FORMATION OF POLAR SPINDLE IN BUFO. 79 



than one-half as large (Fig. 5). This difference is not found at 

 a later period ; for, when the chromosomes are arranged at the 

 equator of the spindle ready to divide, they are considerably 

 smaller than the chromosomes of an earlier period and are all, 

 apparently, of the same size. 



While the chromosomes are being arranged at the equator of 

 the spindle they undergo further changes in form. The polar 

 arms shorten considerably, while the thick knob at the middle 

 increases in size and gradually spreads out laterally, thus forming 

 two wing-like projections on the chromosomes (Figs. 6, 7). In 

 proportion as the lateral wings grow larger the polar arms of the 

 chromosomes become shorter and thinner, so that there can be 

 no question but that this lateral growth takes place at the ex- 

 pense of the rest of the chromosome. In a dorsal view, the 

 wings appear to be spread out flat on the spindle and the chro- 

 mosome has the appearance of a cross in which the polar arms 

 are somewhat longer than the equatorial arms (Fig. 6). In a 

 lateral view, however, the wings are seen to be raised up from 

 the spindle while the polar arms are extended along the spindle 

 fibers. Carnoy and Lebrun (2) have applied the term " oiselet" 

 to this stage in the development of the chromosome. The typ- 

 ical oiselet stage is followed by one in which the body of the 

 "bird" gradually disappears while the wings constantly increase 

 in size (Fig. 7). Very soon, all that is left of the original polar 

 arms is a slight projection on each side of the angle formed by 

 the meeting of the two wings (Fig. 8). In the succeeding stage 

 every trace of the polar arms has disappeared and there are 

 twelve broad V-shaped chromosomes arranged at the equatorial 

 plate with the angle of the V turned in towards the center of the 

 spindle (Fig. 9). Usually, before this last stage is reached, the 

 spindle has come to lie close to the surface of the egg and nearly 

 radial in position. This is by no means invariably the case, how- 

 ever, as sometimes the spindle is still some distance below the 

 surface of the egg when the chromosomes have divided in prepa- 

 ration for the first maturation division. 



Fig. 6 shows part of a section of an egg fixed as soon as pos- 

 sible after the toad was killed. The spindle lies at the periphery 

 of the egg and the chromosomes, with well-developed lateral 

 wings, are at the equator. That this egg and others from the 



