294 BULLETIN OF THE 



poles, and increases with the increase in the distance between the poles, 

 attaining its maximum in the dumb-bell stage, after which (at division 

 of the nuclear band) it wanes and altogether disappears. The elonga- 

 tion of the nucleus and the radial figures are together comparable to the 

 magnetic rod and its influence on iron filings, without, however, necessi- 

 tating the implication of an identity of forces. 



About the time the nucleus begins to elongate, the outline of the egg 

 undergoes for a short time a series of changes in form (pp. 403, 404, 

 417), consisting of low elevations. In the dumb-bell stage the egg elon- 

 gates in the direction of the handle of the dumb-bell, and a circular furrow 

 appears in the plane of division and finally effects the separation of the 

 halves. This constriction is often accompanied by irregular changes of 

 the surface in the form of lobed pseudopodial processes, which here and 

 there arise and disappear. This phenomenon presumably has a causal 

 relation with attractions in the nucleus. 



The further changes, as seen in the interior of the living egg during 

 and after the constriction of the yolk, begin with a separation of the 

 ^ heads of the dumb-bell figure, and a modification of their form. Each 

 is at first flattened in a plane parallel with the division plane, and then 

 becomes a meniscus, the concave surfaces of the two meniscuses facing 

 each other and continuing in connection by a thin pedicel. About the 

 time the constriction divides f>he yolk, there suddenly appears a small 

 clear spot in each half of this pedicel, at some distance from the divis- 

 ion plane, and this spot, which is at first of irregular form, becomes 

 rounded and increases in size ; it is the nucleus of the daughter cell. 

 Next, the radial arrangement of the yolk disappears, then the pedicel 

 vanishes, the nucleus migrates partly into the meniscus, and finally the 

 latter is reduced to two small areas at the sides of the nucleus. The 

 newly formed nuclei in this, as well as in all subsequent stages, are with- 

 out membrane, and consist of a homogeneous substance. Inasmuch as 

 the nuclei are of nearly uniform size for the first few generations, it fol- 

 lows that a considerable increase in the nuclear substance has taken 

 place after each segmentation. 



Osmic acid preparations stained in carmine furnish additional infor- 

 mation on these internal changes. Preparations of eggs somewhat older 

 than those exhibiting the " middle zone " show a ribbon-shaped body in 

 its place. Its ends reach into the middle of the suns, and, owing to the 

 intensified action of the reagents, appear as dark sharply defined streaks. 

 Where the band enters the head of the dumb-bell it presents rodlike 

 thickenings (seitliche Verdichtungszone des Kernbandes). 



