Fig. 2
Diagram illustrating germinal continuity. Through a series of divisions a germ-cell gives rise to a body or a soma and to new germ-cells. The latter, not the body, give rise in turn to the next generation.
Determiners of Characters, Not Characters Themselves, Transmitted.—The fact should be thoroughly understood that the actual thing which is transmitted by means of the germ in inheritance is not the character itself, but something which willdeterminethecharacter in the offspring. It is important to remember this, for often thesedeterminers, as they are called, may lie unexpressed for one or more generations and may become manifest only in later descendants. The truth of the matter is, the child does not inherit its characters from corresponding characters in the parent-body, but parent and child are alike because they are both products of the same line of germ-plasm, both are chips from the same old block.
METHODS OF STUDYING HEREDITY
Before entering into details it will be well to get some idea of the methods which are commonly employed in arriving at conclusions in the field of heredity. Some of these are extremely complex and all that we can do in an elementary presentation is to get a glimpse of the procedures.
Our Knowledge of Heredity Derived Along Three Lines.—Our modern conceptions of heredity have been derived mainly from three distinct lines of investigation: First, from the study of embryology, in which the biologist concerns himself with the genesis of the various parts of the individual, and the mechanism of the germs which convey the actual materials from which these parts spring; second, through experimental breeding of plants and animals to compare particular traits or features in successive generations; and third, through the statistical treatment of observations or measurements of a large number of parents and their offspring with reference to a given characteristic in order to determine the averageextent of resemblance between parents and children in that particular respect.
The Method of Experimental Breeding.—A tremendous impetus was given to the method of experimental breeding when it was realized that we can itemize many of the parts or traits of an organism into entities which are inherited independently one of another. Such traits, or as we have already termed them, unit-characters, may be not only independently heritable but independently variable as well. The experimental method seeks to isolate and trace through successive generations the separate factors which determine the individual unit-characters of the organism. In this attempt cross-breeding is resorted to. Forms which differ in one or more respects are mated and the progeny studied. Next these offspring are mated with others of their own kind or mated back with the respective parent types. In this way the behavior of a particular character may often be followed and the germinal constitutions of the individuals concerned can be formulated with reference to it. Inasmuch as we shall give much consideration to this method in the chapter on Mendelism we need not consider it further here.
The Statistical Method.—The statistical method seeks to obtain large bodies of facts and to deal with evidence as it appears through mathematical analysis of these facts. The attempt of its followers is to treat quantitatively all biological processes with which it is concerned. Historically Sir Francis Galton was the first to make any considerable application of statistical methods to the problems of heredity and variation. Inhis attempts to determine the extent of resemblance between relatives of different degree as regards bodily, mental and temperamental traits, he devised new methods of statistical analysis which constitute the basis of modern statistical biology, orbiometryas it is termed by its votaries. Professor Karl Pearson in particular has extended and perfected the mathematical methods of this field and stands to-day as perhaps its most representative exponent. The system is in the main based on the calculus of probability. The methods often are highly specialized, requiring the use of higher mathematics, and are therefore only at the command of specially trained workers.
Just as insurance companies can tell us the probable length of human life in a given social group, since although uncertain in any particular case, it is reducible in mass to a predictable constant, so the biometrician with even greater precision because of his improved methods can often, when a large number of cases are concerned, give us the intensity of ancestral influence with reference to particular characters.
For example, it is clear that by measuring a large number of adult human beings one can compute the average height or determine the height which will fit the greatest number. There will be some individuals below and some above it, but the greater the divergence from this standard height the fewer will be the individuals concerned.
