The Nucleus in some Unicellular Organisms. 



THE importance of continuous and exhaustive studies concerning 

 the ultimate and irreducible nature, morphology, and function 

 of animal and vegetable cells is universally seen. The cellular 

 elements of tissues began to be observed even by Hooke two centuries 

 and a quarter ago ; and some further advances were made, but it was 

 not until 183 1 that Robert Brown, the great pioneer in botany, took 

 the first great step leading to a practical advancement of the subject. 

 He gave definite knowledge of vegetable cells, and he demonstrated 

 that the nucleus was one of its normal elements. 



It need hardly be remarked that the cell-theory proper had its 

 foundation in the work of Schleiden, but by him it was not extended 

 beyond the structure of plants ; he clearly defined the vegetable cell 

 as the elementary organ which constitutes the sole essential form- 

 element of all plants, and without which a plant cannot exist ; and 

 as consisting, when fully developed, " of a cell- wall composed of 

 cellulose, lined with a semi-fluid nitrogenous coating." 



The cell was thus to Schleiden a vesicle with semi-fluid 

 contents. 



In the year following (1839) Schwann showed that the Animal 

 Kingdom was ultimately as cellular in structure as the Vegetable ; 

 but to the vesicular wall and the semi-fluid lining of Schleiden he 

 added a third element, the nucleus, and he deemed this essential 

 to the history of the cell, at least in some period of its life. 



From this time, the triple elements of the cell were accepted as 

 its normal condition ; but investigation made the continuance of 

 this belief more and more uncertain. It was shown that cells multi- 

 plied by " budding," and that the nucleus underwent fission when the 

 cell divided ; and also that no cell could take origin save from a parent 

 cell. 



Soon it was demonstrated that there was no vital importance 

 attachable to the cell- wall ; and in 1857 Leydig declared it entirely 

 unessential, and defined the cell as a " soft substance enclosing a 

 nucleus." 



Subsequently, Max Schultze contended that the life of a cell 

 might be complete without a nucleus, but the cell was held to be the 

 ultimate morphological unit in which life was manifested. Every 



