Influence of Environment 



223 



On the other hand in certain animal phyla such as the cteno- 

 phores, mollusks, annelids and ascidians isolated cleavage cells 

 give rise only to part of an animal ; in this way one may get a 

 right or left half of an animal (Fig. 77) from right or left cleav- 

 age cells; an anterior half (Fig. 78), or a posterior half (Fig. 

 79) from anterior or posterior cleavage cells; 'or any one of the 

 cells of the 4-cell stage may produce the corresponding quarter 

 of an entire animal. Such cases are known as "mosaic devel- 

 opment." 



There has been much discussion among biologists as to the 



FIG. 76. DOUBLE EMBRYOS OF FROG DEVELOPED FROM EGGS INVERTED WHEN 

 IN THE 2-CELL STAGE. A, twins with heads turned in opposite directions. 

 B, twins united back to back. C, twins united by their ventral sides. 

 D, double headed tadpole. (From Wilson after O. Schultze.) 



