64 Jenkinson, Germinal Layers of Vertebrates. 



layers of the Gastrula are the primary germ-layers, the 

 ectoderm and the endoderm, and are homologous or 

 morphologically similar throughout the series ; the cavity 

 of the Gastrula is the archenteron or primitive gut, and 

 invariably becomes the alimentary canal ; the opening of 

 the cavity is the primitive mouth. With the mesoderm 

 the theory is as such not concerned. Further, this Gastrula 

 is a recapitulation in ontogeny of a phylogenetic form, 

 the Gastraea, the common ancestor of all the Metazoa ; 

 to this primeval form the living Coelenterates are most 

 nearly allied. Before proceeding to examine the cre- 

 dentials of this creed, we may glance briefly at some of 

 its later successors and descendants. 



The planula theory of Lankester — put forward almost 

 contemporaneously — closely resembles the hypothesis of 

 Haeckel, differing only from it in substituting for the 

 Gastrula a Planula or Gastrula without a mouth, in which 

 the primary endoderm arises by delamination instead of 

 invagination. The theory was intended to meet the 

 difficulty presented by the variability in the fate of the 

 blastopore. 



Passing over a few years we find Balfour" returning 

 to a belief in the Gastrula as a form which ' reproduces 

 'with more or less fidelity a stage in the evolution of the 

 ' Metazoa permanent in the simpler Hydrozoa.' Still later 

 developments of the Gastrula theory, restricted, however, 

 to the Vertebrata, are those propounded by Lwoff" and 

 Hubrecht.*' Lwoff identifies the two primary layers — the 

 animal and vegetative cells — of the vertebrate embryo 

 with the ectoderm and endoderm of the Coelenterate or 

 Gastraea, but part only of the archenteric cavity with the 



11 "Comparative Embryology," vol, 2, ch. 13, 1881. 



12 Bull, soc.imp. nat. AIoscou, n.s., vol. 8, pp. 57, 160, 1894. 



13 " Furchung und Keimblattbildung bei Tarsius spectrum," Verh. k. 

 Akad. Wetensch. Amsterdam, (2), vol. 8, no. 6, 1902. 



