FORMATION OF POLAR SPINDLE IN BUFO. 8/ 



and longitudinal divisions of the chromosomes is not as impor- 

 tant as many investigators have claimed : the division of the 

 chromatin substance would appear to be the important thing, the 

 manner of its achievement quite secondary, as Hertwig (9) has 

 maintained. 



There is, however, the following possibility to be considered. 

 \Yhen the germinal vesicle breaks down, all of the chromo- 

 somes are arranged in pairs, in some cases the ends of a pair 

 of chromosomes have united to form a closed ring. Very soon 

 after this stage the chromosomes break up into granules and 

 all traces of the chromatin substance is lost until innumer- 

 able chromatin granules appear in connection with the first polar 

 spindle. It is conceivable that all of the chromatin granules be- 

 longing to a pair of chromosomes have remained united during 

 this period of the apparent disintegration of the chromosomes, 

 although I have not been able as yet to demonstrate such a 

 union. If such is the case, then the chromosomes of the first 

 polar spindle are bivalent structures, each being composed of the 

 two chromosomes that had become paired at an earlier period of 

 development. On this assumption it is probable that the knob- 

 like thickening in the middle of the chromosomes, shown in Figs. 

 4 and 5, is caused by the fusion of the ends of the two chro- 

 mosomes. In text-figure I, ABC and ACD would represent the 

 two chromosomes united at AC. The subsequent changes in 

 the shape of the chromosomes serve merely to again elongate 

 the original chromosomes (Text-fig. 3) which are finally separated 

 by the division through AC. The first maturation division, 

 therefore, is a reduction division and the second division only is a 

 longitudinal one. It certainly cannot be mere chance that at the 

 time of the breaking down of the germinal vesicle, the chro- 

 mosomes should invariably become arranged in pairs. In light 

 of the most recent investigations on spermatogenesis and ovo- 

 genesis it would seem as if the above explanation must be the 

 true one for the maturation divisions in the egg of Bnfo, although 

 at present I am not able to prove it. I hope that the work I am 

 doing on the spermatogenesis of this amphibian will throw some 

 light on the maturation divisions in the egg. 



