Abstract
The primitive axis of the rat embryo appears during the ninth day after insemination, on the inner concavity of the upper or anti-mesometrial portion of the blastodermic vesicle. Accordingly its sagittal section is U-shaped. In order to isolate definite portions of the axis, the embryonic end of the cylinder is first cut off from the rest by a transection in the region of the amnion; then the embryo is flattened by means of a slit in each lateral wall of the vesicle so isolated. Transverse cuts may now be made at any level of the axis. In the present series the whole posterior half was isolated as a single explant, the anterior half usually being divided into thirds transversely.
The isolated fragments of the embryonic regions of the rat in stages just previous to and during the formation of the embryonic axis are capable of typical differentiation in vitro. The pieces were explanted on cover slip clots of rat plasma and rat embryo or placental extract. The plasma donor was injected with a few cc. of 10% dextrose immediately before the bleeding. The cultures were usually washed or transferred after 48 hours; fixed after 3 or 4 days, photographed in toto and later examined in serial sections.
The designated levels of embryos in stage 14 (medullary plate) and stage 15 (head-fold) round up into little vesicular structures, and differentiate characteristic primitive organs. The anteriormost third of the anterior part of the axis always includes part of the yolk-sac which forms its typical epithelium, blood islands and vascular channels. One or two pulsating myocardial masses invariably develop; histologically these may show little differentiation. Mesenchyme frequently is found; less frequently medullary tube. The second third of the anterior axis usually forms yolk-sac, and always pulsating heart-masses.
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