CYTOLOGY, the science which deals with the structure, development and functions of the cell; of the multiplication of cells and of the relations of cells to organs and tissues. It is distinguished from histology in that histology deals with tissues, while cytology deals rather with the cells of which tissues are composed. A sharp distinction cannot be made between cy tology and morphology, although in general it may be said that cytology is concerned more with individual cells than with organs. Cytol ogy may be called °oil immersion') morphology because microscopes with very high magnify ing powers are required for cytological studies. The terms, cytology and morphology, are used very indiscriminately by many botanists and zoologists. During the past 20 years, most botanical studies with such titles as "The Mor phology of —,)) contain more or less cytology, while only a smaller number of articles pur porting to be cytological are largely morpholog ical. Among the problems which belong indisputably to cytology are the following: The origin, development, structure and functions of the cell and of its various constituents; nuclear and cell division; fertilization; organization of the embryo; the relation of the individual cell to cell complexes; the role of the cell in heredity. An idea of the subject matter of cytology can best be gained by examining, from the cytologi cal standpoint, a few prominent subjects which are also studied by investigators in other fields. See EMBRYOLOGY; FERTILIZATION; HEREDITY; MUTATION.
The A cell has been defined as ma mass of protoplasm with a nucleus in While this is true for many simple forms, there is in addition, in the vast majority of cases, a cell wall surrounding the protoplasm (Fig. 1).
Cells without nuclei were reported frequently about 40 years ago, but at present no animal cells without nuclei are known. Numerous cases of non-nucleate cells were reported by the older botanists, but as lenses and methods of prepara tion have improved, nuclei have been demon strated until the only doubtful cases now known to the botanist are the bacteria. The blue-green algae were long thought to have no nuclei, but nuclei have now been demonstrated in several genera and are doubtless present throughout the group.
Most cells are too small to be seen without a microscope, cells which are visible to the naked eye being rather exceptional. The egg of a bird consists of a single cell, as do the eggs of all animals and plants. The largest plant cells are the internodal cells of the stonewort, Chars, which reach a length of two inches. The largest egg cell described for any plant is that of Dioon, a plant related to the sago palm; this cell reaches a length of about one-fourth of an inch.
The most usual shape of free cells is the spherical, and cells forming a part of a tissue are usually more or less rectangular in form.
When first formed, the cells of the individual animal or plant are very much alike, but as one examines cells farther and farther from the regions where active cell multiplication is taking place, it is seen that the originally similar cells are becoming very unlike. In the higher plants the outer cells become differentiated into pro tective tissue, the innermost into conductive tissue, others into assimilative tissue and still others become reproductive cells. In higher ani mals similar differentiations take place, cells which finally become so different, as those which form nerves, muscle, glands and even the teeth, having been practically alike in the be ginning. Among the unicellular organisms there is often a remarkable differentiation and divi sion of labor, the single cell performing the functions of locomotion, securing food, respira tion, digestion, assimilation, etc. Such differen tiation and the causes which lead to it are among the most important of cytological prob lems.
It is a remarkable fact that while undergoing nuclear division, the cells of animals and plants strikingly resemble one another, even in the behavior of the most minute constituents of the nucleus and protoplasm. (Fig. 2). This must mean that animals have been derived from plants or that structures of amazing similarity have arisen independently in animals and plants.