No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 411 



which appear at only a certain stage of the cell. The true 

 nucleolus may be considered first, then these other structures, 

 or " pseudonucleoli." 



There is always one true nucleolus to each nucleus, and in 

 only two cases out of hundreds of ova examined have I seen 

 two nucleoli (Figs. 57 and 61). The position of the nucleolus 

 within the nucleus is in most cases excentric, seldom central, 

 and never apposed to the nuclear membrane; it apparently lies 

 free in the caryolymph, and is not supported by the chromatin 

 threads. In the youngest, most immature germinal vesicles 

 (I have not studied it in the ovogonia) it is apparently wholly 

 homogeneous, dense, not noticeably refractive, and usually 

 spherical (Figs. 57-61) ; sometimes, however, it shows an oval 

 or more elongate form, and in the latter case its long axis 

 usually coincides with that of the nucleus (Fig. 58) ; it is 

 never irregular in outline. 



The nucleolus always colors differently from the chromatin, 

 when treated with double stains, as follows : 



With the increase in size of the nucleus the nucleolus 

 enlarges, and in such a way that the size of the latter usually 

 preserves its proportion to that of the former ; but as the 

 figures show, this proportion is quite frequently not preserved. 

 What may be termed the first stage of this nucleolar growth 

 consists merely in an increase in the amount of the homogene- 

 ous substance, and between the largest homogeneous nucleoli 

 (Fig. 65) and the smallest (Fig. 57) there is no difference 

 except one of size. 



The second period of nucleolar growth is introduced when 

 vacuoles commence to appear in the substance of the nucleolus 

 (Fig. 62). Since my observations show that these nucleolar 

 vacuoles are derived from small fluid globules which first appear 



