THE VITAL PROPERTIES OF THE CELL 



173 



together, form by their coalescence extensive structures (Fig. 73), 

 which, especially in Worms and Arthropods, serve as a protection 

 to the whole surface of the body. This skin consists chiefly of 

 chitin, a substance which is only soluble in boiling sulphuric acid. 

 In its minute structure it very closely resembles cellulose mem- 

 branes, especially in its stratification, which indicates that growth 

 has taken place by the deposition of new lamellce upon the inner 

 surface of those already formed. 



Occasionally the old chitinous sheaths are ruptured and dis- 

 carded after they have developed beneath them a younger, more 

 delicate skin to take their place ; this process is termed sloughing. 

 Calcium salts may be deposited, by means of intussusception, in the 

 chitinous skin in order to strengthen it. 



Finally, intercellular substances are formed, when numerous 

 cells secrete from their entire surfaces solid substances, which, 

 however, do not remain isolated as in cell membranes, bat which 

 coalesce to form a coherent mass, it being 

 impossible to recognise from which cells 

 the various portions of it originated (Fig. 

 74). Thus, in tissues with intercellular 

 substance, the individual cells cannot be 

 separated from one another, as they can be 

 in plant tissue. In the continuous ground- 

 substance, which may consist of very differ- 

 ent chemical substances (mucin, chondrin, 

 glutin, ossein, elastin, tunicin, chitin, etc.), 

 and which further may be either homo- 

 geneous or fibrous, small spaces are present, 

 w r hich contain the protoplasmic bodies. 

 Now, since the area of intercellular sub- 

 stance in the neighbourhood of the cell 

 space is controlled to a considerable extent 

 by the protoplasmic bodies it contains, it 

 has been called by Virchow (I. 33) a cell territory. Such a cell 

 territory, however, is of necessity not marked off from neighbour- 



.m. 



Wl!& 



I 



^ 



Ca2> 



-Wi) -I -J 



<jg>< 



Fig. 74 Cartilage (after 

 Gegenbaur) : c superficial 

 layer ; b intermediate layer 

 passing into a, typical car- 

 tilage. 



mg ones. 



Amongst the cell products, which may be classed as external or 

 internal according to their position, the muscle and nerve fibres 

 must be mentioned. Being composed of protein substance, they 

 come next after protoplasm in the consideration of the substances 

 of which tissues are composed ; they must be classed with the 



