36 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



■ i t 



COELENTERATE RECONSTITUTIONS 



Decrease in rate of tentacle development and in number of tentacles 

 appearing in a given time from distal to proximal levels of section has been 

 observed in pieces of hydra {P elmatohydra oligactis) .^ Reconstitution oc- 

 curs at all levels of the stem or hydrocaulus of the hydroid Tubularia, 

 which in some species may attain a length of lo cm. or more without 

 ^ — -^ branching. The hydranth primordium develops 



within the perisarc adjoining a cut end by rediffer- 

 entiation of the coenosarc, the tentacles arising as 

 longitudinal ridges and separating from the hy- 

 dranth body progressively from tip to base (Fig. 13). 

 Emergence of the hydranth is brought about later 

 by elongation of the coenosarc proximal to it. Evi- 

 dently hydranth development progresses from the 

 tip of the piece proximally. Further evidence of this 

 course of development is given by the reconstitu- 

 tion of very short pieces into apical parts of hy- 

 dranths. In these as much of the hydranth is 

 formed basipetally as length of piece and scale of 

 organization permit (pp. 344-49). Pieces a centi- 

 meter or more in length usually give rise to a hy* 

 dranth at each end (oral and aboral hydranth) ; but, 

 in general, results of reconstitution vary with length 

 of piece, body-level, and condition of animal.'' 



Rate of hydranth development, as measured by 

 time from section to emergence from the perisarc, 

 is higher at the distal than at the proximal end of 

 the piece, except when very short; the rates at the 

 two ends may then be equal, because the gradient is 

 practically absent. Sometimes, also, the rates maybe 

 almost equal at the two ends of extremely long stem 

 pieces, apparently because the proximal end has become physiologically 

 isolated from the dominant hydranth and has undergone change in physi- 

 ological condition as a result. Some species of Tubularia form buds and 

 new axes under these conditions, that is, when proximal regions come to lie 

 beyond the range of dominance of the apical hydranth or when its domi- 



5 Peebles, 1897; Koelitz, 1910; Weimer, 1928; Rulon and Child, 1937. 

 ^ For experimental data see Driesch, 1897, 18996; Morgan, 19016, 1902a, 190317, 1905, 

 19066, 1908; Morgan and Stevens, 1904; Child, 1907a-/. 



a. i 



mn 



Fig. 13. — Hydranth 

 reconstitution in Tubu- 

 laria; a long hydranth 

 primordium at oral end 

 of a distal stem-level and 

 a short aboral primor- 

 dium from a proximal 

 level. 



