GASTRULATION OF THE VERTEBRATES. 405 
mouth” have been looked upon from the beginning as 
synonyms, the more so as in the phylum of the Coelenterates 
(Polyps, Medusz, Corals, Sea Anemones, etc.),in which we 
find duplication of the layers by delamination in one genus, 
by invagination in others, the primitive mouth or blastopore 
eradually becomes the so-called mouth of the Medusa or 
Hydra, or the oral slit of the full-grown Coral or Anemone. 
It should at the same time be noticed that the oral slit of 
the Anemone (Actinia) undergoes a further complication by 
the formation of a stomodzeum. In consequence of the 
formation of this ectodermal involution, below the level of 
the oral disk, the primitive mouth is displaced to the lower 
Fie. 1.—A gastrula with open blastopore. 
level at which the stomodzum passes into the entoderm. It 
is all the more important to keep this in view as this com- 
plication of the full-grown Ceelenterate has its echo in the 
phylogeny of the higher animals. We thus find occasion to 
insist, already now, on the inadequateness of confusing the 
oral slit of an Actinia with a primitive mouth. 
We shall be able to approach the question of gastrulation, 
as also that of the formation of the germ-layers in Vertebrates, 
with a more open mind if we can provisionally disinterest 
ourselves in the question as to whether delamination or 
invagination was the more primitive process by which a one- 
layered blastula became converted into a_ two-layered 
gastrula. Those who hold with Haeckel that invagination 
has been the prototype of all gastrulation are biassed and for 
