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The Nucleus in some Unicellular Organisms. 
HE 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 1831 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 
