As a part of the A-Z Photographic Glossary of Biological Terms challenge, today it is Y for Yolk as illustrated by this frogspawn from a Vaillant’s frog (Lithobates vaillanti), a frog species found from Mexico through Belize and down to Panama.
We tend to think of an egg yolk as the yellow centre of chickens eggs, but one can be found in the eggs of all birds and most reptiles and insects, where the yolk takes the form of a special storage organ constructed in the reproductive tract of the mother. In many other animals, especially very small species such as some fishes and invertebrates, the yolk material is not in a special organ, but inside the ovum.
Yolks, being mainly stored food, tend to be very concentrated, and in particular tend to be rich in nutrients such as vitamins, minerals, lipids and proteins. The proteins function partly as food in their own right, and partly in controlling the storage and supply of the other nutrients. For example, in some species the amount of yolk in an egg cell affects the developmental processes that follow fertilization. Yolk is not living cell material like protoplasm, but largely passive material.
In frogs, the egg is composed of a series of jellylike layers that protect the developing embryo from desiccation, pathogens and, to a limited extent, predators. Surrounding the embryo is a structure called the vitelline membrane. Oxygen and carbon dioxide diffuse passively through this structure, enabling the developing frog to breathe; because of the lack of an outer shell, the egg must be deposited in the water or a very damp location to prevent drying out.
Frogs lay so many eggs because most, as in the Vaillant’s Frog, do not look after their young and so most will not survive to adulthood. From the several thousand eggs that one female lays, only around five (two tenths of one percent!) will become adult frogs. The rest of the eggs or tadpoles will be eaten by birds, fish, newts, water beetles, dragonflies or simply dry up before hatching.
In the photo above, the black dot is devoid of yolk but rich in cytoplasm and has the nucleus- this is known as the animal pole. Below, the unseen hemisphere of the egg contains the yolk which lacks pigment and is known as the vegetal pole. At the equatorial circumference will be found the place at which the male sperm will penetrate the egg, called the receptive cone. Once fertilized, each tadpole embryo will first eat the jelly that is around it until it is ready to hatch and shortly after hatching, the tadpole will then feed on the remaining yolk to get the nutritious start the tadpole will need prior to metamorphosis into adulthood.