76 CYTOKINESIS. 



A. Movements during Maturation. 

 First Maturation. — The cell movements during maturation result in the segre- 

 gation of yolk and cytoplasm at opposite poles of the egg, and in the transportation 

 of the mitotic figure to the animal pole. While the germinal vesicle is still intact 

 it lies some distance from the periphery and is closely surrounded by yolk spherules, 

 and there is a very incomplete separation of yolk and cytoplasm throughout the 

 entire egg. As soon as the mitotic spindle is formed and the nuclear membrane is 

 broken, there is an area immediately surrounding the spindle and asters free from 

 yolk, but elsewhere in the egg there is an intimate mingling of yolk and cytoplasm. 

 The initial position of the spindle differs in different eggs ; it rarely lies in the chief 

 axis of the egg and may be at right angles to this. Gradually the spindle turns 

 until its axis nearly coincides with that of the egg, and at the same time the whole 

 spindle is moved out toward the surface, until finally the outer end of the spindle 

 comes into contact with the cell membrane, and the surface of the egg is elevated 

 into a papilla at this point. This movement is in part due to the mere lengthening 

 of the nuclear spindle which doubles in length during the process, but in part also to 

 a general movement of the cell substance by which the spindle is turned and carried 

 bodily toward the surface of the egg. At the same time there are movements 

 within the egg which lead to an accumulation of cytoplasm at the animal pole and 

 a movement of the yolk spherules toward the opposite pole. There is no evidence 

 that this movement is due to activity on the part of the nucleus and centrosomes. 

 The initial position of the centrosomes and the direction of the central spindle are 

 not the same in different eggs, and yet the final position of the mitotic figure is the 

 same in all cases ; the centrosomes and asters at the two poles are identical in form, 

 size and staining reactions until the outer pole of the spindle comes into contact 

 with the surface of the egg. When this occurs the sphere and centrosome at the 

 outer pole become flattened against the egg membrane (PL I, figs. 14, 15). At the 

 same time the spindle begins to shorten, and this continues until it is not more than 

 half as long as in the metaphase or early anaphase. At the same time the chromo- 

 somes at the outer pole are crowded into the sphere (fig. 16), and finally they are 

 pushed through this until they come into contact with the opposite cell wall (figs. 

 22, 23). Such a phenomenon is found only in the formation of the polar bodies and 

 must be caused by factors which are wholly different from those which have com- 

 monly been held to be active in the mechanics of mitosis. Neither the contraction 

 of mantle fibres (Van Beneden, Boveri), nor the growth of central spindle fibres 

 (Driiner), nor the chemotropic influence of the centrosome and sphere (Strasburger) 

 will explain the extreme movement of the chromosomes in the first maturation. 

 On the other hand the evidence is conclusive that this extreme movement of the 

 chromosomes is due to the same factor which forces the entire spindle into contact 

 with the egg membrane and then causes the flattening of the sphere and centro- 

 some and the shortening of the spindle. Evidently this factor lies wholly outside 

 of the spindle, since it acts upon the spindle as a whole, and must consist of stresses 



