ENDOTHELIUM IN TISSUE CULTURES 43 
The contrast between the endothelial cells and the liver cells 
is always quite marked, as shown in figures 5 and 12. Some- 
times the endothelial cells have exceedingly long and slender 
processes which extend out from the ends of the cells parallel 
to each other and to the long axes of the cells. 
It is very difficult to determine whether the reticulum formed 
by the endothelial cells is a syneytium or not. In most places 
it is impossible to decide between actual fusion and mere adhesion. 
Occasionally it is clearly evident that the connections are only 
adhesions and that each cell preserves its individuality. This 
together with the fact that cells at the periphery readily become 
isolated from one another, would lead one to believe that actual 
continuity does not exist at any time. Again, under certain 
conditions, cells retract their processes and tend to round up, 
losing all connection with neighboring cells; this also would 
indicate adhesion and not fusion. The difficulty of determining 
the exact status of the exceedingly thin processes that extend 
out onto neighboring cells, especially where the cells are closely 
packed together, has led us in the past to consider that we were 
dealing with a syncytium, but we are inclined now to doubt this, 
both for endothelium and mesenchymal reticuli in general. 
CELL STRUCTURES 
Mitochondria. In young cultures the mitochondria are mostly 
in the form of threads of varying lengths and forms and often 
with no definite orientation. In addition there are often rods 
and granules (fig. 13). Sometimes, even on the first day, they 
may show a more or less radial arrangement about the centriole 
region. The conditions of the mitochondria in different cultures 
of the same age, and even in different cells of the same culture, 
vary so tremendously that it is difficult to give a general descrip- 
tion of their appearance (figs. 18 to 20). No two cells are ever 
exactly alike. As the culture ages the mitochondria tend to 
become more or less radially arranged about the centriole and 
enlarging centrosphere (figs. 14, 15) and the threads tend to 
break up into rods and granules until finally the cells contain 
a large (giant) centrosphere and mitochondrial granules (figs. 
