130 SIMPLE HISTOLOGICAL ELEMENTS. 



2. The fatty degeneration may pass into the pigmentary ; which 

 may also occur spontaneously. Here the coloring matter generally 

 passes from deep yellow to brownish black. 



3. Dropsy of cells occurs if the blood contains an undue amount 

 of water. (Wedl) 



4. Crystals form in cells from the absorption of the watery por- 

 tion of the cell-contents. 



5. Atrophy or involution (Wedl) of cells may occur, in conse- 

 quence of a diminished supply of nutrient fluid. 



PBIMOKDIAL CELLS. 



Schwann discovered that all the tissues of animals, 1 as well as of 

 plants, are developed originally from nucleated cells ; and to these 

 the name of primordial cells has been given. They present nothing 

 peculiar in their microscopic appearance, however ; containing the 

 five elements already mentioned as usually characterizing a cell (p. 

 114). We find in the embryo a mass of cells, for instance, which 

 are to develop bone; another to form muscle; a third, fibrous tissue, 

 &c. But though the microscope does not enable us to detect any 

 original difference in them, their vital properties must differ, as the 

 developmental result demonstrates. 



Schwann's assertion, however, applies only to the tissues proper, 

 and not to the simple histological elements already described. 

 Simple fibre and simple membrane are, for example, lower develop- 

 ments than cells, and are not formed from the latter. On the con- 

 trary, cells have their walls formed of simple membrane, as has 

 been shown. 



The primordial cells, therefore, need no further notice here, since 

 it is in respect to their functions, and not as mere histological ele- 

 ments, that they are peculiar. Besides, the manner in which each 

 tissue is developed from its primordial cells, and the peculiarities of 

 the latter in each case, will be explained in the division of this work 

 upon the " Tissues proper." 



Before proceeding to speak of the fluids, however, certain cells 

 which are either found isolated, or, at least, do not coalesce to form 

 tissues, will be described. 



1 This discovery was announced in 1839. Schleiden had previously shown that 

 the tissues of plants are formed from cells. 



