Mouvements cellulaires dans le développement de l'embryon des amphibiens et des poissons

Abstract

The study of the cellular movements during the pregastrula and gastrula stages, in amphibia and teleosts, shows resemblances and differences in connection with the structure of the egg. The entire egg of amphibia does not show any activity during the pregastrula stages; the teleost one is animated with complex movements, which are sometimes distinctly polarized. This activity appears in the cortical layer, in contact with the yolk. If it does not envelop pure cytoplasm only (embryonic shield) the cortex is inactive like the amphibia's. Thus the motor activity does not depend upon the cortex alone, but probably upon the interaction cortex/yolk. The invagination during gastrulation is not an infolding towards the depth of the egg-wall but brings in individual displacements of the blastomeres: the deep blastomeres proceed, the ones on the others, towards the blastocoel, and into that moving mass, the superficial blastomeres (flask cells), endowed with a polarized motor activity, go deep more quickly. During normal development, the yolk mass of the teleosts might be used as a support for the moving cells, but this part is not absolutely necessary as the invagination accomplishes itself in the expiants deprived from yolk. If the layer of superficial blastomeres (enveloping layer) is suppressed, the invagination cannot be carried out (teleosts). The epiboly is due to the autonomous intrinsic spreading force acquired by the cells of the animal hemisphere at the beginning of the gastrulation. In experimental conditions, a capacity of spreading, which can be compared to an epibolic movement, can be set in evidence in the centre of the pregastrulean egg. The epibolic capacity, in teleosts, is limited only to the cells of the enveloping layer, as it appears in the culture experiments. The realization of the epibolic movement, more complex in teleosts than in amphibia, results from the cooperation of force groups, respectively localized in the segmented and non-segmented portions (vitelline syncytium, perivitelline pellicle) of the egg. These two movements of invagination and epiboly are not connected and may be realized in gastrula separately. The mechanism of the morphogenetic movements sets the following problems: 1. 1. spontaneous mobilization of cells 2. 2. nature of the individual movements and coordination 3. 3. orientation of the movements. In conclusion, these questions are rapidly examined. It is suggested that the initiation of the activity and the orientation might be dependent upon secreted substances.