5 . THE PHYLUM PROTOZOA
167
(2) larger nuclei, usually 2.5-3.0 p in diameter, with intranuclear bar
(Fig. 57A and B), in plasmodia located throughout blood spaces of
oysters within 10 days after infection. The larger nuclei persist throughout the year in plasmodia with frequently vacuolated and granular
cytoplasm. Frequently interspersed with these throughout the year
are plasmodia with very large ovoid or spherical nuclei, ranging up to
7.5 p in diameter, and invariably with prominent intradesmose (Fig.
57B). Occasionally large and very large nuclei are found in the same
plasmodia.
Couch et al. (1966) have described what they believe to be the sporulating and spore stages of M . nelsoni from the digestive diverticula of
naturally infected oysters. Brief descriptions of these extemely rarely
seen stages are given below.
Sporulation and spore. With the onset of sporulation, small plasmodia
and their nuclei increase in size. When the nuclei in enlarged plasmodia
reach three to four times their initial size (Fig. 57F), karyokinesis
commences. A conspicuous bar or spindle appears within the nucleus
during division and the endosome becomes associated with the spindle
and divides (Fig. 57G). Daughter nuclei resulting from initial divisions
are relatively large (3-4 p ) but diminish in size in subsequent stages.
Sporonts (Fig. 57H), each measuring 18-30 p, formed when dividing
nuclei develop into sporoblasts. Sporoblast nucleus, 3-4 p in diameter,
surrounded by cytoplasmic condensation, often includes remnants of
spindle; rarely do sporoblasts include more than one nucleus.
Early sporocysts (Fig. 57I), each measuring 28-54 p, filled with
variable numbers of incompletely formed spores. Immature spores,
each measuring 6 - 8 p in diameter, round, with an orifice, but rarely
with fully formed operculum. Definitive sporocysts, each measuring
28-54 p, contain from eight to about fifty spores.
Mature spores (Figs. 57J and K), ranging from 5.3-10-7p x 4.87-5 p (mode 7.5 x 5.4p), surrounded by refractjle capsule, with or
without projections or appendages; sporal operculum, approximately
1 p high, extends laterally beyond margin of orifice to margin of
capsule; sporoplasm nucleus small, 1.5-2-0 p, with peripheral endosome.
Since sporulation and mature spores are extremely rarely found in
oysters (Haskin et al. (1966) have reported finding only two out of
10 000 infected oysters harboring such spores), and since the spores
described by Couch et al. (1966) were from naturally infected oysters,
some doubt exists as to whether the spores which they have described
are those of M . nelsoni. Relative t o this point, Haskin et al. (1966)
have stated:
167
(2) larger nuclei, usually 2.5-3.0 p in diameter, with intranuclear bar
(Fig. 57A and B), in plasmodia located throughout blood spaces of
oysters within 10 days after infection. The larger nuclei persist throughout the year in plasmodia with frequently vacuolated and granular
cytoplasm. Frequently interspersed with these throughout the year
are plasmodia with very large ovoid or spherical nuclei, ranging up to
7.5 p in diameter, and invariably with prominent intradesmose (Fig.
57B). Occasionally large and very large nuclei are found in the same
plasmodia.
Couch et al. (1966) have described what they believe to be the sporulating and spore stages of M . nelsoni from the digestive diverticula of
naturally infected oysters. Brief descriptions of these extemely rarely
seen stages are given below.
Sporulation and spore. With the onset of sporulation, small plasmodia
and their nuclei increase in size. When the nuclei in enlarged plasmodia
reach three to four times their initial size (Fig. 57F), karyokinesis
commences. A conspicuous bar or spindle appears within the nucleus
during division and the endosome becomes associated with the spindle
and divides (Fig. 57G). Daughter nuclei resulting from initial divisions
are relatively large (3-4 p ) but diminish in size in subsequent stages.
Sporonts (Fig. 57H), each measuring 18-30 p, formed when dividing
nuclei develop into sporoblasts. Sporoblast nucleus, 3-4 p in diameter,
surrounded by cytoplasmic condensation, often includes remnants of
spindle; rarely do sporoblasts include more than one nucleus.
Early sporocysts (Fig. 57I), each measuring 28-54 p, filled with
variable numbers of incompletely formed spores. Immature spores,
each measuring 6 - 8 p in diameter, round, with an orifice, but rarely
with fully formed operculum. Definitive sporocysts, each measuring
28-54 p, contain from eight to about fifty spores.
Mature spores (Figs. 57J and K), ranging from 5.3-10-7p x 4.87-5 p (mode 7.5 x 5.4p), surrounded by refractjle capsule, with or
without projections or appendages; sporal operculum, approximately
1 p high, extends laterally beyond margin of orifice to margin of
capsule; sporoplasm nucleus small, 1.5-2-0 p, with peripheral endosome.
Since sporulation and mature spores are extremely rarely found in
oysters (Haskin et al. (1966) have reported finding only two out of
10 000 infected oysters harboring such spores), and since the spores
described by Couch et al. (1966) were from naturally infected oysters,
some doubt exists as to whether the spores which they have described
are those of M . nelsoni. Relative t o this point, Haskin et al. (1966)
have stated: