406 



THE PROTOZOA 



each containing six nuclei namely, four peripheral and two central 

 (Fig. 167, F ; Fig. 168, F). The reduction-nuclei are 1- ft out in the cold, 

 and die off. From each sporoblast a spore is formed in the following way 

 (Fig. 167, Q J ; Fig. 168, G) : Of the four peripheral cells, two are parietal cells 

 which give rise to the two valves of the sporocyst, the other two as capsu- 

 logenous cells produce the two polar capsules ; the two central nuclei with the 

 mass of protoplasm in which they lie become the amoebula or sporozoite of 

 the spore. The sporozoite is thus at first binucleate, but when the spore 

 is fully formed its two nuclei fuse into one. There can be no doubt that this 

 fusion represents a karyogamy, and that the single nucleus is a synkaryon. 

 The cells which form the valves of the sporocyst and polar capsules degenerate 

 when the spore is completely formed. 



FIG. 168. Spore-formation in Myxdbolus pfeifferi. A, Propagative cell from the 

 plasmodium ; B, division of the propagative cell into two unequal halves ; 

 C, the smaller cell of the preceding stage applies itself as a flattened envelope- 

 cell to the larger ; D, a mass formed by union of two couples of cells similar 

 to those of the preceding stage the pansporoblast ; E, pansporoblast with 

 fourteen nuclei, two representing the envelope-cells, and twelve arisen by 

 division of the two larger cells of the preceding stage ; F, the pansporoblast 

 divided into the two envelope-cells and two masses, sporoblasts, each con- 

 taining six nuclei, which are arranging themselves so that two nuclei are 

 more central, four more peripheral in position ; G, spore in process of forma- 

 tion ; the sporocyst is formed by two parietal cells (p.) ; at the upper pole are 

 the two polar capsules (p.c.) in their capsulogenous cells ; and the lower part 

 of the spore is occupied by the binucleate amoebula (am.) ; H, ripe spore ; 

 in the amoebula the two nuclei have fused into one ; lettering as in the last. 

 After Keysselitz. 



Mercier (765*5), on the other hand, who has also studied the spore -formation 

 of Myxobolus pfeifferi, differs from Keysselitz in his account, more especially 

 with regard to the sexual processes. Ho affirms that the plasmodium contains 

 four zones: (1) A peripheral zone, without nuclei or spores; \2] a zone of 

 nuclei, all similar amongst themselves, which multiply by mitosis ; (3) a zone 

 containing nuclei of different sizes and early stages of spore -formation ; and 

 (4) a central region containing ripe spores. In the third zone differentiation 

 of microgametes and macrogametes takes place. The gametes copulate, and 

 the zygote is a pansporoblast. Its nucleus (synkaryon) divides to form 

 fourteen nuclei, two of which are rejected, while from the remaining twelve 

 arise two sporoblasts, each with six nuclei, and finally two spores. 



