382 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



with that of fixation are very great. In the adult egg, on the other hand, the great 

 frequency with which the discharge is seen can not be explained as due to its more 

 frequent occurrence, for this would indicate a degree of activity, on the part of the 

 nucleolus at this stage for which there is no particle of evidence ; the completion of 

 yolk formation is accompanied by the complete cessation of all nucleolar activity 

 and growth, the nucleolus remaining of the same size and retaining the same ap- 

 pearance as long as the egg remains in the pinnule. 



Professor Chubb states that there appear to him to be only two possible 

 explanations : Either the discharge so frequently seen in the adult egg represents 

 the last which occurred during the waning activity of the nucleolus, and which 

 has since remained unchanged, or, and he thinks this the more probable explanation, 

 the discharge in the adult egg is caused by the contraction of the nucleolus at the 

 moment of fixation, the activity of the nucleolus during the closing period of the 

 egg's growth being insufficient to provide the necessary pressure for a normal 

 discharge of the gradually accumulating material. He has, however, never seen 

 the appearance of a discharge of spherules from the nucleolus of the still growing 

 oocyte after yolk formation has commenced, and he is unable to do more than to 

 suggest that this restriction of the contracting influence of the fixing reagent to 

 the nucleolus of the adult egg may be due to a change in the physical properties 

 of the cortical layers of the nucleolus after the activity of the latter structure has 

 ceased. 



The occurrence of discharged nucleolar spherules in the young oocyte long 

 before there is any obvious indication of the formation of spherules on the chro- 

 matin threads, their almost invariable spatial relation to the destained area of the 

 nucleolus, and their uniform behavior, irrespective of the nature of the fixing 

 reagent, distinguish these spherules sufficiently clearly from those on the chromatin 

 threads. The fact that the nucleolar spherules are less frequently met with in 

 later than in earlier stages indicates that they do not accumulate as such within 

 the nucleus. Equally certain is it that they do not dissolve in the caryolymph, 

 for there is never, at any period, any trace of nucleolar spherules in course of 

 dissolution in the nucleus, although if such a process occurred it would necessarily 

 occupy some time. There remains, therefore, as the only possible fate for the 

 discharged nucleolar spherules, an immediate passage to the cytoplasm. 



In addition to the persistence in the caryolymph of the last nucleolar discharge, 

 the completion of yolk formation is marked by another phenomenon in connection 

 with the nucleolus, namely, the gradual formation, on the surface of this structure, 

 of one or more lens-shaped accumulations. 



As its first appearance this accumulation is extremely thin, tapering away 

 imperceptibly at its edge on the nucleolar surface. It gradually increases in size, 

 becoming more and more hemispherical as it does so, until it may equal more than 

 one-third the total bulk of the nucleolus. Its staining affinity generally resembles 

 that of the destained area of the nucleolus, being usually weakly basophile but 

 showing considerable affinity for the acid stain. The substance of the accumulation 

 is always homogeneous in its staining reactions; it never shows a destained area, 

 and it is never vacuolated. The absence of any connection between nucleolar 



