EXPERIMENTAL EMBRYOLOGY 147 



has not been suggested by an}^ previous author is that the quahtative 

 differences between the various nuclei, as expressed by the nonmigration 

 of vitellophags, might conceivably result in surface-tension phenomena 

 around each nucleus. Such differences, arising from differences in the 

 nuclei themselves, might possibly inaugurate the general plasma stream- 

 ing and also cause the differences between the movements of various 

 nuclear types, perhaps largely by causing the vitellophags not to par- 

 ticipate in the general outward movement or actually to move against it. 

 In favor of the second hypothesis — plasma streaming — there are two 

 positive observations: (a) Strasburger's report that when some of the 

 nuclei anterior to the point of cauterization are delaj^ed from entering 

 the cortical layer in eggs of Calliphora, the inner cortical layer is formed 

 in those regions before the nuclei reach it; (6) Seiler's report (1924) 

 that the eggs of Phragmatohia (moth) hybrids occasionally undergo 

 "cleavage" of the cortical layer to form a pseudoblastoderm even though 

 no cleavage nuclei are present in the egg. His illustrations show clearly 

 that those areas which undergo this "cleavage" process have a con- 

 siderably thickened cortical layer. Both these cases certainly represent a 

 plasma streaming without nuclei, from which it is reasonable to assume 

 that the plasma streaming is the primary factor in these movements. 

 But plasma streaming alone leaves unexplained those cases where vitel- 

 lophags are left behind in the yolk (unless the plasma streaming is local 

 and caused by the nuclei — its general appearance being due to a summa- 

 tion of local effects around each nucleus). The third hypothesis is 

 indefinite. It is included here partly as a possible answer to the occa- 

 sional nonmigration of vitellophags, partly as a possible source of origin 

 of the plasma streaming. 



The factor or factors causing the centrifugal migration of the nuclei 

 must be the same that bring an end to the random orientation of the 

 mitotic figures and arrange the nuclei in the form of a hollow sphere. 

 It would seem that only a centrifugal plasma streaming could accom- 

 plish this. But from what could such a plasma streaming originate? 

 Certainly not directly from randomly oriented nuclei. Perhaps it is 

 unwise to attempt delimitation to any single factor or part, but it seems 

 possible that the stimulus might arise from the distant influence of the 

 cortical layer. The original shape, later movements, and change of 

 shape of the nuclear sphere are not too well correlated with the original 

 thicknesses of the various regions of the cortical layer so it can scarcely 

 be a truly quantitative relationship. Nevertheless Schnetter (1934a, 

 p. 161) points out that there are certain quantitative correlations in the 

 honeybee egg. He describes the outward movement of the nuclei as 

 occurring in waves passing anteriorly and posteriorly from the region of 

 the differentiation center (q.v.), the nuclei first reaching the surface 



