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 sball 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, :01; :03). A detailed ac- 
count of the sexual reproduction of well known types through- 
out the plant kingdom has been recently published by Mottier 
(:04b) 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 Guérin (: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 

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