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CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 333 



centrifuged away from the animal pole (figs. 19 to 21). 2) 

 After centrifuging, the eggs were kept in normal condition from 

 6 to 24 hours, during which time the cytoplasm and nuclei 

 moved to their present positions; the fact that this movement 

 takes place under normal conditions indicates that it is a move- 

 ment back to the normal animal pole. 3) The most important 

 evidence that the maturation pole and not the ectodermal pole 

 is shifted by centrifuging is to be found in those cases in which 

 one polar body lies at the ectodermal pole and the other is more 

 or less distant from it. Such a condition is shown in figures 33, 

 35, 41, 47, 54, 58. In these cases the polar body lying at the 

 ectodermal pole is quite normal in size and appearance, whereas 

 the other polar body is much larger than normal. The plain 

 import 'of this is that a normal first polar body was formed at 

 the animal pole before the egg was centrifuged, that an abnor- 

 mally large second polar was formed at the pole to which the 

 cytoplasm and the spindle were displaced and that the cytoplasm 

 and the germ nuclei have moved back to the animal pole after 

 the eggs were removed from the centrifuge. The animal pole 

 is thus marked by the first polar body and though the second 

 polar body may be formed far from this pole it does not change 

 the original polarity of the egg, nor of the embryo which de- 

 velops from it. These facts constitute, I think, indisputable 

 evidence that the polarity of the egg and embryo is not the result 

 of the formation of polar bodies at a particular point, hut rather this 

 polarity of the egg antedates the maturation and under normal 

 conditions is the cause of the location of the maturation spindles 

 and polar bodies as well as of the ectodermal pole. The maturation 

 pole does not necessarily coincide with the animal pole of the egg, 

 nor does it determine the ectodermal pole of the embryo. 



7. Results of centrifuging after maturation and before the first 

 cleavage {figs. 28 to 30, 59 to 62) 



In normal eggs cytoplasm continues to segregate at the ani- 

 mal pole and yolk at the vegetal pole throughout the periods of 

 maturation, fertilization and early cleavage. The egg nucleus 

 and centrosphere lie close to the animal pole and consequently 



