328 Charles Zeleny. 



blastomeres. An explanation of this has, however, been offered. 

 The first two cleavages being perfect apical-basal ones, the iso- 

 lated blastomeres cannot be expected to show other than perfect 

 larvae, assuming only a slight regulation overcoming lateral 

 asymmetry, notwithstanding the partial cleavage, which after all 

 is only quantitatively partial (=^ or 34 of a pattern). At the 

 same time the fragments of unsegmented eggs can never be said 

 to contain the materials divided accurately with respect to an 

 apical-basal axis because, in the first place, the cut is never per- 

 fectly vertical, and in the second place, the rounding in of the 

 edges after such a cut causes a disarrangement of the materials 

 which must result in unequal distribution at the first cleavage. The 

 ability to regulate such an unequal distribution must of course 

 largely depend upon its extent and character. The greater op- 

 portunity given to fragments of unfertilized eggs to regulate such 

 differences in distribution (if any) before cleavage takes place 

 may to some extent explain the whole character of the cleavage 

 in such fragments without the assumption of a perfectly Isotropic 

 egg, an assumption which is contradicted by the evident polarity 

 of the egg at this period as indicated by the eccentricity of position 

 of the nucleus and the presence of the basal protuberance. The 

 existence of such an apical-basal differentiation in the unsegmented 

 egg was indeed already indicated by Professor Wilson's result on 

 certain eggs in which the basal portion was removed by a hori- 

 zontal cut and which showed the basal quartet of the resulting 

 eight-cell stage much smaller in comparison with the upper than 

 In the normal whole egg. 



The data on localization of formative factors in the egg be- 

 fore cleavage and during the early segmentation stages may, 

 therefore, be provisionally stated In the following form for Cere- 

 bratuliis, If an Intimate relation is assumed between localization 

 of visible materials and localization of formative factors. The 

 unfertilized egg before the beginning of maturation already shows 

 evidences of a polarization which necessitates the assumption of 

 a heterogeneity In material. Upon this basis, and In the same 

 apical-basal direction, later differentiation proceeds. 



During the preliminary maturation stages and after fertlliza- 



