176 HEREDITY AND ENVIRONMENT 



polarity of the egg from which it came (Figs. 

 23,26, 27,28, 38, 39). 



(b) Symmetry. — In many cases the sym- 

 metiy of the developed animal is foreshadowed 

 in the cj^toplasm of the egg. The eggs of 

 cephalopods (Fig. 38) and of insects (Fig. 

 39) are bilaterally symmetrical while they are 

 still in the ovary; in other cases, such as ascid- 

 ians, Amphioxus and the frog, bilateral sym- 

 metry appears immediately after fertilization 

 (Figs. 9, 27, 28) , though in some of these cases 

 there is reason to believe that the eggs are 

 bilateral even before fertilization ; in still other 

 cases bilaterality does not become visible until 

 later in development and we do not now know 

 whether it is present in earlier stages or not; 

 but wherever it can be recognized in the earlier 

 stages it is certain that the bilateral symmetry 

 of the egg becomes the bilateral symmetry of 

 the developed animal. 



(c) Inverse Symmetry. — In most animals 

 bilateral symmetry is not perfect, certain or- 

 gans being found on one side of the mid line 

 and not on the other, or being larger or dif- 

 ferently located on one side as compared with 



