354 LIB3IE H. HYMAN. 



data have been checked by several different methods it seems 

 hardly scientific to dismiss the matter under the word "assumes." 

 Gradients are not assumptions; they are facts. It is legitimate, 

 of course, for any one to question and criticize the interpretation 

 of these facts, and desirable that other possible explanations of 

 them should be suggested ; but to ignore such facts by designating 

 them as assumptions is not the way to arrive at scientific truth. 

 The nature of the axial gradients has been so thoroughly and 

 frequently discussed in numerous papers from this laboratory 

 that presentation of the subject here seems to me superfluous. 



Banus next remarks that the hydroid Tubularia is extensively 

 used to support Child's conceptions. It is scarcely necessary to 

 point out to the zoological world the exaggeration conveyed by 

 this statement, as it is well known that other ccelenterates have 

 been used as extensively and other lower forms much more 

 extensively in accumulating the experimental evidence upon 

 which those conceptions rest. 



We are next informed that Child "has made no measurements 

 of the rate of metabolism of different regions of the stem of 

 Tubularia." We would be pleased to carry out such experiments 

 if Banus would kindly suggest a suitable method. The matter 

 would be relatively simple were it not for the fact that the 

 relative proportions of perisarc and cosnosarc vary in different 

 regions of the stem of Tubularia. It is therefore difficult or 

 impossible to determine the amount of living material in portions 

 of the stem and impossible to establish any basis for comparison 

 of the metabolism of different regions. Naked hydroids, such 

 as Corymorpha, would be required for experiments of this kind. 

 The metabolic rate of different regions of the first zooid of 

 Planaria dorotocephala has been determined and has been found 

 to accord with the metabolic gradient conception. 



Banus then proceeds to discuss the regional differences in 

 rate of regeneration of Tubularia. He says that Child "as- 

 sumes" the existence of such differences. In his summary he 

 states that "the rate of regeneration of the oral hydranth of an 

 apical piece is on the average identical with the rate of re- 

 generation of the oral hydranth of the basal piece"; and further 

 that "there is no evidence of the existence of level or regional 



