444 MONTGOMERY. [Vol. XV. 



contain vacuoles ; and in every respect the nucleolar changes 

 during this stage are the very reverse of the preceding. 



Fourth nucleolar stage. — This is characterized by the gradual 

 degeneration and disappearance of the nuclei (Fig. 235). Small 

 vacuoles arise in them, and these increase numerically, while at 

 the same time the nucleolar substance stains less intensely. 

 Fusion of neighboring nucleoli is very frequent at this time, or 

 perhaps a little time before the nucleoli lose their staining 

 power ; accordingly, in the largest germinal vesicles it is the 

 rule to find a small number of large nucleoli. The nucleoli 

 are not evenly distributed along the periphery of the nucleus, 

 and are often flattened against the nuclear membrane. This 

 nucleolar stage is found only in the largest ovarial eggs, where 

 the nucleus is perfectly regular in outline, without amoeboid 

 processes, and its membrane has attained its greatest thickness. 



Since this species is a protandric hermaphrodite, in which 

 male and female sexual products ripen successively in each 

 gonad, I found it at first difficult to determine whether a young 

 nucleus in a given gonad corresponded to a male or to a female 

 cell. But after comparing briefly the spermatogenesis of the 

 other metanemerteans mentioned in this paper, and finding in 

 them that no nucleus in any stage of spermatogenesis was larger 

 than any of the smallest germinal vesicles here figured, I con- 

 cluded that also in Stichostetmna no male nuclei can attain the 

 dimensions of even the smallest nuclei of our second nucleolar 

 stage, and hence that all these nuclei were correctly concluded to 

 be germinal vesicles, and not nuclei of spermato-genetic stages. 



We notice in the succession of the nucleolar stages described 

 the rhythmic sequence in regard to (i) the position of the 

 nucleoli, (2) their states of fusion and division, and (3) the 

 absence and presence of vacuoles in them ; these successive 

 changes may be expressed as follows : 



Nucleoli. 



Stage. Position. Vacuoles. Fusion, division. 



First . . . peripheral . . . absent fusion ? 



Second . . . central . . . present . . . division, then fusion. 



Third . . . peripheral . . . absent division. 



Fourth . . . peripheral . . . present . . . fusion, then division. 



