No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 497 



Lineus gesserensis. — Cells of the first type: one or two small 

 nucleoli. Cells of the second type : one nucleolus. Cells of 

 the third type : a single nucleolus, or two of unequal size. 



Cerebratuhis lactens. — Cells of the first type : as in the 

 preceding species. Cells of the second type : one or two 

 nucleoli. Cells of the third type : one or two nucleoli, which 

 in one case stained differently. Cells of the fourth type : 

 usually one peripheral nucleolus ; rarely are two present, and 

 then they are unequal in dimensions. 



In all these cells the nucleolus is comparatively small, homo- 

 geneous, and no evidences of nucleolar division were seen. 



IV. GENERAL COMPARISONS AND CONCLUSIONS. 



Here I shall summarize merely the results of my observations 

 on the nucleolus, and compare them with the conclusions of 

 other investigators. Numerous other morphological points 

 have been brought up, however, in the. preceding pages, such 

 as yolk development, differentiation of ova, nuclear divisions, 

 distribution of the chromatin elements in the germinal vesicle 

 at different stages in the growth period, changes in the struc- 

 ture of cytoplasm, etc. 



I. CJiemistry of the Nucleolus. 



I have made no special chemical study of these structures, 

 except what may be learned from their reactions to stains. In 

 the gregarines no substance could be demonstrated which chemi- 

 cally corresponds to the chromatin of the metazoan cell ; ^ but the 

 following table represents the mode of staining of true nucleoli 

 in the somatic and grerm cells of the Metazoa : 



Schwarz ('87) distinguishes in plant cells pyrenin, the sub- 

 stance of the true nucleoli, from the other nuclear substances 



^ That is, not to chromatin in the form of pure nucleic acid. 



