Amphibian Reproduction


Book - The Frog Its Reproduction and Development 3
What Do Bacteria Need to Reproduce? In winter, it digs much deeper and has been recorded at a depth of 4. The reticulated leaf frog Phyllomedusa ayeaye has a single opposed digit on each fore foot and two opposed digits on its hind feet. More than one third of species are considered to be threatened with extinction and over one hundred and twenty are believed to have become extinct since the s. At this time the ovary has been freed of its several thousand mature eggs and contains only oogonia with no pigment and little, if any, yolk.

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How Do Amphibians Reproduce?

The salamander fertilization process can be external or internal. In most cases, the males excrete sperm that the female picks up and stores in her glands. She uses the stored sperm to fertilize the eggs she lays. Most frogs fertilize externally, with the male holding onto the female's back legs and emitting sperm when she begins to lay the eggs.

How Do Amphibians Reproduce? Quick Answer Amphibians reproduce by laying eggs, but the fertilization process differs based on the type of amphibian. What Animals Undergo Metamorphosis? How Do Angiosperms Reproduce?

Full Answer Unlike mammals that give birth to live babies, amphibians must lay their eggs in a suitable environment in order to reproduce. Learn more about Animal Reproduction. How Do Parakeets Reproduce? One might suggest, therefore, that the oviducts may act as accessory excretory ducts, for certainly body cavity fluids must be similarly eliminated. As the egg leaves the ovary it is nude except for the non-living, transparent, and closely applied vitelline membrane.

Thus far it has been impossible to fertilize these body cavity eggs and have them develop. When they are placed in a sperm suspension some will show surface markings which resemble very closely the normal cleavage spindles and the cleavage furrows but none have developed as embryos as yet.

These body cavity eggs are often quite distorted, due to the fact that the ovulation process involves a rupture of the follicle and forcing out of the egg from a very muscular follicle. The egg is literally squeezed from the follicle, through a small aperture. The process looks like an Amoeba crawling through an inadequate hole. Ovulation rupture and emergence of the egg takes several minutes at laboratory temperatures, and is not accompanied by hemorrhage.

By the time the egg reaches the ostium within 2 hours , as the result of ciliary propulsion, it is again spherical. Ciliary currents alone force the egg into the ostium and oviduct. The ostial opening is very elastic and does not respond to the respiratory or heart activity, as some have described. The eggs are simply forced into the ostium, from all angles, stretching its mouth open to accept the egg.

As soon as the egg enters the oviduct and begins to acquire an albuminous mucin-jelly covering, it becomes fertilizable. One can remove such an egg from the oviduct by pipette or by cutting the oviduct 1 inch or more from the ostium, and can fertilize such an egg in a normal sperm suspension. The physical or chemical changes which occur between the time the egg is in the body cavity and the time it is removed from the oviduct, which make it fertilizable, are not yet understood.

As the egg is propelled through the oviduct by ciliary currents, it receives coatings of albumen jelly.

The initial coat is thin but of heavy consistency, and is applied closely to the egg. The egg is spiraled down the oviduct by its ciliated lining so that the application of the jelly covering is quite uniform.

There are, in all, three distinct layers of jelly, the outermost one being much the greater in thickness but the less viscous. The intermediate layer is of a thin and more fluid consistency. There is hyperactivity of the glandular elements of the oviduct just before the normal breeding season, or after anterior pituitary hormone stimulation, so that the duct is enlarged several times over that of the oviduct of the hibernating female.

The presence of the jelly layers on the oviducal or the uterine egg is not readily apparent because it requires water before it reaches its maximum thickness. Eggs sectioned within the oviduct show the jelly as a transparent coating just outside the vitelline membrane. As soon as the egg reaches the water, however, imbibition swells the jelly until its thickness becomes greater than the diameter of the egg.

The function of the jelly is to protect the egg against injury, against ingestion by larger organisms, and from fungus and other infections. Equally important, however, is the evidence that this jelly helps the egg to retain its metabolically derived heat so that the jelly can be said to act as an insulator against heat loss.

