2I 4 



Heredity and Environment 



FIG. 75. DWARF AND DOUBLE EMBRYOS OF Amphioxus. A, isolated blas- 

 tomere of the 2-cell stage segmenting like an entire egg. B, twin gastrulae 

 from a single egg. C, double cleavage resulting from partial separation of 

 the first two cleavage cells. D, E, F, double gastrulae arising from such 

 forms as C. (From Wilson.) 



give rise only to parts 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." 



