GERMINAL MEMBRANE. 
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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