ABSTRACT
In 1923 Robertson revitalised the old theory that reproducing unicellular organisms discharge into the medium a substance which accelerates growth. This theory has of necessity the corollary that the larger the amount of culture medium, the greater the dilution of this substance, and therefore the less acceleration. Robertson maintained that some organisms would not reproduce at all if placed in too large an amount of culture medium. Thereby he confused two separate concepts—one, of a substance necessary for growth, the other, of a substance which accelerates growth. The history of the former concept will be taken up later in connection with the growth of yeast. For the present, discussion will be confined to the production of an auto-catalyst by protozoa.