Galton compared the heights of 204 normal English parents and their 928 adult offspring. In order to equalize the measurements of men and women he found he had to multiply each female height by 1.08.Then, to take both parents into account when comparing height of parents to that of children he added the height of the father to the proportionately augmented height of the mother and divided by two, thus securing the height of what he termed the “mid-parent.” He found that the mid-parental heights of his subjects ranged from 64.5 to 72.5 inches, and that the generalmodewas about 68.5 inches. It should be mentioned that themode, in a given population, represents the group containing the largest number of individuals of one kind; it may or may not coincide with the average. The children of all mid-parents having a given height were measured next and tabulated with reference to these mid-parents. The results of Galton’s measurements may be expressed simply as follows:
The Law of Regression.—It is plain from this table that the offspring of short mid-parents tend to be under average or modal height though not so far below as their parents. Likewise children of tall parents tend to be tall but less tall than their parents. This fact illustrates what is known as Galton’slaw of regression; namely, that if parents in a given population diverge a certain amount from the mode of the population as a whole, their children, while tending toresemble them, will diverge less from this mode. It is clear that the extent of regression is an inverse measure of the intensity of inheritance from the immediate parents; if the deviation of the offspring from the general mode were nearly as great as that of their parents then the intensity of the inheritance must be high; if but slight—that is, if the offspring regressed nearly to the mode—then the intensity of the inheritance must be ranked as low. In the example in question it must be ranked as relatively high. Computations show that as regards stature the fraction two-thirds represents approximately the amount of resemblance between the two generations where both parents are considered.
Correlations Between Parents and Offspring.—In modern researches the conception of mid-parent and mid-grandparent as utilized by Galton has been largely abandoned. It has been found more convenient as well as more accurate to keep the measurements of the two parents separate and to deal with correlations between fathers and sons, fathers and daughters, mothers and sons, mothers and daughters, brother and brother, etc. Professor Pearson and his pupils have found for a number of characters that the correlation between either parent and children, whether sons or daughters, is relatively close. The correlation between brother and brother, sister and sister, and brother and sister, usually ranges a little higher than the corresponding relation between parents and children.
The Biometrical Method, Statistical, Not Physiological.—While biometry may in certain cases go far toward showing us the average intensity of theinheritance of certain characters it can not replace the method of the experimental breeder which deals with particular characters in individual pedigrees. It must be borne in mind that the biometrical method is a statistical and not a physiological one and that it is applicable only when large numbers of individuals are considered in mass. It is most valuable in cases where we are unable sharply to define single characters, due probably to the concurrent action of a number of independent causes, or where experiment is impossible so that we have to depend solely on numerical data gained by observation.
Mental Qualities Inheritable.—Galton showed by this method long ago, and Pearson and his school have extended and more clearly established the work, that exceptional mental qualities tend to be inherited. While on the average the children of exceptional parents tend to be less exceptional than their parents, still they are far more likely to be exceptional than are the children of average parents. By this method Professor Pearson has shown that such mental and temperamental attributes as ability, vivacity, conscientiousness, temper, popularity, handwriting, etc., are as essentially determined as are physical features through the hereditary endowment.
THE BEARERS OF THE HERITAGE
Before we can make any detailed analysis of the inheritance of characters we should have some general idea of the physical structure of animals and particularly some familiarity with the development of an individual from the egg, as well as some knowledge of the nature of the germ-cells.
The Cell the Unit of Structure.—If we examine one of the higher animals, as, for example, the horse, the dog, or man, we find that it is made up of a large number of constituents, such as bones, muscles, nervous elements, blood and other tissues. Each kind of tissue is composed of a number of living units, ordinarily microscopic in size, which are known as cells. A careful examination of various cells reveals that although they may differ greatly in size, shape and minor details, they all alike possess certain well-marked characteristics. Each when reduced to its fundamental form is seen to consist of a small mass of living matter termed protoplasm in which may usually be distinguished two regions—the cell-body orcytoplasm, and thenucleus(Fig. 3,p. 21). Any cell, whether it be of the brain, of the liver, or from any organ of an animal or plant, has this same fundamental structure. In addition, a limiting membrane or wall of some kind is generally present, although it is not a necessary constituent of all cells.
Fig. 3
Diagram of a cell showing various parts.
Unicellular Organisms.—While such a structure as a tree or a horse is composed of countless millions of cells, on the other hand numerous organisms, both plant and animal, exist which consist of only one cell. Yet this cell is just as characteristically a cell as arethe components of a complex animal or plant. It has the necessary parts, the cell body and the nucleus. Moreover it exhibits all of the fundamental activities of life, though in a simplified form, that a complex higher organism does.
