488 
culminating and most perfect illustration 
in the central nervous system, where com- 
paratively early in embryonic life all the 
cells become specialized, and with the ac- 
quisition of specialization they forfeit their 
power of multiplication, the neuroglia cells 
partly, the nerve cells wholly. The growth 
of the brain after early stages depends not 
on proliferation of cells, but chiefly upon 
the increase in size of the individual cell. 
The correctness of this statement is not 
affected, in my belief, by the fact that 
epithelial portions of the medullary tube in 
comparatively late stages may be added to 
the nervous portion, the cells multiplying 
rapidly, as we see at the growing edge 
of the young cerebellum. The brain here 
grows by the addition of cells in the indif- 
ferent stage, but as soon as these cells are 
differentiated they conform to the general 
law and divide uo more (neurones) or 
slowly (glia cells). 
The two types of differentiation produce 
essentially unlike conditions. The pathol- 
ogist may not overlook such unlikeness 
with impunity. The two types pass into 
one another with many intergrades. Hence 
when we consider the possibilities of path- 
ological alteration we must in each case 
seek to determine how far the condition of 
the tissue involved permits cell multiplica- 
tion, as well as differentiation. 
Just as the segmentating ovum divides 
itself into parts, which we name germ lay- 
ers, each of which has its special and ex- 
clusive share in developing the adult tissues, 
so does each of the three germ layers divide 
into parts, each part having its special and 
exclusive roll, and these parts again sub- 
divide until, after the final partition, the 
adult variety is produced. During all these 
changes there is no exchange of roles. It 
will be profitable to let the phenomena pass 
before us in rapid review. 
First, then, the ectoderm. This layer 
early separates into two parts; one to form 
SCIENCE. 
[N.S. Vou. XIII. No. 326. 
the central nervous system, the second the 
epidermis ; the nervous part thereafter 
never forms epidermal structures, the epi- 
dermal part never forms a nervous system. 
The central nervous system retains in part 
a simple epithelial character, but most of 
its walls become nervous tissue; its cells 
pass from the indifferent stage and become 
neuroglia cells or young nerve cells (neuro- 
blasts). Neuroglia cells never become any- 
thing else, and the nerve cells are always 
nerve cells to the end. The primitive epi- 
dermis forms a series of special sensory 
areas and the permanent epidermis. The 
sensory areas, which belong to the olfac- 
tory, auditory and gustatory organs, soon 
become well defined and never produce any 
cell arrangements like those of the epider- 
mis. This last, on the contrary, remains. 
as before stated, rich in undifferentiated 
cells, and gradually produces a great va- 
riety of structures. Most of these, namely, 
the hairs and glands, are small and very 
numerous, while a few like the nails, 
enamel organs and epithelium of the lips 
are larger. No one of these special struc- 
tures, however, converts itself into another. 
The basal layer of the general epidermis 
may perhaps preserve a true embryonic 
quality and have wide differential possi- 
bilities. 
Next, as to the entoderm, which under- 
goes less differentiation than either of the 
other two germ layers, since over a large 
part of its extent it remains throughout life 
a simple epithelium with many cells very 
slightly modified in structure. Wherever 
in it specialization takes place, as in the 
tonsil, thymus, thyroid, esophagus, liver 
or pancreas, each territory of cells keeps 
its characteristics and never assumes those 
of another territory. 4 
Finally as to the mesoderm, in which 
layer variety of differentiation attains its 
maximum. To follow the genesis of this 
variety is most instructive. The mesoderm 
