DEVELOPMENT Ay, 
egg-nucleus unite, the resulting nucleus contains the normal 
number of chromosomes. The meaning of these reduction 
phenomena—highly important from the standpoint of heredity 
—is a much debated subject. 
Fertilization.— As the eggs pass through the vagina, they 
are capable of being fertilized by spermatozoa, previously 
stored in the seminal receptacle. Through the micropyle of 
the chorion one or more spermatozoa enter and a sperm- 
nucleus unites with the egg-nucleus to form what is known as 
the segmentation nucleus. Through this union of nuclear 
Fic. 188. 
Equatorial section of egg of a beetle, Clytra leviuscula. b, blastoderm; g, germ 
band; y, yolk granule; yc, yolk cell.—After LécarILiLon. 
substances the qualities of the two parents are combined in the 
offspring. Needless to say, the minute details of the process 
of fertilization are of the highest biological importance. 
Blastoderm.—lIn an arthropod ovum the yolk occupies a 
central position (centrolecithal type), being enclosed in a thin 
layer of protoplasm. From the segmentation nucleus just 
mentioned are derived many nuclei, some of which migrate 
outward with their attendant protoplasm to form with the 
original peripheral protoplasm a continuous cellular layer, the 
blastoderm (Fig. 188). 
