الفهرس 
Development of the pronephric kidneyOnly 14 pages are availabe for public view
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Abstract
The development of the excretory system of the mosquito fish, Gambusia rrfjnis (Baird & Girard) was studied. The development of the pronephric and mesonephric kidneys and related structures such as the urinary bladder, vascularization and corpuscles of Stannius was investigated in details. The following results were obtained : The pronephros originates on each side of the lmm. long embryo from an unsegmental intermediate cell mass or nephrotome by forming primary then secondary pronephric folds. The primary pronephric folds are formed at the levels of the second, third and fourth somites. After the nephrocoele becomes separated from the coelom of the lateral plate mesoderm, the enclosed cavity of the secondary pronephric fold is dilated to form a single pronephric chamber on each side. The dorsal aorta forms two branches which extend toward and press against the median walls of the two pronephric chambers to form the pronephric glomeruli, one on each side. The pronephric chamber differentiates into Bowman’s capsule that is vascularized by a pronephric glomerulus, developed from the dorsal aorta. The pronephric tubule is formed posterior to each pronephric chamber due to the rotation of the secondary pronephric fold from its posterio-lateral edge. Posteriorly, a solid cord of cells from the intermediate cell mas, extending from the sixth to seventh body segment, develops separately and gives rise to the adage of each pronephric duct. In older stages each pronephric tubule is extended posteriorly to join with the anterior end of the previously developed pronephric duct. It differentiates into different kinds of epithelium and forms large coils. The pronephric glomeruli become more shifted cephalad, mostly due to a physical pressure caused by the developing air bladder in this region. The pronephros persists for a considerable time after the appearance of functional mesonephric tubules . Degeneration of the pronephros bqins in the 8mm. long embryo. The - vanishing pronephric glomeruli are replaced by a lymphoid tissue, and they become disconnccted from the pronephric tubules and disappeared within the mesonephric kidneys. The mesonephric kidney develops in 3.5 mm. long embryo from the unsegmented intermediate cell mass. The intermediate cell mass forms condensations of cells scattered around the coils of the pronephric tubules and at the anterior part of the pronephric ducts between the dorsal aorta and the posterior cardinal vein. The elongation and tubulation of these solid cords lead to the connection of these mesonephric tubule anlagen with the coiled pronephric tubules and the anterior parts of the pronephric ducts. The pronephric ducts are now referred to as the mesonephric or the archinephric ducts and the mesonephric kidney becomes the functional trunk or adult kidney. The mesonephric Bowman’s capsules develop from the hollow free ends of the mesonephric tubule rudiments and differentiate into thin-walled capsules. The mesonephric glomeruli are formed from branches of the renal artery. Lymphoid or intertubular tissue is developed between the coils of the pronephric tubules and the mesonephric tubules of the mesonephros, in a large amount in the anterior region and in the form of narrow strands around the mesonephric ducts posteriorly. The functional mesonephric kidney is broad anteriorly in the form of a triangular structure and extends posteriorly in the form of a narrow strand on each side of the trunk .[Fig. 33) The intermediate cell mass forms the vascular strands and gives rise to the blood vessels . In the young stages, the caudal vein is continued directly with the posterior cardinal vien which leads anteriorly to the posterior cardinal sinuses, establishing the renal portal circulation of the embryo. In the older stages, the dorsal aorta supplies the kidney with oxygenated blood through segmental paired renal arteries while the non-oxygenated blood is supplied to the kidney from the posterior cardinal vein which is branched into two renal portal veins. In the adult stage, the posterior cardinal veins extend anteriorly to the ducts of Cuvier as the right and left posterior cardinal sinuses. The urinary bladder is mesodermal in origin and appears in 3.5 mm. long embryo. In the early stages of development the posterior ends of the pronephric ducts unite together, forming a common urinary duct (common pronephric duct) behind the posterior region of the hind-gut. The epithelium of the common urinary duct is low cuboidal, resembling that of the pronephric ducts. After that, the middle portion of the common urinary duct becomes wide and gives rise to the anlage of the urinary bladder. In the older developnlental stages, the urinary bladder increases in size and becomes very swollen. The wall of bladder in its complete development consists of an inner simple low cuboidal epithelium, a middle connective tissue layer and an outer circular muscle layer . The corpuscles of Stannius develop in 3mm. long embryo, as a pair of thickenings from the intermediate cell mass at the dorsal side of the posteriormost parts of the pronephric ducts. Each of which increases in size, becomes lobulated and is surrounded by a thin connective tissue, forming an oval encapsulated body which is invaded by many blood capillaries.