500 JOHNSON. [Vol. VIII. 



The double nuclear apparatus of Stentor probably expresses 

 the extreme of differentiation between micro- and meganuclei. 

 The latter are highly modified as to structure and, as regards 

 the higher Stentors, as to form ; the former are simplified as 

 much as possible, and reduced to a size so minute that, even 

 with the highest powers, they appear as hardly more than mere 

 specks lying on the surface of the immensely greater mega- 

 nucleus. 



Generalia. — There are few phenomena in cytology more 

 interesting than the apportionment of nuclear functions in the 

 Infusoria to two kinds of nuclei differing in size, structure, and 

 mode of division. The meganucleus is comparable to certain 

 somatic nuclei of the Metazoa that have become highly 

 specialized to subserve some particular function ; e. g., the 

 "giant nuclei" of gland- and excretory cells, found mainly 

 among the Arthropods. The likeness between the two, as 

 shown by the "massive," granular structure, proneness to 

 assume unusual shapes, and amitotic division, are very striking, 

 and strongly suggest a similarity of function. The micro- 

 nucleus corresponds to a metazoan germ-nucleus, and is the 

 bearer of the " immortal " germ-plasm. The Infusorian, then, 

 contains within the scope of a single cell both somatic and 

 germinal elements. Through the exchange and copulation of 

 micronuclei at time of conjugation, the perpetuity of the 

 somatic part is assured in much the same way that the endless 

 new generations of a Metazoan, each with its wonderful somatic 

 development, are made possible by the periodic activity of the 

 germ-plasm, likewise brought into play by the union of a male 

 and female pronucleus, 



B. Fission. 



The important process of self-division, or fission, has received 

 more attention in the genus Stentor than in any other group of 

 Infusoria. We have, in the first place, the surprisingly clear 

 and accurate account by Abraham Trembley ; ^ we have the 

 descriptions and figures by Ehrenberg ('38), Balbiani ('60), 

 Stein ("67), Moxon ('69), Cox ('76), and Schuberg ('9o). The 



1 Phil. Trans. Roy. Soc. xliii, No. 474, p. 180, 1744. 



