552 BULLETIN OF THE 



attracted portion free to move in the direction of the centre of the at- 

 tracting protoplasm, be that the centre of the yolk or a point nearer 

 the animal pole. The case of the migration of the germinative vesicle 

 before its constituents are converted into a spindle would demand no 

 special modification of the assumed forces. The most serious objections 

 to this explanation are the constancy of the distance between the asters 

 during the migration and the entire similarity of the lateral nuclear 

 plates, both in size and behavior, which appears incompatible with their 

 being affected in different degrees by the assumed attraction and re- 

 pulsion. 



Simple contractions on the part of the vitellus might under certain 

 circumstances produce the same result. If the yolk presented in its 

 primary radius a structural condition which offered less resistance than 

 other radii to the progress of a moving body, any contractions of the 

 vitellus would cause the amphiaster to passively advance along this 

 radius until it reached the surface, and finally cause its protrusion. 

 That this is structurally different from other radii is sufficiently obvious 

 in many cases, but I do not know that there is any direct evidence of 

 the condition (more passable) assumed. This would make the whole 

 amphiaster entirely passive as far as regards the migration, and it would 

 afford no explanation of the cause which induces the return of the inter- 

 nal half of the spindle toward the centre of the ^^^ as a female pro- 

 nucleus. It may be of some importance in this connection to know 

 just when this migration of the amphiaster takes place. Whitman has 

 shown that a quiescent stage may intervene between the formation of 

 the first archiamphiaster and that of the first polar globule ; but I am 

 not quite certain how far the ^gg has advanced when this interruption 

 of activities is manifest. It seems most likely from his account that 

 although the " polar figure " (which results from the presence of one of 

 the asters near the surface) makes its appearance, the centre of the 

 external aster does not reach the surface until after the egg emerges 

 from its quiescent state. At least, he does not mention the existence 

 of the " pellucid spot " at this stage, and states that it usually appears 

 from ten to twenty-five minutes after the egg is deposited. It is there- 

 fore probable that the contact of the astral corpuscle with the limiting 

 envelope of the egg takes place during the series of vitelline con- 

 tractions which terminates in the production of a polar cell. These 

 contractions have been shown to assume in Clepsine a most remarkable 

 and uniform appearance, — a constriction advancing from the equator 

 toward the primary pole. It is not to be claimed that the migration of 



