176 SURVEY OF CELL-LIFE. 
molecules between the particles of this membrane takes place 
in such a manner that more molecules are deposited between 
those particles which le side by side upon its surface than 
there are between those which lie one beneath another in its 
thickness, the expansion of the membrane must proceed more 
vigorously than its increase in thickness, and therefore a con- 
stantly imereasing space must be formed between it and the 
nucleolus, whereby the latter remains adherent to one side of 
its internal surface. 
I have made no observations on the formation of nuclei with 
more than one nucleolus. But it is easy to comprehend 
how it may occur, if we conceive that two nucleoli may le 
so close together that the layers which form around them 
become united before they are defined externally, and that by 
the progressive deposition of new molecules, the external limi- 
tation is so effected that two corpuscles are enclosed by it at 
the same time, and then the development proceeds as though 
only one nucleolus were present. 
When the nucleus has reached a certain stage of develop- 
ment, the cell is formed around it. The following appears to 
be the process by which this takes place. A stratum of sub- 
stance, which differs from the cytoblastema, is deposited upon 
the exterior of the nucleus. (See pl. III, fig. 1, d.) In the 
first instance this stratum is not sharply defined externally, 
but becomes so in consequence of the progressive deposition 
of new molecules. The stratum is more or less thick, some- 
times homogeneous, sometimes granulous ; the latter is most fre- 
quently the case in the thick strata which occur in the forma- 
tion of the majority of animal cells. We cannot at this period 
distinguish a cell-cavity and cell-wall. The deposition of new 
molecules between those already existing proceeds, however, 
and is so effected that when the stratum is thin, the entire 
layer—and when it is thick, only the external portion—he- 
comes gradually consolidated into a membrane. ‘The external 
portion of the layer may begin to become consolidated soon 
after it is defined on the outside; but, generally, the membrane 
does not become perceptible until a later period, when it is 
thicker and more defined internally ; many cells, however, do 
not exhibit any appearance of the formation of a cell-mem- 
brane, but they seem to be solid, and all that can be remarked 
