114 



INDUCTION AND ORGANISATIOM 



the mesoderm of the host. It is important to make a clear 

 distinction between these two types of embryonic induction, 

 and to discuss them separately. They are: (1) induction by 

 contact; here the "inductor'' and the material that reacts to 

 it, the so-called ''reaction system", are in immediate contact, and 



Fig. 42. Diagram of a cross section through the dorsal part of an 

 embryo, illustrating the difference between contact induction and 

 progressive induction. The organiser (hatched) influences the over- 

 lying ectoderm and the underlying endoderm by contact induction 

 (arrows drawn in full), but it influences the surrounding mesoderm 

 by progressive induction (interrupted arrows). 



(2) progressive induction, which extends from cell to cell in 

 a coherent layer of cells (Fig. 42). 



In normal development, the material of the dorsal marginal 

 zone, or organisation centre, becomes invaginated during gastrul- 

 ation, and forms the archenteron roof. It becomes located under 

 that precise part of the ectoderm which will later differentiate 

 into the neural plate. The hypothesis is obvious that here, 

 again, "contact induction" plays a role, and that the neural 

 plate is determined under the influence of the underlying 

 archenteron roof. Marx (1925) demonstrated that the latter 

 material does indeed possess inductive powers. He cut out 

 part of the archenteron roof of an older gastrula, and brought 

 it into the blastocoel of a blastula. The slit in the blastula wall 

 through which it was inserted soon healed again (Fig. 43 a). 

 During the gastrulation of the host, the blastocoel was obliter- 

 ated by the archenteron, and the implant then lay immediately 

 under the ectoderm. Therefore, it was in a position to exert 

 such inductive influence as it possessed (Fig. 43 b). This 

 "insertion method" has proved very effective, and it has been 



