398 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



piexes were obtained, while with the blastoporal hp the cephaHzation was 

 obvious. The analysis was yet still guided at that time by the bare opposition 

 between the head and trunk-tail organizers. 



A few years later, Okada's laboratory produced some papers mainly concerning 

 the possible differences between the non-invaginated and invaginated parts of 

 the dorso-marginal material {cf. p. 376). 



In the first group of explantations, performed on Triturus pyrrhogaster, Okada and Hama 

 (1943) compared the fate and the inducing ability of the blastoporal lip excised at six 

 different stages, and the archenteric roof often divided into two or three parts, anterior, 

 middle, posterior. For the uninvaginated material of the younger stages up until the 

 formation of the semicircular blastopore, a marked difference was found between these 

 results and those from affixation in the blastocoele (p. 377). Trunk organs were still 

 obtained, but they were completed in numerous cases by typical acrencephalic structures. 

 The proportion of olfactory placodes and optic vesicles was highest when the uninvaginated 

 part of the crescent-shaped blastoporal lip was used, a fact that may be interpreted by 

 the presence, at this very level, of the prechordal territory. At this same stage, the invagi- 

 nated portion was distinctly less active from the same point of view. From the semicircular 

 blastopore stage on, a decline in the inducing activity of the mostly prechordal material 

 was observed. It seems that no case, either using still un- or invaginated material, provided 

 only acrencephalic inductions. 



In 1945, the same authors made another comparison concerning the regions of the 

 archenteric roof from the semicircular to the circular blastopore stages. The fragments 

 were either inserted in a young neurula and covered with ectoblast of an advanced gastrula, 

 or wrapped in the same tissue. The inductions of acrencephalic type were only few, and 

 were obtained, as in Mangold's experiments, from explants more posterior than had been 

 expected. Somites and other mesoblastic manifestations of induction were regularly 

 missing. Otic vesicles were often induced, especially by trunk chordomesoblast, even quite 

 posterior. 



At about this time, Ter Horst (1948) reinvestigated the fate of explants taken 

 from the still open neural plate. Parts of the neurectoblast and of its chordo- 

 mesodermal substratum were cultivated separately in heteroplastic sandwiches. 

 Most of the results are now easily understandable, and do not need a detailed 

 account. Two of them, however, deserve to be mentioned, (j) An alpestris sandwich 

 provided with the anterior fourth of a cristatus neural plate formed a complete 

 brain; this organ had been formed by the explant, except for its telencephalon 

 and olfactory placodes which had been induced in the alpestris material : a classical 

 case o{ complementary induction. (2) A purely mesoblastic part (without notochord), 

 taken about mid-length of the archenteric roof, developed partly into a neural 

 tube, while the rest mostly failed to differentiate, except for some muscle cells 

 (Fig. 63). This is a typical case of direct neuralization of mesoblastic material. 

 The same event has been observed also by Tondury (1936) and by Dalcq and 

 Lallier (1948). In these last cases, which concerned transplantations of blastoporal 

 lips, diagnosis of the graft was possible thanks to vital staining and it cannot be 

 excluded that the Nile blue could have played a role. This objection does not hold 

 for the observation of Ter Horst, for she only used a heteroplastic technique. 



Gallera used the sandwich method (1948) for a control in his transplantation 

 experiments. He inserted the prechordal neurectoblast between pieces of neurula 

 epiblast. The development of the explants was slower than for grafts made on 

 embryos. With young donors only, neuroidal differentiation took place. With more 



