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When we examine the embryology of the human nervous system, we must start from the important fact, which we have already seen, that the first structure of it in man and all the higher Vertebrates is the simple medullary tube, and that this separates from the outer germinal layer in the middle line of the sole-shaped embryonic shield.
Long before there is any trace of the skull, limbs, etc., in the embryo of man or any of the higher Vertebrates at the early stage in which the whole body is merely a sole-shaped embryonic shield there appears in the middle line of the shield, directly under the medullary furrow, the simple chorda.
In the dorsal wall of this the sole-shaped embryonic shield is developed, and on the under-side of this a shallow groove appears in the middle line, the first trace of the later, secondary alimentary tube. The gut-groove becomes deeper and deeper, and its edges bend towards each other, and finally form a tube.
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