Bernard and Batuschek showed that the greater the wave length of light the less heat passed through the jelly around the frog's egg, in comparison with an equivalent amount of water and under similar conditions. Passage of eggs through the oviduct. The eggs of the frog are greatly distorted as they pass down the oviduct toward the uterus.

They accumulate albumen around them, but, since they spiral down the duct, the albumen jelly is evenly deposited and the eggs become spherical as the jelly swells when the eggs pass from the uterus into the water. Oviducts of the frog under various states of sexual activity.

A Post-ovulation condition, collapsed and dehydrated. B Actively ovulating condition, oviduct full of eggs, edematous. C Oviduct of non-ovulating, hibernating female. Originally, and erroneously, the jelly was thought to act as a lens which would concentrate the heat rays of the sun onto the egg, but since the jelly is largely water, which is a non-conductor of heat rays, this theory is untenable.

One can demonstrate that the temperature of the egg is higher than the temperature of the immediate environment, even in a totally darkened environment. So, the jelly has certain physical functions in addition to those as yet undetermined functions which aid in rendering the egg fertilizable. The egg takes about 2 to 4 hours, at ordinary temperatures, to reach the highly elastic uterus, at the posterior end of the oviduct and adjacent to the cloaca.

Each uterus has a separate opening into the cloaca, and the ovulated eggs are retained within this sac until, during amplexus sexual embrace by the male , they are expelled into the water and are fertilized by the male.

Generally the eggs are not retained within the uterus for more than a day or so. There may be quite a few hours between the time of appearance of the first and the last eggs in the uteri. The maturation process can best be described as it begins, immediately after the normal breeding season in the spring. At this time the ovary has been freed of its several thousand mature eggs and contains only oogonia with no pigment and little, if any, yolk.

Even at this early stage each cluster of oogonia represents a future ovarian unit, consisting of many follicle cells and one ovum. There has been no way to determine which oogonium is to be selected for maturation into an ovum and which will give rise to the numerous follicle cells that act as nurse cells for the growing ovum.

It is clear, however, that both follicle cells and the ovum come from original oogonia. All ova develop from oogonia which divide repeatedly. These pre-maturation germ cells divide by mitosis many times and then come to rest, during which process there is growth of some of them without nuclear division. These become ova while those that fail to grow become follicle cells. However, there are pre-prophase changes of the nucleus of the prospective ovum comparable to the pre-prophase changes in spermatogenesis.

The majority of oogonia, therefore, never mature into ova, but become follicle cells. The process of maturation involves contributions from the nucleus and the cytoplasm.

First, chromatin nucleoli aid in the synthesis of yolk, and second, the breakdown of the germinal vesicle allows an intermingling of the nuclear and the cytoplasmic components. Only a small portion of the germinal vesicle is involved in the maturation spindle so that it may be at this time that the nucleus exerts its initial influence on the cytoplasm.

All cytoplasmic differentiations must be initiated at a time when the hereditary influences of the nucleus are so intermingled with it. Growth Period to Primary Oocyte Stage. Growth is achieved largely by the accumulation of yolk. As soon as growth begins the cell no longer divides by mitosis and is known as an oocyte rather than an oogonium.

The growth process is aided by the centrosome, which is found to one side of the nucleus, and around which gather the granules or yolk platelets.

The chromatin filaments become achromatic and the nucleoli increase in number, by fragmentation, and become more chromatic. Many of the nucleoli, which are concentrations of nucleo-protein, pass through the nuclear membrane into the surrounding cytoplasm during this period.

It is not clear whether this occurs through further fragmentation of the nucleoli into particles of microscopic or sub-microscopic size, and then their ejection through the nuclear membrane. It may occur by the loss of identity and chromatic properties by possible chemical change and subsequent diffusion of the liquid form through the membrane to be resynthesized on the cytoplasmic side of the membrane.

During the growth of the oocyte, further nucleoli appear within the nucleus, only to fragment and later to pass out into the cytoplasm. The presence of chromatic nucleoli in the cytoplasm is closely associated with the accumulation deposition of yolk.

The granules within the cytoplasm extruded fragments of nucleoli function as centers of yolk accumulation and have therefore been named "yolk nuclei. It is not a true cell nucleus. The centrosome and other granular centers lose their identity and the yolk granules then become scattered throughout the cytoplasm.

