Cell Interactions during Growth and Morphogenesis 83 



2. The implanted chorda-mesoderm went ahead and developed pre- 

 cisely as it would have had it been in its normal position in the embryo; 

 that is, it constructed a notochord and somites. In contrast, any other part 

 of the gastrula, when implanted in this manner, did not develop as it 

 would have if it had not been moved, but instead developed in accordance 

 with its new locale. 



The second result showed that indeed there was something special 

 about dorsal-lip tissue. The first result revealed that dorsal-lip cells could 

 induce overlying ectoderm to form brain, nerve cord, and associated 

 parts. When that part of the dorsal-lip tissue that normally underlies the 

 brain was implanted, it induced brain development very well and tail de- 

 velopment very poorly, and when posterior chorda-mesoderm was im- 

 planted, it induced tail very well and brain very poorly. Thus the chorda- 

 mesoderm is a specific inducer. 



The main question, of course, is what compound or compounds does 

 the chorda-mesoderm supply to the overlying ectoderm to produce the 

 organization of the ectoderm tissue? Unfortunately, any of a wide variety 

 of substances from many sources has been found to do this, so wide in 

 fact that the induction appears to have very little specificity. It is almost 

 as if the ectoderm at this stage of development is a gun primed to shoot 

 in a fixed direction and needing only a touch on the trigger to set it off. 

 This "sensitivity" raises two other very interesting questions. 



1. Although it is true that the embryologist can make ectoderm 

 organize itself by treating it with a wide variety of agents, in the actual 

 embryo only dorsal-lip cells can do this. What, then, passes between the 

 chorda-mesoderm and the overlying ectoderm? To answer this question, 

 we label the constituents of dorsal-lip cells with radioactive carbon or 

 sulfur or phosphorus and thus determine what materials pass from the 

 "hot" mesodermal cells to the "cold" ectodermal cells. 



2. If it is true that the embryologist can cause ectoderm to organize 

 itself by applying many agents, why is it in the actual embryo that only 

 one part of the ectoderm normally becomes organized into brain and 

 spinal cord? Why aren't secondary embryos caused to pop out all over the 

 place, having been induced by any of the multitude of materials that 

 normally leak out of cells? Here we return to the problem of the morpho- 

 genetic field discussed in Chapter 4. 



SECONDARY INDUCERS IN EMBRYOS 



As we just described, dorsal-lip tissue induces overlying ectoderm to 

 transform into brain and nerve cord, while it itself forms notochord and 



