It is believed, on the strong though indirect evidence of genetics, and on the basis of certain experiments on developing animals (see EXPERIMENTAL EMBRYOLOGY), that by this process the daughter cells receive nuclei which are exactly identical with one another and with the parent nucleus.
As all the divisions by which the cells which compose the body of a Metazoan are formed, are of this kind, it follows that the nucleus of any one cell in such an animal is the exact equiva lent of that of every other cell, and that the differences which actually exist between such cells depend either on the position which they hold in the body, that is to the influence of their neighbours, or of the outer environment, or to the character of the cytoplasm they received at some cell division.
This conception at once shows the importance of an examina tion of the cytoplasm. Examination of living cells taken from the body of a Metazoan seldom reveals any structures except drops of fat, yolk spheres and similar non-living substances stored in the cell for future use. But by the application of special methods of fixation and staining and a careful use of solvents, it is possi ble to find in all cells several classes of bodies whose behaviour suggests that they are actual portions of that mechanism on which the life of the cell depends. In a few fortunate cases these struc tures can actually be seen in living cells, and are thus probably not, as has been suggested, merely artificial products of the long process of preparation by which they are usually made visible. Thus the cytoplasm of a cell is so complex that it can possess a definite morphology, which might, in part, provide an explanation of the diversity of cells which have been derived from a single source.
It is clear that the most favourable material on which to test such a hypothesis is the segmenting egg of a marine Metazoan, because there the whole process of cell division, and of the grad ual differentiation of cells can be watched under the microscope in living material.
In such animals as the sea urchin Echinus, the egg consists of a surface membrane, which surrounds a liquid cytoplasm within which the nucleus floats a little excentrically. In most cases there is no visible evidence that any part of the cytoplasm differs from any other part. This cell, after fertilization divides re peatedly, and the cells which are formed appear to be all exactly similar up to the time when 64 of them are present. At this
stage the embryo is a sphere with a wall one cell thick sur rounding a liquid-filled cavity. The arrangement of the cells is clearly haphazard, they owe their mutual positions to sliding movements, which, in other cases, can be shown to depend on the ordinary laws of surface tension. But in time this sphere of cells exhibits polarity, one end of it tucks itself within the other, and the animal acquires anterior and posterior ends. Is this polarity the result of an invisible polarity present in the egg or not? The egg of a mollusc or of a polychaet worm usually presents as little evidence of structure as that of an Echinoid, but its development is of very different type. In either case the first division produces two cells which may differ from one another visibly. The second cleavage divides each of these into equal parts, whilst the third is unequal, separating small micromeres from the poles of the large megameres. These cells do not lie in tiers, the quartette of micromeres is so disposed that each one lies in the groove between two megameres. Although this posi tion is that which would naturally be reached by sliding of cells under the forces of surface tension, it does not, in fact, arise in that way, but is predetermined by the direction of the axes of the spindles which bring about the mitoses of the third cleavage.
Thus, in this case it is certain that something within the egg determines both the character and position of the individual blastomeres. Indeed, the fate of each cep'_ in such an embryo is ab solutely determined; it will form a definite structure or part of a structure in the adult.
The fact that, in certain cases at any rate, the fate of a blastomere may be determined from the time of its origin, led to attempts to discover the mechanism which lies hidden in its parent cell. Such a mechanism may involve a localization of definite substances, recognizable by chemical means. If substances which determine the fate of a blastomere be present in definite parts of its parent, it should be possible to remove them artifi cially, and thus inhibit the development of those characters for whose appearance they are responsible.