Meiosis

“ In consequence there are two classes of sperms in man; female producing containing 24 chromosomes, and male producing containing 23 chromosomes. If the factor for color blindness is carried by the X chromosome its inheritance in man works out on the same chromosome scheme and in the same way as does white eye color (or any other sex linked character) in the fly, for the O sperm in man is equivalent to the Y sperm in the fly. ”

“ When a male germ-cell or sperm unites with a female germ-cell or ovum, within the female body, fertilisation of the ovum takes place, and this gives rise to the fertilised germ-cell from which is to arise first the germ-cells or direct descendants of itself, and secondly the embryo in which these germ-cells will come to lie. This happens by the repeated and continued division of the fertilised germ-cell, a division which constitutes growth, and which under suitable conditions of nourishment and protection and exercise will ultimately produce a human being. ”

“ In the sperm-cell there occurs just that failure in the production of chromatin which, according to the hypothesis above sketched out, is to be expected; for, in the process of cell-multiplication, the cells which become spermatozoa are left with half the number of chromosomes possessed by preceding cells: there is actually that impoverishment and declining vigour here suggested as the antecedent of fertilization. It needs only to imagine the ovum and the polar body to be alike in size, to see the parallelism ”

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

“ But the germ cells do not remain undifferentiated; both egg and sperm are differentiated, the former for receiving the sperm and for the nourishment of the embryo, the latter for locomotion and for penetration into the egg. But while the differentiations of tissue cells are usually irreversible, so that they do not again become germinal cells, the differentiations of the sex cells are reversible, so that these cells, after their union, again become germinal cells. ”

“ Thither a number of the male cells have traveled by their own activity; several come in contact with the ovum and one, but only one, actually enters it. Almost at the moment when they touch, the two cells unite so intimately that all trace of the spermatozoon is lost. Fertilization of the ovum, as this event is scientifically termed, has as its main purpose the uniting of the nucleus of a male germinal cell with the nucleus of the female germinal cell. ”

“ Of the 24 chromosomes in each sperm or egg we are here concerned with only one, known as the sex chromosome because, in addition to transmitting other characteristics, it determines the sex of the new individual.
Neither the ovum nor the spermatozoon (the human race is referred to) is capable alone of developing into a new individual. They must join in the process known as fertilization. The sperm penetrates the egg (within the body of the female) and the 24 chromosomes from each source, male and female, are re-grouped in a new nucleus with 48 chromosomes—the full number. ”

“ The oocyte contains 6 chromosomes and the spermatocyte 5 chromosomes which are not yet united into synaptic pairs; the "sex" chromosomes are shown in black and white, two are present in the oocyte, but one is present in the spermatocyte. In the reduction division the synaptic pairs separate, giving rise to two types of spermatozoa, one of which has the sex chromosome and the other lacks it; all ova are alike in this regard. If an egg is fertilized by a sperm without the sex chromosome a male results; if fertilized by a sperm containing the sex chromosome a female results. ”

“ Before fertilization a neck-canal cell is formed by the division of the ovum-nucleus. After the body of a spermatozoid has coalesced with the egg-nucleus the latter divides repeatedly and forms a mass of tissue which grows more vigorously in the lower part of the fertilized ovum, and extends upwards towards the apex of the ovum as a peripheral layer of parenchyma surrounding a central space. By further growth this tissue gives rise to a proembryo, which consists, at the micropylar end, of a sac ”

“ When we come to consider the second spermatocytes, spermatids, and spermatozoa, it is necessary to divide them into two classes, because of the unequal apportionment of the accessory chromosome consequent upon its remaining undivided in the first spermatocyte mitosis. There are formed, accordingly, two numerically equal classes of second spermatocytes—those containing sixteen chromosomes plus the accessory chromosome, and those with merely the sixteen chromosomes. ”

“ It is therefore evident that an egg fertilized by a spermatozoön (1) containing the small member of an unequal pair or (2) lacking one chromosome, must develop into a male, while an egg fertilized by a spermatozoön containing the larger element of an unequal pair of heterochromosomes or the odd chromosome must produce a female.
Whether these heterochromosomes are to be regarded as sex chromosomes in the sense that they both represent sex characters and determine sex, one can not decide without further evidence. ”

“ Now when in the course of the production of egg cells within the mother’s germinal tissue the pairs of chromosomes are pulled apart, each separate cell, and so each egg, will contain the full number of chromosomes, 24, including the sex chromosome. But when the same thing happens in the course of the production of sperm only every other one will have the full number ”

“ We are now ready to go back to the germinal tissue and trace the process of heredity as it actually works out in the developing offspring. In the ordinary cells of the germinal tissue, as we have already seen, the chromosomes are exactly like those in the other cells of the body. Cell multiplication goes on actively in the germinal tissue of both parents; in the mother this leads to the production of germ cells which are called eggs, and in the father to the production of cells that are called sperm. ”

“ In all these cases there are two X chromosomes in the oocytes (and somatic cells) of the female, and after reduction the female gametes or unfertilised ova are all alike, having a single X chromosome or group. On fertilisation half the zygotes have XX and half XY, whether Y is absence of a sex-chromosome, or one of the other Y forms above mentioned. The sex is thus determined by the male gamete, the X chromosome united with that of the female gamete producing female individuals, while the Y united with X produces male individuals. ”

