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Biology
(Part 6)



Morphology - Introduction: (3) Modes of Cell Differentiation

3. Development, then, is a process of differentiation by which the primitively similar parts of the living body become more and more unlike one another.

This process of differentiation may be effected in several ways.

(1.) The protoplasm of the germ may not undergo division and conversion into a cell aggregate; but various parts of its outer and inner substance may be metamorphosed directly into those physically and chemically different materials which constitute the body of the adult. This occurs in such animals as the Infusoria, and in such plants as the unicellular Algae.

(2.) The germ may undergo division, and be converted into an aggregate of cells, which cells give rise to the tissues by undergoing a metamorphosis of the same kind as that to which the whole body is subjected in the preceding case.

The body, formed in either of these ways, may, as a whole, undergo metamorphosis by differentiation of its parts, and the differentiation may take place without reference to any axis of symmetry, or it may have reference to such an axis. In the latter case, the parts of the body which become distinguishable may correspond on the two sides of the axis (bilateral symmetry), or may correspond along several lines parallel with the axis (radial symmetry).

The bilateral or radial symmetry of the body may be further complicated by its segmentation, or separation by divisions transverse to the axis, into parts, each of which corresponds with its predecessor or successor in the series.

In the segmented body, the segments may or may not give rise to symmetrically or asymmetrically disposed processes, which are appendages, using that word in its most general sense.

And the highest degree of complication of structure, in both animals and plants, is attained by the body when it becomes divided into segments provided with appendages; when the segments not only become very different from one another , but some coalesce and lose their primitive distinctness; and when the appendages and the segments into which they are subdivided similarly become differentiated and coalesce.

It is in virtue of such processes that the flowers of plants, and the heads and limbs of the Arthropoda and of the Vertebrata, among animals, attain their extraordinary diversity and complication of structure. A flower-bud is a segments body or axis, with a certain number of whorls of appendages; and the perfect flower is the result of the gradual differentiation and confluence of these primitively similar segments and their appendages. The head of an insect or a crustacean is, in like manner, composed of a number of segments, each with its pair of appendages, which by differentiation and confluence are converted into the feelers and variously modified oral appendages of the adult.

In some complex organisms, the process of differentiation, by which they pass from the condition of aggregated embryo cells to the adult, can be traced back to the laws of growth of the two or more cells into which the embryo cell is divided, each of these cells giving rise to a particular portion of the adult organism. Thus them fertilized embryo cell in the archegonium of a fern divides into four cells, kone of which gives rise to the rhizome of the young fern, another to its first rootlet, while the other two are converted into a placenta – like mass which remains embedded in the prothallus.

The structure of the stem of Chara depends upon the different properties of the cells, which are successively derived by transverse division from the apical cell. An inter-nodal cell, which elongates greatly, and does not divide, is succeeded by a nodal cell, which elongates but little, and becomes greatly subdivided; this by another inter-nodal cell, and so on in regular alteration. In the same way the structure of the stem, in all the higher plants, depends upon the laws which govern the manner of division and of metamorphosis of the apical cells, and of their continuation in the cambium layer.

In all animals which consist of cell-aggregates, the cells of which the embryo is at first composed arrange themselves by the splitting, or by a process of invagination, of the blastodern into two layers, the epiblast and the hypoblast, between which a third intermediate layer, the mesoblast, appears, and each layer gives rise to a definite group of organs in the adult. Thus, in the Vertebrata, the epiblast gives rise to the cerebro-spinal axis and to the epidermis and its derivatives; the hypoblast, and to the epithelium of the alimentary canal and its derivatives; and the mesoblast, to all the intermediate structures. The tendency of recent inquiry is to prove that the several layers of the germ evolve analogous organs invertebrate animals, and to indicate the possibility of tracing the several germ layers back to the blastomeres of the yelk, from the subdivision of which they proceed.





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