SYNGAMY AND SEX IN THE PROTOZOA 



145 



the first division of the micronucleus is an equating division, the 

 second reducing ; so in Operculwia (Enriques, 112), Chilodon 

 (Enriques, 113), Carchesium (Popoif, 125), Didinium (Prandtl, 126), 

 and Anoplophrya (Fig. 74) ; in the last named ihe second division 

 of the micronucleus reduces the chromosomes from six to three, 

 and union of the pronuclei brings the number up to six again. In 

 Carchesium the number of chromosomes is reduced from sixteen to 

 eight. A similar reduction-process has been described by Mulsow 

 (123) in gregarines (p. 335). Hence in these cases the pronuclei 



o 



FIG. 74. Behaviour of the micronucleus during successive stages of the con- 

 jugation of Anoplophrya (Collinia) branchiarum. A, Micronucleus of one 

 conjugant preparing for division ; B, later stage, with six chromosomes dis- 

 tinct ; 0, nuclear spindle, with an equatorial plate of six chromosomes ; 



D, diaster-stage, with six daughter-chromosomes at each pole of the spindle ; 



E, later stage, with the chromosomes at each pole fused into one mass ; 



F, G, H, reconstruction of the daughter-nuclei ; the remains of the spindle 

 between them disappears gradually ; /, the two micronuclei preparing for 

 division ; appearance of six chromosomes in each (one nucleus is seen in 

 profile, the other from one pole) ; J, diaster- stages, showing three chromo- 

 somes at each pole of the spindle (reducing division) ; K, later stage, the 

 chromosomes fused into masses.of chromatin ; L, four granddaughter-micro- 



^nuclei ; M, one of them grows in size, the other three begin to degenerate ; 

 N, division of the persistent micronucleus to form the two pronuclei ; 0, two 

 pronuclei and three degenerating micronuclei. After Collin (50), magnification 

 about 2.000 diameters. 



have exactly half the amount of chromatin contained in the ordinary 

 nuclei, just as in the Metazoa. 



Doflein (111) and Hartmann (116) consider that a process of 

 reduction is absolutely essential to the conception and definition 

 of syngamy, and regard reduction as a criterion whereby true 

 syngamic union of gametes and pronuclei can be distinguished 

 from plastogamic and nuclear fusions which have nothing to do 

 with the sexual process. ' No fertilization without reduction ' 

 (Hartmann). But it must be acknowledged that in a great many 

 cases of gamete-formation in Protozoa a reduction of the chromatin- 



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