Formation of the Layers.
In the last chapter the blastoderm was left as a solid lens-shaped mass of cells, thicker at one end than at the other, its uppermost row of cells forming a distinct layer. There very soon appears in it a cavity, the well-known segmentation cavity, or cavity of von Baer, which arises as a small space in the midst of the blastoderm, near its non-embryonic end (Pl. 7, fig. 1.).
This condition of the segmentation cavity, though already[116] described, has nevertheless been met with in one case only. The circumstance of my having so rarely met with this condition is the more striking because I have cut sections of a considerable number of blastoderms in the hope of encountering specimens similar to the one figured, and it can only be explained on one of the two following hypotheses. Either the stage is very transitory, and has therefore escaped my notice except in the one instance; or else the cavity present in this instance is not the true segmentation cavity, but merely some abnormal structure. That this latter explanation is a possible one, appears from the fact that such cavities do at times occur in other parts of the blastoderm. Dr Schultz[117] does not mention having found any stage of this kind.
The position of the cavity in question, and its general appearance, incline me to the view that it is the segmentation cavity[118]. If this is the true view of its nature the fact should be noted that at first its floor is formed by the lower layer cells and not by the yolk, and that its roof is constituted by both the lower layer cells and the epiblast cells. The relations of the floor undergo considerable modifications in the course of development.
The other features of the blastoderm at this stage are very much those of the previous stage.
The embryonic swelling is very conspicuous. The cells of the blastoderm are still disposed in two layers: an upper one of slightly columnar cells one deep, which constitutes the epiblast, and a lower one consisting of the remaining cells of the blastoderm.
An average cell of the lower layer has a diameter of about 1/900 inch, but the cells at the periphery of the layer are in some cases considerably larger than the more central ones. All the cells of the blastoderm are still completely filled with yolk spherules. In the yolk outside the peculiar nuclei, before spoken of, are present in considerable numbers. They seem to have been mistaken by Dr Schultz[119] for cells: there can however be no question that they are true nuclei.
In the next stage the relations of the segmentation cavity undergo important modifications.
The cells which form its floor disappear almost completely from that position, and the floor becomes formed by the yolk.
The stage, during which the yolk serves as the floor of the segmentation cavity, extends over a considerable period of time, but during it I have been unable to detect any important change in the constitution of the blastoderm. It no doubt gradually extends over the yolk, but even this growth is not nearly so rapid as in the succeeding stage. Although therefore the stage I proceed to describe is of long continuance, a blastoderm at the beginning of it exhibits, both in its external and in its internal features, no important deviations from one at the end of it.