On the invagination of the gastrula

Conclusions

An experimental study of the problem of gastrulation has yielded evidence which shows that the present prevailing theories are inadequate to account for the facts.

Especial attention has been given to the theory ofAssheton, according to which the geometrical relation of nuclei to the cells in the blastula is of fundamental importance to invagination. It was found that an excess of calcium added to the sea water in which the cultures ofS. purpuratus were raised caused a displacement of the nuclei to the inside so that they appeared at the edge of the blastocoele. Lithium larvae on the other hand showed the nuclei disposed toward the periphery in the cells of the blastulae. Since, however, nuclear position did not uniformly correspond to invagination or evagination as the case happened to be, the view that the position of the nuclei determines invagination had to be given up.

A modification ofAssheton's view is suggested, namely, that invagination is caused by cell bridges asymmetrically placed. The eccentric attraction between the cells at the vegetal pole therefore presumably depends upon physical structures and not, asAssheton supposed, upon an attraction between the nuclei.