Ill DIFFERENTIATION 79 



f'uged either slowly for a long time (/= 10 g to 12 g) or 

 rapidly for a short time (/= 228 #), and various stages are 

 used, namely, unfertilized eggs, fertilized but unseguiented 

 eggs, two-celled stages and eight-celled stages. 



1. The eggs are centrifuged while still in the oviduct 

 (presumably during the second maturation division), then 

 removed and fertilized. 



A. Slowly for five hours. The direction of the centrifugal 

 force makes any angle with the egg-axis, and some eggs have 

 the white pole uppermost. The first furrow appears at the 

 normal time, but is often not meridional, the blastomeres 

 being unequal. While normal embryos may be developed, 

 the blastopore sometimes fails to close, and in a few cases 

 only half the egg is segmented and develops into a half- 

 embryo. 



B. Rapidly for thirty minutes. At the centripetal pole 

 which may or may not be the original animal pole there is 

 an extrusion of hyaloplasm. The development of these eggs 

 is as in the previous experiment. 



It is to be noted that in these eggs the axis of centrifuging 

 may make any angle with the original axis, that by the 

 redistribution of yolk and cytoplasm a new polarity may 

 therefore be conferred on the egg, and that the symmetry of 

 the embryo is related to this new and not to the original 

 polarity, the anterior end being developed at the centripetal 

 pole, in precisely the same way as in Pfliiger's forcibly 

 inverted eggs. 



2. Since the perivitelline fluid has been exuded the egg is 

 free to turn inside the jelly membrane, and places itself upon 

 the machine with its yolk-pole outwards. The egg is centri- 

 fuged, from fifteen minutes to one and a half hours after 

 insemination. 



A. Slowly for five hours. The results are very much the 

 same as before ; open blastopores and half-embryos occur. 



B. Rapidly i. Fifteen minutes after insemination for from 

 ten minutes to thirty minutes. Three layers or strata appear, 

 at right angles to the egg-axis (which coincides with the axis 

 of centrifuging) ; at the animal (centripetal) pole is a layer 



