FERTILIZATION 623 



nuclei and the macroconjugant only female functional pronuclei. The 

 other pronuclei are produced as usual, but fail to develop. Similarly, 

 in Metopus signwides (Noland, 1927) one conjugant contributes all of 

 its potentialities to the other, in what may be interpreted as a male ani- 

 mal contributing its life and all its potential gametes to the female. 



The Macronucleus during Conjugation 



In ciliates, the micronucleus is concerned with sexual activity and 

 reproduction and is therefore frequently referred to as the generative 

 or reproductive nucleus and represents the "germ plasm" of the Metazoa. 

 The macronucleus, on the other hand is concerned with metabolism or 

 vegetative activity and is considered the trophic or vegetative nucleus 

 and represents in part the somatoplasm of the Metazoa. The chromatin of 

 these two types of nuclei is combined in the single nucleus of other cells 

 and in the one kind of nucleus found in the multinucleate Opalina. 



The disintegration of the macronucleus at the time of conjugation, 

 in all ciliates with dimorphic nuclei, represents the death of the soma 

 and the end of the genetic unit. Differentiation of the new macronucleus 

 in exconjugants similarly represents the development of the new somatic 

 individual from the zygote. 



In ciliates generally, the old macronucleus shows signs of disintegra- 

 tion during the maturation divisions, and, by the time of crossing of the 

 pronuclei, fragmentation or other evidences of disintegration are well 

 under way. It is during the differentiation of the new macronucleus from 

 the synkaryon that the most rapid breakdown and the final absorption 

 of the old macronucleus occur. This is probably due to the withdrawal 

 from the cytoplasm of all chromatin-building elements by the developing 

 macronuclear Anlage or "placenta." The old macronuclear remnants are 

 possibly used as "fertilizer," or reserve of chemical elements in about the 

 right proportion, for replenishing the cytoplasm and maintaining the 

 equilibrium. 



Before disintegration, the old macronucleus exhibits strange activity 

 in several species of Anoplophrya (Schneider, 1886; Collin, 1909; 

 Brumpt, 1913; Summers and Kidder, 1936), and in two species of 

 Chilodonella (MacDougall, 1936). At about the time of crossing of 

 the pronuclei, the macronucleus in each conjugant elongates and con- 

 stricts in the middle, as one half pushes across the protoplasmic bridge 



