Book - Text-Book of Embryology 1
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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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The Germ Cells
The vertebrate animal body is a complex of numerous types of cells. The great majority of the cells are engaged in carrying on the various activities of daily life. Muscle cells contract and produce motion and locomotion; red blood corpuscles carry oxygen from the lungs to all parts of the body; epithelial cells synthesize and secrete substances which are used in some manner or excrete waste products; nerve cells convey impulses from one region to another and thus bring distant parts into communication. All these are integral parts of the body, working in harmony in response to the demands put upon them. They are usually spoken of as somatic cells (soma-body) because they compose the bulk of the body and are concerned in its specific activities which collectively constitute the general body economy. When death occurs all these cells die and disintegrate without leaving any descendants.
Within the body is another group of cells which differ in certain respects from the somatic cells. They are confined to the genital or sex glands, to the testis in the male and the ovary in the female. They probably play no part in the general body economy; they are concerned in perpetuating the race. During the life of an individual of a given generation they are discharged at certain times from the glands that contain them, and under proper conditions then develop into a new individual of the succeeding generation. For this reason they are known as germ cells. While these cells contain the same visible elements as the somatic cells, that is, nuclear and cytoplasmic components, there are differences in internal organization which make these cells alone capable of producing a new member of the species. Under ideal conditions of reproduction, therefore, they do not die and disintegrate, as do the somatic cells, but are carried along into and with successive generations, always constituting the plasm from which new individuals arise. Each sex has its own peculiar type of cell; the female carries the ovum (ovium, female sex cell or germ cell), the male carries the spermatozoon (spermium, sperm, male sex cell or germ cell).
The ovum is among the largest cells in the animal body, but varies in size from a fraction of a millimeter in some of the invertebrates and in mammals to several inches in the largest birds. The differences in size are due in large measure to differences in the amounts of food or yolk stored within the egg. Taking the human ovum as an example of ova containing a small amount of yolk (deutoplasm) , it is not truly spherical in shape but ovoid, with an average diameter of slightly less than 0.2 mm. As seen in section in the ovary it presents the appearance of the traditional typical cell (Fig. 1). Surrounding the ovum is the zona pellucida, a thick, highly refractive membrane which sometimes shows a faint radial striation. Immediately outside of this membrane one or two layers of the epithelial cells of the Graafian follicle are arranged radially as the corona radiata. The zona pellucida is probably composed of differentiated cytoplasm of the inner ends of these cells. Some investigators have described a delicate mtelline membrane between the zona pellucida and the ovum; others have not observed it. If this is present it is probably a true cell membrane, a product of the egg cytoplasm.
Fig. 1. From a section of the ovary of a 12-year old girl.
- The primary oocyte lies in a large mature Graafian follicle and is surrounded by the cells of the " germ hill " (the inner edge of which is shown in the upper left-hand corner of the figure). Photograph.
The egg cytoplasm (historically called the vitellus, whence the term vitelline that is so frequently used in embryology) is more opaque and more coarsely granular than the cytoplasm of most cells, due to the presence of granules or globules of yolk. These globules are suspended in the cytoplasm and composed of fatty and albuminous substances that are later utilized in the growth of the embryonic cells. It should be added that the composition of the yolk in the human ovum is assumed, but analysis of the yolk of the hen's egg has shown a large percentage of lipins including lecithin, with some proteins also, and a similar composition of the yolk granules in other ova is a reasonable assumption. Lecithin (lekythos), is a term that was used by the ancients to designate the yolk of an egg. The yolk globules are congregated near the center of the cell, surrounding the nucleus, while a zone of cytoplasm nearly destitute of yolk forms the peripheral portion of the ovum. In his recent study of the maturation of the human ovum Thomson describes and illustrates a centrosphere which then disappears after the formation of the second polar body.
The nucleus is situated near but not quite in the center of the ovum amidst the yolk granules. Its volume bears about the same ratio - to the volume of the egg cytoplasm as the nuclear volume of the average somatic cell bears to its cytoplasmic mass. A distinct nuclear membrane encompasses the usual nuclear structures. The chromatin seems rather scanty, the nucleus thus being conspicuously vesicular. The single nudeolus (plasmosome) is intensely stainable, and in a fresh human ovum has been observed to perform amoeboid movement.
