Part 3
The process by which one cell (a mother cell) divides into two cells (daughter cells) is called _mitosis_, and is inaugurated by the _centrosome_ (Fig. 8, A, _at_). The centrosome divides into _two_ centrosomes, which at first remain close together (Fig. 8, B, _sp_), and then gradually separate from one another. Each centrosome becomes the center of a system of fine achromatin fibers arranged round it in a radiating manner and forming what is called the _attraction sphere_; also, at the same time, a spindle-shaped bundle of achromatin fibers, called the _spindle_ (Fig. 8, B, sp), extends between the centrosomes. In the meantime, important changes have been taking place in the chromosomes (hereditary threads) of the nucleus. The chromosomes, which at first are arranged in an apparently inextricable tangle or network, frequently assume U-shaped or V-shaped forms (Fig. 8, C, _chr_), and the nuclear membrane disappears. Sooner or later each chromosome splits longitudinally into two daughter chromosomes, with which the achromatin fibers of the spindle become connected (Fig. 8, D). In this phase of mitosis the split V-shaped chromosomes form a single group called the _equatorial plate_ (chr), and extend across the axis of the spindle. It is to be observed from the diagrams in the figure, that one of the centrosomes has traveled to the opposite pole of the nucleus, thus causing the achromatin fibers of the spindle to extend across the original site of the nucleus. The equatorial plate of split V-shaped mother chromosomes (hereditary threads) thus divides the fibers of the spindle into two parts, one half extending from one centrosome to one group of daughter chromosomes, while the remaining half extend from the other centrosome to the other group of daughter chromosomes. Soon the achromatin fibers of the spindle contract, and in this way separate the two groups of daughter chromosomes, so that one group is drawn towards one centrosome, and the other group to the other centrosome (Fig. 8, E, F, G, H). After the two groups of daughter chromosomes have been drawn to their respective centrosomes, each group assumes the tangle or network phase like the nucleus of the mother cell, and an investing nuclear membrane reappears for each (Fig. 8, I). Thus from the mother nucleus of the mother cell we get two daughter nuclei (I). In a further phase of the mitotic process, a furrow appears on the surface of the protoplasm and surrounds it in the form of a ring. This furrow is in a plane at right angles to the long axis of the spindle, and gradually deepens until the protoplasm is divided into two parts, each segment of protoplasm containing its own nucleus and centrosome; in short, the mother cell has divided into two daughter cells (I).
It will thus be observed that the centrosomes and their achromatin fibers are a beautiful mechanism by which the heredity threads (chromosomes) are exactly divided into two equivalent halves.
There are some cells (_Amœba proteus_, for instance,) which divide in a much simpler manner than by mitosis; in these there is no complicated rearrangement of the chromosomes and no disappearance of the nuclear membrane, the nucleus simply becoming separated into two parts (_Amitosis_).
=The Human Ovum.= The human ovum is a typical cell about one-fifth of a millimeter in size and spherical in shape. It is a nucleated piece of protoplasm possessing an enveloping cell-wall (metaplasm). The protoplasm contains nutrient material or yolk (metaplasm). The _maturation_ of the ovum essentially consists in throwing out half of its chromosomes. In doing this the nucleus (mother) approaches the surface of the protoplasm (Fig. 9, A), and divides, by mitosis, into two daughter nuclei; then the unripe ovum divides into two cells, but of very unequal size. This process is repeated a second time. Thus two small cells are formed which are known as the _polar bodies_ (Fig. 9, A, B, pol. b). The large cell remaining after the formation of the two polar bodies is the _mature ovum_ (B). Its nucleus, which recedes towards the center of the protoplasm, is called the female _pronucleus_ (Fig. 9, B, f. pr.). The pronuclei of mature ova differ from the nuclei of all the other cells of the body in that they only contain half as many chromosomes (hereditary threads).
The ripe _spermatozoid_ (a flagellate sexual cell of the male) corresponds to the ovum of the female. It also contains a pronucleus having only half the number of chromosomes that the other cells of the adult body possess.
