REPROD UCTION 53 



tozoa and ova of the Metazoa. The microgametes are actively 

 motile, relatively small and numerous, while the macrogametes 

 are non-motile as a rule, much larger but fewer (Fig. 25). In 

 Chlamydomonas (Fig. 37), according to Goroschankin, the two 

 gametes begin to fuse at their anterior ends. The flagella are 

 lost and the cytoplasmic and nuclear fusions take place. A new 

 body wall is secreted around the whole. In Volvox, the two 

 gametes may be produced in the same colony, as in Volvox glo- 

 bator, or in different colonies, as in V. aureus and V. perglohator. 

 The union of gametes results in the formation of a zygote. Pan- 

 dorina and Eudorina show similar changes. In the Sporozoa, 

 anisogamy has been observed in numerous species of Eimeria 

 (Fig. 109), Plasmodium (Fig. 120), etc. In the Cnidosporidia, 

 some cases of anisogamy have been reported by certain investi- 

 gators, while others report negative findings. Anisogamy has 

 been suggested to occur in some Amoebaea, particularly in En- 

 damoeba blattae, by Mercier, but this awaits confirmation. 



Conjugation 



Conjugation is a temporary fusion of two individuals of one 

 and the same species for the purpose of nuclear exchanges. This 

 process is found almost exclusively in the Ciliophora. The two 

 individuals may be similar or dissimilar morphologically. The 

 former case is called isogamous conjugation, and the latter 

 anisogamous conjugation. The former appears to occur more 

 commonly than the latter. 



In Paramecium caudatum (Fig. 26) two individuals come in 

 contact on their oral surfaces. The micronucleus in each conju- 

 gant divides twice (b-e), forming four micronuclei, three of 

 which degenerate and do not take active part during further 

 changes (f-h). The remaining micronucleus divides once more, 

 producing a wandering nucleus and a stationary nucleus (/, g). 

 The wandering nucleus in each of the conjugants enters the 

 other conjugant and fuses with its stationary nucleus {h, r). 

 The two zygotes thus formed become separated from each other 

 and are called exconjugants. In each of them, the micronucleus 

 divides three times in succession {i-m) and produces eight nuclei 

 (w), four of which remain as micronuclei, while the other four 

 develop into macronuclei (o). Cytoplasmic fission follows then, 



