Influence of Environment 221 



Other Monstrous Forms. If the temperature or density of the 

 surrounding medium is altered during the gastrula stages the 

 endoderm may be caused to turn out instead of in (exogastrula), 

 thus producing an animal which is turned inside out (Fig. 82). 

 In other cases (vertebrates) the gastrula mouth may fail to close, 

 thus producing animals in which the spinal cord and vertebral 



PCp 



End 



FIG. 82. EXOGASTRULA OF Crcpidula. 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. 



column are split in two (spina bifida) ; or the brain may be forced 

 outside of the head or may be lacking altogether (anencephaly). 

 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 

 abnormal environmental conditions in early stages of ontogeny. 

 Effects of Food. In addition to such monsters, which are in- 

 capable of long life, many peculiar if not abnormal types of ani- 

 mals are produced by the action of unusual environmental stimuli 

 during later stages of development. Gudernatsch found that if 

 tadpoles of the frog were fed on the thyroid gland they trans- 

 formed into minute frogs, scarcely larger than flies, but if fed 

 on thymus gland they grew to be large, dark-colored tadpoles 



