20 KARYOKINESIS. 



right into and through the sphere until they come into contact with the cell wall. 

 In no other mitosis is there such a shortening of the spindle ; in fact, in all other 

 divisions, with the possible exception of the second maturation, the spindle continues 

 to oTow lonacer throughout the whole of the mitosis. A similar shortening of the 

 first maturation spindle has been observed in Ascaris (Boveri '87), Branchipiis 

 (Brauer '92), Opkryotrocha (Korschelt '95), Myzostomum (Wheeler '95), Cerebra- 

 tuliis (Coe '99), PolychcBrus (Gardiner '98), Axolotl and Triton (Carnoy and Lebrun 

 '99, see their figs. 110 and 112). The principal cause of this shortening is to be 

 found in the peripheral movement of the mitotic figure, as will be described in 

 Part II ; its chief result is the production of a much smaller polar body than would 

 be possible if the spindle maintained its maximum length throughout the later stages 

 of division. At the time of its greatest length the first maturation spindle is about 

 one-half as long as the diameter of the egg, and since the division of the cell body 

 always takes place through the middle of the spindle, the first polar body would 

 have a diameter one-quarter that of the egg were it not for this shortening of the 

 spindle. 



I agree with G. Niessing ('99), that the shape of the spindle, i.e., whether it is 

 stout or slender, is due to the quantity and location of the interfilar substance, but 

 this depends upon the degree of contraction of the linin reticulum. Both reticulum 

 and interfilar substance are widely distributed through the nuclear cavity in the 

 early prophase, and at this stage the spindle is very stout ; in later stages, as the 

 reticulum contracts and the interfilar substance passes to the poles, the spindle 

 grows slenderer. In the late anaphase, when the sjjindle becomes shorter, it again 

 grows stouter, figs. 15 and 16. 



The second maturation spindle arises within the centrosome left in the egg at the 

 close of the first maturation. At first it occupies but a small part of the cavity of 

 this centrosome, but it grows rapidly until it fills the whole of it. The outlines of 

 the mother centrosome then disappear and the spindle lies free in the sphere sub- 

 stance. Here it grows rapidly in size, but never becomes more than half as long as 

 the first maturation spindle, though it is relatively stouter. Its mantle fibres are 

 not formed directly from a linin reticulum, since there is no vesicular nucleus, 

 though they may possibly be formed from nuclear material which escaped from the 

 germinal vesicle at the previous division. 



During the prophase, the direction of the first maturation spindle bears no con- 

 stant relation to the egg axis. It may lie obliquely or even at right angles to that 

 axis, figs. 9 and 10, but ultimately it moves into a radial position, fig. 12, et seq. 



The direction of the second maturation spindle, like that of the first, varies 

 greatly, though in all cases it ultimately becomes aj^proximately radial. As in 

 Physa (Kostanecki and Wierzejski '96), the outer pole of the second maturation 

 spindle lies at the very point where the mid-body (^Zwischenkorpcr) of the first 

 maturation spindle was formed. The second polar body is given off immediately 

 under the first, so that the latter becomes separated from the surface of the egg and 

 remains mounted upon the former. This happens irrespective of the initial direc- 



