Book - Text-Book of Embryology 3
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Bailey FR. and Miller AM. Text-Book of Embryology (1921) New York: William Wood and Co.
- Contents: Germ cells | Maturation | Fertilization | Amphioxus | Frog | Chick | Mammalian | External body form | Connective tissues and skeletal | Vascular | Muscular | Alimentary tube and organs | Respiratory | Coelom, Diaphragm and Mesenteries | Urogenital | Integumentary | Nervous System | Special Sense | Foetal Membranes | Teratogenesis | Figures
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When the complex maturation processes described in the preceding chapter are completed, the spermatozoon is ready for union with the mature ovum. This union, which forms the starting point of a new individual in all sexual reproduction, is known as fertilization, and the resulting cell is the fertilized ovum, or zygote.
The details of the process vary in different animals. Its essence is the entrance of the spermatozoon into the ovum and the union of the nucleus of the spermatozoon with the nucleus of the ovum. At the time of its entrance into the egg, the sperm head is small and its chromatin extremely condensed. Soon after entering the ovum, however, the sperm head undergoes development into a typical nucleus, the male pronucleus. This male pronucleus is to all appearances exactly similar in structure to the nucleus of the egg which latter is now known as the female pronucleus. The chromatin networks in both pronuclei next pass into the spireme stage, the spiremes segmenting into chromosomes of which each pronucleus contains one-half the somatic number. The nuclear membranes meanwhile disappear and the chromosomes lie free in the cytoplasm. During these changes in the pronuclei, the amphiaster has formed and the male and the female chromosomes mingle in its equatorial plane. At this stage no actual differentiation can be made between male chromosomes and female chromosomes, the differentiation shown in Fig. 15 being schematic. The picture is now that of the end of the prophase of ordinary mitosis, the somatic number of chromsomes being arranged in a plane midway between the two centrosomes. With the mingling of male and female chromosomes fertilization proper comes to an end. The further steps are also identical with those of ordinary mitosis. Each chromosome splits longitudinally into two exactly similar parts, one of which is contributed to each daughter nucleus, and the cell body divides into two equal parts. There thus result from the first division of the fertilized ovum, two cells which are apparently exactly alike and each of which contains exactly the same amount of male and of female chromosome elements.
The amphiaster of the fertilized ovum appears to develop as in ordinary mitosis. As to the origin of the centrosomes, however, much uncertainty still exists. The middle piece of the spermatozoon always enters the ovum with the head. It has already been shown that one or two spermatid centrosomes take part in the formation of the middle piece. Male centrosome elements are therefore undoubtedly carried into the ovum in the middle piece. It is equally well known, for some forms at least, that the centrosome of the ovum disappears just after the extrusion of the second polar body. In a considerable number of forms the development of the egg centrosome from, or in close relation to the middle piece of the spermatozoon has been observed. The details of fertilization as it occurs in the sea-urchin have been carefully described by Wilson. In cases of this type (Fig. 16) the tail of the spermatozoon remains outside the egg while the head and middle piece, almost immediately after entering, turn completely around so that the head points away from the female pronucleus. An aster with its centrosomes next appears, developing from, or in very close relation to the middle piece. The aster and sperm nucleus now approach the female pronucleus, the aster leading and its rays rapidly extending. On or before reaching the female pronucleus the aster divides into two daughter asters which separate with the formation of the usual central spindle, while the two pronuclei unite in the equatorial plane and give rise to the chromosomes of the cleavage nucleus. In the seaurchin the polar bodies are extruded before the entrance of the spermatozoon. In cases where the polar bodies are not extruded until after the entrance of the spermatozoon the amphiaster forms while waiting for their extrusion, the nuclei joining subsequently. When the sperm head finds the polar bodies already extruded, union of the two pronuclei may take place first, followed by division of the centrosomes and the formation of the amphiaster.
Fig. 15. Diagram of fertilization of the ovum. (The somatic number of chromosomes is 4.) Boveri, Bohm and von Damdoff.
Fig. 16. Fertilization of the eggs of the star-fish and sea-urchin. A, B, C, entrance of the sperm into the cytoplasm (star-fish). D, mature spermatozoon of the sea-urchin; E-H, successive stages in the penetration of the sperm nucleus (♂N) and centrosome (♂C) into the cytoplasm; I-L, stages in the approach of the sperm nucleus (♂N) to the egg nucleus (♀N), the division of the sperm centrosome (♂O) and the first cleavage spindle. Fol, Wilson, from Conklin Heredity and Environment.
