. Carnegie Institution of Washington publication. 122 A HUMAN EMBRYO BEFORE THE APPEARANCE OF THE MYOTOMES. streak. This layer of cells is thinnest behind and lies uniformly close to the ento- derm, to which its cells are attached by numerous fine processes. The constit- uent cells vary considerably in size and shape; most of them possess a number of larger or smaller, partly anastomosing processes, while some seem to have a smoothly rounded cell-body. One finds quite generally a fine, sharp line on that surface of the mesoderm toward the ectoderm, very much like a basement membrane of connect
. Carnegie Institution of Washington publication. 122 A HUMAN EMBRYO BEFORE THE APPEARANCE OF THE MYOTOMES. streak. This layer of cells is thinnest behind and lies uniformly close to the ento- derm, to which its cells are attached by numerous fine processes. The constit- uent cells vary considerably in size and shape; most of them possess a number of larger or smaller, partly anastomosing processes, while some seem to have a smoothly rounded cell-body. One finds quite generally a fine, sharp line on that surface of the mesoderm toward the ectoderm, very much like a basement membrane of connective-tissue origin, the membrana prima of von Spee. There is no indication anywhere of an arrangement of the mesodermic cells in two layers, as has been repeatedly described in the primitive-streak region. The majority of the mitotic figures observed in this specimen are found near, or at a short distance from, the primitive streak; by far the greater number of these occur in the ectoderm and mesoderm, especially in the former; only rarely are they. Fio. 4.—Head process and completion plate. Dorsal view, but in the plane of the sections (i. e., some foreshortening. Cf. fig. 3). On the left is the anterior end of the primitive streak; immediately anterior to this is the dorsal opening of the archenteric canal. The four ventral openings of the canal are seen in the center of the figure; dotted lines indicate that the lumen is indistinct or doubtful. The small dotted ring near the anterior end represents a very doubtful cavity, and just beyond this is shown the line where the entoderm becomes a distinct layer. Marginal lines as before. seen in the entoderm. In those cases in which the axis of the spindle can be deter- mined it is found in nearly all instances parallel to the surface of the ectoderm or mesoderm and at right angles to the median line. As mentioned above, the anterior part of the primitive groove is but the bottom of a deep, median furrow in the blastoderm. This fur
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