SURVEY OF CELL-LIFE. Lae 
is that the external portion of the layer is somewhat more 
compact. 
Immediately that the cell-membrane has become consoli- 
dated, 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 mole- 
cules 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 inter- 
space between the cell-membrane and cell-nucleus is at the 
same time filled with fluid, and this constitutes the cell-con- 
tents. 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 por- 
tion 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 le 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 les 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 
12 
