386 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



nucleolar constituent is concerned, the mode of growth is by passive deposition 

 rather than by osmotic diffusion. 



There are two points concerning the young nucleolus to which attention may be 

 directed: The first concerns its position. In the resting oogonium the nucleolus 

 appears to be alwaj's peripheral in position, whereas in the actively growing oocyte 

 it is for some time invariably central — a fact which scarcely points to the peripheral 

 position of the nucleolus as denoting a cytoplasmic origin for this structure. 



The second point concerns the close relation which the young nucleolus in- 

 variably shows to the chromatin. Both the peripherally placed nucleolus of the 

 resting oogonia and the centrally placed nucleolus of the actively growing ooc^^e 

 are surrounded hj a portion at least of the chromatin. In the oogoniiun this rela- 

 tion is maintained until preparation for mitosis leads to the withdrawal of the 

 perinuclear chromatin. In the young oocyte the gradual subdivision and more 

 equal distribution of the chromatin which accompanies the increasing metabolism 

 of the growing cell more speedily leads to the same result. The fact that in the 

 young oocyte, at a stage which can be but very little removed from the anaphase 

 stages of the preceding oogonial division, there should already be a relatively large 

 nucleolus surrounded by a thin laj'er representing the whole of the chromatin of 

 the cell emphasizes the importance of this relation between the nucleolus and the 

 chromatin. It is evident, however, that though this arrangement may facilitate, it 

 can not be essential to, the formation of the nucleolar substance, for in the growing 

 oocyte the nucleolus more than trebles its diameter after it has lost all direct rela- 

 tion with the chromatin. 



On the other hand, the later behavior of the nucleolus in the oocyte of 

 Antedon offers considerable evidence of a highly suggestive character relative 

 to the significance of this structure in the cell metabolism. One of the most 

 i-triking morphological features which the nucleolus invariably presents is its 

 sphei-ical, or appi-oximately spherical, form — a form in which the minimum 

 surface area is associated with maximum bulk. In this respect the nucleolus 

 offers an instructive contrast to the chromatin, Avhere it is an obvious endeavor 

 to assume a form with as great a sujierficial area as possible. It is impossible not 

 to conclude that this difference between the nucleolus and chromatin indicates the 

 nature of the difference in their physiological significance ; that while the chromatin 

 is active, and probably selectively so. taking what it requires from the caryolymph, 

 the nucleolus, at least in so far as getting fresh material is concerned, is passive, 

 the nucleolar constituents being deposited on, or diffusing through, its surface 

 without any effort on the part of the nucleolus itself. It has already been seen 

 that the further changes which the nucleolar substance undergo take place most 

 actively within the interior of the nucleolus and give no indication that a reaction 

 between the nucleolar substance and the caryolymph is essential to the process. 



Although the immediate source for both the nucleolar constituents must be 

 the caryolymph, yet their very different behavior when nucleolar activity ceases 

 would seem to indicate their derivation from distinct ultimate sources. There 

 are three possible sources for these nucleolar constituents, namely, the chromatin, 



