HISTOLOGY. 17 



The study of Anatomy and Development has brought to 

 fht certain generalizations of wide applicability and great 

 importance. 



1. It has been said that the great majority of living beings 

 present a very definite structure. Unassisted vision and or- 

 dinary dissection suffice to separate the body of any of the 

 higher animals, or plants, into fabrics of different sorts, which 

 always present the same general arrangement in the same 

 organism, but are combined in different ways in different 

 organisms. The discrimination of these comparatively few 

 fabrics, or tissues,, of which organisms are composed, was the 

 first step toward that ultimate analysis of visible structure 

 which has become possible only by the recent perfection of 

 microscopes and of methods of preparation. 



Histology, which embodies the results of this analysis, 

 shows that every tissue of a plant is composed of more or less 

 modified structural elements, each of which is termed a cell ; 

 which cell, in its simplest condition, is merely a spheroidal 

 mass of protoplasm, surrounded by a coat or sac — the cell- 

 wall — wdrich contains cellulose. In the various tissues, these 

 cells may undergo innumerable modifications of form — the 

 protoplasm may become differentiated into a nucleus with its 

 nucleolus, a primordial utricle, and a cavity filled with a wa- 

 tery fluid, and the cell-wall may be variously altered in com- 

 position or in structure, or may coalesce with others. But, 

 however extensive these changes may be, the fact that the 

 tissues are made up of morphologically distinct units — the 

 cells — remains patent. And, if any doubt could exist on the 

 subject, it would be removed by the study of development, 

 which proves that every plant commences its existence as a 

 simple cell, identical in its fundamental characters with the less 

 modified of those cells of wmich the whole body is composed. 



But it is not necessary to the morphological unit of the 

 plant that it should be always provided with a cell-wall. Cer- 

 tain plants, such as JProtococcus, spend longer or shorter peri- 

 ods of their existence in the condition of a mere spheroid of 

 protoplasm, devoid of any cellulose wall, while, at other times, 

 the protoplasmic body becomes inclosed within a cell-wall, fab- 

 ricated by its superficial layer. 



Therefore, just as the nucleus, the primordial utricle, and 

 the central fluid, are no essential constituents of the morpho- 

 logical unit of the plant, but represent results of its meta- 

 morphosis, so the cell wall is equally unessential ; and either 

 the term " cell " must acquire a merely technical significance 



