membrane makes its appearance, probably originating as a transformation either of the cells which constitute the lower part of the cap of the enamel-organ, or of those which form the lower part of the investing capsule or tooth-sac, and the connection of the tooth with the external skin layer is again established through the pericemental membrane. The formation and growth of the cementum of the tooth is thus seen to be a modification of the methods by which the outward growth of the external skin layer is effected.

My reference to the formation of embryonal corpuscles in the transition of the prismatic cells of the cap of the enamel-organ into ameloblasts is spoken of as a kind of physiological necromancy, in which it is necessary to "wholly and completely set aside every law of nature." Well, this is not the first instance in the history of the world of men attributing to magic phenomena which they did not understand. If my critic has not watched the process by which a tissue returns to its embryonal condition, he certainly has much to learn concerning retrograde metamorphosis. This would hardly be the proper time for me to enter into a discussion of the processes of inflammation, but, so far as this wonderful and orderly provision of nature applies to the development of the teeth, I am quite well prepared to demonstrate everything which I have asserted. The photograph on the screen (Plate II., Fig. 9) is from a section cut just prior to the commencement of the formation of either dentine or enamel. In the earlier stages preceding this the columnar layer of cells is plainly seen at the lower part of the enamel-organ. But at this stage they have entirely disappeared, and from them there has been developed a sheet of round embryonal corpuscles. Those constituting the lower layer of this sheet are destined to develop into the ameloblasts proper, which, while somewhat resembling the original columnar layer, are quite different in many essential features, being club-shaped, or larger at the ends and constricted somewhat in the centre. This photograph (Plate II., Fig. 10) is from the side of a tooth somewhat more advanced in development. Enamel has just begun to be formed at the tip of the cusp or cutting edge. Further down on the side, in the region from which this photograph was taken, the enamel-forming cells are just developing from the embryonal corpuscles. The upper portion of this sheet of embryonal corpuscles is developed into the flattened layers known as Nasmyth's membrane, which is clearly seen in any wellmounted specimen. (See also Plate I., Fig. 4.)

Leaving this part of my subject, I call your attention to the

structure of that layer of cells surmounting the tooth-pulp, and known as the odontoblasts. You are all aware that Prof. Heitzmann and his pupils, Drs. Abbott and Bödecker, have represented these bodies as round or oval nucleated corpuscles, between which the dentinal fibrillæ pass. Dr. Abbott, I believe, has stated positively that the dentinal fibrillæ are direct prolongations of non-medullated nervefibres, and they are so represented in his chart. Dr. Bödecker has stated it as his opinion that there is a direct connection. I had long been perfectly satisfied in my own mind that they were holding erroneous views of the origin of the fibrillæ, but it was not until a few months ago that I succeeded in cutting sections so thin that but a single layer of odontoblasts was represented. These specimens have been examined by Prof. Heitzmann, Dr. Bödecker, and Dr. Atkinson. Prof. Heitzmann had hardly more than glanced at one of the sections, when, with that perfect frankness which is so characteristic of this noble searcher after nature's truths, he admitted his mistake. Subsequently, while examining this specimen under my 1-25 inch objective, he thought he could detect very delicate fibres passing between the odontoblasts. But it is certainly established beyond all question that the dentinal fibrillæ proper are prolongations of the odontoblasts. The discovery that there are frequently two and sometimes three fibres projecting from a single cell, and that these cells are connected at their base by roots or processes with the reticulum of the pulp, and the fact that the pulp itself is a form of persistent embryonal tissue, as shown by the researches of Heitzmann and Bödecker, led me to the conclusion that they are a form of ganglion cell. The photograph on the screen (Plate I., Fig. 6) is from the periphery of the pulp of a completely developed tooth. It is plainly seen that the odontoblasts terminate in fibres, and close examination reveals some of them dividing into two or three. But this point is so difficult of elucidation that I shall depend largely upon the sections under the microscopes for a perfect demonstration.

