XENO-INDUCTIONS 



431 



brain and axial mesoderm" (Kuusi, 1951, p. 356). This statement needs, however, 

 a slight attenuation (p. 436). On the other hand, the same xeno-inductor will, 

 by appropriate fractionation, provide us with valuable information (p. 441). 



(c) Regional influence of the host 



Before examining in more detail the nature of the xeno-inductors, it is worth 

 mentioning that their study provided some more information concerning the 

 influence of the primary embryo on the induced structures. This is the classical 

 problem of regionality, especially important with implantations in the blastocoele. 



N£URAL DIFr£RENTIATIONS 

 (or their derivafes) 



FORE-BRAIN REGION 



Tel- and/ or diencephalon 



Eue or ifs fragments 



Nasal pit 



Balancer 



HIND -BRAIN REGION 

 Rt)ombencepholon 

 Ear vesicles 



TRUNK AND/OR TAIL REGION 

 Spinal cord 



UNSPECinC NEURAL 

 Vesicles and/or tiolls 

 Nery or ganglion cells only 



MESODERMAL DIEFERENTIATIONS 

 Notochord 

 Myotomes 

 Proneptiros 

 Fin witl^ mesenctiyme 



LIVER 



113 specimens, 



LIVER* BONE- MARROW BONE- MARROW 



66 specimens 



24 specimens 



f 



20 'tO SC ec 100% H W 60 BO lOOX !0 iO 60 10 lOOX 



Fig. 85. Histograms of the inductions obtained with liver, bone marrow and the tissues 



together. The implants were taken from guinea pig(s) and implanted in the blastocoele of 



newt gastrulae. From Toivonen and Saxen, ig55b. 



Chuang (1939) recognized that this host influence was as real with xeno-inductions 

 as with organizer grafts. Vahs (1956) led a broader and statistically-based enquiry 

 for which a pool of 1327 individual cases of xeno-inductions was examined, 

 without regard to the inductors used. 2631 induced structures were available; 

 they were all classified according to the number of each type located in (I) the 

 head region, (II) the cardiac region, (HI) the fore part and (IV) the hind part 

 of the trunk. The results (Fig. 86) confirm that an influence from the primary 

 system exists, but only concerning acrencephalic structures. In my opinion, the 

 source of this influence must be in the prechordal anlage of the young archenteric 

 roof The fact that other parts do not exert such a control or harmonization, not 

 even for organs as cephalic as the otocysts, fits well with the idea that notogenesis 

 is a more autonomous process and implies a close hierarchy of the involved 

 structures. 



{d) First approaches to the nature of the xeno-inductors 



Attempts to disclose the nature of the induction process by a biochemical analysis 

 of some very active inductors, began with the discovery of xeno-induction, and 

 the passionate work first accomplished in this direction by Fisher, Needham, 

 Waddington, Barth, and their coworkers {cf Brachet, 1950) deserves a tribute 

 of admiration. This long and arduous exploration has resulted during the last 



Literature p. 483 



