628 STUDIES IN GENERAL PHYSIOLOGY 



with the optic ganglion be removed, an antenna will be 

 produced in the place of the eye, while if the eye alone 

 is cut off an eye is regenerated. The presence or absence of 

 the optic ganglion decides whether a regeneration or a hetero- 

 morphosis will follow. 1 



I found, very early in my experiments, that in certain 

 Hydroids a heteromorphosis can be produced without any 

 organ being cut off or any wound being inflicted upon the 

 animal. In Antennularia a Hydroid common at Naples 

 the arrangement and orientation of the organ as well as 

 the direction of growth is dominated by gravitation. The 

 animal consists of a straight vertical stem, which forms 

 stolons at its lower end and which carries small branches 

 with limited growth at regular intervals. On the upper 

 surface of these branches the polyps are found. If such a 

 stem be suspended horizontally in the water the lateral 

 branches which are directed downward and which had 

 finished growing now begin to grow downward very rapidly. 

 At the same time the polyps on these branches disappear. 

 The downward-growing parts no longer resemble the old 

 side-branches but look like roots. A closer examination 

 reveals the fact that they not only possess the morphological 

 appearance of roots but also the physiological reactions of the 

 latter, inasmuch as they are positively geotropic and stereo- 

 tropic, while the branches do not show these forms of 

 irritability. In this case the tissue of the polyps which dis- 

 appeared seems to have been transformed into the tissue of 

 roots. 2 



I made a similar observation shortly afterwards at Woods 

 Hole in another Hydroid, Margelis. When the uninjured 

 points of a stem of Margelis are brought in contact with a 

 solid body the point of the stem assumes the form and 



1 HERBST, Archivfttr Entwickelungsmechanik, Vol. IX (1899), p. 215. 



2 Part I, p. 191. 



