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, /. 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 Amoebge, 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 Muscinese and Vascular 
plants, but more often from Thallophytes, is a product of diff'erentiation of the proto- 
plasm, is sufficiently evident, not only from its chemical behaviour {'vide 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 Hyoscymmis niger, according to A Weiss in the Sitzungsberichte der kais. 
Akademie der Wissenschaften zu Wien, vol. LIV, July 1866. 
