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 composed 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, 



