![]() ![]() This is due to less quantity or lack of yolk in micromeres and megameres having more yolk present on the vegetal end. Micromeres divide more rapidly than the megameres.After 32-cell stage, cleavages become less regular and difficult to follow.Two horizontal divisions following this, results in the formation of a 32-celled stage.Two more vertical divisions (fourth cleavage set) take place forming 16 cells (8 megameres and 8 micromeres).Out of these, 4 upper smaller ones are called micromeres and 4 lower larger ones are called megameres. The third division is horizontal passing above the equator forming 8 unequal blastomeres.Second division is also vertical but at right angle to the first resulting in 4 blastomeres.The furrow extends from animal pole to the vegetal pole (lower end). The first division is vertical and divides the zygote into two blastomeres.So, the cleavage is called holoblastic but unequal. In frog, the division is complete but the cells formed are unequal.The zygote now divides through a process called cleavage or segmentation.The cell thus formed is called zygote and the nucleus is called zygote nucleus which has diploid chromosome number, 26.After the second maturation division giving off second polar body, the fusion of male and female nuclei takes place.The first polar body was already present below the vitelline membrane.One sperm penetrates one ovum and as soon as the male pro-nucleus enters the ovum, a second polar body is budded off from the ovum.Fertilization in frog is external, as it occurs in water.When the female sheds ova (several hundred) through her cloaca, the male deposits spermatic fluid over them and fertilization occurs.The male comes over the back of the female and firmly clasps her body by his forelimbs.Frogs copulate in water by a sexual embrace called amplexus or pseudo-copulation. During breeding season (rainy season), the croaking of male attracts the female frog.Ova are larger than sperms, non-motile and nearly spherical.The sperms are microscopic, thread like and motile, 0.03mm in length.Sperms and ova (eggs) are produced by respective gonads in males and females by meiosis.Sequential events in the development of frog: Growth: Increase in size and weight through cell division and cell enlargement.Morphogenesis: Growth and differentiation of form and structure.Organogenesis: Development and differentiation of different organs from primary germ layers.Gastrulation: Movement and rearrangement of cells.Blastulation: Formation of hollow ball of cells.Morulation: Formation of solid ball of cells.Cleavage: Repeated divisions taking place in the zygote.Fertilization: Fusion of male and female gametes to form a zygote.Mating: Transfer of male gametes inside female’s body by copulation.Gametogenesis: Formation of gametes or sex cells.Sequence of events before and during embryonic development in living organisms: You may also want to see: The process of fertilization and development of embryo in angiosperms The development of embryo takes place in an orderly sequence and follows the same fundamentally similar sequence of events in almost all sexually reproducing organisms.The branch of biology dealing with the study of embryo is known as embryology.Embryo is the earliest stage of development, while it is within the egg or reproductive organs of mother until hatching or birth.Zygote divides and re-divides to form an embryo.In sexual reproduction, development of organisms begins with a fertilized egg or zygote. ![]()
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