64 PERMANENT TISSUES OF 
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 mem- 
brane, 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 Soripehed of cells, we are now required to prove, that 
the tissues do not only originate from cells in this general man- 
ner, 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 transforma- 
tions. 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 al- 
ready (in the Introduction) seen such changes in plants, for 
example, 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 con- 
nected 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 de- 
gree of development at which the cells must arrive, in order 
to form a tissue. The complete retention, or relinquishment, 
