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he development of the mesonephros of the pink salmon, Oncorhynchus Gorbuscha (Walbum) Newstead, James Duncan

Abstract

The development of the kidney of the pink salmon was traced from the earliest stages to the year-old finger-ling. The development of the pronephros has been treated separately. The mesonephros develops from the intermediate cell mass of the twelfth to the thirty-ninth segments. Differentiation of the unsegmented intermediate cell mass closely follows segmentation of the somites. The vascular strand is established beneath the hypochord by migration of cells from the intermediate mesoderm of either side. A solid cylindrical duct rudiment forms as a fold of the dorsolateral aspect of the mass on either side. Mesonephrogenic material separates as a narrow bridge of cells passing from one duct to the other between the cardinal vein and the aorta. Primary mesonephrogenic rudiments appear as condensations of the bridge cells closest to the ducts. Tubule rudiments are formed by elongation of the condensations. The free end of each tubule rudiment dilates to form a thin walled Bowman capsule whose lumen later extends into the tubule rudiment and finally opens to the duct. A glomerulus develops in the Bowman capsule as a solid invagination of cells from the vascular strand. This mass of cells subsequently becomes vascularised. Differentiation of the tubule into regions follows the appearance of the glomerulus. Secondary and subsequent tubule generations are similarly formed but open into previously formed primary tubules. The intertubular spaces become filled with myeloid tissue richly supplied with blood sinuses. The myeloid tissue is derived from the vascular strand. Three pairs of corpuscles of Stannius appear early in development as outgrowths of the segmental duct epithelium. Smaller unpaired corpuscles appear later to make the total number twelve. The fully differentiated nephron has five regions:- 1. a Bowman capsule and glomerulus 2. a short neck segment which opens directly from the Bowman capsule 3. a segment characterised by its low columnar epithelium and high brush border 4. a segment with tall columnar cells and a low brush border 5. a segment of low simple columnar cells. Several nephrons enter a collecting tubule. At least three generations of tubules open to the segmental duct by way of a common collecting duct formed by modification of the primary collecting tubule. Involution of the pronephros commences at about the time the fry normally enter the water at the river mouth. Retention in fresh water does not appear to affect the time of onset of involution. The pronephric region becomes very richly permeated with blood sinuses and may serve as a blood storage organ in older fish. In the oldest specimens studied arterial blood is supplied by paired arterioles in the intersegmental septa. Venous blood from the tail passes directly into a large median vein which passes anteriad in the kidney tissue. This vein originates as the right postcardinal vein in the young embryo. The left postcardinal does not develop posterior to the pronephros. Portal blood from the dorsal and ventral musculature reaches the kidney by way of venules in the myosepta. In the posterior region two large median arteries (arteria primitiva mesenterica) pass through the kidney to the hind gut. In the pronephric region the large median coeliaco-mesenteric artery passes through the kidney to supply the viscera. Glomerular counts made on samples taken from fresh and salt water over a period of two months suggest that the increased osmotic concentration of the marine environment has the effect of retarding the rate of development of new glomeruli in a logarithmic relation to length.

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