270 THE CELL-THEORY 



the next of a substance which swells up in water {b) ; and the outer- 

 most of a different, but not exactly defined, substance («). We may 

 call one of these portions " cell-membrane," and another intercellular 

 substance, but they are assuredly all nothing but differentiated portions 

 of one and the same periplast. 



Woody tissue presents precisely the same phenomena, the inner 

 layers of the periplastic substance having, very generally, a different 

 composition from the outer. 



Morphologically, we have already noticed the lamination of the 

 periplastic substance, and we may mention its fibrillation, a process 

 which takes place almost invariably in the inner layers of the periplast, 

 .and to which the well-known spirality of the so-called secondary 

 deposits must be referred ; but a more important process for our; 

 present purpose is what we have called Vacuolation ; the development 

 of cavities in the periplastic substance independent of the endoplast, 

 and which, to distinguish them from the cells, may conveniently be 

 termed Vacuoles. In the youngest vegetable tissues there are no such 

 cavities, the periplastic substance forming a continuous solid whole ; 

 and it is by this vacuolation, which occurs as the part grows older, 

 that all the intercellular passages are formed, and that many cells 

 ■obtain that spurious anatomical independence to which we have 

 adverted above. The exaggerated development of the vacuolse in the 

 pith of the rush converts the periplastic substance, with its proper 

 endoplastic cavities, into regular stellate cells. (Fig. i, B.) 



Sufficient has been said to illustrate the differentiation of the 

 primitive vegetable structure into its most complex forms. If we turn 

 to the animal tissues, we shall find the same simple principles amply 

 sufficient to account for all their varieties. 



In the plant, as we have seen, there are but two histological 

 elements — the periplastic substance and the endoplasts, cell-wall and 

 intercellular substance, being merely names for differentiated portions 

 of the former ; cell-contents, on the other hand, representing a part of 

 the latter. In the animal, on the other hand, if we are to put faith in 

 the present nomenclature, we find cell-wall, intercellular substance, and 

 cell-contents, forming primitive elements of the tissues, and entering 

 into their composition as such : there have been no small disputes 

 whether the collagenous portion of connective tissue is intercellular 

 substance or cell-wall, the elastic element being pretty generally 

 admitted to be developed from distinct cells. Again, it appears to 

 be usual to consider the fibrillae of striped muscle as modified cell- 

 contents, while the sarcolemma represents the cell-walls. The hyaline 

 substance of cartilage is asserted by some to be cell-wall, by some to 



