78 



BULLETIN 120, UNITED STATES NATIONAL MUSEUM. 



nuclei of other organisms. It is this observation, and the compara- 

 tive nuclear studies thus suggested, which give the key to the inter- 



a 



Fig. 48. — Protoopalina mitotica : a, an ixdividdal of the usual sort, X 460 



DIAMBTERS ; 6, A QUADRINUCLEATB INDIVIDUAL WHICH HAS, PROBABLY ABNORMALLY, 

 DELAYED ITS FISSION, X 460 DIAMETERS ; C, A DAUGHTER CELL, X 360 DIAMETERS ; rf, 

 ITS NUCLBUSj X 1,485 DIAMETERS ; C, NUCLEUS OF ANOTHER DAUGHTER CELL, WHICH 

 SHOWS THB MACROCH BO MATIN SPINDLE FIBERS. (C, d, AND C, AFTER MBTCALF, 1912.) 



pretation of speciation developed in the present paper, and it is the 

 nuclear conditions found in these comparative studies, which give 

 rise to the suggestions in section 6 of this paper, as to the origin of 