Importance of Cell-Theory.—This discovery that every living thing is a single cell or an aggregation of cooperating cells and cell-products is one of our most important biological generalizations because it has brought such a wide range of phenomena under a common point of view. In the first place, the structure of both plants and animals is reducible to a common fundamental unit of organization. Moreover, both physiological and pathological phenomena are more readily understood since we recognize that the functions of the body in health or disease are in large measure the result of the activities of the individual cells of the functioning part. Then again, the problems of embryological development have become much more sharply defined since it could be shown that the egg is a single cell and that it is through a series of divisions of this cell and subsequent changes in the new cells thus formed that the new organism is built up. And lastly, the problem of hereditary transmission has been rendered more definite and approachable by the discovery that the male germ is likewise a single cell, that fertilization of the egg is therefore the union of two cells, and that in consequence the mechanism of inheritance must be stowed away somehow in these two cells.
Heredity in Unicellular Forms.—In unicellular animals one can readily see how it is possible for anindividual always to give rise to its own kind. One of the simplest of the single-celled animals is theAmeba(Fig. 4,p. 24).
The ameba eats and grows as do other animals. Sooner or later it reaches a size beyond which it can not increase advantageously, yet it is continuously taking in new food material which stimulates it to further growth. Here then is a problem. The ameba solves this difficulty by dividing to form two amebæ. Such a division is illustrated in Fig. 4,p.24. First the nucleus divides, then the cell-body. When the two new amebæ separate completely each renews the occupation of eating and growing. But what has become of the parent? Here, where once existed a large adult ameba are two young amebæ. The parent individual as such has disappeared, yet there has been no death, for we have simply two bits of living jelly in place of one. They will in turn repeat the same process, so will their offspring, and thus, barring accident, this growth and reproduction, or overgrowth as we may regard it, may go on forever, as far as we know. Here the problem of heredity, or the resemblance of offspring to parent, is not a very complicated one. The substance of the cell-body and cell-nucleus divides into two similar halves, so that each descendant has the substance of the parent in its own body, only it has but half as much. It differs from the parent, not in quality or kind, but in size.
Fig. 4
Six successive stages in the division ofAmeba polypodia(after Schulze). The nucleus is seen as a dark spot in the interior.
Reproduction and Heredity in Colonial Protozoa.—There are enormous numbers of these single-celled animals existing in all parts of the world. Some are simple like the ameba, others are very complex in structure. Many, after division, move apart and pursue wholly independent courses of existence. On the other hand we find a modification appearing in some which is of the greatest importance. After division instead of moving apart the two cells may remain sideby side and divide further to form two more, these in turn may divide and thus the process goes on until there is formed what is known as a colony. Each cell of such a colony resembles the original ancestral cell because each is a part of the actual substance of that cell. As in the ameba, the first two cells are the ancestral cell done up in two separate packets, and thus finally the full quota of cells must be so many separate packets of the same kind of material. Inasmuch as each is but a repetition of its original ancestor, it can, and at times does, produce a colony of the same kind as that ancestor produced.
Conjugation.—At longer or shorter intervals, however, we find that two individuals, on the disruption of the old colony, instead of continuing the routine of establishing new colonies through a series of cell divisions, very radically alter their behavior. They unite and fuse into a single larger individual. This process is calledconjugation. We find it occurring even in some species of ameba. The conjugating cells in some colonies are alike in size and appearance, in others different.
Specialization of Sex-Cells.—A beautiful sphere-shaped colony known asVolvoxis to be found occasionally in roadside pools. Depending on the species ofVolvoxto which it belongs, the colony may be made up of from a few hundred to several thousand individuals arranged in a single layer about the fluid-filled center of the sphere and bound together by a clear jelly-like inter-cellular substance. Each individual cell also connects with its neighbors by means of thin threads of living matter. One of the largestspecies isVolvox globator, one edge of which is represented in Fig. 5,p. 27. Mutual pressure of the cells gives them a polygonal shape when viewed from the surface. Each cell, with a few exceptions to be noted immediately, bears two long flagella, whip-like structures which project out into the water. The lashing of these flagella gives the ball a rotary motion and thus it moves about. When the colony has reached its adult condition and is ready to reproduce itself, certain cells without flagella and somewhat larger than the ordinary cells become more rounded in outline and increase considerably in size through the acquisition of food materials. They are then known as egg cells or ova. Each ovum finally enters on a series of cell-divisions forming a mass of smaller and smaller cells which gradually assumes the form of a hollow sphere like the parent colony. The young colonies thus formed drop into the interior of the parent colony to escape later to the outside as independent swimming organisms when the old colony dies and disintegrates.
