26 J. T. CUNNINGHAM. 
(10), is simply that part of the blastopore which has just 
closed; part of it, the extreme anterior part, may be formed 
during segmentation, but the posterior part must be due to 
actual concrescence. It will be shown below that the primitive 
streak in all Vertebrates represents concrescence of the edges of 
the blastoderm. 
The points which are to be emphasised in all this are that the 
whole edge of the blastoderm in Teleosteans and apparently also 
in Amphibia is invaginated, and that the whole edge is therefore 
homologous with the lip of the ancestral blastopore. Unfor- 
tunately there is still some uncertainty concerning the relation 
of the posterior end of the blastopore to the embryo. In 
Teleosteans, as far as can be seen in the living ovum (vde figs. 
9 and 22), the posterior end of the blastopore is on the dorsal 
surface ; and it has always been stated that in the frog the end 
of the blastopore was entirely embraced by the medullary fold 
and formed the medullary canal. But Sedgwick and Miss 
Johnson (8 and 9) have lately attempted to show that in 
Triton part of the blastopore extends round to the ventral 
surface. Miss Johnson’s figures are somewhat diagrammatic. 
If the proposition is correct that the posterior end of the 
blastodermic aperture in the newt remains open the question 
immediately arises, Is the whole edge of the blastoderm in the 
newt inflected? The answer to which question cannot be 
found in Miss Johnson’s figures. If it is not, then the blasto- 
pore (primitive) and the edge of the blastoderm in the newt are 
not the same thing. If it is, then it may prove true that the 
anus is derived from the posterior end of the blastopore, and 
has passed from an original terminal position forwards on the 
ventral surface by a continuous process during ancestral 
history without ever closing up. ‘The process may easily have 
taken place without causing any alteration in other systems of 
organs except the shortening of the intestine. This would 
mean that the closure of the medullary canal posteriorly 
divides the open part of the blastopore into two parts, one 
forming the neurenteric canal, the other remaining open to 
the exterior and persisting as the anus. 
