ONTOGENESIS 143 



free-swimming individuals (see fig. 54) come to a 

 temporary resting position with one flagellum touch- 

 ing solid matter. They then sway toward each 

 other, meet, and fuse into one. Their flagella dis- 

 appear, and a thick covering or cyst is secreted about 

 the fused mass. After a period of rest, the contained 

 protoplasm fragments into a number of spores, or 

 into individuals like the parents, which escape from 

 the cyst as typical " monads " of a smaller size. 

 Other Bodos show a certain difference in size between 

 individuals of the same culture. Conjugation occurs 

 apparently at random between these dissimilar in- 

 dividuals and between similar ones. 



It is perhaps more usual for the fully developed 

 microorganism, instead of fusing directly with 

 another similar individual, to fragment into a 

 number of minute, active, individuals called micro- 

 gametes, which conjugate with one another. The 

 latter procedure must be of advantage to the species, 

 since on account of the much larger number of new 

 individuals produced at one time, the race is not so 

 likely to be exterminated. 



One of the most primitive examples is Stephano- 

 sphcera, which consists of a colony of eight flagellate 

 individuals (fig. 55) arranged in a plate within a 

 gelatinous sphere of which they form the equator. 

 In reproduction, each cell of the colony divides into 

 sixteen or thirty -two smaller individuals (gametes), 

 all of the same size, which break out of the gelati- 

 nous-sphere, swim away, and conjugate, two by two, 

 to form a zygote. Each zygote divides into four 



