This latter part not only forms thepronephros and mesonephros, but also by a ventral extension gives origin to the walls of the vertebrate body-cavity or metacoele.
For this reason the pronephros is said to be formed, in part at least, from a portion of the coelom situated more ventrally than the purely somatic part which gives rise to the mesonephros.
According to the much more probable and reasonable view, the pronephros represents the oldest and original excretory organs, while the mesonephros is a later formation.
The great distinction of thepronephros is that it, and it alone, forms the segmental duct.
Further, the pronephros was said to arise from an outbulging of the somatopleuric mesoblast, which extended over a limited number of metameres, and was not segmental, but continuous.
Next comes the question, why was the pronephros not repeated in the meristic repetition that took place during the early vertebrate stage?
For my own part I should expect to find evidence of a former existence of an atrial cavity rather in the pigment round the pronephros and its duct than in the duct itself.
The kidney is represented by a highly primitive pronephros and no trace exists of an air-bladder or lung.
Sedgwick has pointed out that the pronephros is always present in types with a larval development, and either absent or imperfectly developed in those types which undergo the greater part of their development within the egg.
The position of the pronephros in relation to the Wolffian body is shewn in fig.
The pronephros may in fact now be described as a slightly convoluted duct, opening into the body cavity by three groove-like apertures, and continuous behind with the rudiment of the true Muellerian duct.
In most Teleostei the pronephros and mesonephros coexist through life, and their products are carried off by a duct, the nature of which is somewhat doubtful, but which is probably homologous with the mesonephric duct of other types.
In Elasmobranchii the original opening of the segmental duct forms the permanent opening of the Muellerian duct, and no true pronephros appears to be formed.
The anterior end remains open to the body cavity, and forms a pronephros (head kidney).
The chief structural peculiarity of the pronephros is the absence from it of Malpighian bodies with the same relations as those in the meso- and metanephros; unless the structures found in Myxine are to be regarded as such.
In both forms the ducts unite behind with the cloaca, and a pronephros of the Teleostean type appears to be developed.
In Myxine the excretory system consists (1) of a highly developed pronephroswith a bunch of ciliated peritoneal funnels opening into the pericardial section of the body cavity.
The coiled and branched tubes of which the pronephros is composed open on the ventral side of the anterior portion of the segmental duct, which in old individuals is cut off from the posterior section of the duct.
In connection with its anterior extremity certain structures have been found in the Fowl, which are probably, on grounds to be hereafter stated, homologous with the pronephros (Balfour and Sedgwick).
Diagrammatic views of the pronephros of Lepidosteus.
They appear to me, however, to shew that in the normal types of Teleostei the organ usually held to be the pronephros is actually nothing of the kind.
The ciliated canal is similar in its relations to the peritoneal canal of such a segmental tube, and the coiled portion of the pronephros resembles the secreting part of the ordinary segmental tube.
It may be noted that in Lepidosteus the whole pronephros has exactly the character of a single segmental tube of the mesonephros.
Transverse section through anterior part of trunk of an embryo on the ninth day after impregnation, shewing the pronephros and pronephric chamber.
In the region between the pronephros and the front end of the fully developed part of the mesonephros very rudimentary tubules had become established.
On the accomplishment of these changes, the pronephros is essentially provided with all the parts typically present in a segment of the mesonephros (woodcut, fig.
At a later stage we find that there is connected with the pronephros on each side, a cavity--the pronephric cavity--into which a glomerulus projects.
In Teleostei thepronephros has precisely the same characters, except that the cavity in which the glomerulus is placed is without a peritoneal canal.
My observations on Teleostei are clearly not sufficiently extensive to prove that the larval pronephros never persists in this group.
There is no trace of a pronephros at all in the rabbit.
Tabulating these facts-- In the adult male: Pronephros disappears.
The mode of development of the pronephros; though the pronephros of Lepidosteus has primitive characters not retained by Teleostei.
A pronephros of the Teleostean type present in the larva.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "pronephros" in a variety of sentences. We hope that you will now be able to make sentences using this word.
