522 ROSS G. HARRISON 



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 Ram6n 

 y Cajal had stated his theory of chemotaxis to account for the 

 outgrowth of the nerve fiber. ^ Inexplicable 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 confinnation of Cajal's theory, that the active 

 factor in the spinning of the nerve fiber is a small mass of amoe- 

 boid protoplasm at the end of a cell process. The formation of 

 long fibers by means of this mechanism was found to be a char- 

 acteristic 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. ^ 



No rigorous proof of the reaction of moving cells to 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 sources.^ Prior to this L. Loeb,^° 



6 La retine des vertebras. La Cellule, T. 9, 1892. 



7 Anat. Rec, vol. 1, 1907; Jour. Exp. Zool., vol. 9, 1910. 



^ 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. 



^ Burrows, Jour. Exp. Zool., vol. 10, 1911; Carrel and Burrows, Jour. Exp. Med., 

 vol. 14, 1911; Holmes, Univ. California Pub., Zoology, vol. 11, 1913. 



1° Arch. f. Entw. Mech., Bd. 6, 1897; Bd. 13, 1902; Zeitschr. f. Krebsforschung, 

 Bd. 5, 1907. 



