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Heredity and Environment 



eel 



FIG. 80. Two LARVAE OF Styela which were centrifuged in the 4-cell 

 stage thereby changing the position of various organ-forming substances. 

 Nervous system (ns), eyes (E), notochord (ch) and muscles (ms) have 

 been displaced, and the larva has been turned inside out, the endoderm 

 (end) being outside and the ectoderm (ect) inside. 



is slight the developed individual may show only the beginnings 

 of a division into two, as in two-headed forms ; if the division of 

 the egg or embryo is complete two separate and perfect individ- 

 uals may be formed from an originally single oosperm. When two 

 individuals are formed from a single egg they have exactly the 

 same heredity and accordingly they are always of the same sex 

 and are so similar in appearance that they are known as "identical" 

 or "duplicate" twins (Fig. 81, right end). On the other hand 

 twins which develop from different eggs do not have the same 

 heredity and may differ in sex as well as in other features ; they 

 are known as "fraternal" twins. 



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 

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

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



