Mitosis

“ This process of division is named mitosis: the figure which the nucleus presents while it is undergoing division we call a mitotic figure. Counting the dividing nuclei, we may determine that in a thousand cells there are a given number which have nuclei in process of division, and such a number I propose to call "the mitotic index." ”

“ The same economy which, working under the control of natural selection, has produced the complexly differentiated tissues of the higher metazoa, which has led to the sexual differentiation between the conjugating gametes and thus to the sexual differentiation of the parents, has resulted in the production of mitosis. ”

“ Mitosis is a continuous process; the following stages of the process are designated only for convenience. During interphase the cell is busy metabolizing, synthesizing new cellular materials, and preparing for self-duplication by synthesizing new chromosomes. In prophase the chromosomes, each now composed of two identical strands called chromatids, shorten by coiling, and the nucleolus and nuclear membrane disappear. During metaphase the chromosomes line up in one plane near the cell equator. ”

“ Judging from all available evidence, the granules are self-propagating units; that is, they can grow and reproduce themselves. So that what really happens in mitosis in the splitting of the chromosomes is a precise halving of the series of individual granules of which each chromosome is constituted, or in other words each granule has reproduced itself. Thus each of the two daughter cells presumably gets a sample of every kind of chromosomal particle, hence, the two cells are qualitatively alike. ”

“ The essence of fertilization is, therefore, the union of two germ nuclei only differing from one another in that they are derived from separate individuals. [43] Since the number of chromosomes appearing in mitosis is solely dependent on the number which originally entered into the composition of the nucleus (Boveri’s Law of Chromosome-Constancy) , it follows that, in the mitotic figures of the developing embryo, the chromosomes will be half maternal, half paternal in origin ”

“ Figure 6 Stages of the mitotic cycle in a hypothetical cell with four chromosomes.
Mitosis
One of the most remarkable characteristics of cells is their ability to grow and divide. New cells come from preexisting cells. When a cell reaches a certain stage in its life, it divides into two parts. These parts, after another period of growth, can in turn divide. In this way plants and animals grow to their normal size and injured tissues are repaired. Cell division occurs when some of the contents of the cell have been doubled by replication, or copying (to be discussed later) . ”

“ The tails are always imperfect. The distribution and varying numbers of these degenerate spermatozoa make it impossible to interpret their condition as due to the absence of the accessory chromosome, as Miss Wallace does in the spider. The only probable explanation, it seems to me, is imperfect mitosis. Cases where more or less chromatin is left behind in the cytoplasm, especially in the first spermatocyte mitosis, are very common, and such cases as those shown in figures 149 and 150 are not rare. The giant cells, so far as I have been able to trace them, do not develop into spermatozoa. ”

“ Hence, then, there is congruity between the above interpretation and certain general truths displayed by animal organization at large. We may infer that in a way parallel to that just indicated, cell-evolution was, under one of its aspects, a change from a stage in which the exciting substance and the substance excited were mingled with approximate uniformity, to a stage in which the exciting substance was gathered together into the nucleus and finally into the chromosomes: leaving behind the substance excited, now distinguished as cytoplasm. ”

“ Speculation aside, however, we may say that the two functions of the chromatin do not exclude one another, but that the general activity which originates from it may be but a lower phase of that special activity caused by fertilization. [2]
§ 74g. Here we come unawares to the remaining topic embraced under the title Cell-Life and Cell-Multiplication. We pass naturally from asexual multiplication of cells to sexual multiplication—from cell-reproduction to cell-generation. The phenomena are so numerous and so varied that a large part of them must be passed over. ”

“ The influence of the “cells” upon one another, the subordination of the cell’s growth, division and differentiation, to the requirements of the whole organism—seen in normal growth, but nowhere more strikingly than in development and regeneration,—is, however, very difficult of explanation in terms of the cell-theory as this was, until quite recently, generally understood. The very elaborate regional differentiation of the protoplasm often seen in the Protozoa sufficiently indicate that multicellular structure is no essential condition for complex regional differentiation. ”

“ For the problems with which we are here chiefly concerned, the problems of heredity, it is enough to know that the cells of multicellular organisms possess an extremely complex apparatus for division, whose chief importance lies in the fact that through it the chromatin units of the nucleus are divided into precisely equal parts, and so separated from each other that one half forms one daughter-nucleus, the other half the other. ”

“ If the first division of a reproductive cell set apart one mass to lie dormant for a time and ultimately to form the reproductive cells of the new generation, while the other mass, exactly of the same kind, developed directly into the new organism, then heredity would simply be a delayed case of what is called organic symmetry, the tendency of similar living material to develop in similar ways under the stimulus of similar external conditions. ”

“ With the discoveries that organisms are built up of morphologically equivalent protoplasmic units, or cells; that both egg and sperm are cells, also; that the nucleus, especially the chromatic substance, is the part of the cell chiefly if not wholly concerned with the inheritance of the individual and specific characters and their distribution in the developing organism; more than all, with the discovery of the essential nature of fertilization, new theories were devised to interpret the still puzzling problem of individual and specific differentiation. ”

“ Each plant, each animal, varies to a greater or lesser degree from its immediate ancestors and may vary to a very great degree. This factor in the lives of plants and animals is called variation. Heredity and variation are the cornerstones on which all the work in the improvement of plants and animals, including man himself, are built.
The Bearers of Heredity.—We have seen that somewhere in every living cell is a structure known as a nucleus. In this nucleus, which is a part of the living matter of the cell, are certain very minute structures always present, known as chromosomes. ”

