RECONSTITUTIONAL PATTERNS IN EXPERIMENT 413 



that section of one or both nerves might abolish the dominance in the 

 crustacean Alpheus; but because of rapid nerve regeneration, autotomy 

 of appendages following nerve section, and high death rate it was not 

 regarded as demonstrative. It seems not improbable that afferent im- 

 pulses from one appendage may inhibit efferent impulses to the other and 

 so prevent its complete development, but with an equal start in regenera- 

 tion neither may be able to inhibit the other. Perhaps the most interesting 

 question in connection with these asymmetries is that of their origin in 

 egg or embryo. 



EXPERIMENTAL DETERMINATION OF PATTERN AND DOMINANCE 

 BY ENVIRONMENTAL FACTORS 



The differential effects on pattern already present of general uniform 

 exposure to various chemical and physical environments were discussed 

 in chapters v-vii. The present section is chiefly concerned with experi- 

 mental determination and alteration of pattern by differential exposure 

 to environmental factors. 



POLARITY IN RELATION TO DIFFERENTIALS IN OXYGEN TENSION 



Many hydroids are extremely susceptible to low oxygen tension in sea 

 water. It has been shown that change from flowing to standing sea water 

 brings about degeneration and death of hydranths and reconstitution of 

 stolons instead of hydranths from apical as well as other regions in a num- 

 ber of hydroid species (pp. 172-75). Stolons can live and grow at much 

 lower oxygen tension than hydranths. With other species, less susceptible 

 to low oxygen in water, low concentrations of cyanide and various other 

 inhibiting agents determine reconstitution of stolons instead of hydranths. 

 Plumularia pieces in very low concentrations of cyanide and also in stand- 

 ing water reconstitute nothing but stolons from both distal and proximal 

 cut ends and from cut ends of lateral branches; but in flowing, well-aerated 

 water hydranth-stem axes develop from all cut ends. According to Barth 

 (19386), decrease in oxygen tension below 4.5 cc. per liter decreases mark- 

 edly the rate of hydranth reconstitution in Tubularia, and an increase 

 above that level increases rate and size of primordium (see also Torrey, 

 191 2). Local removal of perisarc from pieces of Tubularia stem may 

 bring about reconstitution of a single hydranth or of two with opposed 

 polarities from the exposed coenosarc (Zwilling, 1939). In long pieces of 

 Tuhularia stems with proximal ends exposed to high oxygen tension, distal 

 ends to boiled water, nitrogen, CO2 or a mixture of 90 per cent oxygen, 