The Fertilized Ovum Termed a Zygote.—After a number of generations of such asexual reproduction, sexual reproduction occurs. The ova arise as usual. Certain members of the colony, on the other hand, go to the other extreme and divide up into bundles of from sixty-four to one hundred twenty-eight minute slender cells, each provided with flagella for locomotion. When mature these small flagellate cells, now known asspermatozoa, escape into the interior of the parent colony and swim about actively. Ultimately each ovum is penetrated by a spermatozoon, the two cells fuse completely and thus form the singlefertilized ovumorzygote. The body-cells of the mother colony finally disintegrate. After a period of rest each zygote, through a series of cell-divisions, develops into an adult Volvox. In some species of Volvox a still further advance is seen, in that instead of both kinds of gametes being produced in the same colony, the ova may be produced by one colony and the spermatozoa by another. Here, then, we have the foreshadowings of two sexes as separate individuals, a phenomenon of universal occurrence among the highest forms of animal life.
Fig. 5
Volvox globator(from Hegner after Oltmanns). Half of a sexually reproducing colony:o, eggs;s, spermatozoa.
Advancement Seen in the Volvox Colony.—In the Volvox colony there is a distinct advance over the conditions met with in various lower protozoan colonies in that only certain individuals of the colonytake part in the process of reproduction and these individuals are of two distinct types; one is a larger, food-laden cell or egg and the other a small, active, fertilizing cell. The motile forms are produced in much greater numbers than the eggs, plainly because they have to seek the egg and many will doubtless perish before this can be accomplished. This disparity in number is only a means of insuring fertilization of the egg. The remaining cells of the body carry on the ordinary activities of the colony such as locomotion and nutrition and have ceased to take any part in the production of new colonies.
Natural Death Appears With the Establishment of a Body Distinct from the Germ.—Volvox is an organism of unusual interest because in it we see a prophecy of what is to come. Although still regarded as a colony of single-celled individuals, it represents in reality a transition between the whole group of unicellular animals termed protozoa and the many celled animals characterized by the possession of distinct tissues, known asMetazoa. Moreover, it shows an interesting stage in the establishment of a body orsomadistinct from special reproductive cells which have taken on the function of reproducing the colony. In such colonial forms natural death is found appearing for the first time, the reproductive cells alone continuing to perpetuate the species. Then again Volvox represents an important step in the establishment of sex in the animal kingdom for in its sexual reproduction the conjugating cells known asgametesare no longer alike in appearance but have become differentiated into definite ova and spermatozoa.
In Volvox as in the other organisms which we have studied we find that all of the cells including the germ-cells are produced by the repeated division of a parent cell, and consequently each must contain the characteristic living substance of that parent. Many other forms might be cited to illustrate reproduction in single-celled animals, whether free or in colonies, but all such cases would be practically but repetitions or modifications of those we have already examined.
Specialization in Higher Organisms.—If we pass on to the higher animals and plants which are not single cells or colonies of similar cells but organisms made up of many different kinds of cells, we find a pronounced extension of the phenomenon met with in Volvox. Instead of each cell executing independently all of the life relations, certain ones are set apart for the performance of certain functions to the exclusion of other functions which are carried on by other members of the aggregation. Thus the organism as a whole has all the life relations carried on, but, as it were, by specialists.
Sexual Phenomena in Higher Forms.—In the reproduction of multicellular organisms, one sees likewise but a continuation of the phenomena exhibited in Volvox. Ordinarily, each new form is produced by the successive divisions of a single germ-cell which in the vast majority of cases has conjugated with another germ-cell. In the development of the egg, as the divisions proceed, groups of cells become modified for their particular work until the entire organism is completed. During development certain cells are set apart for reproduction of the form just as they were inVolvox. These two kinds of reproductive cells in multicellular organisms are derived ordinarily from two separate individuals known as male and female, though there are some exceptions. The main difference between these cells which will have to unite to form a single fertile germ-cell, is that they have specialized in different directions; one is small and active, the other large, food-laden and passive. But with two such germ-cells coming as they do from two individuals, one the male, the other the female, it is obvious that the actual living substance of which each germ is composed will be distinctive of its own parental line and that when the germs unite these distinctive factors commingle, hence the complications of double ancestry arise.
