168 C. M. CHILD. 



by Dr. Hyman with similar results. But the relative amount of 

 alimentary tract tissue in pieces of a given weight taken near 

 the head is less than in pieces taken near the mouth, so that even 

 in animals which have been starved for some time the difference 

 in total oxygen consumption or total CO 2 production of pieces 

 from different regions is not strictly comparable with the regional 

 differences in susceptibility of the body-wall. Where it has been 

 possible thus far to use these methods we have found that the 

 axial gradients demonstrated by other methods are also gradients 

 in oxygen consumption and CO 2 production. 1 



Gradients in Organs. By one method or another or by several 

 different methods, the existence of physiological gradients in 

 various axiate organs or parts of many organisms has been 

 demonstrated, e.g., in various reproductive axes and in the 

 "hairs" of algse (Child, 'i6c, e, '170, J 2Oa), in the larger, slow 

 moving flagellum of Noctiluca, in the tentacles of hydrozoa 



1 Allen ('20) has recently reported failure to find differences in CO 2 pro- 

 duction corresponding to the antero-posterior gradient in pieces of Planaria 

 agilis. This is a different species from the one used in this laboratory, but 

 conditions are probably not fundamentally different in the two. It may be 

 pointed out, however, that the differences in CO 2 production in pieces from 

 different body-levels in Planaria dorotocephala are not very great (Robbins 

 and Child, '20) and that Allen's failure to find such differences in P. agilis is 

 probably due to the fact that the activity of the alimentary tract was not suffi- 

 ciently decreased by starvation to permit the differences in the body wall to 

 appear. A study of P. agilis is now being made in this laboratory by Dr. 

 Hyman and it has been found that the alimentary tract of this species is con- 

 siderably larger in relation to the size of the animal than in P. dorotocephala. 

 An individual of P. agilis of given size is able to take in a much larger quan- 

 tity of food than a P. dorotocephala of the same size. In well fed individuals 

 of P. agilis oxygen consumption per unit of weight is only about two thirds 

 that in well fed individuals of P. dorotocephala of the same size. Much if 

 not all of this difference is undoubtedly due to the fact that a much larger 

 proportion of the total weight of P. agilis than of P. dorotocephala consists of 

 food in the alimentary tract or of reserves which are taking little or no part 

 in respiration. Moreover, as might be expected, P. agilis starves and under- 

 goes reduction much less rapidly than P. dorotocephala and Allen's failure to 

 find an increase in rate of oxygen consumption in the later stages of starvation 

 was due merely to the fact that the starvation period in his experiments was 

 -not long enough. Dr. Hyman has found that an increase in oxygen consump- 

 tion does occur in this species as well as in P. dorotocephala and P. maculata 

 in the later stages of starvation, but under similar experimental conditions 

 the rise begins considerably later and progresses more slowly in P. agilis than 

 in the other species. 



