368 



ELIOT R. CLARK 



distance of cell B from muscle edge is 0.078 mm., or 29.5 

 per cent of the entire width. On May 26, seventeen days later, 

 three days after the stage shown in fig. 11, cell B\ one of the 

 daughter cells of cell B is 0.29 mm. distant from the muscle edge, 

 or 58 per cent of the entire width, 0.50 mm. Thus this cell has 

 moved through 37.5 per cent of the width of the fin, an actual 

 distance of 0.188 mm. Calculated in a similar way, the cell P 

 is found on May 3, 13.5 and on May 26, 14 per cent of the distance 

 from muscle edge to fin margin; it has, then, remained practically 

 stationary. Many striking instances of relative change in posi- 

 tion may be seen by following any two or three selected cells 

 through figs. 7 to 11, such as cells E, N and F. 



A. May. 20 



B May 2 I 



C May 22 



D May23 



Fig. 13 Four successive drawings of the same mesenchyme cell, taken from 

 the series of studies represented in figs. 2 to 12, to show the method of wandering 

 of the mesenchyme cell. The cell here shown is the one labelled G in figs. 2 to 

 12; stage D in this figure corresponds to cell G in fig. 10. The dotted cell in D 

 is the same as A. It is superimposed to show the change in position. Enlarge- 

 ment 270 times. • 



The exact way in which this change in position of the cells 

 takes place is shown by a comparison of successive drawings 

 of the same cell. Cell B, in figs. 2 to 9, furnishes a good example, 

 since the several stages here are separated by short intervals of 

 time. In fig. 13 a single cell is shown, in four successive stages 

 at daily intervals. It will be seen that change in position is 

 effected by a movement of the protoplasm. Fine processes are 

 sent out from the main body of the cell, such as 3, in fig. 13. These 

 processes may be quite temporary, and may be withdrawn sub- 

 sequently, without attaining more than a hair-like thickness. 



