352 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



process) to join its pronephros or its myotomes^ (2) In the second, quite famous 

 possibihty (Spemann's organizer transplantation), the graft sinks or invaginates, 

 incorporates itself into the ventral middle layer and not only develops into 

 notochordal material and some somitic parts, but also causes the lateral plates of 

 the host to become notochord, somites and pronephros. The reaction is generally 

 completed by the induction of a neural plate, which will be considered later. 



Other operations performed on the same early stages, e.g. rotations of the 

 animal half (Fig. 51, p. 383), or of a large dorsal piece (Figs. 49, 50, pp. 381-382) ; 

 cause juxtaposition of the median (chordo-somitic) material and the more ventral 

 parts of the mesoblastic layer (Dalcq, 1940, 1941a, 1942). If they are realised 

 early enough, the chances are that a typical series oi notoch.ord-^orm\.e.s-nephrotome 

 and lateral plate will be established. This phenomenon is accomplished, as Nile- 

 blue vital staining has ascertained, by some recasting of the tendencies present in 

 the graft and an elevation of potential in the adjacent material. 



These results, and several others, attest the reality of an inductive influence taking 

 place inside the middle layer. However, these are only experimental data. One might 

 ask whether a similar mechanism plays a role in normal development. We do find 

 such a process in the awakening action exerted by the duct of the pronephros on the 

 mesonephritic blastema. There the necessity of a contact or at least of a narrow prox- 

 imity (p. 464) has been demonstrated (Cambar and Gipouloux, 1 956) and its imme- 

 diate consequence is the enrichment of the cells in RNA, their proliferation, and their 

 subsequent arrangement in canaliculi (O'Connor, 1939; Van Geertruyden, 1946; 

 and, for the exception found in axolotl embryos, Nieuwkoop, 1948). This shows that 

 intra-mesoblastic induction is not only experimental. It may well be thought to exist 

 inside the middle layer, as a conservative safeguard of the hierarchized potentials. 



We should now inquire how this induction is accomplished. Here, we do not 

 know any details concerning the immediate consequences of the relevant opera- 

 tions. Other types of induction, as we shall see, have been studied more accurately. 

 Although it has not been demonstrated directly, it is very likely that some agent 

 is transmitted from the inductor to the reacting cells. 



A considerable change in the fate of mesoblastic material can also be obtained 

 at a later stage, when the neural plate is already beginning to form. In grafting 

 and isolation experiments, it has been shown that somitic material has a tendency 

 to transform into pronephros, especially if it is adjacent to lateral plate material; 

 the presence of notochord favors the persistence of myogenic cells (T. Yamada, 

 1937, 1940). Recently, a rather large amount of somitic material (about five 

 presumptive somites) has been cultivated, wrapped in ectoblast. It yields an 

 appreciable quantity of myoblasts, pronephros, limb bud, lateral plates, blood cells^ 



' As far as I know, this migration of a homograft toward the corresponding dorsal material 

 of the host has been described in works made in my laboratory first by Minganti (1949) 

 and then, more explicitely, by Benoit (1952). This enigmatical process is probably equivalent 

 to the "homing instinct" which insures recolonization of the bone marrow of irradiated 

 animals by injecting a suspension of normal lymphoid and red blood cell-forming tissue 

 (see Mitchison, 1958). 



^ For an experimental analysis of the conditions necessary for the differentiation of the 

 erythroblasts, see Finnegan, 1953. 



