DYNAMICS OF MORPHOGENESIS 69 



mass of new embryonic tissue the new head arises. This head 

 then develops from the earhest stages of morphogenesis, while 

 in other regions of the piece, except at the posterior end, the 

 original structure undergoes more or less alteration but does not 

 completely disappear in a regression to embryonic cells. In 

 figure 2 the region of head-formation is indicated by the shaded 

 region at x, the region of tail-formation by z, and those regions 

 of the piece which retain the chief features of their structure, 



We have already seen that the higher the rate of metabolism 

 in y following section, the less likely is a head to arise from x, 

 and vice versa, and it has been pointed out that this inverse re- 



Figure 2 



lation between head-frequency and rate of metabohsm in other 

 regions of the piece, can mean only that the new head develops, 

 not in correlation with and under the control of the piece as a 

 whole, but, so to speak, in spite of it. 



In short, the facts already cited, and a large body of evidence 

 still to be presented, point directly to the conclusion that head- 

 formation in a headless piece is not the restitution of a missing 

 part but the first step in the development of a new individual. 

 If this is the case, then head-formation in a piece is essentially 

 the same process as in embryonic development. There the head, 

 or more specifically the cephalic nervous system, is the first region 

 or organ of the individual to become morphologically visible. 

 So far as we can determine, these early stages of head-develop- 

 ment are not dependent upon conditions in other regions of the 

 egg or embryo, but the head-region takes the lead in development. 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 17, NO. 1 



