. Contributions to embryology. Embryology. 118 A HUMAN EMBRYO BEFORE THE APPEARANCE OF THE MYOTOMES. right. Separating these prominent folds (plate 3, figs. 2 and 3) lies, in their caudal halves, the anterior end of what we may call the primitive groove, here very deep and narrow. The groove between these folds in their cei)halic jiortion is much shallower and finally lost. Near the posterior end of this shallower gioovc, whicii is continued forward without distinct interruption from the ])rimitive gnxne, hut in a plane sUghtly to the right, lies the minute dorsal opening of the archenteric ca
. Contributions to embryology. Embryology. 118 A HUMAN EMBRYO BEFORE THE APPEARANCE OF THE MYOTOMES. right. Separating these prominent folds (plate 3, figs. 2 and 3) lies, in their caudal halves, the anterior end of what we may call the primitive groove, here very deep and narrow. The groove between these folds in their cei)halic jiortion is much shallower and finally lost. Near the posterior end of this shallower gioovc, whicii is continued forward without distinct interruption from the ])rimitive gnxne, hut in a plane sUghtly to the right, lies the minute dorsal opening of the archenteric canal—so-called chordal canal—slightly to the left in the bottom of the groove. The anterior third of the blastoderm is in general slightly convex, but its sur- face is broken up by many small, irregular folds to whicli one can attach no signifi- cance. It is certain that the ectoderm in the anterior half of the embryo has suffered more distortion than any other part. The i-esult has been an obliteration, as far as they may have been indicated, of the early medullary folds anteriorly, coujiled with what seems to be their accentuation and prolongation posteriorly. That these last- mentioned folds, occupying the center of the blastoderm, have anything to do with the medullary folds is, considering the stage of develojmient, very doubtful. The posterior ends of these folds, especially on the right, have a remote resemblance to the caudal lobes of a later date. Fio. 2.—Blastoderm and axial struc- tures in dorsal view. X about 40. For more exact details of the bead process see text^figures4 and 5. The location of the cloacal membrane is indicated by dotted outline. Close to the posterior tip of the amnion is the allan- tois; in section and near it, in the body-atalk, are sections of two vessels. The marginal lines have reference to the location from wliich were taken the photo- (?rapha (P.) and drawings (D.) shown on plates 2 and 3. The amnion lies close to the embryonic ectoder
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