1 70 



organisers: inducers of differentiation 



is no regulation and nothing resembling a pluteus is formed. Further 

 consideration of this and related phenomena is reserved for Chap. ix. 

 Lastly, a region of cytoplasm essential for subsequent differentia- 

 tion of the embryo has been discovered in Insects. As mentioned 



Fig. 84 



The activating centre in early development of the dragon-fly Platycnemis. 

 a, The egg was constricted tightly (some nuclei already present in posterior 

 portion). No development anterior to constriction, owing to inability of the 

 activating substance to reach it, b. The egg was constricted loosely, and the 

 activating substance was able to diffuse, and development occurs in the anterior 

 portion, c. Constriction behind the activating centre does not interfere with 

 development, d. Normal embryo resulting from c. e, f, Constriction and ex- 

 clusion of nuclei from the activating centre: no development. (From Morgan, 

 Experimental Embryology , Columbia University Press, 1927, after Seidel.) 



in Chap, v (p. 128), the hinder end of the tgg of the dragon-fly con- 

 tains a region, whose destruction, or isolation by means of a con- 

 striction, prevents development of the embryo. Before this region 

 can exert its activity, it is necessary that nuclei should reach it ; if 

 the nuclei are prevented from doing so (by a constriction drawn 



