MARcH 29, 1901.] 
After these preliminary explanations it 
is possible to define the evening’s task with 
precision. It is twofold. First, to pre- 
sent some of the more important concep- 
tions derived from embryological study in 
regard to the processes of cell differentia- 
tion. Second, to suggest some of the bear- 
ings of these conceptions on the problems 
of pathology. 
PART I, NORMAL DIFFERENTIATION. 
Under this head I propose to discuss 
three fundamental ideas : 
First, of the undifferentiated cell. 
Second, of the progress of differentiation. 
Third, of the changes which may succeed 
differentiation. 
The fertilized ovum is an undifferentiated 
being, although it has a very complex or- 
ganization, and contains besides the proto- 
plasm a store of nutritive material, the 
so-called yolk or deutoplasm. As there is 
_ only one nucleus, there can be no variety 
of nuclei ; the term undifferentiated, there- 
fore, applies to the protoplasm, which seems 
to have a uniform essential structure 
throughout, although the masses and 
strands of protoplasm may exhibit charac- 
teristic peculiarities, especially in relation 
to the distribution of the yolk. In the 
adult, on the contrary, the protoplasm of 
the cells of different tissues offers many 
varieties of essential structure, which can 
often be readily distinguished under the 
microscope. It isa legitimate conclusion 
that the absence of visible peculiarities of 
the intimate structure of egg protoplasm, 
by which one part may be distinguished 
from another, corresponds to uniformity of 
structure throughout the egg, excepting, of 
course, certain special characteristic ar- 
rangements, as, for example, the centering 
about the centrosome, which may occur in 
any cell. 
We have also direct experimental proof 
that the egg is uniform throughout, or to 
SCLENCE. 
483 
use a better phrase, that the egg is isotropic. 
Pfluger, in 1884, proved that the side of 
the frog’s egg, which normally develops 
into the ventral surface of the embryo, can 
be made to develop into a perfectly typical 
dorsal surface. ‘The frog’s egg has a small 
white area, which normally lies underneath, 
the larger darkly pigmented area of the 
egg alone showing from above. Out of the 
dark area the back with the nervous sys- 
tem and other parts takesits origin. If the 
eggs, freshly fertilized, are fastened with 
the white side up, then the white side 
produces an absolutely normal back and 
nervous system, normal as to form and . 
function, though lacking the typical pig- 
mentation. These observations were con- 
firmed by Born, who further discovered 
- that the segmentation nucleus always rises 
towards the upper side of the egg, and that 
the position of the nucleus determines 
which part of the ovum shall become the 
dorsal side of the embryo. Another set 
of experiments by Oskar Schultze demon- 
strated that both the unpigmented and the 
pigmented sides of the same egg could be 
made to produce dorsal structures. 
Another class of experiments, which were 
firsts made by Hans Driesch, have demon- 
strated that the earliest cells (segmentation 
spheres, blastomeres or cleavage cells, as 
they are variously called) produced by the 
ovum preserve the undifferentiated quali- 
ties of the parent egg, and may develop in 
one way or another according to circum- 
stances. The egg of a sea urchin divides 
into two cells, each of which multiplies 
and normally gives rise to half* of the 
body of the animal. By somewhat violent 
shaking the two cells may be artificially 
separated ; each cell may then develop into 
a complete larval sea-urchin, but of half 
the normal size only. Similar experiments 
have since been made by several investi- 
gators, who have obtained like results with 
* Tt would be safer to say supposedly about half. 
