MONOCYSTIS. 101 
end-to-end manner observed also in Gregarina. In repro- 
duction two individuals (gametocytes) become associated 
inside a common cyst. The nucleus of each divides up 
repeatedly, and the daughter nuclei migrate to the surface of 
the cell, where each becomes surrounded by a little mass of 
protoplasm. Each of the gametocytes has thus given rise 
to a number of gametes, while there remains over a mass 
of residual protoplasm which has not been used up during 
this process. The wall between the two gametocytes now 
breaks down and the gametes conjugate in pairs, forming 
zygotes. It is probable that of each pair of conjugating 
gametes one is derived from each gametocyte. Each zygote 
Fic. 44.—Life history of Monocystis.—After Biitschli. 
1, Young Gregarine lying within a sperm mother cell of earthworm. 
2. Association of two Gregarines within a cyst, ready to form gametes. 
3. Numerous spore-cases (pc. pseudonavicelle) within a cyst. 
4. A spore-case with eight spores (s.) and a residual core (74). 
secretes a membrane and becomes a spore-case. The 
nucleus divides up, and eight elongated spores are formed 
round a residual core. The spore-case now takes its typical 
shape and is known as a pseudonavicella. The spores are 
considerably larger than those of Grvegarina. Eventually, in 
the alimentary canal of another earthworm the cyst bursts, 
the spore-cases are extruded, the spores emerge from their 
firm chitinoid cases. The young spore (sporozoite) is like 
a bent spindle (falciform), and seems next door to being 
flagellate. It bores into a mother sperm cell, and from this 
it afterwards passes as an adult into the cavity of the 
seminal vesicles. Intracellular parasitism and copious food 
naturally act as checks to activity, and the adult is sluggish. 
The allies of Monocystis occur chiefly in “Worms,” 
Tunicates, and Arthropods; none are known in Vertebrates. 
