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Plate 4. Normal villous ova and pathologic pre-villous ova

Fig. 16. A maid-cross section of the embryo and adjacent chorion of a 14-day ovum. The amniotic cavity is now a ﬂattened cone and is completely enclosed by the thin amniotic membrane above and the primitive ectoderm below. The yolk-sac has now appeared and has already outstripped the amniotic cavity in size although the former has probably not been present for more than 36 ours. Carnegie 7801, section 12-1-3, X 100.

Fig. 17. A mid-cross section of the embryo and adjacent chorion of a 16-day ovum. The doublelayered amnion is attached by mesocblast to the adjacent chm-ion. The amniotic cavity is approximately equal in size no the yolk-sac cavity. The latter is bounded by the endoderm above and the double-layered yolk-sac elsewhere. Carnegie 7802. section 44-3-5. X 100.

Fig. 18. A mid-cross section of a pathologic, pre-villous ovum of uncertain age but probably 11 to 12 days old Its embryonic mass and hence the amnion are absent although there is an exioocoelomic membrane and some mesoblastic tissue lining the chorionic cavity. Carnegie 7771, sec tion 3-4-I. x 100.

Fig. 19. A mid-cross section of a pathologic ovum of uncertain age. Its relatively normal embryo is equivalent to that of a 14- or possibly 15-day specimen although its tropho-blast is defec tive, especially toward the right side. Note the relatively poor mesoblastic formation associated this region in contrast to the abundant mesoblast associated with the better, but still defective trophoblast on the left side. Carnegie 7800, section 15-1-3, X 35.