SECTIONAL TRANSACTIONS.—D. 385 
object and its background ; (2) the obliteration of light and shade by counter 
lighting and shading ; and (3) the breaking up of form by means of a super- 
imposed disruptive pattern. These principles are those actually found 
to operate widely in nature. 
Disruptive patterns are considered in relation to the habits, posture 
and habitat of the animals exhibiting them ; a special aspect of disruptive 
colouration, described as coincident disruptive colouration, has for its 
essential feature the extension of the pattern across separate but adjacent 
parts of the body. ‘This greatly strengthens the disruptive effect by uniting 
parts morphologically separate, by concealing otherwise conspicuous organs, 
and by replacing the real form by a superimposed apparent configuration. 
The effect may further be intensified by actual modification of form and by 
diffusion of outline. The above principles have important applications 
in relation to camouflage in modern warfare. 
Dr. R. H. THouLess.—Protective colouration as a problem in the psychology 
of perception (3.45). 
The Gestalt theory of perception makes possible a psychological state- 
ment of the principles of the concealing colouration of animals in terms of 
the organisational properties of a perceptual field. ‘The perceptual field 
is organised in such a way that parts of it are more or less segregated as 
separate configurations (Gestalten) seen as figures against a background. 
The tendency of such separate parts of the perceptual field to appear as 
mental units may be strong or weak. Strong (or insistent) configuration 
is determined by such factors as the possession of a definite, continuous and 
smooth boundary, difference in brightness (tone value) from the background, 
movement of the configuration as a whole relative to the background, 
etc. Cryptic colouration makes the configuration proper to the animal 
adopting it (the body configuration) as weak as possible. Disruptive 
colouration makes an alternative configuration much stronger than the body 
configuration. Flash colours may be a special device for dealing with the 
insistency of a configuration moving relative to the background. An 
attempt will be made to illustrate some of the relevant properties of visual 
configurations by epidiascope projection. 
Friday, September 6. 
Dr. J. A. Kircuinc.—The osmotic function of contractile vacuoles (10.0). 
Contractile vacuoles occur in all fresh-water Protozoa, and in some marine 
ones. In Peritricha the rate of output of fluid is increased by a decrease, 
and decreased by an increase in the osmotic pressure of the external medium. 
A contractile vacuole probably prevents excessive swelling of the body by 
ejecting water as fast as this comes in through the body surface by osmosis, 
and is probably more important for this purpose in fresh-water forms. How- 
ever, when marine Peritricha are transferred to dilute sea-water the contractile 
vacuole, by its increased output, probably controls the body volume. 
Suppression of vacuolar activity with cyanide leads to an immediate increase 
in the body volume ; and removal of the cyanide results in a rapid recovery 
of the contractile vacuole followed by a shrinkage of the body. Vacuolar 
action must involve the separation of water from salts, and therefore the 
expenditure of energy, which is probably obtained directly by an oxidative 
process. ‘The inhibitory effect of cyanide is immediate, as in the case of 
the selective process of the kidney tubules, and not delayed as in the case 
