OF ALL ANIMAL TISSUES.
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, according 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 frequently 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 [1] 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. ]
