According to Braun (No. [120]) three independent communications are to be distinguished in Birds. These are best developed in the Duck. The first of these is a small funnel-shaped diverticulum leading from the neural groove through the hypoblast. It is visible when eight mesoblastic somites are present, and soon disappears. The second, which is the only one I have myself investigated, is present in the embryo duck with twenty-six mesoblastic somites, and is represented in the series of sections ([fig. 104]). The passage leads obliquely backwards and ventralwards from the hind end of the neural tube into the notochord, where the latter joins the primitive streak (B). A narrow diverticulum from this passage is continued forwards for a short distance along the axis of the notochord (A, ch). After traversing the notochord, the passage is continued into a hypoblastic diverticulum, which opens ventrally into the future lumen of the alimentary tract (C). Shortly behind the point where the neurenteric passage communicates with the neural tube the latter structure opens dorsally, and a groove on the surface of the primitive streak is continued backwards from it for a short distance (C). The first part of this passage to appear is the hypoblastic diverticulum above mentioned.
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 contin164uity 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.
It is probable, from the above description, that the front portion of the primitive streak in the bird corresponds with that part of the lips of the blastopore in Elasmobranchii which becomes converted into the tail swelling and the lining of the neurentic canal; while the original groove of the front part of the primitive streak appears to be converted into the posterior diverticulum of the neural canal. The hinder part of the primitive streak of the bird corresponds, in a very general way, with the part of the blastopore in Elasmobranchii, which shuts off the embryo from the edge of the blastoderm (vide p. [64]), though there is of course no genetic relation between the two structures. When the anterior part of the streak is becoming converted into the tail swelling, the groove of the posterior part gradually shallows and finally disappears. The hinder part itself atrophies from behind forwards, and in the course of the folding off of the embryo from the yolk the part of the blastoderm where it was placed becomes folded in, so as to form part of the ventral wall of the embryo. The apparent hinder part of the primitive streak is therefore in reality the ventral and anterior part[65].
It has generally been maintained that the primitive streak and groove become wholly converted into the dorsal portion of the trunk of the embryo, i.e. into the posterior part of the medullary plate and subjacent structures. This view appears to me untenable in itself, and quite incompatible with the interpretation of the primitive streak given above. To shew how improbable it is, apart from any theoretical considerations, I have compiled two tables of the relative lengths of the primitive streak and the body of the embryo, measured by the number of sections made through them, in a series of examples from the data in Gasser’s important memoir (No. [127]). In these tables each horizontal line relates to a single embryo. The first column shews the number of somites, and the second the number of sections through the primitive streak. Where the primitive streak becomes divided into two parts the sections through the two parts are given separately: the left column (A) referring to the anterior part of the streak; the right column (P) to the posterior part. The third column gives the number of sections through the embryo. The first table is for fowl embryos, the second for goose embryos.
| No. of Somites. | No. of sections through the Primitive Streak. | No. of sections through the Embryo. |
|---|---|---|
| 0 | 29 | 7 |
| 0 | 45 | 10 |
| 0 | 39 | 23 |
| 2 | 30 | 30 |
| 4 | 30 | 30 |
| A P | ||
| 5 or 6 | 10 + 17 = 27 | |
| 8 | 12 + 20 = 32 | 48 |
| 12 | 13 + 10 = 23 | |
| 14 | 9 + 12 = 21 | |
| 18 | 10 + 7 = 17 | 70 |
| 8 + 4 = 12 | ||
| 8 + 3 = 11 |