214 GENERAL ZOOLOGY 



the sperm-nucleus. As the sperm-centrosome with its sperm- 

 aster (star-like arrangement of egg-cytoplasm fibers) is draw- 

 ing the sperm-nucleus inward, division of the centrosome 

 takes place and a spindle begins to form (Fig. 104, D). At 

 the same time the sperm-nucleus enlarges to the size of an 

 egg-nucleus. The new spindle with its enlarged sperm- 

 nucleus moves forward until it comes in contact with the 

 egg-nucleus (Fig. 104, j), which we remember has lost its 

 centrosome and spindle. 



The fibers of the spindle penetrate the nuclear membranes 

 of both egg-nucleus and sperm-nucleus, and drag into the 

 middle plane of the spindle all the chromosomes, the same num- 

 ber from each nucleus. By this act fertilization is completed, 

 and the new individual begins its existence as an embryo. 



Development. The spindle that is formed at the comple- 

 tion of fertilization is called the cleavage spindle. After the 

 chromosomes divide (Fig. 104, 7' 1 ), and are drawn toward 

 either pole, the young embryo cleaves or divides through the 

 middle, so that in each cell there is an equal amount of cyto- 

 plasm. There is also in each cell a nucleus containing chro- 

 mosomes, half of which came from the egg-nucleus and half 

 from the sperm-nucleus. Each of the two cells divides again, 

 making four cells. The polar cells disappear after a time. 



In the eight-cell stage in the earthworm there is the begin- 

 ning of a cavity at the center of the small sphere. As the 

 number of cells increases the cavity becomes relatively larger, 

 although the diameter of the hollow sphere 'is no greater than 

 the diameter of an unfertilized egg. When the cavity reaches 

 its maximum relative size the cells are arranged in a single 

 layer just beneath the original egg-membrane. This is called 

 the blastula stage. In this stage, as seen in Fig. 105, B, a sec- 

 tion through the blastula shows that the upper cells are smaller 

 than the lower cells. The larger cells flatten and soon begin 

 to bend inward, as shown in Fig. 105, D. The effect of this 



