Book - The Frog Its Reproduction and Development 2

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Rugh R. Book - The Frog Its Reproduction and Development. (1951) The Blakiston Company.

Frog Development (1951): Introduction | Rana pipiens | Reproductive System | Fertilization | Cleavage | Blastulation | Gastrulation | Neurulation | Early Embryo Changes | Later Embryo or Larva | Ectodermal Derivatives | Endodermal Derivatives | Mesodermal Derivatives | Summary of Organ Appearance | Glossary | Bibliography | Figures
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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Chapter 2 - General Introduction to the Embryology of the Leopard Frog Rana pipiens

The leopard frog, Rana pipiens (photographs by C. Railey).

The embryology of most of the Anura (frogs and toads) is essentially the same. However, since the leopard frog, Rana pipiens, is so abundant and is most generally used for embryologicai, physiological, and morphological studies, the following description will be specifically of this form. Where there are differences in closely related forms that may be used in embryology, those differences will be indicated in the text.


Rana pipiens (Schreiber) has a widespread distribution over entire North America. It hibernates in marshes or pools and seems to prefer the swampy marshlands for breeding in the spring. It may be found in hay fields where there are many insects, but it remains close to a supply of relatively calm water.


When these frogs are sexually mature they measure from 60 to 110 millimeters from snout to anus, the female being about 10 mm. longer than the male of the same stage of maturity. The body is slender and the skin is smooth and slimy, due to a mucous secretion of the integument. The general color is green, except when the animal is freshly captured from hibernation. At this time the chromatophores are contracted by the cold and the frogs have a uniform light brown color. Extending backward from the eyes are a pair of light colored elevations known as the dorsal plicae, between which are two or three rows of irregularly placed dark spots. Each spot has a light (generally yellow) border. On the sides of the body these spots are usually smaller and more numerous, and on the legs they are elongated to appear as bands. Occasionally one may find a dark spot on the tip of each eyelid. Sometimes a light colored line, bordered below by a dark stripe, occurs along the jaw and extends posteriorly below the tympanic membrane and above the forearm. The ventral aspect of the body is always shiny and white.


The period of germ cell formation occupies much of the long interval between the annual spring breeding seasons. Breeding occurs normally between the first of April and the latter part of May, depending upon the latitude and upon variations in the temperature. Therefore Rana pipiens breeds for about two months, as the temperature rises, from Texas to Canada. In any case, breeding generally occurs before the frogs have had an adequate opportunity to secure food. This means that they must call upon what reserves of fat and glycogen they did not consume during the extended period of hibernation. The interval from egg-laying to metamorphosis of the tadpole is about 75 to 90 days, this phase of development being completed well before the next hibernation period.


Eggs generally are shed early in the morning, and Rana pipiens will lay from 2,000 to 3,000 of them. The bullfrog, Rana catesbiana, has been known to lay as many as 20,000 eggs. These are laid in heavy vegetation to which their jelly coverings make them adherent. Eggs are found usually in shady places, floating near the surface in rather shallow water.


Fertilization takes place during amplexus, the term for sexual embrace of female by male, as the eggs are laid (oviposition) by the female. The cleavage rate depends upon the temperature of the environment and there may be a lag of from IVi to 12 hours between the fertilization of the egg and the appearance of the first cleavage furrow. Often frogs are misled by an early thaw and proceed to shed and fertilize their eggs, and then the pond freezes over. Such eggs usually can withstand a brief (1 to 2 days) freezing without serious effects. Once cleavage has begun, it must proceed (within certain limits of speed) until the egg is divided into progressively smaller and smaller units, first known as blastomeres and later as cells. The first cleavages are quite regular. Since the egg has so much yolk the division of parts of the egg becomes very irregular after about the 32cell stage. The cleavage planes in the early stages may be altered by unequal pressures applied to any egg within a clump of eggs.


Early development of the frog's egg.


The blastula develops an eccentric cavity because the animal pole cells are so different from the large yolk cells of the vegetal hemisphere. However, the end of the blastula stage is the end of the cleavage stage, although cell division goes on throughout the life of the embryo, the larva, and finally the frog.


