254 FOUNDATIONS OF BIOLOGY 



old theories of preformation and epigenesis confront us to- 

 day as a result of recent research. 



The reader already recognizes the fertilized egg as a cell, 

 with its nucleus comprising a complex of quite definite ele- 

 ments the chromosomes contributed jointly by the two 

 gametes. To this extent, then, the nucleus and therefore the 

 egg exhibits a ready-formed structural basis which (as we 

 have already suggested, and will have occasion to elaborate 

 later) seems to be definitely related to characters which 

 appear in the offspring. 



Turning to the egg cytoplasm, we are confronted with 

 conditions which are not so uniform but nevertheless highly 

 suggestive. In the first place, before fertilization the egg 

 possesses a definite polarity, expressed, for example, in the 

 position of the nucleus and the distribution of food material 

 (yolk), pigment granules, and vacuoles. This polarity is 

 traceable, in part at least, to the polarity of the oogonia, and 

 through them to the germinal epithelium. In brief, the egg 

 as a whole is organized; the invisible organization of the 

 fundamental matrix of the cytoplasm being revealed, in part, 

 by the disposition of various elements of the cell. Now in 

 some cases this cytoplasmic organization remains essentially 

 undisturbed at fertilization, and persists as that of the 

 zygote, while in others it is superseded sooner or later by a 

 reorganization which establishes that of the new organism. 

 Herein, apparently, is to be sought the explanation of the 

 difference in behavior in potentialities of various types 

 of eggs during cleavage stages. Clear-cut examples of the 

 two chief types will serve to bring the main facts before us. 



The first type is well illustrated by the egg of a Mollusc, 

 Dentalium, and a primitive Chordate, Cynthia. The egg of 

 the latter shows at the first division five clearly differentiated 

 cytoplasmic regions. For the sake of simplicity these may 



