164 



FERTILIZATION OF THE OVUM 



of conjugation is a union of the nuclei of the conjugating cells. 

 Among the unicellular plants both the cell-bodies and the nuclei 

 completely fuse. Among animals this may occur ; but in many of 

 the Infusoria union of the cell-bodies is only temporary, and the con- 

 jugation consists of a mutual exchange and fusion of nuclei. It is 



Second fission. 



First fission, alter separation. 



Difierentiation of micro- and 

 macronuclei. 



Separation of the gametes. 



> Division of the cleavage-nu- 

 cleus. 



Cleavage-nucleus. 



Exchange and fusion of the 

 germ-nuclei. 



Germ-nuclei. 



, Formation of the polar bodies. 



Union of the gametes. 



Fig. 81. — Dingrnm showin? the history of the mictonuclei during the conjugation of Para- 

 vimciuin. [Modified from MauPAS.] 



A' and 1" represent the opposed macro- and micronuclei in the two respective gametes; circles 

 represent degenerating nuclei ; black dots, persisting nuclei. 



impossible within the limits of this work to attempt more than a 

 sketch of the process in a few forms. 



We may first consider the conjugation of Infusoria. Maupas's 

 beautiful observations have shown that in this group the life-history 

 of the species runs in cycles, a long period of multiplication by cell- 

 division being succeeded by an "epidemic of conjugation," which 

 inauo-urates a new cycle, and is obviously comparable in its physio- 



