No. I.] POLYCHOERUS CAUDATUS. 83 



In the stage shown in Fig. 7 the nucleolus has reached 

 its maximum development, and from now on it decreases 

 rapidly in size. As it decreases the structure of the whole 

 nucleus changes. A network of granular substance (Fig. 8) 

 resembling the chromatin granules of a still later stage occupies 

 the whole nucleus, while occasionally fragments of this net- 

 work stain as deeply as the chromatin granules of a later 

 stage. For the most part, however, the affinity for color is 

 much less than in the riper nucleus. The nucleolus has rela- 

 tively diminished in size, but not in its staining qualities. It 

 is to be noted that this structure never lies in the center of the 

 nucleus, but always near the nucleolar wall. In Fig. 9, which 

 represents the nucleus when fully ripe and ready to take part 

 in the formation of the first maturation spindle, the nucleolus 

 may be seen lying in close contact with the nuclear wall, com- 

 pletely broken up into fragments which are but faintly tinted 

 by a stain for which in an earlier stage this structure exhibited 

 the greatest affinity. On the other hand, the substance of the 

 nucleus shows a distinct granular reticulum of chromatin par- 

 ticles which now for the first time stains deeply. In following 

 the successive stages of the nucleus of the ovarian ovum (Figs. 

 5 and 6) through Figs. 7, 8, and 9, one cannot but be impressed 

 with the fact that as the nucleolus diminishes in size and inten- 

 sity of affinity for stain, the nucleus acquires these very quali- 

 ties. This would suggest that the chromatin is built up at the 

 expense of the nucleolus, rather than that the nucleolus is a 

 by-product of the nucleus. 



Formatioti of the Polar Bodies. 



Before entering on the description of the formation of these 

 bodies it is necessary to make clear in what sense certain terms, 

 which have been applied by different authors to different struc- 

 tures, are used in this paper. The organ which in this ovum 

 presides over karyokinesis is a clear vesicle in which a dark 

 staining spot is formed only after the amphiaster is well devel- 

 oped. In accordance with the terminology adopted by Wilson 

 (13), I shall call this vesicle the centrosphere, and the spot 



