No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 487 



it probably does not assimilate the mature secretion corpuscles, 

 since when the latter are produced, as we shall see, the nucleus 

 commences to retract in size and to withdraw its processes. As 

 the nucleus increases in size its chromatin reticulum becomes 

 looser, as if it were elastically stretched by the expansion in 

 volume of the nucleus; the chromatin is continued into the 

 ramifying processes of the nucleus. 



The nucleolar changes during the prophase are as follows : 

 in the immature cell there is invariably a single rather large 

 nucleolus, which occupies a more or less central position in the 

 nucleus (Figs. 178-18 1, 184) ; it may be either oval or spindle 

 shaped, and most frequently contains one or several small vac- 

 uoles. Its substance appears homogeneous after treatment with 

 corrosive sublimate, granular after the action of the fluid of 

 Flemming, and has no limiting membrane ; in all its stages 

 within the nucleus it stains very intensely, though always dif- 

 ferently from the chromatin. Now as the nucleus increases 

 in volume so also does the nucleolus, though at first at a rela- 

 tively more rapid rate than does the former ; and in growing 

 larger it gradually becomes more elongated, rod shaped, and at 

 this stage is most frequently in contact with the nuclear mem- 

 brane (Fig. 182). When it has taken up this peripheral posi- 

 tion its period of most rapid growth commences, so that at 

 this stage there is a proportionately greater amount of nucleo- 

 lar substance in the nucleus than at any other period in its 

 history. When it is apposed to the nuclear membrane it has 

 at first more or less the form of a rod (often of a slightly curved 

 rod), but as its substance commences to increase in volume this 

 rod shape gradually becomes changed and the nucleolus becomes 

 bent inwards (towards the center of the nucleus), frequently in 

 the form of a V, an S, or a W, though there is marked vari- 

 ability in regard to the form it may assume, since no two nucle- 

 oli can be found at this stage which have exactly the same form 

 (Fig. 189). It is about this time that the nucleolus attains its 

 greatest staining density. Then this large and irregularly 

 shaped nucleolus leaves the nuclear membrane and begins to 

 fragment into pieces, which are very irregular in shape and 

 variable in number and size ; the nucleolus may show thereby 



