Reconstruction of the epithelial core of the alimentary canal typical of the median members of stage 17. It will be noted that the epithelium has acquired a pattern which one can recognize many details approximating the regional forms of the mature canal; the elements of this epithelial pattern are already present at stage 16. Antedating stage 16 is a level with yet simpler expression of the same parts, and so on, back to the primordial system. Thus starting from the primary form this precocious epithelium of the gut progresses in its development step by step, always in the direction of its inherent plan and at no place deviating from it. Considering the relative backwardness of the other coats of the gut during this period, one must attribute a high degree of intrinsic potency to the different regions of the epithelium.

In stage 17 the vermiform appendix becomes marked off form the caecum. In the kidney the pelvis has become branched. The intestinal loop herniates into the umbilical cord without rotation or coils.

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