Question

Select 2 organisms (e.g. invertebrate or vertebrate)that undergo different patterns of cleavage.For each of your selected organisms, (e.g. sea urchin, snails, C. elegans, Drosophila) describe in detail developmental changes through cleavage, blastula & gastrula stages.

Answer 1

Sea Urchin undergoes holoblastic cleavage and chicken undergoes meroblastic cleavage.

Developmental stages of Sea Urchins

1. When the egg is unfertilized, it is surrounded by a yolk membrane, and the yolk material is distributed evenly throughout the cytoplasm of the egg, which is referred to as isolecithal egg..
2. Soon after fertilization, the egg gets surrounded by a fertilization membrane in order to prevent other sperms from penetrating the egg.
3. During the cleavage phase, the entire fertilized egg undergoes a holoblastic or a complete cleavage, where the egg is equally divided into two blastomeres.
4. The subsequent cleavage takes place perpendicular to the first cleavage, further dividing the two equal cells into four equal blastomeres.
5. The third cleavage takes place in an equatorial plane, which is perpendicular to the first two planes of cleavages, resulting in eight equal cells.
6. After this stage, cleavage becomes asymmetrical and subsequent cleavages result in the generation of different sized cells.
7. The cleavages finally end in the generation of a morula, which comprises of 128 cells and is roughly the same size as the initial fertilized egg.
8. After this, rearrangement of the cells take place to give rise to a blastula comprising of a central cavity known as the blastocoel. At this stage, the fertilization membrane disappears, and one side of the blastula thickens to form the vegetal plate.
9. The cells at the vegetal plate undergo invagination where they migrate inwards, giving rise to the formation of the ectoderm and endoderm.
10. Following this, cells at the base of the gastrula undergo ingression and start moving inwards to form the mesoderm, which eventually gives rise to the skeleton and the body cavities.
11. The gastrulation stage finally ends in the generation of the larva of the sea urchin, which goes on to further develop and form the entire organisms.

Developmental Stages of Chickens

1. The egg of chickens is telolecithal, meaning that the yolk is unevenly distributed in the egg. The pole of the egg with a lower concentration of the yolk is known as the animal pole, and the opposite pole is known as the vegetal pole.
2. Following fertilization, the egg membrane rapidly develops into the fertilization membrane.
3. Telolecithal eggs undergo meroblastic or partial cleavage, where the yolk is not incorporated into the developing embryo.
4. The development of the embryo occurs in the blastodisc at the animal pole, which is separated from the rest of the egg by the germinal cavity.
5. The blastula eventually forms the blastocoel, which comprises of the epiblast at the surface and the hypoblast in the centre.
6. The formation of a groove or the primitive streak marks the start of gastrulation in the embryo.
7. This groove is surrounded by the blastoderm and ends in a primitive pit or the Hensen's node. In front of this node, lies the notochord, which later gives rise to the formation of the neural plate and the neural groove.
8. Following further development, the head folds become visible, followed by somites and paired blocks that later form the vertebrae, ribs, skin, and dorsal muscles.
9. The position of the embryo with respect to the egg yolk also changes as more and more organs are formed and become clearer, eventually leading to the formation of a complete bird.