No. 2.] ORGANIZATION OF THE EGG OF UN 10. 235 



believe that this diminution of the asters just before the union 

 of the germ-nuclei is much greater than has been described." 



II. The Maturation of the Egg. 



I have not observed the breaking down of the germinal 

 vesicle ; but from the study of ovarian eggs I have found that 

 before extrusion it lies excentrically near the animal pole. In 

 the earliest stage of the first maturation spindle observed (not 

 figured) the two centrosomes lie near the periphery in the 

 neighborhood of the animal pole, and at the same distance 

 from it ; the chromosomes lie between them, but much nearer 

 the center of the ^gg, so that the spindle is bent in the middle. 

 The asters are barely indicated in this stage. The axis of the 

 spindle straightens by the inward migration of the centro- 

 somes to the level of the chromosomes, so that the whole 

 spindle lies very little above the center of the 0.%^. Then 

 follow the rotation, elongation, and peripheral migration of 

 the entire spindle (PI. XXIV, Figs. 1,2, 5, 6, and 9). 



I. TJie Chrovio somes in Maturation. 



I have paid relatively little attention to this subject, as I 

 was chiefly concerned with the study of cytoplasmic phe- 

 nomena. However, some of the observations are of interest. 

 Sixteen chromosomes are formed in the germinal vesicle. In 

 the earliest stage seen they are apparently typical tetrads, 

 which, however, elongate before they become arranged in the 

 equatorial plate. Each chromosome then takes on the form 

 of a longitudinally split rod with two constrictions. The first 

 division is at right angles to the axis of the rod, and passes 

 midway between the two constrictions. The chromosomes 

 take on the very characteristic forms represented in PI. XXIV, 

 Figs. I, 2, 5, and 9; material accumulates at the two ends of 

 each chromosome, and, as in a heterotypic division, also in the 

 middle. The chromosome finally parts in the center of this 

 median accumulation. As the chromosomes diverge towards 

 the poles, they undergo typical changes in form, until, when 



