322 HEREDITY AND ENVIRONMENT 



stimulating the egg to begin development, may 

 itself die or remain inactive, in which case the 

 hereditary traits which develop are those of the 

 mother only. In many animals unfertilized 

 eggs may be stimulated to begin development 

 by a great variety of changes in the medium, 

 all such cases being known as "artificial 

 parthenogenesis." 



3. Modifications of Development after 

 Fertilization. — Environmental changes, acting 

 upon the oosperm after fertilization, or upon 

 the embryo, may produce an almost infinite 

 variety of abnormal types of development, but 

 so far as known none of these modifications be- 

 comes hereditary. It seems probable that 

 changes in hereditary constitution take place 

 in the main before fertilization and especially 

 during the maturation divisions. 



If the cleavage cells are separated from one 

 another in the 2-cell or 4-cell stage each of 

 them may give rise to an entire animal (Fig. 

 70) ; in this way two complete animals may 

 be derived from a single egg of a star-fish or 

 sea urchin, of an amphioxus, and of several 

 other animal types. If the frog's egg is turned 



