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Simultaneously with the construction of the medullary tube we have in the Amphioxus-embryo the formation of the chorda, the coelom-pouches, and the mesoderm proceeding from their wall. These processes also take place with characteristic simplicity and clearness, so that they are very instructive to compare with the vermalia on the one hand and with the higher vertebrates on the other.
During the second day the Amphioxus-embryo undergoes few other changes. The number of primitive segments increases, and generally amounts to fourteen, some forty-eight to fifty hours after impregnation. It now begins to nourish itself independently, as the food material stored up in the ovum is completely used up.
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