PHYLUM PROTOZOA 51 



within the same genus. Thus in Paramecium, the common 

 species P. caudatimi has a single micronucleus, P. aurelia has 

 two, P. polycarium has four, and P. multimicronucleata may have 

 as many as eight. In spite of its importance in normally direct- 

 ing reproductive processes, the micronucleus is not an absolute 

 essential in cell life, for amicronucleate races of Paramecium, 

 Didinium, and other ciliates have been cultured for long periods. 



Reproduction is by binary fission, usually in the free state 

 but in some forms accompanying encystment. Preparatory to 

 fission the micronucleus divides by mitosis and the macronucleus 

 by amitosis. In several ciliates, chromosomes of a fixed number 

 have been observed in the dividing micronucleus though never 

 in the macronucleus. Mouth and other structures are frequently 

 duplicated in the dividing individual before fission is completed. 



Conjugation of varied forms occurs in this group. It always 

 results in reorganization of the protoplasm involving the absorp- 

 tion of the macronucleus by the cytoplasm and reorganization 

 of a new macronucleus and micronucleus from products of a 

 fertilization nucleus. The old idea of conjugation as essential 

 to prevent senescence has been shown to be unfounded, for 

 Woodruff has carried a race of Paramecium through 11,000 

 generations without opportunity for conjugation. This he 

 accomplished by keeping isolated individuals under observation 

 and removing one after each body division. 



In the Peritricha, of which Vorticella is an example, a modified 

 form of conjugation between, a microconjugant and a macro- 

 conjugant occurs. By two or three successive divisions, some 

 individuals produce four or eight microconjugants which after 

 acquiring an aboral ring of cilia become free-swimming. One of 

 these microconjugants fuses with an ordinary individual, which 

 is the macroconjugant. In brief, the micronucleus of the 

 microconjugant undergoes division to form eight parts of which 

 seven degenerate and only one persists. Later, this one divides 

 into two. During this same time the micronucleus of the 

 macroconjugant has divided into four parts of which three 

 degenerate and the remaining one redivides to form two nuclei. 

 In both conjugants, the macronucleus has undergone disinte- 

 gration. In each conjugant, one of the two pronuclei which 

 originated from the micronucleus disappears. The remaining 

 nucleus of the microconjugant passes over into the cytoplasm of 

 the macroconjugant and fuses with the single nuclear mass 



