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Heredity and Environment 



Isolation of Cleavage Cells. If the cleavage cells are separated 

 from one another in the 2-cell or 4-cell stage each of them may 

 give rise to an entire animal (Fig. 75) ; in this way two com- 

 plete animals may be derived from a single egg of a star-fish or 

 sea-urchin, of an amphioxus, and of several other animal types. 

 If the frog's egg is turned upside down in the 2-cell stage, double- 

 headed or double-bodied embryos may result (Fig. 76). In such 

 cases each cleavage cell is said to be totipotent, that is, it is ca- 

 pable of giving rise to an entire animal. 



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.) 



