RECONSTITUTIONAL PATTERNS IN EXPERIMENT 405 



stage of development. And if this is the case, the physiological isolation 

 making possible development of a new zooid is not complete isolation. 

 That it is not complete is further indicated by the fact that contraction re- 

 sulting from stimulation of an advanced head may extend posteriorly be- 

 yond an early fission zone but may be blocked at an advanced zone. 



In 5. leucops head development from an anterior cut end is very fre- 

 quently completely inhibited, apparently by the rapid development of a 

 fission zone and head at a more posterior level. This development may 

 have begun before section but not have become visible, or may be initiated 

 by isolation of the piece and, being more rapid than reconstitution at the 

 anterior cut end, inhibits the latter (Van Cleave, 1929). No case of reduc- 

 tion and destruction of zooids or of head reconstitution from anterior cut 

 ends was observed in more than a hundred pieces of a Japanese species 

 resembling S. leucops, cut in various relations to visible fission zones; but 

 new fission zones developed very rapidly in the pieces, and their domi- 

 nance apparently inhibited reconstitution at the anterior cut end. These 

 species differences suggest that if head reconstitution at a cut end is slow, 

 as compared with head development at a fission zone, the latter may 

 inhibit the former. 



Destruction of zooids and inhibition of head reconstitution at a cut 

 end in an animal in which head development occurs rapidly at a fission 

 zone are of considerable interest as effects of obliteration or reversal of a 

 gradient by change in position of the dominant region. In S. grande 

 this reversal determines not only inhibition of development but complete 

 loss of the epithelial character of the body layers and their separation 

 into isolated cells. In S. leucops and the Japanese species the reversal of 

 dominance has less extreme effects; it inhibits head reconstitution with- 

 out destruction of zooids. The question may be raised whether the pres- 

 ence of a gradient is necessary for maintenance of the epithelial character 

 of the body layers in S. grande. The isolated cells are not killed but have 

 evidently lost pre-existing relations to each other. In this connection cer- 

 tain other suggestive cases may be recalled. When the gradient of the 

 axis of the alga Griffithsia is obliterated by differential inhibition with 

 chemical agents, the cells separate but are not necessarily killed, and with 

 return to normal environment may reconstitute new axes (Child). With 

 decrease or obliteration of the gradient in the blastula of Phialidiiim loss 

 of epithelial character and immigration into the blastocoel of cells from 

 all parts of the blastula wall, instead of from the basal region only, occur 

 (p. 167). The entoderm of echinoderm gastrulae and exogastrulae shows 



