MUSEUM OF COMPARATIVE ZOOLOGY. 193 



inserted arc corresponds to a circle having approximately the diameter 

 of the outer stellate sphere. The resistance oft'ered by the yolk en- 

 velope is, however, so considerable as to cause a decided flattening of 

 the external half of the peripheral stellar sphere. It also results from 

 this resistance that the outer ends of the radiate filaments of the outer 

 star are curved away from the polar axis, and finally bent backward, — 

 much as the hairs of a soft brush would be when gently forced against 

 the concave surface of a highly curved watch-crystal. The effect of this 

 curvature in the filaments is at first quite deceptive ; for it unmistak- 

 ably suggests to the observer that from the apex of the protuberance 

 there is a funnel-shaped depression extending to near the centre of the 

 radiate figure, — a depression in form something like that of the corolla 

 of a morning-glory. This illusion is further heightened in specimens 

 which have been stained, by the fact that this apparently invaginated 

 portion is onty slightly tinged in comparison with other portions of 

 the stellate figure, or the yolk (Fig. 43). It is, however, very certain 

 that no such depression actually exists, as one may be convinced by care- 

 fully rotating the egg and focusing the instrument so that the centres 

 of both stellate figures are seen distinctly at the same time. The axis 

 of the spindle then lies in a plane parallel with that of the microscope 

 stage, and its whole length is exactly in focus. If there were such a 

 depression at the surface which lies in the continuation of the spindle 

 axis, it should be observable in the outline of the yolk ; but, on the 

 contrary, the very transparent portion of the peripheral star shows a 

 sharply defined outline, convex externally, and continuous at each end 

 with the outline of the rest of the yolk (Figs. 43, 48). 



When the egg has been subjected to the action of acetic acid, the 

 very fulness of the outline at the animal pole, as contrasted with the 

 more or less shrivelled and irregular outline of the rest of the vitellus, 

 is to me an indication of the high state of mechanical tension to which 

 this portion of the surface is subjected. 



At this time the superficial portion of the protuberance seems to be- 

 come diff'erentiated into a thin membrane with double contour, which is 

 continuous at the margin of the elevation with the less conspicuously 

 differentiated outer layer of the rest of the vitellus. This membrane is 

 at first of uniform thickness over the whole surface of the protuberance. 

 It is only in a subsequent stage that it assumes a different and peculiar 

 appearance. 



As already indicated, the radiating fibres of the peripheral aster suf- 

 fer a bending back, which changes their original direction more or less, 



VOL. VI. — NO. 12. 13 



