Influence of Environment 



229 



0. H 



FIG. 81. DIAGRAM SHOWING THE DIFFERENT TYPES OF UNION OF DOUBLE 

 HUMAN MONSTERS, each being produced by a partial division of a single 

 egg or embryo. If the division is a complete one, duplicate twins are 

 formed, as shown by the figures at the right end of each line. (From 

 Wilder.) 



In some cases eyes are wholly lacking, in others the two eyes fuse 

 together into a single one as in the fabled Cyclops (Fig. 83). 

 Practically all such cases of monstrous . development are due to 



End 



FIG. 82. 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. 



