414 MONTGOMERY. [Vol. XV. 



substance of these vacuoles is a differentiation of the nucleolar 

 ground substance. We may assume, then, that this explana- 

 tion of the genesis of the nucleolar vacuoles is the correct one, 

 and now proceed to explain the changes in the nucleolus dur- 

 ing the successive development of its vacuoles. If we take the 

 size of the nucleolus as a general criterion (though it is not an 

 infallible one, since there are considerable individual differences 

 in different nucleoli (cf. Figs. 62, 65, 80) ) of the stage of 

 the nucleolus, the process of assimilation of the nutritive glob- 

 ules from the nucleus by the nucleolus seems to be in general 

 as follows : first, one or two globules are taken into the nucleo- 

 lus, and later when others (apparently a varying number) are 

 also taken up into it, we reach a stage when the nucleolus 

 contains a number of fluid vacuoles (the assimilated nutritive 

 globules) (Figs. 64 and 70). Then these vacuoles commence to 

 fuse together (Figs. 63, 66, 72), finally by their fusion giving 

 rise to one large vacuole, which fills about three-quarters of the 

 space of the nucleolus, and always lies excentrically within the 

 nucleolus (Figs. 68, 69, 73, "]•], 79). The nucleolus has now 

 attained its greatest dimension and is either perfectly spher- 

 ical, or more usually ovoid in shape. Its large excentric vacuole 

 is encircled by a peripheral layer of the primitive homogeneous 

 ground substance of the nucleolus, which has undergone no 

 structural or chemical change. This layer of ground sub- 

 stance becomes necessarily thinner as the vacuole becomes 

 larger, i.e., as the pressure from within becomes greater. But 

 since the large vacuole lies peripherally, the peripheral sub- 

 stance of the nucleolus remains thickened at that point opposite 

 the vacuole, and this thickened portion of the nucleolar wall 

 has most frequently the form of a concavo-convex lens (or on 

 a cross-section, of a half moon), the concave side of which 

 borders upon the vacuole. This thickened part, as the remain- 

 ing portion of the peripheral layer of the nucleolus at this stage, 

 is in every respect identical with the ground substance of the 

 nucleolus in earlier stages, before vacuoles had made their 

 appearance in it ; and the total amount of the substance of the 

 peripheral layer seems to be equal to the amount of the homo- 

 geneous substance of the nucleolus at the end of the preceding 



