180 SURVEY OF CELL-LIFE. 
formation of the cell-contents frequently follows, giving rise to 
a formation of new products in the cell-cavity. In most of 
the hollow cell-nuclei, the contents become paler, less granu- 
lous, and in some of them fat-globules, &c., are formed. (See 
pages 173, 4.) We may therefore say that the formation of cells 
is but a repetition around the nucleus of the same process by 
which the nucleus was formed around the nucleolus, the only 
difference being that the process is more intense and complete 
in the formation of cells than in that of nuclei. 
According to the foregoing, then, the whole process of the 
formation of a cell consists in this, that a small corpuscle (the 
nucleolus) is the earliest formation, that a stratum (the nucleus) 
is first deposited around it, and then subsequently a second 
stratum (substance of the cell) around this again. The sepa- 
rate strata grow by the reception of new molecules between 
the existing ones, by intussusception, and we have here an illus- 
tration of the law, in deference to which the deposition takes 
place more vigorously in the external part of each stratum than 
it does in the internal, and more vigorously in the entire ex- 
ternal stratum than in the internal. In obedience to this law 
it often happens that only the external part of each stratum 
becomes condensed into a membrane (membrane of the nucleus 
and membrane of the cell), and the external stratum becomes 
more perfectly developed to form a cell, than the nucleus does. 
When the nucleoli are hollow, which, according to Schleiden, 
is the case in some instances in plants, perhaps a threefold 
process of the kind takes place, so that the cell-membrane 
forms the third, the nucleus the second, and the nucleolus the 
first stratum. Probably merely a single stratum is formed 
around an immeasurably small corpuscle in the case of those 
cells which have no nuclei. 
Varieties in the development of the cells in different tissues. 
Although, as we have just seen, the formative process of the 
cells is essentially the same throughout, and dependent upon a 
formation of one or many strata, and upon a growth of those 
strata by mtussusception, the changes, on the other hand, which 
the cells, when once formed, undergo in the different tissues, 
are, in their phenomena at least, much more varied. They 
may be arranged in two classes according as the individuality 
