eriments on the frog's egg (in the _Schles. Gesell. f.
vaterl. Kultur_, 15th Feb. 1884), also the important researches of M.
Nussbaum and Gruber (followed up later by Verworn, Hofer and Balbiani)
on Protozoa, and other experimental work" (pp. xi.-xii.).

In 1888 appeared a famous paper by W. Roux,[489] in which he described how
he had succeeded in killing by means of a hot needle one of the two
first blastomeres of the frog's egg, and how a half-embryo had developed
from the uninjured cell. Some years before[490] he had enunciated, at
about the same time as Weismann, the view that development was brought
about by a qualitative division of the germ-plasm contained in the
nucleus, and that the complicated process of karyokinetic or mitotic
division of the nucleus was essentially adapted to this end. He
conceived that development proceeded by a mosaic-like distribution of
potencies to the segmentation-cells, that, for instance, the first
segmentation furrow separated off the material and potencies for the
right half of the embryo from those for the left half. He had tried to
show experimentally that the first furrow in the frog's egg coincided
with the sagittal plane of the embryo,[491] and his later success in
obtaining a half-embryo from one of the first two blastomeres seemed to
establish the "mosaic theory" conclusively.

Roux's needle-experiment aroused much interest, especially as Weismann's
theory of heredity was then being keenly discussed. Chabry had published
in 1887 some interesting results on the Ascidian egg,[492] which strongly
supported the Roux-Weismann theory. Considerable astonishment was
therefore caused by Driesch's announcement in 1891[493] that he had
obtained complete larvae from single blastomeres of the sea-urchin's egg
isolated at the two-celled stage. He followed this up in the next
year[493] by showing that whole embryos could be produced from one or more
blastomeres isolated at the four-cell stage. Similar or even more
striking results were obtained by E. B. Wilson on _Amphioxus_,[494] and
Zoja on medusae.[495] Driesch succeeded also in disturbing the normal
course and order of segmentation by compressing the eggs of the
sea-urchin between glass plates, and yet obtained normal embryos.
Similar pressure-experiments were carried out on the frog by O.
Hertwig,[496] and on _Nereis_ by E. B. Wilson,[497] with analogous results.

In 1895 O. Schultze[498] showed that if the frog's egg is held between