228 THE AMERICAN NATURALIST. [VOL. XXXIX. 



developed at other periods of the life history, or to the primitive 

 conditions always found with the origin of sex. These exceptional 

 processes will be collected and described under the heading 

 "Asexual Cell Unions and Nuclear Fusions," following this 

 portion of the paper. 



The union of gametes is generally termed fertilization. The 

 evolution of the sexual process always tends towards a differ- 

 entiation of the two sexual cells, one becoming more richly 

 stored with food material and containing more protoplasm than 

 the other. This latter gamete is always considered the female 

 and is said to be fertilized when the male gamete, either as a 

 motile sperm or reduced simply to a sperm nucleus generally 

 with some accompanying protoplasm, fuses with it. The most 

 evident morphological feature of fertilization is the close union 

 of the gamete nuclei so that the chromosomes of both enter into 

 the mitotic figure with which the new generation begins. 



We shall not discuss the various forms of gametes nor their 

 habits in different types of sexual reproduction. They have 

 been described in two articles by the author on the origin and 

 evolution of sex in plants (Davis, :oi ; : 03). A detailed ac- 

 count of the sexual reproduction of well known types through- 

 out the plant kingdom has been recently published by Mottier 

 (: O4b) under the title " Fecundation in Plants " a term which 

 he prefers to fertilization. This paper gives in English the 

 most extensive summary of our knowledge of the subject up to 

 the date 1902 and will be read with especial interest as the 

 most available expression in English of Strasburger's general 

 views on the significance of the events connected with sexual 

 reproduction. 



A recent paper of Guerin (: 04) is confined to an account of 

 fertilization in the phanerogams which are treated in considerable 

 detail. His discussion of double fertilization and parthenogene- 

 sis is of especial interest and will be taken up later. 



Our purpose is to divest from the events of sexual cell unions 

 and nuclear fusions all secondary and unessential processes and 

 to outline, as are now understood, the fundamental phenomena. 

 And to make the subject more plain we shall try to compare 

 in their essentials the events of fertilization in plants with those 



