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. 



Koelliker, 1817-1905, the veteran embryologist, for so 

 many years 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, tnat 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 

 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 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 gastrsea theory. 



Beginning of the Doctrine of Germinal Continuity.- 

 The conception that there is unbroken continuity of germinal 