The frog's egg will serve as an example of an ovum with a moderate amount of yolk suspended in the Cytoplasm, yet enough yolk to produce a definite and visible effect upon the organization of the cell and to influence strongly the future processes of development. The female frog deposits the eggs in clusters in quiet water where they may be observed resting on the bottom or sticking to leaves and twigs. The eggs are enclosed in a jelly-like substance, each cell with its own gelatinous capsule or membrane (Fig. 2). Each egg is spherical and measures from 1.5 to 3 mm. in diameter, depending upon the species of frog. Externally something more than one-half of the cell is black owing to the presence of pigment granules, and the remainder is nearly white. If the eggs have been free in the water for a few minutes the dark sides are turned upward. A delicate vitelline membrane, not easily seen, surrounds each ovum. This is a true cell membrane, a product of the egg cytoplasm. Outside of this is a tough membrane called the chorion and then the gelatinous capsule, both being secondary egg membranes produced by the cells of the oviduct and not by the cytoplasm of the ovum.
If the egg is bisected through the centers of the dark and light areas the two halves are exactly alike. The cut surface of either half shows three substances: pigment, cytoplasm and yolk. The pigment forms a superficial layer which coincides with the dark superficial area. It is a product of cytoplasmic activity without any known importance in future development. The portion of the egg not covered by pigment contains a large amount of yolk, in fact more yolk than cytoplasm, in the form of globules of different sizes. The remainder of the egg contains some yolk but the cytoplasm is excessive. Therefore we may speak of the cytoplasmic or animal pole and the yolk or vegetal pole of the egg, the former approximately indicated on the surface by the dark area and the latter by the light area. The yolk has a slightly higher specific gravity than the cytoplasm, which accounts for the fact that if the egg is left free in its natural medium the dark pole turns upward. An egg like this in which more yolk is accumulated at one side than at the other is known as a telolecithal ovum as distinguished from one of the homolecithal type in which the yolk granules are distributed uniformly or nearly so, as in the mammalian ovum.
The nucleus of the frog's ovum is proportionately smaller than in the case of an egg with a small quantity of yolk. It is conspicuously eccentric, situated nearer the animal than the vegetal pole. Being thus situated it obviously tends to occupy the center of the cytoplasmic mass. The nuclear membrane encloses the usual nuclear components; the chromatin is rather scanty and numerous small nucleoli (plasmosomes) are present.
The freshly laid hen's egg may be chosen as an example of a large ovum with a relatively great quantity of yolk (Fig. 3) . The shape is characteristic. The outer covering is the shell, a calcareous substance. If the shell is broken the tough shell-membrane appears; this is a double layer with a considerable air space between the layers at the larger end of the egg. Enclosed by this membrane is the thick layer of albuminous substance with a denser twisted portion, the chalaza, at each end of the egg. All these structures are secondary egg membranes secreted around the ovum proper by the epithelium of the oviduct during its passage through that organ.
The ovum proper consists of the large spherical mass of yolk, 25 mm. or more in diameter, and a small disk of cytoplasm, 3 or 4 mm. in diameter, which rests upon the yolk. If the unbroken egg is allowed to lie in one position for a minute or two the disk will be found uppermost when the shell is opened owing to the slightly higher specific gravity of the yolk. At the time of laying, however, development has proceeded for several hours, for fertilization normally occurs in the oviduct before the secondary egg-membranes are deposited. The ovum proper must be examined in the ovary or immediately after its escape therefrom in order to see it before development begins. At this time the yolk mass is quite similar to that of the egg after laying, and the small disk of cytoplasm containing a single flat nucleus is attached to one side of the yolk. While a few small yolk granules are suspended in the cytoplasm, there is an abrupt transition from the cytoplasmic disk to pure yolk. By far the greater part of the yolk contains no cytoplasm but consists solely of nutritive substances which are later carried to and assimilated by the growing embryo.
