Ill EXPERIMENTAL MODIFICATIONS 427 



thinks, a mass effect. On the contrary, the means which evoke an acrencephalon 

 are cytolytic (Hohfreter) or even lethal, and their consequence is not only 

 denaturation of the proteins, as currently admitted, but probabh' the production 

 of an extra-normal inductor. If we think therefore in terms of latent potency 

 in the ectoblast, it can only be a tendency toward notogenesis. 



As revealed by micro-surgery, the region where the factors conditioning noto- 

 genesis are at their peak is able to evolve into the prechordal inductor. This 

 process can be hampered by lithium and by early centrifugation, while it can be 

 favored by several different operations. 



The possibility of eliciting characteristic aspects of neural crest differentiation 

 by the introduction of definite aminoacids allows the best prospect for further 

 elucidation of primary induction. 



D. Xeno-indiidions 



When it had been recognized in Spemann's laboratory (Bautzmann, Hohfreter, 

 Mangold and Spemann, 1932) that the inductive property was not limited to 

 the "organizer"' but could be evoked in other parts of the germ by life-stopping 

 treatments, the way was opened for a more sensational generalization which 

 included practically all tissues or organs of animals, vertebrates or even inverte- 

 brates, directly or after appropriate alteration (Hohfreter, ig34b, c). 



This discovery made possible a new approach to the nature of the inducing 

 process and provoked a movement of investigations of a still-growing interest 

 (compare with the accounts of Brachet, 1944, 1950). Their results are of consider- 

 able variety and intricacy, but, in the recent period, they decidedly tend towards 

 clarification. 



{a) Fresh foreign tissues 



The use of fresh, untreated inductors is rather limited since many of them exert 

 a deleterious effect on the reacting material (inserted in the blastocoele or 

 wrapped in a piece of ectoblast from a young amphibian gastrula). Moreover, 

 the implanted tissue is likely to undergo aseptic autolysis which may cause 

 undesirable complications. Nevertheless, pieces of fresh organs have been used 

 successfully in several cases: 



Salamandra liver, inducing a lens (Hohfreter, 1934b); lizzard {Lacerta agilis) liver, inducing 

 a tail (Hohfreter, 1934b); kidney of the same animal, inducing some nervous cells (Hoh- 

 freter, 1934b); mouse heart, inducing, in clear relation with the implant, a brain and 

 nervous masses, or a hind-brain with otocysts, or neuroid parts, or a small tail (Hohfreter, 

 1934b); mouse liver, inducing a notomerit (Hohfreter, ig34b); mouse kidney, inducing the 

 same complex (Hohfreter, 1934b) or, sometimes, an acrencephalon (Chuang, 1940); 

 newt liver, inducing either acrencephalon or notomerit structures (Chuang, 1940); perch 

 and plaice lieart muscle, inducing a lens (Toivonen, ^945) ; frog abdominal skin, inducing 

 chordomesoblast only (Okada 1948); mouse bone marrow, inducing myotomes, myoblasts, 

 and notochord, sometimes with a spinal cord (Kawakami and Mifune, 1955) ; mouse spleen 

 stroma, inducing either an acrencephalon and olfactory placodes or myoblasts and noto- 

 chord (Kawakami and Mifune, 1955); cortical parts of mouse kidney, inducing hind-brain, 



Literature j>. 4H3 



