Factors in Regeneration 335 



erate a polyp at the tip. In these, it is impossible to tell where 

 the stolon ends and where branch begins. It is nearly always 

 difficult to tell in advance whether a large growing shoot will 

 ultimately become a branch or a stolon. Stolons in my experi- 

 ments never regenerated stolons; when cut, only hydranths were 

 produced at the cut ends, irrespective of the level at which the 

 cuts were made. 



When stems are subjected to adverse conditions, the cut ends 

 may regenerate new tissue, which becomes surrounded by a sticky 

 perisarc, which does not differentiate into perfect hydranths. 

 The new tissue is really a modified stem, which under these adverse 

 conditions may increase in length and is then called a "stolon;" 

 when it is amputated or when it grows into a favorable environ- 

 ment the distal end frequently differentiates into a hydranth. 

 When a large number of stems were placed in a shallow dish of 

 water containing much debris, and the water was left undisturbed 

 for many days, very few polyps appeared, while a remarkably 

 large number of stolons were produced. When the water was 

 frequently changed, however, the "stolons" invariably bore polyps. 

 As the colony grows older, the perisarc of the stolons becomes 

 thicker, less plastic and encrusted with debris. If the conditions 

 remain constant, the "stolon "functions permanently as an anchor- 

 ing organ and can no longer of itself produce polyps, though it 

 has the ability to do so, if cut and removed to a favorable environ- 

 ment. "Stolons" are not limited to any particular region nor is 

 their formation influenced by gravity, size of the piece, kind of 

 regeneration at opposite end, etc. Their presence or absence, in 

 this hydroid at least, is not an indication of the presence or absence 

 of certain internal changes and is, therefore, useless as an index 

 of the polarity of the stem. 



RATE OF REGENERATION^^ 



The term rate is here used to designate the interval between 

 amputation and that point in the differentiation of the regenerating 



'''Loeb ('91) determined rate of development and rate of growth for different hydroids. Morgan and 

 Stevens '04, Journ. Exp. Zool.,made careful observations on the rate of development at basal and oral 

 ends of pieces. 



