No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 413 



may conclude, then, that when the nucleus is comparatively 

 small, and when no yolk or only small yolk globules are pres- 

 ent in the cytoplasm, the nucleus derives a nutritive substance 

 from the cytoplasm, which is closely similar to that composing 

 the youngest yolk globules ; but when the nucleus has grown 

 large, and the cytoplasm is packed with large yolk globules, it 

 has the power to take up these larger globules also.^ 



To return, then, to the second stage of nucleolar differentia- 

 tion. This stage does not commence when the nucleolus has 

 attained a certain size, but may commence in some nucleoli 

 earlier than in others; and again it is not marked by a particular 

 stage of development of the yolk in the cytoplasm. The fluid 

 vacuoles probably stand in a genetic relation to the small 

 nutritive globules found in the nucleus, which have been 

 just described. That is, these globules of the 'nucleus pene- 

 trate into the nucleolus and then constitute the fluid vacuoles 

 of the latter structure. I have reached this conclusion after 

 observing that the vacuoles of the nucleolus and the small 

 nutritive globules within the nucleus always stain in exactly 

 the same way. This assumption is further strengthened by 

 the fact that, when the nutritive globules lie in the nuclear 

 sap at some distance from the nucleolus, they have invariably 

 a spherical form ; but in those numerous cases where they may 

 be seen apposed to the outer surface of the nucleolus they 

 become flattened against the surface of the latter, as if the 

 nucleolus were (figuratively speaking) a loadstone which 

 attracts them to itself (Figs. 63, 69, 75). If this origin of 

 the vacuoles of the nucleolus were not the true one it would 

 be difficult to explain their mode of genesis, since there appears 

 to be no other substance within the nucleolus from which they 

 could be derived, and there is no reason for supposing that the 



1 The intensity in the staining of the yolk globules increases with their size, and 

 the largest stain much more deeply than does the nucleolus. During all the 

 earlier growth stages the nuclear membrane is retained, and it is seldom, and 

 then only slightly, irregular in outUne ; therefore the yolk cannot be taken up by 

 the mechanical aid of amoeboid processes of the nucleus, but its substance must 

 osmotically penetrate the nuclear membrane. And as I mentioned above, it does 

 not seem probable that the yolk globules retain their shape while penetrating this 

 membrane, but diffuse through it in the form of an irregular fluid mass, and then 

 in the nucleus this fluid becomes re-formed into globules. 



