THE PROTOZOA 139 



development can take place until these sporozoites are transferred 

 to another worm. The eaxct method in which this takes place is 

 not known, but it is plausibly suggested that when the earthworm 

 dies and disintegrates the cysts are left unaffected in the soil ; or, 

 again, when a worm-eating bird swallows the earthworm it is able to 

 digest it, but not to injure the resistant spores, which pass through 

 the alimentary canal and are scattered on the ground with the 

 excrement. Another worm swallows the soil containing some of 

 the spores, and so in a chance manner becomes infected. The 

 sporozoites are probably released in the alimentary canal, whose 

 digestive juices are able to dissolve the tough wall of the sporocyst, 

 but it is not yet known how they reach the sperm sacs. Once 

 here they enter the sperm mother cell and the cycle begins 

 all over again. 



We see a considerable difference between the method of 

 reproduction in Monocystis as compared with either Amceba or 

 Paramcecium. In the latter two forms the multiplication always 

 takes place by binary fission, i.e. division of the nucleus into two, 

 followed by a corresponding splitting of the cytoplasm. The former 

 animal, when it divides either as a sporoblast or as a gametocyte, 

 behaves differently. The nucleus, first by repeated indirect division, 

 gives rise to eight or a large number of daughter nuclei, and not 

 until the nuclear divisions are quite complete and the daughter 

 nuclei have migrated to the periphery does any separation of the 

 cytoplasm occur. Then as many separate small cells as there were 

 nuclei are formed simultaneously. This type of multiplication is 

 distinguished as multiple fission, as opposed to binary fission. 

 Another interesting difference is to be found in the life histories, 

 for in Monocystis we first encounter in a simple form the phenomenon 

 known as " alteration of generations " or metagenesis. In the first 

 place we have the sexual generation ending with a large number of 

 gametes which unite to form zygotes, and in the second place the 

 zygotes themselves multiply asexually to produce eight sporozoites. 

 It is to be noted, however, that once the sporozoites have been formed 

 they do not divide any further, as commonly occurs in other allied 

 forms, even when they enter into their feeding stage in the sperm 

 sacs. This particular sort of life history with its alternation is 

 termed digenetic, in contradistinction to the simple one in Amceba, 

 which is said to be monogenetic. 



The two gametocytes each produce a large number of gametes 

 which fuse in pairs to form zygotes in a way suggestive of the fertili- 

 sation of the ovum by the sperm in the higher animals. The two 

 gametes, however, are, as far as we can see, exactly the same in size 

 and structure, and we cannot distinguish a male and a female form 



