324 HEREDITY AND ENVIRONMENT 



the same heredity and may differ in sex as well 

 as in other features; they are known as "fra- 

 ternal" twins. 



If the temperature or density of the sur- 

 rounding medium is altered during the gas- 

 trula stages the endoderm may be caused to 

 turn out instead of in (exogastrula), thus pro- 

 ducing an animal which is turned inside out 

 (Fig. 77). In other cases (vertebrates) the 

 gastrula mouth may fail to close, thus pro- 

 ducing animals in which the spinal cord and 

 vertebral column are split in two (spina 



FIG. 77. Exogastrula of Crepidula. The endoderm (end) 

 has been turned out instead of in, thus leaving the digestive 

 layer of cells on the outside of the body; ShG, shell gland, 

 V, velum. 



