MONOGRAPH OF THE EXISTING CKINOIDS. 387 



the cytoplasm, and the nutritive plasma of the parent organism by which the 

 egg is bathed. 



Apart from the improbability of the nutritive plasma, as we understand it, 

 exercising a selective action in depositing food material onto the nucleolus, this 

 source for the ground substance of the nucleolus is excluded by the remarkably 

 constant relation which the size of the nucleolus always shows to the stage of 

 the egg's growth, a relation very difficult to explain if the nucleolus passively 

 receives the bulk of its substance directly from a source outside the cell. On the 

 other hand, the cytoplasm is equally excluded as a source for the ground sub- 

 stance of the nucleolus, for this substance continues to be deposited on the surface 

 of the nucleolus after the completion of yolk formation, at a time, therefore, when 

 practically the whole of the cytoplasm has been converted into metabolically 

 inactive yolk spherules. There remains, therefore, only the chromatin as the 

 source for the ground substance of the nucleolus, a source already indicated by the 

 intimate relation this structure shows to the nucleolus in the young oocyte, in which 

 the as yet relatively inactive metabolism permits of a massed arrangement of the 

 chromatin. 



The origin of the basophile constituent of the nucleolus presents more diffi- 

 culty. There is reason to believe that the activity of the chromatin is continued 

 for some time at least after the simultaneous completion of yolk formation and 

 cessation of nucleolar activity. The only structure, therefore, which in its behavior 

 shows a direct temporal relation to the cessation of nucleolar activity is the cyto- 

 plasm. On the completion of yolk formation there is not only an abrupt cessation 

 of nucleolar activity, but there is also a sudden failure in the supply of the baso- 

 phile nucleolar constituent, a fact which certainly points to the cytoplasm as the 

 source for this substance. 



It is not, however, only the completion of yolk formation but also the com- 

 plete cessation of all cytoplasmic activity, which coincides with the termination 

 of nucleolar activity. The production of the basoj^hile nucleolar substance shows 

 no relation to the commencement of yolk formation, but is produced continuously 

 throughout the growth of the egg. This substance must be the result, therefore, 

 not of yolk formation itself but of a much more fundamental vital process of the 

 cytoplasm, a conclusion which is supported by the fact that we already have, in 

 the peripheral spherules presently to be described, an obvious by-product of the 

 actual procass of yolk formation. 



That the nucleolus deals with the waste products of cytoplasmic activity rather 

 than with the nutritive material in the caryolymph is also indicated by the ulti- 

 mate fate of the discharged materinl described below. 



The nucleolus of Antedon would thus appear to be derived from the chromatin 

 and to function as the excretory organ of the cell, the waste products of cyto- 

 plasmic activity undergoing their final changes within its substance and being 

 discharged, now, presumably, inert and harmless, into the cytoplasm, where they 

 slowly dissolve. 



It has been shown that throughout the growth of the oocyte the nucleolus 

 periodically discharges deeply basophile spherules into the caryolymph. It has 



