212 



DYNAMICS OF LIVING MATTER 



He mentions that in the short pieces less "polarity" exists than in the 

 long pieces; but as the conception of heteromorphosis is the opposite 

 of polarity, Morgan's statement is only a different way of expressing 



the same fact. I remember that when Miss 

 Bickford made her experiments on regeneration 

 in Tubularians in my laboratory, I was struck 

 with the fact that in the very small pieces cut 

 out of a stem the polyps at the oral and the 

 aboral end developed practically simultaneously. 

 Here, too, polarity was less pronounced in small 

 pieces than in large pieces. I believe the reason 

 for this lies in the role which processes of the 

 character of a current or a flow of material play 

 in these phenomena. The red pigment, and 

 possibly other substances which are of importance 

 for regeneration, gather not in one point but in 

 an area of the length of several millimeters, 

 where a polyp is to be formed. If the regener- 

 ating piece is in itself only a few millimeters 

 long, the pigment must remain scattered equally 

 over the small piece, and hence the polarity 

 must disappear. Something similar may occur 

 in the case of a Planarian. If the piece is 

 very small, the head-forming material will remain equally distributed 

 through the whole length of the piece, and hence the chance for the 

 simultaneous formation of the head at either end is greater than in a 

 large piece. 



It is a general experience that in order to get a duplication of organs, 

 the regenerating animal must be split into two pieces. Thus, in order 

 to obtain two larvae from one egg, the egg must be cut into two, or the 

 heap of cells must be separated into two parts. One might believe that 

 in order to get two heads in front of a Planarian, the front end would 

 have to be separated into two by a longitudinal incision. Two heads, 

 however, often develop in front of a Planarian whose head has been 

 cut off without such an incision. The explanation is obvious on the 

 basis of Sachs's hypothesis. If the gathering of certain substances at 

 the front end is the cause of the formation of a head, and if we assume 

 with Bardeen that these substances are carried to the cut by the circu- 

 latory system, it is comprehensible that in two different spots at the 

 front end substances necessary for the formation of the head may 

 gather. 



FIG. 55. AFTER VAN 

 DUYNE. 



