78 NATIONAL TRENDS IN BIOLOGY 



ing a period of some twenty years, were performed on the 

 developing newt. He found, as in other embryonic ani- 

 mal development, that after the fertilized egg has divided 

 into a number of undifferentiated cells, the upper cells 

 grow down and cover the lower. A circular zone of down- 

 growth is thus formed, marked by a definite rim or lip. 

 This down-growth begins at one point of the circle (on 

 what is to become the dorsal portion of the embryo) and 

 spreads round, gradually, to complete the circle. This 

 first active region is called the "dorsal lip." Spemann's 

 reasoning led him to believe there was some causal rela- 

 tion between this dorsal lip and the beginning of differen- 

 tiation among the various cells. 



In order to demonstrate his theory, he worked out 

 an interesting technique by which he transplanted a small 

 portion of the dorsal lip into some abnormal position in 

 another developing embryo. The result bore out his sup- 

 position, for not only did the animal which had the graft 

 made upon it develop its own set of normal organs, but 

 in addition, it developed a second set in relation to the 

 graft. 



Spemann concluded that the portion transplanted had 

 some inherent potentiality of growth which had thus 

 formed what might be called a "secondary embryo." He 

 again put his conclusion to the test by transplanting a 

 portion of the dorsal lip of an animal which had consider- 

 able pigment in it to one which had none, so that the 

 growth of the two tissues could be observed distinctly. 



