574 



CONTINUITY OF LIFE 



what takes place when a college student 

 begins his training to be a heart specialist. 

 At first he receives a general education, 

 learning as much as he can about the var- 

 ious fields of human endeavor. He then 

 concentrates on the natural sciences, gain- 

 ing a broad background in biology, physics, 

 and chemistry. Entering medical school 

 next, he learns about the entire body and its 

 functions. Finally, he assigns himself the 

 task of learning all he can about the heart 



The principle of induction 



What is the guiding influence that directs 

 these cells into a single course? That it must 

 have something to do with the neighboring 

 cells was seen in the preceding discussion. 

 Certain ingenious experiments, again per- 

 formed by Spemann 25 years ago, throws 

 some light on the answer to this question. 



It was demonstrated that by transplant- 

 ing cells located just above the blastopore 



Fig. 23-13. If a few cells from the region above the blastopore of an amphibian embryo are 

 removed and placed in some other region of a second embryo, they will produce a nerve 

 cord just as they would have done had they remained in their original location. The opera- 

 tion is performed in the two left figures and the results are shown in the middle (early) 

 and right (later) figures. 



and the way it works. He may even go far- 

 ther and become specialized in certain dis- 

 eases of the heart. Thus his education has 

 gone from the general to the specific. Like- 

 wise in the developing embryo, the job of 

 the first cleavage cells is a general one, and 

 as development proceeds the tasks become 

 more and more specific until finally by the 

 time the animal is ready to take on the busi- 

 ness of living on its own, the tasks have all 

 been assigned and, with few exceptions, 

 the cells can do little else but perform 

 those functions. 



in amphibian embryos to an area in a sec- 

 ond embryo that would normally form 

 something else, belly, for example, a second 

 embryonic axis was set up in the new site, 

 even though the embryo had one of its own 

 (Fig. 23-13). The transplanted cells sank 

 below the surface and induced a nerve cord 

 above the area, just as they would have 

 done if left in the original embryo. Such 

 cells were thought of as the organizing cen- 

 ter of the embryo and hence were called 

 the organizers. 



This was further illustrated in the forma- 



