THE ACRASINA
B . M. S H A F F E R
Department of Zoology, Cambridge University, England
I. Introduction
109
A. Deviations from the Mean
109
B. The Pseudoplasmodium
I l l
IT. The Non-Social Phase
113
A. The Cells
113
B. Organization
116
I I I . The Onset of the Social Phase
118
A. Food Shortage
118
B. The Initiators
119
C. The Founders
129
D. Sexuality
131
IV. The Aggregation
134
A. The Cells
134
B. Organization
138
C. Acrasin
154
D. The Nature of Intercellular Bonding
157
V. TheGrex
160
A. The Cells
160
B. The Whole
171
References
179
I. Introduction
A. Deviations from the Mean
'There are five kinds of human beings: male, female—' and, added
Sir Ronald Fisher after a pregnant pause, 'X sperm, Y sperm, and
ovum.' Most multicellular organisms, of course, have a phase in their
life cycle in which the individuals are separate nonadherent cells, intercellular adhesion reappearing soon after fertilization. Likewise a number
of organisms classified as Protista may have a multicellular phase; and
of these the Acrasina—Acrasieae to botanists—the cellular slime moulds
or collective amoebae, pass back and forth most conveniently between
phases in which the cells are solitary or nonsocial, and aggregated or
social. During the first phase, the cells feed on bacteria; during the
second, in which there may be some cell division but no increase of mass,
the cells first collect together, and then the aggregates erect stalked fruiting bodies of varying degrees of complexity (Fig. 1). The Acrasina are
well suited for studying many aspects of cell properties, behaviour, and
109