452 MONTGOMERY. [Vol. XV. 



the nucleolus is usually nearer the center of the nucleus than 

 in those more mature stages where the nucleus lies near the 

 periphery of the cell. But in the more mature stages the 

 nucleolus may lie at the animal pole, or the vegetal pole, or 

 at one side of the nucleus, so that no coincidence between 

 the position of the nucleolus and the age of the nucleus can be 

 determined. Thus the nucleolus stands, e.g., in no relation to 

 the animal pole of the more mature nucleus, that pole where 

 amoeboid processes are produced (Figs. 204 and 209). The 

 ground substance of the nucleolus is dense and homogeneous, 

 and stains quite deeply ; the nucleoli of the smaller germinal 

 vesicles stain, as a rule, less intensely. In the ground sub- 

 stance of all the nucleoli more or less numerous fluid globules 

 occur, which stain very faintly or not at all, and their presence 

 gives a vacuolated appearance to the nucleolus ; those within 

 the same nucleolus are of unequal size, and among them two 

 or three usually occur which far exceed the others in size. 

 Occasionally there is one large central vacuole (Fig. 206), but 

 as a rule the larger ones are peripheral, and may produce prom- 

 inences of the surface of the otherwise perfectly smooth and 

 spherical nucleolus (Figs. 209 and 212). In one large vacuole 

 (Fig. 212) a finely granular mass was found, though this may 

 have been an artefact. Since in the smaller nucleoli these 

 vacuoles are less numerous and smaller in size, it would seem 

 probable that in stages antecedent to those found by me the 

 nucleolus may be wholly devoid of such vacuoles. The nucleo- 

 lus has no enveloping membrane, for what at first view appears 

 to be such a structure careful study shows to be merely the 

 result of refraction. 



In addition to the single large nucleolus described, there are in 

 the most mature nuclei also from about one to five minute nuclei 

 (Fig. 209). These vary somewhat in size, are perfectly spher- 

 ical and homogeneous, without vacuoles, and stain more deeply 

 than the larger one. Sometimes they are found in close con- 

 tact with the nuclear filaments {cf. the nucleoli of the second 

 generation in Tetrastefnma catcnulatiim and the observations 

 of Ruckert ('92) on the germinal vesicles of Selachii). These 

 probably have no genetic relation to the large nucleolus, since 



