ticular time during pregnancy for its development, and this is known as its "critical period" during which the developing embryo, or fetus, has the greatest sensitivity to environmental influences. During the third and fourth weeks, for example, major development can be discerned with reference to the somites (muscle blocks of the body), the heart, brain, spinal cord, and both the inner and outer ear. If these systems are disrupted in this period for any reason, they may not develop properly and can lead to birth defects. This is thus the time in a pregnancy where caution has to be exercised in the diet, stress levels, and general health of the mother (Browne, 1996, www.bio.mq.edu/au/school/museum/exhibitions.html).
Early in the formation of the brain. By the eighth day after conception, cells on the inside of the blastocyst detach themselves and form a flat plate across the inside known as the embryonic disc. Within a week, some of these cells differentiate so that the disc is soon made up of two distinct layers, the ectoderm and the endoderm. The ectoderm eventually becomes the nervous system, as well as skin, hair, and fingernails. The endoderm forms the respiratory and digestive tracts and related
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