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an elongation phase and a 
constriction phase (Raikov). The elongation phase 
is likely driven by both the assembly and slid-
ing of microtubules, which may assemble inside 
the macronuclear envelope (e.g., Tucker, Beisson, 
Roche, & Cohen, 1980; Williams & Williams, 
1976) or outside the macronuclear envelope (e.g., 
Diener, Burchill, & Burton, 1983). 
 While duplication of all cell constituents occurs 
during the cell cycle, developmental biologists 
and systematists have been particularly fasci-
nated by the duplication of the cortical compo-
nents. Lynn and Corliss (1991) have separated 
this development into cortical somatogenesis and 
cortical stomatogenesis : the replication of the com-
ponents of the somatic cortex and the oral cortex, 
respectively, which are often highly co-ordinated 
 processes. Frankel (1989) has provided a detailed 
review of these processes from the perspective of 
a developmental biologist. In particular, ciliate 
 systematists have long been fascinated with the 
 ontogeny of the oral apparatus (see Chapter 1 ; 
Corliss, 1968; Fauré-Fremiet, 1950a, 1950b). 
Foissner (1996b) has provided a detailed discus-
sion of the comparative stomatogenesis of ciliates, 
but see also Tuffrau (1984). 
 Briefly, the conspicuous elements of cortical 
 somatogenesis that have attracted attention are 
the kinetosomes , contractile vacuole pores , and 
 cytoproct . Ciliates were one of the first groups 
of organisms to be investigated for replication of 
kinetosomes, demonstrating that the “daughter” 
kinetosome developed in close proximity to and in 
a well-defined relationship with the parental kine-
tosome (Allen, 1969; Dippell, 1968). This proc-
ess, now called cytotaxis or structural guidance ,
is responsible for the precise positioning of new 
cortical units (Aufderheide, Frankel, & Williams, 
1980; Frankel, 1989). Kinetosomal replication can 
occur throughout the cell cycle or be confined to 
a period close to the time of cytokinesis and be 
highly correlated with cortical stomatogenesis. 
Initiation of kinetosomal replication undoubtedly 
involves participation of gene products that diffuse 
through the cytoplasm: for example, the product 
of one such gene, sm19+ , appears to be involved 
in kinetosomal replication in Paramecium (Ruiz, 
Garreau de Loubresse, & Beisson, 1987). New 
 contractile vacuole pores (CVPs) are typically 
replicated at cell division, although in some cili-
ates with large numbers of contractile vacuoles the 
replication process may be uncoupled from cell 
division. In Tetrahymena , the proter develops new 
CVPs adjacent to somatic kineties in a predictable 
location in its posterior right quadrant, defined 
by the “ central angle ”. This angle is a manifesta-
tion of a mechanism that places the new pores in 
a roughly proportional fashion in relation to the 
total number of somatic kineties (Frankel; Nanney, 
1980; Nanney, Nyberg, Chen, & Meyer, 1980b). In 
Chilodonella species, a proportioning mechanism 
may also exist, but in this case the many contrac-
tile vacuole pores , which are distributed over the 
ventral surface, are newly placed in both proter and 
opisthe, apparently in relation to major features 
of the cortex, such as somatic kineties, the oral 
region, and the boundaries of the ventral surface. 
During this somatogenesis in Chilodonella , the old 
contractile vacuoles and their pores dedifferentiate 
and disappear (Kaczanowska, 1981; Kaczanowska, 
Wychowaniec, & Ostrowski, 1982). The old cyto-
proct , since it is typically in the posterior end of 
the cell, is inherited by the opisthe, and a new 
cytoproct develops in the appropriate position in 
the proter, presumably positioned by mechanisms 
similar to those specifying the position of CVPs. 
 In addition to these conspicuous cortical ele-
ments, we should remember that all other organelles 
are typically duplicated during each interfission 
period – mitochondria , extrusomes , Golgi apparati , 
 ribosomes , lysosomes , and all the smaller molecu-
lar constituents not visible as discrete entities by 
the microscopist. 
 Cortical stomatogenesis is literally the forma-
tion of a mouth. This process is usually the most 
conspicuous cortical ontogenetic event, since the 
oral region is generally the most obvious corti-
cal differentiation. Since the oral apparatus was 
historically considered highly significant as a taxo-
nomic feature, its development in different taxa has 
preoccupied ciliate systematists . In the chapters 
that follow, stomatogenesis of each of the classes 
is briefly characterized, based on the primary lit-
erature and the comprehensive review of Foissner 
(1996b). Stomatogenic patterns are now divided 
into five major types with subtypes – apokinetal , 
 parakinetal , buccokinetal , telokinetal , and mixoki-
netal (Corliss, 1979; Foissner, 1996b). However, all 
subtypes within a pattern of stomatogenesis should 
not be regarded as diversifying from an ancestral 
type: they should not be considered as homologous . 
Rather, the several kinds of telokinetal stomatogen-
esis probably have evolved independently in dif-
ferent classes as the morphology of these ciliates 
diversified. For example, cyrtophorids , prostomate-
ans , colpodeans , and litostomateans all exhibit dif-
ferent kinds of telokinetal stomatogenesis (Foissner, 
1996b), but molecular phylogenetic analyses clearly 
demonstrate that these classes are not closely 
related. Thus, typifying stomatogenesis using this 
classification system should be viewed only as a 
descriptive approach, enabling a systematic charac-
terization of the existing diversity. It may be of phy-
logenetic significance in relating groups within the 
classes. More complete definitions of these kinds of 
 stomatogenesis can be found in the Glossary (see 
Chapter 2 ) and in Foissner (1996b). 
 The conspicuousness of stomatogenesis has also 
attracted the attention of developmental biologists 
who have investigated a variety of its aspects. The 
primordium or anlage for the new oral apparatus 
may be positioned by mechanisms that are influ-
enced by the global properties of the cell, ensuring 
that the new oral apparatus is placed in some pro-
portional manner in relation to the whole (Frankel, 
1989; Lynn, 1977b). However, the assembly of the 
oral apparatus adds a level complexity to cortical 
developmental processes as it encompasses at least 
three levels of biological organization – organelles 
(e.g., kinetosomes ), organellar complexes (e.g., 
 membranelles , polykinetids ), and organellar systems
(e.g., the entire apparatus itself). There is a complex 
interplay of controls at these levels and different 
4.5 Division and Morphogenesis 113
114 4. Phylum CILIOPHORA – Conjugating, Ciliated Protists with Nuclear Dualism
processes that coordinate assembly at each level 
(Frankel, 1989). Furthermore, this development 
takes place in the context of the cell, so that the 
entire apparatus, both in terms of the size of each 
oral polykinetid, for example, and sometimes the 
numbers of oral polykinetids are strongly related, 
for example, to cell size (Bakowska & Jerka-
Dziadosz, 1980; Bakowska, Nelsen, & Frankel, 
1982a; Jacobson & Lynn, 1992). Thus, systema-
tists must be aware of all of these potential con-
straints on oral development when they consider 
which aspects of the process and which features 
of the differentiated oral apparatus are significant 
from a systematic perspective. For example, are the 
differences in number and size of oral polykinetids 
in two isolates of a genus evidence of different 
species or of the phenotypic plasticity of these 
components in a single species as it varies in cell 
size? Even somatic structures, such as numbers of 
somatic kineties, can be strongly correlated with 
 cell size (Lynn & Berger, 1972, 1973). 
 Finally, once the cells have separated, there are 
often significant morphogenetic processes