AUTHOR'S PREFACE ix 



is a section entitled " The Cell as the Elemental Germ of an 

 Organism," in which both the older and more recent theories of 

 heredity are dealt with. 



The second part of the complete work, which is to deal with the 

 tissues, will be of about the same length, and will form to a greater 

 extent a supplement to the Text-book of Embryology. For in 

 addition to a description of the tissues, especial emphasis will be 

 laid upon their origin of histogenesis and upon the physiological 

 causes which underlie the formation ; the other side of the process 

 of development, that is to say, histological differentiation, will also 

 be discussed. 



In the account, which I have endeavoured to make as intelligible 

 as possible, scientific views have primarily guided me. What I 

 have striven to do to the best of my ability is, to fix the scientific 

 stand-point occupied at present by the doctrines of cell and tissue 

 formation. Further, I have tried to delineate the historical course 

 of the development of the more important theories. With regard 

 to disputed points I have frequently compared various opinions. 

 If, as is natural, I have placed my own views in the foreground, 

 and, moreover, if I have occasionally differed from the views and 

 explanations of prominent and highly-esteemed scientists whose 

 opinions I value extremely, it is only due to them to say that I do 

 not on that account consider the conceptions preferred by me to be 

 unconditionally correct, still less do I wish to belittle the views 

 from which I differ. Antagonistic opinions are necessary to the 

 life and development of science; and, as I have observed in 

 studying the history of the subject, science progresses most 

 rapidly and successfully in proportion to the diversity of the 

 opinions held by different authorities. As is only human, almost 

 all observations and the conclusions deduced from them are one- 

 sided, and hence continually need correction. How necessary then 

 must this be in the subject of the present inquiry, that is to say, 

 in the cell, which is a marvellously complicated organism, a small 

 universe, into the construction of which we can only laboriously 

 penetrate by means of microscopical, chemico-physical and experi- 

 mental methods of inquiry. 



OSCAR HERTWIG. 



Berlin, October, 1892. 



