398 PATTERNS AND PROBLEMS OF DEVELOPMENT 



INDUCED REGRESSION IN CERTAIN HYDROIDS 



Removal of the distal half of the hydranth primordium of Tubularia 

 (see Fig. 13 [p. 36]) in its earlier stages may lead to complete regression 

 of the proximal part and development of a new complete primordium in 

 the usual relation to the end of the piece. In this case the gradient is 

 altered by the activation following the second section, and a new hydranth 

 pattern is determined, partly in the cells forming the proximal part of 

 the first primordium, partly in cells farther proximal. The new pattern 

 obHterates that which had already begun to develop. Results of such op- 

 erations, however, differ with level of section and stage of development 

 of the first primordium. If section is near the distal end, only more or less 

 reorganization of the distal region of the primordium may occur; or if 

 the primordium is advanced in development, regeneration of the part re- 

 moved is the usual result. Regression of the primordium sometimes re- 

 sults in both Tubularia and Corymorpha from section immediately proxi- 

 mal to it in early stages or in its proximal region. Disappearance of the 

 original primordium is followed by development of a shorter one in the 

 distal region of the piece and, in some cases, of another at the proximal 

 end. In these cases activation following section at the proximal end of 

 the early hydranth tends to establish a new dominance and reverse the 

 gradient pattern over more or less of its length; and the reversal obliterates 

 the former pattern, even though it has become morphologically visible, 

 and determines a new pattern at one or both ends.^' 



REGRESSION OF FISSION ZONES 



Fission may be inhibited and the fission zone obliterated in planarians 

 by section and regeneration of a head a short distance anterior to the 

 zone. Here there is no directly visible regression, but the region which 

 represented the anterior part of a posterior zooid is reorganized into a 

 prepharyngeal region. If or when fission occurs in such an individual after 

 reconstitution, it is at a level much farther posterior. In some species of 

 Stenostomum section and reconstitution of a head a short distance anterior 

 to a fission zone already visible, but in early developmental stage, will 

 bring about regression of the zone and reorganization of the region con- 

 cerned into a part of the body of the reconstituted individual. Similar 

 regression and reorganization of early fission zones has been brought about 

 in the same way in several species of microdrilous oligochetes by various 

 experimenters. A section anterior to the zone is much more effective than 

 posterior section (E. H. Harper, 1904). 



" Driesch, 1897, 19026; Peebles, 1900; Child, n)ogd. 