The gastrula is an embryo having two primary germ layers, the epiblast (presumptive ectoderm and mesoderm) and the endoderm.


Gastrulation in the frog.


The second layer is continuous with the first and develops by integrated movements of sheets of cells. There results the formation of a new and second cavity known as the gastrocoel, or archenteron, which is the primary embryonic gut cavity. The opening into this cavity is the blastopore, and is located in the approximate region of the posterior end of the gut cavity, or the region of the anus.


The process of gastrulation in the frog is completed by providing also the third germ layer, or the mesoderm, and the notochord, which come from the epiblast. The notochord is the axis around which the vertebral column will be built. The mesoderm will give rise to the bulk of the skeleton and muscle, to the entire circulatory system, and to the epithelium which lines the body cavity.

Early development of the frog embryo. (Top) Development of the axial central nervous system. (Bottom) Development of the external gills and operculum.

Shortly after gastrulation the embryo elongates and develops a dorsal thickening known as the medullary plate. This thickened ectoderm will give rise to the entire central nervous system. It closes over dorsally to form the neural and the brain cavities. These continuous cavities are later almost obliterated by the growth and expansion of their walls to form abundant nervous tissue. Extensions of this central nervous system grow out into all parts of the body and all organs as nerves.

Early development of the frog embryo. Ventral view of the 11 mm. larva ( 12 days' development at 25° C).


This neurula stage then develops surface ciliation and acquires the characteristic shape of a tadpole having a head, body, and extending tail.


The embryo is designated as that stage of the frog development before the appearance of a mouth or external gills. There appears a pair of oral sucker-like structures on the head. Shortly after the embryo emerges from its jelly capsule (i.e., hatches), it develops external gills for respiration, and is then known as a larva or tadpole. Embryonic studies include the tadpole because, until metamorphosis is achieved, the organ systems are not all developed to the adult form. The external gills of the tadpole function for a short time and then are replaced by internal gills. The external gills are covered by an operculum or posterior growth of the hyoid arch, with but a small pore or spiracle remaining on the left side of the head. This is the only channel of egress for water from the pharynx and out over the internal gills within the branchial chamber.

Development and absorption of the external gills of the frog larva.

From the first day of hatching, when the total water content of the tadpole is about 56 per cent, there is a very rapid rise in the water content until the fifteenth day after hatching, when it reaches the maximum point of 96 per cent water (Davenport, 1899, Proc. Bost. Soc. Nat. Hist.) This imbibition accounts for the apparent acceleration of growth of the newly hatched tadpole, but it is not related to an increase in total mass since there is no immediate intake of food. The growth rate is affected by various environmental factors such as space, heat, available oxygen, and pressure. The tadpole soon begins voracious feeding on a vegetarian diet.


Shortly after hatching, finger-like external gills develop rapidly on the posterior sides of the head and these constitute the only respiratory organs. Simultaneously with the opening of the mouth a series of visceral clefts (gill slits) develop as perforations in the pharyngeal wall, and their walls become folded to form internal gills. The external gills gradually lose their function in favor of the internal gills. They then become covered over by a posterior growth of tissue known as the operculum. There remains but a single excurrent pore, the spiracle, on the left side at the posterior margin of the operculum. There are but few changes in the respiratory system from this stage until metamorphosis begins at about IVi months. The internal gills lose their function in favor of lungs at metamorphosis and this allows the aquatic tadpole to become a terrestrial frog. When the tadpole begins to develop its lungs it frequently comes to the surface for air. The forelimbs begin to grow through the operculum, and, about 2.5 months after the eggs are fertilized, the hind legs begin to emerge and the tadpole is ready for the critical respiratory and excretory changes that accompany metamorphosis.

Metamorphosis of the frog, Rana catesbiana. {Reading from left to right, top and bottom) : Tadpole; tadpole with hind legs only; tadpole with two pairs of legs; tadpole with disappearing tail, ready to emerge from water to land; immature terrestrial frog; mature frog.

