The nuclear genetic factors, or genes, remaining dormant throughout the period of cleavage, begin to manifest themselves during gastrulation, and in ever-increasing measure control the process of development from this stage onward. This does not mean that the process taking place during cleavage are not affected by genes but the effect of the genes is [...]
The gastrulation is the most active phase of embryonic development during which extensive or profound morphogenetic activities of different types of blastomeres occur and rate of metabolism of gastrula as a whole becomes rapidly increased. Its metabolism can be studied under following headings: (a) Catabolism- The morphogenetic movements during gastrulation cause an increased expenditure of [...]
Gastrulation is a nicely integrated, dynamic process one, which is controlled largely by intrinsic forces bound up in the specific physicho-chemical conditions of the various presumptive organ forming areas of the late blastula and early gastrula. These internal forces in turn are correlated with external conditions. In most vertebrates, it includes different kinds of morphogenetic [...]
The correct interpretation of gastrulation is impossible without a knowledge of the position which the presumptive germinal layers occupy in the blastula. This position may be ascertained in various ways. A chart showing the fate of each part of an early embryo, in a particular blastula, is called fate map. The fate maps of different [...]
In the blastula, most of the blastomeres which have to become differentiated into the cells of three fundamental germinal layers- ectoderm, endoderm and mesoderm of the future embryo, occur in a single layered blastoderm and at the external surface of the blastula. To occupy their respective positions in the embryo the blastomeres of presumptive organ [...]
The cleavage occurs more readily in the active cytoplasm than the yolk-laden cytoplasm or deutoplasm of the egg. The yolk which occurs in all animal eggs in little (e.g., microlecithal eggs) or larger (e.g., mesolecithal and macrolecithal eggs) amounts and remains distributed evenly (e.g., isolecithal eggs) or unevenly (e.g., telolecithal eggs) in the ooplasm of [...]
The cleavage is a phase of intensified chemical activity and a great amount of turn over of molecules occurs during its metabolism. The metabolism of cleavage can be studied under following two headings: A. Catabolic activities of cleavage- The cleavage requires large amount of chemical energy in the form of ATP molecules for its completion. [...]
By studying the cleavage in the eggs of different animal groups, it has been concluded that in all animal eggs, the cleavage seems to be governed by certain basic principles, rules of laws. Certain fundamental rules or laws of cleavage are following: 1. Sach’s law- In 1877, Sach proposed following two laws: a) Cells tend [...]
During cleavages, differnt cleavage furrows may divide the egg from different planes. Few important cleavage planes are following: 1. Meridional plane- When cleavage furrow passes through the centre of animal-vegetal axis and bisects the both poles of the egg, then such plane of cleavage is called meridional plane, e.g., Rana pipiens, Lepidosteus osseus and first [...]
The rate of cleavage varies from species to species. For example, in the gold fish, divisions follow each other continuously at regular intervals of 20 minutes. The inter-cleavage interval of frog is one hour and of mouse is 10 to 12 hours. In most of these cases, the cleavage rate remains rapid (synchronous) during completion [...]
Dr. Amlan Kumar Mitra, Biology Guide Since 2009
