In this article we will discuss about the formation of primary organ rudiments in amphioxus and amphibians.
Primary Organ Rudiments in Amphioxus:
Immediately after the germinal layers have taken up their positions in the inside and on the surface of the gastrula, the next phase of development sets in. The sheets of epithelium which were representative of the various parts of the future animal become broken up into discrete cell masses of diverse shape which can be called the primary organ rudiments.
The term “primary” indicates that the structures in question are not final; they are actually complex in nature, and a further subdivision of the cell masses, or at least of most of them, is necessary before every single organ and structure of the adult animal appears as such.
In the interior of the embryo the presumptive materials of the notochord, the mesoderm, and the gut become separated from one another by crevices appearing along the boundary lines of each. The strip of cells lying mid-dorsally rounds itself off forthwith and becomes a cylindrical cord of cells, the notochord, which still differs from the same organ of the adult Amphioxus in the structure of its cells.
As the presumptive material of the mesoderm becomes separated from both the notochord and the endoderm, it breaks up into a series of roughly cuboidal masses of cells lying on each side, one behind the other, along the length of the animal’s body. These blocks of mesodermal cells are called the mesodermal segments. In connection with their formation.
Just before the mesodermal cells become separated from the endoderm and notochord, a longitudinal groove appears on the inner surface of the mesodermal bands, that is, on the surface toward the archenteron. The groove becomes drawn out into each of the mesodermal segments, so that a pocket-like invagination is formed in each.
At the same time, crevices cut in between adjoining segments from the outside, separating them from each other. When the mesodermal segments eventually become separated from the endoderm and notochord, the pocket-like invaginations in each become completely closed off from the cavity of the archenteron and become small cavities inside the mesodermal segments.
In a later stage of development, these cavities expand and become the secondary body cavity or coelom of the adult animal. The derivation of the cavities of the mesodermal segments from the archenteric cavity is distinct in the case of the most anterior segments, but the posterior mesodermal segments are solid masses of cells at first and acquire cavities later as a result of the separation of the cells in the middle.
When the notochord and the mesodermal segments dissociate themselves from the endodermal material, the free edges of the endoderm approximate each other and fuse along the dorsal midline. The endoderm thus becomes converted into a closed sac. The cavity of this sac becomes the lumen of the alimentary canal.
In the ectoderm the presumptive nervous system material becomes separated from the surrounding presumptive epidermis in the form of an elongated plate, the neural plate. The neural plate sinks below the level of the remainder of the ectoderm and is then covered by the free edges of the epidermal epithelium.
The neural plate forthwith rolls itself into a tube, the lateral edges folding themselves upward and fusing along the midline. The neural plate is thus transformed into the neural tube, which becomes the spinal cord of the animal. The neural tube does not close completely at the anterior end but leaves an opening, the neuropore, which remains patent until the later stages of development.
The epidermal epithelium which covers the neural plate by shifting over its surface is derived from the sides and from the area posterior to the neural plate. Because the hind end of the neural plate borders on the blastopore, the posterior portion of epidermis covering the neural plate is derived from the region below (ventral to) the blastopore.
As the epidermis shifts forward over the surface of the neural plate, it also passes over the blastopore. The blastopore becomes cut off from the exterior and opens into a space lined by the walls of the neural tube. The canal thus formed, which connects the blastopore, and therefore the archenteron, with the cavity of the neural tube, is called the neurenteric canal.
The canal persists for only a short time, until the cavity of the neural tube (later becoming the central canal of the spinal cord) becomes completely separated from the cavity of the archenteron, which in turn becomes the cavity of the alimentary canal. The cavity of the alimentary canal later acquires communication with the exterior by means of the oral and anal openings which break through the body wall, but this does not take place until a very much later stage of development.
After the formation of the spinal cord, the notochord, the gut, and the mesodermal segments, the early embryo of Amphioxus possesses most of the essential features of the organization of chordate animals. The features which are still lacking are a mouth, an anus, and the gill clefts. The gill clefts, as well as the mouth, are developed in all chordates at a comparatively late stage.
A stage of development in which an embryo possesses a spinal cord, notochord, gut, and segmented mesoderm recurs with great tenacity in all vertebrates, and that the subsequent stages of development in all classes of vertebrates are much more similar than are the processes of cleavage and gastrulation.
Primary Organ Rudiments in Amphibians:
At the time of the closure of the blastopore, the separation of organ rudiments in amphibians has gone somewhat further than in Amphioxus; the chordomesodermal mantle has, to a great extent, separated itself from the endoderm, and the mesoderm has attained its definitive position between the endoderm and the ectoderm.
To make the separation complete, the notochord becomes split at its anterior end from the prechordal plate, and the prechordal plate itself separates from the adjoining endoderm. The endoderm forthwith closes under the prechordal plate. The foregut is then surrounded on all sides by endodermal cells.
In the mid-gut region the free edges of the endoderm approach each other in the midline and fuse, thus completely enclosing the mid-gut. The chordomesodermal mantle becomes subdivided into its two components – the notochord in the middle and the mesoderm on both sides.
As the notochord is separated from the mesoderm, the strand of notochordal cells becomes converted into a cylindrical body, which is round in cross section. The mesoderm, at the same time, becomes subdivided into a series of mesodermal segments. Contrary to what is found in Amphioxus, however, not the whole of the mesoderm becomes segmented but only its dorsal part, the part adjoining the notochord.
