Fig. 76.—The Egg and Early Stages in its Development. (After Gegenbaur.)
Fig. 77 shows a stage in the development of one of the mollusks just as the differentiation of cells has commenced.
The Nucleus.—To the earlier observers the protoplasm appeared to be a structureless, jelly-like mass containing granules and vacuoles; but closer acquaintance with it has shown that it is in reality very complex in structure as well as in chemical composition. It is by no means homogeneous; adjacent parts are different in properties and aptitudes. The nucleus, which is more readily seen than other cell elements, was shown to be of great importance in cell-life—to be a structure which takes the lead in cell division, and in general dominates the rest of the protoplasm.
Chromosomes.—After dyes came into use for staining the protoplasm (1868), it became evident that certain parts of it stain deeply, while other parts stain faintly or not at all. This led to the recognition of protoplasm as made up of a densely staining portion called chromatin, and a faintly staining portion designated achromatin. This means of making different parts of protoplasm visible under the microscope led to important results, as when, in 1883, it was discovered that the nucleus contains a definite number of small (usually rod-shaped) bodies, which become evident during nuclear division, and play a wonderful part in that process. These bodies take the stain more deeply than other components of the nucleus, and are designated chromosomes.
Fig. 77.—An Early Stage in the Development of the Egg of a Rock-Limpet. (After Conklin.)
Attention having been directed to these little bodies, continued observations showed that, although they vary in number—commonly from two to twenty-four—in different parts of animals and plants, they are, nevertheless, of the same number in all the cells of any particular plant or animal. As a conclusion to this kind of observation, it needs to be said that the chromosomes are regarded as the actual bearers of hereditary qualities. The chromosomes do not show in resting-stages of the nucleus; their substance is present, but is not aggregated into the form of chromosomes.
