Amphibian Embryo (1963)

[Music] With the beginning of each cycle, of reproduction. Frogs and salamanders, in fact, most amphibia, return to water at spawning time to lay their eggs. Certain kinds of salamanders engage in stimulatory behavior called amplexus. Fertilization does not take place at this time. This activity merely prepares them to deposit eggs and sperms. These cotton-like masses deposited by male salamanders contain sperms. Eventually, they will be picked up by a female, and the sperms will fertilize the eggs internally. These are fertilized salamander eggs. Like salamanders, frogs also engage in stimulatory amplexus, but fertilization occurs outside the body. After the frog eggs are laid and fertilized, they become firmly attached to some object in the water. Toad and frog eggs are quite similar in appearance to salamander eggs. For comparative study, we will observe embryionic development of the toad and of the salamander. First, we will look at the fertilized toad egg. This toad egg is about 1/16th of an inch in diameter. Like all eggs, it is composed of cytoplasm, yolk, and a nucleus which contains chromosomes. Heavier yolk fil granules grouped in the lower part of the egg cause a definite polarity. An animal pole in the upper hemisphere and a vegetital pole in the lower. The materials in the animal pole area will give rise mainly to the anterior or front parts and those of the vegetital pole to the posterior or hind regions of the embryo. Soon after fertilization, the toad eggs or zygot begin to divide or cleave. This action, which usually takes several hours, is speeded up here by means of time-lapse photography. The cleavage line sweeps downward from the animal pole to form two cells called blastoirs. The second cleavage is again a vertical division at a right angle to the first. The third cleavage on a horizontal plane produces eight blasttomirs. Then the eight cells divide into 16 and so on. Once again let us watch this dynamic process from the beginning. Notice in particular that the yolk laden cells in the vegetital pole divide at a slower pace than cells in the animal pole. As a result, fewer and larger cells form in the vegetital area. Now observe another blastula developing. With each division, the number of cells increases while at the same time the size of individual cells decreases. A diagrammatic section through the blastula reveals a cavity called the blastoil which was formed during early cell division. The animal pole above the blastoil is generally composed of two to four layers of small cells while the vegetital pole has yolk laden cells that are larger and heavier. We return again to the outside of the blastula to observe the development of a crescent-shaped ridge of active cells. This is the beginning of the internal organization of cells which will form the primitive gut. This period of development is known as gastrolation. As the cells move downward, they overgrow the slower dividing yolk cells and form a lip of cells around an opening called the blastopore. Another view of a blastula shows how the active pigmented cells of the animal pole are turning inward to form the crescent-shaped dorsal lip of the developing blastopppore. Then the pigmented cells spread downward and over the yolk cells. For a while a yolk plug is visible. Later the anal opening will be located in this area. Once again, a section of the early gastrica reveals that during the time when the dorsal lip of the blasapore begins to develop, the blastoil is being crowded out by the cells of the primitive gut called the arenteron. Eventually, some of the cells around the arteron become the mesoderm. This layer located between the ectoderm and the endoderm will give rise to formation of the skeleton muscles and circulatory system. Cells forming the ectoerm will develop into the outer covering and nervous system while the endoderm will form the digestive tract. When development has reached this point, gastrolation has been completed. As growth continues in some species, each embryo may move out of the jelly mass. However, it remains enveloped in a delicate membrane while it continues its development into the neurola stage. When the blastoapore has nearly closed over the yolk plug, the neural plate and neural folds begin to develop as forerunners of the spinal cord and brain. From the neural plate and folds, the entire nervous system will develop. When the neural folds converge, they will form a canal. A cross-section view reveals that the combined cells of the neural plate and neural foss have united to form a neural tube. The ectoerm, the mesoderm and the endoderm continue to develop and the arenteron is converted into a gut. An ample supply of yolk cells sustain embryionic growth. After the process of gastrolation, each embryo has slowly elongated along the anterior posterior axis. All the while, embryionic movement continues within the confinement of the transparent jelly. Within each capsule, embryos develop almost in unison. The jelly mass is now more fluid. As an embryo elongates, blocks of sommites appear along both sides of the neural tube. These sommites become the backbone, muscles, and other tissues. Each embryo continues a spinning motion within its membrane by means of psyia covering the exterior of the embryo. The muscles contract and soon a toad larvae has hatched. Later, as a tail bud develops, wiggling movements become more vigorous. The larvae continues to grow at the expense of more yolk cells. Additional somites appear, also an eye bulge and gills. Development continues. Yet the larvas are still sightless, still without a mouth. But other organs are rapidly being formed. At last, the mouth, the eyes, and gills are complete. With each heartbeat, blood in the gills rushes through the delicate network of capillaries, exchanging gases in the process of respiration. The young tadpole, toad or frog, is now completely dependent for survival on its own behavior in its environment. Within a few days, the gills are overgrown with a shieldlike skin fold, and so they become located internally. Finally, the tadpole becomes amphibious. The gills are replaced with lungs for breathing air. Since the salamander is closely related to the frog and the toad, its embryionic development is very similar. For comparison and review, we return to watch the development of a fertilized salamander egg. Again, hours are transformed into seconds. Here you are looking down on the animal pole of a salamander egg as it begins to divide. Like the toad egg, the pigment at the animal pole is distributed only on the surface of the egg. This becomes more apparent as cleavage lines cut inward to reveal the light colored cytoplasm. With each cleavage, the cells increase in numbers without at the same time increasing the size of each individual cell. To make it easier to follow an individual cell division as it occurs, concentrate your attention on a particular cell and watch it divide and redivide. Now you are looking from underneath the egg at the vegetital pole. Again, as in the case of the toad egg, gastrolation begins with a crevice of inward moving active cells. Watch how the crescent-shaped wave of cells forms the dorsal lip of the blasapore. See how the cells extend around more and more to complete finally the circle which surrounds the blasapore. Remember, it is these smaller inward flowing cells which will eventually give rise to a gut, bone, muscle and other tissues. Next, embryionic development enters the neurola stage. Here is a dorsal view of the embryo. The anterior end is up, the posterior is down. Again, using time lapse, development that normally takes about 24 hours will be seen in a matter of seconds. At the middle of the embryo, a slight groove is the first indication of a developing neural plate. Soon, neural folds begin to show. They appear on each side of the embryo and grow upward toward us, arching up and over the neural plate to meet finally and fuse into a neural tube. Later, the tube forms the brain and spinal cord. The embryo begins to move by means of psyia. This is another embryo as seen from the side at the stage where the neural foss have just closed. Observe it grow and elongate. Here again, our time-lapse camera compressed the embryionic development from approximately 3 days to something less than a minute. As elongation progresses, the head can be recognized. Then the eye bulge and the gills. Soommites become apparent too. From these, you recall vertebrae, muscles, and other tissues will develop. At last, the larvae is ready to hatch. This is shown in normal speed photography. The salamander is encased in its jelly capsule. Soon it will hatch. The gills have developed. Gills and flowing blood take on the function of respiration. Unlike the frog or the toad, a salamander larvae hatches with decided adult characteristics. Finally, limbs develop and the larvae passes through an aquatic existence before it changes into a gillless adult for life on land. Later, the salamander will return to a nearby stream to breed and to perpetuate the species.