Development of frog

Structure of a sperm or spermatozoon

  

Sperm or male gamete is small, motile, microscopic and thread like structure. It is developed by the process of spermatogenesis. The body of sperm is divided into three parts- the head, middle piece and the tail. The acrosome helps in penetration to the ovum or egg by secretion of tissue dissolving enzyme. The middle piece consists of two short centrioles and the mitochondria.The sperms are haploid and are discharged in large numbers.

Structure of ovum (egg)

The ovum is an egg cell or female gamete that is immobile and has a haploid nucleus. It is divisible into two parts: the upper pigmented animal pole or animal hemisphere and the lower, the yolk ladden non-pigmented area, the vegetal pole or vegetal hemisphere.The entire egg remains covered by a double walled membranous structure, the outer of which is albuminous vitelline membrane and the inner plasma membrane.

Copulation

Almost all frogs fertilize the eggs on the outside. The male holds the female around the waist in a mating hug called amplexus. He fertilizes the eggs as the female lays them. Amplexus can last hours or days.Both eggs and sperms are discharged through the cloaca.

Fertilization

Fertilization occurs in the rainy season.In frogs, the fertilization is external i.e. outside the body in water. Male frogs produce croaking sounds , copulation occurs and females lay eggs in bundles.After the release of ovum and spermatozoa, spermatozoa floats toward ovum.Many spermatozoa gets attached to single ovum.Here, only one spermatozoa penetrates inside the ovum.

Formation and Development of Embryo

1. Cleavage or segmentation

-During embryonic development, the zygote undergoes mitosis, resulting in the formation of smaller cells that become partitioned off by cytokinesis. As cleavage continues, cells in the animal pole divide more rapidly than those in the vegetal pole, forming a hollow ball of thousands of cells called the blastula with a fluid-filled cavity called the blastocoel. Transcription of zygote genes does not occur until the blastula contains approximately 4,000 cells. Until then, all activities are run by gene products deposited by the mother when she formed the egg.

2.Blastulation

• As micromeres divide faster than megameres, the incipient blastocoel gradually increases , enlarges and shifts towards the animal hemisphere. Finally, a large hemispherical cavity is formed called blastocoel. This stage of embryo is called balstula. It is filled with a fluid secreted by the surrounding blastomeres which contains carbohydrates and albumen that provides nutrition to cells of the embryo.

3.Gastrulation

• The blastulation is followed by gastrula formation . For this, a series of changes is taken place in blastula. These changes are completed in following four steps epiboly, imboly, migration of micromeres and rotation of gastrula. 

4.Formation of three germ layers

• After the formation of diploblastic gastrula, the segmentation advances i.e. divides rapidly around. All three germ layers: outer ectoderm, inner endoderm and the middle mesoderm are formed. 

5.Formation of coelom

• After the formation of mesoderm, this mesodermal sheet splits from the central region and forms a cavity known as coelom. Coelom is the body cavity which is formed by the splitting of the mesoderm.

6.Further development

• After the formation of nerve cord, notochord and coelom, the embryonic cells divide repeatedly to form pre-tadpole and tadpole larval stage which spends most of its life in water by developing gills for respiration, fins for locomotion, etc. But later it undergoes metamorphosis and the frog leads the amphibious life.