380 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



history of the oocyte, the irregularly scattered chromatin strands showing no 

 further spatial relation either to the nucleolus or to the nuclear membrane. 



As the chromatin threads lose their radial arrangement they become more 

 deeply stained and sharply denned, and now for the first time bear distinct, though 

 as yet small, deeply stained spherules. Often the strands appear to be composed 

 solely of these minute spherules. Sometimes, however, appearances suggest that 

 the latter are really arranged on a fine, deeply stained, and apparently homogeneous 

 axial thread the zigzag and branched character of which gives to the chromatin 

 strands a " mossy " or " filagree " appearance. During the period preceding the 

 commencement of yolk formation, and thus during the progressive increase in the 

 basophile reaction of the cytoplasm, the chromatin is always relatively small in 

 quantity and is often very faintly stained. As yolk formation progresses, however, 

 both the quantity and the staining capacity of the chromatin greatly increase, and 

 at the same time the spherules on the chromatin threads grow in size until, in the 

 adult egg, they may equal the spherules of the yolk. 



The staining capacity of these chromatin spherules and the size they ulti- 

 mately attain varies considerably in different individuals and is also to some extent 

 dependent on the mode of fixation, and in material fixed solely with sublimate 

 solution there is no trace of either chromatin or chromatin spherules. On the com- 

 pletion of yolk formation the condition of the chromatin remains unchanged as 

 long as the egg remains in the pinnule. 



In the younger oocytes the nucleolus, which is at this stage invariably central 

 in position, is homogeneous in appearance and, after most fixing reagents, deeply 

 basophile. As growth proceeds, however, this uniform staining reaction is lost, 

 the nucleolus showing a tendency to part with the basic stain during extraction. 

 This destaining is always from within outward and may, with moderate extraction, 

 be so extensive as to include the whole nucleolus. Usually, however, there is a 

 considerable cortical region which tenaciously retains the basic stain. The readi- 

 ness with which this destaining takes place is mainly determined by the condition 

 of the nucleolus itself at the moment of fixation and is independent of the age 

 of the egg ; eggs of the same age and in the same ovary differ greatly in this respect. 



The more faintly staining internal region of the nucleolus is never spherical 

 in form, and its size, though mainly due to the condition of the nucleolus itself, is 

 partly determined by the extent of extraction the slide has undergone; more- 

 over, its contents are distinctly acidophile, far more so than is the caryolymph. It 

 is evident, therefore, that the faintly staining area of the nucleolus is not of the 

 nature of a vacuole. 



True vacuoles, spherical in form and with clear, unstained contents, do, how- 

 ever, occur in the nucleolus. Their occurrence, like that of the destained area, is 

 quite inconstant, for they are sometimes present, sometimes absent, at all stages of 

 the egg's growth, though they are far more rarely met with in the earlier than in 

 later stages. They are always much smaller than the faintly staining area, and 

 although they may be present throughout the whole body of the nucleolus when 

 this is deeply basophile, they are almost or entirely confined to the periphery when 

 a considerable destained area is present. 