Structure of the Cell.—Before we can understand certain necessary details of the physical mechanism of inheritance we must inquire a little further into the finer structure of the cell and into the nature of cell division. A typical cell, as it would appear after treatment with various stains which bring out the different parts more distinctly, is shown in Fig. 3,p. 21. Typical, not that any particular kind of living cell resembles it very closely in appearance, but because it shows in a diagrammatic way the essential parts of a cell. In the diagram, there are two well-marked regions; a centralnucleusand a peripheral cell-body orcytoplasm. Other structures are pictured but only a few of them need command our attention at present. At one side of the nucleus one observes a small dot or granule surrounded by a denser area of cytoplasm. This body is called thecentrosome. The nucleus in this instance is bounded by a well-marked nuclear membrane and within it are several substances. What appear to be threads of a faintly staining material, thelinin, traverse it in every direction and form an apparent network. The parts on which we wish particularly to rivet our attention are the densely stained substances scattered along or embedded in the strands of this network in irregular granules and patches. This substance is calledchromatin. It takes its name from the fact that it shows great affinity for certain stains and becomes intensely colored by them. This deeply colored portion of the cell, the chromatin, is by most biologists regarded as of great importance from the standpoint of heredity. One or more larger masses of chromatin or chromatin-like material, known aschromatin nucleoli, are often present, and not infrequently a small spheroidal body, differing in its staining reactions from the chromatin-nucleolus and sometimes called thetrue nucleolus, exists.
Cell-Division.—In the simplest type of cell-division the nucleus first constricts in the middle, and finally the two halves separate. This separation is followed by a similar constriction and final division of the entire cell-body, which results in the production of two new cells. This form of cell-division is known assimpleordirect division. Such a simple division, while found in higher animals, is less frequent and apparently much less significant than another type of division which involves profound changes and rearrangements of the nuclear contents. The latter is termedmitoticorindirectcell-division. Fig. 6,p. 33, illustrates some of the stages which are passed through in indirect cell-division. The centrosome which lies passively at the side of the nucleus in the typical cell (Fig. 6a,p. 33) awakens to activity, divides and the two components come to lie at the ends of a fibrous spindle. In the meantime, the interior of the nucleus is undergoing a transformation. The granules and patches of chromatin begin to flow together along the nuclear network and become more and more crowded until they take on the appearance of one or more long deeply-stained threads wound back and forth in a loose skein in the nucleus (Fig. 6b,p. 33). If we examine this thread closely, in some forms it may be seen to consist of a series of deeply-stained chromatin granules packed closely together intermingled with the substance of the original nuclear network.
As the preparations for division go on the coil in the nucleus breaks up into a number of segments which are designated aschromosomes(Fig. 6c,p. 33). The nuclear membrane disappears. The chromosomes and the spindle-fibers ultimately become related in such a way that the chromosomes come to lie at the equator of the spindle as shown in Fig 6d,p. 33. Each chromosome splits lengthwise to form two daughter chromosomes which then diverge to pass to the poles of the spindle (Figs. 6eandf,p. 33). Thus each end of the spindle comes ultimately to be occupied by a set of chromosomes. Moreover each set is a duplicate of the other, because the substance of any individual chromosome in one group has its counterpart in the other. In fact this whole complicated system of indirect divisionis regarded by most biologists as a mechanism for bringing about the precise halving of the chromosomes.
Fig. 6
Diagram showing representative stages in mitotic or indirect cell-division:a, resting cell with reticular nucleus and single centrosome;b, the two new centrosomes formed by division of the old one are separating and the nucleus is in the spireme stage;c, the nuclear wall has disappeared, the spireme has broken up into six separate chromosomes, and the spindle is forming between the two centrosomes;d, equatorial plate stage in which the chromosomes occupy the equator of the spindle;e,f, each chromosome splits lengthwise and the daughter chromosomes thus formed approach their respective poles;g, reconstruction of the new nuclei and division of the cell body;h, cell-division completed.
The chromosomes of each group at the poles finally fuse and two new nuclei, each similar to the original one, are constructed (Figs. 6gandh,p. 33). In the meantime a division of the cell-body is in progress which, when completed, results in the formation of two complete new cells.
As all living matter if given suitable food, can convert it into living matter of its own kind, there is no difficulty in conceiving how the new cell or the chromatin material finally attains to the same bulk that was characteristic of the parent cell. In the case of the chromatin, indeed, it seems that there is at times a precocious doubling of the ordinary amount of material before the actual division occurs.
