113 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEEAL. 



Inasmuch as KowalewL&i,* from the results of his embryological 

 work, drew the Conclusion that the nervous layer and embryonic skin 

 of Insects and Vertebrates are homologous, and that the germinal 

 layers of Amphioxus and Vertebrates correspond with those of 

 Molluscs (Tunicata) or worms, he was in agreement with the long 

 recognised fact that anatomical transitional forms and intermediate 

 links between the different groups or types of animals exist, and that 

 these latter do not represent absolutely isolated planes of organization, 

 but the highest divisions in the system, and he only gave in reality 

 an embryological expression to the claims of the descent theory. In 

 fact, the conclusion which Kowalewski reached was completely 

 correct viz., that the homologies of the germinal layers in the 

 different types afford a scientific basis for comparative anatomy and 

 embryology, and must be recognised as the starting-point for the 

 proper understanding of the relationships of the types. For this 

 position we find amongst the vertebrata proofs at every step. 



But while his own comprehensive embryological experiences inspired 

 Kowalewski, the founder of the theory of the germinal layers, with a 

 prudent reserve, other investigators, inclined to bold generalization, 

 appeared at once with ready theories, in which the results of embryo- 

 logical investigations were interpreted in accordance with the theory 

 of descent. Among these E. Haeckel's gastrasa theory is especially 

 prominent, which raises no less a claim " than to substitute, in the 

 place of the classification hitherto received, a new system based on 

 phylogeny, of which the main principle is homology of the germinal 

 layers and of the archenteron, and secondarily on the differentiation 

 of bhe axes (bilateral and radial symmetry) and of the ccelom." 

 E. Haeckelf designated the larval form used as the point of depar- 

 ture the Gastrula, and believed to have found in it the repetition 

 in embryonic development of a common primitive form, to which the 

 origin of all Metazoa may be traced back. To this hypothetical 

 prototype, which is supposed to have lived in very early times during 

 the Laurentian period, he gave the name of Gastrcea, and called the 

 ancient group, supposed to be widely scattered and to consist of 

 many families and genera, by the name Gastrceadce. From this sup- 

 position was deduced the complete homology of the outer and inner 



* A. Kowalewski, "Embryologische Studien an Wiirmcrnund Arthropod en." 

 Petersburg, 1871, p. 58-fiO. 



f E. Haeckel, " Gastraeatheorie." Jen. nat. Zeitschrifr, 1874.'' For criticism 

 see C. Glaus, " Die Typenlehre und Haeckel's sogenannte Gastrseatheorie," 

 Vienna, 1874. 



