244 



Embryogenesis: Progressive Differentiation 



choice of this term, which we, like Spemann 

 ('38) himself, shall use only in a provisional 

 sense, was based upon the discovery that the 

 tendency of self-organization of the graft 

 was combined with the induction of host 

 tissue to form an integrated whole. By as- 

 similating adjacent host cells into its meso- 

 dermal system and by inducing additional 



self-organized blastoporal material (Fig. 

 82c). (2) The action of the graft may go 

 further by causing "complementary induc- 

 tions" (Mangold, '32). In this case, the host 

 cells supplement the axial system of the 

 graft by adding to it entire rows of somites, 

 large segments of the notochord, and a pro- 

 nephric and neural system not containing 



Fig. 82. Diagram of the transplantation of a piece of the upper blastoporal lip into another gastrula {a, b) 

 and the self-differentiations plus inductions of the graft (c, d) . In c, the tissues derived from the graft are 

 shown in black, and the induced tissues in white. (Original.) 



structures from the overlying ectoderm the 

 graft could give rise to a more or less com- 

 plete secondary embryo (Figs. 82, 92). There 

 are two somewhat different ways whereby 

 the host can participate in the construction 

 of the secondary formation: (1) A true "as- 

 similative induction" (Spemann and Geinitz, 

 '27) may occur when the host cells become 

 harmoniously incorporated into the grafted 

 tissues thus forming sectors of the notochord, 

 somites, pronephros and neural tissue of the 



any grafted cells. Both modes occur usually 

 combined (Spemann and Mangold, '24; Leh- 

 mann, '32; Bavitzmann, '29, '33). The ceph- 

 alocaudal axes of the secondary embryo and 

 of the host may be oriented at any angle to 

 each other, but several workers observed a 

 striking preference of the graft for develop- 

 ing its main axis parallel to that of the host, 

 even if its original orientation was at a wide 

 angle to it (Geinitz, '253; Spemann, '31; 

 Lehmann, '32). Probably the invaginating 



