narrow limits of the science of botany, for it is owing to this that 

 the cell-theory was first applied to animal tissues. For it is just 

 in animal cells that the nuclei stand out most distinctly from 

 amongst all the other cell-contents, thus showing most evidently 

 the similarity between the histological elements of plants and 

 animals. Thus this little treatise of Schleiden's, in 1838, marks 

 an important historical turning-point, and since this time the 

 most important work, in the building up of the cell-theory, has 

 been done upon animal tissues. 



Attempts to represent the animal body as consisting of a large 

 number of extremely minute elements had been made before 

 Schleiden's time, as is shown by the hypotheses of Oken (I. 21), 

 Heusinger, Raspail, and many other writers. However, it was 

 impossible to develop these theories further, since they were 

 based upon so many incorrect observations and false deductions, 

 that the good in them was outweighed by their errors. 



It was not until after some improvements had been made in 

 optical instruments, during the years from 1830-1840, that work 

 justifying the application of the cell-theory to animal tissues was 

 accomplished. 



Purkinje (I. 22) and Valentin, Joh. Miiller (I. 20) and Henle 

 (I. 11), compared certain animal tissues with plant tissues, and 

 recognized that the tissue of the chorda dorsal is, of cartilage, of 

 epithelium and of glands, is composed of cells, and in so far is 

 similar in its construction to that of plants. Schwann (I. 31), 

 however, was the first to attempt to frame a really comprehensive 

 cell-theory, which should refer to all kinds of animal tissues. 

 This was suggested to him by Schleiden's " Phytogenesis," and 

 was carried out by him in an ingenious manner. 



During the year 1838 Schwann, in the course of a conversation 

 with Schleiden, was informed of the new theory of cell-formation, 

 and of the importance which was attached to the nucleus in plant- 

 cells. It immediately struck him, as he himself relates, that 

 there are a great many points of resemblance between animal and 

 vegetable cells. He therefore, with most praiseworthy energy, 

 set on foot a comprehensive series of experiments, the results 

 of which he published in 1839, under the title, Mikroscopische 

 unter suchung en uber die Uebereinstimmung in der Structur wtd 

 dem Wachsthum der Thiere und Pflanzm. This book of Schwann's 

 is of the greatest importance, and may be considered to mark an 

 epoch, for by its means the knowledge of the microscopical 



