Book - The Frog Its Reproduction and Development 10

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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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Chapter 10 - A Survey or the Later Embryo or Larva

External Features

The external changes in shape of the frog embryo and early larva are continuous. The body is elongated and a posteriorly directed tail bud and tail develop, just dorsal to the original position of the blastopore. This tail carries with it an extension of the notochord, myotomes, and blood vessels as well as the pigmented epidermis of the body. It shows contractions of the < -shaped muscle blocks even before the time of hatching.

The previously described surface changes are further accentuated. The yolk mass accounts for the ventral bulge in the belly region. Anteriorly the pronephric and gill bulges are more prominent. The olfactory pits appear in the original placodes, dorsal and slightly lateral to the stomodeum. All four branchial grooves are developed and the rudiments of the external gills are beginning to grow from the upper levels of the first and second branchial arches.

The embryo hatches when it measures about 6 mm. in total length. Since no food is ingested during this period, the interval between the fertilization of the egg and hatching depends almost entirely upon the temperature of the environment during development. The hatching process probably is accomplished by the aid of temporary glands in the sucker region. These glands presumably elaborate an enzyme which aids in digesting away the surrounding jelly coverings, allowing the larva to escape. The jelly itself is not a food for the larvae, or tadpoles, even though after hatching they often are seen attached temporarily to the empty jelly capsules.

The pre-hatching embryos show constant swimming movements due to the presence of surface cilia. At this stage, and immediately after hatching, there is also considerable muscular movement and the entire larva (tadpole) may show occasional coil and S-shaped body contractions which are entirely muscular.

Development of the external gills of Raiia pipiens.

Adaptations for respiration become a more and more important biological function as the embryo develops organ systems. Thus the third and fourth visceral arches develop finger-like external gills and the fifth visceral arch gives rise to a rudimentary pair of such gills. The mouth opens through to the gills and a constant current of water is taken into the mouth and pharynx. This water passes out through the second and third branchial clefts (otherwise designated as the fourth and fifth visceral clefts) and over the associated external gills. In this way these thin-walled, ectodermally covered, and highly vascular gills are able to exchange CO.j and Oo. Subsequently, when the tadpole develops a set of internal gills, the hyoid arch gives rise to a posteriorly directed flap-like membrane which covers the degenerating external gills. This is called the operculum. On the left side of the head the operculum remains open at its posterior margin, to allow the egress of water. This opening is known as the spiracle. The opercular flaps from the two sides fuse ventrally to envelop the gill or opercular chamber within. Surrounding the mouth are a pair of horny jaws and lips, covered by horny rasping papillae. These are derived from the corneum and consist of rows of tooth-like horny denticles which are replaced frequently. Cornification begins at the 11 mm stage.

Tadpole with external gills.

Feeding becomes important as the yolk is being consumed more rapidly. The tadpole begins to use its oral accessories to obtain food, which is almost exclusively vegetation, until after metamorphosis. The intestine, developed from the midgut, is a long, thin, coiled tube having the appearance, through the thin abdominal wall, of a watchspring. If stretched out straight, this larval gut often measures about nine times the length of the body of the tadpole.


Under normal conditions of temperature (i.e., 20°-25° C.) and food supply, the tadpole of Rami pipiens will reach metamorphosis in 75 to 90 days. This period can be extended by keeping the larvae in an environment cooler than normal or it may be shortened by keeping them warmer and feeding them thyroid hormone or dilute iodine which tends to accelerate the changes attending metamorphosis.

There are four major areas of change during metamorphosis. First, the respiratory system, which has already gone through an external and an internal gill phase, now changes to a lung type of respiration. This develops concurrently with the change from an aquatic to a terrestrial environment or habitat, characteristic of amphibia. Second, the horny jaws are lost, the mouth widens, and the gut shortens to about two or three times the length of the body. There are parallel changes in the histology of the gut to take care of the change in diet. Third, the two pairs of legs develop and the tail is lost by regression. The hind legs appear some time before metamorphosis and the forelimbs are pushed through the opercular membrane just before emergence of the tadpole from the water. Fourth, certain endocrine glands function actively and the definitive gonads appear. There are also those changes v/hich are necessarv in the transformation of the tadpole into the frog. The larval skin is shed, along with the horny jaws. The mouth becomes a large horizontal slit instead of a simple oval opening. The gill clefts are all closed.

The 7 mm. frog larva in serial frontal sections.

There is thus developed a frog in miniature, once metamorphosis is achieved and the embryonic stages of development are passed. It will be necessary now to return to the earlier developmental stages and treat the various derivatives of the three primary germ layers. The student is cautioned not to lose sight of the fact that the embryo is a composite whole, even though we are discussing the development of one or another of the various systems. There is functional integration which must be implied, while we are studying an isolated system in detail.