Centrifugal Force upon Beetles^ ^KS^ 545 



of the mollusk, Cumingia: "In general, a resting nucleus may 

 be forced to the lighter pole of the cell owing to the presence in 

 the nucleus of nuclear sap, but the chromosomes and the spindle 

 are more difficult to move, since they have nearly the same specific 

 gravity as cytoplasm. When they move they do so as a whole, 

 which shows that the spindle figure when present is a definite 

 structure/' 



It is seldom that mitotic figures are found in sections of beetles' 

 eggs and none was present in any of the many centrifuged eggs that 

 I have examined. The nucleoli of the centrifuged eggs of Chry- 

 somelid beetles seemed not to be affected, but were found in all 

 parts of the nuclei irrespective of the direction of the centrifugal 

 force. The nucleolus is heavier than the nuclear sap in the ova 

 of the lobster. Its eccentric position was noted by Bumpus ('91, p. 

 225); later Herrick ('95, p. 155) proved that it falls to the lower 

 side of the nucleus "like a shot within a tennis ball." Lyon 

 ('07, p. 168) reports that the germinal vesicle is forced to the 

 light end when unmatured eggs of Asterias are centrifuged, but 

 that the nucleolus is heavier. 



The germ-cell determinants. Figs. II, 12 and 13 are from 

 longitudinal sections through the posterior ends of eggs which 

 had been centrifuged one hour, two hours and four hours respect- 

 ively. They show that the pole-disc moves en masse toward the 

 heavy end of the egg and that it carries with it the " Keimhaut- 

 blastem" in which it is suspended. In Fig. 11 there is a slight 

 indentation in the surface at the posterior end; in Fig. 12 the pole- 

 disc has penetrated farther into the yolk, leaving an open pathway 

 {pt) behind it. This pathway is really unbroken, but appears 

 cut across in the figure. A third stage is shown in Fig. 13, where 

 the pathway has become closed and the group of germ-cell deter- 

 minants is on its way toward the anterior pole. The eggs (Series 

 C.B. 4) were oriented with their posterier ends toward the axis 

 of rotation. No definite conclusion could be reached concerning 

 the comparative specific gravity of the pole-disc, but a section 

 through an egg centrifuged with its anterior end toward the center 

 (C.B. 3, b) leaves little doubt that it lies between that of the gray 

 cap and the yolk. The fact that the pole-disc moves as a whole. 



