Burrows: Study of Body Cells. 487 



this movement noted that the sells never moved directly outwards 

 into the liquid but always at its surfaces. In Harrison's earlier 

 studies he had attempted to cultivate fragments of the neural tube of 

 frog embryos in hanging drops of serum. No evident activity was 

 observed. Success was attained only when these fragments were 

 placed in lymph which clotted about them. In studying the move- 

 ment of the cells in lymph Harrison then noted that the cells moved 

 always in contact with the fibrin fibrils, and in his later analysis of 

 the culture of the Lewises he found the cells migrating always in 

 contact with the surface of the cover glass or on the free surface of 

 the medium. Harrison^^ termed this phenomenon, as L. Loeb-'^ had 

 done, "stereotropism." He considered these cells apparently highly 

 complex systems whose mechanism for movement is regulated 

 through such contacts. Both authors thought this a common prop- 

 erty of all cells. 



After a careful study of the movement of these embryonic cells in 

 liquid medium. I noted, however, that these cultures of the Lewises 

 were applicable only for the movement and growth of the cells of 

 the younger embryos. No activity is seen about the normal frag- 

 ments of heart muscle or, other mesenchyme cells of older embryos 

 at the surface of the liquid or in contact with any solid. As I 

 have stated above, blood plasma or fibrinogen is the one common 

 substance of the body capable of stimulating activity in them. 



From these facts it seems evident, therefore, that it is not solids 

 in a general sense that are necessary for the movement of these 

 cells, but specific adsorbing substances. The younger embryonic 

 cells differ from the older ones in that they may move at the sur- 

 face of the salt solution and liquid media as well as in the clot. 

 The older cells had lost this property. This leads me then to 

 analyze more carefully this movement at the surface of the me- 

 dium. I measured the position of the cells moving at the cover- 

 glass surface with the micrometer of the microscope. I found that 

 these cells were not in contact with the cover glass, as Harrison, 

 Lewis-s and L. Loeb had thought, but that they lay often a con- 

 siderable and measurable distance below it. Again, previous to the 

 movement of the cells on the surfaces of the hanging drop, these 

 surfaces change. They became covered by a film or scum, which 

 made them appear leathery. The cells in every case moved, grew 

 and divided in this film of material. This substance is not the 

 coagulating substance "L" liberated by the other cells. It is son e- 

 thing new, which acts evidently in place of and antagonistically to 



