The eggs of hens occasionally do not have this "egg-pod," or "egg-shell"; nor do those of the turtle, nor the "roe" of fish these being encased in tough, elastic "egg-cases" ; and the human ovum also has a very elastic "egg-case" which is called the zona peUudda.
To one unfamiliar with physiology the word "cell" is almost meaningless, but for our present purpose some knowledge of it is essential.
Not to be misleading, it must be pointed out that almost all cells are invisible to the naked eye, and the enormous eggs of birds, though single cells in every respect, are only wonderful and exceptional examples of modification for a particular design.
The body of a man contains untold millions of cells, all practically microscopic in size and with a great divergence of function; but in the beginning, when the germ was conceived, his existence sprang from a single nucleated cell.
The fundamental type of a cell is a minute mass of granular protoplasm having a cell wall which limits it, and a nucleus and nucleolus, though the cell walls and nuclei are not essential constituents of all cells.
Represents a typical cell.
A, Granular protoplasm (vitellus or yolk);
B, cell-wall or zona pelluclda;
C, nucleus with nucleolus, called also germinal vesicle with germinal spot.
(From Gray's "Anatomy".)
Cells are modified in various ways to perform the different functions of nutrition, sensation, automatic and spontaneous motion and reproduction, each cell being an independent organism which enters into the formation of tissue by association with other cells. Thus we have bone-cells, blood-cells, lymph-cells, fat-cells, cartilage-cells, muscle-cells, nerve-cells, mucus-cells, etc., and in addition to these the cells which are concerned in reproduction, e.g., the ova and spermatozoa.
Structurally or morphologically, the ovum is similar to other cells; but physiologically it is vastly different, since it is capable, if impregnated, of developing an organism which is the counterpart of the parent.
From this semi-fluid, almost homogeneous cell which constitutes the ovum there are developed all the myriads of cells of the body. The higher we ascend in the scale of life the smaller the ova become, until in tK. human female this minute "egg", or ovum, measures only T in. in diameter.
The ovum discharged at each menstrual period, being incapable of locomotion like that of the spermatozoa, is directed by the Fallopian tube, on the side corresponding to the ovary from which it came, into the uterus.