254 Prof. E. Hack el on the Position of the 



Within about a year (1867) my pliylogenetic hypotheses 

 received a welcome confirmation by the iiijportant embryo- 

 logical investigations of Kowalevsky, which made their ap- 

 pearance in the interval. This meritorious natm'alist, who 

 for the first time attacked the most difficult questions of 

 comparative ontogeny at their root, and who, by his brilliant 

 discoveries as to the identical ontogeny of Aviphioxus and the 

 Ascidia, bridged over the greatest gap hitherto existing within 

 the animal kingdom, showed at the same time that in the 

 most different groups of animals the primordial course of 

 development of the embryo is the same, and especially that 

 the germ-lamella theory, previously firmly established only 

 among the Vertebrata, also applies to the Invertebrata of the 

 most various groups*. In a more detailed memoir which has 

 recently appeared, these views are further developed f. 



That the primordial germ-lamella3 of the higher animals are 

 to be com})ared with the two permanent formative membranes 

 of the Acalephffi or Nematophora (the entoderm and exoderm) 

 was shown as early as 1849 by Huxley:]:, the discoverer 

 of those membranes. In Kleinenberg's thoughtful and sug- 

 gestive monograph of Hydra, this comparison is more closely 

 demonstrated, and at the same time employed in favour of the 

 view of the monophyletic origin of the animal kingdom. 



The anatomy and developmental history of the Calci- 

 spougias, as described by me, have furnished proof that the 

 sponges also belong to the circle of this stock-relationship, 

 and that indeed in them the two primordial germ-lamellaj are 

 retained through life in the ])urest and simplest form. The 

 development of the Calcispongise from the Gastrula is of 

 decisive significance for this theory. / regard the Gastrida as 

 the most imjwrtant and significant emhryonie form in the whole 

 animal kingdom. It occurs among the Sponges (in Calci- 

 spongi^ of all the three families), the Acaleph^ [Cordylo- 

 phora. Medusa, Siphonophora, Ctenophora, Actiniae), the 

 Vermes {Phoronis, Sagitta, Euaxes, Ascidia, &c.), the 

 EcHiNODERMATA (Asterida, Echinida), the Mollusca {Lym- 

 nceus), and the Vertebrata [Amjjhioxus). Embryonic forms 

 which may be derived without difficulty from the gastrula also 

 occur among the Arthropoda (Crustacea and Tracheata). 

 In all these representatives of the most various animal stocks 



* Entwickelimgsgescliiclite des Amphioxiis lanceolatus, 1867 (M^m. de 

 I'Acad. de St. Petersb. tome xi. no. 4). 



t Embiyologische Studien an Wiirmern und Arthropoden, 1871 (ibid, 

 tome xvi. no. 12). 



X " On the Anatomy and Affinities of the Medusae," Phil. Trans. 1849, 

 p. 426. 



