Marcu 29, 1901.] 
a classification as the basis of all his further 
study of the science. How much this re- 
form is needed is indicated by the many 
writers who put glioma under the head of 
connective tissue tumors, although gliomata 
arise from the ectoderm, and connective 
tissue arises from the mesoderm. Such a 
classification is on a par with the ancient 
system which put the whales among the 
fish, for it is not going too far to say that it 
is impossible that connective tissue should 
produce a, glioma, because the two things 
belong in different classes. Another note- 
worthy violation of embryological law is 
offered by the classification of all muscle- 
tumors under one head, ‘ myoma,’ although 
smooth and striated muscle fibers are ge- 
netically and structurally distinct, with no 
intermediate or connecting forms of tissue, 
and with only a slight physiological resem- 
blance. As regards epitheliomata; they 
should be studied in relation to their layer- 
ship, and it is reasonable, in my judgment, 
to expect that they will be found to have 
very distinctive characteristics according to 
the germ-layer from which they take ori- 
gin, for the layership of a tissue governs the 
normal differentiation and therefore prob- 
ably also the abnormal. I believe that the 
first competent investigation in this field 
will mark a new epoch of pathological 
science. When the epoch comes our mor- 
phological sense will no longer be shocked, 
as for instance by the application of the 
name adenoma to an epithelioma of an 
organ like the kidney, which is in no sense 
a gland. 
I should like to urge especially the study 
of the layership of the various cancers. 
Can we safely assume that there is only one 
kind of cancer? May it not well be that 
ectodermal, mesothelial and entodermal 
cancers are separate kinds ? 
Next as to undifferentiated cells. The 
cells of this sort have the power of mul- 
tiplication in a high degree, and they have 
SCIENCE 
495 
the possibilities of increasing their size and 
of undergoing further differentiation, and 
their occurrence in the adult is of the. 
utmost pathological significance. Such 
cells exist in four important parts, (1) in 
the basal layer of the epidermis and in 
corresponding portions of the epidermal ap- 
pendages ; (2) in the adult mesenchyma or 
connective tissue; (3)in many parts of the 
adult mesothelium, especially of the epithe- 
lia of the genito-urinary tracts ; (4) in the 
entodermal epithelium of the gastro-intesti- 
nal tract. It is significant that it is pre- 
cisely from these parts that the develop- 
ment of many rapidly growing tumors 
takes place, and it is further significant 
that the least differentiated or specialized 
of all, namely, the mesenchymal cells, are 
the ones which produce the greatest 
variety of tumors—as the following list 
recalls : myxoma, myoma (but not rhab- 
domyoma), fibroma, lipoma, chondroma, 
osteoma and sarcoma. Angioma presum- 
ably belongs in a different category. The 
mesenchyma still-exhibits, by the formation 
of its characteristic tumors in the adult, its 
embryonic capacity to transform itself in 
varied ways. 
Further insight into pathological develop- 
ment may be gained from the tissues or 
cells which have undergone differentiation, 
but do not attain a high grade of specializa- 
tion. The endothelium of blood-vessels, 
the endothelium of lymph-vessels, the red- 
blood cells, the leucocytes and the neurog- 
lia are examples of this class. All the cells 
of the kind just enumerated have advanced 
in organization beyond the embryonic state, 
but have retained the power of cell multi- 
plication. When they multiply they pro- 
duce cells like themselves, so that we might 
describe them as so many histological 
species each capable of reproducing its own 
kind. In accordance with this conception, 
derived from the normal development, is 
the pathological fact that each of these 
