56 HANDBOOK OF PROTOZOOLOGY 



produces eight nuclei, of which seven degenerate and the re- 

 maining one divides once more. In the stalked individual the 

 micronucleus divides twice, forming four nuclei, of which three 

 degenerate, the other dividing into two. During these changes 

 the cytoplasm of the two conjugants fuses completely. The 

 wandering nucleus of the smaller conjugant unites with the 

 stationary nucleus of the larger conjugant, the other nuclei 

 degenerating. The single nucleus thus produced divides several 

 times to form a number of nuclei, from some of which macro- 

 nuclei are differentiated. Later the organism undergoes divi- 

 sion. 



Meiotic Division 



In the foregoing paragraphs, references have been made to 

 the divisions which the nuclei undergo prior to fusion. In all 

 Metazoa, during the development of the gametes, the gameto- 

 cyte nuclei undergo a reduction division, or meiosis, by which 

 the number of chromosomes is halved; that is, each fully 

 formed gamete possesses one-half the number of chromosomes 

 typical to the species. The zygote stage restores the number. 



In the Protozoa, meiotic division takes place presumably 

 prior to or during sexual reproduction, but the process is under- 

 stood only in a few species. Among the ciliates in which con- 

 jugation is common, meiosis seems to take place in the second 

 micronuclear division, although in a few forms, for example, 

 Oxytricha fallax, according to Gregory, the actual reduction 

 takes place during the first division. Prandtl (1906) was the 

 first to note the reduction in number of chromosomes in the 

 Protozoa. In Didinium nasutum, he observed sixteen chromo- 

 somes in each of the daughter nuclei during the first division, 

 but only eight in the second division. Since then, the fact that 

 the reduction is accomplished at the time of the second micro- 

 nuclear division has been confirmed in Chilodon uncinatus (four 

 to two chromosomes) by Enriques in 1908 and by MacDougall 

 in 1925; in Carchesium polypinum (sixteen to eight; Popofif, 

 1908); Uroleptus mohilis (eight to four; Calkins, 1919). In 

 various species of Paramecium and other ciliates, the number of 

 chromosomes is so large, possibly more than one hundred, that 

 it has been impossible to observe the stage of actual reduction. 



