STRUCTURAL BASIS OF THE HUMAN BODY. 



15 



It has been already mentioned that in animal cells an envelope or cell- 

 wall is by no means always present. In adult vegetable cells, on the 

 other hand, a well-defined cellulose wall is highly characteristic; this, it 

 should be observed, is non-nitrogenous, and thus differs chemically as 

 well as structurally from the contained mass. 



Moreover, in vegetable cells (Fig. 8, B), the protoplastic contents of 

 tli3 cell fall into two subdivisions: (1) a continuous film which lines the 

 interior of the cellulose wall; and (2) a reticulate mass containing the 



FIG. 8. (A). Young vegetable cells, showing cell-cavity entirely filled with granular protoplasm 

 enclosing a large oval nucleus, with one or more nucleoli. 



(B.) Older cells from the same plant, showing distinct cellulose- wall and vacuolation of proto- 

 plasm. 



nucleus and occupying the cell-cavity; its interstices are filled with fluid. 

 In young vegetable cells such a distinction does not exist; a finely gran- 

 ular protoplasm occupies the whole cell-cavity (Fig. 8, A). 



Another striking difference is the frequent presence of a large quan- 

 tity of intercellular substance in animal tissues, while in vegetables it is 

 comparatively rare, the requisite consistency being given to their tissues 

 by the tough cellulose walls, often thickened by deposits of lignin. In 

 animal cells this end is attained by the deposition of lime-salts in a matrix 

 of intercellular substance, as in the process of ossification. 



Forms of Cells. Starting with the spherical or spheroidal (Fig. 9, a) 

 as the typical form assumed by a free cell, we find this altered to a poly- 

 hedral shape when the pressure on the cells in all directions is nearly the 

 same (Fig. 9, b). 



Of this, the primitive segmentation-cells may afford an example. 



The discoid shape is seen in blood-cells (Fig. 9, c), and the scale-like 



FIG. 9. Various forms of cells, a. Spheroidal, showing nucleus and nucleolus. 6. Polyhedral. 

 c. Discoidal (blood-cells), d. Scaly or squamous (epithelial cells). 



form in superficial epithelial cells (Fig. 9, d). Some cells have a jagged 

 outline (prickle-cells) (Fig. 13). 



Cylindrical, conical, or prismatic cells occur in the deeper layers of 

 laminated epithelium, and the simple cylindrical epithelium of the intes- 

 tino and many gland ducts. Such cells may taper off at one or both 



