At first, the mesenteron freely communicates with the exterior by the opening of the blastopore.
There is in this way formed a tube on the floor of which the blastopore opens behind, and which is itself open in front.
The position of the blastopore is the same as in other types, viz.
This rim is a very important structure, since it represents the dorsal portion of the lip of the blastopore of Amphioxus.
This we consider to be the posterior end of the blastopore of the embryo.
The lip of the blastopore has moreover become inflected for its whole circumference.
The final fate of the blastopore is nearly the same as in Amphioxus.
The force of this consideration can best be displayed by a short summary of the fate of the blastopore in different forms.
These thickened tracts of ectoderm in Peripatus and a few other forms can be clearly seen to surround the blastopore.
The fate of the blastopore is so variable that it is difficult even to classify the cases which have been described.
blastopore blas·to·pore (blās'tə-pôr')
The opening into the archenteron formed by the invagination of the blastula to form a gastrula.