450 MONTGOMERY. [Vol. XV. 



It is probable that the two nucleoli of such nuclei have not 

 arisen by division from a single nucleolus, but are nucleoli 

 which have been developed at different points in the nucleus 

 and which are destined to fuse together later and form a single 

 one. This assumption was based upon the observation of 

 nuclei where two nucleoli lie at opposite poles of a nucleus 

 (Fig. 1 66) and each is apposed to the nuclear membrane, or 

 where only one occupies such a peripheral position, the other 

 being in the center of the nucleus (Fig. 164). In one figure 

 (Fig. 165) we see a nucleus in which the two nucleoli lie near 

 the center, close together, which might denote the beginning 

 of such a fusion. On a little reflection this explanation of the 

 presence of two nucleoli will appear quite allowable. In the 

 more usual mode of development a larger nucleolus is formed 

 at the periphery of the nucleus, wanders towards its center, and 

 then much smaller masses of nucleolar substance are similarly 

 formed and later fuse with the large nucleolus ; while in the 

 cases under consideration two nucleoli of nearly equal size are 

 produced, either simultaneously or in succession, and these 

 afterwards fuse together. These two nucleoli of nearly equal 

 size cannot be division products of a single primitive nucleolus, 

 since two nucleoli are never found in the larger germinal 

 vesicles. 



The nuclear sap of the smaller germinal vesicles does not 

 stain at all ; in the larger ones (Figs. 168, 173-17S' I77) it 

 does, and the explanation for this staining may be given by the 

 assumption that there is a dissolution of nucleolar substance 

 throughout the whole nucleus, i.e., of that substance of the 

 assimilated yolk balls which does not enter into the formation 

 of the nucleoli. During the mitotic stages no constituents 

 of the nucleus stain except the nucleolus and the chromatin 

 filament, but these do not stain in the same manner. 



At first sight the heteronemertean Linens seems to differ 

 markedly from all the metanemerteans here examined, in that 

 it contains a single, enormous germinal spot. But in Linens, 

 though a single large nucleolus is first formed, it nevertheless 

 grows by the addition to it of much smaller nucleolar globules 

 {Nut. Gl., Figs. 168, 174, 177) which have the same method of 



