GROWING LYMPHATICS AND THE MESENCHYME 365 



fibrillae is carefully followed, it is seen to lead to one of the larger 

 easily visible processes. The behavior of these minutest fibrillae 

 has not been considered in the present study — the drawings 

 including only the processes which are readily seen. Between 

 the larger processes communications are rare, though here and 

 there undoubted connections may be seen. The branched mesen- 

 chyme cells have a uniform distribution, an equal amount of 

 space separating the neighboring cells. 



Near the edge of the muscle, where the distance between the 

 two layers is greatest, the cells are arranged in three or four layers. 

 Near the fin margin, where the fin is so thin that the two layers 

 of skin are almost in contact with each other, the cells form but 

 a single layer. Between these two extremes there are all grada- 

 tions. In making records of the cells a color scheme was used to 

 designate roughly the cells in the different layers. Thus, those 

 nearest the observer were drawn in black, those farthest away in 

 green, and those in the middle in red. In the reproductions, 

 the form has been changed, as indicated in the legend of figs. 2 

 to 11. With th : s crude scheme, it was found to be very easy to 

 identify each individual cell, with its main processes, in the suc- 

 cessive stages. 



What now, are the visible changes which occur in these cells 

 during the growth of the tail? 



As successive stages are compared it is soon to be noted that 

 changes are continually going on. On the one hand, branches 

 become thicker and longer, and new branches are sent out On 

 the other hand, branches become shorter and thinner, until they 

 may entirely disappear. The thickened central portion, too, is 

 continually changing shape, now extending further and further 

 along one or more branches, while retreating from others. The 

 picture becomes much clearer when it is found that, by a summa- 

 tion of these processes, the cell may actually shift its position, 

 or wander. The evidence for this conclusion does not rest upon the 

 observation of a single cell or a group of two or three cells; nor 

 does it depend upon the selection of some supposedly stable 

 structure to serve as a fixed point — such as a chromatophore 

 which may itself shift position. It is only certainly to be proved 



