482 
themselves into new orders, which we call 
natural laws. As little as a description of 
the people of the United States with no in- 
formation as to their history could satisfy 
a serious thinker, so little can descriptions 
of fully developed structures satisfy an. 
earnest pathologist. An innate, intense 
mental impulse is continually driving us 
forward in the search for causes, and obedi- 
ence to this impulse is one of the main 
factors in scientific progress. All this is 
familiar, trite even, but may serve to fix 
our starting thought, namely, that we are 
to study causes. 
Our attention is to be directed to the con- 
sideration of what embryology can teach us 
in regard to the causation of organization, 
and then to the application of those teach- 
ings to pathology. 
This plan will exclude from our discus- 
sion many of the aspects of embryology 
which appeal most strongly to pathologists. 
We must omit from our study at least three 
groups of interesting phenomena, to wit: 
First, the arrests of development; second, 
the teratological formations, monstrosities 
and mis-developments, which will, however, 
have to be included ultimately even in the 
precise field we are about to study; third, 
the so-called teratoma, or to use a more 
recent term, embryoma. I may say in 
passing that I find it very difficult to accept 
the hypothesis that these remarkable struc- 
tures arise by a parthenogenetic develop- 
ment of ova, retained in the parent body. 
Professor Bonnet’s hypothesis is more legiti- 
mate, but towards that also my attitude is 
one of sceptical agnosis. Bonnet suggests 
that one of the early segmentation cells 
(blastomeres) may become isolated and 
retarded in its development, remaining as 
an inclusion in the feetal tissues, and after- 
wards develop and produce a variety of 
tissues, as isolated blastomeres have been 
shown by experiments on the lower animals 
to be capable of doing. 
SCIENCE. 
[N. 8. Vou. XIII. No. 326. 
Teratological formations fall, it seems to 
me, naturally into three fairly definite di- 
visions: (1) those due to necrosis of the 
tissues, which apparently rarely if ever 
takes place uniformly throughout the em- 
bryo; (2) those due to gross mechanical 
disturbances of the development, conse- 
quent upon failure of the proper correla- 
tion of the growth of parts; monstrosities 
of this division are probably the most com- 
mon; (3) errors in the differentiation of 
the tissue or pathological histogenesis. It 
is only phenomena of the first and second 
divisions of teratology which we can safely 
drop from view, while those of the third 
division—errors in differentiation—we must 
bear uninterruptedly in mind. 
One more preliminary explanation is nec- 
essary. The range of pathological changes 
is not so great as to reach equality with em- 
bryological; developments. In the normal 
embryo we deal with the evolution of com- 
plete organs together with all their accom- 
panying varied and complex modifications 
of tissue. In pathology, on the contrary, 
we deal not with organs, but with modifi- 
cations of tissues, with histogenesis. The 
statement will not seem too absolute if it 
is recalled that we have excluded arrests of 
development and monstrosities from our 
discussion. 
Histogenesis is the common territory in 
which the pathologist and embryologist have 
—to borrow a legal phrase—an undivided 
interest. Itis unfortunate that our tendency 
has so long been to attempt an unnatural 
and impossible partition of the territory, 
which has resulted only in a division of our 
forces into two camps, between which has 
reigned little interest and less sympathy. 
I venture to regard your invitation to ad- 
dress you to-night as a wish, which I fully 
share, to secure fuller cooperation between 
the two camps of workers, who are both 
striving to lay bare the laws which govern 
the differentiation of cells. 
