34 



PROTOZOA 



isi 



\-.in 0. 



Conjugation. — The nuclear elements of Paramecium are of two 

 kinds. The nutritive or somatic nucleus (macronucleus), consider- 

 ably larger than the reproductive or sexual nucleus (micronucleus), 

 is responsible for the meta- 

 bolic activities of the animal. 

 If a Paramecium is cut in 

 two, the part containing the 

 two nuclei will survive, but 

 the other half will die. The 

 reproductive nucleus as- 

 sumes importance in conju- 

 gation and sexual reproduc- 

 tion and the nutritive nucleus 

 then temporarily disappears 

 (Fig. 1 1 ) . Parameciu7n aurelia 

 has two reproductive nuclei. 

 Preliminary to conjuga- 

 tion the oral surfaces of two 

 animals are united by a pro- 

 FiG. \iA. Lacry- toplasmic bridge. The re- 

 maria sp. productive nucleus goes 



through the maturation pro- 

 cess, divides twice formingfour gametes, three of 

 these degenerate, the survivor divides, forming 

 a resident/^;w«/^ pronucleus and a j-;;2^//migrant 

 male pronucleus . Each male pronucleus passes 

 across the protoplasmic bridge into the endo- 

 plasm of the other animal and fuses with the 

 female pronucleus, thus fertilizing it, forming 

 the zygote nucleus. After mutual fertilization, 

 the fusion nucleus of each animal divides by 

 indirect division (see p. 500 for mitosis) into 

 two and finally into eight nuclei equal in size. 

 Four of these develop into nutritive or somatic 

 nuclei, and four into reproductive or sexual ambiguum. (After Stem. 



nuclei. Transverse divisions (simple fissions) F''^";^ Calkins 5/./o^j oj 

 . . . . , 1 • 1 . . the Protozoa. Courtesy 01 



result m four animals, each with a nutritive L^a and Febiger ) 



and a reproductive nucleus. 



Conjugation has been supposed to be necessary to prevent 



degeneration of Protozoa, but experiments previously mentioned 



i! 



Fig. \iB. Spirostomum 



