332 EDWIN G. CONKLIN 



pole at the lower side in the figure. The probable identity of 

 the cleavage cells is indicated by the labeling and the arrows. 



On the whole then it may be concluded that changes in the 

 maturation pole induced by centrifuging have no lasting influ- 

 ence on the polarity of the egg and do not modify the normal 

 type of cleavage; such modifications of cleavage as do occur 

 may be attributed to, (1) the prevention of the return of nuclei 

 and cytoplasm to the animal pole by long continued centrifug- 

 ing or by the formation of division walls, (2) abnormal distri- 

 bution of cytoplasm and yolk to the first two blastomeres and 

 the permanent separation of these substances by partition walls, 

 or (3) the partial separation of blastomeres of the 2-cell or 4-cell 

 stages. 



6. The maturation pole does not determine the animal pole of the 

 egg nor the ectodermal pole of the embryo {figs. 35 to 58) 



Probably in all animals the polar bodies are formed at the 

 animal pole of the egg and the latter becomes the ectodermal pole 

 of the embryo. However these experiments prove that the 

 polar bodies may be forced to form at any point whatever on 

 the surface of the egg without changing in the least the location 

 of the animal or ectodermal pole. Thus in figures 35 to 41, 

 which represent eggs which were centrifuged for 4 hours and fixed 

 6 hours later, the original animal pole is clearly indicated by 

 the position of cytoplasm and nuclei and yet one or both of the 

 polar bodies were extruded at a distance from this pole and after- 

 ward cytoplasm and nuclei returned to the original animal pole. 

 In figures 47 to 54 the center of the plate of micromeres (ecto- 

 meres) is the ectodermal pole and yet this lies some distance 

 from one or both of the polar bodies. 



The evidence that it is the maturation pole and not the ani- 

 mal pole which has been moved in these eggs is the following: 1) 

 The polar bodies are larger than normal, showing that they were 

 formed during centrifuging; many of them contain cytoplasm and 

 oil, thus proving that they were formed at the centripetal pole; 

 it is certain that maturation spindles m the early stages may be 