“ To make clear the application of the terms chemical and mechanical to the problem of Genetics the nature of that problem must be more fully described. In its most concrete form this problem is expressed in the question, how does a cell divide? If the organism is unicellular, and the single cell is the whole body, then the process of heredity is accomplished in the single operation of cell-division. Similarly in animals and plants whose bodies are made up of many cells, the whole process of heredity is accomplished in the cell-divisions by which the germ-cells are formed. ”

“ This is the fundamental cause of the fixity of species, which means the production of offspring having the same specific characteristics as their parents. How, under these conditions, the mutability of species is brought about must be discussed later. It is first of all necessary to inquire more closely into the composition of the chromatin, and to study the special phenomena of cell-growth in connexion with conjugation, where new and extraordinary features come to light.
A chromosome is not, or is not usually, a simple body. ”

“ But in no case is the cytoplasm a negligible factor—in no case does it serve merely as food for the chromosomes. The entire cell, nucleus and cytoplasm, is concerned in heredity and differentiation.
D. The Mechanism of Development
Development consists in the transformation of the oosperm into the adult. What is the mechanism by which this transformation is effected? There is progressive differentiation of the germ into the developed organism, but by what process is this differentiation accomplished? ”

“ For the supposition that the nucleus is the hereditary mass transmitting the characters of the species necessitates the conclusion, in the case of unicellular forms, that the hereditary mass remains in possession of all the rudiments of the cell while it passes through the various phases of its cycle of development. ”

“ It is that this universal trait of the Metazoa and Metaphyta, must be ascribed to the primitive action and re-action between the organism and its medium. The operation of those forces which produced the primary differentiation of outer from inner in early minute masses of protoplasm, pre-determined this universal cell-structure of all embryos, plant and animal, and the consequent cell-composition of adult forms arising from them. ”

“ Cells are liable to all the evolutionary changes that the organism as a whole is subject to.
Studies of embryology have shown that the fusion of biparental reproductive cells results in the formation of a complete new individual which, at the time of the fusing of the two conjugating cells, called gametes, or germ cells, inherits the characteristics of each parent and its ancestors.
The determination of the sex of the cell, plant, or animal, depends upon the presence of extra male or female sex-chromosomes, or sex-determinant fibers of the cell nucleus. ”

“ But some of the hereditary units in each cell produced are active. They multiply, grow and migrate out of the nucleus, and get among the units of the enveloping protoplasm. During this activity they undergo physical and chemical changes and effect corresponding differentiations in the protoplasm of the cell. Thus, through many mitoses and many differentiations of the protoplasm of cells, we finally derive from the fertilized ovum all the cells that constitute the adult body, such as muscle cells, glandular cells (as liver cells, kidney cells, etc.) ”

“ We must, therefore, represent the possibilities of exchange between the nucleus, in its capacity of bearer of the hereditary mass, and the protoplasm as being such that all the rudiments are not simultaneously in activity, but that some of them can remain latent for a time. ”

“ Section of the egg of the beetle, Calligrapha, showing the pigment at one end where the germ cells will later develop as shown in the other two figures. (After Hegner.) At every division of the cell the chromosomes also divide. Half of these have come from the mother, half from the father. Every cell contains, therefore, the sum total of all the chromosomes, and if these are the bearers of the hereditary qualities, every cell in the body, whatever its function, has a common inheritance. ”

“ The complex mechanism exhibited in the nucleus during segmentation is very wonderful. The fertilized ovum begins to divide, the nucleus passing through a series of complicated changes whereby its chromosomes undergo a lengthwise division—a division that secures an equable partition of the substance of which they are composed. With each successive division, this complicated process is repeated, and the many cells, arising from continued segmentation of the original cell, contain nuclei in which are embedded descendants of the chromosomes in unbroken succession. ”

“ A determinant exists for every part of the adult organism which is capable of being different in different individuals. And, lastly, each determinant consists of a number of ultimate particles [Pg ix] called biophores, which eventually pass into the protoplasm of the cells in which they come to lie and direct the vital activities of these cells. A most important part of the theory is what it supposes to occur during the embryological development of the individual. The mass of germplasm derived from the germplasm of the parent lies in a mass of ordinary protoplasm. ”

“ We are not to cross species or varieties, or even accidental plants. We must cross unit-characters, and consider the plants only as the bearers of these units. We may assume that these units are represented in the hereditary substance of the cell-nucleus by definite bodies of too small a size to be seen, but constituting together the chromosomes. ”

“ The union of the two pronucleii is plainly to produce a nucleus which shall contain chromosomes, and hence hereditary traits from each parent and the subsequent splitting of these chromosomes and the separation of the two halves into daughter nucleii insures that all the nucleii, and hence all cells of the adult, shall possess hereditary traits derived from both parents. Thus it comes that, even in the adult, every body cell is made up of chromosomes from each parent, and may hence inherit characters from each. ”

“ From the materialistic point of view, nobody knows why protoplasm acts as it does, least of all, why some masses of protoplasm act one way, and exact duplicates act differently. But if, on the other hand, we look beyond the facts and methods of physics and chemistry, and even beyond the most plausible theories of genetics, we can readily explain this remarkable action of the cells as the result of the will of an ever acting, omniscient, almighty God. Certainly nothing else is adequate to explain the behavior of living cells. ”