The source of all yolk for the growing ova is originally the digested food of the female. This nutrition is carried to the ovary by way of the blood system and conveyed to the nurse or follicle cells and thence to the oocyte. The yolk is at first aggregated around yolk nuclei, then concentrated to one side of the nucleus.

Finally it assumes a ring shape around the nucleus between an inner and an outer zone of cytoplasm. Subsequently the nucleus is pushed to one side by the ever-increasing mass of yolk so that eventually there is an axial gradient of concentration of oval yolk platelets from one side of the egg to the other.

The smaller platelets are found in the vicinity of the nucleus, in the animal hemisphere. The larger platelets are located toward the vegetal hemisphere. There is an increase averaging from to per cent in the total lipoid substance, neutral fat, total fatty acids, total cholesterol, ester cholesterol, free cholesterol and phospholipin content of the ovaries of Rana pipiens occurring during the production and growth of ova Boyd, The primary oocyte may show a slight flattening of the surface directly above the region of the nucleus.

These growth changes and the unequal distribution of pigment, yolk, and cytoplasm are the first indications of polarity or a gradient system within the egg. When the polarity is well established, the cytoplasm, the superficial melanin or black pigment, and the nucleus are all at the animal hemisphere pole. The light colored yolk is more concentrated toward the vegetal pole.

The egg is then regarded as a telolecithal egg. During this phase of egg maturation there is a drain on the metabolism of the frog which requires an excess of food intake because the materials for egg growth must be synthesized from nutritional elements received from the vascular system of the female. For Rana pipiens this period of most active feeding comes during the summer when the natural foods, insects, worms, etc. During the growth of the oocyte in general there are important changes occurring within the nucleus germinal vesicle of the egg.

Thirteen pairs of chromosomes may be seen in synizesis contraction , converging toward the centrosome at the "yolk nucleus" stage. A little later the nuclear membrane develops sac-like bulges, the nucleoli are scattered, and there is a colloidal chromosome core which almost fills the entire nucleus. The chromosomes themselves are small and almost invisible. When the primary oocyte is about half its ultimate size, there appear definite sacs on the nuclear surface.

The fragmented nucleoli are located at the periphery of the lobulated nuclear membrane, and the chromosome frames have become relatively large. The chromosomes, by this time, have reached their maximum length and possess large lateral loops.

Finally, in the fully grown nucleus of the primary oocyte the nuclear sacs are very prominent, and the nucleoli appear in clusters in the center of the egg, surrounding the chromosome frame. This frame is a gel structure which gives rise to the first maturation spindle, containing 13 pairs of slightly contracted chromosomes. These structural features can be observed in the living germinal vesicle if it is removed from the oocyte and placed in isotonic and balanced salt medium, omitting the calcium ion.

A minute amount of NaHoPO, is added to shift the pH toward the acid side, which makes the chromosomes the more visible. Or, the chromosomes may be. Stained with crystal violet in a calcium-free medium.

Amphibian cells are among the largest in the animal kingdom and the frog's egg nucleus is large enough to see with the naked eye. It can be removed with considerable ease and examined beneath the binocular microscope. Before the time of hibernation the eggs that are to be ovulated for the next spring are in the fully grown primary oocyte stage, having their full complement of yolk, cytoplasm, and pigment. Externally more than one-half of the egg appears densely black, due to surface pigment granules, while the rest is creamy white.

The nucleus is prepared for the maturation divisions. Such an egg measures about 1. The surface layer of the amphibian egg is formed before fertilization and it is definitely not hyaline, as it is in some Invertebrate eggs. It contains many small yolk grains and irregular accumulations of spherical, black pigment granules. With each cleavage, subsequent to fertilization, this superficial coat is divided between the blastomeres, being an integral part of the living cell.

There is no clear-cut demarcation between this surface coat and the inner cytoplasm and yolk. It is believed that these growth changes of the egg are under the influence of the basophilic cells of the anterior pituitary gland, which cells are greater in number at this time than at any other.

