2 6 2 THE POPULAR SCIENCE MONTHLY. 



cause which produces the cells of the dorsal cord could not be 

 different from the one that gives origin to the vegetable cells." 

 The two men went together to the amphitheatre of anatomy to 

 examine the nucleuses in question, and Schleiden recognized a 

 complete resemblance between them and the nucleuses of the 

 cells of plants. " From that time," Schwann continues, " all my 

 efforts were directed to finding proof of the pre-existence of the 

 nucleus in the cell." And he goes on to tell how his views were 

 confirmed as his researches advanced. At the time Schwann thus 

 undertook to show that all the organs are of cellular origin, the 

 structure of most of them was very imperfectly known. The 

 application of the microscope to researches in animal histol- 

 ogy was of recent introduction, and everything was to create. 

 Schwann did not shrink from the tremendous task which opened 

 up before him ; and what he had done first for the cartilages and 

 the dorsal cord, he tried in succession for all the other bodily tis- 

 sues ; and in all he had the joy of seeing his idea confirmed. 



Schwann came upon many new discoveries in the course of 

 these investigations. He first compared the egg to a cell, and rec- 

 ognized cells in the globules of the blastoderm; described the 

 stellar pigmentary cells, the layers of the nail, the development of 

 feathers, the nucleuses of the prisms of the enamel, those of the 

 smooth and striated muscles, the fibers of the dental pulp, the 

 cells destined to be transformed into fibers of the crystalline, etc. 

 He called attention to the envelope of the nervous fibers which 

 bears his name as the sheath of Schwann — all of which discover- 

 ies have been confirmed by modern research armed with its more 

 perfect technic and superior instruments. The theory of the cell 

 as the primordial element of all the tissues was hereafter to serve 

 as the Ariadne's thread to the numerous investigators who devoted 

 themselves to the study of morphology, ' and was to help them 

 explain the infinite variety of organic forms. It gave a definite 

 purpose to the application of the microscope to investigations in 

 anatomy and physiology. It was the foundation of modern physi- 

 ology? and all the morphological progress accomplished during 

 nearly the past half-century has grown out from it. Except for 

 its having familiarized the conception of the constitutional unity 

 of living matter, and having declared the principle that every 

 cell is the product of another cell, the doctrine of selection and 

 descent could not, in the opinion of Edward Van Beneden, have 

 gained ground. Its salutary influence in pathological anatomy 

 and the advance of physiology was immediate and great. Acting 

 in another direction, it put an end to the theory of a special vital 

 force, which was in full sway when it was first promulgated, and 

 raised up that of physico-chemical action, which has taken its 

 place. How was it possible to reconcile the notion of cellular 