Fig. 3. Diagram of a vertical section through an unfertilized hen's egg. Bonnet.
The presence of the large quantity of yolk in the ova of birds and reptiles is correlated with the long period during which embryos of these animals undergo development within their shells before hatching and attaining ability to get their own food. In the case of the frog the moderate amount of yolk in the egg serves as food for the growing embryo until it becomes a free-swimming larva or tadpole. An embryo of a mammal develops for a long period in the uterus of its mother from an ovum with scanty yolk, but provision is made for drawing nourishment directly from the maternal blood during this time.
A simple classification of ova is made on the basis of the amount and distribution of the yolk content. The term meiolecithal is used, to designate ova in which the yolk granules are few (many invertebrates, Amphioxus, mammals). Mesolecithal ova are those which contain moderate quantities of yolk (amphibians). Ova that possess large yolk content are classed as polylecithal (certain fishes, reptiles, birds) . It has been stated earlier in the chapter that in case the yolk is accumulated in greater quantity toward one pole the ovum is telolecithal, while in case of nearly uniform distribution it is homolecithal. The yolk has a slightly higher specific gravity than the cytoplasm, in consequence of which the animal pole of the egg turns. upward, except in most of the teleost ova where the yolk is composed of oil droplets that are lighter than the cytoplasm. In many insect eggs the yolk is centrally placed and the cytoplasm forms an outer layer; these are known as centrolecithal ova.
Compared with the ovum the spermatozoon is an exceedingly small cell bearing little resemblance to the ordinary or typical cell. It is so small in most animals that the ovum of the same species exceeds it in bulk several hundred thousand times. Its peculiar shape and structure are correlated with its high degree of motility, the cytoplasm being drawn out into a long slender tail or flagellum which in the living cell is lashed about and thus drives the whole cell along. All spermatozoa of vertebrates are of the flagellate type, the human spermatozoon serving as an example.
With the usual preparation the human spermatozoon shows a head, a middlepiece or body, and a tail, measuring in total length from 50 to 60 micra. On side view the head is nearly oval, usually a little narrower at the front end; on edge it appears pear-shaped. The nucleus is situated in the head, nearer the attachment of the body, and a thin layer of cytoplasm, the galea capitis, surrounds the nucleus and is continued forward as the acrosome. The head is about 4.5 micra in length, 2 to 3 in width and 1 to 2 in thickness, being much smaller than a red blood corpuscle. The body is attached to the broader end of the head and is cylindrical, measuring about 6 micra in length. Sometimes a narrower portion, the neck, is visible at the point of attachment. Without sharp demarkation the body continues into the slender tail which runs to a point and measures from 40 to 50 micra in length.
Special preparations of spermatozoa reveal other details of structure (Fig. 4). The body contains a delicately fibrillated cord, the axial thread, which is continued throughout the tail, narrowing to a point at its terminus. Surrounding the axial thread is a capsule of cytoplasm which, however, does not extend to the tip of the tail, thus leaving the axial thread naked for a short distance. In the body the cytoplasm contains a spiral fiber, perhaps of a mitochondrial nature, winding round the axial filament; other mitochondria also are present. The body contains the centrosome which takes the form of a double structure; one part, the anterior end knob, is attached to the posterior surface of the head close to the nucleus, the other part, the posterior end knob, is situated a little farther back. A derivative of the centrosome, as shown during development of the spermatozoon, is the end ring which marks the boundary between body and tail.
Spermatozoa of other animals, both vertebrates and invertebrates, show a great variety of forms. A few of these are illustrated in Fig. 5. Some are simple in form and structure, others are complex and even bizarre. Almost throughout the series, however, there is some structure Head* that lends itself to the function of motility.
