This passage does not long remain open, but after its closure, when the tail-end of the embryo has become folded off from the yolk, a third passage is established, and leads round the end of the notochord from the closed medullary canal into the postanal gut. It is shewn diagrammatically in fig. 106, _ne_, and, as may be gathered from that figure, has the same relations as the neurenteric canal of the Ichthyopsida. In the goose a passage has been described by Gasser, which appears when about fourteen or fifteen somites are present, and lasts till twenty-three are formed. Behind its opening the medullary canal is continued back as a small diverticulum, which follows the course of the primitive groove and is apparently formed by the conversion of this groove into a canal. It is at first open to the exterior, but soon becomes closed, and then atrophies. [FIG. 104. FOUR TRANSVERSE SECTIONS THROUGH THE NEURENTERIC PASSAGE AND ADJOINING PARTS IN A DUCK EMBRYO WITH TWENTY-SIX MESOBLASTIC SOMITES. A. Section in front of the neurenteric canal shewing a lumen in the notochord. B. Section through the passage from the medullary canal into the notochord. C. Section shewing the hypoblastic opening of the neurenteric canal, and the groove on the surface of the primitive streak, which opens in front into the medullary canal. D. Primitive streak immediately behind the opening of the neurenteric passage. _mc._ medullary canal; _ep._ epiblast; _hy._ hypoblast; _ch._ notochord; _pr._ primitive streak.] In the chick there is a perforation on the floor of the neural canal, which is not so marked as those in the goose or duck, and never results in a complete continuity between the neural and alimentary tracts; but simply leads from the floor of the neural canal into the tissues of the tail swelling, and thence into a cavity in the posterior part of the notochord. The hinder diverticulum of the neural canal along the line of the primitive groove is, moreover, very considerable in the chick, and is not so soon obliterated as in the goose. The incomplete passage in the chick arises when about twelve somites are present. It is regarded by Braun as equivalent to the first formed passage in the duck, but I very much doubt whether there is a very exact equivalence between the openings in different types, and think it more probable that they are variable remnants of a primitive neurenteric canal, which in the ancestors of those forms persisted through the whole period of the early development. The third passage is formed in the chick (Kupffer) during the third day of incubation. In Melopsittacus undulatus the two first communications are stated by Braun (No. 120) to be present at the same time, the one in front of the other.