=Fertilization.= The male sexual cell (_spermatozoid_) is vastly smaller than the female sexual cell (ovum). Having a _flagellum_ (Fig. 9, B, s), it moves about, like a tadpole in water, and seeks the ovum. When it comes in contact with the ovum it penetrates into its interior (usually only one doing so), as indicated at (Fig. 9, B, s). The tail or _flagellum_ of the spermatozoid fuses with the protoplasm of the ovum, and disappears from view. Its _pronucleus_ (C, m. pr.), accompanied by its _centrosome_ (C, m. c.), approaches the _female pronucleus_ (f. pr.) of the ovum (Fig. 9, C, D). Finally the male and female pronuclei coalesce to form a single nucleus (Fig. 9, E, n. os.). The centrosome of the ovum persists for awhile and then disappears; that of the spermatozoid remains in the ovum, and is the agency by which cell multiplication, through _mitosis_, takes place.
[Illustration: FIG. 9.—Diagram illustrating the maturation and fertilization of the human ovum. A, one polar body is formed and a second is in process of formation; B, both polar bodies are formed and a spermatozoid is penetrating the ovum; C and D represent the approach of the male pronucleus towards the female pronucleus; E indicates the amalgamation of the two pronuclei to form the nucleus of the oösperm (segmentation nucleus); pol. b, polar bodies; pol. c, centrosome of the polar body; chr. p, chromatin of the polar body; f pr, female pronucleus; p, protoplasm; p p, peripheral protoplasm (but _not_ cell wall); f c, female centrosome; m c, male centrosome; m pr, male pronucleus; n, os, nucleus of the oösperm (first stage of a human being).]
The cell resulting from the coalescence of the male pronucleus with the pronucleus of the ovum, and which is only one fifth of a millimeter in size, is the first stage in the existence of a human being. Man thus starts his career as a Protozoan-like creature,—as a unicellular animal. The fertilized ovum is called the sperm-egg (_oösperm_), and contains now the normal number of _hereditary threads_ (chromosomes); for those of the male have been added to those of the female.
=Segmentation of the Oösperm.= The fertilization of the ovum imparts to it a wonderful stimulus, so that the oösperm divides, by mitosis, into two cells, these two into four, the four into eight, the eight into sixteen, these into thirty-two, and so on repeatedly, until a large number of comparatively small cells are formed (Fig. 10). This mass of cells is spherical in shape, and the little round cells towards the surface project in such a way as to give to the mass an appearance somewhat similar to the fruit of the mulberry, whence it is termed the mulberry body or _morula_ (Fig. 10, 4). In the morula stage of his existence man resembles the solid colony of protozoans represented by _Pandorina_. The cells of the morula next become arranged regularly in a single layer at the circumference, by which the embryo assumes the form of a hollow sphere, and is known as the _blastula_ (Fig. 10, 5). This phase of man’s existence is quite suggestive of _Volvox_.
Soon one side of the blastula is invaginated or pushed in, as one would push in one side of a hollow india-rubber ball. The result of this invagination (called _gastrulation_ technically) is the formation of a sort of cup. This is the _gastrula_ (Fig. 10, 6) phase of man’s existence. It is a higher and fundamentally different phase of existence than either the morula or the blastula. It corresponds, not to a Protozoan, but to the higher Metazoan. It possesses the fundamental anatomical qualities of a low Cœlenterate (Polyp).
The mode of gastrulation is different in man from that just described, and varies in different animals, but the essential point common to all is the formation of a double cellular-membrane; the outer membrane being called _epiblast_ or ectoderm, and the inner one _hypoblast_ or endoderm, the enclosed cavity being the primitive digestive cavity. These two layers are the primary germinal layers. A third layer is subsequently formed between them, by the agency of one or both of them, and is called the _mesoblast_. From these three simple membranes, which are composed exclusively of cells, are formed all the complex tissues and organs of the adult man. The _epiblast_ develops into the nervous system (brain, spinal cord, and nerves), and the cuticle, hair, etc. The _hypoblast_ develops into the cellular parts of the digestive canal, the liver, lungs, etc. The _mesoblast_ develops into the muscles, bones, ligaments, blood-vessels, etc. To trace the details of this evolution of a human being from the microscopic _oösperm_ is a fascinating and instructive study, but is beyond the limits and purpose of this little book. It will well repay further careful study by the reader.
