GERMINAL MEMBRANE. 55 



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 velk-cavitv 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 j 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 j Sthly, 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, 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 



