13 



case the ancestral evolution is most closely followed. Con- 

 sequently, although it is generally admitted that some 

 embryonic stages in the development of Metazoa probably 

 repeat the unknown transition forms, still there is great 

 difference of opinion amongst zoologists as to which em- 

 bryonic forms actually represent the ancestral Metazoa. 

 Haeckel * has founded upon the prevalence of the embryonic 

 stage known as the Gastrula (fig. 8) throughout many 



Fig. 8. Gastrula stage in the development of a Metazoon ; a, epiblast ; c, hypoblast ; 

 bl, blastopore ; en, archenteron. 



groups of the Metazoa, his " Gastrea " theory, which is, 

 that the Gastrula stage in embryology represents the Gastrea, 

 an ancestral organism formed of two layers of cells, the 

 outer epiblast and the inner hypoblast, enclosing a central 

 cavity which communicates with the exterior at one end 

 (see fig. 8). Lankester,! on the other hand, considers that 

 the Planula (fig. 9), an embryo formed like the Gastrula 

 of two layers of cells, but differing from it in having no 

 opening, is more probably the far back common ancestor 

 of the Metazoa. Eecently Butschli | has brought forward 

 arguments in favour of the " Placula " (fig. 10) — a simple 



* SUidien zur Gastrcea-theorie, Jena, 1877. 

 + Notes on Embryology and Classification. London, 1877. 

 X Aiinals and Magazine of Natural History^ for May, 1884. 



