496 
species of cells produces tumors of its own 
kind. This is a familiar conclusion as 
regards the endothelium, both of the blood- 
vessels and of the lymph-vessels, and also as 
regards the neuroglia. It seems to me that 
the excessive multiplication of leucocytes 
may properly be classed in the same cate- 
gory as the growths resulting in angioma 
and glioma. I do not know whether or not 
an excessive and abnormally rapid produc- 
tion of red blood cells may occur so as to 
occasion a special and distinct disease. In- 
creased production of red blood cells (ery- 
throcytes) is, of course, well known to occur, 
but I understand that a distinct disease 
of this origin is, as yet at least, not recog- 
nized. Itis not improbable, however, that 
such a disease exists—we should, I suppose, 
name it erythrocytosis or haematoma. 
Wecan now distinguish two main groups 
of new formations ; first, those with marked 
eytomorphosis, or changes in cell structure, 
as, for example, myoma, lipoma, chon- 
droma, etc.; second, those without marked 
cytomorphosis, the cells of the new growth 
resembling those of the parent tissue as, 
for example, angioma and glioma. 
Members of the first group have been 
termed heteroplastic. 
Members of the second group have been 
termed homoplastic. 
Accepting these terms, we may say tumors 
are either heteroplastic or homoplastic. From 
the standpoint of the embryologist these 
terms are much more than convenient ad- 
jectives ; on the contrary, they denote dif- 
ferences of a fundamental character, upon 
which we must base a large part of our 
notions about pathological differentiation. 
Finally, as to the differentiated cells. 
We have just considered cells which have 
reached a low degree of differentiation, and 
therefore will now give our attention only to 
the most highly differentiated. Of these 
the nerve cells, or as they are now termed, 
the neurones, stand highest and are char- 
SCLENCE. 
[N.S. Von. XIII. No. 326. 
acterized not only by the great specializa- 
tion of their organization, but also by the 
complete loss of their ability to multiply 
by cell-division. The neurones are then 
extremely unlike the embryonic cells and 
they represent the extreme end of that 
scale of which the undifferentiated cell is 
the beginning. Itis, therefore, very signifi- 
cant that neurones do not form tumors. 
Neuronoma, as such a tumor would be 
called, does not occur, so far as hitherto 
recorded—and if, as is possible, a neuro- 
noma should be found, we should have to 
explain it not as a tumor-growth of neu- 
rones, but as the result of proliferation of 
indifferent cells, which subsequently be- 
came differentiated into neurones. The 
so-called neuromata of pathologists do not 
here come into consideration because they 
are merely accumulations of growing axis- 
cylinders. 
Liver cells and striated muscles also rep- 
resent a very high differentiation. It is 
possible that with more exact knowledge 
we shall be able to state that these ele- 
ments also cannot produce tumors, although 
there may be tumors of the liver and of 
striated muscle-fibers. Possible, because 
Cohnheim’s famous theory of tumor origin 
from persistent embryonic tissue may be, 
though not generally applicable, available in 
these two instances. The adoption of this 
view would furnish an explanation of sev- 
eral familiar facts ; of the fact that we do 
not find tumors formed by differentiated 
liver cells; of the fact that cancer of the 
liver arises usually from the bile-ducts, 
which have a simple and little differentiated 
epithelium ; of the fact that myoma of the 
cardiac and of the developed skeletal mus- 
cles is exceedingly rare; of the fact that 
rhabdomyomas so occur that their origin 
may be attributed to inclusions of portions 
of embryonic muscle plates. As regards 
primary epithelioma of the liver, it is 
claimed that it arises usually from the bile- 
