168 HEREDITY AND ENVIRONMENT 



rounding the vegetative pole usually becomes 

 the endoderm or inner cell layer. The axis 

 which connects these poles, the chief axis of 

 the egg, becomes the gastrular axis of the em- 

 bryo and in every great group of animals bears 

 a constant relationship to the chief axis of the 

 adult animal. The polarity of the developed 

 animal is thus directly connected with the 

 polarity of the egg from which it came (Figs. 

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



(b) Symmetry. In many cases the sym- 

 metry of the developed animal is foreshadowed 

 in the symmetry 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 

 (Fig. 27, 1, 2), 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 



