44 WARREN H. LEWIS 
16, 19). ‘The process is essentially the same as that already 
shown for fibroblasts and mesenchymal cells (W. H. Lewis, 
19, 720). 
I have before me seventy photographs (at 1450 diameters) of 
endothelial cells from this series, which were taken especially for 
the mitochondria, and it is difficult to select therefrom a few 
that will adequately convey to the reader an idea of the great 
variability displayed by these cells. In some cultures varying 
numbers of cells, as early as the third day, contained only granu- 
lar mitochondria. ‘This condition, however, may be delayed; 
some cells, even as late as the tenth day of cultivation, still 
showed thread-like, as well as granular and rod-shaped, mito- 
chondria. Most of the cells, even up to the tenth day, contained 
all three types, granules, rods, and threads, varying in length 
thickness, and form. Variations in the amount of mitochondrial 
substance were sometimes quite marked. Presumably the mito- 
chondrial complex is much more constant under the normal 
conditions within the embryo, and the great variations in the 
cultures are to be attributed to the variations in the abnormal 
conditions of the cultures. They do not appear to be due to 
differences in hydrogen-ion concentration from pH 6.4 to pH 7.6, 
nor to the age of the embryo, since great variations may occur in 
two successive cultures of the same series or even in the same 
culture. The most probable factor seems to be in the thickness 
of the medium drop and the accompanying evaporation in the 
air chamber, with consequent variations in the oxygen supply 
to the cells, dependent on the thickness of the fluid separating 
them from the air in the chamber of the hollow slide. 
Granules and vacuoles. Granules and vacuoles, which have 
a marked affinity for neutral red, methylene blue, and brilliant 
cresyl blue, gradually accumulate in these cells and vary as 
much as they do in the fibroblasts and mesenchyme cells (Lewis 
and Lewis, 715; W. H. Lewis, ’19, ’20). The rate at which they 
make their appearance varies in different cultures; as do like- 
wise the relative proportions and the size of the granules and 
vacuoles (figs. 21 to 23). We are still of the opinion that these 
granules and vacuoles bear a relation to degenerative changes 
