RECONSTITUTIONAL PATTERNS IN EXPERIMENT 411 



nant region may be said to develop in spite of the isolated piece. The 

 reconstitution of a subordinate part, such as a posterior end, apparently 

 involves no such conflict. Nerve stimuli not only do not inhibit it but 

 may be necessary for its occurrence, probably by their effect on metabolic 

 level of the cells concerned. Local and temporary dominance may exist 

 in the new posterior end, but it does not induce extensive reorganization 

 in more anterior regions. Regeneration of appendages apparently re- 

 sembles more closely posterior reconstitution than it does head reconsti- 

 tution. There may be local dominance in the regenerating tissue, either 

 distally or proximally, but it does not induce any considerable reorganiza- 

 tion in the parts from which regeneration takes place; for example, the 

 regenerating amphibian hmb has little effect on the limb stump, except 

 adjoining the level of section. 



We have seen that new dominant regions can develop in hydroids, 

 planarians, and annelids in the absence of other parts; and there is at 

 present no evidence that other parts of the body, when present, contribute 

 to the completeness of their development; they tend, rather, to inhibit 

 it, if they have any effect. In the later course of development, more par- 

 ticularly in the higher animals, relations of parts may change, local pat- 

 terns of dominance and subordination may arise, and parts originally 

 developing independently of certain other parts may later be dependent 

 on, or affected by, those parts in one way or another. For example, it has 

 been shown by many investigators that in vertebrates absence or addition 

 by implantation of peripheral parts in embryonic or later stages may 

 bring about hypoplasia or hyperplasia in some part or parts of the central 

 nervous system.--' Such effects, of course, have nothing to do with pri- 

 mary developmental pattern; but they indicate that peripheral parts may 

 acquire, after they appear, some degree of dominance over the further 

 development of parts previously independent of them. 



DOMINANCE IN COMPENSATORY REVERSALS OF ASYMMETRY 



An experimentally reversible dominance of an appendage on one side 

 of the body over an appendage of the same segment on the other side 

 has been found in certain annelids and arthropods. The serpulid poly- 

 chete Hydroides dianthus possesses two opercula, one highly developed 

 and functional in closing the tube when the animal withdraws into it, 

 the other rudimentary. Removal of the functional operculum is followed 



^•J Shorey, 1909; Detwiler, 1921, 1923, 1926, 1927, 1936, particularly chap, viii and citations 

 given there; Hamburger, 1934, 1939; Kappers, 1934; May, 1927, 1933. 



