Question

1. Define holoblastic radial cleavage, making sure to include the influence of yolk. Indicate the common name of an organism that exhibits this type of cleavage.

2. Define meroblastic discoidal cleavage, making sure to include the influence of yolk. Indicate the common name of an embryo that exhibits this type of cleavage.

3. Define epiboly as a mechanism for gastrulation and then briefly describe one specific example of how epiboly is used in relation to ectoderm. In addition, indicate two derivatives of ectoderm in vertebrates.

4. Define ingression as a mechanism for gastrulation and then briefly describe one specific example of how ingression is used in relation to mesoderm. In addition, indicate two derivatives of mesoderm in vertebrates.

Answer 1

Answer :- The above given questions is all about the cleavage in zygote.

1. Holoblastic radial cleavage. :- Holo means the full or complete , and radial means in radial symectry. so, in radial holoblastic cleavage is the cleavage takes place in full zygote and the blastomeres arrange in the upper tier directly and next to the lower tier. In this type of cleavage the egg pruduce with very little amount of yolk. Deuterosomes show this type of cleavage in echinoderms and vertebrates.

2. Meroblastic discoidal cleavage :- Mero means partial of not full , and discoidal means in the form of disc. so, in meroblastic discodial cleavage is the cleavage takes place in partial zygote and furrow produce by cleavage do not penetrate the yolk. disc of cells are producedk nown as blastodisc. In this type of cleavage the egg produce with large amount of yolk. Telolecitha eggs of birds and reptiles exhibit this type of cleavage.

3. Gestation is the period of embryonic development and epiboly is define as the movement of layer ( yolk syncytial) and blastodisc around the cell of yolk. This epibolic movement result in tissue spreading and thinning in vertebrates and invertebrates species. Amphibians shows the mechanism of epiboly in relation to ectoderm as during gestulation drives the expansion of the cells of ectoderm close to the blastopores. Derivatives of ectodermic layer in vertebrates are give rise to nasl cavity, skin , hairs, nails, etc.

4. Ingression is a mechanism in gestulation, in which many changes takes place in location or in position of cells of development. This process involve in breakdown of attachment of neighbouring cells of epiblast and produce mesenchyme cells. however ingression of these cells result in formation of mesoderm like in birds and mammals. Derivatives of mesoderm layer in vertebrates are formation of connective tissues, muscles, subcutaneous layers, etc.