250 ELIOT R. CLARK AND ELEANOR LINTON CLARK 
season, in order, chiefly, to follow the same leucocytes throughout 
the process. This we succeeded in doing, and found in every 
instance that the leucocytes revert, after the extrusion of the 
oil, to typical rounded or amoeboid leucocytes. We feel morally 
certain that had we seen these leucocytes in sections, we should 
have labelled them connective-tissue cells. 
Again, when leucocytes were first seen emerging from lym- 
phatic capillaries, they often gave the appearance of pulling 
away from the endothelial cells—an interpretation which might 
very readily be made from sections. However, continuous 
observation of lymphatic capillaries showed that the leucocytes 
always entered the lymphatic vessel, from the outside, that they 
moved a variable distance along the lumen and then passed 
out through the endothelial wall. In their movements among 
the connective-tissue cells, again, leucocytes frequently pass over 
them so closely that only continuous observation shows that 
they are not being formed in some way by the connective-tissue 
cells. 
While we hesitate to generalize from a set of observations 
restricted to a single species of a lower vertebrate, still our 
results are so definite and so positive that it seems fair, at least, to 
advance the hypothesis that similar tissues in other animals 
react similarly. Certainly, one must be very skeptical toward 
conclusions as to cell transformations which are based on studies 
of fixed material, especially when such studies yield such a 
variety of conflicting views. . 
One matter remains to be cleared up by further studies, 
namely, the possibility of the transformations of one type of 
leucocyte into another. Owing to the great difficulty in seeing 
the outlines of the nuclei in the cells studied, we did not attempt - 
to distinguish, in the living, between the different types of 
amoeboid cells, except for the pigment cells, and our studies 
of total mounts of fixed specimens are still incomplete. Some 
investigators hold that polymorphonuclear leucocytes may be 
transformed into mononuclear cells (Janowski, Metchnikoff). 
A suspicious fact in this connection is the oft-repeated obser- 
vation that, early in inflammation, polymorphonuclears predom- 
