In many cells, especially vegetable ones, will also be observed clear spaces termed vacuoles. These vacuoles contain water with various chemical substances held in solution, which serve the purpose chiefly of food-reservoirs.
The nucleus also is formed of threads called nuclear or chromatin threads (chromosomes), the interstices of which are filled with hyaloplasm or achromatin. In the nucleus can also be observed the nucleolus.
The protoplasmic and nuclear threads show various structural modifications in different regions and under different physiological states of the cell.
As will be observed later on, the nuclear threads are of special interest to the student of heredity. They may in one phase of cell-activity look like one thread forming an inextricable network, while in other phases they may look like thick, short, distinct rods.
The centrosome ([Fig. 1]), with its enveloping attraction-sphere, constitutes another fundamentally important part of the cell. It is especially concerned with the phenomena of cell division and multiplication.
Just as the living body consists of an infinitely complex figure of living mosaics termed cells, so the cell itself consists of an infinitely complex figure of still smaller living mosaics called, by Spencer, Physiological Units. These units have been given different names by various writers, viz.: by Darwin, gemmæ (gemmules); by de Vries, pangennæ; by Hertwig, idioblasts; by Weismann, biophors, etc., etc.
Like the atom of the chemist and the molecule of the physicist, the physiological unit of the biologist is merely at present an intellectual conception, yet it is, at the same time, an intellectual necessity and plays a very important part as the theoretical component of many vital questions. Just as the cells are the visible units of life, so the physiological units are the invisible units.
The physiological activities of cells are those that pertain to their nutrition and reproduction.
The nutrition of cells includes all processes that are subservient to their life and well-being, such as irritability, contractility, absorption of food, its digestion and assimilation, secretion, etc.
In consequence of the wonderful nutritive activities of cells, we may well speak of them as marvelous magicians. Hertwig, following Haeckel, speaks of many cells as being builders. In the same spirit, we can say that multitudes of cells are expert chemists, artists, sculptors, mathematicians, and so on, in that they make all the myriad chemical products of organic nature, such as spices, pigments, sugars, starches, acids, perfumes, and numerous other substances; they paint in colors that rival the hues of the rainbow; they construct all of the beautiful forms in the animal and plant worlds; and they draw lines as straight and curves as graceful as the most expert mathematician.