Dr. \V. tSalon.sky on llackeVs Gastra;a Theory. 27 



cesses, and not judge of the ontogenetic facts merely from the 

 opinion derived from the anatomical facts. If we desire to 

 take an ohjective view, we cannot «ay that when two different 

 modes of development " occur in very nearly allied forms " 

 they are of no consequence to us on account of their relation- 

 ship*. If the phylogenetic fundamental law is correct, the 

 relationshipof animals must only be ascertained from ontogeny; 

 otherwise the idea of relationship, which is derived only from 

 tectological facts, is a preconceived opinion. 



In now concluding my remarks, I hope in these few words 

 to have furnished the factual evidence : — 



1. That the most important factor in the ontogeny of 

 animals is the first differentiation of the germ-lamella. 



2. That this differentiation commences in different animals 

 at different periods of their development, and in most cases 

 leads to the Plamda-form, which occurs in all animals, 

 either in the pure form (in most animals) or in a modified 

 form (Vertebrata and some invertebrate animals), and even 

 exists as free-living animal forms. In many instances the 

 Phiniila-iorm may be overleaped and replaced by the Di- 

 hlastuhi. 



3. That the development of the stomachal cavity is a later, 

 secondary developmental phenomenon, which occurs in diffe- 

 rent animals in different stages of development, and cannot 

 take a place in the idea of the fundamental form of develop- 

 ment. 



4. That therefore the Gastrula-iorm. cannot be accepted as 

 a fundamental form in the developmental history of all 

 Metazoa ; and, consequently, 



5. That the problematical form " Gastrcea^^ cannot be 

 accepted as the " stock-form " for the higher animal stocks. 



EXPLANATION OF PLATE V. 



Figs. 1, 2, 3. Three developmental stages of the oyster (original) ; Ex. 

 exoderm ; En. entoderm ; V. velum ; «S'. shell ; M. invagina- 

 tion of the anterior intestine ; D. intestine. 



Fiff. ^. Diblastula of an Ascidian (from Kowalevsln- : " Weitere Studien," 

 &c., in Arch, fiir milvr. Anat. Bd. vii. Taf. x. fig. 5): Ex. exo- 

 derm ; En. entoderm. 



Fig. 5. Diblastula oi Hiidrophilus (fromKowalevsky: "Embrvol. Studien," 

 &c., in M^m. de I'Acad. de St. Petei-sb. tome xvi. Taf. ix. 

 fig. 20) : Ex. exoderm ; En. entoderm ; Nd. nutritive %"itellus. 



Fig. 6. Ga.^friila of an Ascidian (from Kowalevsky, /. c. Taf. x. fig. 6) : 

 Ex. exoderm ; En. Entoderm. 



Fig. 7. Transverse section through the embryo of Hydrophilm (from 

 Kowalevsky, /, c. Taf. ix. fig. 2G) : Ex. exoderm ; En. entoderm. 



* Hackel, Die Kalkschwamme, Bd. i. p. 467. 



