490 
epithelial lining for the cavities of the 
thorax and abdomen. It is literally the 
sane epithelium in the four cavities, for 
they were originally one, with a single con- 
tinuous epithelium. It is well also in our 
nomenclature to recognize the important 
fact that the epithelium is radically, be- 
cause genetically, distinct from the endo- 
thelium of the blood vessels and lymphatics, 
aud the application of the term ‘ endothe- 
lium’ to the covering of, for instance, the 
peritoneum, leads and can lead only to 
confused bad thinking. If mesothelium be 
employed as suggested, clearness will be 
gained. 
Coming back now to the subject of the 
mesoderm, let us note that when a striated 
muscle fiber is produced a striated muscle 
fiber it always remains, and it never be- 
comes anything else; the ovary never 
changes. In short, with the mesoderm as 
with the ectoderm and entoderm, we see 
the fate of the cells once fixed to be there- 
after unchangeable as to the kind of differ- 
entiation. 
Our hasty review is worse than imper- 
fect, yet is sufficient to impress upon us the 
great law that differentiation in any direc- 
tion terminates the possibility of differen- 
tiation in any other direction. In accord- 
ance with this law we encounter no in- 
stances, either in normal or in pathological 
development, of the transformation of a cell 
of one kind of tissue into a cell of another 
kind of tissue, and further we encounter 
no instance of a differentiated cell being 
transformed back into an undifferentiated 
cell of the embryonic type with varied po- 
tentialities. 
Thus far I have expressed myself some- 
what as if there were two sharply defined 
conditions, the differentiated and the un- 
differentiated. To give such an impression 
would be to create error, since differentia- 
tion is a slowly progressive and wholly 
gradual change in the cell. We must look 
SCIENCE. 
(N.S. Vou. XIII. No. 326. 
upon each step in the process of differentia- 
tion as establishing narrower limits for 
future changes. Thus, when in the spinal 
cord neuroblasts diverge from the glia cells, 
they are not specialized into different classes 
of neuroblasts; such specialization comes 
later. So in the mesenchyma after the 
embryonic cells have changed and large 
numbers of them have become connective 
tissue cells, these last still are capable of 
various further differentiations, and may, 
therefore, be said to have been arrested in 
their development at a stage of partial dif- 
ferentiation. This quality of the connec- 
tive tissue cells is, from the pathological 
standpoint, one of the most important. facts 
known to us concerning the structure of 
the body. 
Having now elaborated, as far as time 
permits, our conception of the nature of 
differentiation, let us turn to our third 
fundamental idea, which concerns the 
changes which succeed differentiation. 
These changes are very unlike the con- 
structive changes which precede them, for 
they are destructive. They fall into three 
main groups: 
1. Changes of direct cell death. 
2. Necrobiosis,* or indirect cell death 
preceded by changes in cell structure. 
3. Hypertrophic degeneration or indirect 
cell death preceded by growth and struc- 
tural change of the cell. 
Of direct cell death no discussion is here 
necessary, for the fundamental idea, which 
I wish to emphasize, is that necrobiosis and 
hypertrophic degeneration are normal proc- 
esses, which invariably occur in the normal 
body and play in many cases important 
roles in the life history of the individual. 
Without necrobiosis and degeneration on 
a large scale, the normal round of human 
life would be impossible. It is singular 
that in treatises on normal anatomy and 
*Tt is a matter for regret that so awkward a term 
as ‘necrobiosis’ should have become current. 
