SUBKINGDOM PROTOZOA. 37 



it contracts to a round mass and surrounds itself with a sec- 

 ond cyst within the first. At the surface of the encysted 

 animal from eight to thirty minute spherical and highly re- 

 fractive bodies appear which are the spores, and by the 

 bursting of the cyst they, with the remains of the protoplasm 

 in which they arose, escape to the exterior and soon begin to 

 develop. Losing its spherical shape each spore becomes 

 amoeboid ; then, drawing in all the pseudopodia but one, 

 which elongates and becomes a flagellum, it passes from the 

 Rhizopod to the Flagellate stage ; and finally the flagellum is 

 withdrawn, cilia appear, and the animal gradually assumes 

 the adult form. Spore-development somewhat similar to this 

 has been observed also in Vorticella, and special interest at- 

 taches to it as probably indicating the line of descent of the 

 Infusoria. 



Conjugation is a frequent process among the Infusoria, 

 where it seems to have a rejuvenating rather than a strictly 

 reproductive function. If prevented, and fission goes on 

 through a number of generations, marked degeneration en- 

 sues ; while if it be allowed, the same number of generations 

 may be produced without any signs of degeneration. The 

 process consists of a renewal of the nuclei and micronuclei of 

 the conjugating forms, and the process as it occurs in Colpid- 

 ium colpoda may be described thus. Two individuals come 

 into contact by the anterior portions of their body, actual 

 fusion of the two protoplasms taking place at the point of 

 contact. The micronucleus in each individual then enlarges 

 and divides, the two thus formed subsequently dividing again, 

 so that each of the conjugating individuals contains four 

 micronuclei and one nucleus. One of the four micronuclei 

 iu each individual now divides, and one of the two thus 

 formed (the male pronucleus) crosses over to the other indi- 

 vidual and unites with the other product of the division, the 

 female pronucleus, there being thus a mutual interchange of 

 micronuclei. The individuals now separate and resume their 

 independent existences, and a rearrangement of the nuclear 

 structures accompanied by fission takes place. The three 

 micronuclei which did not take part in the formation of the 

 pronuclei of conjugation degenerate, as does also the original 



