Waldeyer (Henle und v. Pfeufer's Zeitschrift, xxxiv. Band, für 1869) has given a different account of the origin of the layers. There is no doubt, however, in opposition to his statements and drawings, that from the very first the hypoblast is distinct from the mesoblast, which is, indeed, most conspicuously shewn in good sections; and his drawings of the derivation of the mesoblast from the epiblast are not very correct.
The changes which have been described are also clearly shewn by means of silver nitrate. Whereas, at first this reagent brought out no outline markings of cells in the lower layer, by the eighth to the twelfth hour the markings (Pl. 1, fig. 3) are very plain, and shew that the hypoblast is a distinct coherent membrane.
In section, the cells of the hypoblast appear generally very thin and spindle shaped, but the outlines brought out by the silver nitrate shew that they are much expanded horizontally, but very irregular as to size, varying even within a small area from 1/4000 - 1/400 of an inch in the longest diameter.
At about the twelfth hour they are uniformly smaller a short way from each extremity of its longer axis than over the rest of the blastoderm.
It is, perhaps, fair to conclude from this that growth is most rapid at these parts.
At this time the hypoblast, both in sections and from a surface view after treatment with silver nitrate, appears to end abruptly against the white yolk. The surface view also shews that its cells are still filled with highly refractive globules, making it difficult to see the nucleus. In some cases I thought that I could (fig. 3, a) make out that it was hour-glass shaped, and some cells certainly contain two nuclei. Some of the cells (fig. 3, b) shew re-entrant curves, which prove that they have undergone division.
The cells of the epiblast, up to the thirteenth hour, have chiefly undergone change in becoming smaller.
In surface views they are about 1/4000 of an inch in diameter over the centre of the pellucid area, and increase to 1/2000 of an inch over the opaque area.
In the centre of the pellucid area the form of the epiblast cells is more elongated vertically and over the opaque area more flattened than was the case with the original upper layer cells. In the centre the epiblast is two or three cells deep.
Before going on to the further changes of the blastodermic cells it will be well to say a few words in reference to the origin of the mesoblast.