366 ELIOT E. CLARK 



by observing a considerable number of cells, by keeping records 

 of the distance of these cells from relatively fixed spots and from 

 one another, and by watching them over periods of time long 

 enough so that the shifting in position is beyond question. More- 

 over the shifting must be such as cannot possibly be accounted 

 for by the general growth expansion, of the entire organ, or by 

 a mere mechanical dragging of the cell. 



That there is a genuine wandering of the cell is shown by a 

 study of the series of drawings from which figs. 2 to 11 are taken. 

 The region selected was large enough, the number of cells watched 

 was large enough, the length of time and number of successive 

 observations were great enough and the changes in shape and 

 position of cells were marked enough to exclude all other inter- 

 pretations. A general idea of the relative amount of shifting 

 is given in fig. 12, which shows diagrammatically the amount and 

 direction of change in position of cells between fig. 9 and fig. 11 

 (May 3 and May 23) . Here we see that, while certain cells 

 such as A, P, U have remained in practically the same spot, 

 others such as B, C, I or their daughter cells B l and 2 , C l and 2 , 

 I > and 2 have made long excursions while still others, such as 

 F, S have made short excursions. While the difference in loca- 

 tion of such a cell as B (B 1 ), with reference to the muscle edge 

 may be in part accounted for by the general expansion of the 

 entire fin, measurements show that the change is very much 

 greater than could be accounted for in such a way. In fig. 9 (May 

 3) the width of the fin from muscle edge is 0.38 mm. The 



Fig. 12 Diagram to show the amount and direction of change in position of the 

 mesenchyme cells shown in the figs. 2 to 11. The same form for the cells is used — 

 solid black, dots, and cross-lines to indicate the different layers. The squares 

 represent the position of the cells on May 3 (fig. 9), the circles their position 

 twenty days later, on May 23 (fig. 11). Since the fin increased in size during this 

 time, the positions of the cells in fig. 3 were measured with reference to the muscle 

 edge as a horizontal and a vertical passing through the base of the lymphatic 

 sprout, and were referred to a similar horizontal and vertical in the diagram of 

 the later stage, with increases to correspond to the growth of the tail. Where 

 mitotic division has taken place, the line, which shows the change in position, 

 forks. The arrows represent the direction of wandering. In cases where cells 

 came into the field, which was being watched, after May 3, the square is omitted 

 (cf. cells 13, 15, 16, etc.) 



