6O FRANK R. LILLIE. 



stance ; and when one considers carefully the data presented, 

 one cannot avoid the conclusion that the direction of polarity is 

 unmodified by the centrifugal powers employed. It must there- 

 fore depend on some configuration or heterogeneous physical or 

 chemical properties of the ground substance established early in 

 the history of the egg, and which is not essentially disturbed by 

 centrifuging. This subject will be examined in the third part of 

 the present paper. 



The conclusion stated above was clearly presented in my 

 paper of 1906. Since then Morgan (1908) has come to the 

 same conclusion for the egg of Arbacia. In his last paper Mor- 

 gan furnishes a satisfactory demonstration that polarity is un- 

 modified in the egg of Arbacia by centrifuging, by showing that 

 the micropyle which bears a constant relation to polarity in the 

 normal egg has the same constant relations to the polarity of 

 centrifuged eggs, as proved by formation of the micromeres and 

 archenteron at the pole opposite the micropyle whatever the di- 

 rection of the centrifugal force, and therefore whatever the position 

 of the centrifugal zones and the form of the cleavage modified 

 (in Arbacia) by tkese zones. I therefore take Morgan's result to 

 be a complete confirmation of my original and present position. 



The demonstration that the direction of polarity is unaltered 

 by centrifuging involves the assumption that the configuration of 

 the ground substance remains the same. In other words, that 

 there is a definite architecture in the ground substance, which is 

 the basis of the localization pattern in normal development (see 

 Science, N. S., XXVII., June 12, 1908). This also involves a 

 new conception of protoplasmic streaming as seen in ova, for this 

 must be interpreted as granule movement exclusively, and not 

 actual flowing. Similarly, the movements produced by the cen- 

 trifuge cannot be mass movements of entire protoplasmic areas, 

 but only granule movements through the ground substance. In 

 the third part of the present paper, I shall consider these con- 

 clusions with some care. 



Observations on Nereis. 



In the summer of 1908, I had the opportunity of comparing 

 the eggs of Nereis linibata with reference to these points, and I 



