36 THE TISSUES OF PLANTS 



lusca. At first these parts consist of cells more or less 

 closely connected together, either by a general enveloping 

 membrane, or by an intercellular substance which holds 

 them together by its adhesive properties. 



Fig. 4. 





a- i C cC 



Pig. 4 represents a portion of one of the animal layers 

 included between the calcareous laminae of a bivalve shell ; 

 in which are shown at a, nuclei forming in the midst of a 

 plastic fluid prepared and elaborated by the cells of a pre- 

 vious generation; h, the same advanced to the condition of 

 incipient cells ; c, the cells more developed but still sur- 

 rounded by the fluid ; d, the cells in close contact with 

 each other, and rendered polygonal by mutual pressure. 



These last cells in the enamel of the teeth attract phos- 

 phate of lime into their cavities, whilst those which form 

 the shelly covering of the moUusca become filled with cal- 

 careous matter. The walls of the cells now disappear and 

 there is a coalescence of their cavities, so that the solid mass 

 appears altogether homogeneous, retaining not a single 

 trace of its cellular origin. 



In some cases, however, the cellular character of the 

 tissue is maintained throughout the; life of the animal. 

 Thus, what is commonly called fat, consisl^ of a mass of 