The coming together of ovum and spermatozoon is apparently determined in some cases by a definite attraction on the part of the ovum toward the spermatozoon. This attraction seems to be of a chemical nature, but is often not limited to the attraction of spermatozoa of the same species. Foreign spermatozoa will be attracted and will enter the ovum if they are physically able to do so. The entrance of these spermatozoa may even start the process of cleavage, though such cleavage is usually abnormal and does not progress very far. That this attraction is not dependent upon the integrity of the ovum as an organism is shown by the fact that small pieces of egg cytoplasm free from nuclear elements exert the same attractive force, so that spermatozoa are not only attracted to them, but will actually enter them. In other cases the stimulus for fertilization is obviously one of contact. The spermatozoa of some fishes will swim around at random until they touch any object when they become attached and are unable to escape. Fertilization in these cases is therefore a matter of chance favored by the enormous number of sperms produced, and by the special breeding habits which insure a close proximity of sperms and eggs.
Of eggs which are enclosed by a distinct membrane, the vitelline membrane, some (e.g., those of amphibians and of mammals) are permeable to the spermatozoon at all points; others have a definite point at which the spermatozoon must enter, this being of the nature of a channel through the membrane the micropyle. In some instances a little cone-shaped projection from the surface of the egg, the attraction cone, either precedes or immediately follows the attachment of the spermatozoon to the egg (Fig. 15). Instead of a projection there may be a depression at the point of entrance.
There seems to be no question that but one spermatozoon has to do with the fertilization of a particular ovum. In mammals only one spermatozoon normally pierces the vitelline membrane although several may penetrate the zona pellucida to the perivitelline space. Should more than one spermatozoon enter such an egg as, for example, in pathological polyspermy the result is an irregular formation of asters and polyasters (Fig. 17), and the early death of the egg either before or soon after a few attempts at cleavage. In some insects, and in selachians, reptiles and birds, a number of spermatozoa normally enter an ovum, but only one goes on to form a male pronucleus.
The ovum thus not only exerts an attractive influence toward spermatozoa, but it apparently exerts this influence only until the one requisite to its fertilization has entered, after which it appears able to protect itself against the further entrance of male elements. As to the means by which this is accomplished little is known, although several theories have been advanced. It may be that when the single spermatozoon necessary to accomplish fertilization has entered the ovum, it sets up within the ovum such changes as to destroy the attractive powers of the ovum toward other spermatozoa, or as even to prevent their entrance. In the case of eggs where the spermatozoon enters through a micropyle, it has been suggested that the tail of the first spermatozoon remaining in the opening might effectually block the entrance to other spermatozoa; or the passage of the first spermatozoon might set up such mechanical or chemical changes in the canal as would prevent further access. In most cases of eggs which have no vitelline membrane previous to fertilization, such a membrane is formed immediately after the entrance of the first spermatozoon, a natural inference being that this membrane may prevent the entrance of any more spermatozoa. Biologists, however, are inclined to discredit the view that the fertilization membrane is a protection against polyspermy.
Fig. 17. Polyspermy in sea-urchin eggs treated with 0.005 per cent, nicotine solution. O. and R. Hertwig, Wilson.
- B, Showing ten sperm nuclei, three of which have conjugated with female pronucleus. C, Later stage showing polyasters formed by union of sperm amphiasters.
The time and place of fertilization are matters of scientific interest and practical importance. In the lower vertebrates, fishes and amphibians, the female discharges the ova into the water at the breeding season and the male likewise discharges the spermatozoa. The sperms swim about and come in contact with and penetrate the ova shortly after they are discharged. If fertilization does not occur both kinds of germ cells soon begin to disintegrate, neither kind remaining alive as a rule for more than a few hours. Among these animals the medium in which fertilization occurs is necessarily water, and since it takes place outside of the animal body it is called external fertilization.
In reptiles, birds and mammals the spermatozoa enter the genital tract of the female and there come in contact with and enter the ova. This is internal fertilization, but the medium in which it occurs is fluid the secretions of the female genital tract. A fluid medium is essential because the progress of the sperm depends upon its flagellate activity. In reptiles and birds the spermatozoa move through the genital passages to the ovarian portion of the oviduct where they enter the ova before the secondary eggmembranes, the albumen and the shell, are deposited. After fertilization development begins at once and, in birds at least, continues until the egg is laid and exposed to the lower external temperature. If it has been fertilized, the egg at the breakfast table has undergone a considerabled degree of development, the small white disk on the surface of the yolk attesting this phenomenon.
In mammals the bulk of evidence shows that fertilization occurs as a rule in the upper third of the oviduct, that is, the third nearest the ovary, the spermatozoa having advanced from the vagina through the uterus and lower portion of the oviduct against the current created by the action of the cilia on the epithelial lining of these structures. Development begins at once and while it is in progress the ovum (as it is still named even after development has set in) is carried down the oviduct and into the uterus where it becomes attached to or embedded in the mucous membrane and continues its transformation into an embryo. In the human also fertilization probably takes place in the great majority of cases in the upper (outer) third of the oviduct (Fallopian tube) . The time required by the spermatozoa to reach this region after insemination has not been determined with accuracy. It is supposed that they advance into the oviducts within a few hours after insemination. If ovulation has occurred prior to this and a mature ovum is moving through either oviduct, fertilization may take place soon after cohabitation.
