Hull ZoBlogical Laboratory, University of Chicago T W O FIGURES
In earlier papers (Child '11 b, '11 c, ' 11 e) attention has been called to the fact that the frequency of head-formation in isolated pieces of Planaria dorotocephala varies with size of piece, region of body and various external conditions which can be controlled experimentally. This difference in capacity, together with the possibility of comparing rates of metabolism in different pieces by the susceptibility method (Child '13 a), affords a means of determining why some pieces produce a head and others do not and since the formation of the head is the first step in the development of the new individual we have an answer to the question why some pieces give rise to new wholes and others do not.
I. THE TIME OF HEAD-DETERMINATION
Since it can be determined experimentally within wide limits in various ways whether a piece of Planaria dorotocephala shall give rise to a head or not (Child 'llb), it is evident that the fate of the piece as regards head formation is not fixedly and finally determined at the time of isolation of the piece from the parent body. The first step in the analysis of the factors concerned in head determination is t o find when the determination of the head occurs. This can be readily and very simply done by the following method.