76 BIOLOGICAL LECTURES. 



into contact with the surface of the egg. Under these circum- 

 stances it is difficult to conceive what causes the spindle to 

 rotate into a radial position and to move bodily toward the 

 surface, unless it be a general movement of the cytoplasm, and 

 the fact that throughout the egg the separation of yolk and 

 cytoplasm is going on at the same time is additional evidence 

 in favor of such a general movement of the cell body. 



During the fertilization similar movements of the egg con- 

 tents are apparently taking place ; the polar segregation of yolk 

 and cytoplasm goes on during the approach of the germ nuclei, 

 and, as during maturation, appears to be due to movements of 

 the cytoplasm. The spermatozoon usually enters near the vege- 

 tal pole, and is carried through almost the whole diameter of the 

 egg to the animal pole, but it may enter at any place except 

 the protoplasmic area immediately around the animal pole. If 

 the sperm enters at the vegetal pole, its course toward the 

 animal pole is nearly straight ; if it enters elsewhere, its course 

 is curved, and the nearer the point of entry to the animal pole 

 the greater the curvature. An aster is formed in advance of 

 the sperm nucleus and precedes it in its course through the 

 egg. The egg nucleus and aster lie very near the animal pole 

 and do not move from this position; they are surrounded by an 

 area of protoplasm free from yolk. The sperm nucleus and 

 aster in their advance through the yolk leave no path behind 

 them; either they are carried along by a general movement of 

 the cell contents, or the yolk is pushed out of their way, to close 

 in again behind them immediately after they have passed. The 

 germ nuclei and asters approach each other as I described in a 

 former lecture at this laboratory ('94), and when the two are close 

 together they lie in an area entirely free from yolk, except that 

 a few spherules are usually found between the two nuclei or 

 asters. These spherules, which are separated from all the rest of 

 the yolk, appear to have been carried before the sperm elements 

 in their advance. After the origin of the cleavage centrosomes 

 the remnants of the asters are carried to a point above the nuclei 

 and immediately under the polar bodies, where they disintegrate 

 and are scattered as coarse granules, a process which will be 

 described more fully when we come to consider the cleavage. 