Chromosomes Constant in Number and Appearance.—With some minor exceptions, to be noted later, which increase rather than detract from the significance of the facts, the chromosomes are always the same in number and appearance in all individuals of a given species of plants or animals. That is, every species has a fixed number which regularly recurs in all of its cell-divisions. Thus the ordinary cells of the rat, when preparing to divide, each display sixteen chromosomes, the frog or the mouse, twenty-four, the lily twenty-four, and the maw-worm of the horse only four. The chromosomes of different kinds of animals or plants may differ very much in appearance. In some they are spherical, in others rod-like, filamentous or perhaps of other forms. In some organisms the chromosomes of the same nucleus may differ from oneanother in size, shape and proportions, but if such differences appear at one division they appear at others, thus showing that in such cases the differences are constant from one generation to the next.
Significance of the Chromosomes.—The question naturally arises as to what is the significance of the chromosomes. Why is the accurate adjustment which we have noted for their division necessary? The very existence of an elaborate mechanism so admirably adapted to their precise halving, predisposes one toward the belief that the chromosomes have an important function which necessitates the retention of their individuality and their equal division. Many biologists accept this along with other evidence as indicating that in chromatin we have a substance which is not the same throughout, that different regions of the same chromosome have different physiological values.
When the cell prepares for divisions, the granules, as we have seen, arrange themselves serially into a definite number of strands which we have termed chromosomes. 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. To use a homely illustration we may picture the individual chromosomes to ourselves as so many separate trains of freight cars,each car of which is loaded with different merchandise. Now, if every one of the trains could split along its entire length and the resulting halves each grow into a train similar to the original, so that instead of one there would exist two identical trains, we should have a phenomenon analogous to that of a dividing chromosome.
Cleavage of the Egg.—It is through a series of such divisions as these that the zygote or fertilized egg-cell builds up the tissues and organs of the new organism. The process is technically spoken of ascleavage. Cleavage generally begins very shortly after fertilization. The fertile egg-cell divides into two, the resulting cells divide again and thus the process continues, with an ever-increasing number of cells.
Chief Processes Operative in Building the Body.—Although of much interest, space will not permit of a discussion in detail of the building up of the special organs and tissues of the body. It must suffice merely to mention the four chief processes which are operative. These are, (1) infoldings and outfoldings of the various cell complexes; (2) multiplication of the component cells; (3) special changes (histological differentiation) in groups of cells; and (4) occasionally resorption of certain areas of parts.
The Origin of the New Germ-Cells.—On account of the unusual importance from the standpoint of inheritance, which attaches to the germ-cells, a final word must be said about their origin in the embryo. While the evidence is conflicting in some cases, in others it has been well established that the germ-cells are set apart very early from the cells which are todifferentiate into the ordinary body tissues. Fig. 7A,p. 38, shows a section through the eight-celled stage ofMiastor, a fly, in which a single large, primordial germ-cell (p. g. c.) has already been set apart at one end of the developing embryo. The nuclei of the rest of the embryo still lie in a continuous protoplasmic mass which has not yet divided up into separate cells. The densely stained nuclei at the opposite end of the section are the remnants of nurse-cells which originally nourished the egg. Fig. 7B,p. 38, is a longitudinal section through a later stage in the development ofMiastor; the primitive germ-cells (oög) are plainly visible. Still other striking examples might be cited. Even in vertebrates the germ-cells may often be detected at a very early period.
Significance of the Early Setting Apart of the Germ-Cells.—It is of great importance for the reader to grasp the significance of this early setting apart of the germ-cells because so much in our future discussion hinges on this fact. The truth of the statement made in a previous chapter that the body of an individual and the reproductive substance in that body are not identical now becomes obvious. For in such cases as those just cited one sees the germinal substance which is to carry on the race set aside at an early period in a given individual; it takes no part in the formation of that individual’s body, but remains a slumbering mass of potentialities which must bide its time to awaken into expression in a subsequent generation. Thus an egg does not develop into a body which in turn makes new germ-cells, but body and germ-cells are established at the same time, the bodyharboring and nourishing the germ-cells, but not generating them (Fig. 2,p. 13). The same must be true also in many cases where the earliest history of the germ-cells can not be visibly followed, because in any event, in all higher animals, they appear long before the embryo is mature and must therefore be descendants of the original egg-cell and not of the functioning tissues of the mature individual. This need not necessarily mean that the germ-cells have remained wholly unmodified or that they continue uninfluenced by the conditions which prevail in the body, especially in the nutritive blood and lymph stream, although as a matter of fact most biologists are extremely skeptical as to the probability that influences from the body beyond such general indefinite effects as might result from under-nutrition or from poisons carried in the blood, modify the intrinsic nature of the germinal substances to any measurable extent.
