36 INTRODUCTION. 



these are united into organs has been less examined, yet on this head 

 also, much has been added of late, especially in man, whose individual 

 organs with the exception of the nervous system, the higher organs of 

 sense, and a few glands (the liver, blood-vascular glands), have been 

 almost exhaustively investigated. If the like progress continue to be 

 made, the structure of the human body will in a few years be so clearly 

 made out, that, except perhaps in the nervous system, nothing more of 

 importance will remain to be done with our present modes of investiga- 

 tion. With comparative Histology it is otherwise ; hardly commenced, 

 not years but decades will be needed to carry out the necessary investi- 

 gations. Whoever will do good work in this field must, by monographs of 

 typical forms embracing their whole structure from the earliest periods 

 of development,* obtain a general view of all the divisions of the ani- 

 mal kingdom, and then, by the methods above described, strive to 

 develop their laws. 



As regards the general propositions of Histology, the science has 

 made no important progress since Schwann, however much has been 

 attained by the confirmation of the broad outlines of his doctrines. 

 The position that all the higher animals at one time consist wholly of 

 cells and develop from these their higher elementary parts, stands firm, 

 though it must not be understood as if cells, or their derivatives, were 

 the sole possible or existing elements of animals. In the same way, 

 Schwann's conception of the genesis of cells, though considerably 

 modified and extended, has not been essentially changed, since the cell 

 nucleus still remains as the principal factor of cell-development and of 

 cell-multiplication. Least advance has been made in the laws which 

 regulate the origin of cells and of the higher elements, and our acquain- 

 tance with the elementary processes which take place during the for- 

 mation of organs must be regarded as very slight. Yet the right track 

 in clearing up these points has been entered upon ; and a logical inves- 

 tigation of the chemical relations of the elementary parts and of their 

 molecular forces, after the manner of Bonders, Ludwig, and others, com- 

 bined with a more profound microscopical examination of them, such 

 as has already taken place with regard to the muscles and nerves, 

 further, a histological treatment of embryology, such as has been at- 

 tempted by Reichert, Vogt, and myself, will assuredly raise the veil, and 

 bring us, step by step, nearer to the desired though perhaps never to 

 be reached, end. 



8. The aids in studying Histology may here be best shortly ad- 

 verted to. With respect to the literature of the subject, the more impor- 



* [See a very praiseworthy monograph of this kind by Leydig, Beitrage zur Mikrosko- 

 pischen Anatomie und Entwickelungs-geschichte der Rochen u. Haie, 1852. (Microscopic 

 Anatomy and Development of the Rays and Sharks.) TRS.] 



