266 LILLIE. [Vol. XVII. 



is the primary factor in these cases ? and that this organization 

 must include at least bilateral symmetry and possibly definite 

 proportions ? 



In his " Inadequacy of the Cell Theory of Development," 

 Whitman has given a picture of the way in which crystallization 

 of the embryo sets in in the undifferentiated mass of cells com- 

 posing the blastodisc of the pelagic fish-egg. " It is well known 

 that the transformation of the blastodisc just before the appear- 

 ance of the germ-ring is quite rapid, at least in the pelagic 

 fish-egg, and also quite independent of cell formation. The 

 discoidal germ-mass suddenly thins out, but not uniformly in 

 all parts. The half of the disc in which the embryo is to be 

 formed remains thick, anticipating, as it were, the axial concen- 

 tration which is to follow, while the half lying in front of this 

 is rapidly reduced to a thin epithelial membrane. This irgional 

 differentiation of the outer layer and the concomitant formation 

 of the germ-ring, including the forward movement of the embry- 

 onic plate (' head process '), which advances in an axial direction 

 to the very center of the disc, are indubitably accomplished, 

 not by the aid of cell formation, but by formative processes 

 of an unknown nature, but, nevertheless, real and all-control- 

 ling. Cell formation, to be sure, goes on, but it seems to 

 me certain that it has no directive influence on the formative 

 process. The cleavage runs on from beginning to end, regularly 

 or irregularly, without modifying in any essential way the form 

 of the blastodisc. All at once, when this segmentation has 

 been carried to a certain point, the transformation sets in and 

 goes rapidly on, without interrupting cell formation, but to all 

 appearance quite independently of it." 



The view that the cytoplasm of the tgg possesses at least a 

 definite bilateral orientation is not in the least weakened by 

 cases of non-determinate cleavage. In such cases all one can 

 say is, that cell formation is not directly adapted to purposes 

 of differentiation. But is this view disproved by such observa- 

 tions as those of Wilson ('95) and Ziegler ('96) that the orienta- 

 tion of the embryo may bear no definite relation to the polarity 

 of the egg .'' We must inquire here what is meant by this 

 expression, polarity of the egg. Two things, clearly, are meant 



