Explain the results and reasons of the following experiments. 2½×4=10
i) When Hydra is transversely sliced into annular segments.
ii) The scientist McKinnel (1962) in his experiments transplanted the
blastula nuclei into enucleated eggs.
iii) Spemann and his coworkers grafted a piece of prospective neural
ectoderm from an early gastrula stage donor embryo in newt of genus
Triturus into a region of host embryo where belly skin should develop.
iv) In chick embryo, a piece of thigh mesoderm is transplanted under the
wing ectoderm.
i) When Hydra is transversely sliced into annular segments.
Answer- It is also observed that if the hydra is transversely sliced into annular segments, then each segment gives rise to a hypostome distally (anteriorly ) and a basal part proximally (posteriorly).Sometimes however, it has been noticed that a short segment, cut just below the
hypostome, will regenerate a hypostome with mouth and ring of tentacles at both ends, giving rise to a 'bipolar hydra'.
Even in the normal course of life hydroid polyps are in a 'continuous state of
regeneration'. The cells comprising the tentacles and hypostome and the basal pedal disc get continuously worn out. discarded and replaced by cells shifting distally and proximally from an area called the growth zone, located below the ring of tentacles.
The growth zone is easily identified by the presence of large number of interstitial cells
In Hydra, thus there is a steady migration of cells from the middle of the body in both direction to the hypostome and tentacles and to the peduncle and foot.
Regeneration process begins with the closure of the wound by contraction of the epitheliomuscular cells of the epidermis. After this the damaged surface is covered by ectoderm which forms a sheet of epithelial cells. In the next few hours more than the usual number of cells are given off from the growth zone towards the cut surface of the damaged area. which may be towards the oral or the aboral side of the body. Regeneration of each piece will occur to form complete miniature hydras even in complete absence of cell division in either piece.
ii) The scientist
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