STRUCTURE OF CELLS. 



existence as a natural structure is (as already remarked, p. x) at best but 

 doubtful. 



Fig. VII. Fig. VIIT. 



I - 



Fig. VII. DIAGRAM OP VEGETABLE CELLS. 



a, a', cellulose wall ; 6, cell-contents ; c, c', nucleus ; d } contents shrunk away from the 

 cell-wall. 



Fig. VIII. DIAGRAM OF ANIMAL CELLS (OP CARTILAGE). 



a, a', capsule ; 6, cell-contents ; c, nucleus ; d y cell-contents shrunk away from the 

 capsule. 



Certain animal cells have a structure conformable throughout with that 

 just indicated as belonging to vegetable cells. Thus, the cartilage-cell has 

 an outer envelope or capsule (fig. vin., a, a'), not indeed composed of cellulose 

 but of animal matter, comparable nevertheless with the vegetable cellulose- 

 wall, and in like manner continuous with the intercellular substance. The 

 cell-contents (6), nucleus (c), and nucleolus or nucleoli, correspond. Finally, 

 it has been held that there is here also a fine interior envelope correspond- 

 ing to the primordial utricle of the vegetable cell, and it may be added that 

 the agreement extends to the doubt entertained of the reality of such a 

 membrane in either case. 



But many animal cells have no exterior capsule answering to the cellulose 

 vegetable cell- wall, and accordingly represent only the parts of the vegetable 

 cell which are within that enclosure ; that is, the nucleus and the mass of 

 cell-contents. The chyle-, lymph-, and pale blood-corpuscles, the mucus- 

 and salivary corpuscles, and the cells that first appear in the embryo, may 

 be adduced as examples. Some cells of this description have or acquire a 

 fine membranous envelope closely investing their surface, and corresponding 

 to the supposed primordial utricle ; but many of them appear to be naked, 

 and to consist of a nucleus surrounded by a mass of soft matter the so- 

 called cell-contents often containing solid granules which are held together 

 by a tenacious substance. In these cases the matter at the surface is more 

 condensed than in the interior, where not unfrequently it is more or less 

 fluid. There is moreover a well-defined superficial outline, but there is 

 wanting an internal contour-line marking oif an envelope distinct from the 

 contained substance. Such bodies, it is true, may sometimes show appear- 

 ances indicating the presence of an envelope, when they are treated with 

 certain chemical re-agents ; but in such cases the apparent envelope may be 

 produced by the action of the substances employed. At the same time, it 

 must be borne in mind that various cells which in their young state have no 

 envelope, distinctly acquire one at a later stage of their existence ; and 

 this occurrence is especially to be observed in cells which are destined to 

 form consistent structures, such as epidermis and epithelium. In these 



