INTRODUCTION. 



be made, the structure of the human body will in n 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 investigation. With comparative 

 Histology it is otherwise; hardly commenced, not years but 

 decades will be needed to carry out the necessary investigations. 

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

 of typical forms embracing their whole structure from the 

 earliest periods of development, 1 obtain a general view of all 

 the divisions of the animal 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 con- 

 siderably 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 acquaintance with 

 the elementary processes which take place during the formation 

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

 track in clearing up these points has been entered upon; and a 

 logical investigation of the chemical relations of the elementary 

 parts and of their molecular forces, after the manner of Donders, 

 Dubois, Ludwig, and others, combined with a more profound 

 microscopical examination of them, such as has already taken 

 place with regard to the muscles and nerves, — further, a his- 

 tological treatment of embryology, such as has been attempted 

 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. 



[> See a very praiseworthy monograph of this kind by Leydig, Beitrage zur 

 Mikroskopischen Anatomic und Entwiekelungs-geschichte der Roche n u. Ilaie, 1S52- 

 (Microscopic Anatomy and Development of the Rays and Sharks.) — Eds.] 