I have stated in my recent series of papers in the Dental Cosmos that, since the announcement of my discovery of the ganglionic nature of the odontoblasts, I have frequently been asked how I explain the formation of dentine. Do these cells perform a double use? Are they elements of sensation, and also for the elaboration of the dentine? It would seem to be almost a self-evident conclusion that the material for maintaining the continued integrity of the dentine must be provided by these cells, for the living matter (the fibrilla) which fills the canaliculi is a prolongation of these cells, as we have

seen. If the supply of nutritive material for maintaining the continued integrity of the dentine comes from this source, why may we not logically infer that it was originally formed in this way?

A study of the physiological significance of the primary germinal tissues again finds practical application here.

72

FIG. 17.

We have seen that in many of the low forms of animal existence, the tissues of which are represented in higher forms while in an embryonal condition, where differentiation has made little progress, a multiplicity of function is performed by the same elements or cells. Pulp-tissue, therefore, being a persistent form of embryonal tissue, we should expect to find it performing a multiplicity of functions. The odontoblasts, while composed of what is essentially neural matter, are probably at the same time the active elementary agents in

FIG. 18.

secreting the material for the formation and continued integrity of the dentine. Such instances of multiplicity of function are by no means rare in the lower forms of functioning bodies. In the myo-epithelial cells of Hydra, as seen in the view on the screen (Fig. 17), we have an instance of contractile fibres originating in epithelial cells. In the Protozoa there are found elements which perform the function of nerves and muscles at the same time. In the neuro-epithelial sense cells of Aurelia Aurita we have almost a perfect duplication of the odontoblasts. These are specially modified epithelial cells of the digestive tract. "They consist," says Balfour, "of an outward process which is prolonged into a fine hair, while their inner ⚫ extremity terminates in fine processes which join a delicate network of protoplasmic fibres." One might easily mistake this for a description of the odontoblasts. (Fig. 18.)

Gentlemen, I hope I have made these matters plain to you, and I trust the contribution will prove, for the honor of the profession which I love, some slight addition to our rather scanty knowledge of an obscure subject. I also trust that in what I have said I have given no cause for offense. I have spoken earnestly and somewhat pointedly, because I could not but see that my critics seemed to hold the opinion that a knowledge which has grown out of the practical investigations of years could be brushed aside by a few hastily pre

pared conclusions drawn from a superficial examination of inferior microscopical preparations. If they are willing to do the drudgery incident to this kind of work, and will go over the ground in the plodding manner that is necessary, I think we shall not be very far apart at the end. I now invite your attention to the exhibition under the microscopes, in which you will doubtless find the most interesting feature of the demonstration.

DISCUSSION.

Dr. ATKINSON: The demonstrations that have been made before us show so clearly the justification of the interpretations which have been made of the mode of development of the tissues, by a very few who were willing to walk in the way of understanding obedience to the presence that revealed it to their minds in this investigation, that I wish to refer, somewhat in detail, to the clear demonstration of how the mistakes have been made in previous interpretations, and to display the folly of men attempting to collate from the recorded statements of other observers, without personal experience in making sections themselves.

The history of how this came before us is a little peculiar. At the meeting of the New Jersey State Society at Asbury Park, Dr. Cryer presented me with his work which embodies his views on this subject. I characterized it to him as utterly erroneous and shameful for a teacher to present. His reply was this: "I was not sure that I was right, but I was determined to bring it before just such men as you, that if I were not right I might have the benefit of better intelligence than I possessed." I was induced to send a copy, of which he gave me three, directly to the essayist of the evening; and he informed me that the moment he received it the influence upon him was such that he could not do other than work faithfully in what he had been before convinced was the true path; and it is to these incessant currents of energy and study that have been so much decried by some who call themselves scientific par excellence, that we are indebted for what we have had.

I cannot find words to express my admiration for the manner in which the essayist has exposed the errors of those investigators who have taken advantage of their own mistakes and unskillfulness to prove their positions. He has shown how, in the pressing down of the cover-glass on the fluid in which the specimen is imbedded, they have separated tissues which should be joined.