Blastopore

Circular space of yolk-surface left uncovered at the vegetative pole of an egg during the spreading of the blastoderm, by epibole. Also the gastrula mouth or arch is tom a. This term is applied to structures not strictly homologous. (1) The primitive gastrula mouth (gastropore, arch is to me, gastrostome). (2) The yolk-area last exposed behind the neurenteric canal, and the postanal vesicle. of Ichthyopsid a. (3) The yolk last inclosed in the egg of Sauropsida, separated by at least one structure (primitive groove) from the primitive blastopore. (The primitive groove is homologous with the second kind of blastopore.) (4) The Mammalian blastopore, a modified gastropore (blastopore of Van Beneden). It is believed that in most animals both the permanent mouth and anus are formed out of the blastopore, but the subject is yet obscure.