THE BIOLOGY OF THE CELL SURFACE 



they are in the centrospheres where they fuse to form 

 five or six droplets; these by farther fusion become one 

 hyaHne mass, the resting nucleus. 



Concomitantly with this behavior of the nucleus and the 

 movement of the hyaline droplets, the centrospheres 

 steadily increase in size and the astral rays become more 

 extensive until the moment that the hyaline droplets enter 

 the centrospheres. Thereupon the centrospheres diminish 

 in size and the astral rays lose definition and shorten. Dur- 

 ing their period of growth the centrospheres are spherical; 

 their change to hemispheres marks the beginning of decrease 

 in size. 



In the meantime the egg exhibits changes in shape. 

 While the hyaline droplets are moving toward the poles of 

 the transparent dumb-bell shaped area, the egg maintains 

 its original spherical form. With the entrance of the hya- 

 line droplets into the centrospheres, when centrospheres and 

 asters have begun to wane, the egg quickly elongates in the 

 long axis of the transparent region. About one minute later 

 the egg divides by a furrow forming at right angles to the 

 egg's long axis. Thus the egg at first a sphere becomes an 

 ellipsoid which divides and then becomes two spheres. 



The site of the future cleavage furrow is marked as early 

 as the stage in which the transparent ellipsoid area is con- 

 verted into a dumb-bell shape. Already moved by the 

 fertilization-reaction, the various cytoplasmic inclusions, 

 especially the yolk, are further distributed by the growth 

 of the sperm-aster; and later, by the formation and exten- 

 sion of the amphiastral system. The conversion of the 

 ellipsoid area to the dumb-bell shape is caused by the move- 

 ment of yolk-spheres in the equatorial plane of the Qgg 

 from the periphery toward the centre. The shifting of 

 the oil-drops can not be easily followed in the living egg. 

 The pigment granules, after fertilization definitely fixed to 

 the outer rim of cytoplasm, at the time of cleavage extend 



256 



