r 



The study of Anatomy and Development has brought to 



;-ln certain generalizations of wk'e applicability and great 

 importance. 



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

 present a very definite structure. ( H , 

 diua iv dissection suffice to separate tin- I. 

 higlu-r animals, or plants, into lahnc.s of different ft rto, \\lii.-h 

 always present the same general arrangenienl in : 

 organism, but are combined in dinYivnt \\a\s in dill- 

 organisms. The discrimination of these comparative! N 

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

 first step toward that ultimate analysis of visible stru- 

 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 "//- 

 wall which 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 ua- 

 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 tin- 

 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 i n< -e as a 



simple cell, identical in its fundamental characters with the less 

 modified of those cells of which the whole body is comp<ed. 



But it is not necessary to the morphological unit of the 

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

 tain plants, such as Protococcus, spend longer or shorter ; 

 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 



