-All tetrapods have the same body plan, one head and four
limb arrangements. This chapter also introduces a new term atavism, which is the
reappearance of traits that had disappeared generations ago.
-The sixth day after conception, the zygote which has been
duplication eventually forms a blastocyst which is embedded into the lining of
the uterus wall. After implantation the cells begin individualizing. Within each
organism a hollow tube, consisting of three layers is formed. One end of the
tube becomes the mouth , the middle section becomes the digestive track and the
other end becomes the anus. The three layers of the tube then constitute the various
organs of the organism.
- The specific differentiation of the tube into these three
major regions is due to the presence/ absence of a certain set of chemicals
which are produced by the cells themselves. The region with a high
concentration of these chemicals turns into the head; subsequently the region
with a low concentration becomes the anus.
- The same genes in every organism result in the formation
of these chemicals and thus in the formation of such tubes. Sea anemones have sides, a front and a
back. Even though they are radially
symmetrical (humans and most other animals are bilaterally symmetrical), they
still have a head region and a tail region.
Those same genes that form the head and tail ends are in other animals,
including humans.
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