82 EMBRYOLOGY 



anterior end of the primitive streak of a duck embryo is transplanted into the 

 presumptive epidermis of a chick embryo, it is found that the duck tissue 

 will self-differentiate into duck notochord. In doing so it induces the chick 

 epidermis to form a chick neural tube. The experiment shows that the 

 anterior end of the primitive streak functions as an organizer for the develop- 

 ment of the nervous system in birds and therefore not only is homologous 

 with but has the same function as the amphibian dorsal lip of the blastopore. 



The actual process of stimulation of a neural plate by an organizer in 

 these cases is called embryonic induction. Embryonic induction may be defined 

 in its broadest aspect as the process by which one group of cells stimulates a 

 second group of cells to differentiate. This principle is repeated again and 

 again during the development of the egg. Earlier chapters have shown how 

 the pituitary induces ovulation and how the sperm induces early develop- 

 ment. And now the formation of the first structure, the neural plate, the 

 primitive nervous system, is induced by the cells of the dorsal lip of the 

 blastopore. 



The concept of the organizer brings up a number of questions. Is the 

 organizer a single factor and, if so, how can it stimulate the many structures 

 of the nervous system? Or is it composed of a large number of separate 

 inducing agents, each of which stimulates a single structure? Is the organizer 

 a hormone, is it a chemical compound? Can it be extracted? Can we induce 

 the development of the nervous system by a simple chemical stimulus as we 

 induced fertilization by chemical agents? These are some of the questions 

 that we shall consider in the next chapter. 



