153 ERNST HAECKEL. 



ance. I consider this homology so extremely important that 

 I accept in this evidence the monophyletic origin of the six 

 groups of higher animals from the common root-form of the 

 Gastraea, and place them all together as germ-lamella 

 animals (Metazoa or Blastozoa) as opposed to the primitive 

 animals which have not yet arrived at the germ-lamella 

 stage. This consideration forms the nucleus of the Gastraea- 

 theory, the most important consequences of which will be 

 subsequently developed. 



Huxley had, in 1849, already asserted that the two per- 

 manent foundation membranes of the Acalephse, entoderm 

 and exoderm, were truly homologous to the germ-lamellse of 

 the higher animals, see his excellent dissertation *' On the 

 anatomy and the affinities of the Medusse.''^ Subsequently, 

 Kowalevsky has laboured in a series of very suggestive 

 ontogenetic works, to extend this homology over the largest 

 part of the animal kingdom, and to show that (with few 

 exceptions) the two well-known original germ-lamellae of 

 the Vertebrata also make their appearance in invertebrate 

 animals of the most different groups. His brilliant dis- 

 covery of the identical ontogenesis of the Amphioxus and 

 the Ascidia (1867), one of the most significant and sug- 

 gestive discoveries of modern zoology, was especially im- 

 portant in this respect.^ This homology of the two primor- 

 dial germ-lamellfe, and the organs immediately originating 

 therefrom is carried out furthest, but, at the same time, 

 in a partially restricted manner, in Kowalevsky's latest 

 work, the ' Embryological Studies on Worms and Arthro- 

 poda' (1871). This theory has next met with the most 

 acute discernment and the most decided advocacy from 



' 'Philosophical Transactions,' 18i9, p. 425 : — "A complete identity of 

 structure connects the ' foundation membranes ' of the Medusae with the 

 corresponding membranes in the rest of the series ; and it is curious to remark, 

 that throughout, the inner and outer membranes appear to bear the same 

 physiological relation to one another as do the serous and mucous layers of 

 the germ ; the outer becoming developed into the muscular system and 

 giving rise to the organs of offence and defence ; the inner, on the other 

 hand, appearing to be more closely subservient to the purposes of nutrition 

 aad generation." 



^ The significant importance which we attribute to Kowalevsky's 

 discoveries, which were corroborated by Kupffer, rests, in our opinion, on 

 two points. First, because tlie deep gulf between the Vertebrata and 

 Invertebrata, hitherto considered impassable, and a chief impedient to the 

 theory of descent, is thereby tilled up. Secondly, the original ontogenetic 

 development of the Vertebrata, as well as of tlie most dissimilar Inverte- 

 brata, from the gastrula, with their common descent from the Gastraea, is 

 thereby also proved. All the attempts which have recently been made by 

 different authors to dispute the fact of this fundamental discovery, or to 

 weaken its signification, appear to be so feeble, that they need no refutation. 



