NUCLEOLAR STRUCTURES OF THE CELL. 163 



a close chemical resemblance to yolk globules, we have every 

 reason for supposing that in these objects, if not generally, the 

 nucleolar substance is nutritive and extranuclear in origin. 



5. Behavior in Nuclear Divisions. In cases of amitosis it 

 is the general rule, as I conclude after a thorough review of the 

 literature on the subject, that division of the nucleolus accom- 

 panies or precedes that of the nucleus. I have seen prepara- 

 tions showing this very clearly in cells of the ovarial follicle of 

 Gryllns ; Dr. Conklin kindly demonstrated these preparations 

 to me, and allowed me to mention here this result of his as yet 

 unpublished observations. 



In mitosis, on the other hand, it is the general rule that 

 the nucleolus disappears by dissolution during the prophases, 

 dissolving either in the caryolymph or in the cytoplasm after 

 the disappearance of the nuclear membrane. Such a dissolu- 

 tion may be either gradual, or the nucleolus may first fragment 

 into pieces, and then these dissolve. In the first maturation 

 division of the ovum of Piscicola the dissolved nucleolar sub- 

 stance is easily demonstrable by the aid of suitable stains in 

 the caryolymph, and after the disappearance of the nuclear 

 membrane seems to commingle with the cytoplasm. 



In some few cases, relatively speaking, the nucleolus may 

 not at all or only partially become dissolved in mitosis, and so 

 be .cast out into the cytoplasm, where it may be discerned 

 through the metaphase. This has been observed, for instance, 

 in the ova of Aequorea (Hacker), Chaetopterus (Mead), Allolo- 

 bophora (Foot, whose preparations I have had the pleasure of 

 seeing), and Myzostoma (Wheeler), though in plant cells this 

 phenomenon would appear to be more usual than in animal 

 cells. 



In still fewer cases the nucleolus divides in the equatorial 

 plate along with the chromosomes. Such cases, which could 

 not, however, be proved with certainty, were observed by me 

 in the ovogonia of Linens and Polydora, and more or less 

 similar cases have been noted by a few other writers. But 

 such a phenomenon is very unusual, and it is very doubtful in 

 such cases whether the nucleolus always divides regularly. It 

 might be regarded as a case of a nucleolus extruded whole into 



