114 CHROMOSOMES DURING SYNGAMY IN PROTOZOA 



less rounded cliromosomes. Each chromosome splits into two, so that 

 the daughter plates and finally the daughter nuclei also contain twenty- 

 two pairs of chromosomes. Each resulting nucleus then undergoes two 

 maturation divisions. In the first of these at the equatorial plate stage 

 there are twenty-two pairs of rounded chromosomes, but when the 

 daughter plates form the chromosomes do not split, as in the preceding 

 nuclear division. One chromosome of each pair passes to each daughter 

 plate, which thus contains only twenty-two chromosomes instead of 

 twenty-two pairs. The process is similar to that shown at Fig. 4, except 

 that in the place of the four chromosomes there are forty-four. Of the 

 resulting nuclei, one degenerates and the survivor divides by mitosis as 

 before. During this division twenty-two chromosomes appear at the 

 equator of the spindle, and each divides, so that each resulting nucleus 

 has twenty-two chromosomes. After union of the gametes, the zygote 

 nucleus has forty-four chromosomes. During all these divisions the 

 chromosomes are long filaments at the commencement of nuclear division, 

 but they gradually retract and finally become roughly spherical, in which 

 form they are arranged as the equatorial plate. 



In connection with the conjugation of ciliates, similar reduction 

 processes have been described. In these Protozoa, as explained above, 

 it is only the micronucleus which takes part in syngamy, the macro- 

 nucleus degenerating. The micronucleus in one individual divides to 

 form two nuclei, and these again to form four. Of these four, three 

 degenerate. The remaining one divides again, so that each of the two 

 associated ciliates contains two nuclei. One of the nuclei in each indi- 

 vidual now passes over to the other and unites with the stationary nucleus, 

 after which the ciliates separate. Here, again, if the number of chromo- 

 somes in the uniting nuclei has not been reduced, it is evident the zygote 

 nuclei will have double this number. Several observers have maintained 

 that the first of the three divisions of the micronucleus is really a reducing 

 division. Hertwig (1889) noted that in Paramecium aurelia, the nucleus 

 of which has a large number of chromosomes during division, the nuclei 

 which unite have approximately half the number of chromosomes seen 

 in the ordinary divisions of the micronucleus during reproduction by 

 fission. Calkins and Cull (1907), in the case of Paramecium caudatum, 

 noted that the number of chromosomes in the ordinary dividing nucleus 

 is about 165. During the first two divisions of the micronucleus during 

 conjugation there is a reduction in the number to about half this. On 

 account of their large number it is difficult to count the chromosomes 

 accurately. Prandtl (1906) found that in Didinium nasutum the first 

 division of the micronuclei during conjugation was associated with the 

 reduction of the chromosomes from sixteen to eight. In CoUinia 



