MICROSCOPIC STRUCTURE 
3d 
and possibly also for the contention that a transformation occurs from 
hemal to lymph nodes, and vice versa. For, as already stated, such a 
transformation might well be assumed to occur, because an actual series 
of nodes with but slight structural differences — except circulatory — be- 
tween the successive nodes, can be formed with true hemal and lymph 
nodes, as extremes. In such a series it would be entirely impossible to 
differentiate many of the intermediate nodes from external appearances, 
or by a cursory microscopic examination; and it should be remembered 
that injection methods alone made positive identification possible in 
many cases of these apparently transitional forms. 
In those nodes in which the blood corpuscles were not intermingled 
more or less uniformly with the parenchyma, they formed masses which 
contained some lymphocytes, especially at the periphery of the masses. 
These local accumulations of blood-cells — blood islands — vary in size 
from small groups of cells to larger masses which may compose the 
greater part of the area of the node. There are nodes, for example, con- 
taining a single blood island practically co-extensive with the node, and 
forming perhaps nine-tenths or more of the area of the section (figs. 15, 
18 and 19). Others, on the contrary, may contain hundreds of minute 
blood islands which may nevertheless form but an insignificant fraction 
of the total volume of the node. When small, numerous and isolated 
these islands often give a spotted appearance to the cross-section under 
low-power magnification. If, on the contrary, they are very large, they 
are often ill-defined, generally irregular in shape, and more or less con- 
tinuous with each other, with the blood in the peripheral blood space, and 
with the large central — internal — spaces, if present. A union of the per- 
ipheral and more external blood spaces into central collecting spaces was 
never observed. 
It is evident, of course, that a peripheral blood space can exist only 
when sufficient lymphatic tissue is preserved; for when the latter 
is greatly reduced in quantity, all boundaries are completely ef- 
faced, thus allowing the blood islands or spaces to merge. Hence as 
more and more of the lymphatic tissue of the node disappears and the 
total hemal area becomes larger and larger, a stage is finally reached 
in which the peripheral and central blood islands or spaces merge into 
an area in which only remnants of the once predominant lymphatic tissue 
are found. But whether these blood islands in the parenchyma were 
large or small, the blood was only very rarely found to be in a state of 
evident degeneration, except in nodes which were largely or almost 
wholly devoid of lymphatic tissue. 
The cellular content of the blood islands did not vary markedly from 
that of the veins ; although at the periphery of the islands, and especially 
