586 FERTILIZATION 



the other divides again into two very unequal parts. The smaller de- 

 generates and the larger is the functional pronucleus, which fuses with 

 the pronucleus of the other member of the pair. Superficially, this type 

 of reduction is strikingly similar to polar body formation in metazoan 

 oogenesis, although the actual reduction in chromosome number was 

 not established by Dobell. The assumption that the two nuclear divisions 

 preceding syngamy are reduction divisions seems reasonable, consider- 

 ing the almost universal occurrence of two divisions in the maturation 

 of gamete nuclei. However, we are not justified in concluding that re- 

 duction in chromosome number occurs, unless positive determinations 

 can be made of the chromosome number before and after reduction. In 

 Copromonas there is obvious reduction in the amount of chromatin 

 when all but one product of the two divisions disintegrate; but reduc- 

 tion in chromosome number has not been demonstrated, although it 

 must occur somewhere in the life cycle, if chromosomes exist in this 

 species. From the genetic angle, this is a critical point and should be 

 determined if possible. After syngamy the zygote may encyst or it may 

 develop directly into a vegetative form. 



Hologamous fertilization has been reported in a few other members 

 of the Plasmodroma, but is not confined to this group if Brumpt's 

 (1909) description of fertilization in the parasitic ciliate Balantidium 

 coli is correct. In this case two individuals come together and are in- 

 vested by a common membrane, as in pseudoconjugation of gregarines. 

 But the two balantidia then fuse completely and permanently. Other 

 workers have not supported Brumpt's description and, although the 

 details difi^er with species and author, Jameson (1927), Scott (1927), 

 and Nelson (1934) agree that conjugation and not copulation is the 

 form of sexual union found in this ciliate. 



The occurrence of fertilization has been reported for a number of 

 flagellates, but in very few cases has the evidence been convincing ex- 

 cept for the Phytomonadida. Goldschmidt (1907) gave a detailed de- 

 scription of a sexual cycle in the animal flagellate Mastigella, similar 

 in type to those described for Arcella and other Rhizopoda. If substanti- 

 ated, another close link between the Pantostomatida and the Rhizopoda 

 will be established. According to Goldschmidt, vegetative forms de- 

 velop into macrogametocytes and microgametocytes, the nuclei of which 

 give off chromatin into the cytoplasm. These chromidia in turn produce 



