POLARITY AND BILATERALITY OF THE ANNELID EGG. 6 1 



found that, just as in Chcctoptcrus, the place of formation of the 

 polar globules is independent of centrifugal stratification, and that 

 the cleavage follows the polarity marked by the polar glob- 

 ules and is not modified in its direction by the position of the 

 yolk arbitrarily imposed by the centrifuge. Morgan has shown 

 the same thing for the lamellibranch, Cuwingia (Science, Vol. 

 XXVII., p. 496, 1908), so that now we have at least four forms 

 in which this principle is firmly established. 



II. BILATERALITY. 



The second main question that I propose is whether or not 

 bilaterality comes under the same category as polarity. And 

 this resolves itself into two questions: (i) Is bilaterality, like 

 polarity, a function of the ground substance, and (2) When does 

 it arise ? 



i. The first morphological evidence of bilaterality in the eggs 

 of Chcstoptems and Nereis is found in the first cleavage. The 

 spindle takes an excentric position, and the cleavage is therefore 

 unequal ; the smaller cell is anterior and the larger cell posterior 

 in position. The course of the entire development is thus deter- 

 mined not only with reference to the bilateral axis, but also with 

 reference to the proportions of embryonic organs. 



If the bilaterality were a result of a kind of equilibrium of the 

 protoplasmic inclusions (nucleus and granules), it is clear that 

 the variations in position of the nucleus and granules produced 

 by centrifugal force should involve change of direction of bilater- 

 ality. If, on the other hand, bilaterality is a function of the ground 

 substance, changes in the position of the nucleus and granules 

 should not materially affect the direction of bilaterality, though it 

 might conceivably modify the inequality of the cleavage which is 

 our index of bilateral organization. However, if the normal pro- 

 portions of the first two cells and the meridional direction of the 

 cleavage plane are maintained in centrifuged eggs in spite of the 

 abnormal distribution of granules between cells that would be 

 thus induced, no other evidence would be necessary to show that 

 the seat of bilateral organization is in the ground substance. 



This is in fact the case, both in Cluztopterus and Nereis : What- 

 ever the distribution of yolk and granules induced by centrifuging, 



