MUSEUM OF COMPAEATIVE ZOOLOGY. 183 



men like a planet, and at its side the two polar globules, of the same 

 exquisite outline, are poised like a pair of satellites. A careful exami- 

 nation, even with reflected light, shows that the halves of the sphere are 

 not quite alike. Notwithstanding the disappearance of the crescent as 

 a distinct feature, the animal half still appears more glassy and not so 

 white as the vegetative hemisphere, yet the one passes into the other 

 without any abrupt transition. In transmitted light the distinction is 

 even more apparent. Near this more translucent animal pole one may 

 sometimes see a faint circular spot (Fig. 6) of still greater clearness, 

 fading away gradually on all sides, but more often only a general trans- 

 lucency of this half of the yolk is noticeable. Even this gradually 

 vanishes, and there follows a period of still greater obscurity. The gran- 

 ulations are grouped in ill-defined shadowy masses, which one feels like 

 comparing to clouds, and again are re-grouped and re-distributed, ap- 

 parently without definite order or effect. Sometimes, however, these 

 changes have been observed to have considerable regularity. The most 

 common appearance, though this has been seen only a few times, is that 

 of a nearly equatorial zone (Figs. 21, 33, 36, 51) of protoplasm, from 

 which the vitelline granules are almost wholly eliminated. The position 

 of this zone varies somewhat in different eggs, and in the same egg has 

 been observed gradually to alter in form and extent. Soon after its 

 appearance, it is seen as a band of narrow surface exposure, but of great 

 depth, so that in optical section (Fig. 33) it extends to near the centre 

 of the vitellus, becoming thinner the nearer it approaches this point. 



The outline thus presented in a section is that of two narrow wedges 

 with their bases lying in the surface of the yolk near its equator, and 

 their apices directed toward, and almost reaching, each other. In the 

 course of twenty or thirty minutes the zone has become broader (Fig. 36), 

 and a sectional view shows that the deeper edge, corresponding to the apex 

 of the wedge, has become much rounded, so that the zone is now limited 

 to the more superficial portion of the yolk. After twenty minutes more 

 have elapsed, it has become still further restricted in its centripetal ex- 

 tension (Fig. 34), and is rapidly becoming indistinct on account of the 

 encroachment of yolk granules. But these changes, as well as the less 

 regular fleecy appearances which are more frequently observable, are ac- 

 companied by no corresponding alterations in the contour of the vitellus 

 as a whole : the latter stills retains its simplicity of form. 



Such changes as have just been traced are not the only ones going on 

 within the yolk at this time, although it is only in favorable cases that 

 one has a view of other possibly more important phenomena. The 



