144 THE PROTOZOA 



In Protozoa the chromosomes are seldom so sharply defined as 

 in Metazoa, and consequently it is difficult or impossible to deter- 

 mine their number. Many cases in wnich a fixed number of 

 chromosomes is alleged to occur, as in Trypanosoma noctuce (Schau- 

 dinn, 132), cannot be accepted without question in the present 

 state of our knowledge. On the other hand, in all groups of the 

 Protozoa, where the sexual processes have been carefully studied, 

 the union of the gamete-nuclei has been found to be preceded in 

 a great many cases by two successive divisions of each nucleus, with 

 one or the other of the following results : either the successive 

 formation of two reduction-nuclei,* which are cast out of the cell 

 or absorbed without dividing further, while the third persists as 

 the pronucleus of the gamete ; or the production of four nuclei, all 

 of which, or only one of them, persist as pronuclei. These reducing 

 divisions in Protozoa suggest forcibly a comparison with those of 

 the Metazoa, and from this analogy it may be further inferred that 

 in Protozoa also the chromatin of the conjugating pronuclei has 

 undergone a reduction to half the specific quantity ; but it is 

 seldom possible to confirm this inference by accurate enumeration 

 of the chromosomes. In the case which has been the most care- 

 fully studied of all others, that, namely, of Actinosphcerium, Hertwig 

 (64) found the number of chromosomes in the first reduction- 

 spindle to be between 120 and 150 ; in the second reduction-spindle 

 the nuniber was about the same, but the chromosomes were about 

 half the size of those in the first reduction-spindle. Moreover, in 

 both the reducing divisions of Actinosphcerium the chromosomes 

 in the equatorial plate divide to form the daughter-plates, as in 

 ordinary karyokinesis, whereas in the reducing divisions of Metazoa 

 the individual chromosomes are not divided, but merely sorted out. 

 Hence it would appear that in Actinosphcerium, and probably many 

 other Protozoa, the reduction of the chromatin in the pronuclei 

 is effected by more direct, though perhaps less exact, methods 

 than in the highly-perfected process seen in the Metazoa. 



Nevertheless, a few cases are known among Protozoa in which 

 the small number of chromosomes permits of their being accurately 

 counted, and in which they are seen to be reduced to half the usual 

 number in the maturation-divisions of the gametes. In Pelomyxa 

 the first division reduces the chromosomes from eight to four ; the 

 second division, however, is equating, and no further reduction 

 takes place (p. 150). In some Infusoria it has been observed that 



* These reduction-nuclei are sometimes termed " polar bodies," by analogy 

 with the maturative process of the Metazoan ovum, but the term is to be avoided 

 in this connection, as it places upon these divisions an interpretation which is at 

 least highly doubtful ; the polar bodies of Metazoa are sister-cells of the ovum ; 

 the reduction-bodies in Protozoa are simply nuclei which are extruded or absorbed. 

 It is certainly not justifiable in fact, and probably no more so in theory, to regard 

 their formation as abortive cell-division. 



