SURVEY OF CELL-LIFE. 177
is that the external portion of the layer is somewhat more compact. |
Immediately that the cell-membrane has become consolidated, its expansion proceeds as the result of the progressive reception of new molecules between the existing ones, that is to say, by virtue of a growth by intussusception, while at the same time it becomes separated from the cell-nucleus. We may therefore conclude that the deposition of the new molecules takes place more vigorously between those which lie side by side upon the surface of the membrane, than it does between those which lie one upon another in its thickness. The interspace between the cell-membrane and cell-nucleus is at the same time filled with fluid, and this constitutes the cell-contents. During this expansion the nucleus remains attached to a spot on the internal surface of the cell-membrane. If the entire stratum, in which the formation of the cell commenced, have become consolidated into a cell-membrane, the nucleus must lie free upon the cell-wall; but if only the external portion of the stratum have become consolidated, the nucleus must remain surrounded by the internal part, and adherent to a spot upon the internal surface of the cell-membrane. It would seem that the portion of the stratum which remains may be disposed of in two ways: either it is dissolved and forms a part of the cell-contents, in which case the nucleus will lie free upon the cell-wall as before; or it gradually becomes condensed into a substance similar to the cell-membrane, and then the nucleus appears to lie in the thickness of the cell-wall. This explains the variety in the position of the nucleus with respect to the cell-membrane. According to Schleiden, it sometimes lies in the thickness of the membrane in plants, so that its internal surface, which is directed towards the cell-cavity, is covered by a lamella of the cell-wall. In animals it also sometimes appears to be slightly sunk in the cell-membrane; but I have never observed a lamella passing over its inner surface ; on the contrary, in almost all instances it hes quite free, adherent only to the internal surface of the cell-membrane.
The particular stage of development of the nucleus at which the cell commences to be formed around it varies very much. In some instances the nucleus has already become a distinct
