IV PREFACE. 



No attempt has been made to discuss the numerous theories bearing 

 upon the subject. Whenever theoretical matters are touched upon the 

 object has been chiefly to suggest probable lines of investigation. I 

 have not hesitated, however, to express my own opinion in all cases 

 in which my special field of study has given me a first-hand knowledge 

 of the subject-matter. 



To designate the sexual process which consists in the fusion of sex- 

 ually differentiated cells, or gametes, and especially the fusion of their 

 nuclei, the term fecundation has been used instead of fertilization 

 fecundation being the equivalent of the German Befruchtung and 

 the Yrenchfecondation. 



It has been necessary, of course, to copy numerous figures from the 

 papers of other investigators, but in every case due credit is given. 



In the citation of literature in the text the author is referred to by 

 the year in which his work was published. No attempt has been made 

 to give a complete bibliography, and no doubt many valuable refer- 

 ences have been omitted. 



The author is indebted to Professors W. Belajeff, H. O. Juel, 

 F. Oltmanns, S. Ikeno, and to Dr. H. Klebahn, Dr. A. H. Trow, 

 Dr. H. Wager, Dr. S. Hirase, and Dr. V. H. Blackman, for re- 

 prints of their papers, from many of which illustrations have been 

 borrowed, and especially to Professor R. A. Harper for helpful 

 suggestions. 



DAVID M. MOTTIER. 



INDIANA UNIVERSITY, August, 1902. 



