Cut exhibiting modifications that affect the accessory nucleus. Referred to on page 26 of this number of The Monist, in M. Binet's article "The Immortality of Infusoria."

[Illustration: CONJUGATION OF CHILODON CUCULLULUS.]

A, beginning of conjugation; b, mouth; n, nucleus; nu, nucleolus; v. c., multiple contracticle vesicles.

B, division of the nucleolus into two segments, nu', nu'; the nucleus n begins to show signs of regression.

C, each of the two individuals in conjugation contains two nucleolar segments, brought near together, of which one probably comes from the individual opposite by course of exchange, and will fuse with the segment not exchanged, to form a compound segment (Maupas).

D, division of the segment into two portions which grow to unequal sizes; the larger, nn, will become the new nucleus, the smaller, the nucleolus of the new formation, nun.

E, the old nucleus, n, reduced to a small pale and rumpled mass, is replaced by the new nucleus nn, near by which is seen the new nucleolus nun.