PROGRESS IN PROTOZOOLOGY

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the preliminary designation "connecting material". Such attempts have
been started just recently.
It deserves to be mentioned that the uppermost trichocyst tip (adjacent
to the cell membrane) is always devoid of MIP; the "annulus" MIPs
surround the fusion site proper somewhat below, so that this structure
(like the "rings" within the cell membrane) seems to be situated outside
the fusion zone proper.
Exocytotic Systems in Other Protists and in Metazoa
"Rosette" type MIP clusters (without a "ring") have been observed
also over extrusomes of heliozoa (Davidson 1976) and sporozoa
(Eivieria: Dubremetz and T o r p i e r 1978, Toxoplasma: Porchet
and Tor pier 1977: Plasmodium: Dubremetz et al. 1979, Bannister,
personal communication). In the case of sporozoa exocytotic
events are believed to (participate in the host cell penetration (Eimeria:
J e n s e n and Edgar 1976; Toxoplasma: Nichols and O'Connor
1981). During conjugation of the ciliate Euplotes the contents of ampullae
are released by exocytosis and again "rosettes" were found at these sites
(L u p o r i n i, personal communication). Nothing alike was reported for
amoebae (for Entamoeba see Henley et al. 1976). There are so far
no data available on the occurrence of "connecting material" in these
cases.
Among metazoa, exocytotic sites are characterized by conspicuous
MIP aggregates only in motor endplates (c.f. Meldolesi et al. 1978).
For many other systems it was claimed that MIPs would be removed
from the fusion sites on a large scale before fusion (O r c i and Perrelet
1978). This view has recently been challenged for methodical
reasons (Plattner 1981). Careful freeze-fracture experiments with
various gland cells also showed that exocytotic fusion events in several
mammalian gland cells might occur without any or with only a small
MIP shift (Tanaka et al. 1980), so that MIPs might here also be
located quite closely to the fusion zone. We arrive to the following conclusion.
If some membrane (integrated and/or associated) proteins would
stay close to the fusion site, they could exert a positive modulatory
effect, e.g., by exerting a Ca 2 +-channel function as mentioned above or
by changing the fusogenic properties of the tightly apposed membranes
(to mention just two possibilities). Supporting evidence along these lines
came recently from work with liposomes which fuse in the presence of
physiological Ca 2 + concentrations only when supplemented with certain
proteins (Z i m m e r b e r g et al. 1980), whereas previous work on lipo-
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