Page:Popular Science Monthly Volume 80.djvu/557

Rh (1839-1843) which he brought forward was able to formulate a definite cell theory for plants; later when this theory was applied to animal tissues and developed by Schwann and Virchow it became an influence as great as that of the theory of evolution, in the development of modern biology.

Schwann, who was at the time an assistant of Müller, received directly from Schleiden the impulse to compare animal and vegetable cells. While carrying out for Müller the experimental study of nerve and muscle, necessary for the proper preparation of his chief's great book on physiology, he became interested in the histological study of these structures and it was at this time that he described the nerve fiber sheath which now bears his name. Once, when he was dining with Schleiden in 1837, the conversation turned to the nuclei of vegetable cells, Schleiden's description of these recalled to Schwann similar structures which he had seen in animal tissues. The resemblance between the animal and plant cells was, without loss of time, confirmed by both observers and the result was Schwann's famous paper (1839) on the accordance in structure of animal and plant tissues.

It is difficult for the student of to-day, thoroughly drilled concerning the details of cell structure in his courses in normal and pathological histology, to realize that only a little over 70 years ago the essential feature of the animal cell, the nucleus, was not recognized, and that it was a botanist who first brought the subject to the attention of a physiologist. Medicine in all its phases has advanced rapidly along the path thus opened up by Schleiden and Schwann. To-day we are interested above all other things in the chemistry of the cell, but from the time of Schwann to the time of Pasteur the study of the morphology of the cell in health and in disease was one of the chief interests of scientific medicine.

It is not to be supposed, however, that Schwann had the conception of the cell which we have to-day. He, as Schleiden before him, made faulty observations and drew faulty conclusions. The important features of Schwann's work were the recognition of the nucleus, not the cell wall, as the important part of the cell, the demonstration of the union or grouping of the cells to form tissues, and the demonstration that the distinctive cells of the tissues of the adult develop from the undifferentiated cells of the early embryo. The misconceptions of the early histologists were natural when we recall the great technical