FERTILIZATION 



283 



nucleus divides, forming a large vegetative nucleus and a smaller gener- 

 ative nucleus. The former moves to the surface of the cell and degener- 

 ates, while the latter gives rise to the gamete nuclei in the following 

 manner. After becoming elongated and incompletely divided it buds 

 off four "reduction nuclei" two from one end and then two from the 



FIG. 113. Conjugation in Paramcecium. (After Morgan, 1913.) 



other. It then completes its division to form the two gamete nuclei, 

 or pronuclei, while the four reduction nuclei degenerate. After lying 

 apart for some time the two pronuclei approach each other and fuse ; 

 sexual union thus takes place between sister nuclei. 



The complicated nuclear behavior occurring at the time of conjugation 

 in Paramcecium caudatum (Fig. 113) may best be described in the words 



