IV PREFACE. 



For manifest reasons, references and bibliography have been 

 omitted, except in connection with statements where mention of the 

 name of the authority has seemed desirable. The author wishes to 

 express his obligation to the writings of Kolliker, Ranvier, Schwalbe, 

 Waldeyer, Retzius, Stohr, Flemming, O. Hertwig, Schaefer, Golgi, 

 Ram6n y Cajal, and others, as well as to many papers found in the 

 Archiv fur mikroskopische Anatomic and other journals. 



G. A. P. 



UNIVERSITY OF PENNSYLVANIA, 



PHILADELPHIA, September 30, 1893. 



