178 SURVEY OF CELL-LIFE. 
vesicle ere it occurs; the germinal vesicle, for example; in 
others, and this is the most common, the nucleus is still solid, 
and its development into a vesicle does not take place until a 
later period, or perhaps the change never occurs at all. When 
the cell is developed, the nucleus either remains stationary at 
its previous stage of development, or its growth proceeds, but 
not in proportion to the expansion of the cell, so that the 
intermediate space between it and the cell-membrane, the cell- 
cavity, is also constantly becoming relatively larger. If the 
growth of a cell is impeded by the neighbouring cells, or if 
the new molecules added between the existing particles of 
the cell-membrane are applied to the thickening of the 
cell-wall instead of to its expansion, it may occur that the 
nucleus becomes more vigorously expanded than the cell, and 
gradually fills a larger portion of the cell-cavity. An example 
of this was brought forward at page 23, from the branchial 
cartilages of the tadpole; on the whole, however, such instances 
are very rare. As the nuclei, in the course of their develop- 
ment, and especially in such instances as that just mentioned, 
continually lose their granulous contents and become pellucid, 
and as in some cases, the germinal vesicle for example, other 
corpuscles, such as fat-globules, &c., may be developed in these 
contents of the nucleus (a circumstance which never occurs 
with respect to the cell-cavities) it is often difficult to distin- 
guish such enlarged nuclei from young cells. The presence 
of two nucleoli is often sufficient to enable us to distinguish 
such an enlarged hollow nucleus. The observation of the 
stages of transition, between the characteristic form of the 
cell-nucleus and these nuclei which so much resemble cells, 
will also aid us in obtaining the information desired. As in 
the case of the germinal vesicle, however, a positive decision 
can only be obtained by demonstrating that such a nucleus 
has precisely the same relation to the cell that an ordinary 
cell-nucleus has; that is to say, that such a nucleus is formed 
before the cell, that the latter is formed as a stratum around 
it, and that the nucleus is afterwards surrounded by the cell. 
Whether the nucleus undergoes any further development, as 
the expansion of the cell proceeds, or not, the usual result is 
that it becomes absorbed. ‘This does not take place, however, 
