(1) The increase in size of the blastoderm.
(2) The diminution in size of the segmentation cavity, both relatively and absolutely.
(3) The appearance of a portion of the blastoderm projecting beyond the rest over the yolk. This projecting rim extends for nearly half the circumference of the yolk, but is most marked at the point where the embryo will shortly appear. I will call it the “embryonic rim.”
These points are still better seen from sections than from surface views, and will be gathered at once from an inspection of Pl. 3, fig. 4.
The epiblast has become still more columnar, and is markedly thicker in the region where the embryo will appear. But its most remarkable feature is that at the outer edge of the “embryonic rim” (er) it turns round and becomes continuous with the lower layer cells. This feature is most important, and involves some peculiar modifications in the development. I will, however, reserve a discussion of its meaning till the next stage.
The only other important feature of this stage is the appearance of a layer of cells on the floor of the segmentation cavity.
Does this layer come from an ingrowth from the thickened edge of the blastoderm, or does it arise from the formation of new cells in the yolk?
It is almost impossible to answer this question with certainty. The following facts, however, make me believe that the newly formed cells do play an important part in the formation of this layer.
(1) The presence at an earlier date of almost a row of nuclei under the floor of the segmentation cavity (Pl. 3, fig. 1).
(2) The presence on the floor of the cavity of such large cells as those represented in fig. 1, bd, cells which are very different, as far as the size and granules are concerned, from the remainder of the cells of the blastoderm.