. The elements of Embryology,. 62 THE SECOND DAY. [chap. the sides. One of the results of this doubling up of the blasto- derm to form the head is the appearance, below the anterior extremity of the medullary tube, of a short canal, ending blindly in front, but open widely behind (Fig. 16, D), a cul de sac in fact, lined with hypoblast reaching from the extreme front of the embryo to the point where the splanchnopleuric leaf of the head-fold (Fig. 16, F. S^)) turns back on itself. This cul de sac, which of course becomes longer and longer the farther back the head-fold is carried, is the rudim
. The elements of Embryology,. 62 THE SECOND DAY. [chap. the sides. One of the results of this doubling up of the blasto- derm to form the head is the appearance, below the anterior extremity of the medullary tube, of a short canal, ending blindly in front, but open widely behind (Fig. 16, D), a cul de sac in fact, lined with hypoblast reaching from the extreme front of the embryo to the point where the splanchnopleuric leaf of the head-fold (Fig. 16, F. S^)) turns back on itself. This cul de sac, which of course becomes longer and longer the farther back the head-fold is carried, is the rudiment of the front end of the alimentary camd, the foregut, as it might be called. In transverse section it appears to be flattened horizontally, and also bent, so as to have its convex surface looking downwards, (Fig. 18 al). At first the anterior end is quite blind, there being no mouth at all; the formation of this at a subsequent date will be described later on. At the end of the first half of the second day the head- FiG. 16. Diagrammatic Longitudinal Section thkough the Axis of an Embryo. The Kection is supposed to be made at a time whea the head-fold has com- menced but the tail-fold has not yet appeared. N. C. neural canal, closed in front but as yet open behind. Ch. nntochord, not reaching to the extreme'front, pnd not as yet lully formed beliind. The section being taken in the middle line, the protovertebrse are of course not shewn. In front of the notochord is seen a mass of uncleft mesoblast, which will eventually fonn part of the skull. D. the commencing fore- gut or front part of the alimentary canal. F. So. Somatopleure, raised lip in its peripheral portion into the amniotic fold Am. Sp. Splauchno- pleure. At Sp. it forms the under wall of the foregut; at F. Sp. it is turning round and about to run forward. Just at its turning point the cavity of the heart Ht is being developed in its mesoblast. 2V- pleuroperitoneal cavity. A epiblas+, B mesoblast, C hypoblast
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