Being Well Born Part 2

=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 side by 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 repet.i.tion 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 called _conjugation_. 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 s.e.x-Cells.--=A beautiful sphere-shaped colony known as _Volvox_ is to be found occasionally in roadside pools. Depending on the species of _Volvox_ to 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 largest species is _Volvox 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 ma.s.s of smaller and smaller cells which gradually a.s.sumes 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 as.e.xual reproduction, s.e.xual 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 as _spermatozoa_, 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 single _fertilized ovum_ or _zygote_. 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 s.e.xes as separate individuals, a phenomenon of universal occurrence among the highest forms of animal life.

[Ill.u.s.tration: FIG. 5

_Volvox globator_ (from Hegner after Oltmanns). Half of a s.e.xually 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 colony take 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 as _Metazoa_. Moreover, it shows an interesting stage in the establishment of a body or _soma_ distinct 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 s.e.x in the animal kingdom for in its s.e.xual reproduction the conjugating cells known as _gametes_ are 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 ill.u.s.trate reproduction in single-celled animals, whether free or in colonies, but all such cases would be practically but repet.i.tions or modifications of those we have already examined.

=Specialization in Higher Organisms.--=If we pa.s.s 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 p.r.o.nounced 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.

=s.e.xual 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 in Volvox. 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 pa.s.sive. 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 central _nucleus_ and a peripheral cell-body or _cytoplasm_. 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 the _centrosome_. 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, the _linin_, 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 called _chromatin_. 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 ma.s.ses of chromatin or chromatin-like material, known as _chromatin nucleoli_, are often present, and not infrequently a small spheroidal body, differing in its staining reactions from the chromatin-nucleolus and sometimes called the _true 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 as _simple_ or _direct 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 termed _mitotic_ or _indirect_ cell-division. Fig. 6, p. 33, ill.u.s.trates some of the stages which are pa.s.sed through in indirect cell-division. The centrosome which lies pa.s.sively at the side of the nucleus in the typical cell (Fig. 6_a_, 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. 6_b_, 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 as _chromosomes_ (Fig.

6_c_, 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 6_d_, p. 33. Each chromosome splits lengthwise to form two daughter chromosomes which then diverge to pa.s.s to the poles of the spindle (Figs.

6_e_ and _f_, 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 division is regarded by most biologists as a mechanism for bringing about the precise halving of the chromosomes.

[Ill.u.s.tration: 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. 6_g_ and _h_, 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 one another 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 const.i.tuted, 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 ill.u.s.tration 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 a.n.a.logous 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 as _cleavage_.

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, s.p.a.ce 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 to differentiate into the ordinary body tissues. Fig. 7_A_, p.

38, shows a section through the eight-celled stage of _Miastor_, 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 ma.s.s 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. 7_B_, p. 38, is a longitudinal section through a later stage in the development of _Miastor_; the primitive germ-cells (oog) 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 ma.s.s 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 body harboring 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.

[Ill.u.s.tration: FIG. 7

A--Germ-cell (_p. g. c._) set apart in the eight-celled stage of cleavage in _Miastor 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 of _Miastor_; the primordial egg-cells (_oog__{3}) are conspicuous (after Hegner).]

=Germinal Continuity.--=The germ-cells are collectively termed the _germinal protoplasm_ and 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 of _germinal 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 pa.s.sing through a process of what is known at _maturation_, 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 more about 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 ident.i.ty 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 we have 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 the _diploid_ number of chromosomes, the gametes the reduced or _haploid_ number.

[Ill.u.s.tration: 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, _reduction_ in 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. 9_b_, p. 42), a process known technically as _synapsis_, and then apparently one member of each pair pa.s.ses entire into one new daughter cell, the other member going to the other daughter cell (Fig.

9_c_, p. 42). In the pairing preliminary to this _reduction 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.

[Ill.u.s.tration: FIG. 9

Diagram to ill.u.s.trate spermatogenesis: _a_, showing the diploid number of chromosomes (six is arbitrarily chosen) as they occur in divisions of ordinary cells and spermatogonia; _b_, the pairing (synapsis) of corresponding mates in the primary spermatocyte preparatory to reduction; _c_, each secondary spermatocyte receives three, the haploid number of chromosomes; _d_, division of the secondary spermatocytes to form _e_, spermatids, which transform into _f_, spermatozoa.]

=Maturation of the Sperm-Cell.--=In the maturation of the male gamete the germ-cell, now known as a _spermatogonium_, increases greatly in size to become a _primary spermatocyte_. In each primary spermatocyte the pairing of the chromosomes already alluded to occurs as indicated in Fig. 9_b_, p.

42, where six is taken arbitrarily to indicate the ordinary or _diploid_ number of chromosomes, and three the reduced or _haploid_ number. The division of the primary spermatocyte gives rise to two _secondary spermatocytes_ (_c_), the paired chromosomes separating in such a way that a member of each pair goes to each secondary spermatocyte. Each secondary spermatocyte (_d_) soon divides again into two _spermatids_ (_e_), but in this second division the chromosomes each split lengthwise as in an ordinary division so that there is no further reduction. In some forms the reduction division occurs in the secondary spermatocytes instead of the primary. Each spermatid transforms into a mature spermatozoon (_f_). The spermatozoa of most animals are of linear form, each with a head, a middle-piece and a long vibratile tail which is used for locomotion. The head consists for the most part of the transformed nucleus and is consequently the part which bears the chromosomes.