food receptacles projecting at right angles to the doors, while on the
lower edge of each door is an iron staple used to receive a sliding bar
which could be operated from the observer's bench as a means of locking
the doors after they had been closed. The space beyond the exit doors
was used as an alleyway for the return of the animals to the starting
point.

It will be necessary at various points in later descriptions to refer to
these several figures. But further description of them will be more
readily appreciated after a careful examination of the ground plan of
the apparatus presented as figure 17 In accordance with the labelling of
this figure, the experimenter enters the apparatus room through doorway
16, passes thence through doorways 17 and 10 to the large cage Z, from
which he has direct access to the animals and can bring them into the
apparatus. The multiple-choice mechanism proper, consisting of nine
similar boxes (nine were used instead of twelve as a matter of
convenience of construction, not because this smaller number is
otherwise preferable) is labelled F. These boxes are numbered 1 to 9,
beginning at the left. This numbering was adhered to in the recording of
results throughout the investigation. The other important portions of
the apparatus are the runway D, from which the subject at the
experimenter's pleasure could be admitted through doorway 12 to the
large response-chamber E; the alleyways G, H, and I, by way of which
return to the starting point was possible; the observation bench C, with
its approach step 13; and the observer's writing table A.

In the construction of this large apparatus, it was necessary to make
provision for the extremely destructive tendencies of monkeys and
anthropoid apes,--hence the apparent cumbersomeness of certain portions.
It was equally necessary to provide for the protection of the observer