GERMINAL MEMBRANE.
öo
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 proble¬
matical 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 in¬
vestigation, whether the cleaving of the yelk described by
Baer, Busconi, 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