GERMINAL MEMBRANE
plained in the same manner; for instance, no formation of yelk-globules can go on at that point at which the germ-vesicle and the stratum for the germinal membrane are in connexion with the layer of cells, but at that spot there must be a gap in each stratum of yelk-globules, which by the increasing thickness of the yelk-substance becomes a canal, necessarily conducting from the yelk-cavity towards the germinal membrane, and into which cells from the yelk-cavity become crowded. Now are these globules of the proper yelk-substance cells? I cannot prove decisively that they are so; the following arguments, however, render it probable: 1st, because Baer believes that he observed. an external membrane in some of them; 2dly, because, when ruptured at a particular spot by the compressorium, they at once pour out a large portion of their contents without the pressure being increased ; 3dly, because, notwithstanding that they lie close together in the yelk and flatten against one another, they do not run together; 4thly, because they so closely resemble some of the cells of the yelk-cavity which are furnished with granulous contents; 5thly, because they, like cells, appear to have an independent growth. These reasons are sufficiently strong to render it probable that the yelk-globules have a cellular structure, though they cannot be received as decisive of the point. However, inasmuch as they all form the contents of a larger cell, it is not absolutely necessary for our purpose that they should be distinctly proved to be cells. Both the indubitable cells of the yelk-cavity, and those problematical ones of the proper yelk-substance, have an independent growth in a fluid, and within another cell. They are cells within cells. For although the formation of new cells takes place only at the outside, yet they are still separated from the organized substance, not only by the cell-membrane of the entire ovum, but also by the layer of cells which is situated immediately beneath it. We here, then, meet with an instance of just such a formation and independent growth of cells within a fluid as was expressed by the fundamental phenomenon previously laid down. It is a point open to investigation, whether the cleaving of the yelk described by Baer, Rusconi, and others, in the development of the lower animals, the ova of frogs for example, may not also depend upon a process of cell-formation, two cells being developed
