minute granules — generally of chlorophyll- 

 are to be seen in a ^streaming' motion, 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 motion is also ap- 

 parently increased five hundred-fold. Within even a very slender thread, the granules 

 near one another not unfrequently flow in opposite directions. Chlorophyll-granules 

 often appear to be in motion on the surface of slender threads ; it may nevertheless 

 be assumed with certainty that they also are enclosed in the substance of the thread ; 

 but, being very prominent, are covered by only a very thin lamella of it. 



Those movements of protoplasm which produce changes in the internal grouping 

 of the protoplasm of the cell may be compared to the displacements of the mass 

 which, in the case of naked Amoebae, change the external contour, and cause its 

 creeping motion. In the case of circulating protoplasm, the firm cell-wall hinders the 

 change of contour, as well as the change of place of the whole mass ; but the large 

 sap-cavity allows of internal changes of position of larger or smaller portions. 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 Muscineae and Vascular 

 plants, but more often from Thallophytes, is a product of difl:erentiation of the proto- 

 plasm, is sufficiently evident, not only from its chemical behaviour [njide supra, under a), 

 but also from its participation in the processes of cell-formation (see Sect. 3). On 

 the other hand, it must be borne in mind that, once formed, it constitutes a character- 

 istically organised 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 pro- 

 toplasm-mass; subsequently its surface becomes firmer without taking the form of a 

 special skin ; in the interior there appear usually two or three (sometimes more) large 

 granules, called Nucleoli, which, however, are often wanting. The nucleus has, at the 

 time of its origin, generally already attained its permanent size, or nearly so ; its growth 

 is never proportional to that of the cell; while in young tissue-cells (Fig. r, p. 2) it 

 usually occupies a large portion of the cell-cavity, in mature cells its mass is progressively 

 smaller in proportion to that of the whole cell. Its further development consists 

 in its obtaining a firmer outer layer, and in the formation of small vacuoles and 

 nucleoli ; only rarely does it grow for a longer time ; its substance may become 

 frothy from the further formation of vacuoles, and sometimes it exhibits a circulation 

 in the interior of the firmer enveloping layer, as in a cell ^ The nucleus always 

 remains enclosed in the substance of the protoplasm ; if this latter forms vacuoles, or 

 developes the circulation already described, the nucleus remains enveloped in a coating 



^ In young hairs of Hyoscyajnus fiiger, according to A Weiss in the Sitzungsberichte der kais, 

 Akademie der Wissenschaften zu Wien, vol. LIV, July 1866. 



