CHAPTER XI 



RECONSTITUTIONAL PATTERNS IN RELATION TO 

 EXPERIMENTAL CONDITIONS 



RECONSTITUTIONAL development provides a wealth of mate- 

 rial showing establishment of new developmental patterns in 

 - definite relations to experimental conditions. New patterns can 

 be determined experimentally not only in isolated pieces of mature indi- 

 viduals but in eggs and early embryonic stages of some forms. From some 

 of these experimentally determined patterns and the ways in which they 

 are determined we can learn something about the real beginnings of de- 

 velopment and how these beginnings can be initiated. The egg at the be- 

 ginning of embryonic development usually has already an established 

 pattern, often with considerable regional differentiation and relatively 

 stable. Except in those cases showing capacity for reconstitution of pat- 

 tern, most eggs and early embryonic stages tell us little concerning the 

 beginnings of developmental patterns. 



NEW PATTERN IN RELATION TO SECTION 



In the preceding chapter it was shown that in coelenterates, planarians, 

 nemerteans, and annelids new dominant regions may be determined by 

 the activation adjoining level of section, that they are self-differentiating, 

 and that they may act as inductors of a new polarity. Even when this 

 polarity is in the same direction as that of the original individual, it is a 

 new polarity, at least as far as alteration of the pre-existing gradient and 

 pattern extends from the dominant region. The unipolar reconstitutions 

 of Tuhularia and Corymorpha in Figure 113, A-D (p. 334), and Figure 116, 

 A, B, E, G (p. 346), are cases in point. The bipolar forms of Figures 113 

 and 116 are cases in which activation at both ends of the piece was suffi- 

 cient to determine a new polar pattern over a greater or less distance. 

 Corymorpha pieces often give rise to multipolar forms, particularly in 

 short pieces from levels near the basal end, from animals in poor physio- 

 logical condition, under experimental inhibiting conditions, and some- 

 times from differential exposure. At least some of the polarities in these 

 are localized in a definite relation to the region of section, but some may 



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