In the tubules of the mammalian testis, where the spermatogenic cells develop into the mature spermatozoa, the sperms are not motile. They acquire some degree of motility in the tubules of the epididymis and the highest degree only after they are mixed with the secretions of the prostate gland and other accessory sex glands. They are active in the fluid of the female genital tract where they swim against the current produced by the cilia of the epithelium -lining the tract. Their rate of progress has been variously estimated from 1.5 to 3.5 mm. per minute. It is not known how long spermatozoa remain alive in the female genital tract. They have been found in the vagina seventeen days and in the cervix of the uterus eight days after cohabitation, and in one case where the oviducts were removed more than three weeks after cohabitation active sperm cells were found but whether they were capable of fertilizing an ovum could not be determined. Spermatozoa can endure considerable variation in temperature; they are most active in a slightly alkaline medium but die quickly in an acid medium. The number of spermatozoa produced by an individual is almost incomparably greater than the number of ova. It has been estimated that only about 400 ova reach maturity during the reproductive period of a little more than 30 years in a woman, while a single ejaculation of semen may contain two hundred million spermatozoa.
Fig. 5. Various types of spermatozoa. from Kellicott, General Embryology.
- A, B, A teleost; C, D, bird; E, F, snail; G, Ascaris; H, an annulate; /, bat; /, opossum; K, rat; L, salamander; M, N, O, P, crustaceans, k, End knob; w, middle piece; u, undulatory membrane.
Significance of Germ Cell Organization. One feature of this has already been mentioned in connection with the morphological differences between the male and female germ cells: The spermatozoon is adapted for locomotion while the ovum is passive and frequently laden with yolk. This diversity in structure is truly correlated with a physiological division of labor. The two cells must unite before development of a new organism can proceed; the egg is non-motile and contains nutriment for the future embryo, the sperm by virtue of its motility approaches the egg and finally enters it.
Another feature of organization is embodied in the chromatin. The chromatin is a visible substance and is regarded as the inheritance material. Its constitution is such that it determines in large measure the course of development of the embryo arising from the united germ cells and the qualities or characters of the adult. Parts of the chromatin contain or comprise factors which give rise to certain characters in the developed organism. These factors, or genes as 'they are frequently called by students of heredity, are not visible things but are probably expressed in the physico-chemical nature of the chromatin. There is ample evidence for their presence, upon which is based the modern theory of heredity or Mendelian inheritance. One set of factors is present in the ovum and another in the sperm. Their relation to the chromosomes and their behavior will be considered in the two succeeding chapters.
There are certain characters of the embryo that are derived directly from the cytoplasm of the ovum ; so chromatin is not the only germ cell substance that influences development. Since these characters come from the female parent and not from the male, this is sometimes called maternal inheritance as distinguished from Mendelian inheritance. The cytoplasm of the sperm seems to be useful only as a temporary locomotor apparatus. The egg cytoplasm is so organized that it becomes potent in determining the course of development. In the case of an ovum that contains a moderate amount of yolk, as in the frog, or a large quantity, as in the bird, there is an obvious polar differentiation or polarity which is visibly expressed in the distribution of the cytoplasm and yolk. This polarity of the egg determines the polarity of the future adult animal. It will be seen in a later chapter that the egg of Amphioxus is bilaterally symmetrical, and that the bilateral character of the developing animal follows upon that of the egg. This is true also of the frogs and fishes. Other evidence of the internal organization of the egg cytoplasm in certain invertebrates is seen in collections of various pigments in the ova; and it is possible to predict accurately the part of the embryo that will be derived from the portion of the cytoplasm containing a given pigment. These few examples are sufficient to indicate that cytoplasmic organization of the ovum determines in a measure the course of development of the future embryo.
- Next: Maturation
References for Further Study
CONKLIN, E. G.: Heredity and Environment in the Development of Men. 1920.
KEIBEL, F. and MALL, F. P.: Manual of Human Embryology. Vol. I, Chap. I, 1910.
KELLICOTT, W. E.: Text-book of General Embryology. Chap. Ill, 1913.
WALDEYER, W.: In Hertwig's Handbuch der vergleichenden und experimentellen Entwickelungslehre der Wirbeltiere. Bd. I, Teil I, Kap. I, 1906. Contains extensive bibliography.
WILSON, E. B.: The Cell in Development and Inheritance. 2d Ed., 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. (2021, April 12) Embryology Book - Text-Book of Embryology 1. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Text-Book_of_Embryology_1
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