What Is an Amphibian?
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 theca interna, plus the limited covering of the theca externa, and the follicle cells together comprise the ovarian follicle. 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. Crespo a Marcela F. Each group of similar cells is derived presumably from a single original spermatogonium, by the processes of mitosis and meiosis. The skin is semi-permeable , making them susceptible to dehydration, so they either live in moist places or have special adaptations to deal with dry habitats. After fertilization, the innermost portion liquifies to allow free movement of the developing embryo.
What Is the Fertilization Process?
Juvenile amphibians are entirely aquatic, with gills and tails, but once they mature, they develop lungs and legs and split their time between land and water. To unlock this lesson you must be a Study. Login here for access. Did you know… We have over college courses that prepare you to earn credit by exam that is accepted by over 1, colleges and universities. You can test out of the first two years of college and save thousands off your degree. Anyone can earn credit-by-exam regardless of age or education level.
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Want to watch this again later? Amphibians are organisms that spend part of their lives developing in water before they're able to live on both land and in water. This unique ecological characteristic means they have a different reproductive strategy than humans. What Is an Amphibian? Salamanders and Newts Salamanders and newts usually reproduce during the winter months. Longtailed Salamander Toads Males call out to females by croaking, so every time you hear a toad or frog calling, you are really hearing them signal to potential mates that they are ready to reproduce!
In the amplexus position, the male grasps on to the female from behind. Frogs Frogs reproduce very similar to toads. Frog eggs are laid in water in high numbers.
Try it risk-free No obligation, cancel anytime. Want to learn more? Select a subject to preview related courses: Caecilians Caecilians are sometimes called blindworms; they are a legless amphibian that resembles a worm.
Caecilians look a bit like worms. Lesson Summary We've learned that there are four groups of amphibians - toads, frogs, salamanders and newts, and caecilians, and each of them uses a slightly different method to reproduce.
Salamanders and newts use a spermatophore so that eggs can be fertilized internally without sexual intercourse.
Toads and frogs use the amplexus position to signal the male and female to simultaneously release eggs and sperm so external fertilization can take place. Caecilians use internal fertilization and often give live birth.
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Like this lesson Share. Browse Browse by subject. Upgrade to Premium to enroll in High School Biology: Enrolling in a course lets you earn progress by passing quizzes and exams. Take quizzes and exams. Earn certificates of completion. You will also be able to: Create a Goal Create custom courses Get your questions answered. Upgrade to Premium to add all these features to your account! Email us if you want to cancel for any reason. Responding to sex stimulation, the spermatozoa become free from their Sertoli cells and are forced from the lumen of the seminiferous tubule into the related collecting tubule.
These collecting tubules are small and are lined with closely packed cuboidal cells. They join the vasa efferentia which leave the testis to pass between the folds of the mesorchium and thence into the Malpighian corpuscles of the kidney.
From this point the spermatozoa pass by way of the excretory ducts, the uriniferous tubules, and into the mesonephric duct ureter which may be found attached to the lateral margin of the kidney. Within the excretory system the spermatozoa are immotile, due to the slightly acid environment.
They are carried passively down the ureter to the slight dilation near the cloaca, known as the seminal vesicle. Within the vesicle the spermatozoa are stored briefly in clusters until amplexus and oviposition occur.
At oviposition the male ejaculates the spermatozoa into the neutral or slightly alkaline water where they are activated and then are able to fertilize the eggs as they emerge from the cloaca of the female. During the normal breeding season amplexus is achieved as the females reach the ponds where the males are emitting their sex calls. During amplexus there are definite muscular ejaculatory movements on the part of the male frog, coinciding with oviposition on the part of the female.
Amplexus may be maintained by the male for many days, even with dead females. As soon as the eggs are laid and the male has shed his sperm, he goes through a brief weaving motion of the body and then releases his grip to swim away. The frogs completely neglect the newly laid eggs.
In the male frog the ureter is not directly connected with the bladder, as it is in higher vertebrates. It is possible that the bladder in the Anura may be an accessory respiratory and hydrating organ, particularly in the toads, where water may be stored during migrations onto land. The male frog also has a duct, homologous to the oviduct of the female, known as the "rudimentary oviduct" or Miillerian duct.
This duct normally has no lumen, and is very much reduced in size so that it may be difficult to locate. There is experimental evidence that this duct may be truly a vestigial oviduct since it responds to ovarian or female sex hormones by enlarging and acquiring a lumen.
At the anterior end of the testes of some Anura e. This structure is said to respond to the removal of the adjacent testis or to the injection of female sex hormones by enlarging to become structurally like an ovary. Occasionally isolated ova have been found within the seminiferous tubules of an otherwise normal testis, suggesting the similar origin and the fundamental similarity of the testis and the ovary.
Finally, attached to the anterior end of the testis of the hibernating frog may be seen finger-like fat bodies corpora adiposa which represent stored nutrition for the long period of hibernation, and for the pre-breeding season when food is scarce. Under the microscope these fat bodies appear as clusters of vacuolated cells, and are not to be confused with the mesorchium. It is believed that they, as well as the gonads, arise from the genital ridges of the early embryo. The fat bodies tend to be reduced immediately after the breeding season, only to be built up again as the time for hibernation approaches.
The mature female frog is generally larger than the male of the same age and species, the Rana pipiens female measuring from 60 to 1 10 mm. The sexually mature female has a body length of at least 70 mm. It can be identified by the absence, at any season, of the dark thumb pad; the inability to produce lateral cheek pouches resulting from the croaking reaction; a flabby and distended abdomen; and the presence of peritoneal cilia.
These cilia are developed in the female in response to the prior development and secretion of ovarian hormones. The ovaries of the frog are paired, multi-lobed organs, attached to the dorsal body wall by a double-layered extension of the peritoneum known as the mesovarium.
This peritoneum continues around the entire ovary as the theca externa. 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.