STUDIES WITH THE CENTRIFUGE 



41 



tilization to the first cleavage. These eggs have been sectioned 

 and stained with Delafield's haematoxyhn and orange G. The 

 yolk hemisphere takes up the orange stain and the protoplasmic 

 band and such part of the cap as has not dissolved out in the 

 alcohols are purple. 



The egg pronucleus just after centrifuging lies in the center of 

 the protoplasmic band and its position is apparently unchanged 

 after fertilization. Eggs which were killed ten or fifteen minutes 

 after fertilization show the sperm head, sometimes with its aster. 



Fig. a 



It lies comparatively near the periphery of the egg but bears no 

 relation to the stratification, being found sometimes in the yolk 

 and sometimes in the protoplasm. From this it is evident that 

 the point of entrance of the sperm is not in any way determined 

 by centrifuging. 



Twenty minutes after fertilization the male pronucleus is 

 enlarging and fusing with the egg pronucleus (fig. a). As the 

 sperm head may enter from any side of the egg with reference to 

 the stratification so it may fuse with the egg pronucleus on any 

 side. Thus as the centrosomes divide moving to the poles of the 

 nucleus the spindle may form at any angle to the stratification. 

 Fig. 2 shows an egg in which the spindle has formed at right angles 

 to the stratification and from which a parallel cleavage would 

 result. The fact that such a large proportion of the cleavages 

 are perpendicular to the stratification is due to the tendency of 

 the spindle to form in the direction of least resistance. In cen- 

 trifuged eggs it is very probable that the upper boundary of the 