During the growth period the vitelline membrane appears on the surface of the oocyte as a thin, transparent, non-living, and closely adherent membrane. It is formed presumably by a secretion from the egg itself, aided by the surrounding follicle cells. It appears to be similar in all respects to the membrane of the same name found around the eggs of all vertebrates. Ovulation, or the liberation of the egg from the ovary, is brought about by a sex-stimulating hormone from the anterior pituitary gland.

Just before and during the normal spring breeding period there is a temporary increase in the relative number of acidophilic cells in the anterior pituitary. It is believed that this is not coincidental but a causal factor in sex behavior in the frog. However, until such time as extracts of specific cell types can be made, this will be difficult to prove conclusively. Attempts on the part of the male to achieve amplexus are resisted by the female not sexually stimulated.

However, such a female can be made to accept the male by injecting the female with whole anterior pituitary glands from other frogs. It is very probable that environmental factors such as light, temperature, and food may act through the endocrine system to prepare the frogs for breeding when they reach the swampy marshes in the early spring, after protracted hibernation.

The pituitaries of the hibernating frogs do contain the sex-stimulating factor, but apparently to a lesser degree than the glands of frogs approaching the breeding season. The injection of 6 glands from adult female frogs will cause an adult female of Rana pipiens to ovulate as early as the last week in August, some 8 months before the normal breeding period. One or two such glands will accomplish the same results if used early in. Another explanation for this may be offered, namely that the ovary itself may become more sensitive to such stimulation as the breeding season approaches.

The ovulation process itself consists of the rupture and the emergence of eggs from their individual follicles. The surface of the egg, separated from the body cavity by only the non-vascular theca externa, is first ruptured and then the egg slowly emerges through the small opening.

Since the egg is known to contain a peptic-like enzyme, it is believed that the pituitary hormone may activate this enzyme to digest away the tight and non-vascular covering. Then by stimulation of the smooth muscle fibers of the cyst wall theca interna the process of emergence is completed.

The relation of the pituitary to smooth muscle activity has long been established clinically. It is true that the ovarian stroma shows undulating contractions at all seasons, irrespective of sex activity. An egg will emerge at any time from a surgically ruptured follicle.

If an ovulating female is etherized and the body cavity is opened, the ovary may be removed and placed in amphibian Ringer's solution and the ovulation process may be observed directly. This will go on for several hours after all connections with the nerve and blood supply are cut off. From the initial rupture of the theca externa until the egg drops free into the body cavity there is a lapse of from 4 to 10 minutes at ordinary laboratory temperatures.

The first maturation division occurs at the time of ovulation. The heterotypic chromosomes i. Movement of the chromosomes is identical with that found in ordinary mitosis. The outermost group of telophase chromosomes are pinched off, with a small amount of cytoplasm and no yolk, to comprise the first polar body. The innermost telophasic group of chromosomes remain within a clear i.

These changes occur as the egg leaves the ovary and before it reaches the oviduct. Possibly the same forces which bring about follicular rupture also influence this maturation process. The second maturation division begins without any intermediate rest period for the chromosomes, at about the time the egg enters the oviduct. There may be a variation in time up to 2 hours for eggs to reach the ostium, depending upon the region of the body cavity into which they are liberated.

Thus the stage of maturation of different eggs within the oviduct may vary considerably. There is a longitudinal division of the chromosomes of the egg which are lined up in metaphase on the second maturation spindle, the axis of which is at right angles to the egg surface.

Since the spindle is primarily protoplasmic, and is made up in part of fibers which may be contractile , the space occupied by the spindle will be free of yolk.

Since it is peripherally placed, and represents a slight inner movement after the elimination of the first polar body, the surface layer of the egg is slightly de-pigmented just above the spindle region. This situation is exaggerated in aged eggs, a relatively large de-pigmented area of the cortex appearing toward the center of the animal hemisphere.

Maturation is not completed until or unless the egg is activated by sperm or stimulated by parthenogenetic means. However, every egg reaching the uterus is in metaphase of the second maturation division, awaiting the stimulus of activation to complete the elimination of the second polar body. Discussion View source History. About Translations Rugh R. This page was last modified on 12 April , at Privacy policy About Embryology Disclaimers. Google Translate - select your language from the list shown below this will open a new external page.

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