JuLy 14, 1899.] 
into the finer details of its structural ele- 
ments. Genetic continuity, the origin of 
like from like, may now safely be regarded 
as a demonstrated fact in the case of all 
existing organisms and of all cells; it hardly 
falls short of the same degree of certainty 
as applied to the nucleus; it is probable in 
the case of various forms of plastids in 
plant cells; while the centrosome is now 
being weighed in the balance with the evi- 
dence for the moment apparently accumu- 
lating on the negative side. 
Up to this point we have been dealing 
with matters of observed fact. The next 
and final step was, however, taken in the 
region of pure speculation, which had in the 
meantime been at work building upwards 
from hypotheses regarding the basic compo- 
sition of protoplasm. Brucke’s suggestion, 
that the cell might be a congeries of bodies 
more elementary than itself, found a much 
fuller expression in Herbert Spencer’s theory 
of physiological units; but it was Darwin’s 
theory of pangenesis that laid the real basis 
for what followed in the works of De Vries, 
Wiesner, Weismann and Hertwig. The 
common feature in all these later views is 
the conception of protoplasm, not as a 
homogeneous substance or mixture of 
substances, but as made up of a host of 
elementary ultra-microscopical corpuscles 
(‘pangens,’ ‘ biophores,’ etc.), specifically 
different, capable of assimilation, growth 
and multiplication, and arising by division 
of preexisting bodies of like kind. Devel- 
oped as a purely theoretical hypothesis, and 
within somewhat narrower limits by Dar- 
win, this conception was expanded and 
brought into more direct relation with ob- 
served fact, especially by De Vries and 
Wiesner, who showed how the assumption 
of such elementary self-propagating corpus- 
cles at the basis of living matter enabled 
us to bring all the observed phenomena of 
genetic continuity under a common point of 
view. The fundamental hypothesis itself 
SCIENCE. 30 
—i.e., the genetic continuity of the ulti- 
mate morphological units has, however, 
always remained, and still remains, a pure 
assumption, incapable of direct proof or 
disproof; for, with the exception of Alt- 
mann and a few of his followers, all are 
agreed that such elementary corpuscles, if 
they exist, must lie beyond the limits of 
microscopical vision. Altmann, however, 
has sought to identify the elementary units, 
or ‘ bioblasts,’ with the visible protoplasmic 
granules; and, in his writings, the series of 
Latin aphorisms initiated by Redi culmin- 
ates in the saying, omne granulum e gran- 
ulo(!), but this conclusion has not been 
taken very seriously by most other investi- 
gators. ? 
I have given this very brief sketch of the 
theoretical side of the question merely as an 
introduction, and shall dwell no farther on 
it at this point, since my main purpose is to 
ask attention to the visible, as opposed to 
the hypothetical invisible, structure of 
protoplasm. <A subject so vast, displaying 
so great a conflict of opinion, must be very 
briefly treated within the limits of a single 
lecture ; and I shall, therefore, confine the 
discussion in the main to the protoplasm of 
the echinoderm-egg, which is accessible to 
every one, has been made a classical object 
through the studies of such leaders of re- 
search as Flemming, Bitschli and Hertwig, 
and illustrates as clearly, perhaps, as any 
other the various interpretations of proto- 
plasmic structure that have been given. 
In thin sections of well-preserved ma- 
terial the protoplasm of a star-fish or sea- 
urchin egg gives the appearance, under a 
high power, of a fine meshwork or frame- 
work composed of innumerable minute 
granules, or microsomes, suspended in a 
clearer, less deeply staining, continuous 
substance (Figs. 1, a,and 4). The spaces 
of the meshwork, which measure from one 
to nearly two microns, are occupied by a 
third substance, clear, homogeneous, and 