Fig. 7
A—Germ-cell (p. g. c.) set apart in the eight-celled stage of cleavage inMiastor americana(after Hegner). The walls of the remaining seven somatic cells have not yet formed though the resting or the dividing (M p) nuclei may be seen;c R, chromatin fragments cast off from the somatic cells.
B—Section lengthwise of a later embryo ofMiastor; the primordial egg-cells (oög3) are conspicuous (after Hegner).
Germinal Continuity.—The germ-cells are collectively termed thegerminal protoplasmand it is obvious that as long as any race continues to exist, although successive individuals die, some germinal protoplasm is handed on from generation to generation without interruption. This is known as the theory ofgerminal continuity. When the organism is ready to reproduce its kind the germ-cells awaken to activity, usually undergoing a period of multiplication to form more germ-cells before finally passing through a process of what is known atmaturation, which makes them ready for fertilization. The maturation process proper, which consists typically of two rapidly succeeding divisions, is preceded by a marked growth in size of the individual cells.
Individuality of Chromosomes.—Before we can understand fully the significance of the changes which go on during maturation we shall have to know moreabout the conditions which prevail among the chromosomes of cells. As already noted each kind of animal or plant has its own characteristic number and types of chromosomes when these appear for division by mitosis. In many organisms the chromosomes are so nearly of one size as to make it difficult or impossible to be sure of the identity of each individual chromosome, but on the other hand, there are some organisms known in which the chromosomes of a single nucleus are not of the same size and form (Fig. 8,p. 41). These latter cases enable us to determine some very significant facts. Where such differences of shape and proportion occur they are constant in each succeeding division so that similar chromosomes may be identified each time. Moreover, in all ordinary mitotic divisions where the conditions are accurately known, these chromosomes of different types are found to be present as pairs of similar elements; that is, there are two of each form or size.
Pairs of Similar Chromosomes in the Nucleus Because One Chromosome Comes from Each Parent.—When we recall that the original fertilized egg from which the individual develops is really formed by the union of two gametes, ovum and spermatozoon, and that each gamete, being a true cell, must carry its own set of chromosomes, the significance of the pairs of similar chromosomes becomes evident; one of each kind has probably been contributed by each gamete. This means that the zygote or fertile ovum contains double the number of chromosomes possessed by either gamete, and that, moreover, each tissue-cell of the new individual will contain this dual number. For, as wehave seen, the number of chromosomes is, with possibly a few exceptions, constant in the tissue-cells and early germ-cells in successive generations of individuals. For this to be true it is obvious that in some way the nuclei of the conjugating gametes have come to contain only half the usual number. Technically the tissue-cells are said to contain thediploidnumber of chromosomes, the gametes the reduced orhaploidnumber.
Fig. 8
A—Chromosomes of the mosquito (Culex) after Stevens.
B—Chromosomes of the fruit-fly (Drosophila) after Metz.
Both of these forms have an unusually small number of chromosomes.
In Maturation the Number of Chromosomes Is Reduced by One-Half.—This halving, or as it is known,reductionin the number of chromosomes is the essential feature of the process of maturation. It is accomplished by a modification in the mitotic division in which instead of each chromosome splitting lengthwise, as in ordinary mitosis, the chromosomes unite in pairs (Fig. 9b,p. 42), a process known technically assynapsis, and then apparently one member of each pair passes entire into one new daughter cell,the other member going to the other daughter cell (Fig. 9c,p. 42). In the pairing preliminary to thisreduction division, leaving out of account certain special cases to be considered later, according to the best evidence at our command the union always takes place between two chromosomes which match each other in size and appearance. Since one of these is believed to be of maternal and the other of paternal origin, the ensuing division separates corresponding mates and insures that each gamete gets one of each kind of chromosome although it appears to be a matter of mere chance whether or not a given cell gets the paternal or the maternal representative of that kind.