Metamorphosis in the leopard frog, Rana pipiens, occurs in from 75 to 90 days after the egg is fertilized, generally in the early fall and at a time when the food becomes scarce and the cool weather is impending. Metamorphosis is one of the most critical stages in frog development, involving drastic changes in structure and in function of the various parts of the body. The tadpole ceases to feed; loses its outer skin, horny jaws, and frilled lips; the mouth changes from a small oval suctorial organ to a wide slit and is provided with an enlarged tongue; the eyes become enlarged; the forelimbs emerge; the abdomen shrinks; the intestine shortens and changes histologically while the stomach and liver enlarge; the diet changes from an herbivorous to a carnivorous one; the lungs become the major respiratory organs with the moist skin aiding; the mesonephros assumes greater function; the tail regresses; sex differentiation begins; and the tadpole crawls out of the water as a frog.


We may now summarize the steps in the development of the frog as follows:

  1. Fertilization of the egg
  2. Formation of the gray crescent due to pigment migration
  3. Early cleavage
  4. Blastula stage — coeloblastula (see Glossary) with eccentric blastocoel
  5. Gastrulation
    1. Early — crescent-shaped dorsal lip
    2. Middle — semi-circular blastoporal lip
    3. Late — circular blastoporal lip
  6. Neurulation
    1. Early — medullary plate
    2. Middle — neural folds converging
    3. Late — neural tube formed and ciliation of embryo
  7. Tail bud stage — early organogeny
  8. Muscular response to tactile stimulation
  9. Early heart beat, development of gill buds
  10. Hatching and gill circulation
  11. Mouth opens and cornea becomes transparent
  12. Tail fin circulation established
  13. Degeneration of external gills, formation of operculum, development of embryonic teeth
  14. Opercular fold over branchial chamber except for spiracle; internal gills
  15. Prolonged larval stage with refinement of organs
  16. Development of hindlimbs, internal development of forelimbs in opercular cavity
  17. Projection of forelimbs through operculum, left side first
  18. Absorption of the tail and reduction in size of the gut
  19. Metamorphosis complete, emergence from water as miniature, air breathing frog


The rate of development of the egg and embryo will depend upon the temperature at which they are kept. The approximate schedule of development at two different temperatures is given below.

Insert Table here


(From "Stages in the Normal Development of Rana pipiens," by Waldo Shumway. Reprinted from Anat. Rec, 78, No. 2, October 1940.)
(From "Stages in the Normal Development of Rana pipiens," by Waldo Shumway. Reprinted from Anat. Rec, 78, No. 2, October 1940.)
(From "Stages in the Normal Development of Rana pipiens," by Waldo Shumway. Reprinted from Anat. Rec, 78, No. 2, October 1940.)


Those who wish to carry the tadpoles through to later development, and even through metamorphosis into frogs, must begin to feed them at about the time the external gills appear. The food consists of small bits of green lettuce or spinach leaves, washed thoroughly and wilted in warm water. The water in the finger bowls, or larger tanks, must be cleaned frequently to remove debris and fecal matter and to prevent bacterial growth. If the tadpoles are not crowded they will grow faster. After about 10 days the numbers should be reduced to about 5 tadpoles per finger bowl of 50 cc. of water. After metamorphosis, the young frogs must be fed small living worms (Enchytrea) or forced-fed small pieces of fresh liver or worms.


Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
Frog Development (1951): Introduction | Rana pipiens | Reproductive System | Fertilization | Cleavage | Blastulation | Gastrulation | Neurulation | Early Embryo Changes | Later Embryo or Larva | Ectodermal Derivatives | Endodermal Derivatives | Mesodermal Derivatives | Summary of Organ Appearance | Glossary | Bibliography | Figures

Reference

Rugh R. Book - The Frog Its Reproduction and Development. (1951) The Blakiston Company.


Cite this page: Hill, M.A. 2017 Embryology Book - The Frog Its Reproduction and Development 2. Retrieved September 24, 2017, from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Frog_Its_Reproduction_and_Development_2

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