It results, then, for the whole living world, that the morphological unit—the pri mary and fundamental form of life—is only an individual mass of protoplasm, in which no further structure is discoverable; that Independent living forms may present brit little advance on this structure; and that all the higher forms of life are aggregates of such morphological units or cells, variously modified. All that is at present known tends to the conclusion that, in the complex aggregates of such units of which fill the higher animals and plants consist, no cell has risen otherwise than by becoming separated from the protoplasm of n pre-existing cell.
2. In the course of its development, every cell proceeds from a condition in which it cloSely resembles every other cell, through stages of gradually increasing diver-scrim until it reaches the condition in which it pres.enta the characteristic features of the elements of a special tissue. The development of the cell is, therefore, a gradual progress from the general to the special condition. The same holds good of the development of the hoe" as a whole. However complicated one of the higher animals or plants may be, it begins its separate existence under the .form of a nucleated cell, which by division becomes converted into an aggregate of similar cells; the parts of this aggregate, follow ing different laws of growth and multiplication, give rise to the rudiments of the organs; and the parts of these rudiments again take on those modes of growth and multiplication and metamorphosis which are needful to convert the rudiment into the perfect structure. The development of the organism as a whole repeats the development of the cell. It is progress from a general to a special form, resulting from the gradual differentiation of the originally similar morphological units of which the body is composed. When thu steps. of the development of two animals are compared, the number of the steps that are similar to one another will be found proportioned to the closeness of the resemblance of the adult forms; sv it follows that tho more cloSely any two animals arc allied at full gron th of structure, the later are their embryonic conditions distinguishable; a law that is alike in both plants and animals.
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. The protoplasm of the germ may neg undergo division and conversion into a cell aggregate; but in various parts of its outer and inner substance may be metamorphosed directly into those physically and chemi cally different materials which constitute the body of the adult. This occurs in such
animal life as that of infusoria, and in such plants as the unicellular alga.% But 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 case just mentioned. The body, formed in either of these ways, may, as a whole, undergo metamorphoses by differentiation of its parts, and the differentiation may take place without reference to any axis of symmetry, or it niay have reference to such an axis. In the latter ease, the parts of the body which become distinguishable may correspond on the two sides of the axis, making bilateral symmetry. or may correspond along several lines paralled with the axis, making 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 niay not give rise to symmetrically or unsymmetrically disposed processes, which are appendages, in the general sense of the word. 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, hut some coalesce and lose their primitive distinctness; and when the appendages and the segments into which they are subdivided similarly become differ entiated and coalesce. By such processes the flowers of some plants and the heads and limbs of some animals attain their extraordinary diversity and complication of structure. A flower-bud is a segmented body or Xis, with a certain number Jf whorls of append ages; 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 is, lit like manner, made of segments, each with its pair of appendages. which, by different iation and confluence, are converted into feelers and variously modified oral appendages of the adult.