Gamete

“ The shape of the gametes is not amoeboid, as is that of the twenty-four to thirty-six-hour-old schizont, but round or oval. The full-grown gametes have the pigment distributed and the chromatin in a single aggregation—just the opposite of nonsexual parasites. The male gamete stains a light grayish blue and has a very large amount of chromatin, usually centrally placed. The female gamete stains a pure blue, has only about one-tenth as much chromatin as plasma, with the chromatin often placed at one side. The pigment of the female gamete is dark brown while that of the male is yellowish brown. ”

“ The asexual zoospores may quickly and readily produce a new individual. The gamete, generally speaking, must fuse with its kind or else die. We have seen that primitive gametes may germinate without conjugation but the resulting plants in the cases best known are weaker than normal individuals. We also know that the lower stretches of the plant kingdom furnish abundant illustrations of parthenogenesis, that is, the power of an egg cell to develop without fertilization. ”

“ Every gamete contains small definite bodies known as chromosomes, and the number of these is, with few known exceptions, constant for the gametes of a given species. On the fusion of two gametes the resulting cell or zygote has therefore a double structure, for it contains an equal number of chromosomes brought in by the paternal and by the maternal gamete—in the case of a plant by the pollen grain as well as by the ovule. By a process of repeated division the zygote gives rise to a plant (or an animal) whose cells apparently retain the double structure throughout. ”

“ One of these corresponds to a polar body in the metazoon egg, the other is the gamete, or germ, nucleus. In each cell this nucleus collects about itself, possibly through the secretion of nuclear material which transforms the surrounding stuff, a denser zone of protoplasm, which, with the nucleus, forms a gamete within the body of the parent cell. The two gametes thus formed fuse while in the space which their formation has left in the parent somatic, or nurse cells. ”

“ An agamete is a germ cell not specialized for union with another complementary cell, or, in other words, it is a reproductive cell incapable of syngamy, e.g. a spore. A gamete, on the other hand, is a reproductive cell (germ cell) specialized for the production of a zygote (a synthetic or diploid germ cell) by union with a complementary cell, e.g. an egg, or a sperm.
Nonsexual cytogenic reproduction is of three kinds, according to the nature of the agamete. When a unicellular organism gives rise to two new individuals by simple cell-division, we have fissiparation or binary fission. ”

“ The condition of heterogamy and its advance over isogamy is the result of several well-defined factors at work wherever sex is present. It is advantageous to specialize one gamete for the purpose of holding as much nourishment as possible, thereby providing well for the vegetative possibilities of the next generation. The most effective way to accomplish this end is to reduce the number of female gametes produced in each oogonium, and the best results will obtain when all the protoplasm from such a mother cell goes to a single female gamete. ”

“ It was in this Alga that the conjugation of self-motile gametes was first discovered by Pringsheim, 1869. When conjugation is about to take place, each cell divides into sixteen, as in vegetative reproduction, but the 16 × 16 cells all separate from one another (Fig. 45 C, female gametes, and D, male gametes) , and swarm solitarily in the water. The male are, most frequently, smaller than the female, but otherwise they are exactly alike ”

“ This is a subject quite independent of the origin of sex and deserves the separate treatment that we are now to present.
Primitive gametes are sexual cells so similar in size, form and internal structure that they cannot be distinguished as male or female. They are found in a number of Fig. 1. Gametes and Gametangia of (a) Cladophora, (b) Ulva. the lower groups of algae and have the general appearance and same mode of formation as the zoospores from which they have been derived. ”

“ For in each of these respects the individual is double; and so to get a true picture of the composition of the individual we have to think how each of the two original gametes was provided in the matter of height, hair, colour, mathematical ability, nail-shape, and the other features that go to make the man we know. The contribution of each gamete in each respect has thus to be separately brought to account. ”

“ The female gametes of this species have a limited period of motility. They swim about slowly and shortly come to rest, attaching themselves. The male gametes are motile for a much longer time. While at rest the female gametes attract the males which hover around (see Fig. 3, a) , with the result that one fuses with a motionless cell and fertilizes it. This is an exceedingly interesting condition and furnishes the last link in the chain of stages through which gametes pass in their sexual evolution. ”

“ The parent-individuals of the microgametes, or microgametocytes, are distinguished as a general rule by clearer protoplasm, free from coarse granulations, and a larger nucleus, more rich in chromatin. The macrogametocytes, on the other hand, usually have coarsely granular cytoplasm, rich in reserve food-stuffs, and a relatively small nucleus. The gametocytes produce the gametes by methods that vary according to the degree of specialization of the gametes. ”

“ It is often found that not only are the gametes differentiated, but that their immediate progenitors may also exhibit characters which mark them off from the ordinary or indifferent individuals of the species. In such cases the parent-forms of the gametes are termed gametocytes, and they may differ amongst themselves in characters which render it possible to distinguish those destined to produce microgametes from those which will produce the other sex. ”

“ We know little if anything of the relation between the two conjugating cells or gametes, of the real nature of the attraction that causes them to approach each other and ultimately unite together. We have, it is true, some evidence that one cell affects the other by some chemical action, as for instance in the fact that the mobile male gametes of a fern are attracted to a tube containing malic acid, but this may be merely an influence on the direction of movement of the male gamete, while there are cases in which neither cell is actively mobile. ”

“ Nearly {279}always the containing cells, often called gametes, are unlike: the sperm-cell being the male product, and the germ-cell the female. But among some Protozoa and many of the lower Algæ and Fungi, the uniting cells show no differentiation. Sexuality is only nascent.
There are very many modes and modifications of modes in which these cells are produced; very many modes and modifications of modes by which they are brought into contact; and very many modes and modifications of modes by which the resulting fertilized germs have secured to them the fit conditions for their development. ”

