Aves, the primitive streak and the neurenteric canal of amniote blastoderms.
As an instance of this striking phenomenon I may mention the neurenteric canal of avine embryos, and the anterior neuropore of Ascidians.
An important fact to note is that the blastopore is included in this overgrowth of epiblast, so that the neural tube remains for some time in open communication with the archenteron by means of a posterior neurenteric canal.
It is thus possible that the neurenteric canal is due to the conjunction of a posterior neuropore with the blastopore, i.
These medullary swellings join together over the furrow, and form a tube; in this case, again, the neural or medullary tube is at first open in front, and connected with the primitive gut behind by the neurenteric canal.
In the earlier stages the alimentary tube is found to be entirely closed, after the closing of the primitive mouth; it only communicates behind by the neurenteric canal with the medullary tube.
The medullary tube would be originally a chemical sensory organ, a dorsal olfactory tube, taking in respiratory-water and food by the neuroporus in front and conveying them by the neurenteric canal into the primitive gut.
The majority are developed in relation to the communication which exists in the embryo between the neural canal and the alimentary tract--the post-anal gut or neurenteric canal.
The post-anal dimple, a shallow depression frequently observed over the tip of the coccyx, may be due to traction exerted on the skin at this spot by the remains of the neurenteric canal, or by the caudal ligament of Luschka.
The germ-layers become confluent, but, unlike the sturgeon, the flattening of the dorsal germ-ring does not permit the formation of a neurenteric canal.
The caudal tip in this stage appears, however, to resemble that of the shark, and, as far as can be inferred from surface views, a neurenteric canal persists.
It consists of a simple dilated anterior end leading into a straight tube, the lumen of which is much larger than that of the ultimate spinal canal, and terminates by way of the neurenteric canal in the anus.
In the arthropod the intestine terminates in the anus; in the vertebrate embryo the canal of the spinal cord terminates in the anus by way of the neurenteric canal.
The neurenteric passage persists but a very short time after the complete closure of the medullary canal.
It is interesting to notice the remarkable relation of the notochord to the walls of the neurenteric passage.
At a later period, as has been shewn by Gasser[455], there is a distinct rudiment of the neurenteric canal in the chick, and a complete canal in the goose.
Such a pit is unusual, but we think it may be regarded as an exceptionally early commencement of that most variable structure in the chick, the neurenteric canal.
In the lizard the early appearance of the neurenteric canal at the front end of the primitive streak clearly shews that here also the primitive streak can take no share in forming the neural plate.
In the succeeding stages the medullary groove becomes continued back to the opening of the passage, which then becomes enclosed in the medullary folds, and forms a true neurenteric passage.
While at the same time the parallel between the blastopore of Elasmobranchii and of the Sauropsida, is rendered more complete by the discovery of the neurenteric passage in the latter group, which was first of all made by Gasser.
In the median line the notochord arises as a ridge-like thickening of the hypoblast, which becomes very soon quite separated from the hypoblast, except at the hind end, where it is continued into the front wall of the neurenteric passage.
We could detect no true neurenteric canal, although a more or less complete fusion of the germinal layers at the tail end of the embryo may still be traced.
The sections immediately behind the last we have represented shew a rudiment of the neurenteric canal of the same form as that first figured by Gasser, viz.
Section shewing a deep pit in front of primitive streak, probably an early indication of the neurenteric canal.
The medullary groove is continued as a deepish groove up to the opening of the neurenteric passage, which thus forms a perforation in the floor of the hinder end of the medullary groove (vide Series B, figs.
Moreover, the mouth would have had much the same relation to this imaginary anterior neurenteric canal that the anus has to the actual posterior one.
As a rule it is slit-like in form and closes completely, but in most cases one portion of it remains open longer than the rest, as the neurenteric canal.
The medullary canal continues for a short distance (about fifteen sections of five microns thickness) posterior to the opening of the neurenteric canal.
Again, we find the neurenteric canal in just the same form and situation in the amphibia.
The tenacity with which the useless neurenteric canal has been transmitted down to man through the whole series of vertebrates is of equal interest for the theory of descent in general, and the phylogeny of the chordonia in particular.
The neurenteric canal then leads directly, in the shape of a narrow arch-shaped tube (Figure 1.
Behind, it passes by the neurenteric canal into the primitive gut or primitive groove.
The chordula-embryo of all the vertebrates is characterised by the dorsal medullary tube, the neurenteric canal, which passes at the primitive mouth into the alimentary canal, and the axial chorda between the two.
A little later, when the primitive mouth is closed, the narrow neurenteric canal (Figure 1.
The connection which the neurenteric canal (Figure 1.
The point of transition is the remarkable neurenteric canal (Figure 1.
The enigmatic neurenteric canal is a very old embryonic organ, and of great phylogenetic interest, because it arises in the same way in all the chordonia (both tunicates and vertebrates).
The uncovered patch of yolk in the blastoporic area soon becomes closed over; and on the formation of the medullary canal the usual neurenteric canal becomes established.
Primitive streak immediately behind the opening of the neurenteric passage.
On the formation of the medullary groove and its conversion into a canal, the blastopore opens into this canal, and gives rise to a neurenteric passage, leading from the neural canal into the alimentary tract (fig.
The neurenteric canal is established; and in front and behind the epithelium of the ventral wall of the mesenteron has begun to be formed.
The anus becomes formed, and a neurenteric canal is also established (Scott).
In the Chordata it is also placed posteriorly to the anus, and, remarkably enough, remains, in a large number of forms, for some time in connection with the neural tube by a neurenteric canal.
It forms at first a simple hypoblastic tube extending from near the front end of the body, where it terminates blindly, to the hinder extremity where it is united with the neural tube by the neurenteric canal (fig.
The canal connecting the neural and alimentary canals has already been described as the neurenteric canal, and represents the remains of the blastopore.
It will be convenient at this point to say a few words as to what is known of the further fate of the neurenteric canal, and the early development of the allantois.
The morphological significance of the postanal gut and of the neurenteric canal has already been spoken of in Chapter XII.
At the lip of the blastopore the medullary cord is continuous with the hypoblast, thus forming the rudiment of a neurenteric canal.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "neurenteric" in a variety of sentences. We hope that you will now be able to make sentences using this word.
