292 THE BIOLOGY OF HYDRA : 1961 



iial colonies, which do approach exponential increase in hydranth 

 number. One can escape this new dilemma by doing what the colon- 

 ies do, namely by introducing secondary stolons at intervals. If one 

 adds such secondary stolons at appropriate times, one can make the 

 growth of the model colony closely approach exponential. I do not 

 know as yet whether or not this is the way colonies maintain ex- 

 ponential growth. 



Laboratory colonies appear to develop in accord with the mod- 

 el. This has been determined by measuring every relevent variable 

 of the pattern of individual colonies, a task much facilitated by the 

 use of a marking technique. If colonies are dipped into trypan blue, 

 the perisarc is stained a deep blue while the tissue is unstained 

 and unaflFected. When such a colony is grown in the absence of 

 trypan blue, all new growth is colorless while that part of the colony 

 present as perisarc at the time of marking remains blue. Thus new 

 growth can be precisely measured as separate from old. The meas- 

 urements support the picture of colony formation just described, 

 except that branch tubes appear to grow more slowly, or at about 

 one-eighth the rate of stolon tubes. But upright tubes grow at al- 

 most exactly one-half the rate of stolon tubes. 



In conclusion, it has been possible to reduce the development of a 

 Cordylophora colony to the growth and branching of a series of 

 tubes: stolons, uprights, and side branches. The parameters of col- 

 ony shape may be summarized in tabular form: 



*These tubes also differ from stolons in that they bear hydranths and only develop 

 at growing tips. 



A model has been developed integrating many of these aspects 

 of asexual colony development, and the development of indi- 

 vidual colonies studied in relation to the model. 



