372 ELIOT R. CLARK 



on the character of the non-dividing cells (fig. 14 G) and slowly 

 wander away from each other. Cells Z 1 and Z 2 , fig. 11, show 

 these same two daughter cells on the day following. 



The position of the cell in the fin at the time of division is of 

 interest. A study of fig. 12, in which the position at the time of 

 division is indicated by the point at which the line showing the 

 course of the cells forks, shows that more divisions occur in 

 the thicker portion, of the fin near the muscle than in the thin 

 part near the edge. Here cells 2, 3, 4, 5, 6, 7, 8 and L remain 

 undivided. Of the cells in the thicker portion, those in the inte- 

 rior appear more likely to divide than those near the surface — 

 the cells A, P, U, R, S and V remain undivided. In all, nearly 

 one-half of the cells observed underwent division during twenty- 

 three days. None of the daughter cells arising from division, 

 underwent further division during this period. 



As a summary of the results of this series of observations on 

 the growing mesenchyme cells the following may be said. All 

 the mesenchyme cells in a selected strip of the fin expansion extend- 

 ing from muscle edge to fin margin were carefully watched and 

 recorded from day to day over a period of twenty-three days. 

 During this time the larva increased in length from about 5.5 

 mm. to 8.5 mm., while the width of the ventral fin expansion 

 increased from .38 to .50 mm., at the portion selected. It was 

 possible to identify each cell throughout this period, and to study 

 its growth changes. The cells were found to possess two striking 

 properties, those of amoeboid movement and of mitotic division. 

 Every mesenchyme cell that was present in the area selected at 

 the beginning of the observations, as well as every cell which 

 wandered into this area from between the muscle layers or from 

 the more anterior portion of the tail during the observations, 

 maintained always its indentity as a mesenchyme cell. Although 

 the cells were actually watched for hours at a time, during periods 

 of active growth changes, and though a number of mitotic divisions 

 were watched throughout, there was never the slightest indica- 

 tion of the transformation of one of these mesenchyme cells into 

 a cell of another type. The mesenchyme cell remained mesen- 

 chyme cell. The two daughter cells quickly resumed the typical 



