MUSEUM OF COMPARATIVE ZOOLOGY. 553 



the spindle is accomplished directly by the movement of this wave-like 

 constriction, for the pellucid spot is seen before the wave has begun to 

 approach the primary pole of the egg. It is only probable that the 

 migration is accomplished by contractions of the yolk, of which this is 

 a special manifestation, and moves toward the animal pole, because that 

 radius corresponds with the line of least resistance. Evidences of con- 

 traction are not so marked in Limax, but still they are exhibited in 

 alterations of the general form of the egg, and especially in the con- 

 stancy with which the primitive axis is shortened. (Compare also 

 Fig. 55.) 



But whether it be the result of a repulsion, due to physical or chemi- 

 cal conditions of the substances concerned, or simply of the contraction 

 of the yolk, the amphiaster certainly appears in this movement to be 

 more acted upon than acting. There must be some influence operat- 

 ing from behind to account for the deflection of the rays of the outer 

 aster, and it is probably the same force which induces the shortening 

 of the spindle observed by 0. Hertwig in Asteracanthion just before the 

 formation of the polar globule. 



The physiological significations which have been attached to the 

 polar globule have differed widely, from an important determining in- 

 fluence upon the course of subsequent events in segmentation to a 

 meaningless exudation of liquid from the yolk. While its morphological 

 place is well established, it does not of necessity follow that its function 

 is explainable from the same data. It may not be as rational now to 

 say that it is without meaning, as it was when Rathke pronounced that 

 verdict ; but its being a cell will not be found sufficient evidence that it 

 is not the means of removing useless or undesirable material. It soon 

 undergoes disintegration, and certainly has no further importance in 

 the economy of the embryo : these have been brought forward as evi- 

 dences to support this view. That its present functional importance 

 consists only in the removal of certain substances from the egg receives 

 further support from the continuance of the process of removal in cases 

 (Batrachia, etc.) where the cell condition is no longer maintained. But 

 an objection to any view which discovers in this phenomenon only 

 the removal of worn-out material is that the removed substance par- 

 takes of the nature of the cell in every essential particular. It cer- 

 tainly embraces nuclear substance and more or less granular protoplasm. 

 The changes which accompany its formation are, save in one or two 

 minor points, like those which accompany ordinary cell division ; its 

 nuclear substance assumes the same conditions (though not so promptly) 



