process, whatever it may be, a process of physiological induction, we may apply a similar name to a process which seems closely analogous to, if it is not really identical with, the process we are now considering. I refer to some highly remarkable observations which were published a year or two ago in Mr. Darwin's work on "Insectivorous Plants". It is there stated that, while looking at a linear series of excitable cells with the microscope, Mr. Darwin could observe the passage of a stimulus along the series, the protoplasm in the cells immediately stimulated first undergoing aggregation, then the protoplasm in those next adjacent doing the same, and so on. Now, the protoplasm in each cell was separated from the protoplasm in the adjacent cell by the walls of both the cells ; yet, notwithstanding there was no observable anatomical continuity between these masses of protoplasm, a disturbance set up in any one of the series of masses immediately set up, by some process of physiological induction, a sympathetic disturbance in the immediately adjacent masses.
This, then, is one case that seems to be comparable with the case of physiological induction in the nerve-fibres of Aurelia, and I think it may be well for physiologists to keep awake to the fact that a process of this kind probably takes place in the case of these nerve-fibres. For it thus becomes a possibility which ought not to be overlooked, that in the fibres of the spinal cord, and in ganglia generally, where histologists have hitherto been unable to trace any anatomical or structural continuity between cells and fibres, which must nevertheless be supposed to possess physiological or functional continuity—it thus becomes a possibility that in these cases no such anatomical continuity exists, but that the physiological continuity is maintained by some such process of physiological induction as probably takes place among the nerve-fibres of Aurelia.
Before quitting the histological part of the subject, it is desirable to state that at about the same time as Mr. Schäfer's work was communicated to the Royal Society, two other papers were published in Germany on the same subject. One of these papers was by Messrs. Hertwig, and the other by Dr. Eimer. Both memoirs display a large amount of patient research, and describe the character and distribution of the nervous tissues in various species of Medusæ. These authors, however, do not describe the nervous network which has been described by Mr. Schäfer. I may add the interesting fact that the nervous tissues in Medusæ appear to be exclusively restricted to the body-layer which is called the ectoderm, and which is the structural homologue of that body-layer in which the nervous tissues of all the higher animals are known to have their origin during the life-history of the embryo.
Proceeding now to state some further results of various physiological experiments, I shall begin with the department Stimulation. And first to take the case of a physiological principle which I observed in the jelly-fish, and which has also been found to run through all ex-