The mesodermal layer has been thickened here owing to the convergence of the mesoderm toward the dorsal side of the embryo during gastrulation. This thickened part now becomes segmented by a series of transverse crevices cutting into the mass of mesodermal cells and separating them.
The thinned out lateral and ventral parts of the mesodermal mantle on each side do not become subdivided into segments, and these parts of the mesoderm are known as the lateral plates. The mesodermal segments in amphibians, called somites, are not fully homologous to the primary mesodermal segments of Amphioxus.
Each somite is separated from the adjoining somites but remains connected to the dorsal edge of the lateral plate. The strand of cells connecting the somite to the lateral plate is known as the stalk of the somite, and these cells, give rise to the excretory system of the embryo.
At about the same time that the segmentation of the dorsal part of the mesoderm is occurring, the lateral plate mesoderm becomes split into two layers, an external layer applied to the ectoderm known as the parietal layer, and an internal layer applied to the endoderm, the visceral layer. The narrow cavity between the two layers is the coelomic cavity or coelom.
In later stages the cavity expands and becomes the body cavity of the adult animal. Small cavities also appear in the somites, but these are obliterated later and disappear without a trace. There is never any connection between the coelomic cavities in the mesoderm of the amphibians and the archenteric cavity.
The presumptive nervous system has been traced previously to the stage when it forms an approximately oval-shaped area on the dorsal side of the embryo. The neural area covers the areas of the notochord and the somites in the middle and posterior parts of the embryo and the prechordal plate in front.
The anterior end of the neural area is underlain by part of the endodermal lining of the foregut. After the closure of the blastopore, the presumptive area of the nervous system becomes differentiated from the rest of the ectoderm in the form of the neural plate. The ectodermal epithelium of the neural plate becomes thickened by the further concentration of the epithelium toward the dorsal side of the embryo.
At the same time, the cells of the neural plate change in shape; they become elongated and arrange themselves into a columnar epithelium. Thus, they are different from the cells of the epidermis, which remain more or less cuboidal and arranged as a stratified epithelium usually two cells thick.
Superficially the neural plate becomes visible because of a concentration of pigment at the edges of the plate. Soon, however, the edges of the neural plate become thickened and raised above the general level as neural folds. A shallow groove may be seen at the same time along the midline of the neural plate, separating it into right and left halves.
The neural plate continues to contract in a transverse direction, especially in its posterior parts. The neural folds become higher, so that eventually the neural plate is converted into a longitudinal depression, bordered laterally and in front by the neural folds.
Subsequently, the folds make contact in the midline and fuse, beginning from the point corresponding to the occipital region of the embryo and progressing forward and backward. In this way the neural plate is transformed into the neural tube. The cavity of the neural tube is broadest in the anterior part of the tube, which later develops into the brain with the brain cavities.
In the posterior part of the neural tube, the cavity is much narrower. It later becomes the central canal of the spinal cord. Sometimes the central canal is not found in the posterior part of the neural tube at the time of the closure of the neural folds, and in this case the tube is hollowed out later by the separation of the cells in the middle of the organ.
The formation and closure of the neural plate is known as neurulation, and the embryo, in the stages when it possesses a neural plate, is called a neurula.
In frogs, the neural folds reach posteriorly to the level of the blastopore or even slightly beyond it. As a result the blastopore, now reduced to a narrow canal, is surrounded by the neural folds. When the folds fuse, the external end of this canal is found in the floor of the neural tube. In most frogs, by the time the neural tube is formed, the blastopore becomes completely occluded by the fusion of its edges.
In some amphibians, however, particularly the clawed frog, Xenopus, the canal remains open and becomes the neurenteric canal, which for some time provides a direct communication between the posterior end of the archenteron and the neural tube. The neurenteric canal is later interrupted and disappears without a trace.
After the blastopore ceases to provide an opening for the gut to the exterior, a new opening breaks through at the posterior end of the embryo, slightly below the position of the blastopore. This opening becomes the anus. In urodeles, however, the blastopore remains outside the neural folds and becomes directly transformed into the anal opening.
After the neural folds have fused in the midline, the neural tube separates itself completely from the overlying epidermis. The free edges of the epidermis fuse, so that the epidermis becomes continuous over the back of the embryo. A certain number of cells, however, do not become included either in the neural tube or in the epidermis.
These cells can be traced back to strips running all along the crest of the neural folds. After the neural tube has become separated from the epidermis, these cells are found as an irregular flattened mass between the neural tube and the overlying epidermis.
This mass of cells is the neural crest. It may be continuous across the midline at first, but soon the mass of cells separates into a right and a left half, lying dorsolateral^ to the neural tube. The neural crest cells have a special part to play in the development of the embryo.
During neurulation, the embryo already begins to stretch in length. At the same time, it is flattened from side to side and also becomes smaller in a dorsoventral direction, but the volume of the embryo does not change appreciably. The stretching is greatest in the posterior part of the embryo. As the neural tube takes part in this general stretching, it becomes still more attenuated in its posterior parts, so that the difference between the presumptive brain and the presumptive spinal cord is increased.
The part of the embryo above the blastopore becomes elongated beyond the blastopore, and this elongation becomes the rudiment of the tail, known as the tail-bud. The pharyngeal pouches are formed as lateral out-pushing’s of the foregut, and in later development they give rise to the gill clefts. The mouth later breaks through at the anterior end of the foregut.