During the last five years a small group in this Laboratory has been studying the potentialities of the flagellar apparatus of
Chlamydomonas reinhardii
for the study of growth and morphopoiesis. The results have been recorded in a number of papers (Randall
et al
. 1964; Warr
et al
. 1966; Hookes, Randall & Hopkins 1967; Randall
et al
. 1967). It is self-evident that the understanding of both growth and morphopoiesis in physical chemical and biological terms will most readily be brought about by the examination of the least complex biological systems, i. e. those that contain the fewest chemical and structural entities and are under the control of a strictly limited number of genes; not for preference a multicellular organism or organ, or even a single cell; but rather a virus or organelle. One would hope by this means to limit the number of genes implicated to something less than 10
2
. An outstanding analysis of the morphopoiesis of T-even bacteriophages has been carried out in recent years by workers in Geneva and Pasadena (see, for example, Kellenberger 1964; Kellenberger & Boy de la Tour 1965; Epstein
et al
. 1963; Edgar & Wood 1966). In the present investigations the methods and techniques of genetics, of optical and electron microscopy (in conjunction with optical diffraction studies of electron micrographs), radioautography and, to some extent, biochemistry have been used. During the last 15 years the structure of cilia and flagella, their basal bodies and associated cortical structures in various organisms (see, for example Fawcett 1961; Gibbons & Grimstone 1960; Satir 1965; Ringo 1966, 1967; Cavalier-Smith 1967; Allen 1967) have been investigated in some detail. Figure 20, plate 8, illustrates the appearance of the living organism in motion and figure 21 the fine structure of the flagellar apparatus (
FA
) of the alga
Chlamydomonas reinhardii
, a biflagellate member of the Chlorophyceae. The
FA
consists essentially of two normally motile flagella
F
, external to the cell (figure 20) and two internal basal bodies (
BB
). Each basal body is joined to its flagellum by a transition region
TR
(figure 21). The basal bodies (
BB
) lie in the main diametral plane of the organism and are joined by a fibrous band (
FB
) (figure 21). The external flagellum, bounded by a membrane continuous with the plasma membrane, contains the axoneme and its subsidiary structures. We shall be concerned chiefly with the basic features of the axoneme of the flagellum, i. e. the two central fibrils and the nine outer pairs. Each of these fibrils is apparently tubular in form and generally similar to the microtubules found in many types of cell (e. g. Tilney & Porter 1965; Tilney & Porter 1967; Peters & Vaughin 1967; Hepler & Newcomb 1964). The appearance in the electron microscope of an intact fragment of tubule (outer pair) from a disrupted negatively stained flagellum of
C. reinhardii
is shown in figure 24
a
, plate 9. The structural interpretation of micrographs of flagellar tubules has been discussed by Grimstone & Klug (1966) and Hookes, Randall & Hopkins (1967) and will be referred to subsequently. For fuller details of the fine structure of the
FA
of
C. reinhardii
, as a whole, reference should be made to Ringo (1966; 1967) and to Cavalier-Smith (1967).
Coordinated beating of algal flagella is mediated by basal coupling
