WITH 5 FIGURES.
ROUX' first pointed out that the material which forms the embryo of the frog is laid down in the black-white ring around the equator of the egg, and that the embryo is formed by a process of concurrence in that this material grows over the white hemisphere. If this is prevented, as in his case of asyntaxia medullaris, a half embryo develops on either side of the equator of the egg. ROUX'S observations and experiments were confirmed by Morgan in a somewhat similar series of experiments.
Hertwig" was able to produce the bilateral half embryos by allowing the eggs to develop in salt solutions. I have seen many similar forms from eggs of rana sylvatica and rana.palustris which were kept on ice for a prolonged period of time. I n such abnormal forms the lips of the blastopore which fail to grow over the large yolk plug differentiate into these modified half embryos with a central nervous system, muscle, the chorda, and the roof of the archenteron.
More recently Morgan has investigated the question of the location of the embryo-forming substances and concludes that the material is first in the upper hemisphere of the developing frog's egg and is later carried downward into the germ ring.