a history of science-4-第24章
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cation。 This friend was Dr。 Theodor Schwann； professor of physiology in the University of Louvain。
At the moment when these observations were communicated to him Schwann was puzzling over certain details of animal histology which he could not clearly explain。 His great teacher； Johannes Muller； had called attention to the strange resemblance to vegetable cells shown by certain cells of the chorda dorsalis （the embryonic cord from which the spinal column is developed）； and Schwann himself had discovered a corresponding similarity in the branchial cartilage of a tadpole。 Then； too； the researches of Friedrich Henle had shown that the particles that make up the epidermis of animals are very cell…like in appearance。 Indeed； the cell…like character of certain animal tissues had come to be matter of common note among students of minute anatomy。 Schwann felt that this similarity could not be mere coincidence； but he had gained no clew to further insight until Schleiden called his attention to the nucleus。 Then at once he reasoned that if there really is the correspondence between vegetable and animal tissues that he suspected； and if the nucleus is so important in the vegetable cell as Schleiden believed； the nucleus should also be found in the ultimate particles of animal tissues。
Schwann's researches soon showed the entire correctness of this assumption。 A closer study of animal tissues under the microscope showed； particularly in the case of embryonic tissues； that 〃opaque spots〃 such as Schleiden described are really to be found there in abundanceforming； indeed； a most characteristic phase of the structure。 The location of these nuclei at comparatively regular intervals suggested that they are found in definite compartments of the tissue； as Schleiden had shown to be the case with vegetables； indeed； the walls that separated such cell…like compartments one from another were in some cases visible。 Particularly was this found to be the case with embryonic tissues； and the study of these soon convinced Schwann that his original surmise had been correct； and that all animal tissues are in their incipiency composed of particles not unlike the ultimate particles of vegetables in short； of what the botanists termed cells。 Adopting this name； Schwann propounded what soon became famous as his cell theory； under title of Mikroskopische Untersuchungen uber die Ubereinstimmung in der Structur und dent Wachsthum der Thiere und Pflanzen。 So expeditious had been his work that this book was published early in 1839； only a few months after the appearance of Schleiden's paper。
As the title suggests； the main idea that actuated Schwann was to unify vegetable and animal tissues。 Accepting cell…structure as the basis of all vegetable tissues； he sought to show that the same is true of animal tissues； all the seeming diversities of fibre being but the alteration and development of what were originally simple cells。 And by cell Schwann meant； as did Schleiden also； what the word ordinarily impliesa cavity walled in on all sides。 He conceived that the ultimate constituents of all tissues were really such minute cavities； the most important part of which was the cell wall； with its associated nucleus。 He knew； indeed； that the cell might be filled with fluid contents； but he regarded these as relatively subordinate in importance to the wall itself。 This； however； did not apply to the nucleus； which was supposed to lie against the cell wall and in the beginning to generate it。 Subsequently the wall might grow so rapidly as to dissociate itself from its contents； thus becoming a hollow bubble or true cell； but the nucleus； as long as it lasted； was supposed to continue in contact with the cell wall。 Schleiden had even supposed the nucleus to be a constituent part of the wall； sometimes lying enclosed between two layers of its substance； and Schwann quoted this view with seeming approval。 Schwann believed； however； that in the mature cell the nucleus ceased to be functional and disappeared。
The main thesis as to the similarity of development of vegetable and animal tissues and the cellular nature of the ultimate constitution of both was supported by a mass of carefully gathered evidence which a multitude of microscopists at once confirmed； so Schwann's work became a classic almost from the moment of its publication。 Of course various other workers at once disputed Schwann's claim to priority of discovery； in particular the English microscopist Valentin； who asserted； not without some show of justice； that he was working closely along the same lines。 Put so； for that matter； were numerous others； as Henle； Turpin； Du…mortier； Purkinje； and Muller； all of whom Schwann himself had quoted。 Moreover； there were various physiologists who earlier than any of these had foreshadowed the cell theorynotably Kaspar Friedrich Wolff； towards the close of the previous century； and Treviranus about 1807； But； as we have seen in so many other departments of science； it is one thing to foreshadow a discovery； it is quite another to give it full expression and make it germinal of other discoveries。 And when Schwann put forward the explicit claim that 〃there is one universal principle of development for the elementary parts； of organisms； however different； and this principle is the formation of cells；〃 he enunciated a doctrine which was for all practical purposes absolutely new and opened up a novel field for the microscopist to enter。 A most important era in physiology dates from the publication of his book in 1839。
THE CELL THEORY ELABORATED
That Schwann should have gone to embryonic tissues for the establishment of his ideas was no doubt due very largely to the influence of the great Russian Karl Ernst von Baer； who about ten years earlier had published the first part of his celebrated work on embryology； and whose ideas were rapidly gaining ground； thanks largely to the advocacy of a few men； notably Johannes Muller； in Germany； and William B。 Carpenter； in England； and to the fact that the improved microscope had made minute anatomy popular。 Schwann's researches made it plain that the best field for the study of the animal cell is here； and a host of explorers entered the field。 The result of their observations was； in the main； to confirm the claims of Schwann as to the universal prevalence of the cell。 The long…current idea that animal tissues grow only as a sort of deposit from the blood…vessels was now discarded； and the fact of so…called plantlike growth of animal cells； for which Schwann contended； was universally accepted。 Yet the full measure of the affinity between the two classes of cells was not for some time generally apprehended。
Indeed； since the substance that composes the cell walls of plants is manifestly very different from the limiting membrane of the animal cell； it was natural； so long as the； wall was considered the most essential part of the structure； that the divergence between the two classes of cells should seem very pronounced。 And for a time this was the conception of the matter that was uniformly accepted。 But as time went on many observers had their attention called to the peculiar characteristics of the contents of the cell； and were led to ask themselves whether these might not be more important than had been supposed。 In particular； Dr。 Hugo von Mohl； professor of botany in the University of Tubingen； in the course of his exhaustive studies of the vegetable cell； was impressed with the peculiar and characteristic appearance of the cell contents。 He observed universally within the cell 〃an opaque； viscid fluid； having granules intermingled in it；〃 which made up the main substance of the cell； and which particularly impressed him because under certain conditions it could be seen to be actively in motion； its parts separated into filamentous streams。
Von Mohl called attention to the fact that this motion of the cell contents had been observed as long ago as 1774 by Bonaventura Corti； and rediscovered in 1807 by Treviranus； and that these observers had described the phenomenon under the 〃most unsuitable name of 'rotation of the cell sap。' Von Mohl recognized that the streaming substance was something quite diff
