MUSEUM OF COMPARATIVE ZOOLOGY. 615 



direction, it is doubtful if the localization of the food material is compe- 

 tent to explain all the polar phenomena of the egg. In early stages 

 these may find expression in a variety of manifestations. The migra- 

 tion of the germinative vesicle toward a definite point in the surface ; the 

 radial position assumed by the maturation spindles ; the waves of con- 

 striction which precede the formation of the polar globules, and the in- 

 equality in the sizes of the latter ; the union of the pronuclei at a point 

 nearer the primary than the secondary pole and the consequently (1) ec- 

 centric position of the first segmentation spindle ; the appearance 'of the 

 first segmentation furrow earlier at the primary than at the opposite 

 pole ; the formation of pseudopodia-like elevations, — often most con- 

 spicuous at the primary pole ; the accumulation of finely granular pro- 

 toplasm at the secondary/ pole after the elimination of the polar globules ; 

 and the appearance of "polar rings" and "ring rays" (Clepsine) at both 

 ends of the primitive axis, — are all indications of a polar differentiation 

 of the egg. The eccentric position of the germinative vesicle might in 

 many cases be induced by a regulated distribution of nutritive sub- 

 stance, and the point at which the polar cells appear might be predeter- 

 mined by the relation of the egg to its sources of nutrient supply ; the 

 direction of the wavelike constrictions, the region of greatest pseudo- 

 podal activity, and even the position of the spindle axis, might also be 

 dependent on the same (nutritive) conditions. But it must be in oppo- 

 sition to the obstructive properties of the nutritive substance, that the 

 elevation of protuberances at the secondary pole, the formation of the 

 aboral '' polar ring," etc., take place. 



A differentiation in the substance of the egg at a period preceding the 

 accumulation of deutoplasm, and regulating its distribution, must proba- 

 bly be assumed, and to this are to be referred the polar phenomena 

 which appear later. What may be the immediate cause of this hypo- 

 thetical earlier differentiation remains to be discovered. It is possible 

 that the topographical relation of the egg (when still in an indiflerent 

 state) to the remaining cells of the maternal tissue from which it is differ- 

 entiated has an important influence in determining this axial condition. 

 It would certainly be interesting to know if that phase of polar differen- 

 tiation which is manifest in the position of the nutritive substance, and 

 of the germinative vesicle, bears a constant relation to the free surface 

 of the epithelium from which the egg takes its origin. If, in cases 

 where the egg is directly developed from epithelial cells, this relation- 

 ship were demonstrable, it would be fair to infer the existence of corre- 

 sponding, though obscured relations, in those modified cases where (as, 



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