648 The American Naturalist. [August, 
by a quantity of food-material, surrounded by a rather dense 
cell-wall. The ovum is said to be ripened or “matured” for 
the reception of the spermatozoon, by the extrusion of two 
small “ polar bodies,” containing both chromatin and hyaline 
protoplasm, and separating off by karyokinetic division. 
After maturation is complete, a single spermatozoon normally 
penetrates ; then a reaction immediately sets in in the cell- 
wall of the ovum, which prevents other spermatozoa from 
entering. The head of the spermatozoon and the nucleus of 
the ovum now fuse together to form a single nucleus, which 
obviously contains the hereditary substance of two individ- 
uals. This is the starting point of the segmentation or distri- 
bution process above described, and it follows that the fertil- 
ized ovum at this stage must contain its typical complement 
of chromatin, archoplasm, etc., for the whole course of growth 
to the adult. 
How shall we connect these phenomena of fertilization with 
the facts of heredity? The most suggestive enigma in con- 
nection with the fertilizdtion process has been the meaning of 
the two polar bodies, especially since Van Beneden demonstrated 
that they contained chromatin. For twenty-five years specu- 
lation has been rife as to why the ovum should extrude a por- 
tion of its substance in two small cells; why not in one cell? 
Why not in a larger number? Thanks to the intense curios- 
ity which these polar bodies have aroused, and to the great 
variety of explanations which have been offered for them, we 
have arrived to-day at a solution which links the higher ani- 
mals with the lower, breaks down the supposed barrier between 
the sexes, and accords with the main external facts of hered- 
ity. 
Tt seems to me best to disregard the order of discovery, and 
to state the facts in the most direct way. First,a few words as 
to the speculations upon the meaning of the polar bodies. 
The early views of fertilization’ were naturally based upon 
the apparent significance of this process in the human species, 
in which the sexes are sharply distinguished from each other 
in their entire structure, and the reproductive cells are also 
*See also the introduction of Weismann’s last essay, Amphimixis. 
