Frog: Fertilisation

FERTILIZATION IN FROGS

The process of copulation is immediately followed by the process of fertilization. The fertilization is external and in water. Each egg falling in water has a small conical protuberance from the equitorial plane. This conical protuberance is called reception cone. The tip of the reception cone is thin walled and projecting out of the albuminous layer. Normally, each egg is surrounded by innumerable sperms. The first sperm that reaches the tip of the reception cone dissolves the tip with the help of an enzyme secreted by the acrosome on its head and makes its way in to the egg. The head and middle piece of the sperm enter the egg, while the vibratile tail is cut off and remains outside the egg.
During fertilization a series of changes occur in the egg. The nucleus of the secondary oocyte divides into two nuclei.
(A)One of these nuclei remains in the egg cell. It is known as female pronucleus. The other nuclei is pushed out through the egg cell membrane. This nucleus settles as second polar body. It remains near the first polar body.
(B)The reception cone becomes flattened and closed. The cell content of the egg with egg cell membrane shrinks. The cell content of the egg gets fully separated from the egg membrane. The egg membrane is now termed as fertilization membrane.
(C)The head of the sperm that has entered the egg changes its shape. The head of the sperm becomes a round male pronucleus.
(D)The male pronucleus moves towards the female pronucleus and finally fuses with it to form a dioploid (2n) nucleus called zygote nucleus. The melanin granules settle deeper from the surface exactly on the opposite side of the site of entry of the sperm in the egg. As a consequence a much less dark (grey) colored semilunar patch called grey crescent is formed. The egg having grey crescent formed in it is recognized as a fertilized egg or zygote.

IMPORTANCE OF FERTILIZATION

Generally the unfertilized egg does not develop into embryo and produce new animal. Fertilization is thus essential. In the process of fertilization the haploid egg and the haploid sperm produced during gametogenesis meet and fuse with each other. This establishes the diploid state in the zygote. Thus, the specific diploid number of chromosomes of the species is maintained. The maturation of the egg cell is completed only after the entry of the sperm in it. The process of fertilization adds new vigor and activeness in the egg cell. The egg cell becomes active for embryo development through the process of fertilization. The process of fertilization brings about the combination of characters of both the parents in the off springs. The grey crescent formed in the process of fertilization determines the future anterioposterior axis and the axis of bilateral symmetry in the spherical zygote.