428 INTRODUCTION TO CYTOLOGY 



however central or nearly so. . . . This areola, or nucleus of the cell as perhaps 

 it might be termed, is not confined to the epidermis, being also found not only 

 in the pubescence of the surface, particularly when jointed, as in Cypripedium, 

 but in many cases in the parenchyma or internal cells of the tissue, especially 

 when these are free from the deposition of granular matter. 



After Brown's announcement, observations on nuclei in various 

 tissues multiplied rapidly, and it was not many years before nuclei 

 came to be recognized as important components of practically all 

 organisms. 



The Cell Theory. — The generalization that the bodies of organisms 

 are regularly composed of cells and their products, and that the cell 

 units are of prime importance in determining growth and differentiation, 

 was made by R. J. H. Dutrochet (1776-1847) in France and by M. J. 

 Schleiden (1804-1881) and T. Schwann (1810-1882) in Germany. In 

 addition to the statement quoted in an earlier paragraph, Dutrochet 

 says in his Recherches Anatomiques et Physiologiques sur la Structure 

 Intime des Animaux et des Vegetaux et sur leur Motilite (1824): 



. . . The globular corpuscles which make up all the tissues of animals are 

 really globular cells of an extreme smallness, which are united only by cohesion. 

 . . . This uniformity of ultimate structure proves that organs really differ one 

 from the other only in the nature of the substances which are contained in the 

 vesicular cells of which they are composed. . . . Growth results from the 

 increase in the volume of the cells, and from the addition of new little cells. 



Dutrochet noted that certain structures, notably feathers, are made up 

 of dead cells, and that others represent cell secretions. Moreover, he 

 recognized the cell as a unit of physiological function. He did not, how- 

 ever, make a clear distinction between true cells and other globules 

 visible in tissues (see Rich, 1926). 



Schleiden and Schwann made their formulation of the cell theory more 

 or less jointly after comparing their observations on plants and animals. 

 In his Beitrdge zur Phytogenesis (1838) Schleiden says, 



, . . every plant developed in any higher degree is an aggregate of fully 

 individualized, independent, separate beings, even the cells themselves. Each 

 cell leads a double life: an independent one, pertaining to its own development 

 alone; and another incidental, in so far as it has become an integral part of a 

 plant. It is, however, easy to perceive that the vital process of the individual 

 cell must form the first, absolutely indispensable fundamental basis, both as 

 regards vegetable physiology and comparative physiology in general. . . . 



Schwann stated the theory in concise form in 1838, and 1839 he pub- 

 lished a full account under the title, Mikroskopische Untersuchungen 

 iiber die Uebereinstinimung in der Struktur und dem Wachsthum der 

 Thiere und Pflanzen. In this classic work a great variety of animal cells 

 are carefully described and figured, and the cell stands forth as an indi- 



