286 



HARRY DEAL TORREY. 



It should be noticed especially that all the heteromorphic 

 pieces used in this series were short, and none were above medium 

 diameter. In not a single case, tinder these conditions, did the 

 polarity fail to reverse in one of the portions into which the hetero- 

 morphic pieces were divided. 



The third series shows the importance of this consideration of 

 length. Three heteromorphic pieces, absolutely and relatively 

 much longer than those of the second series, one of them con- 

 siderably larger than the other two, were sec- 

 tioned as shown in Fig. 9. Five days after 

 section, all six pieces w r ere heteromorphic which 

 indicated that in none of them, whether distal 

 or proximal, was development at the wound 

 dominated by the conditions, existing at the 

 other end of the piece. 



The same fact is brought into clear relief 

 by a comparison of the following figures. Of 

 14 segments representing the distal half of the 

 column of 14 polyps of moderate size, 12 were 

 heteromorphic in 3 days. Of 81 very short 

 segments from several small polyps, only 6, 

 j/ or 7.4 per cent., became heteromorphic. 



Further, of 13 segments of approximately 

 the same length and diameter as the pieces 

 obtained by cutting the heteromorphic pieces 

 in series 2, 8 became heteromorphic. Of 15 similar segments, 

 10 became heteromorphic. 



Besides these figures, there is a mass of evidence, obtained by 

 repeated experiments on large numbers of individuals, demon- 

 strating that the presence of the original hyd ninth on a segment 

 of the column inhibits the development of a proximal hydranth. 

 It is clear, then, in the light of the facts cited in this section, 

 (i) that reversals of polarity profound enough to effect entire seg- 

 ments of the column as units are readily produced in Corymorpha; 

 and (2) that the stage of differentiation at one end of a piece will 

 under certain conditions control differentiation at the other end. That 

 reversals of polarity, in cases of heteromorphosis, are often shown, 

 by form changes, to affect considerable areas of the column, 

 without aid from the knife, will appear in the following section. 



FIG. 9. 



