The Protoplasmic Theory of Life

Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: 14 CELL THEORY BEFORE 1860. This may be given up to the period of its full development by Dr. Lionel Beale, and after that a review of the present state of knowledge upon these, in some respects, rival theories. This method will, I think, conduce to clearness of understanding the subject, better than the...
More Description

Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: 14 CELL THEORY BEFORE 1860. This may be given up to the period of its full development by Dr. Lionel Beale, and after that a review of the present state of knowledge upon these, in some respects, rival theories. This method will, I think, conduce to clearness of understanding the subject, better than the strictly chronological method in which both are mingled together. Schleiden, who was the founder of the cell theory, though by him restricted to plants, defines the vegetable cell as the elementary organ which constitutes the sole essential form-element of all plants, and withoutwhich a plant cannot exist; and as consisting, when fully developed, of a cell wall composed of cellulose, lined with a semi-fluid, nitrogenous coating. With him, therefore, the cell consisted of two parts, viz., a vesicle and semifluid contents. In plants the cell forms are distinct, and easily recognized, and thus, when the conception of a similar elementary organ was extended to the animal kingdom by Schwann, in 1838, it is not to be wondered at that the cellular form was expected to be universal. Schwann added to Schleiden's two elements a third? the nucleus?which he deemed also of essential importance, and to be present in all cells, if not always, at least in some stage of their existence. On his authority this threefold doctrine of the cell became universally prevalent for a time. I give here Schwann's original definition of his theory, as some points in it have been overlooked or forgotten in the mass of controversial writing this subject has provoked: ? The following admits of universal application to the formation of cells: ?There is, in the first instance, a structureless substance present, -which is sometimes quite fluid, at others SCHLEIDEN, SCHWANN, HENLE. 15 more or less g...