CYTOKINESIS. 77 



in the cytoplasm, probably of active movements, and the fact that throughout the 

 egg the separation of yolk and cytoplasm is going on at this time is additional 

 evidence in favor of such general movements in the cell body. 1 



Second Maturation. — The initial position of the second maturation spindle 

 varies greatly in different eggs ; in some cases it is almost at right angles to the egg 

 axis {e.g., fig. 28), but it always turns so that one pole lies almost in contact with 

 the mid-body between the egg and the first polar body. It is very probable that 

 the spindle turns into the line of least resistance, and the fact that the line of least 

 resistance leads directly from the chromosomes to the mid-body may be due to the 

 persistence in this axis of the spindle remnants of the first maturation. The sec- 

 ond polar body is extruded immediately under the first, so that the latter is raised 

 upon the former and is separated from the egg. This also must be due to the fact 

 that the surface tension is least at this point. Whether this is the result of the fact 

 that the egg membrane is here lifted by the first polar body, or that the surface 

 layer between the egg and the first polar body is here newly formed, or that the 

 cell membrane grows most rapidly over the pole of the spindle cannot be determined 

 by observation alone. 



B. Movements during Fertilization. 



During the fertilization similar movements of the egg contents 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 move- 

 ments of the cytoplasm. The spermatozoon usually enters near the vegetal pole, 

 and is carried through almost the whole diameter of the egg to the animal j)ole, 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. 



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, 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 cleav- 

 age 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 



1 Kostanecki and Wierzejsky ('96) have observed in Physa that the peripheral movement of the 

 spindle, and the separation of the yolk and cytoplasm go hand in hand. 



