PBOTOZOA 28 



the Ciliates during conjugation the nucleus undergoes an 

 elaborate series of divisions, and of the products only one 

 belonging to each gamete migrates and fuses with another 

 belonging to the other gamete ; all the others are absorbed. 

 The cytoplasm does not appear to suffer much change, and in 

 Paramecium, after the nuclear reduction has occurred, the 

 conjugating individuals separate and the original cytoplasm 

 and its orientation are adopted. Moreover, during the phases 

 of growth and cell division which follow, the cytoplasm is 

 divided, together with the nuclei, and the polarity expressed 

 by the cytoplasm is carefully preserved. 



Successive divisions 

 of the two 

 micronuclei 



Fusion 



Reconstruction 



FIG. 9. Diagram to illustrate the nuclear changes during the conjugation of 

 Paramecium. The nuclei which disappear are shown by circles, the 

 surviving nuclei by shading. 



Conjugation is by no means universal. Certain of the 

 primitive Protozoa have found a means of adjustment with- 

 out it, and in many lowly examples chromatin material either 

 has not been found or has a transient appearance. But the 

 more modified Protozoa have become so specialised as to 

 accumulate chemical differences during cell divisions which 

 have to be neutralised by conjugation. 



Distribution. Attention has already been directed to the 

 wide dispersion of Protozoa by wind and current. They form 

 an important part of the floating life (plankton) of fresh waters 

 and the sea, and many species occur in both. In consequence 

 the monographs which have been published on Protozoa may 

 be used for the identification of gatherings of plankton almost 

 anywhere. The student who wishes to pursue the subject in 



