486 
indifferent, so that if it were possible to 
graft a young mesodermal cell on to the 
ectoderm or entoderm, it would become a 
true ectodermal or entodermal cell, as the 
case might be. 
But although we may, so far as we now 
see, regard the cells in the germ layers as 
originally wholly indifferent as individual 
cells, nevertheless, we must not forget that 
as members of a germ layer their potential 
fate is already restricted by the conditions 
of their very layership—if I may coin a 
word for the occasion. Each layer pro- 
duces its own group of tissues and never 
any other. There are indeed apparent ex- 
ceptions to the rule, as, for example, the 
stratified horny epithelium of the cesopha- 
gus, which is strikingly similar to the epi- 
dermis, although in one case the tissue is 
ectodermal, in the other entodermal. We 
have, however, to do only with a resem- 
blance, and the development in the two 
cases is quite unlike—the cesophageal epi- 
thelium in the mammalian embryo being 
ciliated at one stage and exhibiting then 
little resemblance to any stage of the epi- 
dermis. 
Each germ layer has its specific function, 
.its exclusive share in the work of differen- 
tiation. The work of one layer is not done 
by another nor shared by another. It is 
true that attempts are made from time to 
time to upset the validity of this funda- 
mental doctrine, but they have hitherto 
failed to find support or recognition from 
any leading embryologist, and I deem these 
attempts unworthy of serious consideration. 
We are able now to assign almost every 
cell of the normal adult human body to its 
proper germ layer. Our only great uncer- 
tainty is where two layers meet, as do the 
ectoderm and entoderm in the pharynx, .or 
as do the mesoderm and entoderm, where 
the ureter opens into the bladder. With 
these and perhaps a very few other small 
exceptions everything in adult anatomy 
SCIENCE. 
(N.S. Vou. XIII. No. 326. 
can be correctly stated in terms of germ 
layers. The layership of every organ is 
known, save that in the cases of the thymus 
gland, the tonsils and the adrenals author- 
ities are not yet agreed. 
A remarkable attempt to upset the doc- 
trine of the germ layers deserves a brief 
consideration. It was first maintained by 
Goronovitch that the cells forming at least 
a part of the skeleton arose from the ecto- 
derm. The same opinion was expressed 
also on the basis of their own investigations 
by H. Klaatsch and by Miss Platt. Con- 
firmation of these views has not followed, 
but on the contrary, C. Rabl, one of the 
most trustworthy of living observers, main- 
tains that essential parts of Goronovitch’s 
and Klaatsch’s evidence are simply errors 
of observation. Klaatsch’s views were 
based partly on the study of the developing 
teleost fins. R. G. Harrison has shown 
that here the German worker is in error. 
Miss Platt’s observations were made in the 
head region of embryo Necturus. An ex- 
amination of a number of series and stages 
has not enabled me to find the slightest 
evidence in favor of Miss Platt’s conclu- 
sions. H.K. Corning has pointed out that 
the processes alleged by Miss Platt to oc- 
cur in Necturus do not take place in the 
frog, Rana temporaria. We may, therefore, 
I think, safely regard this attempt to over- 
throw the morphological value of the germ 
layers as unsuccessful. I know of no other 
attempt of sufficient importance to be even 
mentioned. 
The importance to pathologists of a thor- 
ough knowledge of the genesis of the tis- 
sues from their germ layers can hardly be 
emphasized too strongly, for it is more than 
probable that all pathological tissues are as 
strictly governed by the law of the specific 
value of germ-layers as are the normal 
tissues. Are there not many pathologists 
whose knowledge of embryology is wholly 
insufficient to meet the practical needs of 
