two
plates and inverted at the two-celled stage there are formed two embryos
instead of one. In the same year T. H. Morgan[499] repeated Roux's
fundamental experiment of destroying one of the two blastomeres, but
inverted the egg immediately after the operation--a whole embryo of half
size resulted. A year or two later Herlitzka[500] found that if the first
two blastomeres of the newt's egg were separated by constriction, two
normal embryos of rather more than half normal size were formed.

The main result of the first few years' work on the development of
isolated blastomeres was to show that the mosaic theory was not strictly
true, and that the hypothesis of a qualitative division of the nucleus
was on the whole negatived by the facts.

Evidence soon accumulated that the cytoplasm of the egg stood for much
in the differentiation of the embryo. A number of years previously Chun
had made the discovery that single blastomeres of the Ctenophore egg,
isolated at the two-celled stage, gave half-embryos. This was in the
main confirmed by Driesch and Morgan in 1896,[501] and they made the
further interesting discovery that the same defective larvae could be
obtained by removing from the unsegmented egg a large amount of
cytoplasm. Conclusive proof of the importance of the cytoplasm was
obtained soon after by Crampton,[502] who removed the anucleate
"yolk-lobe" from the egg of the mollusc _Ilyanassa_ at the two-celled
stage, and obtained larvae which lacked a mesoblast. This result was
brilliantly confirmed and extended some years later by E. B. Wilson,[503]
working on the egg of _Dentalium_. He found that if the similar
anucleate "polar lobe" of this form is removed at the two-celled stage,
deficient larvae are formed, in which the post-trochal region and the
apical organ are absent. He further showed that in the unsegmented but
mature egg prelocalised cytoplasmic regions can be distinguished, which
later become separated from one another through the segmentation of the
egg. The segmentation-cells into which these cytoplasmic substances are
thus segregated show a marked specificity of development, giving rise,
even when isolated, to definite organs of the embryo. Wilson concluded
that the cytoplasm of the egg contains a number of specific
organ-forming stuffs, which have a definite topographical arrangement in
the egg. Development is thus due in part to a qualitative division not
of the nucleus but of the cytoplasm. Corrobo