Opalina mitotica. 85 
nuclei half divided and resting in this mid-mitotie condition, each 
nucleus being dumbbell-shaped and with its chromatin in a late 
anaphase of mitosis. A very slight advance in nuclear division 
before coming to rest would give the quadrinucleate condition, as 
in O. lanceolata. 
In a previous paper I have expressed the opinion that the 
binucleate condition of the four binucleate species then known was 
due to a delay in the division of the body,!) the nuclei dividing 
and the cell-body remaining for a long time undivided, its division 
oceurring later just as the two nuclei are preparing to enter upon 
their next division. In ©. mitotica the two nuclei actually enter 
upon this second division and almost complete it before coming to 
rest, and the cell body still remains undivided. Its division takes 
place just as the second nuclear division is completed, not when it 
is beginning as in O. intestinalis. In O. lanceolata the division of 
the body is still further delayed. The two nuclei not only enter 
upon but complete the second division, until four nuclei are formed, 
and still the body remains undivided. The division of the body 
does not oceur until these four nuclei enter upon the third division. 
The quadrinucleate condition is thus characteristic of this species. 
The multinucleate condition of other species seems to be due to a still 
further delay in the division of the body, while nuclear division con- 
tinues, resulting in many nuclei within one body. To be sure the 
delayed divisions are merely delayed, not entirely suppressed, for, in 
the spring, division of the body takes place rapidly, more rapidly than 
division of the nuclei, until finally uninuclear gametes are formed. 
The Opalinas are only temporarily binucleate. The higher 
Orliata have become permantly binucleate by the complete suppression 
of one division of the cell body. The permanent binucleate condi- 
tion of the higher Ciliata allows differentiation between the nuclei, 
one being active in nutrition (the macronucleus), the other (the 
micronucleus) remaining comparatively inactive chemically, but awa- 
king to activity when division oceurs. The micronucleus is the sole 
carrier of heredity, for the macronucleus takes no share in the 
sexual phenomena, but degenerates. It is evidently easier to form 
1) “It seems probable that the Opalinas were originally uninucleated, 
that the binucleated condition was brought about by the suppression of 
one division of the body when the nucleus divided, and that the multi- 
nucleated condition is still more secondary, being due to further suppression 
of divisions of the body’ (METCALF, 1909, p. 308). 
