420 MONTGOMERY. [Vol. XV. 



Third nucleolar stage. — The nucleoli increase in number, but 

 gradually become smaller and wander towards the periphery 

 of the nucleus (Figs. 154 and 155), until they all lie close to 

 the inner surface of the nuclear membrane. In this stage 

 they attain their maximum staining intensity, as is well seen 

 after the use of Heidenhain's iron haematoxylin, by which 

 they become colored a greenish blue (Fig. 157), while in the 

 previous stages they are brownish yellow, unstained by the 

 haematoxylin. 



Fourth nucleolar stage. — Vacuoles of varying size arise in the 

 nucleoli, and become somewhat irregular (instead of spherical) 

 in outline (Figs. 156 and 158). In numerous nuclei it may 

 be noticed that all the nucleoli lie close to the nuclear mem- 

 brane, except a single one, which is placed nearer the center 

 and differs from the others in not staining with haematoxylin, 

 though it usually contains vacuoles ; it may be a nucleolus 

 which has not developed as fast as the others have (Fig. 

 156). 



All nucleoli in the third and fourth stages are very uniform 

 in size, and smaller and much more numerous than in the 

 second ; since there are no facts which permit us to conclude 

 that new nucleoli are being formed in the last two stages we 

 must consider that in them a division of the nucleoli must take 

 place, and this would explain their increase in number and 

 concomitant decrease in size. The fourth stage would seem to 

 be characterized by the commencement of a degeneration of 

 the nucleoli, if the presence of vacuoles and the irregularity 

 of form may be taken as a criterion of degeneration. Neither 

 in this species nor in the other nemerteans examined have I 

 seen stages showing the formation of the pole spindle, so that 

 I cannot describe the ultimate fate of the nucleoli. But the 

 observations of those who have studied these divisions seem 

 to show that they all disappear before the pole spindles are pro- 

 duced ; and accordingly the phenomena characteristic of our 

 fourth nucleolar period might represent the commencement of 

 these degenerative processes. 



The method of formation of the yolk may next be considered, 

 since the yolk stands in a certain relation to the genesis of the 



