46 PROCEEDINGS OP THE AMERICAN ACADEMY 



Van Beneden was the first to call attention to the fact, " that the 

 time which elapses between two successive phases of cleavage is 

 shorter and sliorter as the cells diminish in volume, and in conse- 

 quence of such diminution."* As only about twenty minutes elapse 

 between the moment of first contact of the pronuclei and the appear- 

 ance of the first meridian cleavage-groove, it is plain that we have no 

 time for an equatorial cleavage to take place between these two 

 events. Thus, at the very outset, we meet with facts that apjiear to 

 be utterly irreconcilable with Hoffmann's account of the origin of the 

 periblastic nuclei. It remains for us to show precisely how these 

 nuclei arise ; and to ascertain wlietlier there are really two very dif- 

 ferent modes of cell-genesis introduced by the division of the first, or 

 of any subsequent, cleavage-ami)hiaster. 



The Growth of the Blastodisc at the Expense of the Periblast. — The 

 moment the coalesced pronuclei enter the amphiastral phase of activity, 

 an important change in the form of the blastodisc sets in, which grad- 

 ually transforms it into a calotte-shaped mass in the course of the first 

 four cleavage-stages. ' These form-changes are especially noteworthy, 

 inasmuch as they confirm and emphasize the fact already stated, that 

 the frst cleavage-ainphiaster cuts the vertical axis of the blastodisc at 

 rigid angles. If this amphiaster had a vertical position, as asserted 

 by Hoffmann, and the accompanying cleavage-plane a horizontal 

 position, then the axis of the blastodisc would lengthen during the 

 process of division. Now just the reverse of this takes place. The 

 axis shortens so rapidly that within five minutes after the amphiaster 

 appears {Ps. tnelanogaster) it is reduced to little more than half its 

 original length. In order to watch the progress of these changes 

 the microscope must have a horizontal position, so that a profile view 

 may be obtained. The inner conical face of the blastodisc flattens 

 very rapidly, and concomitantly the peripheral face becomes slowly 

 more convex. At the end of five minutes (we are now speaking of 

 Ps. melunogaste?') the blastodisc has the form of a double-convex lens, 

 with outer and itnier surfaces of very nearly equal curvature. Two 

 minutes later it has a meniscoidal form, the convex outer face almost 

 or quite in contact with the egg-membrane, and the concave inner 

 surface moulded to that of the vitelline sphere. It holds this form 

 for about five minutes, during which it becomes somewhat thicker at 

 the expense of the periblast. Ten minutes after the disappearance of 



* Van Beneden first noticed this fact in the ovum of Cfammarus locusta, and 

 of the rabLiit, No. 4, p. 46. 



