DEVELOPMENT 147 



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 



FIG. 188. 



Equatorial section of egg of a beetle, Clytra Itzviuscula. b, blastoderm; g, germ 

 band; y, yolk granule; yc, yolk cell. After LCAILLON. 



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. In 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). 



