224 BIOLOGY AND ITS MAKERS 
the development of insects and other invertebrates, and Remak 
is notable for similar work with the vertebrates. As already 
mentioned, he was the first to recognize the middle layer as 
a unit, through which the three germ-layers of later embry- 
ologists emerged into the literature of the subject. é 
Koelliker, 1817-1905, the veteran embryologist, for so 
many ycars a professor in the University of Wiirzburg, carried 
on investigations on the segmentation of the egg. Besides 
work on the invertebrates, later he followed with care the 
development of the chick and the rabbit; he encompassed 
the whole field of embryology, and published, in 1861 and 
again in 1876, a general treatise on vertebrate embryology, 
of high merit. The portrait of this distinguished man is 
shown in Chapter VIII, where also his services as a histologist 
are recorded. 
Huxley took a great step toward unifying the idea of germ- 
layers throughout the animal kingdom, when he maintained, 
in 1849, that the two cell-layers in animals like the hydra 
and oceanic hydrozoa correspond to the ectoderm and 
endoderm of higher animals. 
Kowalevsky (Fig. 68) made interesting discoveries of a 
gencral 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 up to 
that time had been considered an invertebrate, and the effect 
of Kowalevsky’s observations was to break down the sharply 
limited line supposed to exist between the invertebrates and 
the vertebrates. This was of great influence in subsequent 
work. Kowalevsky also founded the generalization that all 
animals in development pass through a gastrula stage—a 
doctrine associated, since 1874, with the name of Haeckel 
under the title of the gastreea theory. 
Beginning of the Doctrine of Germinal Continuity.—. 
The conception that there is unbroken continuity of germinal 
