176 THE ANCESTRY OF VERTEBRATES 



tion of the germinal layers in Vertebrates be not exhaustive, 

 I think it will sufficiently illustrate the reigning confusion 

 and uncertainty on these subjects. The cause of all this 

 controversy is the absence of true insight as a consequence 

 of the lack of a phylogenetic guiding thread connecting the 

 Vertebrates with the lower groups of animals. Observations 

 and facts alone can not help us here. My theory leads to 

 the following conception which, I believe, also proves to 

 be best in harmony with the facts. 



In Invertebrates the blastopore, after having contracted 

 to a narrow opening, passes into the cardiac pore as a con- 

 sequence of the formation of the stomodaeum. In Vertebrates 

 the blastopore, after having contracted to a narrow opening, 

 passes into the neurenteric pore as a consequence of the 

 formation of the medullary tube. In Invertebrates we call 

 gastrula the stage in which the blastopore has contracted 

 to a narrow opening but before the stomodaeum has formed. 

 Thus in Vertebrates we must call gastrula the corresponding 

 stage, being e.g. that of fig. 12 or Plate II, fig. 1. That which 

 lies at the surface here is the primary ectoderm, that which 

 lies in the interior is the primary endoderm, just as in 

 Invertebrates. Thus the roof of the archenteric cavity 

 consists of (primary) endoderm cells just as does the floor, 

 as has been argued already by many investigators who have 

 emphasized the fact that, e.g. in the frogs egg, the cytological 

 character also of the cells of the roof renders it quite evident 

 that they belong to the endoderm, not to the ectoderm. 



Eccentric closure of the blastopore. — However, it cannot 

 be denied that the gastrulation in Vertebrates exhibits 

 certain peculiarities which call for an explanation. The blas- 

 topore does not close in a concentric way. This is the case 

 also in Annelids. The originally wide blastopore here closes by 

 fusion of the two opposite lateral borders, leaving open only 

 the foremost end which passes into the cardiac pore. 

 How the blastopore border while contracting moves over the 

 surface of the egg is not easily to be determined in Vertebrates 

 since we have no fixed landmark. Observing the living 

 egg of a frog in its natural position, in which at first 

 the animal pole is directed right upwards, gives no reliable 

 results since during the gastrulation the centre of gravity 

 of the egg changes its position and causes a rotation of 

 the latter. ' In observing the egg in "Zwangslage" between 

 two glass-plates and making e.g. photographs of it, for 



