III. CILIATA. 



207 



borhood of the cytostome at right angles to the surface and divides 

 into two nuclei, the superficial being called the wandering or 



FIG. 146. Conjugation in Paramcecium. fc, macronucleus; ?i 



stomes. 



micronucleus; o, cyto- 



I. Changes of micronucleus; left sickle stage, right spindle stage. 



II. Second division of micronucleus into primary spindles (1, 5) and secondary 

 spindles (2, 3, 4; 6, 7, 8). 



III. Degeneration of secondary spindles (2, 3, 4; 6, 7, 8); division of primary spindle 

 into male (1m, 5m) and female spindles (I?/;, 5w). 



IV. Exchange of male spindles nearly complete (fertilization), one end still in the 

 parent animal, the other united with the female spindle, 1m with 5w and 5m with Ito; 

 macronucleus broken up. 



V. The cleavage spindle t formed by male and female spindles dividing into the 

 secondary cleavage spindles t', t". 



VI. VII. End of conjugation. The secondary cleavage spindle dividing into the 

 anlage of the new micronucleus (nfc') and that of the new macronucleus, pt 

 (placenta). The fragments of the old macronucleus begin to degenerate. 



Since P. caudatum shows the earlier and P. aurelia the later stages better, these 

 forms have been used, P. caudatum for I-III, P. aurelia for the rest. The differences 

 consist in the existence of one micronucleus in P. caudatum, two in P. aurelia^ and 

 that in the latter the nuclear degeneration begins in I. 



male nucleus, the deeper, the stationary or female nucleus. The 

 male nuclei of the two copulating animals are exchanged, travers- 



