MUSEUM OF COMPARATIVE ZOOLOGY. 229 



are not satisfactorily indicated. In other cases, even before the entire 

 disappearance of the nuclear structure (Fig. 86), a portion of the rays 

 form a continuous, thick, spindle-shaped body, whose fibres converge to- 

 ward the centre of the asters. These interstellate fibres, however, are 

 not traceable to the centres of the stars, but are lost in the margins of 

 the central " areas." The limits of the spindle are much more clearly 

 indicated a little later, when equatorial thickenings appear. Occasion- 

 ally, however, the arrangement of the thickenings in the equator is very 

 irregular (Fig. 92). I believe, but cannot say with certainty, that 

 this indicates an early condition, and that it is followed by a more 

 regular arrangement. Usually the equatorial thickenings are so uni- 

 formly disposed that their combined effect is the same as though re- 

 sulting from a flat disk occupying exactly the equator of the spindle 

 (Fig. 89). The thickness of the latter is somewhat more than half its 

 length (Figs. 83, 89). An optical section corresponding to the equa- 

 torial plane (Figs. 84, 92) shows that the thickenings in the case of 

 this amphiaster, as in that of the archiamphiasters, are arranged in the 

 form of a ring, so that it may be inferred that the nuclear fibres are 

 themselves more numerous near the surface than at the axis of the 

 spindle. 



The changes in the general outline of the vitellus, which have already 

 been noticed in speaking of the living egg, are preserved in those treated 

 with acid. The first change — a lengthening of the axis of the yolk 

 which is parallel to the spindle occurs at about the time the equa- 

 torial zone of thickenings is formed. This prolation of the yolk is quite 

 apparent, as in Fig. 89. 



A comparison with views along the animal radius, however, shows that 

 this change of form is as much due to a flattening at the animal pole, 

 and consequent shortening of the corresponding axis, as to a lengthen- 

 ing in the direction of the spindle. In such a view, either from the 

 animal or from the vegetative pole, the outline of the yolk may remain 

 circular, even when the spindle is completely formed (Fig. 83). 



Up to this time the stellate figures have been constantly increasing 

 in size, though not uniformly on all sides. The rays are traceable for 

 the greatest distance in planes passing through the centres of the stars, 

 and parallel with the equator of the spindle. More than half the rays 

 of each aster are found on the distal sides of these planes. Each of the 

 asters is substantially hemispherical. The rays now stretch out to near 

 the periphery of the yolk, and approach each other along the equatorial 

 plane. The latter thus becomes apparent as neutral territory, where 



