No. 2.] THE EGG OF BUFO LENTIGINOSUS. 3 i 7 



as the apparent union of the two sister portions of each half 

 ring has already occurred and the double segments have come 

 to lie parallel, when the spindle is still some distance below 

 the surface of the &gg (PI. XXIX, Fig. 28). There are cer- 

 tainly not more than twenty-four distinct chromosomes present 

 at the stage of Fig. 28, although I have not been able to make 

 out the exact number satisfactorily. On account of the small 

 size of the chromosomes, it is extremely difficult to follow the 

 changes they undergo, and only a more extended research with 

 an abundance of very favorable material can explain these 

 stages satisfactorily. Cases in which the chromosomes have 

 split longitudinally and then apparently re-fused are not 

 unknown. In Ophryotrocha, Korschelt has found that there 

 is a distinct longitudinal splitting of the four chromosomes 

 during the formation of the first polar spindle, and a subse- 

 quent apparent disappearance of the division, so that, in the 

 equatorial plate of the spindle after it has reached the periph- 

 ery, there again appear but four chromosomes, seemingly 

 forming a tetrad group. The splitting reappears after the 

 chromosomes have separated and are migrating towards the 

 poles of the spindle. 



While the eggs of Bufo are in the upper part of the oviducts, 

 the chromosomes shift their position on the spindle, and in the 

 anaphase (PI. XXX, Fig. 32) the angles of the chromatin loops 

 are turned towards the spindle poles so that their true shape 

 can readily be seen. In favorable cases some of the chromo- 

 somes show their double structure ; but the reappearance of the 

 splitting is best seen in the equatorial plate of the second polar 

 spindle (PI. XXX, Fig. 37). 



After metakinesis, the chromosomes separate in such a man- 

 ner that twelve double chromosomes, one from each group, 

 migrate to each pole. The first polar division, therefore, sep- 

 arates the two chromosomes which oi-iginally fused to form a 

 chromatin ring. 



After the separation of the chromosomes, the distal end of 

 the spindle extends up into a small projection of egg-sub- 

 stance (PI. XXX, Fig. 33) which is then pinched off to form 

 the first polar body. During this process the spindle becomes 



