NUCLEOLAR STRUCTURES OF THE CELL. l6l 



from the nucleus would speak against any direct connection of 

 nucleolar and secretion substances. The nucleus evinces great 

 metabolic activity, and so would appear to play an important 

 part in the formation of the secretion ; but for the production 

 of such activity an intense process of nutrition is necessary, 

 and it would seem to me probable that the nucleoli are in 

 some sense by-products of this nutritive metabolism. There 

 are few other observations on nucleolar changes accompanying 

 secretion stages ; but reference may be made to those of Nuss- 

 baum on gland cells of Arguliis, to the effect that increase in 

 the number of nucleoli is produced by starvation. 



4. Ontogenetic Origin. — How and where does a nucleolus 

 make its first appearance within the nucleus } Obviously it 

 would be incorrect to attempt to decide this point by a study 

 of the reformation of nucleoli within a daughter-nucleus after 

 mitosis, since the objection might well be raised that in such 

 a case nucleolar material discharged into the cytoplasm during 

 mitosis might be taken up again after mitosis, and accordingly 

 that the first origin of this substance is not explained. 



In studying this point I was so fortunate as to find a fit- 

 ting object in the germinal vesicles of Nemerteans. In all the 

 Metanemerteans examined, but not in Linens, the germinal 

 vesicle appears to arise by the increase in size of a nucleus of a 

 connective tissue — a tissue of branching cells of an embryonal 

 character, which has been termed by me " mesenchym." These 

 nuclei of the undifferentiated tissue show no trace of nucleoli. 

 By increase in size of a nucleus and its cell body, and by the 

 rounding off of the latter, an ovarial ovum (first ovocyte) is 

 formed. Now, when the latter has attained a certain size, nu- 

 cleoli make their first appearance, and at first are always closely 

 applied to the inner surface of the nuclear membrane. Simul- 

 taneously the yolk globules begin to segregate within the cyto- 

 plasm ; yolk and nucleoli show nearly the same chemical reaction. 

 But one essential point remains to be emphasized : the yolk sub- 

 stance, when it can first be differentiated by suitable stains, is 

 found to be at some distance from the 'nucleus. These facts 

 would seem to lead to the conclusion that in these cases the 

 nucleolar substance is extranuclear in origin, and that it may 



