Question

1. Explain three important embryonic components that result from gastrulation, including why they are important in bilaterian animals.

2. Describe the basic difference between protostomes and deuterostomes. In which group would humans belong? In which group would you expect to see identical twins, and why?

3.Explain why the notochord is especially important during the embryonic development of chordates.

4.Molecular evidence suggests that hagfish and lamprey should form sister taxa. Explain why many phylogenies suggest that lamprey branched off after hagfish, and would thus be more closely related to jawed vertebrates.

5.Explain why it may have been important for hagfish to have lost their backbone.

6.Explain why the life cycle of lamprey is said to be anadromous. In which part of their life cycle are they parasites?

Answer 1

1. When the single layered blastula proliferates in an organised way and form a multilayered gastrula, this process is known as gastrulation. This gastrulation process gives rise to three important embryonic components which are collectively known as germ layers. The germ layers are nothing but layers of cells. The external germ layer is known as ectoderm, the internal layer is known as endoderm and the germ layer present between the ectoderm and endoderm is known as mesoderm. These three germ layers makes the bilateral organisms triploblastic. Also the mesoderm and endoderm gives a body cavity to the organism known as coelom. But the main function of these germ layers is giving rise to organs by the process of organogenesis. Specific germ layers gives rise to specific organs in the organism. The endoderm gives rise to organs like the colon, the stomach, the intestines, the lungs, the liver, and the pancreas and also form the internal linings of the body and also of certain organs like the gastrointestinal tract, the lungs, the liver, the pancreas,glands that open into the gastrointestinal tract and the female genital organ.

2.

Protostomes	Deuterostomes
All invertebrates belongs to this group.	All chordates and echinoderm belongs to this group.
Blastopore develops in to mouth.	Blastopore developes in to anal opening.
Exhibits determinate cleavage.	Exhibits indeterminate cleavage.
Gut tunnels itself in to embryo and form anus.	Gut tunnels itself in to embryo and form mouth.
Contains solid ventral cord.	Contains hollow nerve cord.
Shows spiral cleavage .	Showa radial cleavage.
Trochophore larva	Dipleurula larva.

Humans belong to the Deuterostomes group as per the most basic feature shown by them, the indetermnate or regular and radial cleavage shown by human embryo.

The deuterostome group can form identical twins, because the cleavage pattern (radial, regular or indeterminate) found in deuterostome can only results in the formation of ideantical twins.

3. Chordates have been named as per the presence of notochord during their embryological development. In some chordates the notochord remains as notochord while in some they develop in to verteral column. The notochord is important for the chordates as it gives support to the embryo through out its development, it helps in formation of nervous system, it is the center for muscle attachment and also is the precursor for the development of vertebral column which is an important of the chordate body and the vertebral column develops during embryogenesis.

4. The lampreys and hag fish both belongs to the jawless fish or agnathans. But before the vertebrate evolution the lampreys have developed hinged jaws and also paired appendages which has divides the lampreys from the jawless hag fish and also helps in sharing most similar characteristics with the jawed vertebrates.The jawless fish are known as cyclostomes, hence the molecular findings shows that both should form the sister taxa while the development of cyclostome especially of lampreys have given proof of sharing the characteristics with the jawed vertebrates. The development of lampreys includes development of cranium,eye, ear, thyroid gland etc. and have resulted in its decline towards the jawed vertebrate.

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