. Chordate morphology. Morphology (Animals); Chordata. ^Bowman's capsule Figure 10-4. Five stages, (A to E), in the development of a meso- nephric tubule. (After Ihle et al, 1927) end of the nephric duct (Figure 10-5). This outpocketing enters the nephrogenic cord; it forms the ureter and from it grow the calyces and several generations of collecting tubes. The nephrons develop from the nephrogenic cord in a sequential fashion as new generations of collecting tubes are produced (Figures 10-6, 10-7). Although the first few genera- tions of nephrons are resorbed, at least 12 generations are pres
. Chordate morphology. Morphology (Animals); Chordata. ^Bowman's capsule Figure 10-4. Five stages, (A to E), in the development of a meso- nephric tubule. (After Ihle et al, 1927) end of the nephric duct (Figure 10-5). This outpocketing enters the nephrogenic cord; it forms the ureter and from it grow the calyces and several generations of collecting tubes. The nephrons develop from the nephrogenic cord in a sequential fashion as new generations of collecting tubes are produced (Figures 10-6, 10-7). Although the first few genera- tions of nephrons are resorbed, at least 12 generations are present at the time of birth. This means more than one mil- lion nephrons are present in each kidney. In the later stages of development, the kidneys move forward, or upward in the case of man, beneath the peritoneum until they near the diaphragm. The kidneys of monotremes remain in the posterior part of the body cavity. The ducts of the kidney, the ureter, and the nephric duct switch positions during development, so that the ureters enter the bladder, while the nephric duct, which is now the male vas deferens (degenerate in the female), en- ters the urethra. The latter e.\tends from the bladder to the outside. The bladder is formed from the allantois. At this point a few definitions appear to be in order, based on structure. The pronephros is composed of tubules formed segmentally at the anterior end of the body cavity. These may open into the coelomic cavity by way of funnels. The tubules unite distally to form a part of, or give rise to, the entire nephric duct. The mesonephros can be defined as having tubules formed from buds of nephrogenic tissue ly- ing medial to the nephric duct. These tubules later estab- lish entrance into the nephric duct. There may be several of these tubules per segment; the two sides of the animal are asymmetric. Peritoneal funnels may develop in relation to these tubules. The metanephros is made up of a system of collecting tubules which grow out from
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