Morphology - Introduction: (1) Most Plants and Animals Are Aggregates of Cells
So far as living beings have a form and structure, they fall within the province of Anatomy and Histology, the latter being merely a name for that ultimate optical analysis of living structure which can be carried out only by the aid of the microscope.
And, in so far as the form and structure of any living being are not constant during the whole of its existence, but undergo a series of changes from the commencement of that existence to its end, living beings have a Development. The history of development is an account of the anatomy of a living being at the successive periods of its existence, and of the manner in which one anatomical stage passes into the next.
Finally, the systematic statement and generalization of the facts of Morphology, in such a manner as to arrange living beings in groups according to their degrees of likeness, is Taxonomy.
The study of Anatomy and Development has brought to light certain generalizations of wide applicability and great importance.
1. It has been said that the great majority of living, beings present a very definite structure. Unassisted vision and ordinary dissection suffice to separate the body of any of the higher animals, or plants, into fabrics of different sorts, which always present the same general arrangement in the organism, but are combined in different ways in different organisms. The discrimination of these comparatively few fabrics, or tissues, of which organisms are composed, was the first step towards that ultimate analysis of visible structure which has become possible only by the recent perfection of microscopes and of methods of preparation.
Histology, which embodies the results of this analysis, shows that every tissue of a plant is composed of more or less modified structural elements, each of which is termed a cell; which cell, in its simplest condition, is merely a spheroidal mass of protoplasm, surrounded by a coat or sac -- the cell wall -- which contains cellulose. In the various tissues, these cells may undergo innumerable modifications of form -- the protoplasm may become differentiated into a nucleus with its nucleolus, a primordial utricle, and a cavity filled with a watery fluid, and the cell-wall may be variously altered in composition or in structure, or may coalesce with others. But, however extensive these changes may be, the fact that the tissues are made up of morphologically distinct units -- the cells -- remains patent. And, it any doubt could exist on the subject, it would be removed by the study of development, which proves that every plant commences its existence as a simple cell, identical in its cells of which the whole body is composed.
But it is not necessary to the morphological unit of the plant that it should be always provided with a cell-wall. Certain plants, such as Protococcus, spend longer or shorter periods of their existence in the condition of a mere spheroid of protoplasm, devoid of any cellulose wall, while, at other times, the protoplasmic body becomes enclosed within a cell-wall, fabricated by its superficial layer.
Therefore, just as the nucleus, the primordial utricle, and the central fluid are no essential constituents of the morphological units of the plant, but represent results of its metamorphosis, so the cell-wall is equally unessential; and either the term "cell" must acquire a merely technical significance as the equivalent of morphological unit, or some new term must be invented to describe the latter. On the whole, it is probably least inconvenient to modify the sense of the word "cell".
The histological analysis of animal tissue has led to results and to difficulties of terminology of precisely the same character. In the higher animals, however, the modifications which the cells undergo are so extensive, that the fact that the tissues are, as in plants, resolvable into an aggregation of morphological units, could never have been established without the aid of the study of development, which proves that the animal, no less than the plant, commences its existence as a simple cell, fundamentally identical with the less modified cells which are found in the tissues of the adult.
Though the nucleus is very constant among animal cells, it is not universally present, and among the lowest forms of animal life, the protoplasmic mass which represents the morphological unit may be, as in the lowest plants, devoid of a nucleus. In the animal, the cell-wall, never has the character of a shut sac containing cellulose; and it is not a little difficult, in may cases, to say how much of the so called "cell-wall" of the animal cell answers to the "primordial utricle" and how much to the proper "cellulose cell-wall" of the vegetable cell. But it is certain that in the animal, as in the plant, neither cell-wall nor nucleus are essential constituents of the cell, in as much as bodies which are unquestionably the equivalents of cells -- true morphological units -- are mere masses of protoplasm, devoid alike of cell-wall and nucleus.
For the whole living world, then, it results: -- that the morphological unit -- the primary and fundamental form of life -- is merely an individual mass of protoplasm, in which no further may present but little advance on this structure; and that all the higher forms of life are aggregates of such morphological units or cells, variously modified.
Moreover, all that is at present known tends to the conclusion, that, in the complex aggregates of such units of which all the higher animals and plants consist, no cell has arisen otherwise that by becoming separated from the protoplasm of a pre-existing cell; whence the aphorism "Omnis cellula e cellula."
It may further be added, as a general truth applicable to nucleated cells, that the nucleus rarely undergoes any considerable modification, the structures characteristic of the tissues being formed at the expense of the more superficial protoplasm of the cells; and that, when nucleated cells divide the division of the nucleus, as a rule, precedes that of the whole cell.
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