318 HEREDITY AND ENVIRONMENT 



only to parts of an animal ; in this way one may 

 get a right or left half of an animal (Fig. 72) 

 from right or left cleavage cells; an anterior 

 half (Fig. 73), or a posterior half (Fig. 74) 

 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 

 development." 



There has been much discussion among bi- 

 ologists as to the meaning of these results. On 

 the one hand it is said that the totipotence 

 of any one of the first four cleavage cells 

 proves that all of these cells are alike and that 

 they have not yet begun to differentiate. On 

 the other hand it is said that a part of an egg 

 may give rise to a whole animal for the same 

 reason that parts of certain adult animals may 

 do the same thing, viz., because they have the 

 power of regeneration. However there are 



veloped after the left half of the egg, A 3 B X , had been killed. 

 B, left half larva from the two left cells of the 4-cell stage, 

 the right cells, <4 S B S , having been killed. The muscle cells 

 (stippled) occur only on one side of the notochord. D, 

 three-quarter larva, the left anterior cells having been killed. 

 E, F, three-quarter larvae, the right posterior cell B, having 

 been killed. 



