56 FRANK R. LILLIE. 



polar bodies appear in the same position that they would have 

 normally, that is, in the primary axis ; in other words, that no 

 redistribution of the granules or nucleus affects the direction of 

 polarity of the egg. If this be- true, then the final conclusion 

 stated in that paper, that polarity is a function or property of the 

 ground substance of the protoplasm, certainly follows. 



This conclusion is of fundamental importance for the entire 

 theory of development, and its basis therefore requires the most 

 careful examination. One thing is clear at the start, viz. : that 

 this point of view affords a complete explanation of the phe- 

 nomena, because the egg is polarized prior to centrifuging, and 

 the centrifugal force acts in all possible directions with reference 

 to the polar axis ; l and this agrees with the fact that the 

 polarity that appears after centrifuging also lies in all possible 

 directions with reference to the axis of stratification. The 

 question is simply this : Is the polarity as marked by the polar 

 bodies and cleavage planes the same polarity in the normal 

 egg and after centrifuging ? Does any other hypothesis agree so- 

 well with the observed facts, and if so, is it for any sufficient rea- 

 son to be preferred to this hypothesis ? 



An alternative hypothesis can lie only in one direction, viz.: 

 that polarity is determined by any point on the surface of the 

 egg to which the maturation spindle happens to go after cen- 



1 The proof of this statement is as follows : In the stage used for the experiments 

 (except where otherwise stated) the first maturation spindle is fixed at the periphery 

 at the stage of the mesophase. Its outer pole marks the center of the animal pole - r 

 this point is also characterized by an entire absence of the ectoplasm, which covers 

 the remainder of the surface of the egg as described in a previous paper ('06). If 

 sections of centrifuged eggs are examined, the maturation spindle is found in about half 

 of the eggs still attached to the surface, at any point in the hemisphere containing the 

 hyaline band. At first sight this would indicate that the eggs oriented themselves in 

 the centrifuge to a slight extent, though it is clear that the animal pole is in some cases 

 90 removed from the ends of the secondary axis. But the fact that fixed spindles 

 are not found in the distal (yolk-laden) hemisphere after centrifuging has another 

 explanation : about half of the eggs after centrifuging have free spindles (/. t. , not 

 attached to the surface), which always lie in the hyaline band, and these represent 

 those eggs whose original animal pole lay in the distal hemisphere in the centrifuge. 

 (Cf. Fig. 8, Cand D. ) The inrush of yolk into the distal hemisphere has torn the 

 spindles loose from the periphery, and their specific gravity has carried them into the 

 hyaline band ; this interpretation of the free spindles is proved by the fact that many 

 of them migrate into the distal hemisphere later in the process of forming the polar 

 bodies (Fig. 2). 



