198 C. M. CHILD. 



CONCLUSION AND SUMMARY. 



It is evident that the formation of stolons in the hydroid 

 species investigated is not dependent on region of stock, gravity, 

 or contact, but rather on a certain degree of inhibition or de- 

 pression, which may be determined either by relations to other 

 parts of the stock or by external factors. Theories of physiolog- 

 ical polarity based on distribution or direction of flow of hypothet- 

 ical "formative stuffs" or upon molecular polarity and orienta- 

 tion afford no satisfactory interpretation of the facts presented 

 in this paper. On the basis of such theories we must assume that 

 placing the animals in standing water or in the experimental 

 solutions must alter fundamentally the distribution or direction 

 of flow of the formative stuffs, or must alter the molecular 

 orientation or polarity in many different ways. But there is not 

 the slightest reason for believing that such changes in conditions 

 could accomplish any of these results. In terms of physiological 

 gradients, however, all the facts are readily and simply accounted 

 for and brought into line with other facts, and the earlier ob- 

 servations concerning such transformations and changes in 

 polarity are likewise easily interpreted. In this field, as in many 

 others the gradient conception affords a basis for the interpreta- 

 tion and synthesis of data which has previously been lacking. 



In view of apparently persistent misunderstanding of the 

 gradient conception, it is perhaps necessary to emphasize once 

 more the fact that it is concerned with the physiology of develop- 

 ment, not with heredity. In other words, the specific protoplasm 

 of Bougainvillea, of Gonothyrcea, or of Obelia, with all its heredi- 

 tary potentialities, whatever these may be, is in each case the 

 basis in which the gradient appears. This conception merely 

 holds that, given this or any other specific protoplasm, the physio- 

 logical gradient is an essential and fundamental factor in the 

 realization of the hereditary potentialities in the form of an axiate 

 individual of the species to which the protoplasm belongs. 



The chief points are summarized as follows: 



I. In various hydroid species the development of stolons can 

 be induced by slightly inhibiting or depressing conditions, e.g., 

 low concentrations of ethyl urethane, MgSO 4 , KNC, HC1, LiCl 

 etc. and in most species even by change from natural conditions 

 to standing water in the laboratory. 



