ORGAN-FORMING SUBSTANCES IN THE EGGS OF ASCIDIANS

Abstract

That egg of any animal is composed of simple, undifferentiated protoplasm is an article of traditional belief with a large number of zo6ologists, and that cleavage of egg is i a mere sundering of homogenous materials capable of any fate is a doctrine which has been given great prominence in recent years. In favor of these commonly accepted views stands a considerable body of experimental work on development of ovum; fragments of eggs or isolated blastomeres in many cases are said to give rise to entire larvae, thus proving, as is usually claimed, that parts of egg or embryo are still undifferentiated at time of experiment. But not all experiments on development have confirmed these conclusions ; some of first and most careful researches of this sort led to directly opposite results. In development of frog's egg Roux found (1883, 85, 87, 89, 92, 93, 94, etc.) that median plane of embryo is determined in egg immediately after fertilization and that the development is, from second cleavage on, a mosaic work of at least four vertical independently developing pieces. In I887 Chabry showed that early cleavage cells of ascidian egg are specified for particular ends and that they develop, if they develop at all, into parts which they would produce under normal conditions. These results were, however, denied on ground of other ex205 Vol. VIII. NO. 4