EARLY STAGES IN THE DEVELOPMENT OF VERTEBRATES, 225 
Birds no trace of it is any longer to be seen. In all those 
vertebrates in which it is present, it closes up and does not 
become the anus of the aduit. The final anus nevertheless 
corresponds very closely in position with the anus of Rus- 
coni. Mr. Lankester has shown (‘ Quart. Journ. of Micro. 
Science’ for April, 1875) that in invertebrates as well as 
vertebrates the blastopore almost invariably closes up. It 
nevertheless corresponds as a rule very nearly in position 
either with the mouth or with the anus. 
If this opening is viewed, as is generally done, as really 
being the mouth in some cases and the anus in others, it 
becomes very difficult to believe that the blastopore can in 
all cases represent the same structure. In a single branch of 
the animal kingdom it sometimes forms the mouth and some- 
times the anus: thus for instance in Lumbricus it is 
the mouth (according to Kowalevsky), in Paleemon (Bob- 
retzky) the anus. Is it credible that the mouth and anus have 
become changed, the one for the other ? 
If, on the other hand, we accept the view that the blasto- 
pore never becomes either the one or the other of these 
openings, it is, 1 think, possible to account for its correspond- 
ing in position with the mouth in some cases or the anus in 
others. 
That it would soon come to correspond either with the 
mouth or anus (probably with the earliest formed of these in 
the embryo), wherever it was primitively situated, follows 
from the great simplification which would be effected by its 
doing so. This simplification consists in the greater facility 
with which the fresh opening of either mouth or anus could 
be made where the epiblast and hypoblast were in continuity 
than elsewhere. Even a change of correspondence from the 
position of the mouth to that of the anus or vice versa could 
occur. The mode in which this might happen is exemplified 
by the case of the Selachians. I pointed out in the course of 
this paper how the final point of envelopment of the yolk 
became altered in Selachians so as to cease to correspond with 
the anus of Rusconi; in other words, how the position of 
the Blastopore became changed. In such a case, if the yolk 
material again became diminished, the Blastopore would 
correspond in position with neither mouth nor anus, and the 
causes which made it correspond in position with the anus 
before, would again operate, and make it correspond in 
position perhaps with the mouth. Thus the blastopore 
might absolutely cease to correspond in position with the anus 
and come to correspond in position with the mouth. 
It is hardly possible to help believing that the blastopore 
