542 



CELL DIVISION IN EGGS OF CREPIDULA. 



the nuclear vesicle is clumped or condensed into a single central mass, or into 

 a dense reticulum of very coarse threads ; the surrounding achromatin 1 is left free 

 from granules of chromatin and yet it stains like oxychromatin and is identical 

 with the material which I have elsewhere (1902) called " chromatic nuclear sap." 

 During mitosis this chromatic sap together with hyaloplasm from the cell 

 body constitutes the archiplasm, which fills the area around the centrosomes 

 radiates along the astral fibers and forms the interfilar substance of the spindle 

 as I have shown in the work just referred to (1902, p. 49). In hypertonic solu- 

 tions this archiplasm is withdrawn from the astral radiations into the spindle and 

 the area immediately surrounding it (figs. 189, 190). At the same time the entire 

 spindle figure shrinks in size, the chromosomes are usually clumped together, and 

 the spindle fibers may disappear. Eggs which have been returned from hyper- 

 tonic solutions to normal sea water lose all appearance of shrinkage within a 

 short time (Exps. 973, 977) . Konopacki found that such eggs assume a normal 

 appearance within five minutes after their return to normal sea water. 



5. Formation of Cytasters and Polyasters. 



(Figs. 183-190, 199, 203, 204, 206, 207, 213, 219, 220.) 



All investigators who have made a cytological study of the effects of hyper- 

 tonic solutions on eggs have emphasized the fact that the appearance of cytasters 

 constitutes one of the most characteristic results. 2 Morgan (1899) in particular 

 holds that the presence of cytasters (his astrospheres) rather than the suppression 

 of cell division is the most distinctive effect of hypertonic solutions. Wilson 

 (1901) believes that these cytasters may contain centrosomes, become the poles 

 of spindles, and in every other way behave as veritable centers of mitotic division. 

 Admittedly the evidence in favor of this view is not entirely conclusive, and 

 Petrunkewitsch (1904) maintains that there is a fundamental distinction in these 

 regards between "nuclear asters" and "cytasters," with which opinion I agree. 



The exact method of the origin of cytasters has not been traced, except by 

 Konopacki (1911), who finds that they arise along the astral radiations and from 



the material of which these radiations are composed 



He finds that they appear 



most frequently before cleavage, more rarely in the 2-cell stage, and never in 

 later stages. My own observations entirely confirm those of Konopacki in these 

 respects. In 1902 I showed that cytasters never form when the germinal -"-"' ° 



intact, and that they form from archiplasm which 

 japed achromatin of the nucleus. 



term "achromatin" as originally introduced by Flemming 



derived in part from 



nucleus other than chromatin. As thus used it is a generic term including oxychromatin 



sap, and 



Since these different substances are 



term which will 



guishable at every 

 or all of them and 



that the term ' ' achromatin 



is reason that the term "acnromaun is usea. . ^ period 



infrequently cytasters appear in eggs under normal conditions, especially during 



1 Not infrequently cytasters appear in eggs under normal conditions, espeu<m.y uu b 

 between fertilization and the first-cleavage (cf. Exp. 996 (1), control on effects of electric curren j 