“ Fig. 8.—Preparation for Mitosis. a, Nucleus of “1/2 blastomere” of Ascaris megalocephala bivalens in resting condition; b and c, nuclei from sister 1/2 blastomeres in preparation for mitosis.
In the same worm (var. univalens) Boveri (1888 and 1899) found that occasional abnormalities in maturation resulted in the suppression of the first polar body and the inclusion of its chromosomes in the second maturation spindle; the egg-nucleus at the time of fertilization thus having two chromosomes instead of one, while the spermatozoon nucleus has only one. ”

“ When it reaches the ovary, the sperm cell penetrates an ovule by making its way through a little hole called the micropyle. It then grows toward a clear bit of protoplasm known as the embryo sac. The embryo sac is an ovoid space, microscopic in size, filled with semifluid protoplasm containing several nuclei. (See Figure.) One of the nuclei, with the protoplasm immediately surrounding it, is called the egg cell. It is this cell that the sperm nucleus of the pollen tube grows toward; ultimately the sperm nucleus reaches the egg nucleus and unites with it. ”

“ If, for instance, all or most of the chromatin brought in by the spermatozoon should be carried into one of the first formed cells along with half of the chromatin of the egg-nucleus, then all the cells that descend from this cell may develop female characters, and all those from the other, male characters. ”

“ Corroborative evidence appears in the contrast which persists in all the higher forms between the relatively large female-cell or germ and the microscopical male-cell or sperm, as also in the absorption of the male cellule by the female cellule. In the sexual cells there is no character in which differentiation goes so far as that of size. [24] The female cell is always much larger than the male; where the former is swollen with the reserve food, the spermatozoa may be less than a millionth of its volume. In the human species an ovum is about 3000 times as large as spermatozoa. ”

“ The one outstanding discovery which has placed the science of Embryology on an absolutely firm basis, and which has made clear so many of the facts, which were previously puzzling, is this: that the germ-cells which give rise to new individuals are themselves produced from pre-existing germ-cells. The entire embryo, or young infant, is derived from one single cell which we have called the fertilised ovum, and that in its turn was derived from the union of two germ-cells, one from the male parent, and one from the female. ”

“ It has not been ascertained, nor is it, perhaps, ascertainable, whether the sperm cells of the male contain in their chromatin a preponderance of male, while the germ cells provide chiefly the female determinants.48 However this may be, it is certain that determinants which severally control the formation both of male and of female structure are always present in every combination of the sperm and germ cells, those which exhibit the greatest energy and vitality probably prevailing in the determination of the sex of the future being. ”

“ The fertilised cell is quite another thing from the unfertilised cell. For if we must regard the spermia as real cells no less than the ova, and the process of conception as a coalescence of the two, we must consider the resultant cell as a quite new and independent organism. It bears in the cell and nuclear matter of the penetrating spermatozoon a part of the father's body, and in the protoplasm and caryoplasm of the ovum a part of the mother's body. ”

“ The Mendelian law of segregation has somewhat lifted the veil in which maturition was wrapped. Every germ cell, being a product of two parent cells, is in one way or other a hybrid. By casting off half of the chromosomes during its maturing, it becomes pure. The gamete is thus always pure and ready for impregnation.
Impregnation.—The process of impregnation is about the same in most many-celled animals. As soon as the head of a single spermatozoön enters the egg-cytoplasm, a new membrane is formed around the ovum which effectually prevents the entrance of any other spermatozoa. ”

“ Parthenogenesis takes place when the whole of the ancestral germ-plasms, inherited from the parents, are retained in the nucleus of the egg-cell. Development by fertilization makes it necessary that half the number of these ancestral germ-plasms must be first expelled from the egg, the original quantity being again restored by the addition of the sperm-nucleus to the remaining half. ”

“ Penetration of the Spermatozoid into the Egg.—In all the higher animals in which the germinal cells are of two kinds, male and female, conjugation takes place in rather a different manner. Here, the female cell or egg only reproduces itself exceptionally by parthenogenesis. It usually contains no chromosomes and often too little chromatin, so that it perishes when conjugation does not occur. ”

“ We have said that two kinds of cells represent reproduction, namely, sperm and germ-cells. These may be furnished by different individuals, or both may be found in one. When both are found in the same individual, the parent is said to be a natural hermaphrodite. A perfect hermaphrodite possesses the attributes of both male and female—uniting both sexes in one individual. Natural hermaphroditic reproduction occurs only among inferior classes of animals, and naturalists inform us that there are a greater number of these than of the more perfect varieties. ”

“ True the germ-cells from the male, i.e. the sperms, differ in appearance when seen under the microscope from those of the female, but there is no reason to believe that there is any difference between them in their capacity, for example, of transmitting the characters of ancestors to succeeding generations. ”

“ But if it may be assumed that human twins, when identical, have been derived from a single egg, it seems to me to be extremely probable that fertilization was also effected by a single sperm-cell. We cannot understand how such a high degree of similarity could have been produced if two sperm-cells had been made use of, for we are compelled to assume that two such cells would very rarely contain identical germ-plasms. ”