278 W. G. YanNaine — JErabryology of Eustylochus. 



The spindle fibers, as in the earlier stages, are few in number and 

 slender, but have become much more regular. The chromosomes 

 generally have still the ring structure though they are now elongated 

 and oval in outline. 



At this time the asters of the spindle are alike in size and appear- 

 ance and it cannot be predicted which one is to pass into the polar 

 body. But soon one pole of the spindle approaches the surface of 

 the e.g^. 



This is accomplished by a movement of the whole spindle, which 

 meanwhile shortens to about one-third of the diameter of the ^^^ or 

 even less. The aster of the outer pole becomes reduced in size, and 

 some of the rays necessarily disappear altogether as the centrosphere 

 approaches and finally comes in contact with the surface of the ^^^ 

 (Fig. 8). Finally the last of the rays at this pole and the centro- 

 sphere itself fade away. 



Division of the Chromatin. 



The shape of the chromosomes in the equatorial plate is very 

 ■ variable, and at first sight it would appear difficult to derive some 

 of the forms from the closed rings. Very similar if not absolutely 

 identical forms are found in all the species of polyclads described by 

 the writers already referred to. Van der Stricht has given the most 

 thorough discussion of the relation of the different forms of chro- 

 mosomes to the original elongated segment and to the ring derived 

 from it. His conclusions are for the most part confirmed or at least 

 supported by what I have observed in Eustylochus and Planocera. 



There is, I think, little room for question that we have here a true 

 case of heterotypical mitosis (Flemming, 2). No longitudinal split- 

 ting can be seen in the elongated beaded segment which is the first 

 stage of the chromosome. The first sign of its approaching division, 

 the cleft or opening through it, does not appear until it has con- 

 tracted into so nearl}'^ round or oval a form that from its shape we 

 cannot do much more than guess which diameter represents the 

 original long axis of the chromosome. If we are to ascertain 

 whether this cleft really is a longitudinal cleavage, an " aequations- 

 teilung " of the chromatin, we must find some other means to deter- 

 mine it. 



The way that the ring opens and divides proves that it consists of 

 two elongated segments, each bent into a semi-circle or U and joined 

 end to end. The junctions of these segments therefore come at 

 opposite points on the ring. The spindle fibers attach themselves 



