370 EDWIN G. CONKLIN 



14. If yolk is forced to the animal pole after the maturation 

 spindle has become attached to the periphery the spindle is 

 stretched in length and when the division plane forms through 

 its equator it cuts off a giant polar body or may divide the egg 

 equally. Giant polar bodies do not develop because they do not 

 receive a spermatozoon, and they do not receive a spermato- 

 zoon because they are formed after the fertilization of the egg 

 and after the entire cortical layer has been rendered imper- 

 vious to the entrance of other spermatozoa (pp. 336-344). 



15. If yolk is forced to the animal pole before the maturation 

 spindle has become attached to the periphery the spindle may 

 be driven to any point on the egg surface and if held there by 

 continued centrifuging either one or both polar bodies may be 

 formed there. Nevertheless nuclei and cytoplasm move back 

 to the animal pole and yolk to the vegetal pole when centrifug- 

 ing ceases and the polarity of the egg and embryo remains un- 

 changed. Therefore the maturation pole does not determine 

 the animal pole of the egg nor the ectodermal pole of the embryo 

 (pp. 335, 336,346, 347). 



16. By centrifuging during cleavage all the yolk maj^ be 

 driven into one daughter cell and most of the cytoplasm into the 

 other one, or by centrifuging early in mitosis the spindle ma}^ be 

 carried out of its normal position so that the first or second 

 cleavage may be equatorial instead of meridional, unequal in- 

 stead of equal. Nevertheless the cells formed by the first two 

 cleavages behave like normal macromeres in that each gives rise 

 to three micromeres (ectomeres) on its animal pole side in the 

 three succeeding cleavages (pp. 349-354). 



17. If the first or second sets of micromeres are forced to form 

 at a distance from the animal pole the succeeding set forms at 

 the animal pole if the pressure is removed. If the unequal 

 cleavages by which micromeres are formed normally are rendered 

 equal by centrifuging the subdivisions of these large "micro- 

 meres" are normal only so far as the cells are concerned which 

 lie nearest the animal pole. This is due to the fact that under 

 normal conditions the upper pole of a spindle is attached to the 

 periphery at the animal pole side of the cell more firmly than 

 is the lower pole of the spindle (pp. 354-356). 



