Ij8 C. M. CHILD. 



some decrease in oxygen content. When development takes 

 place in standing water several centimeters deep without renewal 

 some degree of differential inhibition and modification usually 

 occurs at the bottom while attachment and development of 

 hydranth-stem axes proceed in the normal manner (Child, '25) 

 on the sides of the container near the surface. Here also there is 

 some decrease in pH at the bottom. When development takes 

 place in a layer of water only a few millimeters deep no such 

 differences appear and development is wholly or almost wholly 

 normal if the material is otherwise in good condition. Occasional 

 cases of modification usually occur even in the best material kept 

 as nearly as possible under optimum laboratory conditions. Such 

 modifications doubtless result from factors affecting particular 

 eggs or spermatozoa before development or from poor condition of 

 certain individuals. 



Experiments with the medusae as the inhibiting agents were 

 performed by placing a number of the medusae in small aquaria or 

 bowls with the developmental stages and determining the pH 

 colorimetrically from time to time. 1 The modifications produced 

 by crowding, standing water etc., are exactly similar in character 

 to those produced by the various other agents used. Some forty 

 years ago Metschnikoff ('86) described as variations similar 

 modifications in the development of Mitrocoma. 



As will appear below, with the various agents and conditions 

 used, developmental modifications result from the differential 

 susceptibility of different body levels. All agents and conditions 

 used inhibit development, the higher levels of the gradient being 

 most inhibited. In the lower concentrations differential acclima- 

 tion, or after return to sea water differential recovery may occur, 

 the higher levels of the gradients acclimating or recovering more 

 rapidly or more completely than the lower levels, as indicated by 

 increased developmental activity. 



1 The work of Smith and Clowes ('24) makes it probable that in these experi- 

 ments, as in those with HC1 (see footnote, p. 177) the chief inhibiting factor is the 

 COs resulting from the respiration of the medusae, rather than the increase in 

 H-ion concentration brought about by the dissociation of carbonic acid formed 

 from a part of the CC>2. The fact that the decrease in pH is rapidly reversed on 

 shaking the water with air indicates that CO production is responsible for it, but 

 it is possible that other products of medusa metabolism may play some part in the 

 inhibition of development. 



