On the Nervous System of Cassiopea Xamachana. 131 



which one half of the marginal sense-organs was removed, while from 

 the other half an equal amount of tissue was cut from between the 

 sense-organs. The two halves were insulated by the removal of two 

 diametrically opposite strips of subumbrella ectoderm. In both 

 experiments the rate of regeneration was measured inward from the 

 periphery of a cavity in the center of the disk from which a circular 

 piece of tissue had been removed. 



In both these researches the experiments were carried out with the 

 view of ascertaining the influence of muscular activity and thus 

 indirectly of the nervous system on the rate of regeneration. In each 

 case it was held that there was no constant difference in the rate of 

 regeneration between the active and inactive individuals. 



In the course of my studies, which were taken up primarily to 

 reexamine the work of Stockard and Zeleny upon this point, I dis- 

 covered that the marginal sense-organs influence regeneration inde- 

 pendently of their control of muscular activity. Such an influence of 

 the sense-organs can be accounted for either on the ground that 

 metabolic activities, not expressed by muscular activity, are under the 

 control of the sense-organs, or that a direct trophic influence is exerted 

 by the sense-organs on the regenerating tissues. A series of deter- 

 minations with the ''biometer" of the rate of CO2 production by 

 specimens under different experimental conditions, for which I am 

 indebted to Dr. S. Tashiro, shows that the first of the two alternatives 

 just mentioned offers a satisfactory explanation of the observed facts. 



The course of normal regeneration as shown when a disk of tissue is 

 removed from the center of a medusa disk is that first the ectodermal 

 epithelium from both the exumbrella and subumbrella surfaces grows 

 inwardly over the exposed surface of the mesogloea until that extending 

 from the two surfaces has come in contact. Fusion of the two layers 

 takes place at once and then a sheet of new tissue begins to extend 

 across the cavity left by the removal of the disk of tissue. In this 

 sheet of new tissue no mesogloea layer is at first distinguishable, the 

 two layers of epithelium being in direct contact at their inner surfaces. 



Just as in the embryonic development of all Cnidaria, the mesogloea 

 makes its first appearance as an acellular layer of gelatinous material 

 secreted by the cells of the layers it is to separate. In later stages of 

 regeneration the newly formed mesogloea fuses insensibly with that of 

 the body of the disk at the periphery of the cavity. When the sheet of 

 regenerated tissue reaches the center of the cavity its edges fuse, but 

 leave for more than 24 hours a recognizable scar at the point where 

 fusion took place. In the specimens used in the following experiments 

 this scar was at first eccentrically placed, but within the first day after 

 it had been formed it came to lie centrally in the sheet of new tissue. 

 From this time the most noticeable change was the increase in the 

 thickness of the regenerated tissue, which within two weeks — in speci- 



