GENERAL HISTOLOGY 



63 



mononucleated cells, and in consequence of this are sometimes regarded 

 as the equivalent of one cell, sometimes as equivalent to many cells, and 

 are called sometimes multinucleated giant-cells, sometimes syncytia. In 

 the following pages a multinucleated mass of protoplasm will he consid- 

 ered as a single cell, because a cell is a vital unit, has a physiological 

 individuality, and in this respect a multinucleated mass of protoplasm 

 behaves like a mononucleated. As the tissue cells and the Protozoa show, 

 the plane of organization is not raised in the least by the multinuclearity. 

 A change only begins at the moment when many particles of protoplasm 

 are separated from one another, and many vital units are formed, i.e., 

 when in place of multinuclearity a true multicellularity appears. 



II. THE TISSUES OF THE ANIMAL BODY. 



Definition of Tissue. In the formation of tissues two processes are 

 operative: (i) the multiplication of cells into cell-complexes, and (2) 

 the histological differentiation of cells. A tissue, 

 therefore, can be denned as a complex of differen- 

 tiated cells histologically similar. 



Histological Differentiation. The chief result 

 of the histological differentiation is that the cells 

 have a definite form and definite relations to neigh- 

 boring cells. In addition, there almost always 

 occurs, as a more important feature, the histological 

 modification of the cell. The fact has already been 

 mentioned that the cell uses its food-material, no', 

 only for its own growth, for increase of its proto- 

 plasm, but also for forming substances, plasmic 



products, either in its interior (internal plasmic procl- 



- , 11- FIG. 2v Forma- 



ucts), or more often on its surface (external plasmic tion of muscle t - lbri i s 



products). The histological differentiation is the in the frog (diagram). 



._ , . ,, a, formative cell; b, 



formation of specific plasmic products. A cell in f ormat } ve cell with 



becoming a muscle fibre (fig. 25), continually secretes 

 upon its surface new fibrillae of specific muscle sub- 

 stance until finally the remnant of the formative cell, 

 the 'muscle corpuscle,' is contained in a mantle of 

 muscle fibrillae. In the same way, each tissue, upon histological ex- 

 amination, is seen to be composed of cells and plasmic products. The 

 former control the formation, the renewal, and the sustenance of the 

 tissue; the latter are the agents of its physiological function. The 

 advantages of tissue formation are far-reaching, since in general they 

 are connected with division of labor. So long as the cell unites in itself 



t\vo transversely stri- 

 ated muscle fibrils; 

 <-, formative cell with 

 numerous muscle 



fibrils. 



