Le facteur ‘temps’ dans l’action inductrice du chordo-mésoblaste et l’âge de l’ectoblaste réagissant
Authors: Gallera J. 
Published: April 2021

ABSTRACT
Transplantations were made of pieces of rhombencephalic neurectoderm and presumptive anterior spinal cord to the ventral surfaces of young neurulae of Triturus alpestris. These experiments were made in two principal series: in the first, the ectoderm had been exposed to inductor of the same age; in the second, younger gastrula ectoderm had been placed in contact with the anterior chordal region of the archenteron roof of young neurulae (in which the inductor had developed to a stage where, in a normal embryo, the major processes of induction of the nervous system have been completed). In these two experimental series the ectoderm was withdrawn from inductive action more or less precociously, but always before the process of induction had been completed. These transplantation experiments were compared with the development of rhombencephalic explants excised from young neurulae.
The first manifestations of inductive activity (to which the grafts had been submitted before their transplantation) was always the formation of neural crest, dispersed as ectomesenchyme and pigment cells. From older grafts there developed ganglionic masses and, eventually, irregularly formed small-sized neural vesicles. The formation of the typical rhombencephalon has only been obtained from grafts and explants taken at the beginning of neurulation.
There is a clear quantitative progression of the successive stages of development of the ectoderm corresponding to the duration of inductive activity. The more that contact with the chordo-mesoderm was prolonged, the greater the portion of the graft evolving into neural crest, or, if the inductive activity was sufficiently long, the more voluminous the neural structures in the grafted ectoderm.
The sole striking difference between the results of the two series of experiments is that ectoderm subjected to induction by chordal and somitic material of its own age has never developed acrencephalic complexes of brain and sense organs, while in the second series, in which the young ectoderm was placed in contact with a much older inductor, such structures were formed by a certain number of young and medium aged grafts (left upon the inducing substrate for 16 to 22 hours).
One must thus conclude that during the later stages of gastrulation there is a considerable modification of the inductive power of the chordal region of the archenteron roof. At the beginning of neurulation this embryonic material acquires some capacity to induce the formation of the anterior brain in the ectoderm. It is significant that the chordencephalogenic inductive power is always predominant in the deutomeritic region of the archenteron roof. The induction of the acrencephalon by this material is not possible unless the effects of the acrencephalic inductive activity are dissociated artificially from those provoked by deutencephalogenic inductive activity with which the chordo-mesoderm is always endowed. This last result can be obtained by interrupting the inducing contact sufficiently early and at the right moment.