Maxcu 29, 1901.] 
is found very early to include in vertebrate 
embryos, four kinds of cells, of which the 
most numerous are undifferentiated cells) 
the other three kinds being (1) endothelial 
cells of blood vessels; (2) blood cells; (3) 
sexual cells; all these are precociously 
Specialized; they are few in number, yet 
they are probably the parents of all the 
cells which are produced of their kind 
throughout life. Our present knowledge 
does not permit us to speak with entire 
certainty, but the evidence is strongly in 
favor of the following three conceptions : 
First. That all the endothelium of the 
blood vessels of the adult is descended di- 
rectly, from the endothelium of the first 
blood vessels differentiated in the extra- 
embryonic portion of the germinal area. 
Second. That all the red blood corpuscles 
are descendants from the red-blood-cells of 
the blood-islands of the area vasculosa. 
According to this view the blood forming 
organs, as they are called, merely provide 
sites, where the red cells can multiply, as 
for instance in the mammalian embryonic 
liver or in the adult marrow. 
Third. That the primitive sexual cells 
by their multiplication produce all the cells 
from which the genoblasts, or sexual ele- 
ments proper, male and female, are evolved. 
The future will decide the validity of 
these conceptions. They are very signifi- 
cant, because they assume that there are 
cells which form exclusive classes, and are 
characterized by a special combination of 
qualities, so that while they retain so much 
of the embryonic character as to have still 
the power of rapid multiplication, they yet 
are so specialized that they can only pro- 
duce their like. If the three conceptions 
are established, we shall regard these three 
sorts of cells as almost the first to be 
fully differentiated. We shall also have to 
regard the vascular endothelium as distinct 
not only from the epithelial lining of the 
body cavity, but also from that of the 
SCLENCE. 
489 
lymphatic system. The immense impor- 
tance of such a discovery as bearing upon 
pathological researches and interpretations 
is obvious. 
The next important change in the mes- 
oderm is the development of the main 
body-cavity, which the embryologist des- 
ignates comprehensively as the ccelom. 
The cells, which lie next the body-cavity 
and border it, assume an epithelial arrange- 
ment; this epithelial layer around the 
cwlom is properly named ‘ mesothelium,’ 
and the loose cells about it constitute the 
“mesenchyma.’ We do not have, however, 
at first a true differentiation of mesothelial 
and mesenchymal cells ; all are undifferen- 
tiated, and we can readily demonstrate 
that the cells are interchangeable, differing 
during early stages by their positions in 
relation to one another and to the body 
cavity, but not differing in their essential 
structures or qualities. Thus we find that 
the mesothelium constantly gives off cells 
which join the mesenchyma, and we find 
later that mesenchymal cells may take on an 
epithelial arrangement around any of the 
cavities—and there are many such—which 
arise within the mesenchyma itself in the 
course of further development. 
But, although difference-of arrangement 
does not necessarily indicate differentiation 
of the cells, it does affect the character of 
the differentiation which ensues. As every 
text-book states, the mesothelium gives rise 
to the striated muscles and to the epithelial 
portions of the entire genito-urinary tract, 
and is permanently retained, with slighter 
modifications, as the epithelium of the peri- 
cardium, pleurz and peritoneum. The mes- 
enchyma produces an even greater variety 
since it is the parent of not only all the 
connective and supporting tissue, but also 
of the lymphatic system. 
I venture to turn aside for a moment to 
urge upon you the adoption of the term mes- 
othelium as the correct designation for the 
