390 THE HISTORY OF BIOLOGY 



animals and plants and the problem of their fundamental constituents, and 

 on the other hand that world of minute, independently living creatures that 

 the new instruments made it possible for the eye to see — in collections of 

 water, in infusions on parts of plants (hence Infusoria), and indeed every- 

 where in nature. As a result of these investigations there arises an entirely 

 new conception of the composition of organisms — cytology, or the knowl- 

 edge of cells. An attempt to show the development of this branch of knowl- 

 edge in summary form offers certain special difficulties; as Richard Hertwig 

 strikingly remarks: "The way was paved for the reform of the cell theory 

 through discoveries made in very different spheres and not until late in time 

 concentrated in a focus." We must therefore give a brief summary of these 

 various discoveries, though it should be mentioned in this connexion that 

 many important steps were taken in this field by people who otherwise 

 exercised little or no influence upon scientific progress. For the sake of brevity 

 we must confine ourselves to discussing only the most important achieve- 

 ments and personalities in the history of cytological research. 



Works on -plant-tissues 

 In the beginning of the eighteen-thirties Bichat's tissue theory was still 

 accepted in zoology, though more or less modified by various investigators. 

 In botany it was different. Since the days of Malpighi and Grew it had been 

 known that the wood of plants is composed of cells — minute chambers 

 having more or less thick walls. It was a matter of dispute whether a num- 

 ber of other elements in the plant, especially spiral vessels and bast, were 

 compact or cellular. The first who attempted to compare the composition 

 of animal and plant was C. F. Wolff; he believed, it will be remembered, 

 that the construction of each represents a mass of cell-shaped forms. Among 

 later scientists Blainville produced a theory, mentioned in the foregoing, 

 that the animal organism is composed of cells, but this theory was not very 

 clearly developed and therefore won but little acceptance. The knowledge 

 of cells, however, made steady, if slow, progress, the botanists still leading 

 the way. To start with, it was a question of deciding whether all the parts 

 of the plant consist -of cells, and this led to lively discussion. Among those 

 who contributed to its solution may be mentioned Charles Francois 

 MiRBEL (1776-1854), professor of botany at the Jardin des Plantes, and 

 LuDOLF Christian Treviranus (1779-1864), professor at Bonn, where he 

 succeeded Nees von Esenbeck. Mirbel especially examined the cell-structure 

 in certain mosses, making valuable contributions to the subject, besides 

 which he resolutely maintained the cell's quality as a basis for all structures 

 in the vegetable kingdom. Treviranus, on the other hand, performed a signal 

 service in the observations he made in regard to the regular movements of 

 the cellular contents in a number of vegetable forms; moreover, he observed 

 that the spiral vessels in plants originate in cells which become stratified 



