Relation between Nucleolus and Chromosomes, 59 



some cases. The female pronucleus is formed from the fusion of five or six 

 vesicles, the product of transformation of the eighteen chromosomes remain- 

 ing in the egg after maturation. 



At no stage in the maturation process are all of the chromosomes at the 

 same stage of transformation. Almost every spindle shows several chromo- 

 somes lagging far behind (figs. 64, 67, 68, 73). This explains why the 

 counting of the chromosomes in polar view becomes difficult, not to mention 

 their minute size. The most favorable sections for counts are such as pass 

 longitudinally through the poles of a prophase figure. Since the chromo- 

 somes are scattered through the equatorial plane, sections of from 7 to 10 

 microns often include the major part of all the chromosomes. Such sec- 

 tions almost invariably show 18 chromosomes in prophase (fig. 64). Simi- 

 lar sections through the anaphase stages again invariably show 18 chromo- 

 somes. The number could never be made less than 18; frequently in 

 the anaphase of the first division 20 (occasionally even 24) V-shaped and 

 bilobed chromosomes were counted, but these counts could usually be satis- 

 factorily reduced to the usual number by taking into account the fact that 

 some had prematurely split in preparation for the second mitosis. Polar 

 views of either the prophase or anaphase stage also never showed less than 

 1 8 chromosomes. This exact number could frequently be counted (figs. 76, 

 77, 80), but in such sections even more frequently than in longitudinal 

 sections of the spindle the number counted was 20 (fig. 79), 22, 23 (fig. 

 78), or even as high as 24 chromosomes. It is very evident that prema- 

 ture splittings of bivalent or univalent chromosomes the normal process in 

 the first maturation division would raise the count in such sections and 

 show wide and illusive variations in number. The best evidence shows that 

 the reduced number of chromosomes is 18. 



THE REDUCTION OF THE CHROMOSOMES. 



The maturation phenomena of Asterias forbesii agree with those re- 

 ported by Bryce in Echinus and likewise present a simple case of double longi- 

 tudinal division. My results are at variance with those reported by Tennent 

 (1905) for this same species. Tennent describes the second maturation divi- 

 sion as transverse, separating bilobed chromosomes into chromosomes of glob- 

 ular shape. I have conclusive evidence (figs. 73, 74 ) that the chromosomes 

 resulting from the second polar mitosis are also for some time after their 

 separation bilobed (dumb-bell-shaped) bodies and only subsequently become 

 stubby or globular. The fact that the eggs observed by Tennent had under- 

 gone previous treatment with CO 2 in sea-water, which seems to have greatly 

 retarded the maturation process (40 minutes according to Tennent), may 

 account for the discrepancy in appearance of the chromosomes. It is pos- 

 sible that in the CO 2 treated eggs the bilobed chromosomes had become 

 globular proportionately earlier in consequence of a much slower transit 

 toward the spindle poles than in eggs under normal conditions. 