A careful study of the life-histories of the few unicellular animals and plants mentioned in the preceding pages will help to let one realize the wonderful powers of nucleated pieces of protoplasm (cells). If isolated pieces of protoplasm can accomplish so much, one will not be astonished to learn that many diverse cells associated together in intimate correlations, as occurs in the higher animals, may accomplish results that are profoundly interesting and marvelous. As far as anatomy and physiology alone can reveal, it is the result of millions of cells acting together that makes possible the existence of such a living, sentient, thinking creature as man or the highly intelligent elephant or any other multicellular animal. It is due to the mysterious powers of protoplasm that one little microscopic cell, like the fertilized ovum of a woman, is able to hold all the heritages of the race, and gradually unfold them as it builds up the body into myriads of diverse cells intimately associated together.
All of the wonderful results of Embryology are accomplished through cell-multiplications, cell-differentiations, cell-associations, invagination and evaginations of cell-groups (tissues or organs), and unequal growth of parts (cells or groups of cells).
[Illustration: FIG. 10.—Segmentation of the fertilized ovum and Gastrulation: 4, morula; 5, section through blastula showing hollow sphere; 6, gastrula showing outer layer of cells (epiblast) and inner layer (hypoblast); the 6 is at the mouth of the cavity (enteron) of the gastrula.]
In concluding this brief but, we hope, useful study of a few selected cells, we may say that an eminent English physiologist has made the statement that a student who has not looked through the microscope and observed the circulation of the blood in the web of a frog’s foot is not fit to study medicine. However beautiful, fascinating and instructive the sight of this circulation may be, we are tempted to make the assertion that the student who has not looked through the microscope at some of the superficial ooze from the bottom of any slow-running stream, in summer, and observed the structure and actions of that wonderful little unicellular animal, the Amœba proteus, is still less prepared to study medicine. In this little speck of protoplasm the problems of life are reduced to their simplest forms, for all higher plants and animals may be regarded as groups of more or less modified amœbæ peculiarly associated together. In our studies of the amœba we will be forcibly reminded of a very clever trick which is practiced in India and is called the mango-trick. In this trick a seed is put into the ground and covered up, and after divers incantations a full-blown mango-bush appears within five minutes. We have never met any one who knew how this thing was done, nor have we ever seen a person who believed it to be anything else than a conjuring trick. So it is with the _amœba_, a beautiful and fascinating trickster of nature. We understand some of its activities, interesting and exceedingly instructive, but there are many others beyond our ken. We see the commencement and ending of many of its chemical activities, but there are numerous other intermediary processes that take place in the hidden recesses of the protoplasm, and concerning which we know nothing. It may fall to the lot of some reader of this little book, as a patient and keen observer, to unravel some of these mango-like tricks of the amœba or other unicellular creature.
SECTION II.
HEREDITY WITH VARIATION.
HEREDITY WITH VARIATION.
That an offspring always inherits from its parents many of their characteristics is well known; that it always varies, more or less, from them is also equally well known. Heredity and variation are twin forces that play upon every creature, holding it rigidly true to the parental type or compelling more or less divergence therefrom, according to the strength of the one or other power; so that every creature is the resultant of the activities of these two great parallel forces. Variation is coextensive with heredity, and every living creature gives evidence of the existence of variations.
=Examples of Variations.= No two leaves on a plant are exactly alike; no two children of the same parents give a perfect resemblance; no two individuals of the same species are molded in precisely the same pattern; of the thousands and thousands of faces that we observe in a city in the course of a year, each has some distinctive peculiarity.
