SECOND DIVISION.
Permanent Tissues of the Animal Body.
The foregoing investigation having taught us that the entire ovum, from its first origin up to that period at which, by the formation of the serous and mucous layers of the germinal membrane, the foundation of all the subsequent tissues is laid, exhibits simply a continual formation and more extended development of cells, and having found the primordial substance of the tissues itself to be composed of cells, we are now required to prove, that the tissues do not only originate from cells in this general manner, but that the special basis of each individual tissue is a matter composed of cells, and that all tissues either consist entirely of or are formed from cells which pass through a variety of transformations. These modifications, which some of the cells undergo in the progress of their development to the subsequent tissues, are very important, since thereby the cells not infrequently cease to exist as separate independent structures. We have already (in the Introduction) seen such changes in plants, forexample, in the coalescence of the cell-walls observed by Schleiden in the bark of the Cacti, and the blending of several cells to form a tube in the spiral and lactiferous vessels. This takes place to a much greater extent in animals, and, in general, the higher the importance of a tissue is, the more do the cells lose their individuality. We shall not, however, enumerate these modifications here; we shall become acquainted with them as the result of investigation of the separate tissues, and, at the conclusion of the work, we shall combine them into a connected representation of Cell-life. It is necessary, however, to mention the most important of them at least preliminarily in this place, in order to make a classification of the tissues.
Since all organic structure is primarily formed from cells, the most scientific classification of general anatomy would manifestly be one founded upon the more or less high degree of development at which the cells must arrive, in order to form a tissue. The complete retention, or relinquishment,
