i8o 



PRINCIPLES OF EMBRYOLOGY 



of a normal embryo. In these organs, parts derived from the graft are 

 often closely intermingled with others induced from the host, both to- 

 gether forming a more or less unitary structure. In such cases, the organ- 

 iser has done something more than merely throw the host tissues into a 

 certain developmental pathway; it must have specified in detail the par- 

 ticular structures, and parts of structures, which the competent tissues 

 form. Spemann (193 1, 1938) followed his original discovery of the 

 organiser phenomenon by showing that different regions of the organi- 

 sation centre have different properties in this respect (Fig. 10.4). The 

 presumptive anterior regions tend to induce head structures and the pre- 

 sumptive taU regions tend to induce tails. There is thus a regional 

 differentiation within the organisation centre, and the regional properties 

 of a given part can be transmitted to the competent tissues in contact with 

 it. In the early gastrula, the regional structure, although definitely present, 

 is not yet firmly fixed. In the first place, although an isolated piece of the 



Figure 10.4 



Head and tail organisers in the newt. 1, A fragment of tissue (presumptive 

 anterior mesoderm) is taken from near the dorsal lip of an early blastopore 

 and placed in the blastocoel of another embryo so as to arrive at the anterior 

 [la) or the posterior [ib) region of the host. In either case it induces a head. 

 g, A similar piece taken from a late yolk-plug gastrula may fail to induce a 

 head {2b) unless it is near the host's head (2a). 



