. Chordate morphology. Morphology (Animals); Chordata. dermatome myotome nephrocoel pronephric crest somatopleure. opening of nephric duct into cloaca B Figure 10-21. The origin of the pronephric tubule in Potypterus from the outer layer of the interme- diate mesoderm, A, and the opening of the nephric duct into the cloaca, B. (A, after van den Broek, 1 938; B, after Goodrich, 1 930) the anterior five rudiments have funnels and their crests join as the precursors of the nephric duct. Several more pos- terior funnels are formed but abort, and the crests of these segments are added to the nephri
. Chordate morphology. Morphology (Animals); Chordata. dermatome myotome nephrocoel pronephric crest somatopleure. opening of nephric duct into cloaca B Figure 10-21. The origin of the pronephric tubule in Potypterus from the outer layer of the interme- diate mesoderm, A, and the opening of the nephric duct into the cloaca, B. (A, after van den Broek, 1 938; B, after Goodrich, 1 930) the anterior five rudiments have funnels and their crests join as the precursors of the nephric duct. Several more pos- terior funnels are formed but abort, and the crests of these segments are added to the nephric duct. The nephric duct continues to be extended posteriorly by crest contributions, separated from the intermediate plate, until it reaches the cloaca (Figure 10-21). The most posterior contributions ap- pear to represent the entire intermediate plate, which then hollows out. As segmental coelomic spaces develop, only the second and fifth pronephric funnels remain, and their nephrocoels open laterally into the coelom through peritoneal funnels. Each of these remaining tubules develops a renal corpuscle. With further growth the two units of a side fuse to form a single element having a single glomerulus and a very con- voluted tubule. As this process of fusion goes on, the con- nection between the nephrocoel (now Bowman's capsule) and the coelom is lost. This single pronephric tubule lies in the fifth somite. The mesonephros develops from segmental clumps of nephrogenic tissue, beginning about four somites behind the single pronephric tubule and extending through the 39th somite. The primary tubules differentiate and enter the nephric duct. In the anterior somites of the mesonephros there are usually two glomeruli and as many tubules. In the caudal half there are more glomeruli (as many as five) with the same number of tubules. In the 20th to 36th somites there are peritoneal funnels for each of the tubules. The pronephros has disappeared in a 90-mm larva and peritoneal funnel
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