from an embryo will grow only endoderm cells. Each part seems to carry 

 on according to its position. 



But position is not absolute. It is always in relation to something else. In 

 the embryos of salamanders and frogs and other mammals we soon recog- 

 nize parts that are to become brain, parts that are to become eyes or legs. 

 Is the character of every tissue or organ already fixed at this stage ? In one 

 series of experiments a bit of tissue that would ordinarily have become eye 

 was transplanted to the abdomen of an embryo, and a piece of leg tissue was 

 transplanted to where wing should have developed (see illustration below). 

 The eye-prospect became an eye, and the leg-prospect became a leg. Here 

 the cells developed what we might suppose to have been their "natural" 

 qualities, those belonging to the position from which they were taken. 



A Master Organizer In the gastrula, or cup, stage, the ectoderm and 

 the endoderm run together at the edge (see illustration, p. 351). The upper 

 edge of the opening, called the "dorsal lip", seems to be a special center of 

 protoplasmic activity. If a bit of this tissue is grafted on any part of an 

 embryo, it starts to develop a new embryo. It apparently influences all the 

 surrounding cells so that, as they grow and multiply, the mass shapes itself 

 in relation to these dorsal-lip cells. Chemical study of these cells has located 

 in them special "organizer" substances. 



BERN ARC 



Transplanted 

 eye 



After Viktur Jlaiiiburger 



GROWING ORGANS OUT OF PLACE 



When the eye-bud in a chick embryo was grafted on the side of the abdomen, it de- 

 veloped into a complete eye, although the nerve connections were not established. 

 The transplanted portion developed according to characteristics normal to its tissues 



362 



