396 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



peripheral spherules of the yolk already referred to and yet to be described for 

 a position in actual contact with the vitelline membrane. 



Thus the absence of any participation by the yolk nucleus of Antedon in the 

 cell metabolism is indicated both by the origin of this structure as a region of the 

 cytoplasm onto which has diffused a portion only of the material intermittently 

 discharged from the nucleolus throughout the growth of the oocyte, and by the 

 fact that its subsequent behavior can be entirely explained as due to the influence 

 on a semifluid mass of the varying physical conditions obtaining in the cell. 



The entirely fortuitous manner in which the yolk nucleus arises in Antedon 

 offers a ready explanation of a hitherto puzzling fact, that though this structure 

 is of constant occurrence in those forms in which it occurs at all, it is nevertheless 

 frequently entirely absent in closely allied species. For it is obvious that very 

 slight changes in the constitution of the yolk, or in the mode of its formation, 

 might suffice to remove the conditions under which the diffusion described above 

 could alone take place. 



In the young oocyte, previous to the formation of the definitive yolk nucleus, 

 the cytoplasm is faintly acidophile, showing little or no tendency to retain the 

 basic stain. At about the time of the appearance of the definitive yolk nucleus, 

 however, the cytoplasm commences to show a distinct basophile reaction which 

 steadily increases in intensity as the oocyte grows, until yolk formation commences. 

 The basophile reaction of the cytoplasm, therefore, first becomes marked in eggs 

 of from 30 y. to 40 (/. in diameter, in which the nucleolus measures about 8[i, and in 

 which the yolk nucleus is already deeply basophile. The reaction reaches its 

 maximum in eggs of from 50 p. to 60 ;x in diameter, in which the nucleolus measures 

 9 n to 10 |x, and gradually disappears as yolk formation progresses. 



Unlike the yolk nucleus, the basophile reaction of which shows but little 

 variation with different fixing reagents, the staining capacity of the cytoplasm 

 varies greatly according to the fixing agent employed. The reaction is most 

 marked in material fixed with a saturated solution of sublimate, the cytoplasm 

 often being, with moderate differentiation, so deeply stained as to almost obscure 

 the black yolk nucleus; greater differentiation will, however, always greatly destain 

 the cytoplasm without appreciably affecting the appearance of the yolk nucleus. 



After fixation with sublimate and acetic, the reaction of the cytoplasm is very 

 similar, though slightly less intense, to that seen after sublimate fixation. In 

 Hermann material, however, it is much weaker than with either of the above 

 reagents, the eggs at the commencement of yolk formation staining a faint blue, 

 but the yolk nucleus, though not so intensely stained as after sublimate fixation, 

 is still deeply basophile. In material fixed with bichromate and acetic there is 

 no indication whatever of a basophile staining reaction of the cytoplasm at the 

 commencement of yolk formation, the eggs at this period actually staining more 

 faintly than at any other time, but the yolk nucleus stains as intensely as in 

 sublimate material. 



The interest attaching to this variation in the staining capacity of the cyto- 

 plasm, according to the nature of the fixing reagent employed, as emphasizing 

 the distinct nature of the causes determining this reaction on the part of the yolk 