That fertilization in the human may and sometimes does occur elsewhere than in the upper third of the oviduct is attested by the position of the growing embryo. Occasionally an embryo develops in the abdominal cavity, which probably shows that spermatozoa have passed all the way through either oviduct. In rarer instances development of the ovum sets in on the surface of the ovary or even within a Graafian follicle. It has been stated that fertilization may occur in the uterus, but there is little evidence to support this conclusion.
Significance of Fertilization
The meaning of such a widely occurring phenomenon as fertilization has been interpreted differently by different scientists, and the question is still far from definite solution. There are several views which may be briefly mentioned.
The earlier belief that fertilization was a necessary antecedent to cleavage of the ovum has been destroyed by the evidence of recent years. Loeb and others have been able to induce artificial parthenogenesis in forms reproducing normally by sexual reproduction. Thus cleavage has been started by chemical stimulation in the eggs of many molluscs, echinoderms, ccelenterates, and even in some of the chordates (teleosts and amphibians). By fertilizing pieces of egg cytoplasm containing no nuclear material, parthenogenesis of the sperm has likewise been induced. While cleavage induced in this manner progresses only a short way, the evidence points to the conclusion that fertilization is not an absolutely necessary factor in reproduction although it normally occurs in the great majority of cases.
Another view is that fertilization rejuvenates protoplasm. According to this view protoplasm tends gradually to pass into a state of senility in which its activity is diminished. With the admixture of new protoplasm when fertilization occurs a new period of vigor is initiated. The life cycles of certain Protozoa are brought to the support of this hypothesis. In these Protozoa a long period of reproduction by a series of cell divisions is followed by some form of conjugation in which two individuals come together and exchange a part of their nuclear material. After conjugation protoplasmic activity is renewed and each of the conjugants starts again on a long period of reproduction. It is probable that the admixture of new protoplasm in fertilization among Metazoa produces a similar invigorating effect.
Another interpretation of fertilization is that this process, called amphimixis in this connection, is important as a source of variation. Since the chromatin of different individuals varies more or less, fertilization will produce new combinations and therefore tend to the production of new forms. However, there is very little evidence that forms which reproduce sexually show more variations than those reproducing by parthenogenesis.
In the opinion of most modern investigators the union of the two germ cells, one from each parent, may result in rejuvenation of the protoplasm, it may be a stimulus to reproduction, a controlling factor in variation; but probably no one of these things expresses the whole significance of fertilization, nor can any one of them necessarily be ruled out. The chief interest of the process at the present time is centered around its relation to the phenomena of heredity and is intimately associated with the interpretation of the maturation processes of the germ cells. The fact of heredity is the resemblance between offspring and parents. From the standpoint of fertilization in its relation to heredity the significant point is that the offspring may develop qualities that were the individual possessions of either one parent or the other. The chromatin, regarded as the heredity material, is the only substance which is contributed in equal or approximately equal parts by the two parents. The union of the germ cells brings the chromatin of the parents together in the fertilized ovum or zygote which develops into a new individual. Upon these facts rests the possibility that the offspring may inherit equally from both parents.
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References for Further Study
BUCHNER, P.: Praktikum der Zellenlehre. Teil I, 1915.
CONKLIN, E. G.: Heredity and Environment in the Development of Men. 3d Ed., 1920.
HERTWIG, R.: Befruchtung. In Hertwig's Handbuch der vergleichenden und experimentellen Entwickelungslehre der Wirbeltiere. Bd. I, Teil I, Kap. II, 1903. Contains extensive bibliography.
KELLICOTT, W. E.: Text-book of General Embryology. Chap. V, 1913.
LOEB, J.: Die chemische Entwickelungserregung des thierischen Eies. 1909.
MARSHALL, F. H. A.: The Physiology of Reproduction. 1910.
MINOT, C. S.: The Problem of Age, Growth, Death. 1907.
MORGAN, T. H.: Heredity and Sex. 1913.
MORGAN, T. H.: The Physical Basis of Heredity. 1919.
WILSON, E. B.: The Cell in Development and Inheritance. 1900.
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Text-Book of Embryology: Germ cells | Maturation | Fertilization | Amphioxus | Frog | Chick | Mammalian | External body form | Connective tissues and skeletal | Vascular | Muscular | Alimentary tube and organs | Respiratory | Coelom, Diaphragm and Mesenteries | Urogenital | Integumentary | Nervous System | Special Sense | Foetal Membranes | Teratogenesis | Figures
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Cite this page: Hill, M.A. (2019, July 16) Embryology Book - Text-Book of Embryology 3. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Text-Book_of_Embryology_3
- © Dr Mark Hill 2019, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G