180 ON THE DEVELOPMENT OF THE ANIMAL TISSUES. 



They afford the basis for a general theory of vegetation and organiza- 

 tion which it had hitherto been impossible to frame. Valuable observa- 

 tions had been made in all parts of physiology, and some branches of 

 the science had been brought to a state of high perfection. But as 

 regards the fundamental principles on which all should rest, these it 

 must be confessed were either very unstable or entirely wanting, hence 

 the slight connection which seemed to subsist between different impor- 

 tant observations in parts of the science which were far advanced. 

 These fundamental principles are now obtained. Schwann himself has 

 pointed out with equal lucidity and acuteness, the general conclusions 

 which are to be deduced from the observations of Schleiden and himself, 

 and has framed from them a theory of the organization and vegetation 

 of organized beings. It is not possible to give here more than the 

 principal features of his theory. 



There is one common mode of development observed in the forma- 

 tion of the most different elementary tissues of plants and animals, and 

 that is the development from cells. In a pre-existing structureless sub- 

 stance, which may be situated either within or on the exterior of cells 

 already formed, new cells are developed in a manner regulated by deter- 

 minate laws, and these new cells undergo various modifications and 

 transformations by which they are converted into the elementary or- 

 ganic tissues. In every tissue the new cells are formed only in those 

 parts to which new nutritive matter has direct access. On this alone 

 depends the difference subsisting between the vascular and non-vascular 

 tissues. In the former the nutritive fluid, the liquor sanguinis, is dis- 

 tributed through every part of the tissues, and hence new cells are 

 formed through its substance. In the non-vascular tissues, on the con- 

 trary, the nutritive fluid has access to one surface only, as in the case of 

 the epidermis. Hence in cartilages, also, when they are destitute of 

 vessels the new cells are formed only at their surface, or to a slight 

 depth, namely, as far as the liquor sanguinis, their cytoblastema pene- 

 trates. The expression, growth by apposition, is correct, when under- 

 stood to signify the development of new cells, and not the growth of 

 those already existing ; for in the epidermis new cells are formed only 

 at the inferior surface of the membrane, whilst in the vascular tissues 

 the new cells are developed in the whole substance of the tissue. In 

 both cases, however, the cells themselves grow by intussusception. 

 Cartilage is at first destitute of vessels, and the new cells consequently 

 are formed only in the vicinity of the external surface. But after ves- 

 sels have extended into the medullary canals, the formation of new cy- 

 toblastema and new cells can proceed not only on the surface of the 