The trained eye of the gardener recognizes each hyacinth among hundreds of bulbs; of the shepherd, each sheep in his flock; of the Laplander, each reindeer crowded in his herd like ants on the anthill. In a flock of 1,000 sheep each mother can even recognize a variation in the _voice_ of her own lamb, all alike to _us_.
Every part of an animal is subject to variations, not only in bodily structure, but also in habits and instincts, and these variations are large in amount, numerous and diverse in character. Many observations, experiments and measurements that have been made at various times attest the truth of this assertion. Not only do variations take place in animals and plants under domestication, but also in the wild state.
=Illustrations of Heredity.= _Mental heredity_ can be illustrated by studying the genealogies of such persons as Aristotle, Goethe, Darwin, Coleridge, Milton, etc. Probably the Bach family, of Germany, supply one of the best illustrations of the inheritance of intellectual character that we know of. The record of this family begins in 1550, lasting through eight generations to 1800. For about two centuries it gave to the world musicians and singers of high rank. The founder was Weit Bach, a baker of Presburg, who sought recreation from his routine work in song and music. For nearly two hundred years his descendants, who were very numerous in Franconia, Thuringia, and Saxony, retained a musical talent, being all church singers and organists.
When the members of the family had become very numerous and widely separated from one another, they decided to meet at a stated place once a year. Often more than a hundred persons—men, women and children—bearing the name of Bach were thus brought together. This family reunion continued until nearly the middle of the eighteenth century. In this family of musicians twenty-nine became eminent.
_Inheritance of moral character_ is well established. Heredity, in its relation to crime and pauperism, has been thoroughly investigated by Mr. Dugdale in his most instructive little work entitled _The Jukes_. In this work the descendants of _one_ vicious and neglected girl are traced through a large number of generations. It reveals that a large proportion of the descendants of this woman became licentious, for, in the course of six generations, fifty-two per cent. of the females became harlots and twenty-three per cent. of the children were illegitimate. It shows also that there were seven times more paupers among the women than among the average women of the State, and nine times more paupers among the male descendants than among the average men of the State.
The inheritance of _physical peculiarities_ is so obvious as to need no illustration. Among the ancients the Romans stereotyped its truth by the use of such expressions as the _labiones_, or thick-lipped; the _nasones_, or big-nosed; the _capitones_, or big-headed; and the _buccones_, or swollen-cheeked, etc. In more recent times we read of the Austrian lip and the Bourbon nose.
Questions of heredity and variation are cytological ones—that is, questions of the anatomy, physiology, physiological chemistry, and pathology of cells. The most important part of a cell, as far as these questions are concerned, is the =nucleus=. The nucleus is the physical basis of all the heritages of an organism, from the simplest to the most complex. The nuclear threads may, therefore, very appropriately be termed the _hereditary threads_, or, collectively, the hereditary mass; and the physiological units in them the _hereditary units_. The nucleus is of fundamental importance in the reproduction or multiplication of both unicellular and multicellular animals and plants.
In unicellular creatures multiplication may take place by fission and by conjugation. Both of these processes can be studied by observation of the infusorians. Maupas’s beautiful investigations on these unicellular animals have demonstrated that multiplication by fission may proceed to a prodigious extent for many generations, but that a time comes when the process fails, and the species will become exhausted and die out unless there is a rejuvenation of it by conjugation of individuals. In conjugation two individual infusoria come in apposition with each other, the nucleus in each undergoes subdivision. They reciprocally exchange part of their nuclear contents so that each infusorian comes to contain hereditary threads of two distinct individuals. From these rejuvenated (or fertilized) individuals multitudes of others may be derived by fission until exhaustion again takes place.
Multiplication in multicellular creatures may be accomplished by budding (which is allied to fission), and is exemplified in the plant, hydra, the queen bee (parthenogenesis), etc., and by fertilization (which is allied to conjugation). A knowledge of the phenomena of fertilization of the ovum by the spermatozoid is essential to any understanding of the problems of heredity and variation in mankind. The nuclear threads of the ovum are its hereditary threads—the _groups of maternal hereditary units_; likewise, the nucleus of the spermatozoid contains the _paternal groups of hereditary units_.
