THE NEOSPORIDIA 405 



vegetative nuclei and two spores. Each spore (Fig. 166, L) consists of (1) a 

 sporocyst composed of two valves, each secreted by a parietal cell ; (2) two 

 polar capsules, each secreted by a capsulogenous cell ; (3) a binucleate amcebula, 

 the sporozoite or sporoplasm. When the spore is fully formed, the cells which 

 form the valves and polar capsules are used up and degenerate, remnants 

 only of their nuclei being visible. The spores represent the propagative 

 phase, and the trophozoite with its vegetative nuclei degenerates. 



The Myxosporidia Polysporea differ from the Disporea in that the plasmo- 

 dium contains a great number of nuclei, some of which are purely vegetative 

 in function, others generative ; and the plasmodium produces in its interior 

 numerous spores, which are always formed in couples. This peculiarity is 

 due to the fa t that the two sporoblasts arise from a cell- complex which is 

 termed a " pansporoblast," producing two sporoblasts and two spores. 



Spore-formation in Polysporea has been studied in Sphceroynyxa sabrazesi 

 (not labrazesi), from the gall-bladder of Hippocampus, by Schroder (767 and 768), 

 and in Myxobolus pfeifferi, a deadly tissue -parasite of the barbel, by Keysselitz. 

 Their results are almost identical in each case, except for minor details, and 

 are in the main as follows (Figs. 167, 168). 



The generative nuclei of the plasmodium become the centres of a condensed 

 patch of protoplasm (Fig. 168, A], forming a " propagative cell " (Keysselitz). 

 These cells may multiply with mitosis of the nuclei for a while, but finally 

 proceed to spore -formation. The nucleus of a propagative cell divides into 

 two, a larger and a smaller, and division of the cell follows sooner or later 

 (Fig. 167, A ; Fig. 168, B, C). Two such couples of cells become associated, and 

 the two smaller cells form an envelope surrounding the two larger cells, which 

 by their association form the pansporoblaat (Fig. 167, B, C ; Fig. 168, D). Hence 

 the pansporoblast is from the first a complex of two distinct cells, and not, 

 as was formerly supposed, a single cell. The two cells of the pansporoblast 

 may be termed gamonts, since they give rise ultimately to gametes, but not 

 to gametes alone, like the gametocytes of Ceratomyxa. The cytoplasm of 

 the two gamonts may fuse into oife mass, but the nuclei remain separate and 

 undergo repeated divisions, until the pansporoblast within its envelope 

 contains twelve nuclei, and may consist of as many separate cells (Fig. 167, D ; 

 Fig. 168, E], The nuclei or cells then become arranged in a definite manner ; 

 eight of them take up a peripheral position, four of them place themselves 

 more centrally (Fig. 167, E). The four central cells are the gametes ; their 

 nuclei undergo reducing divisions, and the four cells then pair off into two 

 couples ; in each couple the cytoplasmic bodies of the two cells fuse together, 

 but their nuclei remain distinct. It is probable that in each couple one 

 nucleus is descended from that of one of the two original propagative cells, the 

 other nucleus from that of the other. 



At this stage the pansporoblast divides into two masses, the sporoblasts 



FIG. 167 continued: 



tion -nuclei beginning to degenerate ; the envelope -nuclei are not represented ; 

 K, the pansporoblast beginning to divide into two sporoblasts ; within the 

 envelope are seen also some small bodies of doubtful nature ; F, the two 

 sporoblasts completely separated, between them two residual nuclei ; each 

 sporoblast has six nuclei, four peripheral, two central ; at the two extremities 

 of the sporoblast the polar capsules are beginning to be formed ; 0, one of 

 the two sporoblasts at a later stage, showing two parietal cells, situated 

 superficially ; two nuclei of the capsulogenous cells, each near a polar capsule ; 

 the two germinal nuclei close together at the centre ; and a residual nucleus 

 attached to the surface ; H, I, further stages in the development of the 

 sporoblast ; J, pansporoblast with two spores almost fully formed, and the 

 two residual nuclei ; each spore has a polar capsule (p.c.) at each end, and near it 

 a nucleus of the capsulogenous cell (n.c.) ; two large parietal nuclei (n.p.), in 

 process of degeneration ; and two germinal nuclei (n.g.) ; K, L, M, fully- 

 formed spores ; in K and M the two germinal nuclei are still separate, in 

 L they have undergone fusion ; in M the two polar filaments are extruded. 

 After Schroder (767 and 768). 



