OF ALL ANIMAL TISSUES. 39 
with separate nuclei in most of the tissues, distinct from those 
which have cells around them, but we also find that the 
younger the cells are, the smaller they are in proportion to 
the nucleus. The ultimate destiny also of the nucleus is 
similar to that of the vegetable cells. As in the last named, 
so in most animal cells it is subsequently absorbed, and remains 
as a permanent structure in some few only. In plants, ac- 
cording to Schleiden, the young cells are always developed 
within parent cells, and we have also seen such a development of 
new cells within those already formed in the chorda dorsalis 
and cartilage. If, however, any doubt existed as to whether 
the primary cells of these tissues were formed within previously 
existing parent cells, none such can arise in reference to many 
of the tissues next to be considered. We shall indeed fre- 
quently meet with a formation of young cells within older 
ones, but it is not the rule, and does not occur at all with 
regard to many of them. 
The following admits of universal application to the forma- 
tion of cells; there is, in the first instance, a structureless! 
substance present, which is sometimes quite fluid, at others 
more or less gelatinous. This substance possesses within 
itself, in a greater or lesser measure according to its 
chemical qualities and the degree of its vitality, a capacity to 
occasion the production of cells. When this takes place the 
nucleus usually appears to be formed first, and then the cell 
around it. The formation of cells bears the same relation to 
organic nature that crystallization does to inorganic. The 
cell, when once formed, continues to grow by its own individual 
powers, but is at the same time directed by the influence of 
the entire organism in such manner, as the design of the 
whole requires. ‘This is the fundamental phenomenon of all 
animal and vegetable vegetation. It is alike equally consistent 
with those instances in which young cells are formed within 
parent cells, as with those in which the formation goes on 
' [Strukturlos.—I have ventured to translate this word as above, although I am 
aware it is open to objection. The idea intended to be conveyed by the author is 
that of a substance in which no definite structure can be detected. As the word 
will be frequently used in the following pages, the reader is requested to assign this 
signification to it invariably— TRANS. ] 
