Fig. 9 Albrecht Koelliker. Born 1817.

vertebrate embryology of high merit. His portrait is shown in Fig. 9.

Huxley took a great step towards unifying the idea of germ-layers throughout the animal kingdom, when be maintained, in 1849, that the two cell-layers in animals like the hydra, and oceanic hydrazoa, correspond to the ectoderm and endoderm of higher animals.

Kowalevsky, whose portrait is shown in Fig. 10, made interesting discoveries of a general bearing. In 1866 he showed the practical identity, in the early stages of development, between one of the lowest vertebrates (Amphioxus) and a tunicate. The latter had up to that time been considered an invertebrate, and the effect of