Change is a fundamental characteristic of all living things. The human body during its life cycle accordingly passes through various phases of form and structure. In the earliest embryonic phases of development, the changes are very rapid, decreasing in rapidity during the later fetal stages, but continuing at a diminishing rate throughout infancy, childhood and youth up to the adult. Following the acme of maturity, changes continue which lead gradually to senescence and final death of the body.
This cycle of change in the body depends upon similar changes in its various component organs, each having its own characteristic life cycle. In a few of the organs this cycle is very short, as in some of the organs of the embryo (e. g., mesonephros). Other organs persist only during childhood (e. g., thymus); while the majority continue, with varying degrees of change, throughout postnatal life. The final death of the body is due to the breakdown of some of the essential organs.
A further analysis reveals the fact that the characteristic life cycles of the organs depend ultimately upon similar changes in their constituent tissues and cells. Every cell has a definite life cycle, an early period characterized by rapid and vigorous changes, later periods of differentiation and maturity, followed by stages of degeneration and death. This cycle of cell changes has been designated by Minot as cytomorphosis.
Associated with the process of cell differentiation (cytomorphosis), and even more important as a factor in the morphogenesis of the body, is the process of growth. The developmental changes in form and structure of the body are due largely to the unequal growth of its various parts. Growth, like other changes in the body and its parts, depends ultimately upon the characteristics of the constituent cells.
Fig. 4. - The Ovum op a New-born Child, with Follicle Cells. (After Mertens.) Nucleus
The cell changes during growth may be grouped under two heads. The first, or growth proper, involves merely the enlargement (hypertrophy) of the individual cells and intercellular products. The second includes the multiplication (hyperplasia) of the cells, which is accomplished by mitotic division. Cell division is necessary in cell growth, for otherwise the cell would soon reach a size where its surface (for nutritive, respiratory and excretory purposes) would be inadequate for its mass. In general, however, cell division is most active in the earlier embryonic periods, during which the cells remain small. Later, cell division diminishes or ceases, and growth is due chiefly to enlargement of the cells already present. It is also during the later period, when the cells have ceased rapid division, that the process of cell differentiation and tissue formation is most marked.
The principle of the ratio of surface to mass often applies to the growing organs as well as to the individual cells. To maintain the necessary ratio, the surface area is increased by the formation, through localized unequal growth, of projections (e. g., villi or folds) or invaginations (e. g., glands) from surfaces. Innumerable modifications of this principle occur throughout the process of morphogenesis.
Fig. 5. - Ovum from Ovary of a Woman Thirty Years of Age. cr, corona radiata. n, nucleus, y, yolk, p, clear protoplasmic zone, ps, perivitelline space, zp, zona pellucida. (McMurrich's Embryology, from Nagel.)
Fig. 6. - Stages of Segmentation in the Ovum of the Mouse, x, polar body. (McMurrich's Embryology, from Sobotta.)
While the present work deals primarily with the adult human organism in the stage of maturity, reference is made also to its changes according to age. Although these changes for the various systems of organs are described under the appropriate sections, it is desirable to consider first some of the more fundamental features pertaining to the body as a whole. This applies particularly to the earlier embryonic period, which includes the more general phases of morpho- genesis. No attempt will be made to describe fully the process of development, the details of which are to be found in text-books of embryology.