=Fertilization.= In fertilization, the spermatozoid (a nucleated flagellate cell) penetrates the ovum (a nucleated, encysted cell), its protoplasm mixes with that of the ovum, and its nuclear threads come into relation with the nuclear threads of the ovum; so that the fertilized ovum (a new creature, a veritable microcosm) is still a nucleated cell, but one in which the nucleus is compound, is _hermaphroditic_, in that it contains maternal and paternal threads—that is, maternal and paternal hereditary units which constitute its hereditary mass.
It will be convenient to speak of the maternal and paternal hereditary units in the fertilized ovum as _ancestral hereditary units_.
This hermaphroditic cell passes through complex phases, illustrated by embryology, to the adult. In doing so this hermaphroditic cell (mother cell) first divides into two smaller cells (daughter cells). The mother cell divides in such a way (by mitosis) that one-half of its nucleus and part of its protoplasm goes to one daughter cell and the other half of the nucleus, with the remainder of the protoplasm, goes to the other daughter cell. It is an interesting fact that although the amount of protoplasm which goes from the mother cell to the two daughter cells may be unequal at times, yet the amount of the nucleus in one daughter cell is always exactly equal[2] to that in the other; _so that each daughter cell contains maternal and paternal hereditary masses of equal quantity and quality_, being in fact one-half that of the fertilized ovum. In consequence of the fact that each hereditary unit in the nucleus of the daughter cell can absorb nutriment and grow, it comes about that the nucleus of each daughter cell attains to the size of that of the mother cell. The enveloping protoplasm of the nuclei also grows to a greater or less extent, so that the cells as a whole grow. These two daughter cells go through the same process and form other daughter cells, and so on through all the mitoses of development, until all the myriads of cells of a living organism are produced, _each of which contains maternal and paternal hereditary masses of equal quality and quantity, and also of the same character as that of the fertilized ovum whence they are all derived_.
Apart from their activity in absorbing nutriment and growing, the great majority of the hereditary units in the nuclei of the forming cells remain latent. 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.), nerve cells, skeletal cells (as bone, cartilage, and connective-tissue cells), and the ova and spermatozoids. According to this theory, the _nucleus_ of each cell in the adult animal or plant is _pure hereditary mass_, exactly like that in the nucleus of the fertilized ovum; but the _protoplasm_ of each adult cell that envelopes the nucleus may differ greatly in different cell groups, as in muscle, nerve, cartilage, and the like. Of course, the protoplasm of those cells that develop into the ova and the spermatozoids has differentiated along such lines as to become like that of the ovum and spermatozoid, the junction of which formed the fertilized ovum. These statements hold true for plants and the lower animal organisms, although they cannot be verified for the higher animals. More than likely the pure hereditary masses are present in the body (somatic) cells of the higher organism in latent conditions, but are unable ever to be developed owing to the greater specialization of these cells. It is thus seen that all the cells of many animals and plants can perform their own special functions and at the same time contain all of the hereditary units of the complex organism in a state ready to develop under favoring conditions.
Since the ova and spermatozoids are cells specially differentiated for the purpose of propagating the species by sexual generation, and since their conjugation produces the germ of a new creature, they may very appropriately be spoken of as =germ cells=. Since all the other cells of the adult form the great bulk of the body that envelopes and protects the germ cells, they may be termed the =body cells=, or =somatic cells=.
Suppose that ova, containing maternal and paternal groups of hereditary units, are fertilized by similarly complex spermatozoids, and the process is repeated generation after generation. There will come a time when the fertilized ovum will have a highly complex nucleus composed of _many different ancestral groups of hereditary units_.
One often hears the expression that a child is a chip of the old block; but this is only a very partial truth, for a child is preëminently a composite chip of many old blocks.