We have now to retrace our steps and look somewhat more closely into the discovery of that important body, the _cell-nucleus_. It was an English botanist, Dr. Robert Brown, who, in 1833, during his microscopical studies of the epidermis of orchids, discovered in their cells “an opaque spot,” to which soon afterwards he gave the name of _nucleus_. Schleiden and Schwann’s later researches led them to the conclusion that the nucleus is the most characteristic formative element in all vegetable and animal tissues in the incipient phase of existence. It then began to be taught that there is one universal principle of development for the elementary parts of all organisms, however different, and that is the formation of cells. Thus was enunciated a doctrine which was for all practical purposes absolutely new, and which opened out a wide field of further investigation for the physiologist, and led up to a fuller knowledge of the cell contents. In fact, it became a question as to whether the cell contents rather than the enclosing wall should not be considered the basis of life, since the cell at this time had by no means lost its importance, although it no longer signified the minute cavity it did when originally discovered by Schwann. It now implied, as Schultz defined it, “a small mass of viscid matter, protoplasm, endowed with the attributes of life.” The nucleus was once more restored to its original importance, and with even greater significance. In place of being a structure generated _de novo_ from non-cellular substance, and disappearing as soon as its function of cell formation is accomplished, the nucleus is now known as the central permanent feature of every cell, and indestructible while the cell lives, and the parent, by division of its substance, of other generations of nuclei and cells. The word _cell_ has at the same time received its final definition as “a small mass of protoplasm supplied with a nucleus.” In short, all the activities of plant and animal life are really the product of energy liberated solely through _histolysis_, or destructive processes, amounting to the combustion that takes place in the ultimate cells of the organisms. But there are other points of especial interest involved in the question of cell formation beside those already mentioned. The cell and its contents collectively are termed the _endoplasm_, or when coloured, as in algæ, _endochrome_. With regard to the outer layer of the cell and its growth nothing satisfactory has been clearly determined and finally accepted.