Osborn. — Spongospora subterranea , ( Wallroth ) Johnson . 339 
attained for a short time just previous to spore formation. The reduction 
in Spongospora is completed before this occurrence ; in the Mycetozoa the 
homotype division does not take place till the spore has germinated. 
Should the observations of Prowazek on Plasmodiophora be confirmed, 
the case of that organism is wholly different from the Mycetozoa or Spongo- 
spora. It is diploid from spore to spore, the x generation being limited to 
a short period within the cyst. This may be more in accordance with ‘ all 
Protozoa ’, as Hartmann 1 has observed, but it does make it more difficult to 
trace the homologies between the plasmodia of Plasmodiophora , on the one 
hand, and those of Spongospora and the Mycetozoa on the other. Thus, 
though the evidence regarding Plasmodiophora is to some extent con- 
flicting, it may be assumed that karyogamy in the plasmodium preceding 
the karyokinesis is the normal occurrence in the Plasmodiophoraceae ; it is 
interesting to note that Pavillard 2 has already forecast this in his summary 
of the work upon the group. 
Summary. 
1. Spongospora subterranea is an intracellular parasite of the potato 
tuber, living in the cells in an amoeboid condition, and invading the 
daughter cells as they form in the process of cell-division. 
2. The nuclei of the amoebae divide in an amitotic manner during the 
vegetative phase ; on its conclusion the amoebae fuse to form a plasmodium. 
3. Plasmodium formation is followed by a degeneration and disappear- 
ance of the vegetative nuclei, chromidia appearing in the protoplasm. This 
is the akaryote stage. 
4. On the conclusion of the akaryote stage the nuclei are formed on 
different sites to the previous ones, some of the chromidia being used in the 
process while the remainder degenerate. 
5. Karyogamy occurs between pairs of the nuclei. 
6. Karyogamy is succeeded by a temporary enlargement of the nuclei 
and a contraction of the chromatin, which is possibly a condition of synapsis. 
7. Two karyokinetic divisions of the nuclei follow each other rapidly; 
the first is marked by its length of spindle ; the spindle of the second is 
shorter, with more sharply defined fibres, and has eight chromosomes. 
8. The spores are uninucleate, and are aggregated in rounded masses 
traversed by fissures and marked by irregular depressions, but remaining 
attached in structures known as ‘ spore balls \ 
9. Spongospora \ s a member of the Plasmodiophoraceae, which group 
has many points of relationship to the Mycetozoa, differing chiefly in the 
parasitic habit, the method of division of the vegetative nuclei, and by the 
less constant presence of a flagellum on spore germination. 
1 Hartmann, M. : Archiv f. Protistenkunde, vol. xiv, 1909, p. 284. 
2 Pavillard, J. : loc. cit., 1910, p. 506. 
