HEREDITY AND ENVIRONMENT IN DEVELOPMENT 443 



seen in some invertebrates, where the endoderm may be caused to 

 evaginate instead of invaginating at the time of gastrulation 

 (Fig. 230). 



An extreme example of the effects of chemical changes in the 

 environment is the production of one-eyed or cyclopean monsters 

 in the embryos of fishes through alteration of the chemical constit- 

 uents of the water (Fig. 23 1 ) . By adding magnesium or other salts 

 it is possible to produce embryos in 

 which the two optic vesicles have fused 

 together at the ventral anterior end of 

 the body as a single cyclopean eye. 

 Although such monstrosities are not 

 longhved, their production demonstrates 

 that the organism develops normally 

 only under a given set of conditions. 

 If the sea water were not what it 

 happens to be, fishes might be quite 

 different from what they are now. and 

 would be called normal. The influence 

 of the environmental factor of light 

 upon the formation of chlorophyll in 



plants is seen when a board is left upon trulation in the mollusc Cre- 

 the lawn for a few days and the grass pitlula. 



beneath it becomes colorless. The in- if the temperature or density 



fluence of gravity is seen when a seed of t^e surrounding water s altered 



. , 111 certain ways during gistrula- 



is allowed to begin its growth of "root" tion, the endoderm is caused to 



and "stem" and is then inverted. *">■"«"' '"-^ead .f i:.. producing 



the abnormal condition known as 



Many other examples might be cited, all an "exogastruia" (c/. Fig. 209 E). 

 showing how changes in environmental I'^ti^efie^'-e the ectodermis above 



_ _ _ _ and the endoderm below. (Prom 



conditions bring about differences in Conklln. "Heredity and Environ- 



the development, sometimes within '"ent "copyright loio. by Prince- 



^ ' _ ton University Press, reprinted 



wide limits. The corollary of this is by permission.) 



that the " normal " organism becomes 



what it does because of a certain normal or " average " set 

 of conditions which are fairly constant and hence productive 

 of the kinds of individuals that occur in nature. The adult 

 is what it is because its hereditary organization, which is 

 embodied in some manner in the germ cells, develops under a par- 

 ticular environment. As will be explained in discussing the possible 

 causes of racial development or Evolution in Chapter 21, the 



Fig. 230. — Effect of environ- 

 mental conditions upon gas- 



