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 1 1 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 nephrocoels fuse and enclose this 

 glomus (or glomerulus). The pronephros begins to degen- 

 erate early (6-7 mm). 



The mesonephros arises in somites 12 to 31, plus or minus 

 a few, according to the species. Each primary canal appears 

 first as a clump of cells which hollows out; then it develops 

 a renal corpuscle and gains entrance into the nephric duct. 

 The number of tubules anteriorly is fewer than posteriorly 

 and their interrelationships are simpler. The tubules are not 

 segmentally arranged; there are several per segment. The 

 anterior part becomes connected with the testis by tubule 

 outgrowth from the Bowman's capsules of the Nephrons or 

 kidney tubules (Figure 10-7) and from the testis. 



The metanephros appears first as an evagination of the 

 extreme posterior end of the nephric duct. This evagination 

 grows outward capped by nephrogenic tissue; it branches 

 forward and posteriorly to form the ureter and primary col- 

 lecting branches. By further subdivision and branching of 

 these, as in the mammal, the metanephric kidney is formed, 

 utilizing the nephrogenic tissues associated with somites 32 

 and 33. The number of nephrons formed in reptiles is far 



kidney 



Mullerion duct 



proctodeum 



Figure 10-9. Ventral view of the urogenital system of a young 

 female alligator. (After Reese, 1915) 



less than in mammals, 1500 to 2000 per kidney as opposed 

 to about a million. The earlier generations of nephrons are 

 retained, so even the larger collecting tubules have nephrons 

 entering them. With the establishment of the metanephros, 

 the anterior part of the mesonephros degenerates down to 

 the epididymis of the male or the vestige observed in the 

 female. 



The urinary bladder when present is formed as in the 

 mammal, from the allantoic outgrowth of the hindgut. 



Embryological deve/opmenf: bird The embryological de- 

 velopment of the bird is much better known than that of 

 the reptile and can be studied using standard embryological 

 preparations, the 48, 72, and 96-hour chick serial sections. 

 The pronephric elements first appear between 40 to 48 

 hours of incubation. The most anterior one is in the third 

 postotic somite, and the units extend posteriorly for about 

 12 segments. Of these, the more anterior ones soon de- 

 generate. 



In many vertebrates the pronephric tubules lie anterior to 

 the ducts of Cuvier, usually in an anterior cardinal sinus. 

 In the 48-hour chick, the most anterior tubules lie behind 

 the level of the ducts of Cuvier (7th to 8th somite), the large 

 venous channels draining the dorsal body-wall vessels (the 

 cardinals) into the heart (Figure 11-28). The anterior tu- 



THE EXCRETORY SYSTEM • 297 



