PROTOPLASMIC MOVEMENT. 75 



flowing of the cell substance. These observations are based in 

 the first instance upon the eggs of Crepidula plana, though 

 many other species and genera of gasteropods have been studied 

 with the same results. The phenomena are, therefore, not iso- 

 lated, and some of them are probably of general occurrence. 



a. Movements in the Unsegmented Egg. 



The cell movements during maturation result in the segrega- 

 tion of yolk and cytoplasm at opposite poles of the egg, and in 

 the transportation of the mitotic figure to the animal pole. 

 While the germinal vesicle is still intact it is closely surrounded 

 by yolk spherules, and there is a very incomplete separation of 

 yolk and cytoplasm throughout the entire egg. As soon as the 

 mitotic spindle is formed and the nuclear membrane is broken, 

 there is an area immediately surrounding the spindle and asters 

 free from yolk, but elsewhere in the egg there is an intimate 

 mingling of yolk and cytoplasm. Gradually the spindle, which 

 at first has a position nearly at right angles to the egg axis and 

 some distance from the surface of the egg, turns until its axis 

 nearly coincides with that of the egg, and at the same time the 

 whole spindle is moved out toward the surface, until finally the 

 outer end of the spindle comes in contact with the cell mem- 

 brane, and the surface of the egg is elevated into a papilla at 

 this point. This movement is in part due to the mere length- 

 ening of the nuclear spindle which doubles in length during the 

 process, but in part also to a general movement of the cell body 

 by which the spindle is turned and carried bodily toward the 

 surface of the egg. At the same time there are movements 

 within the egg which lead to an accumulation of cytoplasm at 

 the animal pole and a movement of the yolk spherules toward 

 the opposite pole. There is no evidence that this movement is 

 due to activity on the part of the nucleus or centrosomes. The 

 initial position of the centrosomes and the direction of the cen- 

 tral spindle are not the same in different eggs, and yet the final 

 position of the mitotic figure is the same in all cases; the cen- 

 trosomes and asters at the two poles are identical in form, size, 

 and staining reactions until the outer pole of the spindle comes 



