The Variety of Amphibian Reproduction

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Book - The Frog Its Reproduction and Development 3
Although much of its body freezes during this time, it maintains a high concentration of glucose in its vital organs, which protects them from damage. Many use camouflage to avoid detection, the skin often being spotted or streaked in neutral colours that allow a stationary frog to merge into its surroundings. For example, White's tree frog Litoria caerulea varies between pale green and dull brown according to the temperature, and the Pacific tree frog Pseudacris regilla has green and brown morphs, plain or spotted, and changes colour depending on the time of year and general background colour. Within the vesicle the spermatozoa are stored briefly in clusters until amplexus and oviposition occur. There are, in all, three distinct layers of jelly, the outermost one being much the greater in thickness but the less viscous. Oviducts of the frog under various states of sexual activity. This coat is necessary for retaining the shape of the egg and in aiding in the morphogenetic processes of cleavage and gastrulation Holtfreter.

What Animals Undergo Metamorphosis?

Amphibian Reproduction

Each lobe of the ovary is hollow and its cavity is continuous with the other 7 to 12 lobes. The ovaries of the female are found in the same relative position as the testes of the male but the peritoneum extends from the dorso-mesial wall rather than from the kidneys, as in the male. The size of the ovary varies with the seasons more than does the size of the testis. From late summer until the spring breeding season the paired ovaries will fill the body cavity and will often distend the body wall.

The mature eggs are highly pigmented on the surface of the animal pole, so that the ovary has a speckled appearance of black pigment and white yolk, representing the animal and the vegetal hemispheres of the eggs. There is no appreciable change in the size of the ovary during hibernation, nor is there any observable cytological change in the ova. However, if a female is forced to retain her eggs beyond the normal breeding period by isolating her from males or by keeping her in a warm environment and without food, the ova will begin to deteriorate cytolize within the ovary.

Immediately after the spring breeding season, when the female discharges thousands of mature ova, the remaining ovary with its oogonia to be developed for the following year is so small that it is sometimes difficult to locate. There is no pigment in the tissue of the ovary in the stroma or in the immature ova , and each growing oocyte appears as a small white sphere of protoplasm contained within its individual follicle sac.

The histology of the ovary shows that within its outer peritoneal covering, the theca externa, are suspended thousands of individual sacs, each made up of another membrane, the theca interna or cyst wall, which contains smooth muscle fibers.

This theca interna is derived from the retro-peritoneal tissue. The smooth muscle fibers can be seen histologically and can be demonstrated physiologically. The theca interna surrounds each egg except for the limited area bulging toward the body cavity, where it is covered by only the theca externa. This is the region which will be ruptured during ovulation to allow the egg to escape its follicle into the body cavity.

The theca interna, plus the limited covering of the theca externa, and the follicle cells together comprise the ovarian follicle.

These two membranes make up the rather limited ovarian stroma of the frog ovary, and they contain both blood vessels and nerves. Within each follicle are found follicle cells, with their oval and granular nuclei, derived originally from oogonia. These follicle cells surround the developing oocyte and are found in close association with it throughout those processes of maturation which occur within the follicle.

Enclosed within the follicle cells, and closely applied to each mature egg, is the non-cellular and transparent vitelline membrane, probably derived from both the ovum and the follicle cells. This membrane is developed and applied to the egg during the maturation process so that it is not seen around the earlier or younger oogonia. Since the bulk of the egg is yolk vitellus , this membrane is appropriately called the vitelline membrane. It is sometimes designated as the primary of several egg membranes.

After the egg is fertilized this membrane becomes separated from the egg and the space between is then known as the perivitelline space, filled with a fluid. The fluid may be derived from the egg which would show compensatory shrinkage. As the oocyte matures and enlarges, the follicle cells and membranes are so stretched and flattened that they are not easily distinguished. It is therefore best to study these structures in the immature ovary.

The egg will mature in any of a variety of positions within its follicle, the exact position probably depending upon the maximum blood supply. As one examines an ovary the eggs will be seen in all possible positions, some with the animal hemisphere and others with the vegetal hemisphere toward the theca externa and body cavity. It is believed that the most vascular side of the follicle wall will tend to produce the animal hemisphere of the egg, and hence give it its fundamental symmetry and polarity.

The frog's egg is essentially a large sac of yolk, the heavier and larger granules of which are concentrated at the vegetal pole. There is a thin outer layer of cytoplasm, more concentrated toward the animal hemisphere and in the vicinity of the germinal vesicle or immature nucleus. Surrounding the entire egg is a non-living surface coat, also containing pigment. This pigment is presumably a metabolic byproduct. This coat is necessary for retaining the shape of the egg and in aiding in the morphogenetic processes of cleavage and gastrulation Holtfreter.

Lateral to each ovary is a much-coiled oviduct suspended from the dorsal body wall by a double fold of peritoneum. Its anterior end is found between the heart and the lateral peritoneum, at the apex of the liver lobe. At this anterior end is a slit-like infundibulum or ostium tuba with ciliated and highly elastic walls. The body cavity of the female is almost entirely lined with cilia, each cilium having its effective beat or stroke in the general direction of one of the ostia.

These cilia are produced in response to an ovarian hormone and therefore are regarded as secondary sex characters. They are found on the peritoneum covering the entire body cavity, on the liver, and on the pericardial membrane. There are no cilia on the lungs, the intestines, or the surface of the kidneys except in the ciliated peristomial peritoneal funnels which lead into the blood sinuses of the kidneys.

The abundant supply of cilia of the female means that eggs ovulated from any surface of the ovary will be carried by constant ciliary currents anteriorly toward and into one or another of the ostia. This can be demonstrated easily by opening the body cavity of an actively ovulating frog or by excising a strip of ventral abdominal wall of the adult female, inverting it in amphibian Ringer's solution, and placing on it some of the body cavity eggs.

Any object of similar size or weight, such as pellets of paraffin, will be carried along by the ciliary currents in the original direction of the ostium. These cilia function the year around, and will carry to the ostia any objects of approximately the size and weight of frog eggs that may be placed in the body cavity. 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. The ovarian follicles project towards the lumen of ovary. Such an ovary greatly enlarges. It attains black color with light yellow spots.

Each oviduct is a long narrow and highly coiled tube. It is divided into three parts in accordance with its structure and functions. The anterior end of the oviduct forms a wide and fringed oviducal funnel. The ovoiducal funnel is located on the dorsal side of the lung. The margin and inner surface of the oviducal funnel is lined by ciliated epithelium. The oviducal funnel leads into the oviduct. This oviduct is straight and thin-walled for a short distance.

Thereafter it becomes highly coiled and thick-walled. This coiled oviduct runs posteriorly along the outer side of the kidney. The hinder portion of the oviduct becomes very thin walled. It is sac-like and is called ovisac. The ovisac opens of the posterior end in the dorsal wall of the cloaca by its individual apertures lying anteriorly to the openings of ureters. The cloaca opens to the exterior by a cloacal aperture at the posterior end of the body.

The release of ovum in female is termed as spawning. Subscribe to Post Comments [ Atom ]. Subscribe to Posts [ Atom ]. Wednesday, October 31, Reproductive system of frogs. Reproductive system of frogs. The male frog and the female frog can be distinguished even by their external morphological characters.

Wednesday, October 31, 2012