30 PATTERNS AND PROBLEMS OF DEVELOPMENT 



The chief purpose of the present section, however, is to call attention 

 to some of the graded differences in reconstitutional patterns of experi- 

 mentally isolated parts of more or less mature individuals and particularly 

 to their relations to body-level of origin of the part. The reconstitutions of 

 mature organisms, particularly of those with elongated polar axes, usually 

 show such differences more clearly than those of isolated parts of eggs and 

 early embryos. Experimental analysis of differentials along an axis in the 

 egg and in early embryonic stages, by isolation of parts, is to some extent 

 limited by the presence of the whole axiate pattern in one or a few cells and 

 often also by the regional cytoplasmic differentiations which occur in 

 many eggs. These apparently represent developmental expressions of pat- 

 tern in relation to ovarian environment, maturation, and fertilization, 

 rather than the fundamental pattern itself. For these reasons considera- 

 tion of egg and embryonic reconstitutions is postponed to a later chapter, 

 following presentation of other lines of experimental analysis of patterns 

 of various forms of development. 



For convenience three types of reconstitution may be distinguished: 

 substitution, redifferentiation or reorganization, and regeneration. Sub- 

 stitution is reconstitution in multiaxiate forms by substitution of another 

 axis for one removed. Substitution occurs very commonly in multiaxiate 

 plants in consequence of isolation from a dominant region. Removal of a 

 dominant axis or of its dominant region results in reconstitution of an- 

 other, or of more than one, axis (previously subordinate) into dominant 

 axes; or in initiation and development of one or more new axes — adventi- 

 tious axes — from cells which were previously parts of an axiate pattern 

 already present. In certain conifers, for example, removal of the domi- 

 nant region of the main axis, the vegetative tip, is followed by a turning- 

 upward of one or more of the uppermost lateral branches and its trans- 

 formation from a bilateral to a radial pattern of branching. Bud primordia 

 inhibited by a dominant region of another axis may be activated and may 

 develop following removal or inhibition of that region; or, if no bud pri- 

 mordia are present , adventitious buds may develop following isolation from 

 a dominant region. Substitution may occur in multiaxiate animals. A 

 hydra bud may remain attached and become the apical region if the par- 

 ent body distal to the bud-level is removed.' 



Redifferentiation or reorganization is reconstitution of an isolated part 

 into something else without outgrowth of new tissue from surfaces of sec- 

 tion. A piece of Tuhularia stem, for example, may undergo reconstitution 



3 Weimer, 1928; Rulon and Child, i937«. 



