EXTERNAL FACTORS OF REGENERATION IN ANIMALS 33 



which Loeb's experiments were made, pieces of the stem were sus- 

 pended vertically, some with the apical end upwards, others with the 

 basal end upwards. In nearly all cases roots were formed by both 

 the upper and lower ends. In a few cases, in which the apical end 

 was upwards, a new stem developed at that end. Pieces suspended 

 in a horizontal position also produced roots at both ends. After 

 removing the ends with their new roots from the pieces suspended 

 vertically, I found that roots again appeared at both ends in nearly 

 every case. The difference between these results and those of Loeb 

 may be due to the time of the year at which the experiments were 

 made, or possibly to some other difference, but the results show that 

 the response to gravity is not always so constant as Loeb's results 

 indicate. 



In a few cases in my experiments the basal end of the hydroid was 

 left attached to the stem on which it had grown, and the piece was 

 put into the same aquarium used for the preceding experiments. In 

 those pieces that lay on the bottom of the aquarium, with the stem 

 standing vertically, a new shoot, and not new roots, appeared on the 

 upper end. Other pieces were hung at the top of the water of the 

 aquarium with the stem turned downwards, and the basal, attached 

 end of the piece upwards. These pieces produced neither a stem nor 

 roots from the apical end. The results show that the presence of 

 roots at one end has an influence on the regeneration at the other 

 end. The same thing was shown in one case in which a short piece 

 sank to the bottom of the dish and, developing roots at its basal end, 

 became fixed : a stem grew out of the apical end. 



A number of other experiments that I made, in which pieces of 

 antennularia were fixed to a rotating wheel, gave negative results, 

 since neither roots nor stems appeared on the pieces. The rubbing 

 of the ends of the piece against the water as the wheel turned round, 

 or else the agitation of the water, prevented, most probably, the 

 regeneration from taking place. 



How gravity acts on antennularia has not as yet been determined. 

 The only suggestion that we can offer at present is that it brings 

 about a rearrangement of the lighter and heavier parts of the tissues. 

 A rearrangement of this sort has been demonstrated when the egg of 

 the frog is inverted, and in consequence certain changes are brought 

 about in the development that will be described in another chapter. 



EFFECT OF CONTACT 



The contact of a newly forming part with a solid body has been 

 shown by Loeb in a few cases, at least, to be a factor in regeneration. 

 If a piece is cut from the stem of the tubularian hydroid Tubnlaria 



