484 
other animals, vertebrate as well as inver- 
tebrate. Even more remarkable larve 
have been raised from blastomeres of the 
four cell and eight cell stages of segmenta- 
tion, producing larve of one-fourth and 
one-eighth the normal size. Zoja claims to 
have repeated the experiment successfully 
on the eggs of Clytia and to have obtained 
one-sixteenth larvee. 
The facts offered suffice to illustrate the 
two aspects of our conception of the undif- 
ferentiated condition of living matter. The 
first aspect is morphological and presents to 
us the apparent uniformity of the visibly 
. minute structure of protoplasm. While we 
readily admit that the uniformity may be 
only apparent in the sense that we fail to 
observe fine differences, yet we none the 
less maintain that the uniformity is real, » 
because there is an absence of variations 
of structure comparable to the variations 
which we can observe in the cells of adult 
tissues. The second aspect is physiological 
and offers to our view the wide range of 
possibilities in the future developmental 
history and growth of the protoplasm. The 
fate of the protoplasm of any given part 
of the ovum is not fixed, but if its condi- 
tions of development are changed its fate 
is changed. A few years ago the mosaic 
hypothesis was advanced by W. Roux and 
has been vigorously defended by him. 
According to the mosaic theory, the egg is 
a mosaic pattern, each member of which 
has its predestined history. It is fortu- 
nate for our comprehension of pathological 
process that we are already able to say that 
Roux’s hypothesis is erroneous. 
We must start then with the right con- 
ception of the ovum, every part of the pro- 
toplasm of which is to be regarded as po- 
tentially capable of producing any or all 
the tissues of the adult. 
We turn next to the consideration of the 
progress of differentiation in order to estab- 
lish a second fundamental idea, namely, 
SCIENCE. 
[N.S. Von. XIII. No. 326. 
that it acts as a progressive restriction of 
the further development. Each successive 
stage of differentiation puts a narrower 
limitation upon the possibilities of further 
advance. Applied to pathology, this law 
means that the range of possible patholog- 
ical changes is determined not merely by 
the nature or kind, but also by the stage or 
degree, of the previous differentiation of the 
tissue. 
The eggs of all animals* pass through 
two well-marked phases of development. 
During the earlier and much shorter 
phase, the nuclei are multiplying rapidly, 
while the cytoplasm is growing but little, if 
at all. This period includes the time of 
segmentation, as commonly described, and 
somewhat longer. During this period the 
total bulk of the nuclei in proportion to the 
protoplasm is fundamentally changed. The 
ovum arises from a cell, the ovocyte, which, 
as its last act, grows rapidly ; this enlarged 
cell, by the process of maturation gives rise 
to the female sexual element, which has a 
single nucleus. After the fertilization we 
have an ovum with much protoplasm and 
deutoplasm, but again with only the single 
segmentation nucleus. The development 
of each individual begins, therefore, with a 
cell in which the extreme disproportion 
between the size of the nucleus and of the 
whole cell-body occurs. The first effort of 
development is to correct this dispropor- 
tion by the enormously rapid increase of 
the nuclei, which continues until cells of 
the embryonic type are produced, that is to 
say, cells each with a minimal amount of 
protoplasm around the nucleus. With the 
production of cells of the embryonic type, 
the first phase of development is completed. 
The limits of this phase are very indefinite 
for we observe often that the prodution of 
cells of the type defined may be far ad- 
vanced in one part of the germ, while it is 
*The protozoa are obviously excluded from the 
present discussion. 