“ Our conception of what occurs when a cross is made between two individuals may be illustrated by the diagram which forms fig. 2. Zygotes are here represented by squares and gametes by circles. The dominant and recessive characters are indicated by small plain and black rectangles. Each zygote must contain two and each gamete but one of these unit-characters. Zygotes such as the original parents which breed true to a given character are said to be homozygous for that character, and from their nature such homozygotes must produce identical gametes. ”

“ The cytoplasm of the male gametocytes is pale-staining and the nucleus is elongate, while the cytoplasm of the females is darker and the nucleus is smaller and round. Formation of male gametes from male gametocytes (the so-called process of “exflagellation”) may occur on a slide of drawn infected blood, also fertilization, and formation of the oökinete, as first seen by MacCallum. ”

“ In most forms in which the formation of the gametes from the progamete has been accurately followed, and in which the progamete of both sexes divides twice in forming the gametes, the division of the nucleus presents certain peculiarities. ”

“ The latter, in consequence, grew slowly, and their sex had time to be differentiated, and was determined by the reaction of the host. He further described and drew from life the formation and fertilisation of the gametes, and again showed the extrusion of part of the nucleus and the actual process of fecundation by micro- gametes. ”

“ The result in either case is a “zygote,” a cell formed by fusion of two which divides in the usual way to produce new individuals. The microgamete in this case is the male element and equivalent to a spermatozoon; the megagamete is the female and equivalent to an egg-cell. The zygote is a “fertilized egg,” or oosperm. In some colony-building forms we find that only certain cells produce by division microgametes; and, regarding the colony as a multicellular individual, we may consider these cells as testis-cells and their microgametes as spermatozoa. ”

“ The gametocytes may, however, possess the power of multiplication without change of character for many generations; or, to put the matter in other words, the sexual differentiation may be apparent not merely in the generation immediately preceding the gametes, but in many generations prior to this. Thus a given species may consist of three different types of adult individuals, male, female and indifferent, each multiplying in its own line. ”

“ The gametes are easily recognized. Their length is somewhat greater than the diameter of a red corpuscle. Their chromatin is usually centrally placed, and they contain more or less coarse pigment. The remains of the [p. 194] red cell often form a narrow rim around them or fill the concavity of the crescent.
While the parasites are more easily found in stained preparations, the varieties are more easily differentiated in fresh blood. The chief distinguishing points are included in the following table:
2. ”

“ Nets with strong inclinations to form zoospores can be made to produce gametes by cultivating in a sugar solution in subdued light or darkness. Plants that have no special inclination to form either zoospores or gametes may be decided one way or the other by the illumination, bright light producing zoospores and darkness gametes. It is also fair to say that sometimes the tendency to form zoospores is so strong that a plant will not yield for several generations to the conditions that generally bring about the immediate production of gametes. ”

“ Whatever the details of development, one cardinal fact is clear. Except for the relatively rare instances of parthenogenesis a new individual, whether plant or animal, arises as the joint product of two sexual cells derived from individuals of different sexes. Such sexual cells, whether ovules or ova, spermatozoa or pollen grains, are known by the general term of gametes, or marrying cells, and the individual formed by the fusion or yoking together of two gametes is spoken of as a zygote. ”

“ Owing to the completely dominant nature of the tall character, such a plant is in appearance indistinguishable from the pure tall, but it differs markedly from it in the nature of the gametes to which it gives rise. When the formation of the gametes occurs, the elements representing dwarfness and tallness segregate from one another, so that half of the gametes produced contain the one, and half contain the other of these two elements. ”

“ It is possible that the embryo in the unpollenized chinkapins does not originate from the female gamete at all, but that it originates from a formative budding of other cells in the ovule. We can speak of parthenogenesis only when the embryo originates from a female gamete alone, i. ”

“ Moreover, among the lower Algæ, which show the beginning of sexual differentiation, the smaller gametes, which we must regard as incipient sperm-cells, are sometimes able to fuse inter se, and give rise to a zygote which will produce a new plant. All which evidence is in perfect harmony with the foregoing conclusion; since, if sperm-cells and germ-cells have natures not essentially unlike those of unspecialized cells in general, their natures cannot be essentially unlike each other. ”

“ Male gametes. 1. Young form similar to female one. 2. Half grown form showing central chromatin. 3. Full grown microgametocyte showing large amount of centrally placed chromatin with light blue protoplasm surrounding. 4. Division of chromatin occurring in microgametocyte and developing in wet preparation. Note.—Chromatin division in gametes does not take place until blood is withdrawn. 5. Spermatozoon like microgametes developing from the microgametocyte. This only occurs in wet preparations or in the stomach of the mosquito.
Quartan Parasites ”

“ Seeded Peas. Large circles represent zygotes, small ones, or single letters, gametes. In F1 all individuals are round but contain round and wrinkled gametes. In F2 the ♂ gametes are placed above the square, the ♀ ones to the left, and the possible combinations of ♂ and ♀ gametes are shown in the small squares, the relative numbers of different types being 1 RR : 2 R (W) : 1 WW. ”

“ The simplest possible condition of the gametes is seen in the free-swimming Ciliata, forms which in other respects are the most highly organized of Protozoa; here the individuals which conjugate are only distinguished from ordinary individuals of the species by the fact that their nuclei have undergone very complicated processes of reduction and nuclear elimination. ”