The reaction of embryonic cells to solid structures

striking case of a cellular tropism (taktische Reizbarkeit) in the behavior of the mesenchyme cells of the sea urchin embryo. It must likewise not be forgotten that as early as 1892 R a m h y Cajal had stated his theory of chemotaxis to account for the outgrowth of the nerve fiber. 6Inexplicable as it may seem, very little of a definite nature has been added to our knowledge of this field since the period just cited. In my work on the development of peripheral nerves' it was definitely shown in confirmation of Cajal's theory, that the active factor in the spinning of the nerve fiber is a small mass of amoeboid protoplasm at the end of a cell process. The formation of long fibers by means of this mechanism was found t o be a characteristic of embryonic nervous tissue, but other somewhat different forms of activity of the same general nature were seen in all cells of the frog embryo when cultivated in clotted lymph. 8No rigorous proof of the reaction of moving cells t o definite stimuli was given in the course of these experiments. One of the most striking circumstances noted, however, was that when the lymph clotted firmly the movement was nearly always active, while in the cases in which the medium remained fluid only rounded cells were seen and these failed to undergo characteristic changes of form and locomotion. These observations led to the hypothesis that the cells were positively stereotropic and therefore unable to leave the solid masses to move out into purely fluid media. The work of Burrows with tissues of the chick embryo lent support to this hypothesis and it has since received further confirmation from other source^. Prior to this L. Loeb, l0 a La retine des vert6br6Ei'. La Cellule, T. 9, 1892. 8 Cells with processes of hyaline protoplasm were seen by Roux in the course of his work on cytotropism. The pseudopodia were described by him as occurring on cells that had become attached to the bottom. In one passage Roux states (op. cit. , p. 163) that it is doubtful if any cytotropic movements take place without the solid base.