Invertebrates 



671 



end. Of course any electric current flowing 

 through the stem would also be broken by 

 the ligature since there is no connection 

 between the ends save through the non- 

 cellular perisarc. In an effort to see whether 

 the circulation or the cellular connections 

 transmit the inhibition, two kinds of experi- 

 ments were carried out. 1. Circulation was 

 blocked by means of an oil droplet or an air 

 bubble (Fig. 230). The basal end then was 

 freed of inhibition and regenerated a hy- 

 dranth. 2. A glass tube was inserted into 

 the coelenteron and the tissue ligatured 

 around this tube. The circulatory fluid would 

 pass through the tvibe but the cellular con- 

 nections between the two ends was broken. 

 Under these conditions the basal end was 

 inhibited. Evidently the inhibition is trans- 

 ported through the circulation (Barth, '40; 

 Rose and Rose, '41). 



Continuing with the analysis, now using 

 ascidians, it can be shown that excretory 

 products inhibit regeneration of zooids from 

 pieces of the stolon (Barth and Barth, '50). 

 Four to six pieces were placed with one of 

 their ends in close proximity, but not touch- 

 ing, in the manner of spokes in a wheel 

 (Fig. 231). Thus one end of each piece was 

 exposed to the accumulated excretory prod- 

 ucts of all the others while the opposite 

 end was not so exposed. The ends in close 

 proximity were inhibited and zooids did not 

 form. The opposite free ends developed 

 zooids. Next carbon dioxide was applied in 

 graded concentration to one end of the 

 piece and the end at the highest concentra- 

 tion of carbon dioxide grew out as a stolon 

 while the end at the lowest concentration 

 formed a zooid (Fig. 231). Urea and uric 

 acid also inhibit zooid formation in a like 

 manner. 



Analyzing these simple observations we see 

 that substances pass through the circulation 

 between one end. A, of a regenerate to the 

 other end, B, and that the inhibition of end 

 B is released by isolating it from end A. A 

 therefore either (1) takes away from B sub- 

 stances necessary for the regeneration of a 

 particular structure C; or (2) A produces 

 inhibitory products, /, in such amounts that 

 the concentration of / is too high for the 

 regeneration of C at B. Therefore some 

 other structure, D, regenerates at B instead 

 of C. We have advanced no argument for 

 (1). Indeed the arguments for this possibility 

 are as yet too tenuous (Fig. 230) since sub- 

 strate concentration has not been controlled. 

 However, we have direct evidence that ex- 

 cretory products do inhibit the differentia- 



tion of C but permit the differentiation of D. 

 These observations and experiments make 

 the competition hypothesis attractive for 

 futiire studies, but it seems only reasonable 

 to investigate the effects of the above ex- 

 perimental treatments on electrical potential 

 differences. 



ENVIRONMENTAL FACTORS 



For regeneration to proceed the external 

 medium must meet the usual requirements of 

 temperature adjustment, hydrogen ion con- 

 trol, gaseous exchange, salt balance and 

 osmotic pressure. In addition, in marine 

 organisms especially, the rate of circulation 

 of the sea water and the population density 

 are important factors. The presence or ab- 

 sence of a surface for attachment is a factor 

 in sessile animals. In general the require- 

 ments for regeneration in regard to en- 

 vironmental factors are more rigid than 

 for simple maintenance. For example, Tu- 

 bularia stems remain healthy at low oxygen 

 tension, but fail to regenerate a hydranth. 

 In certain ascidians, stolons will maintain 

 themselves in standing sea water at 28° C, 

 but regeneration of a zooid does not occur 

 unless the temperature is reduced to 23° C. 

 (Jaeger and Barth, '48). 



Temperature is a very important factor in 

 marine organisms during the summer 

 months when much of the work on regener- 

 ation is done. Laboratory temperatures are 

 often too high for regeneration and a lower- 

 ing of the temperature by a few degrees 

 centigrade is then necessary. Temperature 

 effects are related to rate of circulation of 

 sea water in such a way that regeneration 

 will occur at higher temperatures in running 

 sea water as compared with standing sea 

 water. Certain ascidian stolons will form 

 zooids at 28° C. in running sea water but 

 not in standing sea water. Hydroid stems 

 show a similar behavior. A reasonable in- 

 terpretation of the above observations is 

 that inhibitory products accumulate in the 

 cells faster in standing sea water, and pos- 

 sibly the oxygen tension within the cell is 

 lower, than in circulating sea water. If this 

 is so then lowering of the temperature will, 

 by lowering the rate of metabolism, decrease 

 the rate of accumulation of excretory prod- 

 ucts and will also increase the internal 

 oxygen tension. 



Population density. The numbers of indi- 

 vidual regenerating parts, or perhaps better 

 the total mass of cells per unit volume of 

 external medium, is a factor in regeneration. 



