230 BULLETIN OF THE 



the forces inducing the stellar figures have not extended their influ- 

 ence, or serve to hold each other in check. It is along this plane that 

 the deepening fissure passes which ultimately separates the yolk into 

 two equal spheroids. 



But while these changes in the form of the yolk are in progress, 

 the spindle undergoes modifications like those which transpire in the 

 maturation spindles. The equatorial zone of thickenings splits into 

 lateral halves, which migrate each toward the corresponding apex of the 

 spindle. Such, at least, is the inference to be drawn from the fact, 

 that, after the flattening and during the subsequent constriction of 

 the yolk, the equatorial zone is wanting, and in its stead two lateral 

 zones are found, which are farther from the equatorial plane the deeper 

 the constriction. That the migration of these thickenings is compara- 

 tively rapid may be inferred, I think, from an examination of Fig. 90 ; 

 the separation of the two lateral zones there exhibited having been ac- 

 complished between the beginning of the depression at the animal 

 pole and the appearance of a shallow furrow at the opposite side of 

 the yolk. When seen exactly edgewise, the lateral zones present a 

 linear arrangement of the thickenings parallel to the equatorial plane, 

 but a slight deviation in the direction of sight causes each lateral zone 

 to assume an oval outline, one edge of which appears, however, by 

 careful focusing, a little deeper than the other. This is represented in 

 the drawing by making the deeper half of the outline less distinct. 



As the constriction advances from the animal pole, the interzonal fila- 

 ments {Kernfdden, Strasburger) which were left behind by the separating 

 lateral thickenings, are forced before it, and thus become bent so that 

 their convexities are directed toward the vegetative pole. This bend 

 retains for some time the nature of a full uniform curve on the convex 

 side, but on the indented side it soon assumes a more angular ap- 

 pearance (Fig. 93). The interzonal filaments thus continue to be carried 

 forward by the advancing depression of the yolk without surrendering 

 their continuity. In this manner they become lengthened, inasmuch as 

 the position of their extremities remains comparatively uninfluenced by 

 this change. They thus form a sort of V-shaped figure, whose free ends 

 terminate in the lateral zones of spindle-fibre thickenings. In all this 

 process the interzonal filaments appear to play an entirely passive role. 

 Meantime the lateral zones have assiimed the nature of nuclear vacu- 

 oles containing each a number of nucleolar bodies. Whether this has 

 transpired by the accumulation of nuclear sap in the region of the 

 thickenings, now converted into nucleolar bodies, or has resulted from a 



