BLASTODERMIC CELLS HETEROGENEOUS. 45 



as having the significance of a cell ; the yolk repre- 

 senting the protoplasm, the germinal vesicle, the 

 nucleus, and the vitellary membrane, the cell-wall. 

 Gigantic a cell, I may remark, as the yolk of the 

 common hen's egg must, in that case, be looked on, 

 still more gigantic a cell must be the yolk of the 

 ostrich's egg, and more gigantic still must have been 

 the yolk of the eggs of the extinct birds of Mada- 

 gascar and New Zealand. But such a view of the 

 character of the ovum ought not to be entertained. 

 An ovum, on the contrary, must be considered as a 

 much more complex and very highly specialised 

 organism. 



The germinal vesicle is not a nucleus with a 

 nucleolus, but is itself a nucleated cell. Through 

 the combined influence of it and the fecundating 

 cells, the yolk is constituted a special blastema, out 

 of which the blastoderma, or germinal membrane — 

 the structure from which the embryo originates — is 

 developed by a remarkable process of cell-formation. 

 The cells composing the blastoderma are, in their 

 endowments, potential as well as actual, of different 

 kinds, though formed in common from the yolk. 

 But the yolk, it is to be remarked, is a substance of 

 heterogeneous composition, and affords materials for 

 the development of the different kinds of cells, just 

 as a fluid containing different salts in solution yields, 

 on evaporation, different kinds of crystals. Hence, 

 in the development of the embryo, the various 

 tissues and organs have their origin in different 



