No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 447 



nective-tissue cells, lead to the conclusion that the nucleolus 

 first appears in the young germinal vesicle, and more particu- 

 larly, that the substance of the nucleolus is extranuclear in 

 origin, and stands in a genetic relation to the substance of the 

 young yolk balls. The substance of both is homogeneous and 

 stains identically; by fixation in Hermann's fluid, followed by the 

 triple stain of Flemming, the nucleolus and the yolk balls stain 

 a brownish yellow (Fig. 160) ; by fixation in corrosive sublimate 

 and staining in haematoxylin and eosin both structures are 

 colored a yellowish red (Fig. 177). Still more conclusive is 

 the following observation : while the greater number of the 

 yolk balls may lie at some distance from the nucleus, one or 

 several are very frequently in close contact with the outer sur- 

 face of the latter, and yolk balls may even be found which are 

 halfway through the nuclear membrane, or which have com- 

 pletely transversed it and lie within the nucleus (Fig. 160). 

 Thus the nucleolus would seem to owe its origin to the sub- 

 stance of yolk balls which have been taken into the nucleus. 

 The very marked increase in the size of the nucleus and the 

 nucleolus is probably caused by a continued process of yolk- 

 ball assimilation on the part of the nucleus. This may be 

 observed in numerous cases where small globules of yolk- 

 ball substance lie within the nucleus, some at its periphery or 

 close to the nuclear membrane, others flattened against the 

 nucleolus (Figs. 160 and 177). By the use of the haematoxylin- 

 eosin stain the nucleolar substance usually stains a little more 

 intensely than the substance of the yolk balls (Fig. 177) ; this 

 would show that this substance, after being taken up by the 

 nucleus, undergoes a chemical change within the latter. Those 

 yolk balls which are not assimilated by the nucleus remain in 

 the cytoplasm and give rise to the yolk globules, as has been 

 described. Thus the nucleolus probably has an extranuclear 

 origin and represents a portion of the yolk-ball substance taken 

 into the nucleus ; its rapid increase in size is due to the addition 

 to it of other similarly assimilated globules of substance. 



In the largest germinal vesicles seen (though these were 

 not mature) the nucleolus is usually spherical in form, seldom 

 oval, and homogeneous in structure, except that it sometimes 



