390 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



AVhen the stainable area first assumes the regular form of the definitive yolk 

 nucleus it is, as already noted, much less deeply basophile than the original 

 nucleolar spherules from which it has been derived. This faintly staining con- 

 dition of the yolk nucleus does not last long, and in eggs very slightly, if at all, 

 larger, the yolk nucleus is as deeply stained as the original spherules. Accom- 

 panying this increase in staining capacity on the part of the yolk nucleus there 

 is a corresponding, though slower and much less strongly marked, increase in 

 the basophile reaction of the cytoplasm. 



That the basophile reaction of the yolk nucleus is, in the main at least, due to 

 other causes than those determining the corresponding reaction of the cytoplasm 

 has already been suggested by the mode of origin of the yolk nucleus. It is still 

 further indicated by considerable differences in this staining reaction in these 

 two regions of the cytoplasm ; for not only does the yolk nucleus stain deeply before 

 there is any basophile reaction on the part of the general cytoplasm, but the baso- 

 phile reaction of the yolk nucleus is only very slightly influenced by the character 

 of the fixing reagent, while that of the cytoplasm varies greatly, and with some 

 reagents for instance, acetic bichromate may be entirely absent. Moreover, 

 even in material in which the basophile reaction of the cytoplasm is most marked, 

 extraction always greatly destains the latter without appreciably affecting the 

 appearance of the yolk nucleus. 



As the yolk nucleus acquires its deeply basophile character it moves away 

 from the surface of the nucleus, becoming rounder and shorter as it does so, 

 and assumes a position midway between the nucleus and the egg periphery. At 

 this stage the yolk nucleus and, after sublimate fixation, the cytoplasm also, has 

 reached its maximum basophile reaction. Yolk formation now commences, and 

 very soon after this, while yolk formation is still only faintly indicated, the yolk 

 nucleus assumes a peripheral position close to, though not as a rule in actual 

 contact with, the egg periphery. At the same time it loses its regular form and 

 rounded contour, acquiring instead a form roughly determined by the nature of 

 the egg periphery and a somewhat irregular outline. At this period, and while 

 yolk formation is as yet mainly confined to the egg periphery, the yolk nucleus 

 shows a considerable increase in size and accompanying loss of staining capacity. 

 As yolk formation proceeds, the yolk nucleus retains its peripheral position and 

 appears to spread out over the evei'-increasing surface of the egg, becoming con- 

 siderably thinner as it does so, always, however, covering the same proportional 

 area of the egg periphery. 



Neither during its disappearance nor during the earlier stages of yolk for- 

 mation does the yolk nucleus ever show the slightest indication of undergoing a 

 granular disintegration. Its outline, though never showing the even character 

 it presented prior to the commencement of yolk formation, is never so irregular 

 as to be indefinite, and as soon as the loss of staining capacity has proceeded 

 sufficiently far it is seen that the structure of the yolk nucleus in all respects 

 resembles that of the surrounding j^olk, the only difference being the ever-decreas- 

 ing basophile reaction, and it is evident that yolk formation has proceeded in the 

 region of the cytoplasm constituting the yolk nucleus exactly as in other regions 



