44 



MONOCYSTIS. PHYLUM PROTOZOA 



REPRODUCTION 



In the stage which we have just described, the animals are 

 known as trophozoites. When they are full grown, two of them 

 come together and form themselves into a rounded mass without 

 fusing. Around this mass a two-walled cyst is secreted (Fig. i8). 

 Each individual now divides by multiple fission, in which the 



spc 



Fig. 1 8. — The life-history of Monocystis. — After BiitschU. 



1. Young individual (f) lying within a sperm mother cell of an earthworm. 



2. Association of two individuals within a cyst, ready to form gametes. 



3. Numerous spore-cases [sp.c, pseudonavicellae) within a cyst. 



4. A spore-case with eight spores (sp.) and a residual core (rb.). 



YiG, 19. Part of a cyst of Monocystis lumbrici show-ing the two kinds of gametes 



'and the residual protoplasm of one of the parents. — After Hoffmann. 



mitosis resembles that of higher animals in that the centrosome 

 appears outside the nucleus and the nuclear membrane disappears. 

 There arise thus, as in the spore formation of Amceba, a number 

 of small germ cells, a certain amount of residual protoplasm being 

 left, which is absorbed by the germ during their development. 

 The germ cells unite in pairs, in which one member is probably 

 derived from each parent. Thus, although the parents are to all 

 appearance exactly alike, there happens here what is known as 

 cross-fertihsation, such as is found in the vast majority of cases 

 throughout the animal kingdom. 



It is said that in M. magna the germ cells from the two parents 



