FORMATION OF POLAR SPINDLE IN BUFO. 75 



II. THE DISINTEGRATION OF THE GERMINAL VESICLE. 



I have already given in detail a description of the early stages 

 in the breaking down of the germinal vesicle, and as this new 

 material confirms but adds nothing to that description, it will 

 be necessary to give only a brief account of the changes in the 

 egg directly preceding the formation of the spindle. 



At the end of the hibernation period the germinal vesicle lies 

 in the upper hemisphere of the egg. It is round in outline and 

 contains a large number of nucleoli which usually form a ring 

 enclosing the chromatin threads. A layer of granular sub- 

 stance staining differently from the cytoplasm, surrounds the 

 lower pole of the germinal vesicle and extends halfway up each 

 side. This substance appears homogeneous at first and then 

 becomes a compact, fibrous band of uniform thickness. I have 

 called this band a " line of radiation," because, as soon as the 

 nuclear membrane has disappeared in this region, the karyoplasm 

 of the nucleus forms into coarse granules and a pronounced radi- 

 ation extends up into the nuclear substance from the entire 

 length of the fibrous band below. The karyoplasmic granules 

 soon become smaller and more numerous and finally disappear 

 entirely, while the radiation from below continues to increase and 

 often extends nearly to the upper surface of the egg. The rays 

 forming this radiation are very fine, and their outer ends run, 

 apparently, into the coarse network which comes to fill the entire 

 space formally occupied by the germinal vesicle. During these 

 changes, the nucleoli have lost their power of staining and have 

 begun to disintegrate. 



When the nuclear membrane breaks down, twenty-four chro- 

 mosomes, arranged in pairs, are scattered throughout the upper 

 part of the nuclear space. The ends of each pair then unite to 

 form a closed ring near which a small aster usually appears. 

 The aster has no centrosome and its rays rarely touch the chro- 

 matin ring. At the next stage, when the radiation from below 

 has reached its greatest extent, the asters and the chromatin 

 rings entirely disappear. Later, when the radiation has begun 

 to decrease, a large number of small round chromatin granules 

 are found near or on the line of radiation which has been gradually 

 shortening during this period. When the chromatin granules 



