All cells originate or are produced in the same way. Either they are developed free in vegetable or animal fluids, or they are produced in the interior of preceding cells. In all casea they originate in con nection with a substance called protein, which exists in cells, either in the form of a small dark spot called a nucleus, or cytoblast, in the interior of which is a nucleolus, or of an expansion on the interior of the cell, when it is called the primordial utricle. Free cell-development nas been observed to take place in plants, in saccharine and other 1iquids about to undergo the fermentation process, and amongst animals in the chyle, blood, and lymph. The exact mode of the development of cells under these circumstances has not been accurately observed, and the particlea or granules of proteinaceous matter from which they are supposed to originate have not yet been proved to have had their origin independent of other cells. The most common form of cell-development is that in which the cell grows around or from the nucleus or primordial utricle. In the animal kingdom the development of the cell more frequently takes place around the nucleus, whilst in the vegetable kingdom its origin is more frequent from the folding in or contraction of the primordial utricle upon itself, by which means two cells originate in one.
Besides the development of cells around the nucleus and round the investing membrane, or primordial utricle, within the walls of the cell, a multiplication of cells frequently takes place by division of the whole cell. This takes place in many of the lower forms of animals and plants [Puorozoe], and also in the red blood-corpuscles of the embryoea of birds and mammalia, and in the colourless blood-corpuscles of the tadpole. It is probable that further observation will extend our knowledge of this mode of cell-multiplication.
One of the highest problems for the physiology of the present day to solve is, the efficient causes of the phenomena of cell-development. The following propositions have been laid down by Kolliker as an attempt to follow up Schwann's idea of the anslogy between chemical changes in inorganic bodies and those which occur in cells : 1. The nucleus of the cell arises in the first place as a precipitate in an organisable fluid, and afterwards becomes consolidated in such a manner that a special investment and contents with a nucleolus appear. Its development may in this case be compared to that of inorganic precipitates, yet the constantly globular figure and size of the nuclei which are just formed, indicate some essential though not yet recognised condition peculiar to them.
2. In the development of cells by division the cell-nucleua plays exactly the same part which was previously ascribed to the nucleolus, and the occurrence of the formation of cells in this manner demonstrates that chemical conditions are not necessarily concerned therein.
3. In cell-development around portions of contents, and in the cleavage process, the nuclei also operate as simple centres of attrac tion upon a certain mass of blastema, and then follows the formation of a membrane upon the surface of this mass, which is moat simply understood as a condensation of the-blastema.
4. In the cell-development directly around the nucleus the invest ment with blastema is wanting, and the nucleus develops the mem brane immediately around itself.
From what has been previously said, it will be seen that the cells are the active seat of the functions of both animals and plants, and the most conspicuous results of organisation takes place in conse quence of their agency. They not only constitute the mass of the
body, but by their agency alone all the special secretions and products of individual plants and animals are formed. The food is conveyed into the body by cella, the blood of animals is charged with cells, and the functions of locomotion and sensation are carried on by the agency of cells. Nor are these last functions peculiar to the animal kingdom. Contractility and sensibility seem to be the property of the substance (protein) of which the nucleus and primordial utricle are composed. To this substance Mr. Huxley proposes to give the name Endoplast, and thus concludes a lecture on the identity of structure of plants and animals :— " In both plants and animals then there is one histological element, the Endoplast, which does nothing but grow and vegetatively repeat itself ; the other element, the periplastic substance (the cell membrane) being the subject of all the chemical and morphological metamorphosea, in consequence of which specific tissues arise. The differences between the two kingdoms are, mainly, 1, that in the plant the Endoplast grows, and, as the primordial utricle, attains a large comparative size ; while in the animal the Eudoplaat remains small, the principal bulk of its tissues being formed by the periplastic substance ; and, 2, in the nature of the chemical changes which take place in the periplastic substance in each case. This distinction however does not always hold good, the Ascidians furnishing examples of animals whose peri plastic substance contains cellulose.
"The plant then is an animal confined in a wooden case, and nature, like Sycorax, holds thousands of delicate Ariels ' imprisoned within every oak. She is jealous of letting us know this ; and among the higher and more conspicuous forms of plants reveals it only by such obscure manifestations as the shrinking of the Sensitive Plant, the sudden clasp of the Diana; or, still more slightly, by the pheno mena of the Cyclosis. But among the immense variety of creatures which belong to the invisible world she allows more liberty to her Dryads ; and the Protococci, the Volrox, and indeed all the Alga, are during one period of their existence as active as Inaimals of a like grade in the scale. True, they are doomed eventually to shut them selves up within their wooden cages and remain quiescent ; but in this respect they are no worse off than the Polype, or the oyster even." For further information on the subject of Cells, see the articles ANIMAL KINGDOM; BLOOD; HISTOLOGY; BOTANY; CILIA; TISSUES, VEGETABLE; TISSUES, ANIMAL.
(Sharpey, in Quain's Elements of Anatomy ; Kfilliker, Handbook of Human Histology, translated for the Sydenham Society by Huxley and Busk ; Carpenter, Manual of Human Physiology ; Principles of Phy siology; Mohl, On the Vegetable Cell, translated by Henfrey; Schleiden, Principles of Scientific Botany, translated by Lankester ; Schleiden, On Plcytogertesis ; Schwan, On the Identity of Structure in Plants and Animals, translated by H. Smith for the Sydeuham Society ; Quekett, Lectures on Histology; Hassell, Microscopic Anatomy of the human Body ; Todd and Bowman The Physiological Anatomy and Physiology of Man ; Quekett, Catalogue of the Histological Series in the Museum of the Royal College of Surgeons, London ; Quarterly Journal of Micro scopical Science; and Transactions of Microscopical Society, vol. i.)