THE EARTHWORM 213 



After the chromosomes are arranged across the middle of 

 the spindle, the entire spindle moves end on toward the sur- 

 face of the egg. As it does so the chromosomes divide each 

 into halves and are drawn to the poles (Fig. 104, C). The 

 outer pole makes a prominence on the cell-wall, and soon a 

 little body is formed, the spindle dividing across the middle. 

 We 'observe that this little body is really a cell, because it 

 has a nucleus with chromosomes, a little cytoplasm, and a 

 wall about it. It is called the first polar cell. That portion 

 of the spindle which was left in the egg disappears, but the 

 little centrosome still exists. It soon divides into two, and 

 the parts, gradually separating, form a spindle between them. 

 The chromosomes are again arranged on the spindle. Then 

 the spindle (Fig. 104, D) swings around into the plane of 

 the first spindle. As the spindle moves toward the cell-wall 

 again the chromosomes divide as before. A second polar cell 

 is formed beneath or slightly to one side of the first. The 

 process of maturation is completed by this phase. 



Fertilization. In the eggs of many animals the entrance 

 of the spermatozoon appears to be a stimulus for the begin- 

 ning of maturation. Whether the spermatozoon enters before 

 or after maturation has begun, it cannot take part in the 

 process of development until the second polar cell has been 

 formed. When this has taken place in Nereis, the egg-spindle 

 and the egg-centrosomes both disappear, leaving the egg- 

 chromosomes inclosed in a nuclear membrane. 



The spermatozoon on entering the egg leaves the " tail " 

 outside, its locomotor function being at an end. The "head," 

 which is really the nucleus of the spermatozoon-cell, or sperm- 

 cell as we may now call it, becomes rounded and for a time 

 remains quiescent (Fig. 104, B). As indicated in Fig. 104, C, 

 the sperm-cell becomes larger, and is drawn toward the cen- 

 tral region of the egg by a centrosome, which we shall 

 call the sperm-centrosome, because it came into the egg with 



