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 fecuxdation has been used instead of fertzlization— 
fecundation being the equivalent of the German Befruchtung and 
the French fécondation. 
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. 8S. 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. 
Davip M. Mortier. 
Inpiana UNIVERSITY, August, 1902. 
