. Chordate morphology. Morphology (Animals); Chordata. ureter may join the vas deferens before entering the cloaca or the two may open separately. In the turtle the urogenital sinus lies below the rectum. In the lizard, Sphenodon, and snake the bladder has remained in its ventral position, but the ureters and sex ducts open dorsally into the cloaca. The alligator and bird are like the lizard but have no bladder (Figures 10-9, 10-10). The fine structure of the reptilian kidney (or bird) differs from that of the mammal in lacking the pyramids of collect- ing tubules, and in having the kidney div
. Chordate morphology. Morphology (Animals); Chordata. ureter may join the vas deferens before entering the cloaca or the two may open separately. In the turtle the urogenital sinus lies below the rectum. In the lizard, Sphenodon, and snake the bladder has remained in its ventral position, but the ureters and sex ducts open dorsally into the cloaca. The alligator and bird are like the lizard but have no bladder (Figures 10-9, 10-10). The fine structure of the reptilian kidney (or bird) differs from that of the mammal in lacking the pyramids of collect- ing tubules, and in having the kidney divided into a very large number of small units. These units are related to the blood in quite a different way than the pyramids. Both venous and arterial blood enter the kidney. The arterial blood serves the zone of glomeruli at the center of, or along one side of, the unit. The arteries are paralleled by efferent veins draining the units. At the periphery of the unit, or along the side opposite the artery, is the afferent renal vessel bringing venous blood into the unit. The collecting tubules of the unit parallel the afferent veins (see Figure 10-12). Embryological development: reptile The first rudiments develop in myotomic segments 3 to 10 or 12. The funnels or cords of cells appear first in segments 3 and 4 and later in the more posterior segments. The most anterior rudiments disappear, leaving those in somites 5 to about 10 or 12 (10 to 11 in the turtle Chrysemys). The anterior two or three of these open into the coelom through ciliated funnels. The more posterior cords separate from the somatopleure and contribute to the nephric duct cord which extends back from the anterior funnels. The tubular duct like the funnels is produced by hollowing out of the cell cords. The poste- rior end of the nephric duct now grows posteriorly, without segmental contribution, to enter the cloaca. In the turtle (Chelonta) and the alligator, a midline coelomic glomerulus develops. A number of
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