The lower layer cells form, for the most part, a uniform stratum in which no distinction into mesoblast and hypoblast is to be seen.
Both the lower layer cells and the epiblast cells are still filled with yolk-spherules.
The structures at the embryonic rim, and the changes which are there taking place, unquestionably form the chief features of interest at this stage.
The general relations of these parts are very fairly shewn in Pl. 7, fig. 5, which represents a section passing through the median line of the embryonic region. They are however more accurately represented in Pl. 7, fig. 5_a_, taken from the same embryo, but in a lateral part of the embryonic rim; or in Pl. 7, fig. 6, from a slightly older embryo. In all of these figures the epiblast cells are reflected at the edge of the embryonic rim, and become perfectly continuous with the hypoblast cells. A few of the cells, immediately beyond the line of this reflection, precisely resemble in character the typical epiblast cells; but the remainder exhibit a gradual transition into typical lower layer cells. Adjoining these transitional cells, or partly enclosed in the corner formed between them and the epiblast, are a few unaltered lower layer cells (_m_), which at this stage are not distinctly separated from the transitional cells. The transitional cells form the commencement of the hypoblast (_hy_); and the cells (_m_) between them and the epiblast form the commencement of the mesoblast. The gradual conversion of lower layer cells into columnar hypoblast cells, is a very clear and observable phenomenon in the best specimens. Where the embryonic rim projects most, a larger number of cells have assumed a columnar form. Where it projects less clearly, a smaller number have done so. But in all cases there may be observed a series of gradations between the columnar cells and the typical rounded lower layer cells[131].
Footnote 131: When writing my earlier paper I did not feel so confident about the mode of formation of the hypoblast as I now do, and even doubted the possibility of determining it from sections. The facts now brought forward are I hope sufficient to remove all scepticism on this point.
In the last described embryo, although the embryonic rim had attained to a considerable development, no trace of the medullary groove had made its appearance. In an embryo in the next stage of which I propose describing sections, this structure has become visible.