SOME PROBLEMS OF REPRODUCTION. 45 
ea —vim 
r eet 
Siig ot 
re 
tad — 
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Be { 
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Fic. 6.—Schemaof formation of pronuclei in a conjugating Ciliate. »= original 
micronucleus of one of the gametes. ps and w3m = the stationary and 
migratory (female and male) pronuclei respectively. v'm = migratory 
pronucleus from the other conjugate, which lies the other side of the 
dotted line. The pronuclei are supposed to unite in the square Z. The 
dash above indicates rejection-nuclei. 
Some Ciliates have habitually two micronuclei; in this case 
both undergo the first two mitoses to form eight nuclei (u?), 
and seven of these abort, leaving only one to undergo the final 
mitosis; or, again, one of the two micronuclei undergoes the 
first mitosis only before its brood abort. 
The conjugation-nucleus undergoes at least two mitoses, 
and of the four nuclei so formed in the simplest cases two 
become mega- and two micro-nuclei, and at the first fusion of 
the exconjugate one mega- and one micro-nucleus pass to each 
daughter-individual. Though this is the easiest type to 
understand it is not the commonest, but the processes, though 
of interest, are complex and too remote from our subject. 
Some of the progeny of the zygote-nucleus in certain species 
are eliminated as rejection-nuclei—a very significant fact. 
The main peculiarities of the conjugation in the Ciliata 
(apart from the formation of rejection-nuclei, to which we shall 
return) are—(1) the formation of two fertile pronuclei; (2) 
the separation of the gametes after karyogamic union of the 
nuclei has taken place without any transference of cytoplasm. 
After noting that here, at least, conjugation is an essentially 
nuclear process, we proceed to infer from the coexistence of 
conditions 1 and 2 that they are correlated phenomena. Let 
us consider the process after the second mitosis and elimina- 
tion of the three rejection-nuclei. 
